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st 21 International Symposium on Plasma Chemistry (ISPC 21) Sunday 4 August – Friday 9 August 2013 Cairns Convention Centre, Queensland, Australia Three-Dimensional Integration of Micro Solution Plasmas and Its Application to Material Processing in Large Volume Liquid T. Shirafuji1 1 Dept. Physical Electronics and Informatics, Osaka City University, Osaka, 558-8585 Japan Abstract: Three-dimensionally integrated micro solution plasma (3D IMSP), in which microplasmas are generated in Ar gas bubbles in aqueous solution being held in a porous dielectric material, has been proposed. Electric field in the bubbles is calculated with numerical simulation and is confirmed to be high enough for igniting electrical discharge in the bubbles even if the bubbles are surrounded with electrically conductive liquid. A proto type reactor for generating the 3D IMSPs has capability of generating large volume plasma in liquid. The 3D IMSP reactor is applied to material processing, in which the 3D IMSP is found to be able to synthesize gold nanoparticles and to decompose organic substance in water. Keywords: Microplasma, Solution Plasma, Liquid, 3D Integration 2. Numerical Study of 3D IMSP 2. 1 Model Description Figure 1 shows cross-sectional view of the model geometry of an ideal 3D IMSP reactor. The reactor is a glass tube containing porous dielectric material. The electrode is located at the central axis of the porous dielectric material and outer surface of the glass tube. Pores in the dielectric material employed in the 3D IMSP reactor should not be isolated but connected like the structure of a trabecular-bone. This is a very important feature that the porous dielectric material should have, because the gas-liquid mixed medium can flow in the reactor and plasma in the gas bubble can interact with the liquid. In this geometry, however, we must pay attention to the fact that all the bubbles are surrounded by the liquid medium which may be electrically conductive. If the electriModel geometry Grounded 2 mm Porous dielectric filled with gas/liquid mixed medium 2 mm Outer electrode Dielectric tube Inner electrode Tube wall εr = 4 - 10000 Dielectric frame εr = 4 φ 0.8 mm Gas bubble εr = 1 1 mm x 1 mm 10 mm 1. Introduction Plasmas in and in contact with liquid have attracted much attention because of their possible application fields such as nano-materials synthesis, surface modification, water treatment, sterilization, recycling of rare materials, and decomposition of toxic compounds [1]. In our previous work, we have successfully obtained glow discharges in aqueous solution, which have been named as "solution plasmas", and applied this technique to nanoparticles synthesis and modification of the surface of nano-materials [2]. Our solution plasma, however, is ignited in a small volume between two stylus electrodes, actual treatment area or volume should be enlarged for practical industrial applications. In the case of gas phase processes, large area processing is realized by producing large area plasmas. In the case of solution plasma processing, however, large volume plasma in liquid is meaningless, because the most important region is gas-liquid or plasma-liquid interface. Thus, preparation of large number of tiny plasmas (microplasmas [3]), which may be called as "integrated micro solution plasmas", is rather important in the case of solution plasma processing, because we can expect much larger area of the interface between gas (or plasma) and a liquid medium. The preliminary experiments for the integration of the micro solution plasmas were performed in two dimensions [4]. Feasibility study concerning possibility of three-dimensionally integrated micro solution plasmas (3D IMSP) was already performed and reported [5]. An actual 3D IMSP reactor was also constructed and its material processing capability was also confirmed through the experiments of synthesizing gold nanoparticles [6] and decomposing methylene blue molecules [7]. In this paper, I briefly describe how the 3D IMSP reactor works, and also describe how to enhance its performance together with its material processing capability. Liquid εr = 80 σ = 1-1000 uS/cm Probe point X-boundary = Periodic Vapp. Fig. 1 The model geometry for numerical calculation of electric field in the micro bubbles in the 3D IMSP reactor. st 21 International Symposium on Plasma Chemistry (ISPC 21) Sunday 4 August – Friday 9 August 2013 Cairns Convention Centre, Queensland, Australia Electric field (Y-dir) in a bubble 1.5 Liquid 1000 uS/cm Tube eps. 10000 6 Electric field (x10 V/m) 2.0 1.0 1000 100 0.5 10 0.0 -11 10 -9 10 -7 10 -5 10 -3 10 Time (s) Fig. 3 Electric field in the bubble in the reactor being composed of a dielectric tube with various dielectric constants. Fig. 2 (a) Applied voltage between the electrodes, and (b) electric field at the center of the bubble indicated by “Probe point” in Fig. 1. cal conductivity is infinite, we cannot expect any electric field in the bubbles because the surface of the bubbles works as a short circuit to extinguish potential difference on the bubble surface. However, the electrical conductivity of the liquid is finite. For example, typical electrical conductivity of tap water is 100 - 200 uS/cm. In addition to this, the resistance of liquid surrounding the bubble becomes higher than that of bulk liquid because of narrow liquid channel in the porous dielectric material. This means that we can expect potential drop on the surface of a bubble even though the bubble is surrounded by electrically conductive liquid. In order to investigate how much electric field can be obtained and to verify whether we can ignite electrical discharge with the electric field obtained, we have performed numerical calculation of electric field in the model geometry shown in Fig. 1. The dc 5 kV is applied between the central axis and outer electrode. The waveform of the applied voltage is shown in Fig. 2 (a). Electrical conductivity of the liquid has been varied in the range of 1 - 1000 uS/cm. Relative dielectric constant of the liquid is 80, which corresponds to that of water. The framework of the porous dielectric material and the tube is insulator with relative dielectric constant of 4, which corresponds to that of SiO2. The gas bubbles are assumed to be insulator with relative dielectric constant of unity. The shape and size of these compo- nents are shown in Fig. 1. For calculating electric field in such system, we cannot apply simple Poisson’s equation, because the medium is composed of mixture of electrically conductive and dielectric materials. Thus, we have employed an electro-quasi static method [8] for solving Maxwell’s equations in our system by using the commercially available finite-element-method solver, COMSOL multiphysics. Two dimensional spatial profile of the electric field is obtained as a function of time by the numerical calculation, which has been reported in our previous report [5]. In this work, we focus on the dependence on electrical conductivity of liquid. 2.1 Dependence on electrical conductivity of liquid Figure 2(b) shows electric field in the bubble indicated by “Probe point” in Fig. 1. The electric field in the bubble reaches its maximum value around 10-7 - 10-6 s after applying the ramp voltage. Since the dielectric framework and gas bubbles are arranged regularly, all the bubbles have the same electric field. This feature can be seen in our previous report [5]. The formation of high electric field in the bubble is a transient phenomenon as can be understood from Fig. 2(b). After 1 us or longer time, most of the voltage is applied between outer and inner surface of the glass tube, because the equivalent circuit of the 3D IMSP is a series circuit composed of capacitance (glass tube wall) and lossy dielectric (porous dielectric filled with gas and conductive liquid). Because of that, we have employed bipolar pulse voltage source for the purpose of continuous treatment of liquid medium as described in the experimental section of this paper. The maximum value of the electric field depends on electrical conductivity of liquid and varies from 5×105 down to 0.7×105 V/m. The threshold electric field for igniting atmospheric pressure discharge is 3×105 V/m [8]. Thus, according to the Fig. 2(b), we can expect generation st 21 International Symposium on Plasma Chemistry (ISPC 21) Sunday 4 August – Friday 9 August 2013 Cairns Convention Centre, Queensland, Australia 30 mm 3 cm 1 mm Fig. 5 Optical emission from the 3D IMSP. Fig.4 Overview of the 3D IMSP reactor. of plasma if the electrical conductivity of the liquid is less than 100 uS/cm. This will be confirmed in the experimental section. 2.2 Enhancement of electric field in the bubble In our system, applied voltage is divided into two parts. One is the wall of glass tube and the rest is the porous dielectric filled with gas-liquid mixed medium. The voltage on the glass tube wall can be reduced by employing the dielectric tube with thinner wall and/or higher dielectric constant. In this work, we have examined effect of utilizing the dielectric tube with higher dielectric constant. Figure 3 shows electric field in the bubble in the system being composed of the dielectric tube with dielectric constant of 10, 100, 1000 and 10000. These values are not absurd since the PZT has dielectric constant of 1700. Electrical conductivity of the liquid is 1000, for which electric field in the bubble is as low as 7×104 V/m in the case of glass tube. As can be understood from Fig. 3, if we employ the dielectric tube with dielectric constant of 1000, electric field in the bubble can be enhanced up to approximately 1×106 V/m even if the electrical conductivity of liquid is as high as 1000 uS/cm. Since the electrical conductivity of 1000 is an extreme case, the reactor with high-dielectric-constant tube is considered to have potential to handle wide range of liquid media. 3. Experimental According to the results of numerical calculation, we have constructed actual reactor. At this moment, we do not have a dielectric tube with high dielectric constant. Thus, we have employed glass tube for construction of the 3D IMSP reactor. Figure 4 shows the overview of the 3D IMSP reactor. The inset photograph in Fig. 4 is the magnified view of the porous dielectric material. The porous dielectric material is a rod-shaped silica pumice (20 mm diam.), which is inserted in a glass tube. Averaged size of the pores is approximately 0.5 mm in diameter. The pores are not isolated but connected. A metal electrode (SUS 304, outer diam. 1/4 inch), where high voltage pulses are applied, is inserted in the central axis of the pumice rod. It has five holes (diam. 5 mm) to feed gas into the porous dielectric material filled with liquid to be treated. The grounded electrode (metal mesh, SUS 304, wire diam. 0.29 mm, aperture 0.98 mm) is attached on the outer surface of the glass tube. Since we cannot complete contact between the metal mesh and glass tube, we may have atmospheric pressure discharge on the glass tube surrounded with the metal mesh. The voltage is supplied from a bipolar high voltage power source (Haiden, SBP-5K-HF2). The voltage waveform is square-wave pulses with amplitude of 5 kV, frequency of 20 kHz and pulse width of 2.3 us. The gas supplied through the central electrode is Ar, and its flow rate is 1.1 L/min. Liquid temperature is controlled to be 30oC by circulating through the cooling device. 3.1 Confirmation of plasma generation Figure 5 shows the photograph of the reactor during operation using water with electrical conductivity of 200 uS/cm. According to the prediction by numerical calculation, electric field formed in gas bubbles are not enough for igniting electrical discharge if the electrical conductivity of liquid is 200 uS/cm, which can be understood from Fig. 2(b). Underestimation of the threshold electrical conductivity for igniting electrical discharge may be due to the fact that actual size and position of pore sizes and width of liquid flow channel are not regulated in our present 3D IMSP reactor. 3.2 Gold nanoparticle synthesis In order to demonstrate material synthesis capability of the 3D IMSP system, we have performed gold nanoparticle synthesis using HAuCl4 aqueous solution. Figure 6 (a) shows photographs of the HAuCl4 aqueous solution after the 3D IMSP treatment for 0 to 60 min. We can confirm that color of the aqueous solution changes from yellowish transparent toward wine-red color, which is result of plasmon resonance absorption by gold nanoparticles. The st 21 International Symposium on Plasma Chemistry (ISPC 21) Sunday 4 August – Friday 9 August 2013 Cairns Convention Centre, Queensland, Australia (a) 0 AuHCl4 0.15 mM 10 20 30 40 50 60 Time (min) (b) (c) Fig. 6 (a) Aqueous solution of HAuCl4 after 3D IMSP treatment for 0 to 60 min, (b) STEM image of the synthesized particles, and (c) EDS image of the STEM-observation area. 0 10 20 30 40 50 60 min 5 cm Fig. 7 Photographs indicating decolorization due to decomposition of methylene blue molecules in methylene blue aqueous solution treated with the 3D IMSP. size of the particles is 6 ± 4 nm as seen in the STEM image shown in Fig. 6 (b). The EDS mapping image shown in Fig. 6 (c) indicates that composition of the particle is surely gold. Details on the experimental procedure and particle analysis are reported in elsewhere [6]. 3.3 Methylene blue decomposition Figure 7 shows photographs indicating decolorization of methylene blue (MB) aqueous solution due to decomposition of MB molecules in the aqueous solution through the treatment by the 3D IMSP. The decolorization of the MB is known to occur also through a simple reduction reaction to form leuco MB [9], which is known to be easily oxidized through oxidation procedure such as agitation in the air. However, the transparent aqueous solution after the 3D IMSP is not colored again, which means that the MB molecules have been decomposed by the 3D IMSP treatment. Through the comparison of decomposition efficiency of the 3D IMSP to that of conventional method, in which the plasma is ignited between two stylus electrodes in aqueous solution, we have found that the 3D IMSP has 16 times higher decomposition efficiency than that of conventional method. Details are reported in elsewhere [7]. 4. Conclusion 3D IMSP, in which a large number of micro solution plasmas are generated in a porous dielectric material filled with gas-liquid mixed medium, has been proposed and constructed for treatment of large volume liquid medium. Through numerical simulations and experiments using the proto-type reactor, we have confirmed that the 3D IMSP can be generated. The numerical simulation has also predicted that the performance can be enhanced by using a high-dielectric-constant material as the wall of reactor tube. Capability of material synthesis has been demonstrated through gold nanoparticle synthesis using HAuCl4 aqueous solution. Decomposition of an organic substance in water has also been demonstrated through MB decolorization experiments. Its energy efficiency has been found to be 16 times higher than conventional method in which the plasma is ignited between two stylus electrodes in aqueous solution. 5. Acknowledgments This work has been partly supported by the Grant-in-Aid for Scientific Research on Priority Area "Frontier science of interactions between plasmas and nano-interfaces" by MEXT, Japan. 6. References [1] Eds. V. I. Parvulescu, M. Magureanu and P. Lukes, Plasma Chemistry and Catalysis in Gases and Liquids (Wiley-VCH, 2012). [2] O. Takai, Pure Appl. Chem. 80 (2008) 2003. [3] K. Tachibana, IEEJ Trans. Elec. Electron. Eng. 1 (2006) 145. [4] T. Shirafuji, J. Hieda, N. Saito and O. Takai, Proc. 20th Int. Symp. Plasma Chem., 20 (2011) 640. [5] T. Shirafuji and A. Nakamura, Trans. Mater. Res. Soc. Jpn. (2013) in press. [6] T. Shirafuji, J. Ueda, A. Nakamura, S.-P. Cho, N. Saito and O. Takai, Jpn. J. Appl. Phys. submitted. [7] T. Shirafuji and Y. Himeno, Jpn. J. Appl. Phys. submitted. [8] A. Fridman, A. Chirokov and A. Gustol, J. Phys. D: Appl. Phys., 38, R1 (2005). [9] O. Impert, A. Katafias, P. Kita, A. Mills, A. Pietkiewicz-Graczyk and G. Wrezeszcz, Dalton. Trans. (2003) 348.