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UNIVERSITÀ DEGLI STUDI DI MILANO-BICOCCA SCUOLA DI DOTTORATO DI SCIENZE CORSO DI DOTTORATO DI RICERCA IN FISICA E ASTRONOMIA Riccardo A. Siliprandi Atmospheric Pressure Plasmas for Surface Modifications Relatore: Prof. Claudia Riccardi Coordinatore: Prof. Claudio Destri Ciclo XX 2004-2007 to my family Avrò piacere d’esser illuminato e tratto d’errore Simplicio in Dialogo sopra i due massimi sistemi del mondo Galileo Galilei, 1632 Contents 1 2 Introduction 1 1.1 Cold Atmospheric pressure plasmas . . . . . . . . . . . . . . . 1 1.2 Surface modifications with atmospheric plasmas . . . . . . . . 2 1.3 Motivations and thesis outline . . . . . . . . . . . . . . . . . . 3 Atmospheric pressure discharges and surface processes 5 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2 Electrical Breakdown of Gases . . . . . . . . . . . . . . . . . 6 2.2.1 Townsend breakdown mechanism . . . . . . . . . . . . 7 2.2.2 Streamer breakdown mechanism . . . . . . . . . . . . 10 2.3 2.4 Dielectric Barrier Discharges . . . . . . . . . . . . . . . . . . 15 2.3.1 Overview and properties of dielectric barrier discharges 16 2.3.2 Dielectric barrier discharge regimes . . . . . . . . . . . 16 2.3.3 Streamer Discharge Regimes . . . . . . . . . . . . . . 18 2.3.4 Micro-discharge interaction and pattern formation . . 20 Plasma-surface interactions . . . . . . . . . . . . . . . . . . . 21 2.4.1 Gas-phase chemistry and processes . . . . . . . . . . . 21 2.4.2 Surface kinetics and processes . . . . . . . . . . . . . . 24 VII VIII 3 Dielectric barrier discharge devices 29 3.1 DBD device for surface modifications . . . . . . . . . . . . . . 29 3.1.1 Plasma reactor . . . . . . . . . . . . . . . . . . . . . . 30 3.1.2 Pumping system and gas distribution . . . . . . . . . 30 3.1.3 Electric power supply and configuration . . . . . . . . 33 3.1.4 Diagnostics . . . . . . . . . . . . . . . . . . . . . . . . 33 DBD device for streamer regime characterization . . . . . . . 34 3.2.1 Plasma reactor . . . . . . . . . . . . . . . . . . . . . . 34 3.2.2 Diagnostics . . . . . . . . . . . . . . . . . . . . . . . . 35 3.2 4 CONTENTS Plasma and material diagnostics 37 4.1 Optical emission spectroscopy . . . . . . . . . . . . . . . . . . 37 4.1.1 Determination of molecular vibrational temperature . 38 Voltage Current measurements . . . . . . . . . . . . . . . . . 38 4.2 4.2.1 4.3 Implementation of Rogowski coils for measurements nanoseconds current pulses . . . . . . . . . . . . . . . 38 Characterization of the materials surfaces . . . . . . . . . . . 43 4.3.1 Infrared spectroscopy (FTIR/ATR-FTIR) . . . . . . . 44 4.3.2 Atomic force microscopy (AFM) . . . . . . . . . . . . 45 4.3.3 Contact angle measurements and surface energy determination . . . . . . . . . . . . . . . . . . . . . . . . 5 46 Statistical characterization of a streamer discharge regime 51 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 5.2 Statistical characterization of current signal . . . . . . . . . . 53 5.2.1 streamers. . . . . . . . . . . . . . . . . . . . . . . . . . 54 Discharge current regimes . . . . . . . . . . . . . . . . 56 Statistical analysis of temporal behavior . . . . . . . . . . . . 62 5.2.2 5.3 5.3.1 Inter- and intra-bump correlations: surrogate model and Hurst exponents . . . . . . . . . . . . . . . . . . . 62 Temporal correlations between streamers . . . . . . . 68 Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . 75 5.3.2 5.4 Structure of the discharge current: bumps, bursts and CONTENTS IX 6 Characterization of the DBD device in nitrogen atmosphere 79 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 6.2 Experimental setup and methods . . . . . . . . . . . . . . . . 80 6.3 Discharge regimes in Nitrogen Atmosphere . . . . . . . . . . . 80 6.3.1 Characterization of the discharge as a function of injected power . . . . . . . . . . . . . . . . . . . . . . . 6.3.2 6.4 7 sure and gas fluxes . . . . . . . . . . . . . . . . . . . . 85 Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . 89 91 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 7.2 Materials and methodology . . . . . . . . . . . . . . . . . . . 92 7.3 Characterization of the deposition process . . . . . . . . . . . 94 7.3.1 Plasma characterization . . . . . . . . . . . . . . . . . 95 7.3.2 Thin film characterization . . . . . . . . . . . . . . . . 97 Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . 105 Fluorination of polymer surfaces 107 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 8.2 Experimental, diagnostics and methods . . . . . . . . . . . . 109 8.3 Characterization of the fluorine grafting process . . . . . . . . 110 8.4 9 Characterization of the discharge as a function of pres- Deposition process of organosilicon thin films 7.4 8 83 8.3.1 Plasma-phase characterization . . . . . . . . . . . . . 110 8.3.2 Material surface characterization . . . . . . . . . . . . 113 Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . 119 Plasma Application for modification of paper surfaces 9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 9.1.1 9.2 9.3 121 Cellulose and paper . . . . . . . . . . . . . . . . . . . 121 Deposition of organic silicon compounds for hydrophobicity . 123 9.2.1 Experimental setup and diagnostics . . . . . . . . . . 124 9.2.2 Hydrophobicity of treated paper surface . . . . . . . . 125 Fluorination process for oil repellency . . . . . . . . . . . . . 128 9.3.1 Experimental setup and diagnostics . . . . . . . . . . 129 X CONTENTS 9.3.2 Oil repellency of paper surfaces . . . . . . . . . . . . . 129 10 Conclusions 135 Bibliography 139 CHAPTER 1 Introduction 1.1 Cold Atmospheric pressure plasmas Atmospheric pressure plasmas are growing as an interesting alternative to low pressure plasmas for several applications. The possibility to develop continuous processing without the costs of vacuum technologies has attracted in the last decade the efforts of several industries and research groups all over the world. Within the different types of atmospheric pressure non-thermal plasmas, dielectric barrier discharges (DBDs) are the most interesting solution. DBDs are a well known type of gas discharge. They have been widely used in industrial applications like ozone generators, plasma display panels, excimer lamps, volatile organic compounds destruction and surface modifications [1, 2, 3, 4, 5, 6]. DBDs are low temperature, non-equilibrium, transient gas discharges operating in a quasi-continuous discharge regime. They usually consist of two electrodes (at least one of them covered with a dielectric material) to 1 2 INTRODUCTION which an AC high voltage is applied for frequencies generally varying in the range of 102 Hz to 102 kHz. At low pressures DBDs operate in a Townsend breakdown regime [5] generating a diffuse glow discharge. At atmospheric pressure, the realization of a diffuse discharge is restricted to limited conditions of geometry, electrical parameters and gas composition [7, 8, 9, 10, 11], and DBDs operate usually in a streamer discharge in which several narrow discharge filaments are typically formed. The streamer regime constitutes a strongly interacting system of discharges exhibiting cooperative behavior. This leads, under specific conditions, to the formation of coherent spatial configurations that have been observed in different types of experimental setups [12, 13, 14, 6]. However, micro-discharges seem, to some extent, to occur at random within the discharge gap for most applications of DBDs. Despite of the existence of several industrial applications and intense study during decades, DBDs still lack a clear physical interpretation of the discharge regimes and of the complex chemistry involved in the processes. 1.2 Surface modifications with atmospheric plasmas Plasma-surface interaction is a rather complicate process which involves several complex chemical and physical mechanisms [15, 16]. For this reason plasma processing is the subject of study in many research areas like plasma physics, surface science, gas-phase chemistry and atomic and molecular physics. The common theme is the creation and use of plasmas to activate a chain of chemical reactions at a substrate surface. At low pressure the behavior of many chemical processes in presence of a plasma have been the subject of intense research in the recent years and is now a well established industrial process[17, 15, 18, 19, 2]. An example is the semiconductor industry which successfully employs plasma processes for the production of integrated circuits. DBDs are already employed in industry for modification of material surfaces [1]. This application has regarded mainly the processes in atmospheric Air for treatments of polymer surfaces to attain wettability, printability and adhesion properties [1, 20, 21]. Its use in different reactive atmospheres has proved more difficult because of the strong dependence of the discharge regime on the atmosphere composition and the absence of an environment 1.3 MOTIVATIONS AND THESIS OUTLINE 3 sealed from contaminations [1, 22]. Moreover, the behavior of several reactive atmospheres, well known at low pressure, is different to some degree when used at atmospheric pressure. This is due to the great difference between the discharge regimes that can be realized at high pressure and the substantial change in chemical and physical processes both in gas-phase and on the surface [23, 24, 25, 26]. 1.3 Motivations and thesis outline This thesis concerns the study of atmospheric pressure plasmas realized in dielectric barrier discharges and their applications to surface modifications of materials. The first part of the work is dedicated to the study and the characterization of the streamer regime. Streamer development is still subject of intense study and several theoretical models and few experiment describe the development of the single phenomena. However, due to the complexity of the interaction between micro-discharges, a clear physical picture of their behavior as a whole system is not presently available. In this study a novel approach to the problem is used, and the tools of statistical analysis are used to investigate the temporal behavior of micro-discharges through the measurement of current signal. Several interesting feature regarding the discharge dynamics and the temporal propagation of correlations are discussed. The second part of this work is dedicated to the study of plasma processing of material surfaces. These studies are performed using a newly built plasma reactor which gives the possibility to study both the discharge physics and the plasma-surface interaction during continuous processing in a wide range of pressures and compositions of the atmosphere. Two specific processes are studied: a deposition process of thin organosilicon films for the creation of hydro-repellent coatings and a grafting of fluorine atoms process to produce hydro- and oil-repellent properties on the surfaces of organic soft matter. The applications of these processes to the modification of paper surfaces is then studied. The thesis is organized as follows. In Chapter 2 a brief review of the physical foundations of plasma discharges and plasma-surface interactions 4 INTRODUCTION is given. Chapter 3 describes the plasma devices that have been realized and used in the experiments. Chapter 4 describes the diagnostics both for the plasma and materials. The description of study and realization of high bandwidth Rogowski coils for the measurement of fast current pulses is given. Chapter 5 involves the study of the streamer regime and its characterization by means of statistical analysis of current signals. Chapter 6 gives a characterization of plasma discharges in nitrogen atmosphere. Chapters 7 and 8 describe two atmospheric pressure processes of surface modifications. A deposition process of thin organosilicon films and a grafting process of fluorine radicals. Chapter 9 is finally devoted to the presentation of some results concerning the application of studied processes for modification of paper surfaces. CHAPTER 2 Atmospheric pressure discharges and surface processes In this chapter the general concepts of and features of electrical discharges in gases the the plasma-surface processes are briefly reviewed. Attention is focused on physical phenomena interesting for the arguments of the present research. For a more complete insight of the problems reader is advised to refer to literature [5, 2, 15, 1]. 2.1 Introduction One of the simplest way to produce a plasma is applying an electric field to a neutral gas. These artificially generated plasmas can be classified into two main categories: thermal and non-thermal ones. In a plasma, as in any gas, the temperature is determined by the average kinetic energy of its components. However, a plasma can exhibit multiple 5 6 ATMOSPHERIC PRESSURE DISCHARGES AND SURFACE PROCESSES temperatures, usually one for the heavy particles and one for the electrons (Ti and Te respectively) unless sufficient collision occur between them. Because of the large difference in mass between electrons and other particles the temperatures of these two species remains different in many conditions. When Te ≃Ti the plasma is considered in local thermodynamic equilibrium , LTE, (which requires also the absence of chemical gradients) and termed as thermal plasma. These discharges are characterized by high temperature of the gas. Examples of thermal plasma are the Plasma Torches or the fusion plasma devices. Otherwise, when large deviations from LTE are present (i.e. Te > Ti ) the plasma is not thermalized an is called non-equilibrium or nonthermal plasma. The main feature of non-thermal plasmas is that the most part of the electrical energy injected in the system is used for the production of energetic electrons rather than heating the gas, while the neutral species and ions remain relatively cold because of the low energy exchange with light particles. The electrons have enough energy to ionize other molecules and atoms generating excited species, other electrons and free radicals. They can achieve sufficient energy to initiate chemical reactions usually forbidden to standard chemistry in the same condition. Plasma can initiate several physical and chemical processes on material surfaces which can provide an efficiency increase in processing methods and very often can reduce environmental impact in comparison to more conventional processes. 2.2 Electrical Breakdown of Gases Electric breakdown is referred to the process that transforms a non-conducting material to a conducting one when a sufficient strong electric field is applied. Although the breakdown is a rather complicate process that strongly depends on the system conditions, it begins always with an electron avalanche, i.e. the multiplication of some primary seed electrons in cascade ionization when accelerated by the electric field. After this initial stage the following development of the discharge depends on several parameters as gas composition, pressure, distance between electrodes, frequency of the applied field and geometry of the system. For sufficient low pressure the mean free path of the electrons is high and the initial avalanche proceeds until the plasma is generated in the whole discharge gap. For relative High pressure the mean 2.2 ELECTRICAL BREAKDOWN OF GASES 7 Figure 2.1: Townsend electrical breakdown in a gap d with constant electric field E = V /d. Secondary electrons emitted by the cathode generate the multiplication of avalanches [2]. free path of the electrons is drastically reduced and the avalanche ionization can generate a great number of electrons giving rise to a localized space charge which propagates in the discharge gap creating a thin conductive channel named streamer. If no means are taken to limit the current in the system, the the streamer is only the initial stage of the so called arc discharge. 2.2.1 Townsend breakdown mechanism The discharge process at low pressure or for low values of pd products, where p is the pressure and d is the inter-electrode gap distance, is called Townsend1 . For the sake of simplicity consider a system of parallel plate electrodes at a distance d to which is applied a DC Voltage V that provides a constant field E = V /d. The seed electrons generated from an external source (for example cosmic rays or natural radioactivity) are accelerated by the electric field in the gap and reach the anode unless they are lost in the way by ion recombination or interaction with the chamber wall. The greater the external electric field (i.e. faster electrons), the smaller the the fraction 1 From the name of John Sealy Townsend who first introduced this model to explain electrical breakdown in gases. 8 ATMOSPHERIC PRESSURE DISCHARGES AND SURFACE PROCESSES of the electrons lost before they reach the anode. As a result, the electric current i in the circuit, which is proportional to the number of charged species which reach the electrodes, initially increases with increasing voltage V . Beginning at a certain voltage, quite all the charged particles (electron and ions) reach the electrodes and the current reaches a saturation i0 and ceases to depend on V . At this point the discharge is non self-sustaining, i.e. the discharge depends on the presence of an external sources (point A in Figure 2.2). At still higher voltage, the electron impact ionization on neutral gas molecules starts initiating the avalanche process (Figure 2.1) and amplifying the initial current i0 due to the external source. It is convenient to describe the ionization in an avalanche by the Townsend ionization coefficient α that express the electron production per unit length: dne = αne −→ ne (x) = n0e eαx dx (2.1) where x is the distance from the cathode, ne the electron density and n0e is the initial electron density created by the external sources. For simplicity here the electron losses due to recombination and attachment to electronegative molecules are neglected. The current at the anode (and so the current in the closed circuit) is equal to: i = i0 eαd where i0 = en0e and e is the charge of the electron. The primary process of electron impact ionization generates n0e [eαd − 1] ions during the avalanche propagation which become important when the voltage is furthermore raised and hitting the cathode they can generate γn0e [eαd − 1] electrons in the process of secondary electron emission. γ is the secondary electron emission coefficient and it depends on cathode material, state of the surface and electric field (which define ion energy). Taking into account this secondary process the current in the circuit is: i= i0 eαd 1 − γ[eαd − 1] (2.2) which is called Townsend formula and was first derived in 1902 to describe the breakdown process in electric discharges. The transition from non selfsustaining to self-sustaining discharge is controlled by the denominator in Equation (2.2). If µ = γ[eαd −1] < 1 the discharge is still non self-sustaining, but when µ approaches to unity the current grows to infinity and the discharge becomes self-sustaining, i.e. the breakdown occurs. The simplest 9 2.2 ELECTRICAL BREAKDOWN OF GASES Figure 2.2: Voltage-current characteristic of low temperature discharge between electrodes for a wide range of currents. A: region of non-self-sustaining discharge, (BC) Townsend discharge, (CD) subnormal glow discharge, (DE) normal glow discharge, (EF) abnormal glow discharge, (GH) arc discharge [5]. breakdown condition can be expressed as: µ = γ[eαd − 1] −→ αd = ln( 1 + 1) γ (2.3) which means that each primary electron generated in the initial avalanche and lost at the anode is replaced by another electron generated by secondary emission at the cathode. This represents a steady self-sustained current in the homogeneous field Et = Vt /d (point B in Figure 2.2), where Vt is the breakdown voltage and is determined from Equation (2.3) as a function of d an in terms of γ and the known function α(E). In the presented ideal framework the current i for a voltage V = Vt would increase to infinity. Any real circuit of the type described above has an ohmic resistance Ω which sets a limit to the current achievable for a given electromotive force E. In the case Ω is so great that only a really small current can flow through the gap and the electrode gap is small in comparison to electrode dimensions, the field is constant and equal to the field in absence of discharge. The potential will be equal to the breakdown voltage Vt . This stable self-sustained discharge with extremely low current is called Townsend dark discharge (segment BC in Figure 2.2). Let us gradually increase the current. This can be realized by reducing the load resistance Ω 10 ATMOSPHERIC PRESSURE DISCHARGES AND SURFACE PROCESSES or by increasing the electromotive force E. The voltage across the electrodes begins to decrease after a certain current is reached. The fall then stops and the current remains almost constant over a fairly wide range of values (sometimes of several orders of magnitude). This is the so-called normal glow discharge (segment DE in Figure 2.2). The lower part of the transition region (segment CD in Figure 2.2) corresponds to a sub-normal glow discharge. The normal discharge has one remarkable property. As the discharge current is varied, its density at the cathode remains unchanged while changes the area through which the current flows. When Ω or E is varied, the luminous current spot on the cathode surface expands or contracts. When no more free surface is left on the cathode, the current is increased by increasing the voltage, hence extracting more electrons from unit surface area. Indeed, the cathode current density must grow. This discharge is said to be abnormal (segment CD in Figure 2.2). The glow first covers the entire cathode surface facing the anode, then reaches every spot unprotected by dielectric on the lateral and inner surfaces and on the support pin, and only having exhausted these possibilities does it become more extended and intense to a degree typical of the abnormal discharge. When i ∼ 1A, the glow discharge cascades down to an arc discharge which is characterized by high current and low voltage. The segment FG in Figure 2.2 describes the transition, and GH represents the arc discharge. 2.2.2 Streamer breakdown mechanism When the pressure is high an the pd > 100 Torr·cm, the Townsend breakdown cannot describe the discharge development. This mechanism is based on the emission of secondary electrons from the cathode and develops in time of the order 10−5 − 10−3 . For high values of pd the breakdown develops much faster and the independence of the breakdown voltage on the material of the cathode, established by very accurate measurements, is evidence against the participation of cathode processes in the breakdown mechanism. This different breakdown is called streamer breakdown for the thin localized plasma channels that are generated in the process. The concept of streamer was originally developed by Raether [27], Loeb [28] and Meek [29]. Also at high pressure an individual avalanche is a primary and compulsory element of the breakdown mechanism. Consider an avalanche in a 11 2.2 ELECTRICAL BREAKDOWN OF GASES uniform external field E0 between plane electrodes. Let it be initiated by a single electron that leaves the cathode at the time t = 0. The x axis is directed from a point on the cathode to the anode. The radial distance from the x axis is denoted by r. Taking into account the possible formation of negative ions, we find the total numbers of electrons and ions increasing as the avalanche moves forward: dN+ dN− dNe = (α − η)Ne , = αNe , = αNe , dx dx dx Ne = e(α−η)x , N+ = α α (Ne − 1), N− = (Ne − 1), α−η α−η (2.4) (2.5) where α and η are the ionization and attachment coefficients. All the new electrons fly to the anode in a group at the drift velocity vd = µe E0 where mue E0 is the electron mobility. However, free diffusion (De )makes the electron cloud spread around the central point x0 = vd t, r = 0. Taking into account both the effects the electron density can be expressed as: (x − vd t)2 + r 2 ne = (4πDe t)−3/2 exp − + (α − η)vd t 4De t (2.6) The density ne decreases with distance from the moving center following a Gaussian law. The radius of the sphere on which the density is exactly e times less than that at the centre, ne (x0 , 0, t), increases with time (during the progress of the avalanche) by the characteristic diffusion law: rD = p 4De t = s De x0 4 = µe E0 r 4Te x0 . eE0 (2.7) The ions remain practically fixed during the time of flight of the avalanche to the anode. (see Figure 2.3). Thus, they accumulate at each point. The positive ion density is n+ (x, r, t) = Z t αvd ne (x, r, t′ )dt′ , (2.8) 0 In the absence of attachment in the limit t → ∞ and for regions not too far from the axis, an approximate calculation of the integral using Equations 12 ATMOSPHERIC PRESSURE DISCHARGES AND SURFACE PROCESSES Figure 2.3: Formation of a streamer[5]. (2.6) and (2.8) gives n+ (x, r) = α r2 exp αx − πra2 (x) ra2 (x) (2.9) where ra2 (x) is the avalanche radius defined by Equation (2.7). The ion concentration in the trail of the avalanche is growing up along the axis in accordance with exponential (2.4) increase of number of electrons. The qualitative change in avalanche behavior takes place when the charge amplification exp(αx) is high. In this case the production of a space charge with its own significant electric field E ′ takes place. This local electric field E ′ should be added to the external field E0 . Because the electrons are much faster than ions the electrons always run at the head of avalanche leaving the ions behind and thus creating a dipole with the characteristic length 1/α (mean distance for an electron before creating an ion) and charge Ne ∼ exp (αx). The fields E ′ and E0 in front of the avalanche head add up to give a field stronger than E0 . The fields E ′ and E0 in the zone between the centers of the space charges of opposite signs point in opposite directions and the resultant field is weaker than E0 . When the avalanche reaches the anode, the electrons sink into the metal and only the positive space charge of the ionic trail remains in the gap (Figure 2.3). The field is formed by the ionic charge and by its ”image” in the anode. The image in the relatively 13 2.2 ELECTRICAL BREAKDOWN OF GASES distant cathode plays a rather insignificant role. The field close to the anode is less than E0 , but exceeds it farther off. The field reaches its maximum at the axial distance from the anode of the order of one ionization length α. When the number of charges Ne is high, the diffusional spreading of the electron cloud is replaced by their electrostatic repulsion. The law of expansion R(x), is given by: R= 3e 1/3 αx 3E ′ exp . = αE0 3 αE0 (2.10) The fast growth of the transverse avalanche size restricts the electron density in the avalanche by the value: ne = (3Ne )/(4πR3 ) = (αE0 )/(4πe). When the transverse avalanche size reaches the characteristic ionization length 1/α (about 0.1 cm at atmospheric pressure in Air), the broadening of the avalanche head slows down dramatically. Obviously, the avalanche electric field is about the external one in this case (see eq. 2.16). The typical values of maximum electron density in an avalanche are about 1012 − 1013 cm3 . When the avalanche head reaches the anode, the electrons sink into the electrode leaving the ions occupy the discharge gap. At the electron absence, the total electric field is due to the external field, the ionic trail and also the ionic charge image in the anode. The resulting electric field in the ionic trail near the anode is less than the external electric field, but farther off the electrode it exceeds E0 . The total electric field reaches the maximum value on the characteristic ionization distance (about 1 mm from the anode). A strong primary avalanche amplifies the external electric field leading to formation of thin weakly ionized plasma channel, so-called streamer. The avalanche-to-streamer transformation takes place, when the internal field of an avalanche becomes comparable with the external one, that is when the amplification parameter αd is big enough. At a relatively small discharge gaps, the transformation occurs only when the avalanche reaches the anode. Such a streamer is known as the cathode-directed or positive streamer. If the discharge gap and over-voltage are big enough, the avalanche-to-streamer transformation can take place quite far from anode. In this case the socalled anode-directed or negative streamer is able to grow toward the both electrodes. The cathode-directed streamer starts near the anode. It looks like and 14 ATMOSPHERIC PRESSURE DISCHARGES AND SURFACE PROCESSES operates as a thin conductive needle growing from the anode. The electric field at the tip of the anode needle is very high, which stimulates the fast streamer propagation in direction of cathode. Usually the streamer propagation is limited by neutralization of the ionic trail near the tip of the needle. The electric field there is so high, that provides electron drift with velocity about 108 cm/s. One hypothesis states that the decisive role is played by energetic photons that are emitted by atoms excited in the avalanche and produce photoionization in the vicinity of the primary avalanche. (Events of production of electrons at the cathode or far from the trail are unimportant in this context because they result in avalanches similar to the primary one.) Electrons produced by photons initiate secondary avalanches that are pulled into the trail due to the direction of the resulting field. Secondaryavalanche electrons intermix with primary-avalanche ions and form a quasineutral plasma. They also excite atoms, so that new photons are emitted. Secondary-avalanche ions en- enhance the positive charge at the cathode end of the evolved plasma channel. This charge attracts the electrons of the next generation of secondary avalanches, etc. This is how the streamer grows. The process of ionization along the ion trail of the primary avalanche begins at the spot where the positive charge and the field are the highest, that is, at the anode, provided the degeneration condition E ′ ∼ E0 has been reached there. This is the situation shown in Figure 2.3. The anode-directed streamer occurs between electrodes, if the primary avalanche becomes strong enough even before reaching the anode. The streamer propagates in direction of cathode in the same way as cathodedirected streamer. Mechanism of the streamer growth in direction of anode is also similar, but in this case the electrons from primary avalanche head neutralize the ionic trail of secondary avalanches. However, the secondary avalanches could be initiated here not only by photons, but also by some electrons moving in front of the primary avalanche When the streamer channel connects the electrodes, the current may be significantly increased to form the spark or arc discharge which are characterized by high current and low voltage. This would lead to Joule heating of the gas and the generation of a thermal plasma. 2.3 DIELECTRIC BARRIER DISCHARGES 15 Figure 2.4: Examples of dielectric barrier discharge systems[2]. 2.3 Dielectric Barrier Discharges As it has been shown in Section 2.2.2 once the conducting streamer channel is established electrons can flow trough it and sink at the anode until current rises and the streamer converts to a spark. If no means are used to limit the current, the temperature of the gas will rise rapidly due to Joule heating (thermal plasma). The simplest solution to the problem is to place a dielectric barrier between the electrodes which prevents the electrons to reach the electrodes and sink. At this point after the streamer channel is created only a limited current for a short time can flow and the temperature of the gas remains quite low while the electrons posses temperatures of the order of electronvolts. This solution establishes a transient discharge which must be reactivate by the external circuit using an alternating or pulsed current power supply. With such system it is possible to obtain a quasi-continuous regime. These are called dielectric barrier discharges (DBDs). Example of DBD system are shown in Figure 2.4 16 ATMOSPHERIC PRESSURE DISCHARGES AND SURFACE PROCESSES 2.3.1 Overview and properties of dielectric barrier discharges Dielectric barrier discharges have a high number of industrial applications [19, 18, 1, 2] because they operate at strongly non-equilibrium conditions at atmospheric pressure and at reasonably high power levels, without using sophisticated pulse power supplies. This discharge is industrially applied in ozone generation [30], CO2 lasers, and as a UV-source in excimer lamps [31, 32]. In addition, the DBD in air is commonly used to treat polymer surfaces in order to promote wettability, printability, and adhesion [1, 22]. DBD application for pollution control is quite promising, but the largest expected DBD application is related to plasma display panels for large-area flat television screens. Strong thermodynamic non-equilibrium and simple design these distinctive properties of DBD allow hoping on expansion of its applications in low temperature atmospheric pressure plasma chemistry. DBD has a big potential to be a prospective technology of exhaust cleaning from CO, NOx and volatile organic compounds [33, 34]. Successful use of DBD reported in recent research on plasma-assisted combustion may result in new applications [35]. 2.3.2 Dielectric barrier discharge regimes Usually at atmospheric pressure for values of product pd > 100Torr·cm the breakdown is the streamer breakdown (see Section 2.2.2) which leads to the formation of several narrow micro-discharges. The origin of the streamer is a large electronic avalanche creating enough ions to localize the electrical field. It is observed when the gas gap becomes large compared with the electron mean free path. However, yet in 1968, Bartnikas found that helium ac discharges between closely spaced plane-parallel electrodes, metallic or covered with a dielectric layer, can exhibit diffuse glow discharge characteristics [36]. After this first observation several research groups studied this particular regime finding that stable diffuse discharges could be obtained in gases including helium, neon, argon, nitrogen, oxygen, and air [7, 8, 37, 9, 10, 11]. However, this diffuse regimes remains extremely unstable and tends to convert to the more stable streamer regime. A detailed explanation of the operation of diffuse discharges is not known. It is clear, however, that streamers can be avoided by using an applied elec- 2.3 DIELECTRIC BARRIER DISCHARGES 17 tric field below the Meek criterion. The requirement for establishing a stable diffuse discharge is that the number of seed electrons is large enough to cause appreciable overlap and merging of the primary avalanches. This results in a smoothing of the field gradients due to space-charge at the stage when streamer formation would otherwise occur. The governing parameters of this transition are the effective first ionization coefficient ᾱ (which is defined as ᾱ = α − η where η is the electron attachment coefficient) and the secondary electron emission from the cathode γ. The coefficient ᾱ, or bet- ter the quantity ∂(ᾱ/n)/∂(E/n) (evaluated at the breakdown) is bound to the radius of the propagating streamer channel [4, 6, 38]. A low value of this quantity results in a wider streamer channels that overlap more easily to form a diffuse discharge. The increase of streamer radius has also been observed experimentally [39]. However, the fundamental mechanism that ensures the presence of enough seed electrons is the so called Penning ionization [40, 41]. A Penning mixture usually consists of a gas with small admixture of impurities. If the components of the impurity B have a ionization potential lower than the metastable potential of the gas A, then the metastable atoms of the main gas can ionize the molecules of the admixture according to A ∗ +B−→A + B + + e− . (2.11) Usually, the probability of this process is so high that very small admixtures may have considerable influence on the discharge development. For example, in Helium which possess highly energetic metastable levels (e.g. He[23 S] and [21 S]), for the creation of a Penning mixture the background impurities may be enough. The presence of seed electrons lowers the breakdown voltage allowing the discharge to develop without the creation of intense field gradients due to space-charge. However this condition requires that the slope of the voltage versus time is limited. Thus, in presence of a sinusoidal excitation voltage, its amplitude and frequency that allows to obtain a diffuse discharge are limited. Very high values of these parameters induce instabilities which lead to a pure filamentary discharge and limits the discharge power in this regime. Another problem of the diffuse discharge is that the presence of electronegative gases in the mixture can rapidly quench the seed electrons reducing their number. This leads again to the streamer 18 ATMOSPHERIC PRESSURE DISCHARGES AND SURFACE PROCESSES regime, limiting the reactive atmosphere that can be eventually employed in diffuse plasma processes. 2.3.3 Streamer Discharge Regimes The dielectric barrier discharge gap usually includes one or more dielectric layers, which are located in the current path between metal electrodes. Two specific DBD configurations, planar and cylindrical are illustrated in Figure 2.4 . Typical discharge gaps varies from 0.1 mm to several centimeters. Breakdown voltages of these gaps with dielectric barriers are practically the same as those between metal electrodes. If the dielectric-barrier discharge gap is a few millimeters, the required AC driving voltage with frequency 500 Hz to 500 kHz is typically about 10 kV at atmospheric pressure. The dielectric barrier can be made from glass, quartz, ceramics or other materials of low dielectric loss and high breakdown strength. Then a metal electrode coating can be applied to the dielectric barrier. The barrier-electrode combination also can be arranged in the opposite manner, e.g. metal electrodes can be coated by a dielectric. As an example, steel tubes coated by an enamel layer can be effectively used in the dielectric-barrier discharge. In most cases, dielectric barrier discharges are not uniform and consist of numerous micro-discharges distributed in the discharge gap as can be seen from figure 2.5. The physics of micro-discharges is based on an understanding of the formation and propagation of streamers, and consequent plasma channel degradation. The electrons in the conducting plasma channel established by the streamers dissipate from the gap in about 40 ns, while the heavy and slowly drifting ions remain in the discharge gap for several microseconds. Deposition of electrons from the conducting channel onto the anode dielectric barrier results in charge accumulation and prevents new avalanches and streamers nearby until the cathode and anode are reversed (if applied voltage is not much higher than voltage necessary for breakdown). The usual operation frequency used in the dielectric barrier discharges is around 20 kHz, therefore the voltage polarity reversal occurs within 25 µs. After the voltage polarity reverses, the deposited negative charge facilitates the formation of new avalanches and streamers in the same spot. As a result, a manygeneration family of streamers is formed that is macroscopically observed as a bright filament that appears to be spatially localized. It is important 2.3 DIELECTRIC BARRIER DISCHARGES 19 to clarify and to distinguish terms streamer and micro-discharge. An initial electron starting from some point in the discharge gap (or from cathode or dielectric that cover the cathode in the case of well developed DBD) produces secondary electrons by direct ionization and develops an electron avalanche. If avalanche is big enough the cathode directed streamer is initiated (usually from the anode region). Streamer bridges the gap in few nanoseconds and forms a conducting channel of weakly ionized plasma. Intensive electron current will flow through this plasma channel until local electric field is collapsed. Collapse of the local electric field is caused by the charges accumulated on dielectric surface and ionic space charge (ions are too slow to leave the gap for the duration of this current peak). Group of local processes in the discharge gap initiated by avalanche and developed until electron current termination usually called micro-discharge. After electron current termination there is no more electron-ion plasma in the main part of micro-discharge channel, but high level of vibrational and electronic excitation in channel volume along with charges deposited on the surface and ionic charges in the volume allow us to separate this region from the rest of the volume and call it micro-discharge remnant. Positive ions (or positive and negative ions in the case of electronegative gas) of the remnant slowly move to electrodes resulting in low and very long ( 10 µs for 1 mm gap) falling ion current. Micro-discharge remnant will facilitate formation of new micro-discharge in the same spot as the polarity of the applied voltage changes. That is why it is possible to see single filaments in DBD. If microdischarges would form at a new spot each time the polarity changes, the discharge would appear uniform. Thus filament in DBD is a group of microdischarges that form on the same spot each time polarity is changed. The fact that micro-discharge remnant is not fully dissipated before formation of next micro-discharge is called memory effect. The principal micro-discharge properties for most of the frequencies do not depend on the characteristics of the external circuit, but only on the gas composition, pressure and the electrode configuration. An increase of power just leads to generation of a larger number of micro-discharges per unit time, which simplifies scaling of the dielectric barrier discharges. Modeling of the micro-discharges is closely related to the analysis of the avalanche-to-streamer transition and streamer propagation. Detailed 2D-modeling of formation and propagation 20 ATMOSPHERIC PRESSURE DISCHARGES AND SURFACE PROCESSES Figure 2.5: Example of stable pattern formation in a one dimensional DBD in air at a pressure of 500 mbar. of streamers can be found literature [42, 43] where also the mutual influence of micro-discharges is considered [43]. 2.3.4 Micro-discharge interaction and pattern formation Although the DBDs in a streamer regimes have been studied and utilized in industries for several decades, the interaction between micro-discharges is still subject of intense studies and a clear physical picture is yet to be found. In the past decades several experiments have been performed showing that under specific conditions regular pattern can be obtained [44, 14, 13, 12]. These patterns have been modeled using methods that apply generally to pattern formation in nonlinear dynamical systems [45, 46]. Thus, the dynamical interactions between filaments, as well as the chemical and electronic interactions within filaments, needs yet a clear explanation. The development and propagation of a single streamer have been studied both from a theoretical point of view [42, 47, 48] and in few experiments [49, 50]. Also some efforts have been performed to describe the interaction between streamers during their initial stage and propagation [43] but up until now, the only possibility to investigate 3D patterns on the time scale of many breakdowns was Monte Carlo simulation of the micro-discharges distribution [51, 52]. An example of pattern formation in a dielectric barrier discharge is shown in Figure 2.5. What is still to be explained is the role of the interaction between developing discharges and the interaction between the micro-discharge remnants on the dielectric surface and discharge volume (a sort of interaction-through memory effect). In Chapter 5 a different approach based on the temporal analysis is proposed to explain some features and limitations of the memory effect. 2.4 PLASMA-SURFACE INTERACTIONS 21 2.4 Plasma-surface interactions Plasmas are largely employed for the modification of surface properties of materials. Plasma technologies have a great importance in several industrial fields for the optical, physical, and chemical modifications of materials surface. For example about one-third of the processes needed to make a modern semiconductor chip involve a plasma-based process. Indeed, Materials and surface structures can be fabricated that are not attainable by any other method, and the surface properties modifications are unique. In a typical reactive plasma the gas phase chemistry is extremely complex because the highly energetic electrons can activate a great number of reactions. In a plasma the species include neutral atoms and molecules, positive and negative ions, radicals, electrons and photons. These species interact with the surface of materials activating a number of processes which can be reassumed as: reaction of atom or chemical groups insertion (grafting), generation of free radicals on the surface (activation), deposition of a thin layers adherent to the surface (film deposition), chemical or physical ablation of the material surface (etching). Often in reactive plasmas all of the cited processes are present, thus the knowledge both of the gas-phase and surface chemistry is fundamental for the development of plasma applications. 2.4.1 Gas-phase chemistry and processes Describing the complexity of the processes and reaction occurring in a plasma is not a simple task and is far beyond the scope of this introduction (see Ref. [15, 2]). Here are briefly introduced the fundamental processes of a reactive plasma. Ionization processes The fundamental process in a plasma is ionization because it is responsible of its generation and sustainment. There are different kind of such processes. Direct ionization by electron impact is the basic plasma reaction and include the ionizations of non-excited atoms, molecules and radicals. It involves the interaction of an enough energetic electron hitting the other neutral species when its energy is high enough to create an ion-electron pair. 22 ATMOSPHERIC PRESSURE DISCHARGES AND SURFACE PROCESSES Preliminary exited neutral species can undergo further ionization in a stepwise ionization by electron impact. This kind of process is important in thermal or highly energetic discharge when the degree of ionization (ratio of electron and ion density) is high. Ionization by collision with heavy particles can generate electrons during ion-molecular or ion-atomic collisions involving also vibrationally or electronically excited species. Chemical reactions are involved too. Photoionization processes generate electron in the collision process between an heavy particle and a photon. Photoionization is important in thermal plasma and in the propagation process of a streamer channel (Section 2.2.2). Surface ionization with electron emission can be provided by ion, electron or photon collisions or just by surface heating (thermoionic electron emission). One of the most important processes is the secondary electron emission (or Auger emission) involving the neutralization of ions at the surface. Electron and charged particles losses Many processes bring to the loss of a free electron and charged particles. The balance between theses processes and the ionization processes determines the degree of ionization and plasma density. Electron-ion recombination processes involve the neutralization of a positive ion with an electron. It is a highly exothermic reaction which need a channel for accumulation of the energy released during the process. This can lead to molecular dissociation, creation of excited species, photon emission, etc. Especially in presence of an electronegative gas (O2 , CO2 , SF6 , CF4 , etc.) the electron attachment processes are extremely important and are often responsible for the balance of charged particles. An attachment process typically take place in electronegative gases when a molecular fragment (dissociation products) has a positive electron affinity. When the electron attachment processes are involved in the balance of electrons and ions (electronegative gases), the actual losses of charged particles are mostly due to ion-ion recombination processes which are the mutual neutralization of positive and negative ions in binary or three-body colli- 2.4 PLASMA-SURFACE INTERACTIONS 23 sions.These processes can proceed by many different mechanisms and have very high rate coefficients. Finally, as for ionization, must be considered in the balance of charged particle losses the surface recombination processes. These processes are the most important in low pressure discharges because they are usually kinetically limited by the diffusion of charged particles to the walls. Gas-phase chemical reactions Along with the processes described above, in a typical reactive gas a wide variety of chemical reactions are to be considered in the gas-phase chemical equilibrium. The radical production processes are responsible for the creation of extremely reactive species that can interact with other elements of the atmosphere or on the surface. These species are usually extremely important for plasma processing. A wide variety of gas phase chemical reactions involving all the active species in the plasma are also to be considered. Usually the number of these reactions is very high and the complete description of the chemical equilibrium of a reactive plasma can become an overwhelming task. Excited atoms and molecules in plasma Excited species are extremely important in plasma chemical kinetics. High electron temperatures and thus highly energetic electrons, can provide a high excitation rates of different electronically excited state of atoms and molecules by electron impact. If the radiative transition to the ground state is not forbidden by quantum selection rules, such a state is called resonant excited state. It has typically a short lifetime (10−8 ÷ 10−6 sec.) and does not interfere with chemical kinetics. Otherwise, if the radiative transition is forbidden, this state is called metastable excited state and because its lifetime can be very long (10−2 ÷ 102 sec.), it can significantly contribute to the chemical kinetics. In presence of molecules in the plasma an extremely important process is the vibrational excitation of molecules by electron impact. Indeed, in a molecular gas, most of the electron energy can be transferred to the vibrational excitation. For this reason, the vibrational excitation, relax- 24 ATMOSPHERIC PRESSURE DISCHARGES AND SURFACE PROCESSES ation, and reaction of vibrationally excited molecules strongly influence the chemical kinetics of the plasma. Several relaxation processes are important: vibrational-translational (VT) processes convert vibrational energy in kinetic energy of the whole particle and it is the loss mechanism of vibrational energies. Vibrational-vibrational (VV) processes rearrange the energy between vibrational levels and are responsible for the creation of highly vibrationally excited molecules which are extremely influent in the chemical equilibrium. In fact, these molecules can posses enough energy for dissociation and/or other endothermic chemical reactions. The vibrational levels are usually thermalized and a vibrational temperature Tv can be defined. However, because the VT processes are often weakly efficient in non-thermal discharges, molecular vibrations ”trap” the electron energy, and Tv > T0 where T0 is the ion and gas temperature. Also the rotational levels of molecules can be excited by rotational excitation of molecules by electron impact processes. Similarly to vibrational levels, the relaxation of rotational levels can happen through rotationalrotational (RR) or rotational-translational (RT) relaxation processes. However, the probability of RT (and RR) processes in very high because of the smallness of rotational quanta, and, in many non-thermal plasmas, the rates of rotational relaxation processes are comparable with the rate of translational thermalization (TT processes). In this cases the defined rotational temperature Tr ∼ T0 . 2.4.2 Surface kinetics and processes Physical and chemical surface processes are central to plasma processing. Some of these processes, which are important for sustainment of the discharge and its chemical equilibrium, have been described in Section 2.4.1. Indeed, the surface and gas-phase reactions sets are strongly coupled and cannot be considered separately. Here attention is concentrated on those surface processes which are fundamental in plasma processing of materials and on the kinetics of surfaces. 2.4 PLASMA-SURFACE INTERACTIONS 25 Figure 2.6: Typical reaction set for a surface process. Surface kinetics Surface reaction mechanism for most plasma processes are still not well understood and characterized. However, adsorption and desorption of reactive species on the surface are usually part of the complex surface processes. Adsorption is the mechanism that brings an atom (or molecule) to form a stable bond with the surface. There are two kind of adsorption processes: physiosorption, which is the creation of a bound due to the weak attractive Van der Waals forces between the atom and the surface, and chemiosorption which is due to the formation of a chemical bond between the atom (or molecule) and the surface. These two kind of adsorption mechanism are often found in the same system with different regimes favored depending on surface temperature and chemical environment. Desorption is the reverse reaction to adsorption and, in thermal equilibrium, the two reaction must be balanced. In Figure 2.6 is illustrated a typical reaction set for a surface process. Reactive species diffuse or flow to the surface with rate constant K1 , where they are adsorbed (K2 ) and react (K3 ). If the process generates by-products (for example in chemical etching) they can desorb (K4 ) and diffuse or flow into the gas phase (K5 ). In addition, must be considered the desorption without reaction of the reactive species (K6 ) and the backward adsorption of the eventual by-products. This is the most simple reaction scheme that must be considered for the description of a surface process. 26 ATMOSPHERIC PRESSURE DISCHARGES AND SURFACE PROCESSES Etching processes Plasma Etching is a fundamental process for the removal of material from a surfaces. The process can be chemically selective (i.e. can remove a specific type of material leaving others unaltered) and is the only commercially viable technology for anisotropic removal of material (i.e. can remove material at the bottom of a trench while leaving the same material on the side walls unaffected). Most application of plasma etching are in the field of integrated circuit fabrications, but other applications exist (in association with other processes) for polishing, cleaning or sterilization of surfaces. Etch process are typical at lower pressure and comprise three main processes. Sputtering is the ejection of atoms from surface due to energetic ion bombardment. It is an unselective (i.e. sputtering yields do not change too much with material), highly anisotropic process, and is the only one which can remove in-volatile products from a surface. In pure chemical etching the discharge supplies gas-phase etchant atoms or molecules that chemically react with the surface to form gas-phase product following a the scheme shown in Figure 2.6. This process is highly chemical selective. Ion-enhanced etching is a process in which the discharge supplies both etchants and energetic ions to the surface. The energetic ions increase the etching rate and anisotropy but reduce the selectiveness of pure chemical etching. Deposition processes Plasma assisted deposition, implantation and surface modification processes are extremely important for the creation of thin films on surfaces and for the modification of surface properties. Plasma-generated thin films can have unique chemical composition and morphology that are not attainable with conventional chemical vapour deposition (CVD) and other processes. Plasma enhanced chemical vapour deposition (PECVD) consists of a plasma activated set of gas-phase and surface reactions that produce a solid product at the surface. Reaction scheme is rather complicate and can involve also polymerization processes both in the gas-phase and on the surface. A deposition process of this kind is studied in Chapter 7. 2.4 PLASMA-SURFACE INTERACTIONS 27 Plasma grafting Plasma grafting is the insertion through chemical bonding of a specific functional group on the surface. It can be considered a deposition processes which follows the reaction scheme illustrated in Figure 2.6. It starts like a deposition with the generations in the gas-phase of a reactive species which is adsorbed on the surface where form a stable chemical bond. However, are not present reactive species that can start a polymerization process neither in gas-phase nor on surface. This results is the creation of a single molecular (or atomic) layer on the original surface. A grafting process of fluorine atoms is studied in Chapter 8. Plasma Activation What is called plasma activation is usually a combination of an etching and a grafting process that in intended to modify or improve surface properties in order to attain, for example, better adhesion of polymeric webs to coatings, painting, gluing, wetting, etc. Plasma activation can also promote crosslinking and is always present also during deposition processes and increases the bonding of reactive species to surfaces. Grafting of specific functional groups can be promoted (for example polar groups in air plasma treatment of polymer surfaces to attain wettability), and the removal of weakly bounded layers through etching can be attained. 28 ATMOSPHERIC PRESSURE DISCHARGES AND SURFACE PROCESSES CHAPTER 3 Dielectric barrier discharge devices This chapter is devoted to the description of the experimental setups utilized in the research. The plasma device developed for the study of surface process in a controlled atmosphere is described in details as long as the motivations of the choices. The diagnostics are mentioned here only when needed in the description and are discussed more deeply in Chapter 4 3.1 DBD device for surface modifications The main advantage of the atmospheric pressure DBD is its easy adaptability to continuos material processing [1, 2, 3, 20]. The idea behind the realization of this experimental setup is the possibility to study both the discharge physics and the plasma-surface interaction during continuos processing in a wide range of pressures and compositions of the atmosphere. Continuous treatment of web material is a central feature of industrial applications and the roll-to-roll configuration is a compulsory characteristic of 29 30 DIELECTRIC BARRIER DISCHARGE DEVICES the plasma discharge system. 3.1.1 Plasma reactor The dielectric barrier discharge (DBD) device used for the experiments is mostly similar to the typical corona treater already used at industrial level for adhesion improvement. The choice of this configuration is motivated by the ease of development and scaling of this type of configuration. A schematic representation of the experimental setup is given in Figure 3.1. The electrode system consists of two parallel high voltage electrodes and a rotating cylindrical grounded electrode. The high voltage electrodes are two cylindrical rods 230 mm long with a 8 mm diameter, coated with pure (> 99.7%) Al2 O3 sintered ceramic dielectric of 2 mm thickness. The grounded rotating electrode is a void steel cylinder coated with ceramic dielectric of 5 mm thickness. Distance between electrodes can be varied between 0.5 and 5 mm. An electric motor with a controller and a motion vacuum feed-through can rotate the grounded cylinder with tangent speeds between 0.1 and 100 m/min. The electrodes are enclosed in a vacuum chamber (Copra Cube by CCR Technologies Gmbh) 40x40x40 cm where particular gaskets have been employed to avoid leakage both in vacuum and in over-pressure. These particular solution allows to perform experiments without contamination not only in under-pressure, but also in slightly over-pressure. The affordable working pressure range is between 10−1 and 1300 mbar, limited below by the evacuation of impurities and above by the leakage of gaskets. 3.1.2 Pumping system and gas distribution The control of the reactive atmosphere is a key feature of this plasma device. To ensure a minimal concentration of uncontrolled contaminations during the experiments, a double-stage rotary pump (SD-301 by Varian) is used to evacuate the atmosphere to a limiting pressure of 5 · 10−3 mbar. Because this pump cannot work efficiently at high pressure without a considerable overheating, a second dry pump (ZA60 by Rial) is used at higher pressure experiments. The desired pressure is maintained constant balancing the inlet fluxes through the regulation of dosing valve V2 (see Figure 3.1). 3.1 DBD DEVICE FOR SURFACE MODIFICATIONS 31 Figure 3.1: Schematic representation of the DBD reactor. The electrodes are enclosed in a vacuum chamber. A motor with a rotation control (RC) rotates the grounded electrode. A rotary pump (P1) is used to evacuate the chamber and a piston pump (P2) with a dosing valve (V2) are used to stabilize the desired working pressure balancing the inlet fluxes. A Pirani pressure gauge (PG1) and a capacitive gauge (PG2) measure the pressure in the chamber. Three mass flow controllers (MFC1,2,3) with different capacity can mix gases and a controlled evaporator and mixer (CEM) can also mix liquid (MFCL) as vapours in a carrier gas (MFC). An amplified signal generator and a current transformer provide the high voltage to the electrodes. Current and voltage are acquired in a oscilloscope with a specifically designed Rogowski coil (ROG) and a high voltage probe (HVP). Optical emission is acquired through an optical fibre with an UV-VIS spectrometer. 32 DIELECTRIC BARRIER DISCHARGE DEVICES Figure 3.2: Layout of a section of the discharge region. The two high voltage electrodes are in front of a rotating grounded electrode. A polycarbonate injection system guarantee the uniformity of gas and vapour flow over the width of the electrodes. Two pressure gauge are used to measure low and high pressure ranges: a Pirani pressure gauge (Ttr 91 by Leybold) measures the pressure respectively in the range 10−3 ÷ 5 mbar and a capacitive gauge (DI2000 by Leybold) measures the pressure 1 ÷ 2000 mbar. The inlet fluxes are controlled by a gas and vapour mixing system (Figure 3.1). Process gases with high purity level are regulated and mixed using a system of 3 mass f low controllers (El-flow by Bronkhorst) with different capacity. To use liquid precursors at atmospheric pressure a controlled evaporator and mixer (Bronkhorst CEM System) is also attached to the inlet allowing the mixing of vapours with concentration up to the saturation at the given conditions of temperature and pressure in the vacuum chamber. The inlet fluxes generated by the gas and vapour mixing system are injected into the vacuum chamber directly between the high voltage electrodes through an injection nozzle. The nozzle is a polycarbonate shower specifically designed and realized to ensure uniform fluxes on the whole width of the electrodes up to 50 ln /min.. Polycarbonate have an upper limit working temperature around 80 ◦ C and good chemical resistance. A sectional view of the injection nozzle arrangement is shown in Figure 3.2. 3.1 DBD DEVICE FOR SURFACE MODIFICATIONS 33 3.1.3 Electric power supply and configuration The AC is applied by an amplified signal generator with frequencies between 10 KHz and 50 KHz and through a high voltage transformer. The power supply is composed by a current rectifier which brings the line current from 230V AC to a 310 V DC. A transistor switching system create the AC current which is connected to the primary winding of the high voltage transformer. The secondary of the transformer is then directly connected to the electrodes and the whole system consist of a resonant circuit. The voltage applied to the primary winding of the transformer is constant and the voltage applied to the electrodes is controlled by the resonance between the proper frequency of the system and the applied frequency. Usually the complete voltage (and power) range of the device is within a span of few kilohertz. 3.1.4 Diagnostics We characterize the plasma discharges principally by means of optical and electrical diagnostics. The emission spectra of the discharges have been measured with a wide band spectrometer A complete description of instrument and measuring techniques is given in Section 4.1. Both the current and the voltage are acquired in a Nicolet 450 oscilloscope respectively with a specific designed Rogowski coil (see Section 4.2.1) and a high voltage probe (Tektronix P6015A). In Figure 3.3 are plotted the amplitude-frequency and phase shift-frequency response of the Rogowski coil used for current measurements in the experiments with the present setup. Current measurement are performed with two different Rogowski probes. The first probe has a lower bandwidth (5 kHz-25 MHz) and is used to measure the displacement current and longer current pulses without introducing dephasing. The second probe is used to record the shape of the fast current pulses due to micro-discharge formation and has an higher bandwidth in the range 400 kHz-120 MHz. Probe type have been selected and adjusted to necessity of different experiment. A complete description of development and calibration of Rogowski coils is given in Section 4.2.1. 34 DIELECTRIC BARRIER DISCHARGE DEVICES Phase [π] Attenuation [dB] 10 -3 -2 10 10 -1 0 10 1 10 2 10 3 0 -3 -6 -9 -12 0.4 0.2 0 -0.2 -0.4 10 -3 -2 10 -1 0 10 10 Frequency [MHz] 1 10 2 10 Figure 3.3: Amplitude vs. frequency (upper panel) and phase shift vs. frequency (lower panel) response of the two Rogowski coils. Circles represent the values for the ferrite core coil used for displacement current measurements with a 5 kHz-25 MHz bandwidth. Diamonds represent the values for the NiZn core coil used as a reference for the fast current pulses, with a bandwidth in the range 400 kHz120 MHz. The dotted lines in the upper panel represent the usual 3 dB limit to determine the probe bandwidth. 3.2 DBD device for streamer regime characterization In order to use the simplest configuration for the characterization of the streamer regime it has been used a different configuration with respect to the one described in Section 3.1. The parallel rod electrodes allow a better understanding of the discharge properties from the analysis of the currentvoltage signals, however the same amplified signal generator as in Section 3.1 have been used in nearly the same frequency range. 3.2.1 Plasma reactor The DBD device used here consists of two high voltage (center grounded) electrodes working under atmospheric pressure conditions in air. They are constituted by two rod electrodes 290 mm long with a 9 mm square section, coated with pure (> 99.7%) Al2 O3 sintered ceramic dielectric, with an external 15 mm square section and 3 mm thickness. A schematic diagram of the experimental setup is shown in Fig. 3.1. The steel electrodes are cave and a cooling gas flow pass through them 35 3.2 DBD DEVICE FOR STREAMER REGIME CHARACTERIZATION ROG T OSCILLOSCOPE HV HV Figure 3.4: The DBD device is made up of two rod electrodes of square section coated with pure (> 99.7%) Al2 O3 ceramic dielectric. The distance between electrodes is 4 mm. An amplified signal generator and a current transformer (T) provide the high voltage to the electrodes. Current and voltage are acquired with a specifically designed Rogowski coil (ROG) and a high voltage probe (HV). to keep the temperature low. The length of the discharge gap is fixed at 4 mm. The AC voltage is applied using the same amplified signal generator used for the device described in Section 3.1. The applied voltage to the primary winding of the high voltage transformer is constant and the one applied to the electrodes is controlled by the resonance between the proper frequency of the system and the applied frequency. The frequency range spans between 31 kHz and 36 kHz in the affordable voltage (and power) range of the device. 3.2.2 Diagnostics As for the other device both the current and the voltage are acquired with a Nicolet-Multipro oscilloscope. Using a high voltage (Tektronix P6015A) probe, voltage is acquired ( on one of the hot wire and doubled to consider the symmetry of the electric circuit. The current signal is acquired using a specific designed Rogowski coil with a ferrite magnetic core coil and a bandwidth of 50 kHz-70 MHz) and a , respectively. A second NiZn core coil with a bandwidth of 400 kHz-120 MHz have been used (see Fig. 3.5) in order to control the response of the ferrite coil to short current pulses due to streamers. The response of the two coils to the shortest current pulses measured in the experiments were almost undistinguishable from each other. Because of the low frequency range of the generator, lying below the 36 DIELECTRIC BARRIER DISCHARGE DEVICES probe bandwidth, the displacement current response is underestimated and dephased. However, this does not affect our analysis of fast current pulses associated to the discharge current. Probe type have been selected and adjusted to necessity of different experiment. A complete description of development and calibration of Rogowski coils is given in Section 4.2.1. -2 Phase [π] Attenuation [dB] 10 -1 10 10 0 1 10 2 10 3 0 -3 -6 -9 -12 0.4 0.2 0 -0.2 -0.4 -2 10 -1 10 0 1 10 10 Frequency [MHz] 2 10 Figure 3.5: Amplitude vs. frequency (upper panel) and phase shift vs. frequency (lower panel) response of the two Rogowski coils. Circles represent the values for the ferrite core coil used for measurements with a 50 kHz-70 MHz bandwidth. Diamonds represent the values for the NiZn core coil used as a reference for the fast current pulses, with a bandwidth in the range 400 kHz-120 MHz. The dotted lines in the upper panel represent the usual 3 dB limit to determine the probe bandwidth. CHAPTER 4 Plasma and material diagnostics In this Chapter are described the diagnostics used to characterize the plasma discharges and the modifications induced on material surfaces. Both the instruments and the analysis methods are described. 4.1 Optical emission spectroscopy The emission spectra of the plasma discharges have been measured by means of a wide band, low resolution spectrometer (PS2000 by Ocean Optics). The spectrometer, equipped with a 10 µm slit, a holographic grating (600 lines/mm, blazed at 400 nm) and a 1024 pixels CCD, has a resolution of 1.02 nm and a spectral band extending from 200 to 850 nm. Integration time is changed depending on the discharge brightness. Emission spectra of the discharges have been recorded through an UV enhanced optical fiber, connected to the device by a vacuum feed-through. Depending on the gas composition, intensities of the emission lines can allow the calculation of 37 38 PLASMA AND MATERIAL DIAGNOSTICS several properties of plasma discharge and gas-phase chemistry like concentrations, vibrational and electron temperature. 4.1.1 Determination of molecular vibrational temperature When a molecular gas is present in the discharge atmosphere the excitation of its vibrational and rotational levels becomes a dominant process and in some cases the most part of the electron energy is spent in these processes (see Section 2.4.1). Because the vibrational levels are in thermal equilibrium, supposing a Boltzmann distribution it is possible to determine the vibrational temperature Tv . The method depends on the molecule and gasphase chemistry and will be explained for mixtures containing nitrogen in Chapter 6 4.2 Voltage Current measurements Voltage and current measurement are fundamental to plasma discharge understanding. Both signals are digitally acquired in a Nicolet MultiPro (3 channels, 200 MHz, 8 bit, 1 GS/s) or a Nicolet 450 Oscilloscope (4 channels, 200 MHz, 2 GS/s) or a Tektronix TDS 4020 (2 channels, 60 MHz, 1 GS/s) and analyzed with the aid of the computer. Voltage is usually acquired with a wide bandwidth, high voltage probe (Tektronix P6015A, 40 kV, 75 MHz) which allows to recognize eventual fluctuations of the applied sinusoidal voltage. Because of the presence in the current signal of very fast processes due to micro-discharges, particular attention must be paid to the current measurement. The development and calibration of home-made Rogowski coil sensor will be described in the subsequent Paragraphs. 4.2.1 Implementation of Rogowski coils for measurements nanoseconds current pulses Detailed measurements of the current response of a dielectric barrier discharge require particular attention to the bandwidth of the probe. The displacement current response will be at the same frequency of the applied voltage, but the discharge current is generally in pulses with duration from 39 4.2 VOLTAGE CURRENT MEASUREMENTS microseconds down to tenth of nanoseconds depending on the discharge regime [4, 6, 10, 9]. An easy way to measure the current is to introduce a shunt in series with the electrical circuit. The current shunt has a good response bandwidth but the protective circuit needed to avoid damage to measuring instruments can cause distortion of the read waveform. The Rogowski coil is galvanically separated from the main circuit and can be designed for a precise measurement of the nanosecond current pulses typical of the streamer discharge regime[53, 54]. Theory and principle of Rogowski coils A Rogowski coil is a conducting wire that is wound in a spiral around a magnetic or non magnetic core and then returns to the original point. The coil is placed around the conductor to couple the pulse signals. The operating principle was formulated by Rogowski and Steinhaus in 1912 [55]. In the original design Rogowski coils were air cored to avoid saturation of magnetic core when measuring high currents. In the present measurements, currents are constantly far below saturation and it will be shown how the choice of the magnetic core influences the bandwidth of the coils. A schematic representation of a Rogowski coil is shown in Figure 4.1. The current I flowing in a cable generate an electromotive force (emf) E at the output of the coil which is proportional (following Faraday law) to the rate of change of the current: ∂I/∂t. The signal E must be integrated with a passive or active circuit. The high frequency behaviour of the coil, in particular its bandwidth and susceptibility to high frequency oscillations, is significantly influenced by the integration circuit impedance. With the right choice of the configuration of the coil the integrating circuit can be reduced in a simple resistance [56, 57, 58, 53, 54]. A lumped parameter model can be introduced to describe the circuit behavior (Figure 4.2). The variable current i1 (t) produce a magnetic field and the rate of change in current produce a voltage in the coil equal to Ui (t) = M di1 (t) , dt (4.1) 40 PLASMA AND MATERIAL DIAGNOSTICS Figure 4.1: Schematic representation of a Rogowski coil. The current I flowing in a cable generate a emf E at the output of the coil which is proportional to ∂I/∂t and must be integrated with a passive or active circuit. Figure 4.2: The equivalent circuit diagram (lumped parameter) (M , mutual inductance; Ls , self-inductance; Cs , stray capacitance; Cp , turn-to-turn capacitance, which can be ignored in the spaced winding in the design because it is very small; Rs , equivalent resistance of coil; R, integral resistance larger than Rs in the design; U0 (t), voltage of the integral resistance; Ui (t), the induced voltage) [54]. 41 4.2 VOLTAGE CURRENT MEASUREMENTS where M is the mutual inductance between the measured circuit and the coil. The transfer function for the lumped parameter model of Figure 4.2 is: U0 (t) = Ls Cs Rs2 R Ui (t), + (Ls + Rs Cs R)s + Rs + R (4.2) where the generic impedance of the integrating resistance have been considered a pure resistive load. If the integrating resistance R is chosen in order to p have R ≪ Ls /Cs the pole of the transfer function 4.2 move along the real axis so the system does not oscillate and the Rogowski coil is self-integrating between the two poles [53]. For toroidal coils with rectangular square section the lumped parameters can be calculated as: Ls = A d2 µN 2 h log = µN 2 2π d1 l 2 4π ǫK1 Cs = log K1 /K2 (4.3) (4.4) where l is the length of the effective magnetic path, µ is the magnetic permittivity of the core, A is the cross-sectional area of the core, d2 and d1 are the outer and inner diameters of the coil respectively, h is the height of the coil, N is the number of turns in the coil, ǫ is the dielectric constant of the core, K1 = (d2 + d1 )/2 and K2 = (d2 − d1 )/2. From the lumped parameter model represented in Figure 4.2 can be deduced the following equations: fl = fh = 1 R + Rs 1 R ≈ 2π Ls + RRs Cs 2π Ls (4.5) 1 1 1 Ls + RRs Cs ≈ 2π RLs Cs 2π RCs (4.6) where fl and fh are the lower and upper frequency limits respectively. From the above equations is possible to determine which construction parameters must be changed in order to obtain the desired bandwidth. Thou, In order to increase the frequency band, fl should be as low as possible, while fh should be as high as possible. According to equations (4.4), (4.5) and (4.6), it can be seen that to obtain the wider bandwidth Ls should be as large as possible while R should be as small as possible. A bigger inductance Ls can be obtained increasing µ or, more efficiently, increasing N which gives a 42 PLASMA AND MATERIAL DIAGNOSTICS quadratic dependency. With increasing N and decreasing R, the bandwidth will become wider, but the sensitivity will become lower. A balance in these parameters must be attained in order to have the wider bandwidth and ensure a good sensitiveness. me ? m0 N—–2 Although bandwidth of the current transducer can reach the desired range by controlling R and Ls in theoretical analysis, the frequency range and configuration parameters of the magnetic core play important roles in determining bandwidth of the current transducer. First, the frequency range of the magnetic core should include the desired frequency range. Two kind of magnetic core have been used to build the Rogowski coils used in the experiments. For the lower bandwidth (and in order to measure displacement currents of the order of kilohertz) a ferrite material with initial permeability µ = 4300 NA−2 , coercive field strength Hc = 0.19 Oe and saturation flux density Bs = 3900 Gauss, have been used which gives an higher sensitiveness but cannot resolve higher frequencies. For the detection of fastest current pulses due to micro-discharges a Nickel-Zinc core have been used which guarantees an higher bandwidth at the cost of sensitiveness. Its parameters are: initial permeability µ = 1500 NA−2 , saturation flux density Bs = 2800 Gauss, coercive field strength Hc = 15 Oe. Also the geometrical parameters of the core can have some influence the final bandwidth [54], but here they are not considered because in the experiments they are constrained by dimensions of the cables and cannot be varied. From the value of Bs the maximum measured current Imax can be calculated according t [54]: Imax = 0.8Bs l . µ (4.7) In all the experiments and for all the build current sensor the measured current is always under the maximum value (I < Imax ). Calibration of the Rogowski coils The calibration circuit (Figure 4.3) consists of a wide bandwidth signal generator which is connected to a 50Ω non inductive resistor through an RG 58 BNC cable. The cable is split on the resistor side with the hot wire 4.3 CHARACTERIZATION OF THE MATERIALS SURFACES 43 Figure 4.3: The calibration system used to evaluate the frequency and amplitude response of the Rogowski coils. passing through the Rogowski probe to be tested. Acquiring the voltage drop across the resistor and the probe signal it is possible to determine the amplitude response, the phase shift and the sensitiveness of the Rogowki Coil. Several combination of magnetic core and number of turns N have been tested while the integrating impedance have been kept constant to a 50 Ω non-inductive resistance. In Figure 4.4 are showed the Bode plots for some built Rogowski coils. The parameters are the type of magnetic core are indicated in the figure legend. The Bode plots for the specific coils used in the single experiments are showed in Sections 3.1 and 3.2 in the description of the experimental setups. We utilized two different Rogowski probes. The first one with a ferrite magnetic core and a bandwidth of 5KHz-25MHz to detect the displacement current and longer current pulses. The second one with an high frequency NiZn magnetic core and a bandwidth of 250KHz-120MHz to detect the current pulses of single streamers. 4.3 Characterization of the materials surfaces In this section are briefly described the diagnostics used for the characterization of the material surfaces after the plasma treatments. At a microscopic level both the chemistry and the morphology are characterized 44 PLASMA AND MATERIAL DIAGNOSTICS Figure 4.4: Amplitude vs. frequency (upper panel) and phase shift vs. frequency (lower panel) response of some Rogowski coils with different construction parameters and magnetic core. with infrared spectroscopy and AFM/SEM/FIB measurements respectively. Modified macroscopic properties are evaluated with contact angle measurements. 4.3.1 Infrared spectroscopy (FTIR/ATR-FTIR) Fourier transform infrared spectroscopy (FTIR) can be used to identify chemical composition of the realized coatings. FTIR is perhaps the most powerful tool for identifying types of chemical bonds (functional groups). Molecular bonds vibrate at various frequencies depending on the elements and the type of bonds. For any given bond, there are several specific frequencies at which it can vibrate. The wavelength of light absorbed is characteristic of the chemical bond as can be seen and identified in the spectrum. Sometimes transmission measurements cannot be performed on several specimens and a surface analysis must be used. An attenuated total reflection (ATR-FTIR) technique operates by measuring the changes that occur in a totally internally reflected infrared beam when the beam comes into contact with a sample. An infrared beam is directed onto an optically dense crystal with a high refractive index at a certain angle. This internal reflectance creates an evanescent wave that extends beyond the surface of 4.3 CHARACTERIZATION OF THE MATERIALS SURFACES 45 the crystal into the sample held in contact with the crystal. This evanescent wave protrudes only a few microns (0.5 µm- 5 µm) beyond the crystal surface and into the sample. Consequently, there must be good contact between the sample and the crystal surface. In regions of the infrared spectrum where the sample absorbs energy, the evanescent wave will be attenuated or altered. The attenuated energy from each evanescent wave is passed back to the IR beam, which then exits the opposite end of the crystal and is passed to the detector in the IR spectrometer. The system then generates an infrared spectrum. The measurement have been performed with a Nicolet Avatar 360 with a resolution of 4 cm−1 in the range 400÷4000 cm−1 , and equipped with an ATR accessory (PIKE-Technology). 4.3.2 Atomic force microscopy (AFM) The atomic force microscopy (AFM)is a rather recent technique to measure the morphology of surfaces down to nanometer scale resolution. The functional scheme of an AFM is represented in Figure 4.5. The AFM head uses a beam deflection scheme to monitor the cantilever displacement. This scheme is quite simple and permits registration of both normal deflection of the cantilever with sub-angstrom resolution and its twisting angle, so normal and lateral force can be measured simultaneously. A laser beam is focused onto the back surface of cantilever close to tip position, and reflected beam falls onto the quadrant photodiode. Cantilever deflection causes displacement of the reflected beam over sections of the photodiode. An amplified differential signal from the quadrant photodiode permits measurement of angular deviation with the accuracy of less than 0.1 degrees, that corresponds to normal cantilever deflection of the order of 0.05 nm. Among the several techniques used to measure the morphology of surfaces, have been used contact mode and semi-contact (tapping) mode. In contact mode cantilevers touches the surface while scanning in repulsive mode (like a needle of gramophone), but can scratches softer surfaces. The semi-contact mode is a special modulation technique for non-destructive imaging of soft samples as well as of hard. It measures topography by tapping the surface with an oscillating probe tip. The measurements in the present work have been performed with a NT-MDT Solver P47H-pro. 46 PLASMA AND MATERIAL DIAGNOSTICS Figure 4.5: Functional scheme of an AFM. 4.3.3 Contact angle measurements and surface energy determination Contact angle measurements have been performed to evaluate macroscopic properties of material surface such as surface energy and wettability. Static and dynamic measurements have been performed using a video-supported contact angle measuring instrument Dataphysics OCA 20. Wettability and surface energy Wetting [59]describes the ability of a liquid deposited on a solid substrate to spread out or remain confined. When the surface energy of a dry substrate is higher than the energy of the wetted one (by some liquid), the liquid spreads completely on the surface in order to lower its energy (for example, the behaviour of water on a clean glass surface). On the contrary, when the surface energy of a dry substrate is lower, the liquid partially wets the surface forming drops (for example, water on a plastic surface). At the contact line between the three phases (liquid, solid, gas or vapour) the contact angle between the liquid drop and the surface is determined by the equilibrium of the surface tensionsσ (or surface energies) of the interfaces according to the 4.3 CHARACTERIZATION OF THE MATERIALS SURFACES 47 Figure 4.6: Young equilibrium between surface tension determining contact angle. Young’s equilibrium [60] (see Figure 4.6): σlv cos θe = σsv − σsl , (4.8) where the subscripts indicate the inter-phase between liquid (l), solid (s) and vapour (v). The equilibrium contact angle θe is a physical constant depending only on the materials, and in no other way on the particular configuration considered. According to the Owens-Wendt two-parameter model the surface tensions of the solid-vapor and liquid-vapor inter-phases consist of two components: a dispersive one accounting for van der Waals and other non-sitespecific interactions and a polar one accounting for dipole-dipole, dipoleinduced dipole, hydrogen bonding and other site-specific interactions[61]. The surface tensions of the liquid and the solid (in contact with vapour) can be expressed as: d p σsv = σsv + σsv , (4.9) p d + σlv . σlv = σlv (4.10) The solid-liquid interfacial tension can be expressed as [62]: q q p p d d σsl = σsv + σlv − 2 σsv σlv + σsv σlv (4.11) 48 PLASMA AND MATERIAL DIAGNOSTICS Figure 4.7: Contact Angles as a function of drop volume when it is increased (right-hand arrow) and decreased (left-hand arrow) whit a syringe. Advancing and receding contact angle are determined as indicated. Combining equations (4.8) and (4.11) yields: σd + σp (1 + cos θe ) lvq lv = d 2 σlv q d + σsv p p σsv s p σlv . d σlv (4.12) d and σ p in equation The two unknown components of the surface tension σsv sv (4.12) can be determined from the measured contact angles against at least d and two test fluids with known values of surface tension components σlv r p σlv p for different σlv . A plot of left hand side of equation (4.12) versus σd lv liquids yields the dispersive component (square of the y-intercept), the polar component (square of the slope) and consequently the surface tension of the solid-vapor interface σsv from equation (4.10). 4.3 CHARACTERIZATION OF THE MATERIALS SURFACES 49 Contact angle hysteresis According to Young’s equation (4.8) the static equilibrium contact angle θe is related to the surface tension of the solid-vapor and solid-liquid interfaces, and it is ideally a unique property of the material system being considered, but practically a hysteresis often arises depending on how the interfaces form. If a liquid droplet is quietly settled on a solid surface (or if its volume is slightly increased after it settling) thus measured contact angle is larger, even up to several tens of degree, than the angle measured for the same droplet after reducing its volume. The advancing angle θa is the largest contact angle achievable before the wetting line begins to move in the direction of the gas phase and the receding angle θr is the smallest contact angle achievable before the wetting line begins to move in the direction of the liquid phase. Many theories of the contact angle hysteresis have been proposed [63, 64, 65, 66] even if a clear interpretation of this effect still lacks. Hysteresis is usually connected with changes of roughness and chemical heterogeneity of the surface [59]. The measurements of advancing and receding contact angles can give interesting informations on the solid-liquid interactions. With the OCA20 instrument the advancing and receding angles have been measured by modifying the volume of a drop by inflating and deflating liquid with a computer controlled syringe. Recording a movie of the dynamic contact angle it is possible to obtain the estimate of advancing (receding) angle (see Figure 4.7) as the maximum (minimum) value before the drop base diameter increases (decrease). 50 PLASMA AND MATERIAL DIAGNOSTICS CHAPTER 5 Statistical characterization of a streamer discharge regime 5.1 Introduction In this chapter, we investigate the temporal behavior of current pulses for a streamer regime in a DBD at atmospheric pressure. As explained in Chapter 2, at low pressures DBDs operate in a Townsend breakdown regime [5] generating a diffuse glow discharge. At atmospheric pressure, the realization of a diffuse discharge is restricted to limited conditions of geometry, electrical parameters and gas composition, and DBDs operate usually in a streamer discharge in which several narrow discharge filaments are typically formed (see Section 2.3.2). The streamer regime constitutes a strongly interacting system of discharges exhibiting cooperative behavior. This leads, under specific conditions, to the formation of coherent spatial configurations that have been observed in different types of experimental setups [12, 13, 14, 6]. 51 52 STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME However, micro-discharges seem, to some extent, to occur at random within the discharge gap for most applications of DBDs. To our knowledge, the statistical properties of such discharges in air have not been discussed with sufficient insight in literature so far, even if similar studies have been performed in dealing with the so-called partial discharges [67, 68]. In particular, we find the existence of two different streamer regimes as a function s (here V s ≃ 23.6 kV). The two of applied voltage, separated by a value Vpp pp different regimes can be characterized by the first moments of the discharge s. distributions, suggesting a way for determining the separatrix voltage Vpp The peculiar feature of DBDs (see Chapter 2) is that the charge transported by the micro-discharges to the dielectric cannot reach the conducting electrode and accumulates near the surface until the change in the local electric field extinguishes the filament. Because of the slow diffusion of charges on the surface, in the subsequent half-cycle of the driving voltage the locally modified field promotes the formation of a streamer in the same spot. This so called ‘memory effect’ is a dominant feature in DBDs (see Section 2.3.3). A typical streamer has a lateral spatial extension of about 0.1 mm and a duration of the order of nanoseconds depending on the device configuration and type of gas. The presence of the memory effect suggests temporal correlations may be found in the discharge current signal. In particular it is found that in the studied streamer discharge regime the existing residual correlations propagating between the discharge processes (half-cycles) are only an effect of the non-stationariety of the discharge current response, thus, correlations vanish outside the single discharge process. On the contrary, by analyzing the current signal inside the half-cycle, it is found that on time scales of the order of hundreds of nanoseconds (i.e., within a single current burst, in which the streamers develop sufficiently close in time), strong correlations exist which also reveal a peculiar ordered temporal structure of the discharge current signal. The experimental setup and the diagnostics used here are described in Section 3.2 and Section 4.2, respectively. In Figure 5.1(a) is shown a schematic representation. The current signal is recorded at constant intervals of τ0 = 5 ns for a total of 3 × 105 steps, for different applied peak-to-peak external voltages in the range (22÷26) kV. 53 5.2 STATISTICAL CHARACTERIZATION OF CURRENT SIGNAL ROG T OSCILLOSCOPE HV HV (a) Experimental setup. (b) Simplified electrical equivalent circuit. Current [mA] Current [mA] Figure 5.1: The DBD device is made up of two rod electrodes of square section coated with pure (> 99.7%) Al2 O3 ceramic dielectric. The distance between electrodes is 4 mm. An amplified signal generator and a current transformer (T) provide the high voltage to the electrodes. Current and voltage are acquired with a specifically designed Rogowski coil (ROG) and a high voltage probe (HV). 200 100 0 -100 -200 0 10 20 30 40 50 60 70 80 10 20 30 40 50 60 70 80 400 200 0 -200 -400 0 time [µs] Figure 5.2: Typical current signal of a DBD device. The upper and lower panels refer respectively to a low voltage (23 kV) and high voltage (25 kV) current signal, representing the two typical discharge patterns observed in the device. The continuous sinusoidal line is the displacement current of the system. 5.2 Statistical characterization of current signal Because the interest is in a detailed analysis of fast current pulses due to micro-discharges, the displacement current of the system must be determined and subtracted. Because the ionization of the gas is very low, it is suitable to assume that the capacitance of the gas does not change during the discharge process and use a simplified electrical equivalent circuit of the discharge system (Figure 5.1(b)). Thus, the current measured in the system 54 STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME can be considered to be made of two components: a sinusoidal displacement current which does not depend on the presence of the plasma and the response current due to the discharge process [9, 11, 5]. The displacement and discharge currents can be then calculated as dV (t) dt Idisch (t) = Itot (t) − Idisplace (t), Idisplace (t) = Cx (5.1) (5.2) where Cx is a capacitance including both dielectric layers and gas gap, V is the applied voltage, Itot the total measured current, Idisplace the displacement current and Idisch the discharge current. An example of the separation of the two components is given in Figure 5.2. 5.2.1 Structure of the discharge current: bumps, bursts and streamers. Once the displacement current has been subtracted from the signal we are left with a series of discharge patterns of alternating sign. Since we are interested in the discharge amplitudes, we have first checked that both positive and negative discharge values occur with a similar distribution, indicating that we can treat them on the same foot. Then, we change the sign to those negative discharge patterns. However, small negative current values still occur in the series which are due to errors introduced by the sinusoidal fit and the intrinsic errors of the current probe. To this end, we introduce a cut-off threshold for the current, below which it is set to zero. The cut-off value Icut is taken as the minimum value of the current measured within the full time steps, Icut = |min I0 (t)|. Then, the discharge current is taken as I(t) = ( I0 (t), if I0 (t) ≥ Icut , 0 , if I0 (t) < Icut . (5.3) For the present results, we find Icut ≃ 10 mA. An example of the resulting signal within a single half-cycle, which we denote as a discharge bump is shown in Figure 5.3. It can be noted that a bump is composed of several well separated discharge bunches, which we call bursts. The bursts are made of a sequence of single streamers (their structure and temporal behavior will be discussed in detail in Section 5.3.2) 55 5.2 STATISTICAL CHARACTERIZATION OF CURRENT SIGNAL 500 λ=50 mA -1 PDF [mA ] -1 400 IB [mA] 10 300 10 10 200 -3 -5 0 100 200 300 400 IB [mA] 100 0 0 2 4 6 8 10 time [µs] Figure 5.3: Discharge current signal IB (t) within a half-cycle as a function of time t [µs], for a high voltage situation (25.3 kV, see also lower panel of Figure 5.2). The continuous line is the mean current response IB , Eq. (5.4). The inset represents the probability distribution function (PDF) where the straight line is an exponential fit using Eq. (5.5) with λ = 50 mA. which are clustered together. This clustering is a result of the presence of strong short-time correlations in the discharge patterns. We are going to analyse these correlations below in Sect. 5.3.1. In what follows, we perform a statistical analysis of bumps. In our analysis, we consider from Eq. (5.3) values of I(t) only within an effective time interval tmin < t < tmax , for a fixed applied voltage Vpp . To stress this fact, the discharge current within a bump is indicated as IB (t). The lower bound tmin is defined as the lowest time, within a halfcycle, at which I(t) > 0 for the first time, calculated among all bumps. The upper bound tmax is defined as the largest time, within a half-cycle, at which I(t) > 0 for the last time. The total number of bumps, NB , is typically NB ≃ 100 in the recorded interval, while ∆t = tmax − tmin varies in the range 10 [µs] < ∆t < 20 [µs], depending on Vpp . We denote as T the half-cycle period. The continuous line in Figure 5.3 represents the mean discharge current 56 STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME response of the DBD device within a half-cycle. It is calculated as NB 1 X (i) IB (t), IB (t) = NB (5.4) i=1 for tmin < t < tmax , and IB = 0 otherwise, where i denotes the bump index. Note that max{ IB (t) } ≪ max{IB (t)}, where max{IB (t)} is the maximum value of IB (t) within a bump. In the case of the bump shown in Figure 5.3 we find max{IB (t)} ≃ 400 mA, while max{ IB (t) } ≃ 70 mA. The inset to Figure 5.3 displays the probability distribution function of IB . It can be approximately fitted by the exponential form (see also Figure 5.6 and related explanation) valid for IB ≥ 0, P (IB ) = P0 δ(IB ) + PA exp(−IB /λ), λ (5.5) where P0 represents the fraction of zero current values inside the bump, indicating the existence of a characteristic current intensity. Exponentially decaying functions are typical of random systems displaying uncorrelated R∞ fluctuations. Using the normalization condition, 0 dIB P (IB ) = 1, with R∞ the convention that 0 dIB δ(IB ) = 1, one has PA = 1 − P0 , representing the fraction of positive current events, denoted also as activity ratio, inside a bump. We have verified that the PDF is only weakly dependent on the current cut-off. 5.2.2 Discharge current regimes The evolution of the mean discharge current is plotted in Figure 5.4 (lower panel) for different applied voltages. The maximum of IB (t) tends to occur at early times, t ≃(2-3) µs, while only at large voltages the mean response spans the whole bump width (see e.g. curve (e) in Figure 5.4). The upper panel in Figure 5.4 displays the corresponding PDFs, which seem to attain a limiting shape, independent of Vpp , for Vpp > 23.5 kV. This is a first indication that discharge currents may be organized into two different discharge regimes. We explore this possibility further in the following. An important quantity assessing the efficiency of the discharge device is the total charge, Qtot , transferred during each half-cycle of the system. 57 5.2 STATISTICAL CHARACTERIZATION OF CURRENT SIGNAL 23 -1 -1 P(IB) [mA ] 10 10 10 20 -3 0 -5 -7 0 200 400 IB [mA] 100 (a) 22.9 kV (b) 23.0 kV (c) 23.5 kV (d) 23.7 kV (e) 24.7 kV 80 <IB> [mA] 40 Vpp [kV] 10 80 60 λ [mA] 22.9 kV 23.0 kV 23.5 kV 23.7 kV 24.7 kV λ=55 mA 25 24 60 40 20 e dc 0 0 b a 2 4 6 time [µs] 8 10 Figure 5.4: (Upper panel ) PDF’s P (IB ) [mA−1 ] vs IB [mA], for different applied potentials Vpp indicated in the plot. The straight line displays an exponential function with decay constant λ = 55 mA (see Eq. (5.5)) and is shown as a guide. (Lower panel ) Mean discharge currents IB (t) versus time [µs], for the same values of Vpp considered in the upper panel. The total charge can be calculated from the mean discharge current IB (t) as, Qtot = Z tmax tmin dt IB (t) , (5.6) where tmin and tmax are the temporal bounds for bumps. The total charge Qtot is plotted in Figure 5.5, where one can see the emergence of two distinct regimes separated by a threshold value Vpp ≃ 23.55 kV. The latter is consistent with a similar behavior obtained from the shape of the PDF’s 58 STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME 10 3 0.6 I II 2 10 0.4 0.3 1 0.2 10 Activity Ratio <Qtot> [nC] 0.5 0.1 10 0 23 24 Vpp [kV] 25 0 Figure 5.5: Mean total charge Qtot [nC] vs applied voltage Vpp [kV], transferred by the discharge currents within a half-cycle (open circles, left-scale). The dashed and continuous lines are quadratic fits to the numerical data and are shown as a guide to stress the presence of a two-regime discharge pattern in the DBD, separated by a value Vpp ≃ 23.55 kV. Also shown, the activity ratio PA (open squares, right-scale) plotted vs applied voltage. The dashed (continuous) line is a linear (quadratic) fit plotted as a guide to the eye. displayed in Figure 5.4. In support to these findings, we can add that visual inspection of actual discharge patterns show also two qualitatively different behaviors at low and high applied voltages. In the low voltage regime, few moving discharges occur at random along the electrodes. At higher voltages, many discharges covering essentially the full electrode length occur in a fully random fashion. The activity ratio of the DBD, that is the fraction of time within a half-cycle in which a positive current is measured, is also plotted in Figure 5.5. It does not show such a clear change of behavior as Qtot does. Yet, we have found that to obtain accurate fits the data need to be separated into two parts, one below Vpp ≃ 23.55 kV, where just a linear dependence occurs, and one above it, in which a quadratic function is required. In this sense, also the activity ratio reflects the presence of two different regimes. 5.2 STATISTICAL CHARACTERIZATION OF CURRENT SIGNAL 59 We can also mention that an accurate quadratic fit to the data for Qtot can not be obtained for the whole interval of voltages considered. This difficulty reflects to some extent also the presence of an underlying tworegime discharge process. Using the approximate form for the discharge PDF, Eq. (5.5), we comment on the two observed discharge regimes. For low voltage values, Vpp < 23.55 kV, the decay parameter λ increases rapidly with applied voltage (see upper panel in Figure 5.4), i.e. the larger the value of λ the more likely the higher current values are. Similarly, the activity parameter PA (Figure 5.5) increases also, and it does it linearly in regime I. Based on evidence that the charge transported by a single streamer does not seem to depend on the applied voltage [6, 5], we suggest that the increase in height and duration of bursts we observe is due to the increasing number of simultaneous or close-in-time streamers as a function of the applied voltage. In this plausible scenario, new streamers can occur spanning the largely available space on the dielectric without strongly experiencing the repulsive interaction with residual charges deposited from previous micro-discharges within the same half-cycle (bump). For higher voltages, Vpp > 23.55 kV, λ stops growing, indicating that a limiting shape of the discharge PDF has been reached. Thus, an upper number of simultaneous streamers seems to occur, as suggested by the shape of the mean discharge current shown in the lower panel of Figure 5.4, shape which becomes broader in time but reaching a limiting upper value as the voltage is increased. In other words, for low values of Vpp bursts are made up of few streamers and IB remains low, spreading in time. By rising the applied voltage, IB first increases in height up to a limiting value, then its temporal duration starts to grow. The two regimes identified previously can be further characterized by looking at selected moments of order n of the discharge current IB (t), which can be calculated according to, 1 In = ∆t Z tmax tmin dt IBn (t) , (5.7) where here 1 ≤ n ≤ 4 and ∆t = tmax − tmin . The symbol denotes an average over different bumps. In addition to the mean, I1 , and standard STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME 100 100 80 80 σI [mA] <I1> [mA] 60 60 40 20 40 20 23 3 24 25 Vpp [kV] 23 24 25 Vpp [kV] 23 24 25 Vpp [kV] 10 8 FI SI 2 1 0 60 6 4 23 24 25 Vpp [kV] 2 Figure 5.6: First moments of the current signal IB (t) versus applied voltage Vpp [kV]. Shown are: the mean value I1 , standard deviation σI , skewness SI and flatness FI . The horizontal line represents the values of SI and FI for an exponential PDF, Eq. (5.9). The vertical dashed line indicates the separatrix value Vpp ≃ 23.55 kV. deviation, σI , of IB (t), we consider, in order to characterize the discharge current, also the skewness, SI = I3 /σI3 and the flatness, FI = I4 /σI4 . It is worth noticing that the total charge and mean current are related to each other according to Qtot = I1 ∆t. The results are shown in Figure 5.6. As one can see from the figure, the mean and standard deviation display two different regimes clearly separated by the value Vpp ≃ 23.55 kV, consistent with our previous results (see Sect. 5.2.1). The first two moments strongly increase with applied voltage, and tend to stabilize above the value Vpp ≃ 23.55 kV. Higher moments, such as skewness and flatness of the distributions can be compared with the values expected from an exponential PDF, P (I) = 1 exp(−I/λ), λ I ≥ 0, (5.8) yielding the moment of order n, In = λn Z ∞ 0 dy y n exp(−y) = λn Γ(n + 1), (5.9) 160 160 120 120 σδ [ns] <τδ> [ns] 5.2 STATISTICAL CHARACTERIZATION OF CURRENT SIGNAL 80 40 23 24 25 Vpp [kV] 23 24 25 Vpp [kV] 23 24 25 Vpp [kV] 15 3 10 Fδ Sδ 80 40 4 2 5 1 0 61 23 24 25 Vpp [kV] 0 Figure 5.7: First moments of the burst lengths τδ versus applied voltage Vpp [kV]. Shown are: the mean τδ , standard deviation σδ , skewness Sδ and flatness Fδ . The horizontal line represents the values of Sδ and Fδ for an exponential PDF, Eq. (5.9). The vertical dashed line indicates the separatrix value Vpp ≃ 23.55 kV. where Γ(n) is the Gamma function. According to Eq. (5.9), the skewness √ and flatness take the values SI = 3/ 2 and FI = 6, respectively. The latter are displayed in Figure 5.6 by the horizontal lines. The good agreement of the higher moments confirms that the choice of the exponential distribution as an approximation was adequate. The slightly deviation of flatness from the predicted value is due to the poor statistic for high current values. A similar analysis can be performed for the burst duration, denoted here as τδ . The result for the corresponding moments are displayed in Figure 5.7. Again in this case, the change of discharge regime becomes apparent around Vpp ≃ 23.55 kV, consistent with our previous findings. The mean value τδ first increases with applied voltage, while above 23.55 kV the burst length stops growing, suggesting that a saturation number of simultaneous streamers has been reached. A similar behavior is displayed by the standard deviation σδ , telling us that also fluctuations around mean values are bound when Vpp > 23.55 kV. The further increase of the first two moments for higher voltages may be due to the apparent overlap of nearby bursts not resolved with the present diagnostic resolution. Higher moments of distributions are finally compared with the values expected for an exponential PDF, 62 STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME Eq. (5.8), suggesting that the actual PDF deviates a bit from an exponential shape. 5.3 Statistical analysis of temporal behavior The peculiar presence of memory effect in DBDs (see Introduction 5.1 and Section 2.3.2) suggests that temporal correlations may exist also in the apparently random behavior of micro-discharges in the streamer regimes analyzed here and patterns (not visible with the eye) are formed. In this section it will be studied the existence and propagation of temporal correlations both between discharge processes (bumps) and inside the single discharge process. 5.3.1 Inter- and intra-bump correlations: surrogate model and Hurst exponents Inter-bump correlations In the following we deal with the question of correlations between discharges. We consider first correlations between bumps, or inter-bump correlations. To this end, we study the quantity CBi Bj (τ ), representing the correlations between bump Bi and Bj , separated by a time lag τ = kT , where k = |i − j| and T is the half-cycle period, CBi Bj (τ ) = where the symbol ∆t IBi (t) − IBi · IBj (t) − IBj ∆t σ Bi σ Bj (5.10) indicates the average over the N∆ = ∆t/τ0 values of IB (t) present inside each bump. We find that CBi Bj (τ ) ≃ const (i.e. independent of τ ) for k ≥ 1, indicating that a residual correlation is present between any pair of bumps. By averaging over the total number of bump pairs in the signal we obtain the mean residual correlation between bumps as, CBB = X 1 CBi Bj |k≥1 . NB (NB − 1) (5.11) i6=j The mean correlation CBB is shown in Figure 5.10 as a function of applied voltage Vpp , where CBB ≃ (0.2 − 0.3) for Vpp > 23.5 kV. Note 63 5.3 STATISTICAL ANALYSIS OF TEMPORAL BEHAVIOR 2.2 400 IB [mA] 2.4 2.6 2.8 3 3.2 400 300 IB [mA] 500 Surrogate Real <IB> 200 100 300 time [µs] 0 200 100 0 0 2 4 6 8 10 time [µs] Figure 5.8: Surrogate uncorrelated time series generated with the model compared with the original source data. that there occurs a maximum of CBB around Vpp = 23.55 kV, suggesting another way of determining the separatrix voltage between regimes I and II. In order to understand the origin of such correlations, we implement a surrogate model in which fully uncorrelated discharges (streamers), IS (t), occur inside a bump. Several methods exist for the generation of an uncorrelated time series with a specific PDF [69, 70]. A simple rejection method can be used to obtain a stationary time series with a PDF like those represented in Figure 5.4. To make the model more realistic, we take into account the intrinsic non-stationarity of the process, that is represented by the time dependence of the mean discharge response. To do this, we calculate N∆ local PDFs, PL (t), one for each time step inside a bump, obtained from the total number of bumps in the signal, NB . Then, for each point inside a bump, we generate an uncorrelated surrogate current signal IS (t) according to the local PDF, PL (t). In Figure 5.8 the original signal is compared with the surrogate one and it can be observed that the original shape of the bump is well reproduced. In Figure 5.9 are compared the P (IB ) of the surrogate generated time series and it can be observed that the two PDFs overlap very well although a little underestimation of the zero component of the original signal is present. This is probably due to the limited number of data available for the calculation of local PDFs PL (t). A consequence 64 STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME 0 10 10 -2 -1 P(IB) [mA ] Surrogate Real 10 10 -4 -6 0 100 200 300 IB [mA] 400 500 Figure 5.9: Comparison between the PDF of the original source data with the PDF of the surrogate uncorrelated time series generated with the model. is a slightly overestimation of lower current values, which brings, for exam ple, to an increase of about 15% of the calculated Qtot values. However, this little differences do not affect the correlation analysis performed in the following. As is apparent from Figure 5.8, the surrogate signal displays a similar shape as the real current, but it looks more uniformly distributed as the discharge clustering typical of bursts is not implemented in the model. Yet, the clustering is not important for determining the residual correlations between bumps as shown by the correlation values CSS (open diamonds in the Figure 5.10). We conclude that the residual correlation between bumps we observe in the discharge patterns is due to the non-stationarity of the signal. This conclusion is further supported by the calculation of CBB for the detrended signal, that is the discharge current normalized by its mean value, i.e. ID (t) = IB (t)/ IB (t) . A similar definition is applied to the surrogate signal. As clearly seen from Figure 5.10, the cross-correlations between bumps vanish for the detrended signals, indicating that residual correlations are a result of the non-stationarity of the discharge process. Similar behavior 65 5.3 STATISTICAL ANALYSIS OF TEMPORAL BEHAVIOR 2 6 4 0.4 <CBB> 8 I [mA] 100 200 300 0.5 time [µs] Real Data Detrended Real Data Surrogate Detrended Surrogate 0.3 0.2 0.1 0 23 24 Vpp[kV] 25 Figure 5.10: Mean value of the residual cross-correlation CBB between discharge processes as a function of the applied voltage Vpp [kV]. Both real and surrogate signals are shown in comparison with their respective detrended signals. The inset shows an example of the original signal, a surrogate and the mean response function for Vpp = 25 kV. is displayed by the surrogate current. Intra-bump correlations Verified that the memory effect between half-cycle has no influence on the temporal behaviour of the discharge it is interesting to see if correlations survive inside the single discharge process. The role of clustering (bursts) becomes apparent when studying intra-bumps correlations, as we do next. To study intra-bump correlations, or autocorrelations in the discharge signal IB , we apply the method known in literature as the fluctuation analysis (FA) based on Haar wavelets (HW) [71, 72]. We briefly summarize the FAHW method in the following. The FA approach is based on random walk concepts. One regards the fluctuation of the signal, ∆IB (t) = IB (t) − IB (t) , (5.12) as a jump performed by a random walker at time step t (in units of τ0 ), 66 STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME where tmin ≤ t ≤ tmax . Then, one calculates the position W (ti ) of the RW at time ti = iτ0 , with 1 ≤ i ≤ N∆ , as the sum over all previous jumps ∆IB (tj ), tj ≤ ti , i X (IB (tj ) − IB (tj ) ), W (ti ) = (5.13) j=1 which is also denoted as the ‘profile’ of the random walk. Once the profile has been obtained, we study the scaling behavior of W (t) on the time scale τ . To do this, we divide the total number of points inside the bump, N∆ , into consecutive non-overlapping segments of length ℓ ≥ 1, corresponding to the time scale τ = ℓτ0 . Inside each segment m, 1 ≤ m ≤ N∆ /ℓ, we evaluate the average of W according to, ℓ 1X W (t(m−1)ℓ+j ). ℓ Bm (ℓ) = (5.14) j=1 The FAHW approach consists in studying the fluctuations of the profile on the ‘time scale’ ℓ, defined as F12 (ℓ) = 2 Bm+1 (ℓ) − Bm (ℓ) , (5.15) where the subindex 1 in F1 (ℓ) refers to the first-order Haar wavelet, and the average is performed over all consecutive boxes m and m + 1. Higher-order wavelets can be introduced [71], allowing for eliminating possible higherorder trends in the profile. The dependence of F1 (ℓ) on ℓ is expected to obey a scaling behavior of the form, F1 (ℓ) ∼ ℓH , (5.16) which defines the Hurst exponent H. The value H = 1/2 indicates uncorrelated fluctuations, or standard random walk behavior. Cases in which H 6= 1/2 correspond to signals in which autocorrelations are present. If this occurs for ℓ → ∞, one says that the signal features long-time correla- tions. Cases in which H > 1/2 denote persistence, and cases with H < 1/2 anti-persistence. More common situations are those in which a power-law exponent H 6= 1/2 occurs only on finite time scales, typically at short time scales. These methods have been also employed for the analysis of turbulent 67 5.3 STATISTICAL ANALYSIS OF TEMPORAL BEHAVIOR 2 <F1(l)>B 10 H=0.50 H=0.85 1 10 0 10 2 <F1(l)>B 10 H=0.50 H=0.88 1 10 0 10 0 10 1 10 2 10 Time [ns] 3 10 4 10 Figure 5.11: (color online) Fluctuation analysis of intra-bump correlations. Shown is the quantity F1 (ℓ) versus time scale τ = ℓτ0 [ns]. The average of F1 has been performed over all bumps in the signal. The straight lines have slopes H, yielding the Hurst exponents. The time series analysed corresponds to a high voltage case, Vpp = 25.5 kV. The open circles represent the original signals, while the open squares the detrended ones. Upper panel : Real data. The diamonds were obtained by excluding the zero current values, yielding H ≃ 0.5. The vertical line indicates the time scale τ = 165 ns. Lower panel : Surrogate model. The detrended surrogate signal displays uncorrelated fluctuations H ≃ 0.5. behaviors in magneto-plasma devices [73, 74]. Results of the FAHW analysis performed for real discharge currents and for the surrogate model are displayed in Figure 5.11. We observe that within a time scale of the order of 160 ns, the real data display strong autocorrelations with H ≃ 0.85. These correlations reflect the discharge clustering within bursts. The detrended signal behaves similarly as the original one for time scales within bursts, suggesting that bursts clustering is a robust type of correlation, even when non-stationarity of the signal is eliminated. For time scales larger than mean bursts width, i.e. 160 ns, we observe different 68 STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME behaviors between the original and the detrended signal: The original signal seems to display persistent correlations at such long time scales, while for the detrended one fluctuations become flat. The latter behavior may suggest the presence of long-time anti-correlations. These two unexpected behaviors can be shown to be an artefact of zero-current events in the calculation of F1 (ℓ). To show this, we have studied the scaling behavior of F1 (ℓ) for the case of positive current values, by excluding zero-current events from the analysis. The corresponding points are displayed by the open diamonds in the upper panel of Figure 5.11. As expected, current fluctuations between bursts are uncorrelated yielding the standard behavior H = 1/2 (dashed line). The surrogate signal displays an effective Hurst exponent H ≃ 0.88, suggesting strong autocorrelations inside a bump. These correlations are shown to be an artefact of the non-stationarity of the model and vanish for the detrended signal. This correlation analysis suggests that inside the single discharge process (bump) exists a cooperative behaviour of the streamers occurring sufficiently close in time (i.e. inside a burst), however the eventual pattern formed by discharge remnants on the dielectric is destroyed by the subsequent burst which does not retain memory of the previous one. 5.3.2 Temporal correlations between streamers The presence of strong correlations inside the single burst found with the Hurst analysis (Section 5.3.1) suggests that a deeper insight into the discharge process is required in order to understand the nature of these correlations. To this end, a specific, short time scale analysis of the burst structure has been performed. The lower panel of Fig. 5.12 displays the short-time scale of a single typical burst. One can anticipate the existence of an internal structure of the burst by the presence of several emerging peaks, aside from few significant ones, that represent the micro-discharges or the temporal superposition of more micro-discharges. From now on they will be all referred to as streamers. The aim is to extract information about the temporal streamer distribution by performing an accurate fit to the full burst shape using Gaussian functions as the basis set. Thus, the single burst 5.3 STATISTICAL ANALYSIS OF TEMPORAL BEHAVIOR 69 Figure 5.12: Upper panel : A typical discharge pattern [mA] within a half-cycle vs time [µs] for Vpp =25 kV. The continuous line represents the mean (absolute value) discharge response of the system, averaged over all half-cycles in the time series. Lower panel : The internal structure of a single burst taken from the upper panel (zoomed around 4 µs). The Gaussian fits are physically identified as streamers. The fit virtually coincides with the discharge pattern. Here, τa represents the time interval between two adjacent streamers, and τb the time separation between two adjacent bursts. shape IB (t) is written according to, NB X 1 (t − ti )2 , IB (t) = ρi √ exp − 2σi2 2πσi i=1 (5.17) where ρi is the streamer charge, σi the standard deviation and ti the temporal location of the streamer. First it is required that the number of Gaussians to be used be reduced to a minimum. This is done by searching for their possible locations using information from the (numerically evaluated) first 70 STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME 10 0 intra-B (52 ns) inter-B (293ns) 3.7 -6 F=τ ,τ F=log-normal -1 σ P(τ) 10 10 4.8 F=τ Gauss -2 -3 3.7 -6 10 -4 10 10 1 10 2 τ [ns] 10 3 4 10 Figure 5.13: The scaled PDF, σP (τ ), of intra-burst (open circles) and inter-burst (open squares) times vs τ [ns]. The lines are different types of fits to the numerical data. Intra-burst times: power-law fits ∼ τ 3.7 for τ < 50 ns and ∼ τ −6 for τ > 50 ns (continuous line); ∼ τ 4.8 ×Gaussian (dashed line). Inter-burst times: log-normal fit (continuous line). The mean intra-burst and inter-burst times are indicated in parenthesis. The corresponding standard deviations are: σa = 17 ns and σb = 250 ns, respectively. Averages over different applied Vpp ranging from 24.5 kV to 25.5 kV have been performed. (0) derivative of IB (t). The obtained initial locations ti are used to initiate the search. The fit parameters for all Gaussians inside a burst are then determined using a recursive least-square method. The latter is implemented by a random search of the parameter values using a simulated annealing [75] type of strategy. The final fit yields global absolute error of the order of 10−4 mA in most cases, and the fit is generally indistinguishable from the experimental data, as illustrated in the lower panel of Fig. 5.12. The reconstruction algorithm described above allows to perform a detailed study of the statistic of time intervals between discharge processes, which are represented by the quantities τa which is the time interval between two adjacent streamers, and τb which represents the time separation between two adjacent bursts. The aim here is to identify the nature of the correlations individuated in Section 5.3 STATISTICAL ANALYSIS OF TEMPORAL BEHAVIOR 71 5.3.1. A first insight can be observed in the behavior of PDFs of waiting times. In Figure 5.13 are represented the separated distributions of waiting times within bursts τa and inter-burst times τb . Data have been accumulated for applied Vpp voltages ranging from 24.5 to 25.5 kV after having verified the statistical properties do not depend significantly on voltage. It can be observed that the distributions of waiting times inside a single burst and waiting times between bursts follow different distribution laws. While the inter-burst times τb show a fast-decaying typically uncorrelated behavior, which in Figure 5.13 is fitted with a log-normal distribution, the intra-burst waiting times τa show a non-trivial power law decaying character. Although the PDF analysis in not enough to recognize a correlated behavior, the presence of power laws in distributions have already been connected with anomalous behaviors and presence of correlations (for example see literature on fluctuation analysis in magnetized plasmas [76, 77, 78, 79, 80, 81, 73, 74]). This anomalous behavior of τa distribution suggests the presence of an intraburst structure. To show this, two correlation functions are constructed to analyze the time series of the occurrence of streamers (or superposition of contemporary streamers) and the charge transported by them. The first is the temporal pair distribution function g(τ ) and is defined by: ρ̄τ = Z 0 ∆τ dτ g(τ ) = Ns − 1 (5.18) where Ns is the total number of streamers pairs within ∆τ , ∆τ is the time lag limit and ρ̄τ = Ns /∆τ . g(τ ) can be calculated as a discrete quantity: g(τ ) = Ns Ns X ∆τ /dτ X δdτ ((ti − tj ) − τ ) Ns (Ns − 1) (5.19) j=1 i=1,i6=j where dτ is the discrete time interval and δdτ is a Dirac function. The function g(τ ) is constructed identically to the pair distribution function which is used to recognize spatial correlations in liquid or solid state matter [82, 83]. g(τ ) is its temporal equivalent and expresses the probability to find two streamers occurring at a distance τ in time. Note that g(τ ) tends to unity for large values of τ . 72 STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME 3.00 1.0 Cqq(τ) 0.5 2.00 0.0 g(τ) 0 100 50 τ [ns] 150 200 1.00 0.00 0 100 200 300 τ [ns] 400 500 Figure 5.14: The streamers temporal pair correlation function, g(τ ) (open circles), vs time lag τ [ns], from in Fig. 5.13. The dashed line is the fit shown the data y = 1 + 4.6 exp(−τ / τa ), with τa = 52 ns. The inset shows the autocorrelation function, Cqq (τ ) (open circles), of streamer charge transfer Q vs time lag τ [ns]. The horizontal line is a guide. In Figure 5.14 it is represented g(τ ) for the time series of occurrence of Gaussian functions (5.17) obtained with the reconstruction algorithm described above. It is evident a persistent characteristic time interval for the occurrence of streamers. This suggests the presence of a characteristic ”frequency” of occurrence which disappears outside the single discharge burst. The presence of this strong correlation between streamers and its vanishing for time intervals longer than typical burst duration, confirms what was found with the analysis of Section 5.3.1. Furthermore, the presence of a characteristic time interval can be connected to the discharge development in which the occurrence of a streamer is somehow produced by a previous one. A possible interpretation can be given with the following mechanism. When a streamer occurs in some point on the dielectric surface, in its surroundings exist several other ”seed” micro-discharge remnants that may not have yet reached the breakdown conditions. At this point the photons emitted by the excited atoms in the first streamers may ”induce” the 5.3 STATISTICAL ANALYSIS OF TEMPORAL BEHAVIOR 73 other remnants to attain the breakdown condition. This photo-ionization induced mechanism is influenced by many parameters such as the time for the streamer to reach an adequate emission intensity, remnants on the dielectric surface and others, but finally shows a characteristic time which is evident in the temporal structure shown in Figure 5.14. The influence of radiation on micro-discharges has been reported in the past [84, 85]. If one assumes that this excitation mechanism exists, then the τa of Figure 5.14 is its characteristic time. The nature of this interaction, however, cannot be determined by the present analysis as it requires a different diagnostic approach. Another interesting quantity is the charge associated to every Gaussian streamer which is proportional to the number of real micro-dischargers occurring at the same time. To evaluate the presence of temporal correlations in this quantity it is introduced the function: 1 X (qj − q ) · (qj−τ − q ) Cqq (τ ) = , Nτ σq2 (5.20) j where the sum over j indicates the sum on the total number of time steps Nτ and q and σq2 are the mean value and variance of the distribution of charges ρ in the time series. The function (5.20) is normalized to unity by definition and indicates whether or not correlations between transfered charge are present at a certain distance τ in the time series of Gaussian streamers. In the inset of Figure 5.14 is plotted the function Cqq for the data shown in Figure 5.13. It can be seen that a correlation persists on shorter time scales than the g(τ ). This suggests that the number of microdischarges activated by the hypothetical excitation mechanism stated above has wider fluctuations and de-correlates faster within the time length of the single current burst. It is possible to interpret the two functions (5.19) and (5.20) as the description of two aspects of the discharge development. The former describes the temporal connections in micro-discharges formation which is independent on their number and thus it is more independent on geometrical constraints like the electrode dimensions (but still depends on the gap distance and atmosphere composition and pressure). That is, it describes a more general property of the discharge process: i.e. the temporal aspect of a possible reaction mechanism in which the occurrence of a micro- 74 STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME discharge in a certain position in the gap promotes the formation of other micro-discharges within a well defined time interval. Obviously, this can be verified only with a spatiotemporal analysis which is beyond the capabilities of the present diagnostics. The function (5.20), on the contrary, takes into account the number of micro-discharges occurring in the gap and thus can be considered more dependent on electrode dimensions which limit the intensity of current pulses (see Section 5.2.1). With this interpretation g(τ ) should be found independent of the streamer regimes described in Section 5.2.2 while Cqq should depend on them. Unfortunately, the non-interacting (low-voltages) regime has a poor statistical basis to perform the analysis described here and a diagnostic improvement is needed. It is interesting to note a similar behavior of the current signal (called multi-peaks which have been observed in diffuse DBDs [86, 87, 88]). Even if discharge conditions, gas compositions and time scales are completely different, similarly, a propagation mechanism (in this case of the ionization front) has been proposed to explain the effect [89, 87, 90]. The temporal analysis performed above requires to prove that the peculiar structure is due to correlations between streamers. To this end, an uncorrelated model time series is generated and analyzed with the function (5.19). To make the comparison possible the surrogate time series are generated with a rejection method [69, 70] starting from the P (τ ) fit function which is defined as: P (τ )R = 0.061 (τ /52)−3.75 . 1 + (τ /52)9.7 (5.21) Also a Gaussian distribution with same mean value and standard deviation is considered. In Figure 5.15 are compared the generating functions with the PDFs of the obtained series. The time series of streamer occurrence are then obtained by simple integration of the waiting time series. In Figure 5.16 the function g(τ ) is calculated for the surrogate time series and compared with the experimental data. It is evident that in absence of temporal correlations the oscillation of g(τ ) vanishes almost immediately, revealing that the temporal structure of the experimental data is effectively due to the presence of temporal correlations between streamers in the discharge process. This is also confirmed by the behavior of the power spectra 75 5.4 CONCLUDING REMARKS -1 10 Uncorrelated Process <τ>= 51.9 στ = 17.0 Gaussian Function Uncorrelated process <τ>=53.0 στ=18.3 τa Fit Function -1 P(τ) [ns ] -2 10 -3 10 -4 10 0 10 1 10 2 10 τ[ns] 3 10 4 10 Figure 5.15: The PDF, P (τ ), of surrogate waiting time series compared with the generating functions. Circles refers to time series generated with equation (5.21). Diamonds refer to time series generated with a Gaussian function with same mean value and standard deviation. (Top-right legend) Because the time series refers to waiting times between streamers, the negative values are ignored. of g(τ ) (inset of Figure 5.16) where the presence of a characteristic time for the real data is well evidenced. 5.4 Concluding remarks The streamer regime of a DBD in air has been characterized by means of the statistical analysis of the discharge current. The presence of two different discharge regimes has been observed in several quantities both regarding the statistical properties of the current intensity and its temporal behavior. These regimes have been found to be dependent on the applied voltage. It has been shown that below a threshold value of the applied voltage, the streamers generated in the discharge process can span the largely available space on the dielectric without being affected by the repulsive interaction with residual charges deposited from previous micro-discharges. This brings a rapid growth of the charge transferred by the system within a 76 STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME F[g(τ)] 3 2 -2 10 0 g(τ) 19 MHz -1 10 20 40 60 80 f [MHz] 1 Exp. <τ>=52.0 ns στ=17.0 ns Gauss. <τ>=52.0 ns στ=17.0 ns P(τ)R <τ>=53.0 ns στ=18.3 ns 0 0 100 200 300 τ[ns] 400 500 Figure 5.16: The streamers temporal pair correlation function, g(τ ) (open circles), vs time lag τ [ns], from the data shown in Figure 5.13. The squares and diamonds represent g(τ ) for the streamer occurrence time series calculated from the waiting time time series represented in Figure 5.15. (Inset) The power spectrum of the g(τ ) functions single discharge process. For higher voltages, a limited number of simultaneous streamers seem to occur, as suggested by the behavior of the discharge current shape and temporal properties. In this discharge regime, where the streamers strongly interact, the rate at which energy is transferred by the system to the plasma discharge gets slower with increasing voltages. The presence of correlations between discharge processes and within the single discharge process have been studied. With the help of a surrogate model it has been shown that the observed residual cross-correlations between half-cycles are only an effect of the intrinsic non-stationarity of the signal, indicating that no memory persistence is present in the temporal structure of the discharge. Also it has been shown that, within the discharge process, strong correlations are present in the current signal within a short time scale of the order of the mean value of the burst duration. This suggests that the interaction between streamers can act only when they oc- 5.4 CONCLUDING REMARKS 77 cur close in time and the eventual memory left as discharge patterns on the dielectric is destroyed by the subsequent burst. Decorrelation between bursts and bumps promotes uniformity of energy pattern deposition over time. Using newly defined correlation functions, the temporal structure of bursts have been revealed to be extremely correlated and the existence of a characteristic frequency in the occurrence of streamers have been found. This frequency is possibly related to the propagation of discharge in the gap. 78 STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME CHAPTER 6 Characterization of the DBD device in nitrogen atmosphere 6.1 Introduction In this Chapter the newly developed plasma device described in Section 3.1 is characterized in an atmosphere of pure nitrogen. The aim is to find the device capabilities by exploring the control parameters and give a description of the plasma discharge device in nitrogen atmosphere, which is often chosen as carrier gas for the development of plasma processes for applications. The controlled parameter for such device are: the power injected into the system, flux of the nitrogen gas through the injection nozzle and pressure. Plasma discharges in the DBD will be characterized as a function of these three parameters using voltage and current measurements and optical emission spectroscopy. Nitrogen discharges have been the subject of studies already for many 79 80 CHARACTERIZATION OF THE DBD DEVICE IN NITROGEN ATMOSPHERE decades [91, 92, 42]. It can form stable atmospheric pressure discharges and, being an electropositive gas, it does not have the tendency to quench electron activity in the plasma. Moreover, nitrogen is mostly chemically neutral and, for example, does not alter chemical composition of thin film deposited with plasma processes (see Chapter 7), even if active species are formed during the discharge. These species, principally molecules in metastable states, due to their long lifetimes and elevated potential energy can transfer energy to other species, for example a reactive compound added to the mixture. Last, but not least, nitrogen is extremely cheap with respect to rare gases like helium or argon. For all this reasons nitrogen, as a basis of plasma discharges, is often chosen to develop plasma processes for applications [93, 94, 95, 96]. 6.2 Experimental setup and methods The experimental setup and the diagnostic utilized are described in Section 3.1 and Section 4.2, respectively. The experiments have been performed as follows. The discharge chamber has been evacuated with the rotary pump P1 (Figure 3.1) down to 5 · 10−3 mbar to avoid contaminations, then a calibrated flux from the injection system fill the chamber up to the desired working pressure. After the working pressure is reached the dry pump P2 is used to balance the inlet flux and keep the pressure stable. The current and voltage signals are acquired with a time step of 5 ns for a total length of 0.5 ms. The inter-electrode gap has been kept fixed at 2.5 mm. 6.3 Discharge regimes in Nitrogen Atmosphere Usually, in atmospheric pressure DBDs in nitrogen, the discharge regime is a filamentary one even if, under specific conditions, a homogeneous diffuse discharge may be obtained [86, 9, 97]. However, it is generally difficult to obtain and reliably control such homogeneous discharges at atmospheric pressure. For example, minor changes in the electrode configuration or small variations of the amplitude or repetition frequency of the applied voltage can cause a transition from the relatively unstable diffuse mode to that of a much more stable filamentary discharge. For many potential industrial applications, the diffuse behaviour is a severe disadvantage compared to the 6.3 DISCHARGE REGIMES IN NITROGEN ATMOSPHERE (a) Discharge in Nitrogen: flux of 2 ln /min., (b) Discharge in ambient Air: injected power 220W. 2 ln /min., injected power 200W. 81 flux of Figure 6.1: Typical current voltage characteristics for discharges in nitrogen atmosphere and ambient air at high injected power. Lower panel: applied voltage and total current. Upper panel: applied voltage and discharge current according to equation (5.2). easier implementation of filamentary DBDs. Moreover, ways can be found for ensuring that the intrinsic instantaneous inhomogeneity of this random filamentary DBD does not lead to global inhomogeneity. Important informations can be obtained by the analysis of the currentvoltage characteristics. For all the discharges in the present setup, because the ionization of the gas is very low, it is possible to consider that the capacitance of the gas does not change during the discharge process [9, 11]. The discharge current is thus calculated with equations (5.2) following the procedure described in Section 5.2. In Figure 6.1 are compared the typical voltage and current waveforms for discharges in nitrogen and ambient Air. It can be observed that both show the typical current pulses due to streamers (which are also visible by the eye) but substantial differences are present. In air, current bursts are typically short and more intense (see Chapter 5 for a complete description of the streamer regime in air) while in nitrogen they seem to be lower in height and longer in time with a peculiar slowly decaying current tail. This behavior can be connected to the dimension and duration of single micro-discharges. As already discussed in Chapter 5, current bursts are the temporal superposition of more micro-discharges. It has been shown that oxygen admixtures to nitrogen can lead to plasma channel reduction [98, 4, 38]. Thus, the lower intensity of the current bursts observed in nitrogen can be possibly explained by the presence of wider 82 CHARACTERIZATION OF THE DBD DEVICE IN NITROGEN ATMOSPHERE Figure 6.2: Typical emission spectra for discharges in nitrogen atmosphere. Flux of 2 ln /min., injected power 100W, pressure 500 mbar. streamer and, thus, less contemporary streamers developing close in time. Also the different charge transported by the single discharge process must be considered. Moreover, the longer duration of current bursts in nitrogen, characterized by the slowly decaying current tail, suggests the presence of active species with longer lifetime like N2 (A3 Σ+ u ) metastable molecules [99] that maintain active the discharge. The presence of electronegative oxygen gas quenches more rapidly this activity. Useful information on the plasma phase can be achieved from the analysis of emission spectra. Nitrogen is a very active species which has a complex reaction scheme involving electronic, vibrational and rotational excited states along with ionized species. Particularly important is the role of vibrational excitations. The creation of excited vibrational state by electron impact is highly favorable while the relaxation processes (see Section 2.4.1) are less effective. Thus, vibrational states adsorb a large part of the energy and act as a sort of reservoir. These energies are typically high enough to activate chemical reactions with other species [2]. In Figure 6.2 is shown the typical emission spectra of a discharge in nitrogen. The spectrum is shown between 300 nm and 500 nm because outside this region the emission lines are absent or too weak . The spectrum is dominated by the second positive system (SPS) of N2 (C 3 Πu →B 3 Πg ) [100, 101]. Vibrational levels are usually thermalized because vibrational-vibrational transition processes are 6.3 DISCHARGE REGIMES IN NITROGEN ATMOSPHERE 83 very effective. Thus, a vibrational temperature Tv can be calculated from the SPS structure by determining the populations of the vibrational levels of N2 [102, 103, 104, 105, 106] and according to the formula: nN2 (C,ν) = EN (C,ν) X IN2 (C,ν)→N2 (B,ν ′ ) 2 − kT v ∝ e 4 νN2 (C,ν)→N2 (B,ν ′ ) ′ (6.1) ν where ν and ν ′ are the vibrational level index, IN2 (C,ν)→N2 (B,ν ′ ) is the intensity and ν the frequency of the electronic transition between N2 (C 3 Πu ) and N2 (B 3 Πg ) levels of nitrogen. It is interesting to recall that also the electron temperature could be determined by the vibrational population levels [107] and, generally, it increases in the same way as Tv . An emission line from the 2 + 2 + first negative system (FNS) of N+ 2 (B Σu →X Σg ∆ν = 0) is also visible at 391.3 nm. This emission line is usually connected to the electron energy and electron energy distribution function because the ionization threshold, from 2 + 3 a neutral N2 molecule, of the N+ 2 (B Σu ) is higher than N2 (C Πu ). Thus, to a first approximation, the ratio between the intensities of two characteristic lines of FSN and SPS is a monotone function of the electron temperature [108, 109]. The ratio between 391 nm line of FSN and 357 nm line of SPS will be used in the following to estimate the variation of electron temperature as a function of discharge parameters. 6.3.1 Characterization of the discharge as a function of injected power The DBD discharge device is powered by a simple transistor switching system which does not allow to control separately frequency and voltage (see Section 3.1.3). The voltage applied to the electrode is varied by varying the difference of the frequency with respect to the resonance frequency of the system. The generator also provides a measure of the power injected. The generator can support powers up to 600W but in the present experiments the power levels have been kept under 250W to avoid overheating and damage to the polycarbonate injection nozzle. As a first characterization, a current-voltage plot is obtained (Figure 6.3(a)) spanning the available power range of the system. It can be observed that the behavior is not unique in the above range and two regions are evident. By comparing the root mean square (rms) value of displacement and total current as a function of the rms 84 CHARACTERIZATION OF THE DBD DEVICE IN NITROGEN ATMOSPHERE (a) Current-voltage characteristic of the (b) Mean value transferred charge in ¸ ˙ of total DBD device. a half period Qtot . Figure 6.3: Discharge regimes: Voltage-Current characteristic and total transferred charge. Flux of 2 ln /min., pressure 900 mbar. rms has a rough linear applied voltage V rms , it is evident that, while Idisplace rms shows a two-stage behavior. This is the same dependence on V rms , Itot effect that has been characterized for discharges in Air in Chapter 5. In the lower voltage regime new streamers can occur spanning the largely available space on the dielectric without strongly experiencing the repulsive interaction with residual charges deposited from previous micro-discharges within the same half-cycle. In the high voltage regime the micro-discharges are forced to a strong repulsive interaction which limits the possibility to add more streamers to the electrodes. In the same way and similarly to equation (5.6) the two discharge region can be well recognized by measuring the mean value of the total charge transferred by the discharge process which can be calculated as: Qtot = Z 0 T /2 Idisch (t)dt , where the integration is over a half period and the mean (6.2) is calculated over all the half periods recorded in the time series. It is worth to mention that, even if the electrode system is not symmetric, no asymmetries have been found in the measure of Qtot . Qtot has been calculated according to equation (6.2) and the results are shown in Figure 6.3(b). A separatrix rms voltage Vsrms ≃ 5.1kV is found. In Figure 6.4(a) the vibrational temperature Tv is shown, calculated according to equation (6.1). It can be noted that the vibrational temperature 6.3 DISCHARGE REGIMES IN NITROGEN ATMOSPHERE 85 (a) Vibrational temperature as a function of (b) 391nm/357nm line intensity ratio as a power. The solid red line is a linear relation function of injected power. plotted as guideline. Figure 6.4: Variation of vibrational temperature and 391nm/357nm line intensity ratio as a function of the injected power. is of the order of 2500 K, which means that in such discharges also the neutral molecules have thermal energy sufficient to influence directly the chemical kinetics evolution of the gas-phase. A slow decrease of Tv with increasing injected power is evident which means that at higher power levels it is allowed a more pronounced quenching of the excited vibrational levels respect to its ground state. In Figure 6.4(b) the 391nm/357nm line intensity ratio is plotted as a function of the injected power. It is evident that no clear trends can be recognized in the plot because all variations seem to be within the error of the measure. This means that no substantial variations are present for the electron temperature. 6.3.2 Characterization of the discharge as a function of pressure and gas fluxes One of the most interesting capabilities of this DBD device is the possibility to work in a completely controlled atmosphere being the electrodes inside a vacuum chamber. This characteristic allows to work also at lower pressure. In the following the electrical and optical behaviour of the discharge is observed as a function of pressure in the range 50÷900 mbar. It has not been possible to perform experiments below 50 mbar, because the system tends to realize a diffuse discharge that is not confined in the electrode gap. In Figure 6.5 are shown the current and voltage waveforms for exper- 86 CHARACTERIZATION OF THE DBD DEVICE IN NITROGEN ATMOSPHERE (a) Discharge at 50 mbar, 75W (b) Discharge at 200 mbar, 100W (c) Discharge at 500 mbar, 100W (d) Discharge at 900 mbar, 100W Figure 6.5: Current-voltage characteristic for discharges at various pressure. Lower panel: applied voltage and total current. Upper panel: applied voltage and discharge current according to equation (5.2). iments at various pressures. At first glance, it is evident that for lower pressure the discharge seems to occur in a sort of continuous mode while at higher pressure the identification of current bursts is possible. This can be explained by the fact the duration of current pulses due to streamers have been measured to be proportional to inverse square of the pressure [4]. As it is known, when the pressure (number density) is low the ionization by direct electron impact α is lower and a localized space charge (which is the origin of the streamer channel) is not created. Moreover, a fundamental role is played by metastable species which (by Penning ionization, equation 2.11) keep the number of seed electrons high and lower the breakdown voltage. The requirement for establishing a stable diffuse discharge (dominated by Townsend breakdown mechanism) is that the number of seed electrons is large enough to cause appreciable overlap and merging of the primary avalanches. With a better analysis of the discharge current for the 50 mbar 6.3 DISCHARGE REGIMES IN NITROGEN ATMOSPHERE 87 (a) Rms voltage and total charge according (b) Mean value transferred charge in ¸ ˙ of total to equation (6.2). a half period Qtot . Figure 6.6: Rms voltage, total charge and absolute intensity of emission as a function of pressure. Flux of 2 ln /min., power 100 W (75W for 50 mbar). discharge (Figure 6.5(a), top panel) it is possible to see that, even if the discharge seems a single process, several well separated current peaks can be observed. This kind of behaviour is different from the current characteristic of a well developed diffuse discharge [110, 108]. This can be possibly explained by the fact that, even if the lower pressure would allow the development of a diffuse discharge, does not exist an effective process to maintain the number of secondary electrons high enough. Thus, the discharge processes last on smaller time scales giving origin to the observed current peaks. The transition between these two regimes is not clear even if they have been studied as a function of gas composition [111], electrical and geometrical parameters [10, 112] and pressure [108]. A simple visual observation suggests that a diffuse discharge exists up to 300 mbar, but this is due to the superposition of thousands discharge processes. It has been shown that the use of fast cameras can reveal the presence of the streamers [113, 110, 114]. An interesting observation is that a substantial reduction of the duration of the discharge process occurs with increasing pressure. Moreover, the same occurs for the slowly decaying current tail. In Figure 6.6 are shown the behaviours of the rms voltage and total charge transferred according to equation (6.2) as a function of pressure and for a constant injected power of 100 W. As it can be expected [5, 1], the rms voltage increases with pressure because the breakdown voltage increases. The interesting aspect is the behaviour of Qtot which decreases with pres- 88 CHARACTERIZATION OF THE DBD DEVICE IN NITROGEN ATMOSPHERE Figure 6.7: Left scale: Vibrational temperature as a function of pressure. Right scale: 391nm/357nm line intensity ratio as a function of pressure. sure. This means that for the same power level the system at lower pressure transfers more efficiently energy to the plasma. This is also confirmed by the behaviour of the absolute intensity of the discharge (Figure 6.6(b)) which is higher for lower pressure. In Figure 6.7 are shown the behavior of the optical emission spectra by measuring vibrational temperature and mean electron energy variations. The grow of the mean electron energy with the decrease of pressure has been observed elsewhere [108] and it is also evidence that the discharge regime moves toward a diffuse one [114, 99]. The similar behaviour of the vibrational temperature is consistent with the general relation between these two quantities [107]. What is more surprising is that Tv shows a minimum at 300 mbar and increases for increasing pressures while the 391nm/357nm ratio seems to reach a plateau with only a slightly increase with pressure. The behaviour of the latter quantity has already been observed elsewhere [108] for air and has been connected to the transition to filamentary discharge. The observed increase of Tv above 300 mbar has no simple explanation. Assuming that electron temperature does not change too much (as indicated by the 391nm/357nm line intensity ratio), some change in the kinetic equilibrium favours the excitation of vibrational states, moreover, a role of the quenching processes of the vibrational state should be considered. However, a deeper study of the phenomena also with 6.4 CONCLUDING REMARKS 89 Figure 6.8: Left scale: Vibrational temperature as a function of nitrogen flux. Right scale: 391nm/337nm line intensity ratio as a function of nitrogen flux. other diagnostics should be performed in order to give a complete explanation. Finally, it has been observed also the behaviour of the discharge when the inlet fluxes are changed. In Figure 6.8 are shown the behavior of the optical emission spectra by measuring vibrational temperature and mean electron energy variations. It is evident that the adjoint of gas convection does not influence the discharge in a measurable way. Also electrical discharge behavior remains unchanged. 6.4 Concluding remarks The capabilities of the developed DBD device have been verified by exploration of parameter space. A nitrogen atmosphere has been chosen because of its capability to transfer energy to other species and generate reactive environments without influencing too much the chemistry of the processes and being often the best basis for the study and development of plasma processes for applications. It has been observed that the system shows the presence of two discharge regimes as a function of the applied voltage as already observed for the system described in Chapter 5. For higher voltages, because of the limitation in the number of simultaneous streamers, the 90 CHARACTERIZATION OF THE DBD DEVICE IN NITROGEN ATMOSPHERE rate at which energy is transferred by the system to the plasma discharge gets slower with increasing voltages. It has been observed also that a possible change in the quenching mechanism of the vibrational state generates a slightly decrease in the vibrational temperature with increasing injected power. An interesting behaviour of electrical and optical measurements has been observed when the pressure is varied even if a complete transition to a diffuse discharge regime cannot be reached. CHAPTER 7 Deposition process of organosilicon thin films 7.1 Introduction In this chapter a deposition process of thin organosilicon films at atmospheric pressure is investigated as a method to obtain and control hydrophobicity of materials surface. Recently, plasma deposition at atmospheric pressure has become a promising alternative to low pressure plasma enhanced chemical vapour deposition (PECVD) [25, 24, 115, 116, 117, 23, 118]. The main advantages are the possibility to avoid the expensive vacuum systems, to decrease the time of treatment, and to simplify the technological transfer where the processes of production are making in continuous mode. The use of organosilicon compounds as precursors for deposition processes of thin films of silicon compound has been studied for several pur91 92 DEPOSITION PROCESS OF ORGANOSILICON THIN FILMS poses like vapour and gas barrier creation [119, 120, 121], wear and friction reduction [122, 123], anti-corrosion protection [124, 125], biocompatibility [126, 127], hydrophobicity of surfaces [128]. Modification of hydrophobicity properties of surfaces with plasma treatment can be obtained with fluorination processes, coating processes with fluorocarbon of hydrocarbon films. However, these processes may become unstable and show aging by oxidation [129] or other more complex aging processes depending on substrate (see for example Chapter 9). Low surface energies, which mean also hydrophobicity, can be attained with high retention in the coating of methyl groups (CH3 ) which have a non-polar character and tend to repel highly polar water molecules. Starting from an organosilicon precursor like hexamethyl-disiloxane (HMDSO, see Figure 7.1) it it possible to obtain a highly organic deposit taking advantage of the elevated intrinsic stability due to their partly cross-linked Si-O chains (backbone) which results in an enhanced long-term durability (See Chapter 9). Here are presented results regarding the deposition process of thin organosilicon films generated at atmospheric pressure in nitrogen with small admixtures of HMDSO vapours. The plasma source is the DBD described in Section 3.1 which works in a roller configuration and is able to simulate continuos treatments of material surfaces and operates in controlled atmosphere. 7.2 Materials and methodology The liquid HMDSO is introduced as a vapour in small quantities using the evaporator system described in Section 3.1.2. The carrier gas which dilute the vapor is nitrogen and concentrations of HMDSO are varied up to 1.2%. According to data from [130], at a pressure of 1 bar and at room temperature (25 ◦ C), the maximum concentration of HMDSO before condensation occurs is 5.5%. The vacuum chamber is initially evacuated with the rotary pump P1 (Figure 3.1) down to 5 · 10−3 mbar to avoid contaminations, then calibrated fluxes from the injection system fill the chamber up to a working pressure of 900 mbar. Although the chamber is provided with gaskets both for under- or over-pressure, a slightly lower pressure ensure a better insu- 93 7.2 MATERIALS AND METHODOLOGY (a) Chemical representation. (b) Graphic representation. Figure 7.1: Representation of the hexamethyl-disiloxane (HMDSO). Formula: C6 H18 OSi2 . Molecular weight: 168.38 amu. Boiling point: 373 ± 2 K. lation from possible contaminations. Experiments have been performed to verify that the hundred millibar difference does not affects the deposition process. After the working pressure is reached the dry pump P2 is used to balance the inlet fluxes and keep the pressure stable. The carrier nitrogen gas is maintained at 2 ln /min, while the liquid flux is regulated as needed to obtain the desired concentration of HMDSO. In the present study the experiments are implemented at constant power of 170 W injected in the system. The specimens undergoing the treatment are exposed to the plasma at the tangent speed of 1 m/min for 15 times. For this kind of geometric configuration is convenient to use the so called corona dose which is defined Power as D = which has the dimensions of energy electrode width×tangent speed on surface. Thus the experiments are performed with D = 728.6 kJ/m2 . The rather high energy dose has been chosen to reduce errors on experimental measurements (weighting, FTIR, thickness). To estimate the residence time it can be considered the diameter of the rod electrodes (12 mm) as the discharge length1 as a rough estimate of its dimension. Thus, for these experiments, the total residence time is around 21 seconds which is extremely low with respect to low pressure plasma processes to obtain the same results. The plasma discharge is characterized by current-voltage measurements and the acquisition of optical emission spectra (see Sections 3.1, 4.2 and 1 This is actually an over estimate because the typical length of the discharge is less. Thus, the actual value cannot be measured independently on power and other parameters in such DBD devices 94 DEPOSITION PROCESS OF ORGANOSILICON THIN FILMS 4.1). The deposits are characterized with several methods. KBr salt pellets of 8 mm diameter are prepared by compression starting from the powder (Fluka) and exposed to the plasma. The transmission infrared spectra of the pellets are measured with the FTIR spectrometer (see Section 4.3.1) before and after the treatment and their difference is considered. Small sheets (10x4 cm2 ) of low density polyethylene (PE) 0.2 mm thick, is washed in acetone and attached to the grounded rotating electrode. It is used to evaluate the morphology of the deposits using the atomic force microscope (AFM) (see Section 4.3.2). Mass deposition rates were evaluated by weighting larger sheets with an analytical balance before and after the exposition to the plasma with an appropriate mask (15x15 cm2 ). Small pieces (∼10x5 mm2 ) cut from (100) silicon wafer where also exposed to the treatment. Where needed, the specimens were attached to the grounded electrode with tape. 7.3 Characterization of the deposition process As already mentioned in Section 7.1 HMDSO possesses some features that make it extremely effective for the realization of hydrophobic coatings. The main polymerization process is through the creation of Si-O bonds with the creation of a highly cross-linked inorganic backbone. This kind of reactions are chemically favorable once in the plasma are generated radicals by fragmentation of the original compound. For this reason organosilicon compounds can produce better deposition rate than simply organic precursors. From Figure 7.1 it is evident that the monomer is initially highly organic. The retention of initial methyl groups is controlled by discharge conditions. Higher levels of power injected into the system usually promote a higher grade of fragmentation which induce a loss in organic character of the deposit. When a completely inorganic coating is needed (silica-like), for example for the realization of barrier effects, usually oxygen is added to the gas mixture in order to promote the oxidation of the organic compounds. Under specific conditions a nearly complete elimination of organic character can be achieved [122, 119, 120]. In the present experiments, on the opposite, it is searched the highest retention of methyl group in order to obtain the best achievable hydrophobicity. In the following, the best degree 7.3 CHARACTERIZATION OF THE DEPOSITION PROCESS 95 Figure 7.2: Current-voltage characteristic of a discharge in nitrogen with 0.45% of HMDSO vapour. of organic retention is searched by analyzing both the plasma and deposit characteristics. 7.3.1 Plasma characterization Important informations can be obtained by the analysis of the current voltage characteristic. The typical current and voltage waveform of the discharge process is plotted in Figure (7.2). From the presence of fast current pulses it is possible to recognize the typical behaviour of the streamer regime already described in the preceding Chapters. In comparison with nitrogen atmosphere (Figure 6.1(a)), it is evident the streamer have a shorter duration. Moreover, a visual observation reveals thinner plasma channels. Possibly this effect can be similar to that observed in air (see Chapter 6) where the presence of oxygen leads to plasma channel reduction [98, 4, 38]. The uniformity of the treatment is then guaranteed by the mean effect due to the large difference between the time scales of the treatment (seconds), of the discharge process (microseconds) and of the typical duration of the streamers (nanoseconds). It is well proved that uniformity is achieved down to the microscopic scale as it is evident from the roughness analysis (see Figure 7.5). Thus, the presence of a streamer regime dose not undermine uniformity issues usually fundamental in plasma applications. Useful information on the plasma phase can be achieved from the analysis of emission spectra. In Figure (7.3) is depicted the typical emission spectra of 96 DEPOSITION PROCESS OF ORGANOSILICON THIN FILMS the discharge. The spectrum is shown between 300 nm and 500 nm because outside this region the emission lines are absent or too weak . The spectrum is dominated by the second positive system (SPS) of N2 (C 3 Πu →B 3 Πg ) [100, 101]. From the SPS structure it is possible to determine the populations of the vibrational levels of N2 molecules and calculate the vibrational temperature Tv [102, 103, 104, 105, 106] which is an interesting plasma parameter because processes such as vibrational relaxation and excitation can strongly influence plasma chemistry [2]. This is because the vibrational levels are mostly excited by direct electron impact and vibrational-translational relaxation processes are not efficient in converting vibrational energy into kinetic energy (heating of the gas). Thus, energy remains ”trapped” in the vibrational levels which give to molecules a reservoir of energy to activate several chemical reactions (see also Section 2.4.1). The determined values are 2000± 100 K for all concentrations of HMDSO explored. These temperatures are lower than temperature achieved in pure nitrogen atmosphere (∼2700±40 K). This finding suggests that, being Tv a monotone function of the electron temperature [131, 107], also the latter is lower in these discharges. In Figure (7.3) an emission line from the first negative system (FNS) of N+ 2 2 + (B 2 Σ+ u →X Σg ∆ν = 0) is also visible at 391.3 nm. This emission line is usually connected to the electron energy [108, 109], but here is not easily observed because of the presence of the CN bands. The most interesting feature of the spectra is the presence of the CN 2 + violet system at 388 nm and 422 nm (B 2 Σ+ u →X Σg ) which is a consequence of the chemistry of N2 +HMDSO vapour mixtures in the plasma state. A complete evaluation of the concentration of active chemical species, ions or radicals from OES diagnostics requires a detailed modeling of the excitation and quenching processes for each light emitting energy level observed in the spectra. However relative information could be inferred by normalizing the emission intensities of different emitting molecules. The CN line intensity at 387.1 nm has been normalized to that of one of the brightest band of the SPS of nitrogen at 357 nm. In this way the dependency of absolute intensity from high energy electrons density in the discharge region is factored out. Moreover, assuming that electron temperature does not change too much 7.3 CHARACTERIZATION OF THE DEPOSITION PROCESS 97 Figure 7.3: Emission spectrum from the plasma of a mixture of nitrogen and 0.15% HMDSO vapour.Inset: CN(387.1)/N2 (357.7) line intensity ratio as a function of HMDSO concentration. and, since dissociation level is usually very low, that the absolute density of nitrogen is constant, the the intensity ratio should be proportional to the relative concentration of CN during the discharge[132]). We can see in the inset of Figure 7.3 that this quantity shows a stepwise behaviour evidencing a threshold value of concentration around 0.3% after which the CN line intensity abruptly decreases. The presence of cyano radical CN is connected to the fragmentation of initial HMDSO monomer in plasma. The formation of CN requires carbon atoms which can come only by the monomer and are created by consecutive fragmentation of the organic components of HMDSO. The vanishing of the CN emission band possibly means others reaction channels are preferred at higher concentrations of HMDSO and the monomer retains more of the initial organic character. From the analysis of the emission spectra it is possible to suggest the presence of two different discharge regimes in which the chemistry of the plasma changes in some way. This behaviour will be observed in other quantities further in the following. 7.3.2 Thin film characterization Although the plasma phase analysis can give useful information on the development of the deposition process, it cannot give too much hints on the 98 DEPOSITION PROCESS OF ORGANOSILICON THIN FILMS plasma-surface interactions and surface reactions complexity. Usually at atmospheric pressure the role of energetic ions less important and the typical reaction scheme is more like the one described in Section 2.4.2, where the radicals, compounds and ions created in the plasma phase are adsorbed to the surface where chemical reactions take place. The change in the morphology of the depositions have been measured on PE substrates with an atomic force microscope as a function of HMDSO concentration. In Figure (7.4) are compared images of the deposits at different concentration of HMDSO in comparison with the untreated PE surface (a). At a concentration of 0.05% (b) we can see that the deposition process is generate a ”dust” like film with evident nanoscale structures. At a concentration of 0.15% (c) and 0.3% (d) it is possible to see that still some structure is present which is embedded in a structureless deposit. At higher concentrations (e and f) the formation of nanoscale structures is no longer visible and the deposition is extremely smooth. The change in the morphology is evident also in the roughness of the surface which is evaluated from theqroot mean square (RMS) of the heights of the surfaces defined as 2 RMS = h2 − h , which is the standard deviation of probability distri- bution function of heights. The change in the morphology can be explained with the presence of two mechanism of deposition. When the fragmentation of monomer is high, a plasma-phase polymerization with subsequent adsorption and reaction on the surface is predominant with the result of a higher grade of cross-linking and the creation of nanoscale structures. These structures are often observed in other experiments both at atmospheric pressure [24] and low pressure (in this case usually with smaller characteristic dimensions [133]). On the other end, when the concentration of monomer is high a different deposition process takes place. As mentioned before, the residence time of specimens in the discharge region is of 21 seconds while the treatment times lasts for around 10 minutes. Thus, the specimens are exposed, for most of the time, to the neutral atmosphere containing the monomer vapour. Possibly, if the concentration is high enough, the monomer is absorbed on the substrate surface and reacts with the radicals created before by the plasma or is activated when exposed to the plasma in a sort of mechanism of adsorption/reaction polymerization. This creates a smoother, softer, structureless deposit which 7.3 CHARACTERIZATION OF THE DEPOSITION PROCESS 99 Figure 7.4: AFM images of thin films deposited on PE substrate at different concentrations of HMDSO. a=untreated, b=0.05%, c=0.15%, d=0.3%, e=0.45%, f=1.2%. 100 DEPOSITION PROCESS OF ORGANOSILICON THIN FILMS 50 Untreated PE RMS [nm] 40 30 20 10 0 0 0.2 0.4 0.6 0.8 HMDSO [%] 1 1.2 Figure 7.5: Roughness of treated PE substrates estimate with RMS as a function of HMDSO concentration. embeds the morphological structure of the plasma phase polymerization. This second kind of deposit is usually really soft and flexible. Large differences can be found also in the chemical composition of the deposited thin film which have been characterized measuring the infrared absorption spectra. Figure (7.6) shows the spectra of the deposits at different HMDSO concentrations. The spectra show the typical bands already recognized in the literature [134, 135, 124] and indicated in the Figure (7.6). According to the literature the stronger absorption band in the range 10001150 cm−1 can be assigned to the Si-O-Si asymmetric stretching mode. Other typical absorption band can be assigned: the CH3 symmetric bending in Si-CH3 at 1260 cm−1 , the CHx symmetric and asymmetric stretching at 2900-2960 cm−1 , the CH3 rocking in Si-(CH3 )2 at 800 cm−1 and the Si-CH3 rocking vibration in Si-(CH3 )3 at 840 cm−1 . Bands at 800 cm−1 , 840 cm−1 , 1260 cm−1 and 2900-2960 cm−1 indicate retention of methyl group in the plasma deposit, which brings the condition for the creation of hydrophobic surfaces with HMDSO plasma. It is interesting to observe the two peaks at 800 cm−1 and 840 cm−1 . It can be seen a rapid growth of the Si-(CH3 )3 peak against the Si-(CH3 )2 as the concentration of HMDSO grows. Si-(CH3 )3 groups are termination sites in the network structure of the deposited films. The abundance of such groups at higher monomer concentration indicates the films are composed of shorter chains having a less cross-linked structure. Observing the spectra it is evident an increase of all the bands relative to 7.3 CHARACTERIZATION OF THE DEPOSITION PROCESS 101 Figure 7.6: FTIR spectra o the thin films at different HMDSO concentrations. a=0.05%, b=0.15%, c=0.3%, d=0.45%, e=1.2%. The spectra are normalized on the Si-O-Si peak intensity. organic compounds which are responsible for the hydrophobic character of the resulting surface. In order to evaluate in a more quantitative way the increase of the organic character of the deposits, the ratios of the areas of the peaks of interest in the spectrum have been analyzed. To this end, a deconvolution process of the spectra has been performed using Lorentzian function as basis [136]. In Figure 7.7 is represented the resulting fit for a 0.05% HMDSO deposit. Not all the peaks are considered: the areas of interest are marked with pattern fill and named out. A quantitative analysis from an infrared spectra can only be performed by calculating ratios of the areas internal to a single measure [136]. To this end two ratios have been evaluated as a function of HMDSO 102 DEPOSITION PROCESS OF ORGANOSILICON THIN FILMS Figure 7.7: peaks areas recognition with Lorentzian functions from the FTIR spectra of 0.05% HMDSO concentration. The areas of interest are marked with pattern fills and named. concentration. The first is the ratio between the areas at 1260 cm−1 relative to Si-CH3 symmetric bending and the area of the band at 10001150 cm−1 relative to Si-O-Si asymmetric stretching mode [137]. This ratio gives some information on the organic character of the deposits and in particular on the methyl group retention. The second ratio considered is the CH3 rocking in Si-(CH3 )2 at 800 cm−1 and the Si-CH3 rocking in Si-(CH3 )3 at 840 cm−1 . The presence of three methyl groups attached to silicon means a termination of the polymeric chain or cross-linked structure. Two methyl groups attached to silicon are related to compounds of the type Me − (Me2 SiO)n − SiMe3 (or ramifications of them). Compounds of this kind have been observed with gas chromatography analysis of the exhaust of process gases [23, 138]. In Figure (7.8) (left scale) it is plotted the value of the first ratio as a function of the HMDSO concentration. We can observe that the organic part of the thin film grows rapidly up to a saturation value of ∼0.24. This means the retention of methyl groups cannot grow beyond a certain value which possibly represents the limit of stability of the deposit. This is confirmed by the saturation behaviour of the second ratio which represents somehow the degree of polymerization and, thus, the stability of the deposit. This chemical analysis confirms the observation of a softer and 7.3 CHARACTERIZATION OF THE DEPOSITION PROCESS 103 Figure 7.8: Ratio of the area at 1260 cm−1 relative to Si-CH3 and the area at 1000-1150 cm−1 relative to Si-O-Si asymmetric stretching mode(left scale). CH3 rocking in Si-(CH3 )2 and the Si-CH3 rocking in Si-(CH3 )3 at 840 cm−1 (right scale). Figure 7.9: Mass deposition rate as a function of the HMDSO concentration. smoother deposit at higher concentrations. The same behaviour is visible in the mass deposition rate plotted in Figure 7.9. The reaching of a saturation value of ∼0.8 µg/mm2 suggests the growth of the polymer is not simply limited by the quantity of monomer present in the gas phase but it depends on the complex chemistry both in the plasma phase and on the surface. Indeed, this saturation is reached with 104 DEPOSITION PROCESS OF ORGANOSILICON THIN FILMS Figure 7.10: Advancing and receding contact angle with water as a function of HMDSO concentration. The error bars indicate the statistical errors on 5 independent measures. Horizontal dashed lines are, in the same colour the advancing and receding contact angle of untreated PE surface. the increase in monomer concentration while the power is kept constant, and could be interpreted as that the deposition process reaches a power deficient regime in which not enough fragmentation is achieved in plasma phase [139, 140]. However the deposition process here seems much more complicated by the presence of two phases (plasma and neutral atmosphere) alternate during the treatment because of the electrode configuration. The retention of methyl groups can be evaluated by a macroscopic measurement of water contact angles. Using the technique described in Section 4.3.3 it is possible to gather information on the microscopic chemical heterogeneity of the deposit and evaluate the achieve degree of hydrophobicity of the surfaces. In Figure 7.10 are showed the advancing and receding water contact angle measured on treated PE surfaces. Advancing angle, which is connected to the presence of non-polar groups on the surface have a saturation behaviour similar to other quantities observed before, indicating that a limiting retention degree of the initial methyl groups has been reached. The observed receding angles lay always below the value of untreated PE. This behaviour can be connected, on one side to the presence of inorganic SiOx compounds in the deposit which are characterized by a high wetta- 7.4 CONCLUDING REMARKS 105 bility, on the other side can be connected to the presence on the surface of polar compounds affine to water [141] which are due to a plasma activation of the surface and the subsequent reaction with atmospheric oxygen and water [16] (which brings to the creation of polar compounds like hydroxyl, carboxyl or carbonyl groups). This surface effect suggests that together with the deposition process an activation process due to nitrogen is also present. 7.4 Concluding remarks The deposition process of organosilicon thin films with plasma of nitrogen with small admixtures of HMDSO vapour has been characterized. Analyzing the behaviour of several quantities as a function of the HMDSO concentration we have found the deposition mode changes with increasing concentration. For lower values the deposition strongly depends on the concentration itself, while after some threshold value it remains most independent. This behaviour has been observed in several quantities relative both to the plasma phase and to the resulting deposits. The retention of organic compounds in the deposits have been studied at a microscopic and macroscopic level. It has been found that the retention of initial monomer methyl groups saturate with concentration and so does the hydrophobic character of the resulting surface. Stability issues of the resulting deposit will be discussed in Chapter 9 dealing with the application of this process to cellulosic materials (paper). 106 DEPOSITION PROCESS OF ORGANOSILICON THIN FILMS CHAPTER 8 Fluorination of polymer surfaces 8.1 Introduction In this chapter a grafting process of fluorine atoms on polyethylene (PE) surface is investigated at atmospheric pressure as a method to obtain and control hydrophobicity and oil-repellency of materials surface. Plasma induced modifications of materials surface with fluorination (grafting of fluorine atoms) processes and deposition of fluorocarbon thin films have been studied for several applications because of the unique properties that can be attained. For example, fluorination of the polymer surfaces produces hydrophobic surfaces, at the same time, preserving the bulk properties of the materials [142, 143, 144, 145]. Fluorocarbon thin films have been studied for the creation of barrier layer against solvent and fuel permeation [146], adhesion of carbon nano-tubes in composite material [147], bio-compatible materials production [148]. Most of the processes are performed at low pressure even if few reports of atmospheric pressure processes are present 107 108 FLUORINATION OF POLYMER SURFACES [149, 26, 7]. However, at atmospheric pressure, the process is rather far from being well understood both when dealing with the plasma-phase properties and the induced surface modifications. Moreover, experiments on fluorination at atmospheric pressure have never been performed in continuous-mode plasma reactors such as the one used here (see Section 3.1). Plasma processes based on Sulfur hexafluoride (SF6 ) are an effective source of fluorine radicals and fluorination of materials surface can be successfully realized [26, 142, 144, 143]. Fluorination process is a grafting process (see Section 2.4.2) which involves the substitution of an hydrogen atom, bound to carbon, with a fluorine. The basic reaction scheme is the following: hν,e − F• −CH −−−−→ −C• −−→ −CF, Ion,F• (8.1) where the surface can be activated by ions, electrons, photons and other active species generated in the plasma (see Section 2.4.2) and, then, a fluorine radical adsorbed to the surface reacts with a carbon radical forming a stable chemical bond. SF6 is a highly electronegative gas which posses a high dielectric strength (i.e. the minimum electric field strength for breakdown) and is usually used as electrical insulator in high-voltage circuit breaker [150]. For this reason, in the present experiment SF6 is only added in small quantities to argon to obtain stable discharges. Thus, materials surface is exposed to both the effect of argon carrier and SF6 . It must be recalled that when polymer surfaces are brought into contact with chemically inert plasmas (like argon), activation (hydrogen abstraction) and etching of low molecular weight molecules are the main processes occurring at the polymerplasma interface, leading to the formation of radical species on the treated surface. In this case a chemical modification of the treated surface is a consequence to the exposure of the treated specimens to the atmosphere [16], mainly because reacts with oxygen and water forming peroxide and hydroperoxide radicals which then form hydroxyl, carboxyl or carbonyl groups [151]. When polymer surfaces are brought into contact with reactive plasmas (like Ar/SF6 mixtures), grafting of chemical species, simultaneous to surface activation or etching occur leading to surface functionalization. This means that the balance of these two competitive processes will determine the final surface properties. To this end, the key parameter that controls 8.2 EXPERIMENTAL, DIAGNOSTICS AND METHODS 109 this equilibrium is the concentration of SF6 in the gas mixture. In the following, both the plasma-phase characteristic and surface properties will be studied as a function of SF6 concentration. 8.2 Experimental, diagnostics and methods The plasma source used for the experiments is the DBD described in Section 3.1 which works in a roller configuration and is able to simulate continuous treatments of material surfaces and operates in a controlled atmosphere. Small sheets (10x10 cm2 ) of low density polyethylene (PE) 0.2 mm thick, are washed in acetone and attached to the grounded rotating electrode with tape. The inter-electrode gap distance is kept fixed at 2.5 mm. Gas mixture of Ar and SF6 are created with the mixing unit described in Section 3.1.2. The vacuum chamber is initially evacuated with the rotary pump P1 (Figure 3.1) down to 5 · 10−3 mbar to avoid contaminations, then calibrated fluxes from the injection system fill the chamber up to a working pressure of 900 mbar. After the working pressure is reached the dry pump P2 is used to balance the inlet fluxes and keeps the pressure stable. The total gas flux is maintained at 10 ln /min. The experiments are implemented at constant power of 155 W injected in the system. The specimens undergoing the treatment are exposed to the plasma at the tangent speed of 1 m/min. for 8 times. The experiments are performed with a corona dose (see Section 7.2) D = 354.3 kJ/m2 . An estimate of the residence time (see Section 7.2) is around 11.2 seconds which is lower with respect to low pressure plasma processes to obtain the same results. Treatment times have been chosen rather higher than those needed to obtain a good fluorination effect in order to allow a more meaningful comparison of the experiments at different SF6 concentrations. The plasma discharge is characterized by current-voltage measurements and by the acquisition of optical emission spectra (see Sections 3.1, 4.1 and 4.2). The materials surface properties are characterized at a microscopic level by measuring the morphology of the treated PE using the atomic force microscope (AFM) (see Section 4.3.2), and at a macroscopic level by measuring the dynamical contact angle with water (bi-distilled, de-ionized) and α-bromonaphthalene (from Aldrich) and by calculating the resulting surface 110 FLUORINATION OF POLYMER SURFACES energy with its polar and dispersive components (see Section 4.3.3). 8.3 Characterization of the fluorine grafting process The substitution of hydrogen atoms with fluorine can give unique properties to material surfaces. The CF groups show a strong repulsion of hydrogenbond forming molecules like water and other compounds containing hydroxyl, carboxyl or carbonyl groups. This characteristic gives a high hydrophobic property to the surface along with the resistance to organic polar compounds (some oils and solvents). Moreover, the polarity of the CF group also shows repulsion with non-polar compounds like organic molecules and some other oils. This characteristics are the basis of the high chemical stability, and unique properties of fluorocarbon-based polymers. The attainment of these properties on polymer surfaces is the aim of the present research. It is worth to mention that the fluorination process of equation (8.1) involves only few atomic layers of the substrate. This means that a little quantity of fluorine on the surface is needed to obtain the desired properties. For this reason the fluorination process is somehow preferred to deposition processes of fluorocarbon thin films which require longer times and greater quantities of reactive fluorocarbon gases. 8.3.1 Plasma-phase characterization Important information on plasma discharges can be obtained by the analysis of current-voltage (I-V) characteristic. Figure 8.1 shows the current and voltage waveforms for discharges in Ar/SF6 mixtures at various concentrations in comparison with pure Ar discharge1 . In Figure 8.1(a) the pure argon discharge I-V characteristic is shown. By observing the discharge current Idisch , calculated according to equation (5.2), it seems that a diffuse discharge mode is achieved [152]. However, a visual observation reveals the presence of really bright diffuse zones in proximity of the dielectrics connected by wide plasma channels. Anyway, the discharge is rather uniform on the electrode surfaces as also confirmed by the uniformity of surface proper1 The pure argon discharge has a slightly lower power level because the low breakdown voltage may cause the discharge to happen outside the discharge gap. This is a limitation of the present experimental setup. 8.3 CHARACTERIZATION OF THE FLUORINE GRAFTING PROCESS (a) Discharge in pure argon, 88W 111 (b) Discharge in argon and 0.5% SF6 , 155W (c) Discharge in argon and 3% SF6 , 155W (d) Discharge in argon and 13.4% SF6 , 155W Figure 8.1: Current-voltage characteristic for discharges at various concentrations of SF6 in comparison with a pure argon discharge. Lower panel: applied voltage and total current. Upper panel: applied voltage and discharge current according to equation (5.2). ties of the treated PE. Even small admixtures of SF6 (Figure 8.1(b)) changes drastically the discharge which goes into a fully developed streamer regime composed by numerous and thin plasma channels. This is in agreement with the increase of the derivative of reduced effective ionization coefficient with respect to reduced field in Ar/ SF6 mixtures [153] which brings to the reduction of streamer radius (see Section 2.3.2). In comparison with air and nitrogen atmosphere (Figure 6.1), it is evident the streamer have a shorter duration. It is interesting to observe the behaviour of root mean square (rms) quanrms increase in the same way because the I rms tities (Figure 8.2). V rms and Itot tot is dominated by the displacement component. However, by observing the rms and two components of current and considering their rms values Idisplacement rms , it is evident a decrease of the latter in comparison to the former. Idischarge 112 FLUORINATION OF POLYMER SURFACES (a) RMS Voltage (left scale) and total cur- (b) RMS displacement current (left scale) rent (right scale) as a function of SF6 con- and discharge current (right scale) as a funccentration. tion of SF6 concentration. Figure 8.2: Behaviour of rms quantities as a function of SF6 concentration at constant injected power. This means that for the same power level, at higher concentrations of SF6 a lesser part of the current is effectively due to the plasma discharge. Possibly, the SF6 tends to shorten the lifetime of the plasma channel (streamer) reducing the current flow and so the total charge transferred during the discharge rms varying from 230 nC for process which has the same behaviour as Idischarge the pure argon discharge to 56 nC for the Ar/SF6 13.4% discharge. Concomitant with the decrease of the discharge current is the decrease of the brightness of the discharge as it can be observed by the absolute intensity recorded with the spectrometer. In Figure 8.3 the emission spectra of a discharge in argon with 1% of SF6 between 600 nm and 860 nm because outside this region the emission lines are absent or too weak. The spectrum is dominated by emission lines of the argon and no contributions of the brightest emission lines of the fluorine are visible. In the inset of Figure 8.3 are compared the spectra for various SF6 concentrations normalized on the 772.5 nm line of the argon. A drastic change in the distribution of the intensities between all the argon emission lines is evident (as it is also in other parts of the spectra which are not shown). It is known the relative intensities of the lines of the argon emission can be connected through a collisional-radiative model to the electron temperature in the plasma discharge [154, 155, 156] even if a complete validation of these models for the streamer regime has to be found. It is possible to suggest that the quenching of electrons in the presence of SF6 leads to a decrease of the electron 8.3 CHARACTERIZATION OF THE FLUORINE GRAFTING PROCESS 113 Figure 8.3: Emission spectrum of a discharge in argon with 1% of SF6 . Emission is dominated by argon lines and no emission line of fluorine are present. Inset: Comparison between spectra for various SF6 concentrations normalized on the 772.4 nm emission line of the Argon. temperature with increase of SF6 concentration. A lower electron temperature means less effectiveness of the plasma creating radicals both in plasma phase and on the surface. This is possibly a key effect for the less effectiveness of the process at higher SF6 concentrations, as it will be shown in the following. 8.3.2 Material surface characterization A complete characterization of the materials surface treated with a fluorination process is more difficult than the characterization of the deposition process described in Chapter 7. This is because, even if other modifications can be introduced by concurrent etching process due to argon activity, a grafting process involves only few atomic layers on the surface and the presence of fluorine cannot be easily detected. Here the characterization has been performed at a microscopic level with the AFM to measure eventual modification of the morphology of PE surfaces, while the fluorination has 114 FLUORINATION OF POLYMER SURFACES a b c d Figure 8.4: AFM images of PE surfaces exposed to plasma treatment for various concentrations of SF6 in argon and compared with the untreated PE. (a): untreated PE. (b): pure argon plasma. (c):argon with 1% SF6 . (d): argon with 6.7% SF6 been evaluated by indirect measurements of the macroscopic properties of the surface with the techniques described in Section 4.3.3. In Figure 8.4 are compared images of the deposits at different concentration of SF6 in comparison with the untreated PE surface (a). It can be observed in Figure 8.4 that the exposure to pure argon treatments changes the surface morphology to some extent. However, the measurements of roughness does not show significant changes. The root mean square (RMS) of the heights of the surfaces has been measured for different image sizes down to 1x1 µm2 but appreciable changes has not been found. Possibly, the morphological intrinsic roughness of the PE covers other eventual effects. Moreover, the ionic activity is low and the effect of the argon treatment 8.3 CHARACTERIZATION OF THE FLUORINE GRAFTING PROCESS mN m Water α-bromonaphthalene Surface Energy 72.1 44.0 Polar 52.2 0.0 115 Dispersive 19.9 44.0 Table 8.1: Surface tension with polar and dispersive components according to Ref. [157]. is at most an activation of the surface that reacts with oxygen and water when exposed to the ambient atmosphere. Another interesting observation is the presence on the surfaces treated with Ar/SF6 plasma, of spot-like structures. These structures tend to reduce in dimension and vanish with increasing SF6 concentration. The presence of fluorine bounded to carbon on the surface has been evaluated indirectly by measuring the macroscopic effect that the presence of fluorocarbons or polar groups generates. These effect are the repellency or affinity of the surface to water and non polar liquid compounds. By measuring the advancing and receding contact angles (see Section 4.3.3) with liquids of different polar and dispersive character it is possible to evaluate the presence of fluorine or other polar compounds bound to the surface. In fact, Figure 8.4 and the measure of RMS guarantees that roughness does not change significantly between the different experiments and the change in advancing and receding angles is then only connected to the chemical heterogeneity of the surface. The two chosen liquids are water for its high polar components and α-bromonaphthalene which is completely lacking of polar groups and posses only a dispersive component. The use of pure liquids also avoids complications with adsorption kinetics which can influence the dynamic of wetting and de-wetting phenomena [59]. In Table 8.1 are reassumed the surface tensions of the used liquids according to Ref. [157]. The measurement of advancing and receding contact angles can give informations on the presence of affinity or repellency to a liquid deposited on a surface [59, 158, 159, 160]. It is possible to interpret the wetting or de-wetting process as an irreversible process in which some surface energy is dissipated as heat to the environment [161], i.e. some potential energy between atoms or molecules is dissipated as vibration (heat) as bonds are formed or snapped in the process. With this assumption a phenomeno- 116 FLUORINATION OF POLYMER SURFACES Figure 8.5: Advancing and receding contact angle of PE surfaces with water as a function of SF6 concentration in argon. Dotted lines represent in the same colours the advancing and receding angles of untreated PE. logical interpretation of the advancing and receding contact angles can be stressed. When a contact line is de-pinned from the surface (receding), stable bonds between liquid and solid must be broken, so receding angle (θr ) can be connected to the affinity of components between the solid and liquid phases (for example, a low receding angle with water means a high presence of polar groups on the surface forming hydrogen-bonds with water). On the contrary, when the contact line tries to advance, it remains pinned to the surface (advancing) because the liquid must overcome the energy barriers due to repulsion (for example, a high advancing angle θa with water means an high presence of non-polar groups on the surface which repel the highly polar water molecules). A similar kinetic interpretation is given for water on hydro-repellent surfaces in [141], here the concept is extended to a general interaction scheme between liquid and solid phase. Figure 8.5 shows the measured advancing and receding water contact angle of treated and untreated PE surfaces as a function of SF6 concentration. It can be observed that a pure argon treatment lowers both θa and θr indicating that an activation process brings, after exposure to oxygen and water of ambient air, to the grafting of polar groups on the originally nonpolar surface. For higher concentrations the effect of fluorination is evident 8.3 CHARACTERIZATION OF THE FLUORINE GRAFTING PROCESS 117 Figure 8.6: Advancing and receding contact angle of PE surfaces with α-bromonaphthalene as a function of SF6 concentration in argon. Dotted lines represent in the same colours the advancing and receding angles of untreated PE. the increase of θa which, as explained above, is sensible to repulsive interactions (in this case between water and fluorocarbon groups). It is interesting to observe that for SF6 concentrations below 1% θr remains under the untreated PE value indicating that probably the activation process is more effective than the fluorination, thus, the surface still undergoes grafting of polar groups when exposed to ambient air. Another interesting aspect is the decrease of θr for high SF6 concentrations. Possibly, this can be explained with the reduced activity of the discharge due to the SF6 (see Section 8.3.1). In Figure 8.6 are shown the measured advancing and receding α-bromonaphthalene contact angle of treated and untreated PE surfaces as a function of SF6 concentration. As for water contact angles, a pure argon treatment slightly lowers the repelling properties of the surface. The presence of fluorocarbon groups on the surface can be seen even at low concentrations of SF6 as both the advancing and receding contact angles are abruptly increased. This is because both untreated PE and α-bromonaphthalene have only a non-polar character and this affinity cause the liquid to wet very well the surface while the presence of fluorocarbon groups introduce immediately a strong repulsive effect. A far more glaring dependence on SF6 concentration 118 FLUORINATION OF POLYMER SURFACES Figure 8.7: Calculated surface energy of PE surfaces as a function of SF6 concentration in argon. Total, polar and dispersive components are plotted. Dotted lines represent in the same colours the surface energy and its components of untreated PE. of the treatment effectiveness is evident in comparison with water contact angles (Figure 8.5). In particular, the receding θr angle, which is bound to the presence of affine (non-polar) groups on the surface, most return to the value of untreated PE for high concentration. This confirms that an effective fluorination of surface is not reached for concentrations of SF6 too high. The analysis of advancing and receding contact angles for the chosen liquids shows clearly that there exists an optimal concentration of SF6 reactive gas for the maximum effectiveness of fluorination process with respect to the competitive activation process. From the contact angle measurements with two different liquids it is possible to determine the surface energy components of the treated PE substrates (see Section 4.3.3). The determination of surface energy is based on the measure of the equilibrium contact angle of the liquid with the surface. However, an equilibrium contact angle is even hard to define and a generally accepted definition still lacks [162, 163, 164]. It must be stressed that the system can be prepared with an apparent stable contact angle θs with the restriction θr < θs < θa . Usually the advancing contact angle is chosen for the determination of the surface energy [162] as it is done here. In Figure 8.4 CONCLUDING REMARKS 119 8.7 are shown the calculated surface energies of PE surfaces as a function of SF6 concentration in comparison with the untreated PE. Total, polar and dispersive components are plotted. It can be seen that the surface energy remains higher than untreated PE for concentrations below 0.5% SF6 most because a high polar character is created by the activation process. At concentrations of 1% SF6 the polar component most vanishes indicating a high hydro-repellency has been reached. After experiencing a minimum value around 3% SF6 the surface energy increases indicating that the reduction of the dispersive component is less effective at high concentrations. 8.4 Concluding remarks The fluorination process of polymer surfaces with mixtures of Ar/SF6 at atmospheric pressure has been studied. The presence of SF6 strongly modifies the discharge properties converting the diffuse discharge regime of pure argon to a streamer regime. The increase of SF6 concentration also reduces the effective current flowing through the plasma possibly quenching the electrons in th discharge. By analyzing the surface morphology it is found that modifications are introduced by the treatment on the surface on the hundred nanometer scale but these alterations do not affect substantially the roughness of the substrates. The effectiveness of the fluorination process has been evaluated through the analysis of the macroscopic surface properties with dynamical and static measurements of contact angles with water and a non-polar liquid. It is found the effective existence of two competitive processes: an activation process which brings to the grafting of polar groups from the atmosphere and a fluorination process. The former process is more important at low concentrations of SF6 and tends to disappear for higher concentrations. The fluorination process is more effective in a concentration interval also reducing it effectiveness at higher concentrations. The combination of these two effects determine the presence of an optimal SF6 concentration where the surface energy is lower. This process is then interesting for the modification of the properties of organic materials like polymers, fabrics, paper, leather and others in order to obtain resistance to water and oils. 120 FLUORINATION OF POLYMER SURFACES CHAPTER 9 Plasma Application for modification of paper surfaces 9.1 Introduction In this chapter some applications of the process studied in the previous chapters are discussed. Some results obtained with treatments of cellulosic surfaces (paper) are showed. 9.1.1 Cellulose and paper Cellulose is a natural polymer of vegetal origin which is found in wood and plants (for example cotton). Cellulose is a polysaccharide with formula (C6 H10 O5 )n (Figure 9.1 (a)) and it is usually found in plants as microfibrils 2-20 nm diameter and 100-40000 nm long (Figure 9.1 (c)). Cellulose is a linear polymer stabilized by intra- and inter-molecular hydrogen bonding 121 122 PLASMA APPLICATION FOR MODIFICATION OF PAPER SURFACES (Figure 9.1 (b)) which minimizes its flexibility. Cellulose tends to form crystal structures allowing the more hydrophobic ribbon faces to stack. Each residue is oriented 180◦ to the next with the chain synthesized two residues at a time. Although individual strand of cellulose are intrinsically no less hydrophilic, or no more hydrophobic, this tendency to form crystals utilizing extensive intra- and inter-molecular hydrogen bonding makes it completely insoluble in normal aqueous solutions. Figure 9.1: Cellulose: (C6 H10 O5 )n monomer structure (a), polymer structure and hydrogen bond linkage (b), SEM image [165] of cellulose fibres (c). Paper is the most important utilization of cellulose which is typically obtained from wood after removing lignin. The applications of paper products are numerous and do not include only the production of printing paper, but also applications in packaging, filtering, biomedical, construction and more. Paper of pure cellulose is rarely used and, besides the fibres, it may contain fillers such as chalk or china clay, which improve the characteristics of the paper for printing or writing. Also coatings may be applied to the paper web later in the manufacturing process in order to attain, for example, water or oil and grease resistance. Plasma based technologies are an interesting alternative for cellulose and paper modification to the standard chemical treatments. The main advantages of cold plasma technologies are several. Without heating which would result in damage of the soft cellulosic materials, the typical energies of active species are comparable with the values of most common bond energies of organic molecules. Consequently gas-phase and surface-phase plasma-induced 9.2 DEPOSITION OF ORGANIC SILICON COMPOUNDS FOR HYDROPHOBICITY 123 reaction mechanisms can conveniently be developed. The plasma-generated surface modification reactions involve only a thin layer (tenth of nanometers) of the substrates leaving the bulk properties mostly unchanged. Due to the ubiquitous thin-layer nature of the modifications, very small amounts of starting materials are required for the surface modification processes. This is also extremely important for the environmental issues of the modern restrictive regulations which limit the use of some reagents and require the treatment of waste-products. These characteristics allow to give the desired properties to surface layers of paper, depending on the nature of plasma gases and plasma parameters. In the following some applicative results are presented focusing on the achievement of the desired properties rather than discussing the processes in detail. In Section 9.2 the deposition of thin organosilicon films on paper surfaces is used to achieve hydrophobicity. In Section 9.3 a fluorination process is used to obtain oil-repellent paper surfaces. 9.2 Deposition of organic silicon compounds for hydrophobicity In most part of the applications of paper a certain grade of hydrophobicity is required. The hydrophilic character of cellulose is then a problem for applications like liquid recipients, printing and packaging. As a matter of fact, in environments with 50% relative humidity, cellulose adsorbs about 5% of its own weight of water [166]. Due to its fibre network structure, paper is a porous material, and can be covered by polymer films in order to make it impermeable to water. In some applications however, it is desirable to combine permeability to air and water repellency. Currently, water repellency is obtained using solvents and organic reagents which can cause environmental problems. Plasma-based technologies have all the characteristics to solve many of these problems and are, thus, an extremely interesting alternative to conventional methods. The application of plasma to modify cellulosic materials have been studied starting from different active precursors. Plasma polymerization of hydrocarbon monomers [167], fluorocarbon compounds [168, 165, 169] and organic silicon compounds [166, 170]. However, all these experiments have been conducted in low pressure plasma 124 PLASMA APPLICATION FOR MODIFICATION OF PAPER SURFACES reactors and vacuum technology is unadaptable to paper processing that is usually continuous. The use of atmospheric pressure plasmas is indeed a new challenge in the field of paper surface treatment. The aim is to explore the possibility to transfer processes already known in a low pressure environment the high pressure plasma regime and to the continuous processing mode which is typical for web materials treatment. Comparison with low pressure treatment will be shown also in the following. 9.2.1 Experimental setup and diagnostics The experimental setup is described in Section 3.1 and a discussion on the deposition process can be found in Chapter 7. Here the discussion will be mostly on the applicative results on paper surfaces. The process used to modify the properties of paper surfaces is a plasma deposition of thin organosilicon films using nitrogen gas with small admixtures of hexamethyldisiloxane (HMDSO) with the aim of achieve a high retention of the initial organic component of HMDSO and obtain hydro-repellent surfaces. Methods are also similar to those described in Section 7.2 even if the treatment times are greatly reduced. The specimens utilized cover a good range of kind of papers. Have been chosen: a collated low weight paper for food packaging (Paper Type A), a medium weight printing paper (Paper Type B), a low weight filter paper (Filter Paper) and a high weight packaging paper (Packaging paper A). Hydrophobicity is tested directly on the treated surfaces using different methods. The water static contact angle is measured with the optical goniometer (Dataphysics OCA20) described in Section 4.3.3. This instrument is also used to determine water adsorption rate of a water droplet (3 µl) by measuring its dynamical behaviour on the surface. From the geometrical properties it is possible to determine the volume lessening of the drop over time. It has been verified that the adsorption rate is constant over the measurement time and the interface base diameter remains constant without changing too much for different kinds of paper surfaces analyzed. Also the evaporation effect of water is negligible and does not influence the measurement. Thus, this method can give information on the adsorption degree of different paper surfaces. Cobb60 measurement methods is anyway used because it is a more standardized value for paper industries. It consists of exposing the paper surface to a 1 cm thick layer of water for 9.2 DEPOSITION OF ORGANIC SILICON COMPOUNDS FOR HYDROPHOBICITY 125 60 seconds and measuring by weighting, before and after the exposure, the mass of water that has been adsorbed by the paper surface. It is expressed in g/m2 . 9.2.2 Hydrophobicity of treated paper surface As already shown in Chapter 7 the concentration of hexamethyldisiloxane is a key parameter in determining the retention of the organic character of HMDSO. The non-polar character of the methyl groups (CH3 ) present in the original monomer is fundamental for the repellency of water which possesses, instead, a high polar character. In Figure 9.2(a) the behaviour (a) Static contact angle. (b) Water drop adsorption rate. Figure 9.2: Effect of variation of HMDSO concentration for various type of paper surfaces and comparison with a low pressure deposition process. The 0 concentration is the value for untreated surfaces. When the untreated value is not indicated the adsorption is mostly instantaneous and measurements cannot be performed. of water contact angle and water adsorption is showed as a function of the HMDSO concentration. It is evident that even at very low concentrations the contact angle grows to very high values and it is much independent on the concentration. This is possibly due to the roughness of the paper surface which greatly increases the water contact angle1 to a sort of saturation value and, thus, make it dependent on the surface roughness more than on its chemical heterogeneity. On the contrary, the behaviour of water adsorption rate (showed in Figure 9.2(b)) is more sensible to the variation 1 It is known that the roughness of a surface will increase the apparent contact angle if the equilibrium contact angle θe > 90◦ , while it is decreased if θe < 90◦ [59] 126 PLASMA APPLICATION FOR MODIFICATION OF PAPER SURFACES in HMDSO concentrations. It is interesting to observe that the hydrophobic properties of surfaces are dependent on the kind of paper only below a certain saturation value of the HMDSO concentration. In Figure 9.3 the (a) Static contact angle (b) Water drop adsorption Figure 9.3: Effect of tangent speed variation for various type of paper surfaces and comparison with a low pressure deposition process [137]. behaviour of water contact angle and water adsorption is showed as a function of the tangent speed of the web treated which is inversely proportional to the residence time of the specimens in the plasma discharge area. The results show that the measure of the contact angle is mostly independent on speed while the adsorption measure can detect the different behaviour for different speeds. Also for this parameter exists a saturation value which cancels the differences between different papers. It is interesting to compare the behaviour of the papers versus HMDSO concentration and speed. For example, Paper Type B properties strongly depend on HMDSO concentration while is most independent on speed (residence time) variations. The Packaging paper A shows, instead, an opposite behavior being more dependent on speed and less dependent on concentration. This suggests that exists a parameter region in which the behavior of the different substrates is dissimilar. Developing processes in this region would require the optimization of parameter for each specimen. However, exist a region of the parameters in which the behaviour of the different substrates is almost the same. The possibility to find a parameter combination which makes the treatment independent on the paper specimen type is confirmed also by the adsorption measurements with Cobb60 methods showed in Figure 9.4, 9.2 DEPOSITION OF ORGANIC SILICON COMPOUNDS FOR HYDROPHOBICITY 127 Figure 9.4: Water adsorption of different kind of paper surfaces measured with Cobb60 method. A comparison with a low pressure deposition process [137]. where the specimens have been treated ”beyond” the saturation discussed before. it is evident that conditions exist where the water-repellency effect is really similar and does not depend too much on the kind of paper surfaces. Another interesting result showed in Figure 9.4 is the comparison with a treatment with HMDSO plasma in a low pressure device [137]. It is possible to see that almost the same adsorption value are achieved even if the typical treatment times are greatly reduced at atmospheric pressure. In the applications of plasma treatment to industrial processes, of great importance is the stability of the properties of treated surfaces. To this end, have been performed aging experiments on the treated paper surfaces. Specimens have been exposed after a plasma treatment to an atmosphere at 80 ◦ C and 65% relative humidity for 7 days. The results are showed in Table 9.1. It is evident that the thin film deposited is extremely stable and has a good adhesion to paper surfaces. Multiple specimens have been tested also to verify uniformity of plasma treatments in the continuous mode. In conclusion, results for a deposition process of organosilicon thin films on paper surfaces have been shown. There exist regions of the treatment parameters where the water-repellency effect strongly depend on the paper 128 PLASMA APPLICATION FOR MODIFICATION OF PAPER SURFACES Specimen Paper Type A Packaging paper A After treatment 13.6±1 4.5±1 After aging 11.5±1 5.1±1 Table 9.1: Cobb60 measurements [g/m2 ] of paper foils after plasma treatment and after the aging process. The errors are statistical standard deviations on five different specimens. type but it is possible to find a region where this dependency vanishes. Also, the best results achieved with low pressure treatment have been well reproduced with atmospheric pressure treatments. It has also been shown that the resulting water-repellency is extremely stable and is not affected by aging. These results show that atmospheric pressure plasmas are extremely interesting for the development of new industrial applications. 9.3 Fluorination process for oil repellency In many applications of paper water repellency is not sufficient. Specifically, in food packaging applications, also a great resistance to oil and grease is required. Usually these properties are achieved using expensive fluorine based coatings and standard wet chemistry processes. Costs in terms of reagents and dangerous waste by-products disposal are really high and even more restrictive directives of the governments make the use of these traditional methods everyday more difficult. The search for different methods to achieve the same property becomes then a necessity. Plasma treatments have potentially the lowest environmental impact due to the little quantities of reagents needed and the nanometer scale character of the processes. Obtain oil-repellency is not a simple task. While water, which possesses a high surface tension due to strong polar component, can be easily repelled by surfaces with non polar character, oils have usually a low surface tension and cannot be easily repelled. Moreover, oils can be both polar or nonpolar and a choice at a microscopic level for the surface modification is not enough. It is known that fluorine containing compound are able to give oil and grease repellency to surfaces, together with a water repellency due to the repulsion of fluorocarbon to form hydrogen bonds with water. Experiments for the modification of paper surfaces with compounds containing fluorine 129 9.3 FLUORINATION PROCESS FOR OIL REPELLENCY have been performed using CF4 [171, 168, 172], perfluoro-methylcyclohexane [165] or fluorotrimethylsilane [169]. However, as for water repellency (see Section 9.2), all these experiments have been performed with low pressure plasma devices which are not suited for applications in paper industry. 9.3.1 Experimental setup and diagnostics The experimental setup is the same described in preceding Section 9.2. The methods are similar to those described in Chapter 8 and the treatments have been optimized for different kind of papers. The fluorination is obtained in a mixture of argon and sulfur hexafluoride (SF6 ). Here are shown the results regarding only a single kind of paper. and compared with a different kind of low pressure plasma process process [172]. Oil repellency is evaluated measuring the surface energy of paper surfaces. This is achieved with the method described in Section subsec:advrec. A typical method to estimate the oil repellency is the so called Kit Test [173]. It consist of exposing the surface to a drop of a mixture of oils an solvents for determined amount of time. There exist two kinds of Kit Test: non-polar Kit Test which is made up of mixtures of castor oil, toluene and heptane which are all non-polar compounds. Polar Kit Test is made up of mixture of water and isopropyl alcohol which are polar compounds. For each mixture is assigned a number and increasing numbers indicate lower surface tensions. The test is passed if the mixture is not adsorbed by the analyzed surface. 9.3.2 Oil repellency of paper surfaces Fluorination process is a grafting process (see Section 2.4.2) which is typical for plasma discharges in mixtures containing fluorinated gases and involves the substitution of an hydrogen atom, bound to carbon, with a fluorine atom following the reaction scheme: hν,e − F• −CH −−−−→ −C• −−→ −CF, Ion,F• (9.1) where a fluorine radical adsorbed to the surface reacts with a carbon radical activated by the plasma on the surfaces. X-ray photoelectron spectroscopy (XPS) can be used to determine the degree of fluorination achieved in the process. The F/C ratio increases from 0 to 0.45 while the O/C ratio de- 130 PLASMA APPLICATION FOR MODIFICATION OF PAPER SURFACES (a) Untreated paper surface (b) Fluorinated paper surface (c) Comparison of the area relative to carbon carbon bonds for the two energy spectra above (d) Deconvolution of area for treated surface. Figure 9.5: XPS analysis of treated and untreated paper surfaces. The grafting of fluorine atoms to carbon is evidenced. creases from 0.53 to 0.46 indicating that fluorine not only replace hydrogen atoms but also oxygen containing groups which are possibly removed from 9.3 FLUORINATION PROCESS FOR OIL REPELLENCY 131 the surface. In Figure 9.6 is shown the Kit Test values for a paper surface Figure 9.6: Kit Test value as a function of time for an atmospheric pressure fluorination plasma process based on SF6 containing gas mixture and a low pressure plasma process with CF4 gas. as a function of time. Two processes are compared: an atmospheric pressure fluorination plasma process based on SF6 containing gas mixture and a low pressure plasma process with CF4 gas. The main difference in the two processes is that the CF4 gas can activate both the fluorination mechanism of equation (9.1) and a deposition process of fluorocarbon compounds. By adjusting the plasma parameters the process will create a thin film of teflon-like polymer deposited on the paper surface. It is evident that, if the fluorine is replaced directly on the cellulose fibres, a fast aging effect cancels completely the oleo-phobic properties of the surface. However, if a deposition process is added the aging effect is not present. Possibly the aging effect is due to the presence of extensive intra- and intermolecular hydrogen bonding which form the cellulose structure and fibres. When fluorine atoms replace the hydrogen atoms the possibility to realize hydrogen bonds no longer exist and the cellulose structure near the surface becomes extremely unstable. This new mobility allows the fibres to move more freely to find the configuration which minimize surface energy. Possibly, thus, because of the repulsion of fluorocarbon to form hydrogen bonds with water, the fluorine containing fibres rotate to the inside of the surface leaving conventional 132 PLASMA APPLICATION FOR MODIFICATION OF PAPER SURFACES cellulose to be exposed to the atmospheric humidity. As shown in Figure 9.6 Figure 9.7: Kit Test value as a function of time for an atmospheric pressure fluorination plasma process based on SF6 containing gas mixture and the same process used on papers previously treated with a deposition process of organosilicon films. for the low pressure CF4 process, the presence of a deposition mechanism can stabilize the treatment. Thus, a solution to the problem can be found in the utilization of a deposition process which is able to create a stable, well adherent thin film on paper surfaces. To this end, the fluorination process has been used on paper surfaces previously treated with the deposition process described in preceding Section 9.2, which has shown (see Table 9.1) to be extremely stable to aging effects. The result are shown in Figure 9.7. The deposition of organosilicon films has two advantages: it is extremely stable, blocking the cellulose fibres from turning and has a high number of CH groups exposed to the atmosphere (because of the methyl groups retention) being a good basis for the grafting process of fluorine atoms. It is evident that the double treatment is stable and removes the aging effect. In conclusion, it has been shown that an atmospheric pressure plasma can produce oil-repellent paper surfaces using a fluorination process based on SF6 containing gas mixtures. A specific problem with the molecular structure of cellulosic materials generate a fast aging effect which removes completely the attained properties. It has been shown that by combining a 9.3 FLUORINATION PROCESS FOR OIL REPELLENCY 133 deposition process of thin organosilicon films with the fluorination process can remove the aging effect giving stable properties over time. These results show that atmospheric pressure plasmas are extremely interesting for the development of new industrial applications for the substitution of traditional coating processes. 134 PLASMA APPLICATION FOR MODIFICATION OF PAPER SURFACES CHAPTER 10 Conclusions The great interest in cold atmospheric pressure plasmas, in dielectric barrier discharges and their potential for the development of plasma processes is one of the motivations of this thesis. DBDs are rapidly growing as one of the the best choice for atmospheric plasma applications, but the field is still new and unexplored in many aspects both regarding the discharge processes and their applications, for example, in material surface modifications. Another motivation is the lack of a clear understanding of the discharge regimes that can develop in DBDs. The streamer regime, which was the first observed (and utilized) back in 1857 Siemens’s DBD ozonizer, still presents some unclear phenomena particularly when dealing with the interactions between the micro-discharges. In this thesis (Chapter 5) the streamer regime of a DBD in air has been characterized by means of the statistical analysis of the discharge current. The typical time scales of micro-discharges (tens of nanoseconds in this experimental setup) made compulsory the development of suitable diagnostics based on home-made current probes (Rogowski coils) 135 136 CONCLUSIONS able to catch the fast current pulses due to streamers (Section 4.2.1). It has been found that the interaction between micro-discharges determines the presence of two different discharge regimes, depending on the applied voltage, which has been observed in several quantities both regarding the statistical properties of the current intensity and its temporal behavior. One of the great issues of DBDs is the so called memory effect which is due to the presence of the dielectric layers and tends to promote the formation of a streamer in the same spot of the preceding half-cycle. This effect could bring to pattern formation which could affect performance of DBD treatment in application where spatial uniformity is required. To understand if this ”memory” is present in the apparently random streamer regime, the presence of correlations between discharge processes and within the single discharge process has been studied. With the help of a surrogate model it has been shown that the observed residual cross-correlations between half-cycles are only an effect of the intrinsic non-stationarity of the signal, indicating that no memory persistence is present in the temporal structure of the discharge. However, by analyzing the current signal inside the half-cycle, it is found that on time scales of the order of hundreds of nanoseconds (i.e., within a single current burst, in which the streamers develop sufficiently close in time), strong correlations exist which also reveal a peculiar ordered temporal structure of the discharge current signal. This temporal structure has been studied using newly defined correlation functions, which reveal the existence of a characteristic frequency in the occurrence of streamers. This frequency suggests the existence of an excitation mechanism between the streamers which connects their development in the gap. These findings reveal very interesting aspect of the cooperative behaviour of the streamer regime and suggest the possibility to carry on the research maybe using also fast optical diagnostics in order to obtain a clearer picture of the spatiotemporal behaviour of streamers. Atmospheric pressure plasma processing is a leading thematic in development of plasma applications. In the last decades great efforts have been done by many research groups in this field, however still lacks a clear knowledge of the plasma discharge properties and of their interaction with surfaces. In this work a new DBD device has been developed (Chapter 3) which is able to operate continuous treatment of web materials in a wide range of pressures 137 and compositions of the gas mixture. The plasma discharge has been initially characterized in nitrogen (Chapter 6) which is often chosen as a basis of discharges for the development of plasma processes for applications. The capabilities of the new DBD device have been explored varying the control parameters and finding that it can work in a wide range of conditions. Another motivation of this work is the study and development of previously known low pressure processes at atmospheric pressure. The deposition process of organosilicon thin films with plasma of nitrogen with small admixtures of HMDSO vapour has been characterized (Chapter 7). It has been found that concentration is a key parameter in controlling the organic/inorganic character of the resulting deposit. Analyzing the behavior of several quantities as a function of the HMDSO concentration we have found the deposition mode changes with increasing concentration. It has been found that the retention of initial monomer methyl groups saturate with concentration and so does the hydrophobic character of the resulting surface. It has also been found that the DBD device is able to create uniform and smooth deposits even if working in a full developed streamer regime. This kind of process is able to create highly hydrophobic surfaces with lower treatment times in comparison with similar low pressure processes. The fluorination process of polymer surfaces with mixtures of Ar/SF6 at atmospheric pressure has been studied (Chapter 8) and it has been found that the presence of SF6 strongly modifies the discharge properties converting the diffuse discharge regime of pure argon to a streamer regime. It is found the effective existence of two competitive processes: an etching/activation process which brings, after the treatment, to the grafting of polar groups from the ambient air, and a fluorination process in which grafting of fluorine atoms to carbon in the backbone of polymers is realized. The balance of these two processes is controlled by the SF6 concentration. By analyzing the surface morphology it is found that modifications are introduced by the treatment on the surface on the hundred nanometer scale but these alterations do not affect substantially the roughness of the substrates. The effectiveness of the fluorination process has been evaluated through the analysis of the macroscopic surface properties with dynamical measurements of contact angles with water and a non-polar liquid. The result is that this process is effective in the modification of the properties of organic materials 138 CONCLUSIONS making the surface highly hydrophobic and oleophobic. These properties are needed in order to obtain resistance to water and oils. Finally, the studied atmospheric plasma processes have been employed for the modification of cellulosic materials (paper). Some aspects of this research are discussed in Chapter 9. It is found the deposition process of thin organosilicon films is able to produce effective hydro-repellent paper surface and that exists a deposition condition in which very different substrate kinds assume the same surface properties. It has also been found that the resulting water-repellency is extremely stable and is not affected by aging. These results show that atmospheric pressure plasmas are extremely interesting for the development of new industrial applications. It has been found that an atmospheric pressure plasma can produce oil-repellent paper surfaces using a fluorination process based on SF6 containing gas mixtures. A disturbance of the fluorination process with the molecular structure of cellulosic materials generate a fast aging effect which removes completely the attained properties. It has been found that by combining a deposition process of thin organosilicon films with the fluorination process it is possible to remove the aging effect giving stable properties over time. These results show that atmospheric pressure plasmas are extremely interesting for the development of new industrial applications for the substitution of traditional oil-repellent coating processes. In conclusion, it has been shown that suitable diagnostics and experimental and statistical characterization can lead to unveil part of the puzzling aspects concerning dielectric barrier discharges. Also it has been shown, using a newly developed DBD device, that a better understanding of discharge conditions allows to investigate suitable plasma processes to give new surface properties to natural and artificial polymers. 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