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New method for gas identification using a single semiconductor sensor
Sensors and Actuators B 66 Ž2000. 22–24 www.elsevier.nlrlocatersensorb
New method for gas identification using a single semiconductor sensor T. Takada) , T. Fukunaga, T. Maekawa New Cosmos Electric Co. Ltd., 2-5-4 Mitsuya-naka, Yodogawa-ku, Osaka 532-0036, Japan Received 30 July 1998; received in revised form 25 January 1999; accepted 2 March 1999
Abstract Simultaneous measurements of changes in semiconductor resistance D R and in sensor temperature DT gave a new method for gas identification using a single SnO 2-based gas sensor. Gas species with any concentration was represented by a point on a particular curve Fi , which was characteristic of a certain species i, on the two-dimensional space Ž D R, DT .. Thus, the species could be identified as j without uncertainty from a measured point Ž D R m , DTm . on the characteristic curve Fj . We identified gas species of H 2 Ž10–1000 ppm., CO Ž10–500 ppm., C 2 H 5 OH Ž10–5000 ppm. and alkane such as CH 4 Ž10 ppm–2%., C 3 H 8 Ž10–2000 ppm. and i-C 4 H 10 Ž10–1000 ppm., and also discriminated volatile organic compounds of C 6 H 6 , C 6 H 5 CH 3 and C 6 H 4ŽCH 3 . 2 ranging from 2 to 200 ppm. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Gas identification; Sensor temperature; Semiconductor resistance
1. Introduction Gas identification Žor odour identification. is one of the most attractive subjects in the field of gas sensing. Usually plural sensors were used and their signals were analyzed with various processing with computers w1x. But it is still hard to identify gas Žor odour. species in the wide concentration range, especially when the gas concentration changes during measurement. The pattern of signals from the sensors is not similar in the wide concentration range, therefore signal processing becomes complicated and difficult to be settled. Moreover, each sensor shows different characteristics under various circumstances including different temperature and humidity dependences and different long-term stability. This makes the processing more complicated and the gas identification more laborious. We have tried to identify gas species by analyzing multiple information obtained from a single semiconductor gas sensor on exposure to reducing gases. It includes changes in resistance, sensor temperature, capacitance, inductance and so on w2,3x. In this report, we will present a new method for identification of gas species and determination of the concentration with a set of data Ž D R, DT .,
)
Corresponding author.
which were obtained from simultaneous measurements of D R and DT using a sintered SnO 2-based gas sensor.
2. Experimental The sensor structure used in the present study is illustrated in Fig. 1. An alumina substrate is equipped with a pair of interdigital Pt film electrodes on the face and a Pt film heater on the back. The heater simultaneously serves as a Pt film thermometer to measure the sensor temperature change. Sintered SnO 2-based thick films with thickness of about 250 mm were formed over electrodes by a screen printing method. The thick films were calcined at 12008C for 2 h. The sensor and the compensator, which compensated for the influence of the change in ambient temperature, were placed in a chamber of stainless steel and were exposed to a mixture of a reducing gas and air by injecting a certain amount of the reducing gas into the chamber. The temperature and resistance changes were measured simultaneously with the electrical circuit shown in Fig. 1. The Pt thermometer of the sensor and the Pt resistor of the compensator were incorporated into a Wheatstone bridge circuit. The temperature change of the sensor was measured as a change in the output voltage of the bridge circuit. The sensitivity S is defined as S s
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 9 9 . 0 0 4 0 4 - 9
T. Takada et al.r Sensors and Actuators B 66 (2000) 22–24
23
Fig. 1. Schematic drawing of the sensor structure and electrical circuit for the present measurement.
R airrR gas , where R air and R gas are resistances in air and in sample gas, respectively.
3. Results and discussion Simultaneous measurements of D R and DT yielded a new method for gas identification using a single SnO 2based gas sensor. It was observed that the gas species causing a large decrease Ža small decrease. in semiconductor resistance brought about only a small increase or a drop Ža large increase. in sensor temperature as shown in Fig. 2, which denotes results for a sintered SnO 2 calcined at 7008C with a small amount of Pd-additive. CH 4 gave rise to a temperature drop of the sensor and a large resistance change in the whole concentration range examined, and in contrast to CH 4 , H 2 , showed a large temperature increase and a small resistance change. From the practical point of view, the above phenomena would enable one to identify the gas species and to determine the gas concentration with a single semiconductor sensor by making a map on the two-dimensional space Ž S, DT ., which is shown in Fig. 3. Gas species i with any concentration is represented by a
point Ž Si , DTi . on the space. When the concentration changes, the point Ž Si , DTi . moves along a curve Fi characteristic of the species i. If the curve has no crossing with other curves, the species can be identified with a measured point Ž Sm , DTm .. If the point Ž Sm , DTm . of a detected gas is located on a previously calibrated curve Fi-C 4 H 10 , the gas can be identified as i-C 4 H 10 as shown in Fig. 3. In this case, H 2 , CH 4 , and i-C 4 H 10 were easily discriminated from each other in the gas concentration ranging from 100 to 2500 ppm. Once a gas species is identified as i-C 4 H 10 , it is easy to determine the concentration from the concentration dependence of the sensitivity. Figs. 4 and 5 also show other identifications for various gases of H 2 Ž10–1000 ppm., CO Ž10–500 ppm., C 2 H 5 OH Ž10–5000 ppm. and alkane such as CH 4 Ž10 ppm–2%., C 3 H 8 Ž10–2000 ppm. and i-C 4 H 10 Ž10–1000 ppm. and for different volatile organic compounds such as C 6 H 6 , C 6 H 5 CH 3 and C 6 H 4 ŽCH 3 . 2 ranging from 2 to 200 ppm. The present method for gas Žor odour. identification has some advantages compared with the conventional method. It is still hard to identify gas Žor odour. species in a wide concentration range by the conventional method. On the contrary, it is easy to identify any concentration of gas
Fig. 2. Changes in resistance and sensor temperature when exposed to reducing gases at 4708C.
24
T. Takada et al.r Sensors and Actuators B 66 (2000) 22–24
Fig. 3. Map of Ž S, DT . for gas identification.
species by the present method utilizing the characteristic curves on the two-dimensional space Ž D R, DT .. If the gas concentration changes during measurement, it is impossible to identify the gas species by the conventional method. It should be noted, however, that the change in gas concentration during measurement does not bring about any difficulty in the gas identification by the present method. When the concentration changes, the measured point
Fig. 5. Identification of various volatile organic compounds. Working temperature was 4808C.
Ž D R m , DTm . moves on the corresponding part of a curve Fk characteristic of species k. Gas species can be identified with the part of Fk , not merely with the point, and this makes the present identification more accurate. If the curve has crossings with other curves, the species cannot be identified with a measured point in the vicinity of the crossings. Even in this case, it is easy to distinguish Fk from other curves with a part of Fk , and the species can be identified as k. Consequently, the present method for gas identification gets more reliable in case the concentration changes during measurement than in other cases.
References
Fig. 4. Identification of various reducing gases. Working temperature was 4508C.
w1x H. Ulmer, J. Mitrovics, G. Noetzel, U. Weimar, W. Gopel, Odours and flavours identified with hybrid modular sensor systems, Sens. Actuators, B 43 Ž1997. 24–33. w2x T. Takada, T. Fukunaga, New method for gas identification and concentration determination, in: Proceedings of the 23rd Chemical Sensor Symposium, 1996, pp. 69–72, Žin Japanese.. w3x H. Kobayashi, M. Miyoshi, T. Takada, Simultaneous measurements of capacitance and conductance changes of semiconductor gas sensor when exposed to reducing gases, in: Proceedings of the 25th Chemical Sensor Symposium, 1997, pp. 157–160, Žin Japanese..
New method for gas identification using a single semiconductor sensor T. Takada) , T. Fukunaga, T. Maekawa New Cosmos Electric Co. Ltd., 2-5-4 Mitsuya-naka, Yodogawa-ku, Osaka 532-0036, Japan Received 30 July 1998; received in revised form 25 January 1999; accepted 2 March 1999
Abstract Simultaneous measurements of changes in semiconductor resistance D R and in sensor temperature DT gave a new method for gas identification using a single SnO 2-based gas sensor. Gas species with any concentration was represented by a point on a particular curve Fi , which was characteristic of a certain species i, on the two-dimensional space Ž D R, DT .. Thus, the species could be identified as j without uncertainty from a measured point Ž D R m , DTm . on the characteristic curve Fj . We identified gas species of H 2 Ž10–1000 ppm., CO Ž10–500 ppm., C 2 H 5 OH Ž10–5000 ppm. and alkane such as CH 4 Ž10 ppm–2%., C 3 H 8 Ž10–2000 ppm. and i-C 4 H 10 Ž10–1000 ppm., and also discriminated volatile organic compounds of C 6 H 6 , C 6 H 5 CH 3 and C 6 H 4ŽCH 3 . 2 ranging from 2 to 200 ppm. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Gas identification; Sensor temperature; Semiconductor resistance
1. Introduction Gas identification Žor odour identification. is one of the most attractive subjects in the field of gas sensing. Usually plural sensors were used and their signals were analyzed with various processing with computers w1x. But it is still hard to identify gas Žor odour. species in the wide concentration range, especially when the gas concentration changes during measurement. The pattern of signals from the sensors is not similar in the wide concentration range, therefore signal processing becomes complicated and difficult to be settled. Moreover, each sensor shows different characteristics under various circumstances including different temperature and humidity dependences and different long-term stability. This makes the processing more complicated and the gas identification more laborious. We have tried to identify gas species by analyzing multiple information obtained from a single semiconductor gas sensor on exposure to reducing gases. It includes changes in resistance, sensor temperature, capacitance, inductance and so on w2,3x. In this report, we will present a new method for identification of gas species and determination of the concentration with a set of data Ž D R, DT .,
)
Corresponding author.
which were obtained from simultaneous measurements of D R and DT using a sintered SnO 2-based gas sensor.
2. Experimental The sensor structure used in the present study is illustrated in Fig. 1. An alumina substrate is equipped with a pair of interdigital Pt film electrodes on the face and a Pt film heater on the back. The heater simultaneously serves as a Pt film thermometer to measure the sensor temperature change. Sintered SnO 2-based thick films with thickness of about 250 mm were formed over electrodes by a screen printing method. The thick films were calcined at 12008C for 2 h. The sensor and the compensator, which compensated for the influence of the change in ambient temperature, were placed in a chamber of stainless steel and were exposed to a mixture of a reducing gas and air by injecting a certain amount of the reducing gas into the chamber. The temperature and resistance changes were measured simultaneously with the electrical circuit shown in Fig. 1. The Pt thermometer of the sensor and the Pt resistor of the compensator were incorporated into a Wheatstone bridge circuit. The temperature change of the sensor was measured as a change in the output voltage of the bridge circuit. The sensitivity S is defined as S s
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 9 9 . 0 0 4 0 4 - 9
T. Takada et al.r Sensors and Actuators B 66 (2000) 22–24
23
Fig. 1. Schematic drawing of the sensor structure and electrical circuit for the present measurement.
R airrR gas , where R air and R gas are resistances in air and in sample gas, respectively.
3. Results and discussion Simultaneous measurements of D R and DT yielded a new method for gas identification using a single SnO 2based gas sensor. It was observed that the gas species causing a large decrease Ža small decrease. in semiconductor resistance brought about only a small increase or a drop Ža large increase. in sensor temperature as shown in Fig. 2, which denotes results for a sintered SnO 2 calcined at 7008C with a small amount of Pd-additive. CH 4 gave rise to a temperature drop of the sensor and a large resistance change in the whole concentration range examined, and in contrast to CH 4 , H 2 , showed a large temperature increase and a small resistance change. From the practical point of view, the above phenomena would enable one to identify the gas species and to determine the gas concentration with a single semiconductor sensor by making a map on the two-dimensional space Ž S, DT ., which is shown in Fig. 3. Gas species i with any concentration is represented by a
point Ž Si , DTi . on the space. When the concentration changes, the point Ž Si , DTi . moves along a curve Fi characteristic of the species i. If the curve has no crossing with other curves, the species can be identified with a measured point Ž Sm , DTm .. If the point Ž Sm , DTm . of a detected gas is located on a previously calibrated curve Fi-C 4 H 10 , the gas can be identified as i-C 4 H 10 as shown in Fig. 3. In this case, H 2 , CH 4 , and i-C 4 H 10 were easily discriminated from each other in the gas concentration ranging from 100 to 2500 ppm. Once a gas species is identified as i-C 4 H 10 , it is easy to determine the concentration from the concentration dependence of the sensitivity. Figs. 4 and 5 also show other identifications for various gases of H 2 Ž10–1000 ppm., CO Ž10–500 ppm., C 2 H 5 OH Ž10–5000 ppm. and alkane such as CH 4 Ž10 ppm–2%., C 3 H 8 Ž10–2000 ppm. and i-C 4 H 10 Ž10–1000 ppm. and for different volatile organic compounds such as C 6 H 6 , C 6 H 5 CH 3 and C 6 H 4 ŽCH 3 . 2 ranging from 2 to 200 ppm. The present method for gas Žor odour. identification has some advantages compared with the conventional method. It is still hard to identify gas Žor odour. species in a wide concentration range by the conventional method. On the contrary, it is easy to identify any concentration of gas
Fig. 2. Changes in resistance and sensor temperature when exposed to reducing gases at 4708C.
24
T. Takada et al.r Sensors and Actuators B 66 (2000) 22–24
Fig. 3. Map of Ž S, DT . for gas identification.
species by the present method utilizing the characteristic curves on the two-dimensional space Ž D R, DT .. If the gas concentration changes during measurement, it is impossible to identify the gas species by the conventional method. It should be noted, however, that the change in gas concentration during measurement does not bring about any difficulty in the gas identification by the present method. When the concentration changes, the measured point
Fig. 5. Identification of various volatile organic compounds. Working temperature was 4808C.
Ž D R m , DTm . moves on the corresponding part of a curve Fk characteristic of species k. Gas species can be identified with the part of Fk , not merely with the point, and this makes the present identification more accurate. If the curve has crossings with other curves, the species cannot be identified with a measured point in the vicinity of the crossings. Even in this case, it is easy to distinguish Fk from other curves with a part of Fk , and the species can be identified as k. Consequently, the present method for gas identification gets more reliable in case the concentration changes during measurement than in other cases.
References
Fig. 4. Identification of various reducing gases. Working temperature was 4508C.
w1x H. Ulmer, J. Mitrovics, G. Noetzel, U. Weimar, W. Gopel, Odours and flavours identified with hybrid modular sensor systems, Sens. Actuators, B 43 Ž1997. 24–33. w2x T. Takada, T. Fukunaga, New method for gas identification and concentration determination, in: Proceedings of the 23rd Chemical Sensor Symposium, 1996, pp. 69–72, Žin Japanese.. w3x H. Kobayashi, M. Miyoshi, T. Takada, Simultaneous measurements of capacitance and conductance changes of semiconductor gas sensor when exposed to reducing gases, in: Proceedings of the 25th Chemical Sensor Symposium, 1997, pp. 157–160, Žin Japanese..