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Planar heat-wire type CH4 gas sensor
Sensors and Actuators B 66 Ž2000. 243–245 www.elsevier.nlrlocatersensorb
Planar heat-wire type CH 4 gas sensor Baofu Quan a,) , Chengsong Sun b, Zhe Wang a , Haiying Quan a , Lijun Zhou b, Yunpeng Li b b
a Department of Electronic Engineering, JiLin UniÕersity, Changchun 130023, People’s Republic of China Department of Electrical Engineering, Shenyang Polytechnic UniÕersity, Shenyang 110023, People’s Republic of China
Received 30 July 1998; received in revised form 10 February 1999; accepted 29 January 2000
Abstract Planar heat-wire type sensors based on SnO 2 and molecular sieve were fabricated by means of RF sputtering. With the filtering and catalyzing effect of molecular sieve, the sensor can detect CH 4 with high sensitivity and high selectivity. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Planar heat-wire type; Gas sensor
1. Introduction In order to detect the contents of CH 4 in environment air accurately and rapidly, CH 4 sensors with different structures have been investigated. Sintered heat-wire type sensor has been studied widely w1–4x. Afterwards, Kokuen et al. w5x have developed planar heat-wire type i-C 4 H 10 gas sensor on Al 2 O 3 substrate by RF sputtering. We have developed planar heat-wire type CH 4 sensor by RF sputtering.
2. Experimental
2.1. Making Pt film electrode
trode Ža. and the structure of sensors Žb. are shown in Fig. 1.
2.2. Making sensors Sensing coating is based on SnO 2 loaded with Sb, Pt, etc. SnO 2 powder was prepared by neutralizing a hot solution of SnCl 4 with an aqueous ammonia solution, by washing it with de-mineralized water, by drying it at 808C and then grinding. The resulting powder was calcined at 7008C for 2 h to obtain SnO 2 grains of a designated size. In order to fabricate gas sensor elements, Sb, Pt and Sn were added to SnO 2 powder and were mixed with water. The resulting paste was coated on the substrate with Pt film electrode and sintered at 6008C for 2 h. The change of bridge output voltage in air Ž Vo . and that in the sample gas
SiO 2 film of 600-nm thickness was prepared on a clean Si substrate by heat oxidization, then Pt film of 700-nm thickness was formed on SiO 2 by RF sputtering as an electrode and a heater. The conditions for making Pt film are as follows: 1.3 = 10y3 Pa vacuum, 1 Pa Ar gas pressure, negative voltage 80 V DC. The picture of elec-
)
Corresponding author.
0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 0 0 . 0 0 3 6 2 - 2
Fig. 1. Planar heat-wire type sensor structure.
B. Quan et al.r Sensors and Actuators B 66 (2000) 243–245
244
Fig. 2. Dependence of the resistance of the coating layer on Sb 2 O 3 content.
of CH 4 diluted with air Ž Vg . were measured as the sensitivity of sensors Ž DV s Vg y Vo ..
3. Results and discussion 3.1. Characteristics of sensor’s structure As Fig.1Žb. shows, the resistance Ž R e . of planar heatwire type sensor is a function of the coating resistance Ž R c . and the Pt film electrode resistance Ž R p ., i.e.: Re s
Rc Rp Rc q Rp
.
Ž 1.
Since R e and R p can be measured easily, R c can be calculated from Eq. Ž1.. When R c 4 R p , the change of R e cannot be detected obviously, while when R c F R p , sensors can work with high sensitivity. In order to obtain low resistance sensing layer, SnO 2 powder mixed with Sb 2 O 3 Že.g., 5 wt.%. or a small amount of Sn is required, since many oxide vacancies, Vox , or interval, Sn Ži., are formed during the calcination processing. Fig. 2 shows the change of the resistance of the coating layer with the Sb 2 O 3 content measured at room temperature. In order to increase the sensitivity for CH 4 gas, suitable content Pt ŽPt–asbestos. was added to SnO 2 coating layer. Influence of the loaded Pt content on the sensitivity of sensor element is shown in Fig. 3. When the Pt content was about 0.3 wt.%, the sensitivity to CH 4 showed a maximum.
Fig. 3. Dependence of output voltage on the content of Pt.
Fig. 4. Relationship between output voltage and gas concentration. Žv, CH 4 ; `, i-C 4 H 10 ; ^, H 2 ; I, C 2 H 5 OH; %, CO..
3.2. Characteristics of the sensors In order to obtain high selective CH 4 sensor, element ˚ molecular sieve loaded was fabricated with SnO 2 and 4 A with a small amount of noble metal catalysts. The sensitivity of the sensor element is shown in Fig. 4 as a function of the sample gas concentration. It is difficult for large molecular gas Že.g., i-C 4 H 10 , C 2 H 5 OH. to pass through molecular sieve and react with inner sensing coating. Small molecular gas ŽH 2 . will be oxidized on the surface of molecular sieve, which can be attributed to the strong catalyzing effect of noble metal ŽPd. doped in molecular sieve. However, the above effect has little influence on gases of CH 4 and CO. Consequently, the sensor’s selectivity is improved. The output voltage of sensor in 1000 ppm CO, H 2 , CH 4 and i-C 4 H 10 gases, respectively, is shown in Fig. 5 as a function of applied voltages. When heated voltage was higher than 5 V, the sensitivity to CH 4 increased rapidly. In 5000 ppm CH 4 gas, response time was about 30 s and the recovering time was about 1 min.
4. Conclusions Planar heat-wire type gas sensor based on SnO 2 –Pt is sensitive to C 2 H 5 OH, H 2 , CH 4 , i-C 4 H 10 , etc. The sensor ˚ molecular sieve caused the modified by coating 4 A remarkable selectivity of sensor to CH 4 gas.
Fig. 5. Dependence of output voltage on the applied voltage. Žv, CH 4 ; `, i-C 4 H 10 ; ^, H 2 ; I, C 2 H 5 OH; %, CO..
B. Quan et al.r Sensors and Actuators B 66 (2000) 243–245
Acknowledgements The authors thank NSFC Žsubject No. 69774027. and JPSTC for financial support.
References w1x N. Komori, S. Sakai, K. Komatsu, High sensitive methane sensor, Transducers ’87 Ž1987. 591–594.
245
w2x J. Gerblinger, V. Lampe, H. Meixner, I.V. Perczel, J. Giber, Crosssensitivity of various doped strontium titanate films to CO, CO 2 , H 2 O and CH 4 , Sens. Actuators, B 18r19 Ž1994. 529–534. w3x V.V. Malyshev, A.V. Eryshkin, E.A. Koltypin, E.V. Vartolomeev, A.A. Vasiliev, Gas sensitivity of semiconductor Fe 2 O 3 -based thickfilm sensors to CH 4 , H 2 and NH 3 , Sens. Actuators, B 18r19 Ž1994. 434–436. w4x D.D. Lee, Y. Choi, B.K. Sohn, SnO 2 rPt thin film CO gas sensors, Transducers ’87 Ž1987. 626–630. w5x A. Kokuen, K.H. Takahashi, Y.K. Nakanouchi, Hot wire type gas sensor of thin film tin oxide, Chem. Sens. 6 ŽSuppl. A. Ž1990. 45–48, Žin Japanese..
Planar heat-wire type CH 4 gas sensor Baofu Quan a,) , Chengsong Sun b, Zhe Wang a , Haiying Quan a , Lijun Zhou b, Yunpeng Li b b
a Department of Electronic Engineering, JiLin UniÕersity, Changchun 130023, People’s Republic of China Department of Electrical Engineering, Shenyang Polytechnic UniÕersity, Shenyang 110023, People’s Republic of China
Received 30 July 1998; received in revised form 10 February 1999; accepted 29 January 2000
Abstract Planar heat-wire type sensors based on SnO 2 and molecular sieve were fabricated by means of RF sputtering. With the filtering and catalyzing effect of molecular sieve, the sensor can detect CH 4 with high sensitivity and high selectivity. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Planar heat-wire type; Gas sensor
1. Introduction In order to detect the contents of CH 4 in environment air accurately and rapidly, CH 4 sensors with different structures have been investigated. Sintered heat-wire type sensor has been studied widely w1–4x. Afterwards, Kokuen et al. w5x have developed planar heat-wire type i-C 4 H 10 gas sensor on Al 2 O 3 substrate by RF sputtering. We have developed planar heat-wire type CH 4 sensor by RF sputtering.
2. Experimental
2.1. Making Pt film electrode
trode Ža. and the structure of sensors Žb. are shown in Fig. 1.
2.2. Making sensors Sensing coating is based on SnO 2 loaded with Sb, Pt, etc. SnO 2 powder was prepared by neutralizing a hot solution of SnCl 4 with an aqueous ammonia solution, by washing it with de-mineralized water, by drying it at 808C and then grinding. The resulting powder was calcined at 7008C for 2 h to obtain SnO 2 grains of a designated size. In order to fabricate gas sensor elements, Sb, Pt and Sn were added to SnO 2 powder and were mixed with water. The resulting paste was coated on the substrate with Pt film electrode and sintered at 6008C for 2 h. The change of bridge output voltage in air Ž Vo . and that in the sample gas
SiO 2 film of 600-nm thickness was prepared on a clean Si substrate by heat oxidization, then Pt film of 700-nm thickness was formed on SiO 2 by RF sputtering as an electrode and a heater. The conditions for making Pt film are as follows: 1.3 = 10y3 Pa vacuum, 1 Pa Ar gas pressure, negative voltage 80 V DC. The picture of elec-
)
Corresponding author.
0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 0 0 . 0 0 3 6 2 - 2
Fig. 1. Planar heat-wire type sensor structure.
B. Quan et al.r Sensors and Actuators B 66 (2000) 243–245
244
Fig. 2. Dependence of the resistance of the coating layer on Sb 2 O 3 content.
of CH 4 diluted with air Ž Vg . were measured as the sensitivity of sensors Ž DV s Vg y Vo ..
3. Results and discussion 3.1. Characteristics of sensor’s structure As Fig.1Žb. shows, the resistance Ž R e . of planar heatwire type sensor is a function of the coating resistance Ž R c . and the Pt film electrode resistance Ž R p ., i.e.: Re s
Rc Rp Rc q Rp
.
Ž 1.
Since R e and R p can be measured easily, R c can be calculated from Eq. Ž1.. When R c 4 R p , the change of R e cannot be detected obviously, while when R c F R p , sensors can work with high sensitivity. In order to obtain low resistance sensing layer, SnO 2 powder mixed with Sb 2 O 3 Že.g., 5 wt.%. or a small amount of Sn is required, since many oxide vacancies, Vox , or interval, Sn Ži., are formed during the calcination processing. Fig. 2 shows the change of the resistance of the coating layer with the Sb 2 O 3 content measured at room temperature. In order to increase the sensitivity for CH 4 gas, suitable content Pt ŽPt–asbestos. was added to SnO 2 coating layer. Influence of the loaded Pt content on the sensitivity of sensor element is shown in Fig. 3. When the Pt content was about 0.3 wt.%, the sensitivity to CH 4 showed a maximum.
Fig. 3. Dependence of output voltage on the content of Pt.
Fig. 4. Relationship between output voltage and gas concentration. Žv, CH 4 ; `, i-C 4 H 10 ; ^, H 2 ; I, C 2 H 5 OH; %, CO..
3.2. Characteristics of the sensors In order to obtain high selective CH 4 sensor, element ˚ molecular sieve loaded was fabricated with SnO 2 and 4 A with a small amount of noble metal catalysts. The sensitivity of the sensor element is shown in Fig. 4 as a function of the sample gas concentration. It is difficult for large molecular gas Že.g., i-C 4 H 10 , C 2 H 5 OH. to pass through molecular sieve and react with inner sensing coating. Small molecular gas ŽH 2 . will be oxidized on the surface of molecular sieve, which can be attributed to the strong catalyzing effect of noble metal ŽPd. doped in molecular sieve. However, the above effect has little influence on gases of CH 4 and CO. Consequently, the sensor’s selectivity is improved. The output voltage of sensor in 1000 ppm CO, H 2 , CH 4 and i-C 4 H 10 gases, respectively, is shown in Fig. 5 as a function of applied voltages. When heated voltage was higher than 5 V, the sensitivity to CH 4 increased rapidly. In 5000 ppm CH 4 gas, response time was about 30 s and the recovering time was about 1 min.
4. Conclusions Planar heat-wire type gas sensor based on SnO 2 –Pt is sensitive to C 2 H 5 OH, H 2 , CH 4 , i-C 4 H 10 , etc. The sensor ˚ molecular sieve caused the modified by coating 4 A remarkable selectivity of sensor to CH 4 gas.
Fig. 5. Dependence of output voltage on the applied voltage. Žv, CH 4 ; `, i-C 4 H 10 ; ^, H 2 ; I, C 2 H 5 OH; %, CO..
B. Quan et al.r Sensors and Actuators B 66 (2000) 243–245
Acknowledgements The authors thank NSFC Žsubject No. 69774027. and JPSTC for financial support.
References w1x N. Komori, S. Sakai, K. Komatsu, High sensitive methane sensor, Transducers ’87 Ž1987. 591–594.
245
w2x J. Gerblinger, V. Lampe, H. Meixner, I.V. Perczel, J. Giber, Crosssensitivity of various doped strontium titanate films to CO, CO 2 , H 2 O and CH 4 , Sens. Actuators, B 18r19 Ž1994. 529–534. w3x V.V. Malyshev, A.V. Eryshkin, E.A. Koltypin, E.V. Vartolomeev, A.A. Vasiliev, Gas sensitivity of semiconductor Fe 2 O 3 -based thickfilm sensors to CH 4 , H 2 and NH 3 , Sens. Actuators, B 18r19 Ž1994. 434–436. w4x D.D. Lee, Y. Choi, B.K. Sohn, SnO 2 rPt thin film CO gas sensors, Transducers ’87 Ž1987. 626–630. w5x A. Kokuen, K.H. Takahashi, Y.K. Nakanouchi, Hot wire type gas sensor of thin film tin oxide, Chem. Sens. 6 ŽSuppl. A. Ž1990. 45–48, Žin Japanese..