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sensors Article Sensing Properties of a Novel Temperature Sensor Based on Field Assisted Thermal Emission Zhigang Pan 1 , Yong Zhang 1, *, Zhenzhen Cheng 1 , Jiaming Tong 1 , Qiyu Chen 1 , Jianpeng Zhang 1 , Jiaxiang Zhang 1 , Xin Li 2 and Yunjia Li 1 1 2 * State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an 710-049, China; [email protected] (Z.P.); [email protected] (Z.C.); [email protected] (J.T.); [email protected] (Q.C.); [email protected] (J.P.Z.); [email protected] (J.X.Z.); [email protected] (Y.L.) Vacuum Micro-Electronic & Micro-Electronic Mechanical Institute, School of Electronics and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, China; [email protected] Correspondence: [email protected]; Tel.: +86-29-8266-8793 Academic Editors: Christos Riziotis, Evangelos Hristoforou and Dimitrios Vlachos Received: 20 January 2017; Accepted: 22 February 2017; Published: 27 February 2017 Abstract: The existing temperature sensors using carbon nanotubes (CNTs) are limited by low sensitivity, complicated processes, or dependence on microscopy to observe the experimental results. Here we report the fabrication and successful testing of an ionization temperature sensor featuring non-self-sustaining discharge. The sharp tips of nanotubes generate high electric fields at relatively low voltages, lowering the work function of electrons emitted by CNTs, and thereby enabling the safe operation of such sensors. Due to the temperature effect on the electron emission of CNTs, the collecting current exhibited an exponential increase with temperature rising from 20 ◦ C to 100 ◦ C. Additionally, a higher temperature coefficient of 0.04 K−1 was obtained at 24 V voltage applied on the extracting electrode, higher than the values of other reported CNT-based temperature sensors. The triple-electrode ionization temperature sensor is easy to fabricate and converts the temperature change directly into an electrical signal. It shows a high temperature coefficient and good application potential. Keywords: temperature sensor; carbon nanotubes; ionization; emission 1. Introduction Temperature sensors are among the most-widely used sensors in consumer and industrial temperature measurement. The development goal of a temperature sensor usually includes high sensitivity, small footprint, and low power consumption. The potential of a carbon nanotubes (CNTs) based temperature sensor offers great opportunities towards extreme miniaturization and low power consumption [1–5], thanks to the unique nanoscale structure and electrical property [6–11]. Recent studies showed that CNTs could be used to construct temperature sensors. For example, a thermometer can be realized by measuring the thermal expansion of gallium inside a CNT [1], since the height of a continuous unidimensional column of liquid gallium inside a carbon nanotube varies linearly and reproducibly in the temperature range 50–500 ◦ C. Nevertheless, this methodology requires microscopic measurement of the height of the gallium. Alternatively, the CNT-based temperature sensors [2–5] can also be implemented by measuring the conductivity variation of CNT induced by the thermal interaction. However, the application of this type of sensor is potentially limited by its complex fabrication process and low sensitivity. A novel ionization temperature sensor based on CNTs electrodes [12] was capable of overcoming the limitations of the above two types of temperature sensors, but it was only used to detect temperature in N2 at 100 V extracting voltage; meanwhile, Sensors 2017, 17, 473; doi:10.3390/s17030473 www.mdpi.com/journal/sensors Sensors 2017, 17, 473 2 of 7 Sensors 2017, 17, 473 2 of 7 its carbon nanotubes have large diameter and small spaces between nanotubes (see supplementary material Figure which resultsmaterial in smallFigure field enhancement factor, current, andfactor, low nanotubes (seeS1), supplementary S1), which results insmall smalloutput field enhancement ◦ sensitivity—especially below (see supplementary material andsupplementary S2, Figure S2). material In this small output current, and 70 lowCsensitivity—especially below Tables 70 °C S1 (see paper, a triple-electrode CNT-based ionization temperature sensor is fabricated. Differently, the CNTs Tables S1 and S2, Figure S2). In this paper, a triple-electrode CNT-based ionization temperature array is grown by a low-temperature thermal chemical vapor deposition (TCVD) process, enabling sensor is fabricated. Differently, the CNTs array is grown by a low-temperature thermal chemical small diameters and(TCVD) larger spaces As a result, the electric around CNT vapor deposition process,between enablingnanotubes. small diameters and larger spaces field between nanotubes. tips and the emission current density of the tips can be enhanced [13]. In addition, the decrease As a result, the electric field around CNT tips and the emission current density of the tips canofbe the electrode[13]. separation can increase the electric strength of the sensor. The fabricated CNT enhanced In addition, the decrease of the field electrode separation can increase the electric field temperature is capable detectingCNT temperature in ambient at low working voltage. strength ofsensor the sensor. The of fabricated temperature sensoratmosphere is capable of detecting temperature The sensing mechanism of the voltage. CNT sensor explained insensing terms ofmechanism the emission in temperature ambient atmosphere at low working The istemperature of of theCNT CNT and the discharge properties sensor is explained in termsofofair. the emission of CNT and the discharge properties of air. 2. 2. Materials and Methods Materials and Methods Figure 1 shows the schematic illustration ofof the presented CNT-based temperature sensor. The Figure 1 shows the schematic illustration the presented CNT-based temperature sensor. The sensor is comprised of three electrode plates; i.e., a CNT-based cathode, an extracting electrode, and a sensor is comprised of three electrode plates; i.e., a CNT-based cathode, an extracting electrode, and collecting electrode. The two ventilating holes in the cathode make the gas diffuse more easily [11]. a collecting electrode. The two ventilating holes in the cathode make the gas diffuse more easily [11]. AA hole in in thethe extracting electrode is is used to to extract discharge particles. rectangular ditch inin the hole extracting electrode used extract discharge particles.A A rectangular ditch the collecting electrode could reduce reflection-induced loss of positive ions and collect more. In operation, collecting electrode could reduce reflection-induced loss of positive ions and collect more. In theoperation, extractingthe voltage Ue is set higherUthan the collecting voltage Uc , where the potential of the extracting voltage e is set higher than the collecting voltage Uc, where thecathode potential is of 0 V. In this configuration, two electric fields E and E are generated with reversed field direction. 1 2 the cathode is 0 V. In this configuration, two electric fields E1 and E2 are generated with reversed The currents Ic and Ie were recorded high-precision multimeters (NI PXI-4071, National field direction. The currents Ic andbyIe two were recorded by digital two high-precision digital multimeters (NI Instruments Corporation, Austin, TX, USA). One minute after the voltages are applied to the electrodes PXI-4071, National Instruments Corporation, Texas, United States). One minute after the voltages and discharge ofelectrodes air is stable, tenthe current values ten ten current values of Ieofare recorded and arethe applied to the and discharge of of airIcisand stable, current values Ic and ten current averaged as currents I and I , respectively. c e averaged as currents Ic and Ie, respectively. values of Ie are recorded and The presented CNT temperature sensor waswas fabricated according to the steps. Three Si The presented CNT temperature sensor fabricated according to following the following steps. Three substrates were firstly patterned by photolithograph to form the pattern of the cavity with different Si substrates were firstly patterned by photolithograph to form the pattern of the cavity with sizes. The patterned then etched anetched inductively plasma (ICP) plasma etcher. The different sizes. Thesubstrates patternedwere substrates were by then by ancoupled inductively coupled (ICP) etched structure included the cathode with two circular holes of 4 mm in diameter, the extracting etcher. The etched structure included the cathode with two circular holes of 4 mm in diameter, the electrode with a circular hole of 6 mmhole in diameter, the collecting electrode withelectrode a rectangular extracting electrode with a circular of 6 mm and in diameter, and the collecting with a ditch structure with the size of 8 mm × 6 mm × 200 µm (length × width × depth) (Figure 1a–c). rectangular ditch structure with the size of 8 mm 6 mm 200 µm (length width depth)(Figure A 1a–c). metallization layer of Ti/Ni/Au was thenwas sputtered on both sides of the extracting electrode and the A metallization layer of Ti/Ni/Au then sputtered on both sides of the extracting electrode inner walls of the cathode and collecting electrode. Vertically aligned multi-walled carbon nanotubes and the inner walls of the cathode and collecting electrode. Vertically aligned multi-walled carbon (MWCNTs) subsequently grown by thermal vapor deposition on (TCVD) one sideon nanotubeswere (MWCNTs) were subsequently grownchemical by thermal chemical vapor (TCVD) deposition ◦ C [14], with around 20 nm in diameter, 5 µm in length, and separated with a ofone the side cathode at 700 of the cathode at 700 °C [14], with around 20 nm in diameter, 5 µm in length, and separated distance 200 nmofin200 between 1d). The three assembled 50 µm-thick with a of distance nm in(Figure between (Figure 1d). electrodes The three were electrodes were with assembled with 50 polyester films. µm-thick polyester films. Figure 1. Cont. Sensors Sensors2017, 2017,17, 17,473 473 33 of of 77 Figure Schemeofofthe thetriple-electrode triple-electrode carbon nanotube (CNT)-based sensor. (a) cathode the cathode Figure 1. 1. Scheme carbon nanotube (CNT)-based sensor. (a) the after after substrate patterning, Ti/Ni/Au sputtering, and CNT growing; (b) the extracting electrode substrate patterning, Ti/Ni/Au sputtering, and CNT growing; (b) the extracting electrode and (c)and the (c) the collecting electrode substrate patterning andTi/Ni/Au Ti/Ni/Au sputtering;(d) (d) Scanning Scanning electron collecting electrode after after substrate patterning and sputtering; electron microscope microscopeimage imageof ofthe theCNT CNTfilm; film;(e) (e)The Thetest testset-up. set-up. 3. 3. Results Results The studied as depicted in Figure 1e. The effect effect of of temperature temperature on the discharge current of air was studied The fabricated sensor was placed inside a test chamber filled with ambient The fabricated sensor was placed inside a test chamber filled with ambient air air heated heated by by aa resistive resistive wire. wire. The temperature inside the chamber chamber was closed-loop closed-loop controlled controlled by tuning tuning the the voltage voltage of of the the resistive wirewire and measuring the temperature using a k-typeusing thermocouple (chromel–nisiloy). resistiveheating heating and measuring the temperature a k-type thermocouple ◦ C. The steady state of the within thewas time of 1 min, in a range of 20of◦ C1 to 100in (chromel–nisiloy). Thetemperature steady statewas of realized the temperature realized within the time min, a After temperature reached state, Uc of reached 1 V and U values 24 and 100 Vvalues were e withstate, rangethe of 20 °C to 100 °C. Aftersteady the temperature steady Ucbetween of 1 V and Ue with applied, the discharge current Ic and were measured respectively the measured detection temperature betweenand 24 and 100 V were applied, andIethe discharge current Ic and Iefor were respectively ◦ C to and the study of emission properties of the CNTs cathode. When temperature T rose from 20 When for the detection temperature and the study of emission properties of the CNTs cathode. ◦ 100 C, currents Ic and Ie 20 exhibited an°C, exponential with T (Figure 2a,b). When extracting temperature T rose from °C to 100 currents Icincrease and Ie exhibited an exponential increase with T voltage U increased from 24 V to 100 V, currents I and I exhibited an increase with U . Sensitivity (Figure 2a,b). When extracting voltage Ue increased e c from e 24 V to 100 V, currents Ic and e Ie exhibited curves to temperature show verycurves similartoshape, as shown in Figure 2. This indicates the in sensor is an increase with Ue. Sensitivity temperature show very similar shape, as that shown Figure sensitive to temperature and sensor is thus capable of detecting temperature change. Values of Ie of were almost 2. This indicates that the is sensitive to temperature and is thus capable detecting twice of thosechange. of Ic . The temperature coefficients of Figure 2a were according to equation temperature Values of Ie were almost twice of those of Ic.calculated The temperature coefficients of SFigure = ∆I/(∆T IFS ), where ∆T is according the variation temperature, ∆I is the variation of Icis , and is the full 2a ·were calculated to of equation S = ΔI/(ΔT·I FS), where ΔT the IFS variation of 1 at 100 ◦ C and 24 V U (Table 1), scale range of IΔI highest coefficient Smax obtained as 0.04 K−of temperature, is the variation of Ic, and IFSwas is the full scale range Ic. The highest coefficient Smax c . The e higher than theasvalues of−1other temperature sensors [1–5]. was obtained 0.04 K at 100reported °C and CNT-based 24 V Ue (Table 1), higher than the values of other reported CNT-based temperature sensors [1–5]. Table 1. Performance comparison of the temperature sensors. No. No. This paper Article 1 This paper Article 2 Article 1 Article 3 Article Article24 Article35 Article Table 1. Performance comparison of the temperature sensors. Test Range (◦ C) Highest Temperature Coefficient (K−1 ) Test 20–100 Range (°C) 50–500 20–100 −269–147 50–500 20–70 −269–147 20–75 20–60 20–70 −1 Highest Temperature 4.0 × 10–2 Coefficient (K ) –3 2.24.0 × 10 × 10–2 −7.0 × 10–4 2.2 × 10 –3 −1.3 × 10–2 –2 –4 −7.0 × 10 −1.7 × 10 –3 –2 −5.0 × 10 −1.3 × 10 Reference Reference [1] [2] [1] [3] [4] [2] [5] [3] [4] Article 4 20–75 −1.7 × 10–2 –3 [5] Article 5 20–60 −5.0 × 10current density and is obtained The relationship between the logarithm lnje (je is the cathode by je = Ie /Sarea ) and the reciprocal –1/T was studied (Figure 2c). Sarea is the total cross-sectional area relationship thewas logarithm lnje according (je is the cathode current density is obtained by of all The nanotubes on thebetween film, and calculated to Figure 1d (Figure S3and in reference [15]); −6 m 2 . Eight –1/T = Ie=/S3.04 area) and reciprocal studied (Figure 2c).were Sarea is the total cross-sectional of all Sjearea × 10the leastwas squares straight lines fitted to the curves of lnjearea −1/T at nanotubes on the film, and was calculated according to Figure 1d (Figure S3 in reference [15]); S area = different Ue , and the coefficients R-Sq were calculated (Figure 2c) −6 2 3.04 10 m . Eight least squares straight lines were fitted to the curves of lnje-1/T at different Ue, R-Sq = 1 −(Figure (Σ(yi −2c) f i )2 )/(Σ(yi − yav )2 ) (1) and the coefficients R-Sq were calculated R-Sq = 1 ((yifi)2)/((yiyav)2) (1) Sensors 2017, 17, 473 4 of 7 Sensors 2017, 17, 473 4 of 7 where yi denotes lnje , f i denotes the fitting value to lnje , yav denotes the mean value of all lnje , and subscript coefficients R-Sq show that the straight lines are the wherei ydenotes i denotesthe lnjesequence , fi denotesnumber. the fittingThe value to lnje, yav denotes the mean value of all lnj e, and subscript denotes the at sequence number. The coefficients R-Sqemission show thatbehavior the straight are the The best fit line toi the curves various Ue , indicating a thermal oflines the sensor. best fit line to the current curves at variousje U e, indicating a thermal emission the sensor. The 2d), relationship between density and the average electric field E0behavior was alsoofobtained (Figure relationship between current density j e and the average electric field E0 was also obtained (Figure E0 = U /d, where d denotes electrode separation between cathode and extracting electrode. j increased e e E0 = Ue/d,awhere d denotesthermal electrode separation between cathode and extracting electrode. je with 2d), E0 , showing field-assisted emission behavior [16]. increased with E0, showing a field-assisted thermal emission behavior [16]. (a) (b) (c) (d) Figure 2. Effect of temperature on current at different Ue. (a) Collecting current–temperature Figure 2. Effect of temperature on current at different Ue . (a) Collecting current–temperature characteristic and (b) Cathode current–temperature characteristic at 24–100 V Ue; (c) lnje increased characteristic and (b) Cathode current–temperature characteristic at 24–100 V Ue ; (c) lnje effect increased linearly with −1/T at 24–100 V Ue, suggesting the thermal emission behavior and a considerable linearly with − 1/T at 24–100 V U , suggesting the thermal emission behavior and a considerable e of temperature on current density; (d) je increased with E0, suggesting the field emission behavior. effect of temperature on current density; (d) je increased with E0 , suggesting the field emission behavior. 4. Discussion 4. Discussion This section analyzes the experimental results to understand the temperature sensing mechanism the present temperature sensor. Ue is applied, electrons are emitted from This sectionofanalyzes theCNT experimental results to When understand the temperature sensing mechanism the nanotube tips and collide with the gas molecules, generating positive ions [17]. A fraction of the of the present CNT temperature sensor. When Ue is applied, electrons are emitted from the nanotube generated positive ions are extracted from the cathode region through the extracting hole towards tips and collide with the gas molecules, generating positive ions [17]. A fraction of the generated the collecting electrode and form the collecting current Ic. Consequently, Ic is a part of the discharge positive ionsI [18], are extracted current given by: from the cathode region through the extracting hole towards the collecting electrode and form the collecting current Ic . Consequently, Ic is a part of the discharge current I [18], I = eN0ed (2) given by: I = eN0 eαd (2) Sensors 2017, 17, 473 5 of 7 where is the charge, N0 is the number of electrons leaving the cathode in one second, Sensorse 2017, 17,electron 473 5 of and 7 α is the first ionization coefficient. Emission electrons and secondary emission electrons of the CNT where econtribute is the electron N0non-self-discharge is the number of electrons leaving the cathode in one second, and α cathode to Ncharge, state, N determined by the emission 0 . In the 0 is mainly is the first coefficient. electrons and secondary emission electrons the CNT electrons [18].ionization It is known that onlyEmission those electrons with energies greater than the sum ofofFermi energy contribute to Φ N0.can In the non-self-discharge state,nanotubes. N0 is mainlyWhen determined by the emission EFcathode and work function be emitted by the carbon Ue is applied, the work electrons [18]. It is emitted known that only those electrons withΦ energies than the sum of Fermi function of electrons by the CNT is reduced from to Φeff . Φgreater eff is the effective work function, energy E F and work function Φ can be emitted by the carbon nanotubes. When Ue is applied, the and it decreased with increasing E1 at a given temperature (Figure 3a). More electrons with energies work function ofΦ electrons emitted by the CNT is reduced from Φ to Φeff. Φeff is the effective work greater than EF + eff could be emitted by CNTs. It is well known that when temperature rises to function, and it decreased with 1 at a given temperature (Figure 3a). More electrons with ◦ T2 and T3 from T1 (100 C ≥ T3 increasing > T2 > T1 ),Erespectively, the probability of an electron gaining more energies greater than EF + Φeff could be emitted by CNTs. It is well known that when temperature energy increases (Figure 3b) [16]. As a result, more electrons leave the cathode at higher temperature rises to T2 and T3 from T1 (100 °C ≥ T3 > T2 > T1), respectively, the probability of an electron gaining and higher Ue , and form larger emission currentI0 (I0 = eN0 ) [19].The emission current density JSchottky more energy increases (Figure 3b) [16]. As a result, more electrons leave the cathode at higher could be expressed as follows [16]: temperature and higher Ue, and form larger emission currentI0 (I0= eN0) [19].The emission current density JSchottky could be expressed as follows [16]: 2 JSchottky = B0 T exp(−Φeff /kT) (3) JSchottky = B0T2exp(−Φeff/kT) (3) where B0 is the Richardson constant and k is the Boltzmann constant. Since JSchottky is the main source where B0 is the Richardson constant and k is the Boltzmann constant. Since JSchottky is the main source of je , it is approximated in this work that JSchottky ≈ je , and exponential increase Ie with temperature of je, it is approximated in this work that JSchottky ≈ je, and exponential increase Ie with temperature could be obtained. The higher the temperature and Ue are, the larger are the cathode currents. The could be obtained. The higher the temperature and Ue are, the larger are the cathode currents. The experimental withthe theabove aboveanalysis. analysis. experimentalresults resultsininFigure Figure22are arein in good good agreement agreement with (a) (b) Figure 3. (a) The effects of the E1 on Φeff; (b) The effect of temperature on the probability of an Figure 3. (a) The effects of the E1 on Φeff ; (b) The effect of temperature on the probability of an electron electron gaining energy. gaining energy. Additionally, the first ionization coefficient α was also affected by temperature [17], Additionally, the first ionization coefficient α was also affected by temperature [17], α = exp(–Ui/(Eλ))/λ (4) α = exp( Ui/ (Eλ))/λ where λ is the mean free path of an electron, Ui is−the first ionization potential of a gas molecule, and (4) E is electric field. where λ is the mean free path of an electron, Ui is the first ionization potential of a gas molecule, and E λ = kT/(πr2P) (5) is electric field. 2 λ = kT/(πr P)molecules, P is gas pressure. α denotes the (5) where k is the Boltzmann constant, r is the radius of gas impact ionization ability of an electron with a gas molecule, which depends on E, λ, and Ui. Here, E where k is the Boltzmann constant, r is the radius of gas molecules, P is gas pressure. α denotes the does not change when electrode separation and applied voltage are given, and the first ionization impact ionization ability of with molecule, which on E, λ, and Ui . Here, potential Ui is a constant forana electron certain gas. Asaagas result, α changes withdepends λ. Since pressure P increases E does change when separation voltage given, andwith the first ionization with not temperature T at aelectrode fixed volume of theand testapplied chamber, λ doesare not change temperature potential U is a constant for a certain gas. As a result, α changes with λ. Since pressure P increases with i according to Equation (5). Therefore, rising temperature could not affect α in the experiment of this temperature T at a fixed volume of the test chamber, λ does not change with temperature according work. If the temperature sensor operates in an open environment, λ increases with temperature T at to constant Equationpressure (5). Therefore, risingthe temperature could not affect in the experiment of this work. P, and then effect of temperature on α αand discharge current should be If theconsidered. temperature sensor operates in an open environment, λ increases with temperature T at constant pressure P, and then the effect of temperature on α and discharge current should be considered. Sensors 2017, 17, 473 6 of 7 5. Conclusions In summary, a triple-electrode CNT-based ionization sensor was fabricatedby using micro fabrication technology. A vertically aligned multi-walled nanotube array was grown by thermal chemical vapor deposition on one side of the cathode. The relationship of currents Ic and Ie versus temperature was investigated in a wider test range of 20–100 ◦ C and at 24–100 V Ue . The sensor in this work had a sensitivity of 0.04 K−1 at 24 V Ue , which is the highest sensitivity compared to the existing CNT-based resistive temperature sensors. The discharge current Ie and Ic increased with temperature, due to the increased number of electron emission at a given volume of gas mixture. Supplementary Materials: The Supplementary Materials are available online at http://www.mdpi.com/14248220/17/3/473/s1. Acknowledgments: We thank Y.H. Wang for contributions to a part of the experiments. This work is partially supported by the Specialized Research Fund for the Doctoral Program of Higher Education (20130201110007), the Natural Science Basic Research Plan in Shaanxi Province of China (2014JZ017), the Director Fund (EIPE14125) from the State Key Lab of Electrical Institution and Power Equipment of Xi’an Jiaotong University, the Fundamental Research Funds (xkjc2015012) for the Central Universities and the National Natural Science Foundation of China (51577142). We thank International Center for Dielectric Research of Xi’an Jiaotong University, Xi’an, China. 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