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Classification of chemical warfare agents using thick film gas sensor array
Sensors and Actuators B 108 (2005) 298–304
Classification of chemical warfare agents using thick film gas sensor array Nak-Jin Choi a , Jun-Hyuk Kwak a , Yeon-Tae Lim a , Tae-Hyun Bahn b , Ky-Yeol Yun c , Jae-Chang Kim a , Jeung-Soo Huh d , Duk-Dong Lee a,∗ a
d
School of Electronics and Computer Science, Kyungpook National University (KNU), Daegu, Republic of Korea b Department of Sensor Engineering, KNU, 1370 San-Kyuk Dong, Puk-Gu, Daegu, Republic of Korea c Department of Chemical Engineering, KNU, 1370 San-Kyuk Dong, Puk-Gu, Daegu, Republic of Korea Department of Materials Science and Metallurgy, KNU, 1370 San-Kyuk Dong, Puk-Gu, Daegu, Republic of Korea Received 12 July 2004; received in revised form 6 November 2004; accepted 8 November 2004 Available online 23 March 2005
Abstract Semiconductor thick film gas sensors based on tin oxide are fabricated and their gas response characteristics are examined for four simulant gases of chemical warfare agent (CWA)s. The sensing materials are prepared in three different sets such as impregnation, physical mixing (ballmilling) and co-precipitation method. Surface morphology, particle size, and specific surface area of fabricated sensing films are performed by the SEM, XRD and BET, respectively. Response characteristics are examined for test gases with temperature in the range 200–400 ◦ C, with different gas concentrations. Test gases are dimethyl methyl phosphonate (DMMP), dipropylene glycol methyl ether (DPGME), acetonitrile, and dichloromethane which are used simulant gases of chemical warfare agents. These sensors showed sensitivities higher than 50% in 500 ppb of test gases and also good repetition behaviour. Four sensing materials are selected with good sensitivity and stability and are fabricated as a sensor array. And then, principal component analysis (PCA) is adapted to classify objective gas among the four simulant gases. The thick film array sensor shows high sensitivity to CWA gases and four CWA gases are classified by using a sensor array composed of four sensing devices through PCA. © 2004 Elsevier B.V. All rights reserved. Keywords: SnO2 ; Chemical warfare agents; Array; Principal component analysis
1. Introduction Chemical warfare agents (CWAs) are very dangerous for health because of its colorlessness and toxicity. Therefore, fast and correct detection of CWAs is essential to protect human beings. They are divided into four types such as blood, nerve, vesicant, and choking agent [1]. In general, semiconductor gas sensor has many advantages like high sensitivity at low level concentration, good stability against environmental change and low price [2,3]. Many semiconductor sensors have selectivity problem because they show similar response to various gases. So, it is difficult to ∗
Corresponding author. Tel.: +82 53 950 5514; fax: +82 53 939 1073. E-mail address: [email protected] (D.-D. Lee).
0925-4005/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2004.11.022
identity the objective gas. Nowadays, there are many studies to classify various gases by means of adding catalysts, using the sensor array or combining two methods, etc. [2–4]. To add catalysts takes much time to examine optimal catalyst for specific material, because of the existence of a large number of catalysts. But to use sensor array requires smaller time than that, because it can be easily classified by using the array pattern such as neural network and statistical method [5,6]. In this study, semiconductor thick film sensors based on tin oxide are fabricated and their responses to four simulant gases are examined with different amounts of added materials, with different operating temperatures and with different gas concentrations. Materials are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and specific surface area analyzer (BET) analysis. From the above
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Table 1 Amounts of added materials and material synthesis method
Table 2 Measurement simulant gases [1,12]
Added material
Added amounts (wt.%)
Synthesis method
Agent
Chemical gas
Simulant gas
Al2 O3 In2 O3 Pt, Pd ZnO ZrO2
0, 4, 12, 20 1, 2, 3 0.1, 0.5, 1 1, 2, 3, 4, 5 1, 3, 5
Ball-milling Ball-milling Impregnation Co-precipitation Co-precipitation
Blood Nerve
HCN Tabun
Choking Vesicant
CE HN
Acetonitrile DMMP (dimethyl methyl phosphonate) Dichloromethane DPGME (dipropylene glycolmethyl ether)
results, four different sensing materials are selected which have good sensitivity and stability to gases and fabricated as a sensor array. A sensor array was examined to test gases with different operating temperatures and with different gas concentrations, and pattern was extracted from acquired data. Finally, principal component analysis (PCA) that was one of the statistical classification method was conducted to classify four test gases [5].
2. Experiments 2.1. Preparation of sensing materials Table 1 shows gas sensing materials used in this study. Base material of all samples is SnO2 . The sensing materials are prepared in three different sets. (1) The Pt or Pd (0.1, 0.51 wt.%) as catalyst was impregnated in the base material of SnO2 by impregnation method [7]. (2) Al2 O3 (0, 4, 12, 20 wt.%) or In2 O3 (1, 2, 3 wt.%) was added to SnO2 by physical ball milling process [8]. (3) ZnO (1, 2, 3, 4, 5 wt.%) or ZrO2 (1, 3, 5 wt.%) was added to SnO2 by co-precipitation method [10,11]. All samples were dried at 110 ◦ C for 1 h, followed by grinding and calcination at 600 ◦ C for 1 h in the furnace. These powders were further sintered at 700 ◦ C for 1 h and deposited on alumina substrate by screen printing technique.
CE and HN mean phosgene and nitrogen mustard respectively [1].
2.2. Fabrication of device ˚ thickness Pt sensing electrode is deposited with 1000 A by using the DC sputter at the front side of alumina substrate and electrode for heater was screen printed using the Pt paste at the back side. Then device is thermally annealed at 850 ◦ C for 10 min in the electric furnace. The heater resistance is 10 ohms and sensing electrode is designed as interdigitated (IDT) structure. All sensors were tested after stabilizing time at 400 ◦ C for 3 days. Total dimension of device is 7 mm × 10 mm × 0.6 mm and size of sensing film is 6 mm × 4 mm. 2.3. Apparatus and measurement method Table 2 shows measurement gases. Typical chemical agents are shown in Table 2 [1]. To use the real chemical gases is very dangerous and difficult, so simulant gases with similar chemical characteristics are used as test gases in safety. Four simulant gases are liquid phase; therefore; vapor pressures are calculated to control the injected gas concentration. Acetonitrile and dichloromethane adapt Antoine equation as shown in Eq. (1) and DMMP uses Eq. (2). In the case of DPGME, vapor pressures at specific temperature were used
Fig. 1. Measurement apparatus.
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0.3 and 0.05 mmHg at 20 and 25 ◦ C, respectively [11–13]. log10 P = A −
B T +C
(1)
P = 2.844 × 108 × exp
−11500 RT
(2)
where P (mmHg) means vapor pressure, constant A–C are specific values of each chemical. And T (K) means temperature of circulator in Fig. 1. As shown in Eqs. (1) and (2), gas concentration can be controlled by using the change of the circulator temperature.
Table 3 Values of used parameters in Eq. (1)
T range A B C
(◦ C)
Acetonitrile
Dichloromethane
7–23 5.93296 2345.829 43.815
−40–40 4.53691 1327.016 −20.474
Table 3 shows specific values in the Antoine equation. T range means that this equation can be used in the range of the temperature. Fig. 1 shows experimental measurement apparatus. Gas amounts were controlled by using mass flow controllers. Test gas concentration in the chamber can be con-
Fig. 2. Response results for four simulant gases: (a) DMMP, (b) DPGME, (c) CH3 CN, (d) CH2 Cl2 .
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301
Fig. 2. (Continued ).
trolled by air amounts for reference gas and vapor amounts of test reactant. Total mass flow was fixed (1000 ml/min). Circulator fixes the atmosphere temperature of reactant. Variation of sensing film resistance was acquired by using data acquisition (DAQ) board (E6024 NI Co., USA.) as a function of injected gas concentration [14]. 3. Results 3.1. Characteristics of sensing materials Prepared materials were characterized by SEM (Hitachi Co., Japan) for surface morphology and thickness, XRD
(Rigaku Co., Japan) for crystallite size and BET for specific surface area. Surface morphologies of the materials were comparatively uniform [8–11]. Crystallite sizes of fabricated materials by ball-milling method (such as SnO2 –Al2 O3 , In2 O3 ) were from 25 to 45 nm and their specific surface areas were from 6 to 9 m2 /g [9]. But crystallite sizes and specific surface areas of made materials (such as SnO2 –Pt, SnO2 –ZnO, etc.) by impregnation and coprecipitation method were from 5 to 20 nm and from 20 to 30 m2 /g, respectively [7,9,10]. Crystallite sizes of fabricated materials by co-precipitation method were smaller than those of fabricated materials by ball-milling method. So, surface areas of the former were much larger than those of the latter.
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Fig. 3. Photographs of a sensor array: (a) sensing electrode, (b) heater, and (c) screened front side.
In the result of XRD analysis, large peaks of all fabricated materials coincide with SnO2 peaks of JCPDS card [7–10]. Added material peak was not shown because added amounts were very small.
For the CH3 CN and CH2 Cl2 , ZnO and ZrO2 showed better sensitivity. But, Al2 O3 and In2 O3 did not show good sensitivity. From the above results, four materials are selected for a sensor array. Four materials were same as 4 wt.%, Al2 O3 ; 2 wt.%, In2 O3 ; 2 wt.%, ZnO and 1 wt.%, ZrO2 .
3.2. Characteristics of gas response Response properties for four simulant gases are examined with sensing films with different operating temperatures and gas concentrations. The results of response characteristics of test gases are shown in Fig. 2. Gas concentration 0.5 ppm and operating temperature 300 ◦ C were fixed in the above experiments. Most samples showed high sensitivity more than 50% for the test gases. Repetition tests were good, only 3% difference in the results were observed. All data for the samples are not shown in the Fig. 2, which contains only significant data. Sensitivity definition is shown in Eq. (3). Metal and metal oxide-added samples gave much higher sensitivity than SnO2 alone. This indicates that the added materials accelerated the chemical reaction by acting as catalysts. For the DMMP and DPGME, four materials same as Al2 O3 , In2 O3 , ZnO, ZrO2 showed better sensitivity than other materials.
S(%) =
(Ra − Rg ) × 100 Ra
(3)
3.3. Gas classification using a sensor array Fig. 3 shows the photographs of a sensor array. Front side of a fabricated a sensor array has four sensing electrode pairs; back side of it has a heater with 10 ohm resistance. Overall sensor size is 10 mm (width) × 10 mm (length) × 0.25 (thickness) mm and sensing film size is 2 mm (width) × 2 mm (length). Overall consumption power is 2 W at 300 ◦ C operating temperature. Therefore, one sensor consumes about 500 mW because an array has four sensors. A sensor array was examined with different operating temperatures from 200 to 400 ◦ C, with different gas concentrations from 0 to 3 ppm.
Fig. 4. Sensitivity change of a sensor array at different operating temperatures: (a) 250 ◦ C, (b) 300 ◦ C, (c) 350 ◦ C, and (d) 400 ◦ C.
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Fig. 5. Classification figures of gases using the PCA method: (a) PCA for each gas, (b) PCA for four stimulant gases.
Sensitivity graphs at different operating temperatures are shown in Fig. 4. Operating temperature was changed from 250 to 400 ◦ C. Injection concentration was fixed at 0.5 ppm. Responses were changed with operating temperatures. PCA results are shown in Fig. 5. PCA is known as one of the statistical distribution methods. For example, if we assume four parameters, parameters are shown in four dimensions in the graph. That is very difficult and complicated. But PCA reduces two- or three-dimension by extracting significant data from various parameters [6]. Therefore, the graph by PCA method was shown very simple. PCA is adapted to classify the chemical agent gases. Fig. 5(a) shows classification among the various gases and Fig. 5(b) indicates that among the chemical gases. Test gases are four simulant gases and CH4 , C4 H10 , H2 , and CO that are toxic gases, which coexist with chemical agents. In Fig. 5, numbers such as 0.5, 3 mean gas concentrations are injected. And the principal com-
ponent (PC) value, 98.9, indicates importance of specific axis out of whole axis. In other words, assume if sum of whole axis importance was 100, 98.9 number means that X-axis occupies about 99. Although two points in the X-axis exist very near, two points can be classified as other class. As shown in Fig. 5, test gases can be easily classified by using a sensor array.
4. Conclusions A gas sensor array with devices based on tin oxide was fabricated. Sensing materials were analyzed by XRD, SEM and BET and response characteristics were examined for test gases. Metal and metal oxide-added samples gave much higher sensitivity than the SnO2 alone. Finally, a sensor array by using the four selected materials was fabricated and PCA
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was adapted to classify objective gas. A sensor array gave good performance by using the thick film and could be easily classified by using the PCA method.
Acknowledgment The authors acknowledge the financial support of the Confrontation to Chemical and Biological Terror and National Research Laboratory Program of the Ministry of Science & Technology.
References [1] J.C. Lee, Chemical weapons destruction technology (I), J. Korea Solid Wastes Eng. Soc. 16 (3) (1999) 205–216. [2] D.S. Lee, D.D. Lee, NO2 sensing properties of WO3 based thin film gas sensors, J. Korean Phys. Soc. 35 (94) (1999) 1092–1096. [3] N. Yamazoe, N. Miura, Environmental gas sensing, Sens. Actuators B 20 (2–3) (1994) 95–102. [4] J.W. Gardner, Electronic Noses: Principles & Applications, Oxford University Press, New York, 1999, pp. 154–156. [5] W.R. Dillon, M. Goldstein, Multivariate Analysis Method and Applications, Wiley & Sons, New York, 1984. [6] N.J. Choi, C.H. Shim, K.D. Song, B.S. Joo, J.K. Jung, O.S. Kwon, Y.S. Kim, D.D. Lee, Recognition of indoor environmental gases using two sensing films on a substrate, J. Korean Phys. Soc. 41 (6) (2002) 1058–1062. [7] W.S. Lee, J.J. Park, H.K. Jun, S.C. Lee, N.J. Choi, D.D. Lee, W.W. Baek, J.S. Huh, J.C. Kim, A study on the tin oxide-based semiconductor gas sensors for the detection of chemical agent simulants, in: Proceedings of KIChE Meeting, KwangJu, Korea, October 24–25, 2003, p. 129. [8] N.J. Choi, T.H. Ban, J.H. Kwak, W.W. Baek, J.C. Kim, J.S. Huh, D.D. Lee, Fabrication of DMMP thick film gas sensor based on SnO2 , J. Korean Inst. Electrical Electron. Mater. Eng. 16 (12S) (2003) 1217–1223. [9] W.W. Baek, K.Y. Yun, S.T. Lee, N.J. Choi, J.C. Kim, D.D. Lee, J.S. Huh, Characteristics of Zn doped SnO2 based gas sensor in di(propylene glycol) methyl ether(DPGME), in: Proceedings of Conference J. Korea Institute of Military Science and Technology 6, Seoul, Korea, August 26, 2003, pp. 489–492. [10] K.Y. Yun, W.W. Baek, S.T. Lee, N.J. Choi, D.D. Lee, J.C. Kim, J.S. Huh, Gas sensing properties of coprecipitated SnO2 with ZrO2 , in: Proceedings of Conference J. Korea Institute of Military Science and Technology 6, Seoul, Korea, August 26, 2003, pp. 497–500. [11] M.K. Park, S.G. Ryu, H.B. Park, H.W. Lee, G.W. Lee, Modeling for decomposition of CH3 CN in plate type DBD reactor, in: Proceedings of Conference J. Korea Institute of Military Science and Technology 6, Seoul, Korea, August 26, 2003, pp. 819–822, August. [12] NIST Chemistry WebBook, 2003. [13] R.I. Hegde, C.M. Greenlief, J.M. White, J. Phys. Chem 89 (1985) 2886. [14] D.Y. Kwak, LabVIEWTM Control of Computer Based and Measurement Solution, Ohm, Seoul, 2002.
Biographies Nak-Jin Choi received the B Eng degree from Pukoung National University, Korea in 1996 and the master of engineering degree from Kyungpook National University in 1998. Currently, he is enrolled in a doctoral course at the School of Electrical Engineering & Computer Science in Kyungpook National University. His research interests include electronic nose and implementation of a system using microprocessor. Jun-Hyuk Kwak received the B Eng degree from Kyungwoon University, Korea in 2003. Currently, he is enrolled in a master course at the School of Electrical Engineering & Computer Science in Kyungpook National University. His research interests include fabrication of micro gas sensor and a system using microprocessor. Yeon-Tae Lim received the B Eng degree from Yeungnam University, Korea in 2004. Currently, he is enrolled in a master course at the School of Electrical Engineering & Computer Science in Kyungpook National University. His research interests include electronic nose of thick film gas sensor array. Tae-Hyun Bahn received the B Eng degree from Catholic University of Daegu, Korea in 2001. Currently, he is enrolled in a master course at the Department of Sensor Engineering in Kyungpook National University. His research interests include fabrication of thin film semiconductor gas sensor using the Sn, In, Pt, etc. Ky-Yeol Yun received the B Eng degree from Kyungpook National University, Korea in 2003. Currently, he is enrolled in a master course at the School of Electrical Engineering & Computer Science in same university. His research interests include fabrication of thick film gas sensor using ZrO2 , ZnO. Jae-Chang Kim received his MS (1984) and PhD (1998) degree in chemical engineering from Korea Advanced Institute of Science and Technology KAIST). In 1988, he joined the Research Institute of Industrial Science and Technology (RIST) at Pohang. Since 1994, he has been a member of faculty in department of chemical engineering, Kyungpook National University. His current fields of interest are applied catalysis in fine chemistry, hydrocarbon and NOx sensor and H2 S removal. Jeung-Soo Huh received his BS degree and MS degree in Metallurgical in 1983 and 1985 at the Seoul National University, respectively. He received PhD from the Department of Materials Science and Engineering of the Massachusetts Institute of Technology in USA. He is currently a professor at the Metallurgical Engineering, Kyungpook National University, Daegu, Korea. His current research interests are metalorganic chemical vapor deposition and organic thin film gas sensor. Duk-Dong Lee was born in Taegu, Korea on December 21, 1942. He received a BS degree in physics and ME degree in electronics from Kyungpook National University, Daegu, Korea, in 1966 and 1974, respectively, and a PhD degree from Yon-Sei University, Seoul, Korea, in 1984. He is currently a professor at the School of Electrical Engineering & Computer Science, Kyungpook National University, Daegu, Korea. He has performed research on semiconductor gas sensors since 1978, and also performed a research in the field of humidity and optical sensors.
Classification of chemical warfare agents using thick film gas sensor array Nak-Jin Choi a , Jun-Hyuk Kwak a , Yeon-Tae Lim a , Tae-Hyun Bahn b , Ky-Yeol Yun c , Jae-Chang Kim a , Jeung-Soo Huh d , Duk-Dong Lee a,∗ a
d
School of Electronics and Computer Science, Kyungpook National University (KNU), Daegu, Republic of Korea b Department of Sensor Engineering, KNU, 1370 San-Kyuk Dong, Puk-Gu, Daegu, Republic of Korea c Department of Chemical Engineering, KNU, 1370 San-Kyuk Dong, Puk-Gu, Daegu, Republic of Korea Department of Materials Science and Metallurgy, KNU, 1370 San-Kyuk Dong, Puk-Gu, Daegu, Republic of Korea Received 12 July 2004; received in revised form 6 November 2004; accepted 8 November 2004 Available online 23 March 2005
Abstract Semiconductor thick film gas sensors based on tin oxide are fabricated and their gas response characteristics are examined for four simulant gases of chemical warfare agent (CWA)s. The sensing materials are prepared in three different sets such as impregnation, physical mixing (ballmilling) and co-precipitation method. Surface morphology, particle size, and specific surface area of fabricated sensing films are performed by the SEM, XRD and BET, respectively. Response characteristics are examined for test gases with temperature in the range 200–400 ◦ C, with different gas concentrations. Test gases are dimethyl methyl phosphonate (DMMP), dipropylene glycol methyl ether (DPGME), acetonitrile, and dichloromethane which are used simulant gases of chemical warfare agents. These sensors showed sensitivities higher than 50% in 500 ppb of test gases and also good repetition behaviour. Four sensing materials are selected with good sensitivity and stability and are fabricated as a sensor array. And then, principal component analysis (PCA) is adapted to classify objective gas among the four simulant gases. The thick film array sensor shows high sensitivity to CWA gases and four CWA gases are classified by using a sensor array composed of four sensing devices through PCA. © 2004 Elsevier B.V. All rights reserved. Keywords: SnO2 ; Chemical warfare agents; Array; Principal component analysis
1. Introduction Chemical warfare agents (CWAs) are very dangerous for health because of its colorlessness and toxicity. Therefore, fast and correct detection of CWAs is essential to protect human beings. They are divided into four types such as blood, nerve, vesicant, and choking agent [1]. In general, semiconductor gas sensor has many advantages like high sensitivity at low level concentration, good stability against environmental change and low price [2,3]. Many semiconductor sensors have selectivity problem because they show similar response to various gases. So, it is difficult to ∗
Corresponding author. Tel.: +82 53 950 5514; fax: +82 53 939 1073. E-mail address: [email protected] (D.-D. Lee).
0925-4005/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2004.11.022
identity the objective gas. Nowadays, there are many studies to classify various gases by means of adding catalysts, using the sensor array or combining two methods, etc. [2–4]. To add catalysts takes much time to examine optimal catalyst for specific material, because of the existence of a large number of catalysts. But to use sensor array requires smaller time than that, because it can be easily classified by using the array pattern such as neural network and statistical method [5,6]. In this study, semiconductor thick film sensors based on tin oxide are fabricated and their responses to four simulant gases are examined with different amounts of added materials, with different operating temperatures and with different gas concentrations. Materials are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and specific surface area analyzer (BET) analysis. From the above
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Table 1 Amounts of added materials and material synthesis method
Table 2 Measurement simulant gases [1,12]
Added material
Added amounts (wt.%)
Synthesis method
Agent
Chemical gas
Simulant gas
Al2 O3 In2 O3 Pt, Pd ZnO ZrO2
0, 4, 12, 20 1, 2, 3 0.1, 0.5, 1 1, 2, 3, 4, 5 1, 3, 5
Ball-milling Ball-milling Impregnation Co-precipitation Co-precipitation
Blood Nerve
HCN Tabun
Choking Vesicant
CE HN
Acetonitrile DMMP (dimethyl methyl phosphonate) Dichloromethane DPGME (dipropylene glycolmethyl ether)
results, four different sensing materials are selected which have good sensitivity and stability to gases and fabricated as a sensor array. A sensor array was examined to test gases with different operating temperatures and with different gas concentrations, and pattern was extracted from acquired data. Finally, principal component analysis (PCA) that was one of the statistical classification method was conducted to classify four test gases [5].
2. Experiments 2.1. Preparation of sensing materials Table 1 shows gas sensing materials used in this study. Base material of all samples is SnO2 . The sensing materials are prepared in three different sets. (1) The Pt or Pd (0.1, 0.51 wt.%) as catalyst was impregnated in the base material of SnO2 by impregnation method [7]. (2) Al2 O3 (0, 4, 12, 20 wt.%) or In2 O3 (1, 2, 3 wt.%) was added to SnO2 by physical ball milling process [8]. (3) ZnO (1, 2, 3, 4, 5 wt.%) or ZrO2 (1, 3, 5 wt.%) was added to SnO2 by co-precipitation method [10,11]. All samples were dried at 110 ◦ C for 1 h, followed by grinding and calcination at 600 ◦ C for 1 h in the furnace. These powders were further sintered at 700 ◦ C for 1 h and deposited on alumina substrate by screen printing technique.
CE and HN mean phosgene and nitrogen mustard respectively [1].
2.2. Fabrication of device ˚ thickness Pt sensing electrode is deposited with 1000 A by using the DC sputter at the front side of alumina substrate and electrode for heater was screen printed using the Pt paste at the back side. Then device is thermally annealed at 850 ◦ C for 10 min in the electric furnace. The heater resistance is 10 ohms and sensing electrode is designed as interdigitated (IDT) structure. All sensors were tested after stabilizing time at 400 ◦ C for 3 days. Total dimension of device is 7 mm × 10 mm × 0.6 mm and size of sensing film is 6 mm × 4 mm. 2.3. Apparatus and measurement method Table 2 shows measurement gases. Typical chemical agents are shown in Table 2 [1]. To use the real chemical gases is very dangerous and difficult, so simulant gases with similar chemical characteristics are used as test gases in safety. Four simulant gases are liquid phase; therefore; vapor pressures are calculated to control the injected gas concentration. Acetonitrile and dichloromethane adapt Antoine equation as shown in Eq. (1) and DMMP uses Eq. (2). In the case of DPGME, vapor pressures at specific temperature were used
Fig. 1. Measurement apparatus.
300
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0.3 and 0.05 mmHg at 20 and 25 ◦ C, respectively [11–13]. log10 P = A −
B T +C
(1)
P = 2.844 × 108 × exp
−11500 RT
(2)
where P (mmHg) means vapor pressure, constant A–C are specific values of each chemical. And T (K) means temperature of circulator in Fig. 1. As shown in Eqs. (1) and (2), gas concentration can be controlled by using the change of the circulator temperature.
Table 3 Values of used parameters in Eq. (1)
T range A B C
(◦ C)
Acetonitrile
Dichloromethane
7–23 5.93296 2345.829 43.815
−40–40 4.53691 1327.016 −20.474
Table 3 shows specific values in the Antoine equation. T range means that this equation can be used in the range of the temperature. Fig. 1 shows experimental measurement apparatus. Gas amounts were controlled by using mass flow controllers. Test gas concentration in the chamber can be con-
Fig. 2. Response results for four simulant gases: (a) DMMP, (b) DPGME, (c) CH3 CN, (d) CH2 Cl2 .
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301
Fig. 2. (Continued ).
trolled by air amounts for reference gas and vapor amounts of test reactant. Total mass flow was fixed (1000 ml/min). Circulator fixes the atmosphere temperature of reactant. Variation of sensing film resistance was acquired by using data acquisition (DAQ) board (E6024 NI Co., USA.) as a function of injected gas concentration [14]. 3. Results 3.1. Characteristics of sensing materials Prepared materials were characterized by SEM (Hitachi Co., Japan) for surface morphology and thickness, XRD
(Rigaku Co., Japan) for crystallite size and BET for specific surface area. Surface morphologies of the materials were comparatively uniform [8–11]. Crystallite sizes of fabricated materials by ball-milling method (such as SnO2 –Al2 O3 , In2 O3 ) were from 25 to 45 nm and their specific surface areas were from 6 to 9 m2 /g [9]. But crystallite sizes and specific surface areas of made materials (such as SnO2 –Pt, SnO2 –ZnO, etc.) by impregnation and coprecipitation method were from 5 to 20 nm and from 20 to 30 m2 /g, respectively [7,9,10]. Crystallite sizes of fabricated materials by co-precipitation method were smaller than those of fabricated materials by ball-milling method. So, surface areas of the former were much larger than those of the latter.
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Fig. 3. Photographs of a sensor array: (a) sensing electrode, (b) heater, and (c) screened front side.
In the result of XRD analysis, large peaks of all fabricated materials coincide with SnO2 peaks of JCPDS card [7–10]. Added material peak was not shown because added amounts were very small.
For the CH3 CN and CH2 Cl2 , ZnO and ZrO2 showed better sensitivity. But, Al2 O3 and In2 O3 did not show good sensitivity. From the above results, four materials are selected for a sensor array. Four materials were same as 4 wt.%, Al2 O3 ; 2 wt.%, In2 O3 ; 2 wt.%, ZnO and 1 wt.%, ZrO2 .
3.2. Characteristics of gas response Response properties for four simulant gases are examined with sensing films with different operating temperatures and gas concentrations. The results of response characteristics of test gases are shown in Fig. 2. Gas concentration 0.5 ppm and operating temperature 300 ◦ C were fixed in the above experiments. Most samples showed high sensitivity more than 50% for the test gases. Repetition tests were good, only 3% difference in the results were observed. All data for the samples are not shown in the Fig. 2, which contains only significant data. Sensitivity definition is shown in Eq. (3). Metal and metal oxide-added samples gave much higher sensitivity than SnO2 alone. This indicates that the added materials accelerated the chemical reaction by acting as catalysts. For the DMMP and DPGME, four materials same as Al2 O3 , In2 O3 , ZnO, ZrO2 showed better sensitivity than other materials.
S(%) =
(Ra − Rg ) × 100 Ra
(3)
3.3. Gas classification using a sensor array Fig. 3 shows the photographs of a sensor array. Front side of a fabricated a sensor array has four sensing electrode pairs; back side of it has a heater with 10 ohm resistance. Overall sensor size is 10 mm (width) × 10 mm (length) × 0.25 (thickness) mm and sensing film size is 2 mm (width) × 2 mm (length). Overall consumption power is 2 W at 300 ◦ C operating temperature. Therefore, one sensor consumes about 500 mW because an array has four sensors. A sensor array was examined with different operating temperatures from 200 to 400 ◦ C, with different gas concentrations from 0 to 3 ppm.
Fig. 4. Sensitivity change of a sensor array at different operating temperatures: (a) 250 ◦ C, (b) 300 ◦ C, (c) 350 ◦ C, and (d) 400 ◦ C.
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Fig. 5. Classification figures of gases using the PCA method: (a) PCA for each gas, (b) PCA for four stimulant gases.
Sensitivity graphs at different operating temperatures are shown in Fig. 4. Operating temperature was changed from 250 to 400 ◦ C. Injection concentration was fixed at 0.5 ppm. Responses were changed with operating temperatures. PCA results are shown in Fig. 5. PCA is known as one of the statistical distribution methods. For example, if we assume four parameters, parameters are shown in four dimensions in the graph. That is very difficult and complicated. But PCA reduces two- or three-dimension by extracting significant data from various parameters [6]. Therefore, the graph by PCA method was shown very simple. PCA is adapted to classify the chemical agent gases. Fig. 5(a) shows classification among the various gases and Fig. 5(b) indicates that among the chemical gases. Test gases are four simulant gases and CH4 , C4 H10 , H2 , and CO that are toxic gases, which coexist with chemical agents. In Fig. 5, numbers such as 0.5, 3 mean gas concentrations are injected. And the principal com-
ponent (PC) value, 98.9, indicates importance of specific axis out of whole axis. In other words, assume if sum of whole axis importance was 100, 98.9 number means that X-axis occupies about 99. Although two points in the X-axis exist very near, two points can be classified as other class. As shown in Fig. 5, test gases can be easily classified by using a sensor array.
4. Conclusions A gas sensor array with devices based on tin oxide was fabricated. Sensing materials were analyzed by XRD, SEM and BET and response characteristics were examined for test gases. Metal and metal oxide-added samples gave much higher sensitivity than the SnO2 alone. Finally, a sensor array by using the four selected materials was fabricated and PCA
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was adapted to classify objective gas. A sensor array gave good performance by using the thick film and could be easily classified by using the PCA method.
Acknowledgment The authors acknowledge the financial support of the Confrontation to Chemical and Biological Terror and National Research Laboratory Program of the Ministry of Science & Technology.
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Biographies Nak-Jin Choi received the B Eng degree from Pukoung National University, Korea in 1996 and the master of engineering degree from Kyungpook National University in 1998. Currently, he is enrolled in a doctoral course at the School of Electrical Engineering & Computer Science in Kyungpook National University. His research interests include electronic nose and implementation of a system using microprocessor. Jun-Hyuk Kwak received the B Eng degree from Kyungwoon University, Korea in 2003. Currently, he is enrolled in a master course at the School of Electrical Engineering & Computer Science in Kyungpook National University. His research interests include fabrication of micro gas sensor and a system using microprocessor. Yeon-Tae Lim received the B Eng degree from Yeungnam University, Korea in 2004. Currently, he is enrolled in a master course at the School of Electrical Engineering & Computer Science in Kyungpook National University. His research interests include electronic nose of thick film gas sensor array. Tae-Hyun Bahn received the B Eng degree from Catholic University of Daegu, Korea in 2001. Currently, he is enrolled in a master course at the Department of Sensor Engineering in Kyungpook National University. His research interests include fabrication of thin film semiconductor gas sensor using the Sn, In, Pt, etc. Ky-Yeol Yun received the B Eng degree from Kyungpook National University, Korea in 2003. Currently, he is enrolled in a master course at the School of Electrical Engineering & Computer Science in same university. His research interests include fabrication of thick film gas sensor using ZrO2 , ZnO. Jae-Chang Kim received his MS (1984) and PhD (1998) degree in chemical engineering from Korea Advanced Institute of Science and Technology KAIST). In 1988, he joined the Research Institute of Industrial Science and Technology (RIST) at Pohang. Since 1994, he has been a member of faculty in department of chemical engineering, Kyungpook National University. His current fields of interest are applied catalysis in fine chemistry, hydrocarbon and NOx sensor and H2 S removal. Jeung-Soo Huh received his BS degree and MS degree in Metallurgical in 1983 and 1985 at the Seoul National University, respectively. He received PhD from the Department of Materials Science and Engineering of the Massachusetts Institute of Technology in USA. He is currently a professor at the Metallurgical Engineering, Kyungpook National University, Daegu, Korea. His current research interests are metalorganic chemical vapor deposition and organic thin film gas sensor. Duk-Dong Lee was born in Taegu, Korea on December 21, 1942. He received a BS degree in physics and ME degree in electronics from Kyungpook National University, Daegu, Korea, in 1966 and 1974, respectively, and a PhD degree from Yon-Sei University, Seoul, Korea, in 1984. He is currently a professor at the School of Electrical Engineering & Computer Science, Kyungpook National University, Daegu, Korea. He has performed research on semiconductor gas sensors since 1978, and also performed a research in the field of humidity and optical sensors.