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Perovskite oxide of PTCR ceramics as chemical sensors
Sensors and Actuators B 77 (2001) 22±26
Perovskite oxide of PTCR ceramics as chemical sensors Z.-G. Zhoua,*, Z.-L. Tanga, Z.-T. Zhanga, W. Wlodarskib a
State Key Laboratory of Fine Ceramics and New Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China b Department of Communication and Electronic Engineering, RMIT, GPO Box 2476V, Melbourne, Vic. 3001, Australia
Abstract A new CO gas sensor of perovskite-type oxide, semiconducting La-doped BaTiO3 PTCR ceramics was studied. It is capable of detecting P carbon monoxide gas in the concentration range of 1.0% in the high temperature NTCR region near and above the Tmax air (175±4008C) by P means of higher barrier potential of PTCR effect. However, there is only a little resistivity change below the Tmax air but above the Tc. The phenomenon is based on anionic adsorption, the extreme activity of the oxygen atoms at the grain boundaries, in the higher temperature P near and above the Tmax air The lattice oxygen in the surface layer of the PTCR ceramics participates in the sensing reaction, and the itinerant electron come from the conduction band of the n-type semiconducting ceramics bring about to a decrease in resistivity of the material. A rate of resistivity change of more than one decade on switching the atmosphere from air to 1.0% in low ¯ow rate was observed. We conclude that PTCR ceramics can be used as a new potential CO gas sensor, simple structure, middle/lower operating temperature and low cost, in ceramic chemical sensors development. # 2001 Elsevier Science B.V. All rights reserved. Keywords: CO gas sensor; BaTiO3; PTC effect; Perovskite oxide; Adsorption; Oxygen vacancy
1. Introduction It is well known that a number of perovskite oxides (ABO3) were used as gas sensor materials because of the stability in thermal and chemical atmospheres. So, over the last decade, the perovskite oxide ceramics has been created and promoted interest in new chemical sensors. The gas sensing perovskite oxide ceramics could be classi®ed into two groups: the semiconducting ceramics and electrochemical ceramics. And now the semiconducting perovskite oxide ceramics were widely used in the detection of reducing gases (such as CO [1±3], H2 [4], C2H5OH [5]), oxygenic gases (such as CO2 [6,7], NOx [8,9], O2 [10±12], CH3OH [13], CH4 [14]), offensive odorous gases (such as NH3 [15], H2S [20]), easy burning and explosion gases (such as C2H2 [16], C2H4 [16], C3H6 [16], C3H8 [14] and LPG [17]), and toxic gases (such as CO [1±3], H2S [18], Cl2 [18], NO2 [18]), because of low cost and high stability in thermal and atmospheres. Among these toxic gases, CO is the most interesting. It is caused by highly carbon monoxide gas can damage the human body. So, the metal oxide semiconductors as n-type SnO2 [19], ZnO2 [20], In2O3 [21], TiO2 [32], a-Fe2 O3 [31],
*
Corresponding author. Fax: 86-10-6277-1160. E-mail address: [email protected] (Z.-G. Zhou).
HfO2 [33], BaSnO3 [22], and p-type as (Ln M) BO3 [23] have been studied for a long time for detecting CO gas. Recently, for CO sensor development, a few of the researchers now are working on decreasing of working temperature of SnO2 based materials by means of promotion of the activity of CO reaction on the surface [24,25]. And a few of them are working on low price CO gas sensor by means of new application of doped BaTiO3 semiconductor with posistor effect [26]. As mentioned above, the perovskite-type semiconducting ceramics based on La doped BaTiO3 with PTCR effect was used for CO detection. The PTCR ceramics are widely used in industries and civilian as a control device. Now the PTCR ceramics have developed to be a new multifunctional semiconducting ferroelectric ceramics [27]. In this paper, the nature is revealed on CO gas response of PTCR ceramics in the low/middle temperature. The mechanism and the relation between the PTC effect and oxygen adsorption, the adsorption and CO sensitivity, and further work were described. 2. Experimental The n-type semiconducting BaTiO3 PTCR ceramics such as Ba0.92 La0.08 TiO3 was prepared by conventional ceramic semiconducting technology. The AP grade powders of
0925-4005/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 1 ) 0 0 6 6 7 - 0
Z.-G. Zhou et al. / Sensors and Actuators B 77 (2001) 22±26
commercial barium titanyl oxalates (i.e. BaTiO (C2O4)4H2O) as a starting material, La2O3, and MnO2 as donor and acceptor dopants. The starting materials were mixed with suitable properties, after wet-milled, dried and calcined, followed by pressing into pellets of 10 mm diameter and 1 mm thickness at pressure. The specimens were sintered at varied temperatures from 1050 to 12508C in air with soaking time of 0.5±2 h. This yielded sintered bodies with porosity of 15%. The resistivity measurements were conducted on a ceramic pellet with an In±Ga alloy rubbed on both end surfaces as electrodes. This two-probe method was used as in a dc bias of 10 V in varied concentration of CO gas in air, of course, air was used as a carrier gas with ¯ow rate of 30 ml min 1 during working. The measurements were performed on all samples, in the tubular ¯ow quartz-glass chamber with thermocouple, ®rst in air and then successively under the reducing CO gas. Switching the atmosphere from air to the 1.0 vol.% CO gas and return to air was conducted at appropriate temperature. The measurements were performed at 3008C in the 1.0 vol.% concentration of CO gas. The temperature was monitored and the resistance was measured automatically every 30 s. The sensitivity was de®ned by S Rgas Rair = Rair where Rgas and Rair are resistance in 1 vol.% CO and air, respectively.
23
Fig. 2. Sensitivity±temperature relations for concentrations of 1.0% CO gas and N2 in PTCR ceramics.
For easy of presentation, to evaluate the CO gas sensitivity of the PTCR ceramics, the ratio of the resistivity of Fig. 1 was taken as a function of temperature for respective samples, and the sensitivity are shown in Fig. 2. This ®gure indicates that the sensitivity increase with temperature increase to 1.0% CO gas and the sensitivities are 10, 20, 40, 70, 90 and 97% while the temperatures are 175, 185, 200, 225, 250 and 3008C, respectively.
3. Results and discussion
3.2. Response characteristics
3.1. Temperature dependence of the CO gas sensitivity Fig. 1 shows the resistivity±temperature characteristics obtained in air and in CO gas for samples. In this ®gure, remarkable decrease in the resistivity near the temperature P giving the maximum resistivity, Tmax air , are evidently, but whereas just a little changes in the resistivity±temperature P characteristics near the Tmax co were observed for all the samples.
The response characteristics of resistivity to CO gas is another important property to be examined for this PTCR ceramics being considered as a CO gas sensor. From this viewpoint, responses to reducing ambient gas containing 1.0 vol.% concentration of CO gas have been obtained for sample at 3008C. In Fig. 3, the response characteristic to N2 gas is also no changed. The data of Fig. 3 reveal that the resistivity changes on switching the ambient gas from air to the CO gas or N2 gas,
Fig. 1. Resistivity±temperature characteristics obtained in air and CO gas for PTCR ceramics.
Fig. 3. Response characteristics of resistivity to reducing gases containing concentration of 1.0% CO and N2 at 3008C for PTCR ceramics.
24
Z.-G. Zhou et al. / Sensors and Actuators B 77 (2001) 22±26
vice versa, were reversible. The resistivity of the sample exhibits a gas sensitivity of decrease fast with an initial response rate of one decades per 10 min in low ¯ow rate of 30 ml min 1, but decrease slow in last. 3.3. Response mechanism It is well known that while La is adopted as a dopant, the La ions will occupy Ba site and behave as donors owing to the higher valence than Ba. As the La dopant concentration is at a low level, the semiconducting behavior is explained by the following impurity incorporation equation [27] in Kroger±Vink notation: 2La2 O3 4TiO2 ) 4LaBa 4TixTi 12OxO O2 4e (1) Since the resistivity of the PTCR ceramics near and above P Tmax air are con®rmed to be determined by the potential height of the blocking layers formed at the grain boundaries. Any way, no evidence for direct reaction of CO with bare cation sites has been obtained in this study. So, however, the decrease in resistivity of the ceramics seems to be attributed to the lowering of the potential height of the grain boundary blocking layers. It occurs after removal of oxygen atoms adsorbed on the grain surfaces by their reactions with CO molecules (Fig. 4). They are of the following types [28±32]. For oxygen adsorption: Oads CO ) CO2 e
(2)
COg ) COads ) COads e
(3)
COads O2 ) CO2 e
(4)
Forming and reaction with a surface vacancy: COg OxO ) CO2 e VxO CO
1 2 O2
VxO
e ) CO2
(5) OxO
(6)
were Oads and COads are the possible species of oxygen and carbon monoxide adsorbed on the grain surface of PTCR ceramics, e is an itinerant electron, OxO and VxO are the
Fig. 5. Schematic outline of the absorption possibilities of CO by a lattice oxygen ion.
lattice oxygen and oxygen vacancy in lattice of the ceramics, respectively. Eqs. (2)±(4) are the oxygen adsorption process P near but below the Tmax air and Eqs. (5)±(6) are the lattice oxygen reaction and forming a surface vacancy of oxygen P above the Tmax air . For the process of adsorption, vacancies to migrate from the bulk to the surface, are based on the forming of surface oxygen vacancies in the grain boundaries of the PTCR ceramics. A possible mechanism of oxidation of CO by lattice oxygen ions is as shown in Fig. 5. The anion oxygen can act as adsorption center for the CO molecules. In ®ne, it is of interest to note that the PTCR ceramics place in the atmosphere of CO gas, the CO molecule react on chemisorbed oxygen from the grain boundary of the semiconducting ceramics, free electrons as a driving force for this kind of CO gas sensor, it is caused by the transmission of the electron carriers in the conduction band, contribute to a decrease in resistivity of a material. 4. Conclusion
Fig. 4. A model of reaction processes for CO gas absorbed on the surface of grain boundary of PTCR ceramics.
The perovskite, n-type semiconducting PTCR ceramics, is a new perspective CO gas sensing material with their high temperature NTCR region. However, just a little change in P resistivity was observed at temperature below Tmax air but above Tc point. The sensing mechanism for the adsorption between the CO gas and PTCR ceramics are based on the higher potential barrier in grain boundaries of the semiconducting ceramics.
Z.-G. Zhou et al. / Sensors and Actuators B 77 (2001) 22±26
The lattice oxygen in the surface layer of the grain boundaries is thought to be involved in the sensing reaction. As a result, the surface resistivity decreases with decreasing potential barrier, is caused by increased density of electron in the conduction band of the semiconducting ceramics. More generally, much work remains to be done in improving the properties of the sensor in the application level, such as increase the sensitivity and decrease the operating temperature, etc. Any way, it is expected that this new low/ middle operating temperature, simple structure and low cost perovskite oxide CO gas sensor will be appeared in the chemical sensor community soon. Acknowledgements We are grateful to the ®nancial support of the National Natural Science Foundation of China (NSFC) (Contract no. 59672012). We also thank many colleagues of the State Key Laboratory of New Ceramics and Fine Processing, Department of Materials science and Engineering of the Tsinghua University for their help. References [1] M. Kuwabara, T. Ide, Co gas sensitivity in porous semiconducting barium titanate, Am. Ceramic Soc. Bull. 66 (1987) 1401±1405. [2] C.M. Chiu, Y.H. Chang, The structure, electrical and sensing properties for CO of the La0.8Sr0.2CO1 xNixO3 d system, Mater. Sci. Eng. A 266 (1999) 93±98. [3] R. Sorita, T. Kawano, A highly selective CO sensor: screening of electrode materials, Sens. Actuators B 35/36 (1996) 274±277. [4] T. Inaba, K. Saji, H. Takahashi, Limiting current-type gas sensors using a high temperature-type conductor thin film, Electrochemistry 67 (1999) 458±462. [5] L.B. Kong, Y.S. Shen, Gas sensing property and mechanism of CaxLa1 xFeO3 ceramics, Sens. Actuators B 30 (1996) 217±221. [6] T. Ishihara, Y. Nishi, H. Nishiguchi, Y. Takita, Detection mechanism of CuO±BaTiO3 capacitive type CO sensor, in: Proceedings of Ceramic Sensors III, San Antonio, TX, USA, 6±11 October 1997. [7] Y.C. Zhang, H. Tagawa, S. Asakuwa, J. Mizusaki, H. Narita, Solid state electrochemical CO2 sensors by coupling lithium ion conductor (Li2CO3±Li3PO4±Al2O3) with oxide ion-electron mixed conductor (La0.9Sr0.1MnO3), Solid State Ionics 100 (1997) 275±281. [8] G. Martirelli, M.C. Carotta, M. Ferroni, Y. Sadaoka, E. Traversa, Screen-printed perovskite-type thick films as gas sensors for environmental monitoring, Sens. Actuators B 55 (1999) 99±110. [9] H. Yamaura, J. Tamaki, N. Miura, N. Yamazoe, NOx sensing properties of metal titanate based semiconductor sensor at elevated temperature, Engineering Science Reports, Vol. 17, 1995, Kyushu University, Japan, pp. 341±346. [10] Y. Noguchi, H. Kuroiwa, M. Takata, Sensing properties of oxygen sensor using hot sport on La0.8Sr0.8Co0.8Fe0.2O3 ceramic rod, Key Eng. Mater. 160±170 (1999) 79±82. [11] J.P. Lukaszewicz, N. Miura, N. Yamazoe, A LaF3 based oxygen sensor with perovskite-type oxide electrode operative at room temperature, Sens. Actuators B 1 (1990) 195±198. [12] P. Shuk, V. kharton, L. Tichonova, H.D. Wiemhofer, U. Guth, W. Gopel, Electrodes for oxygen sensors based on rare earth manganites or cobaltite, Sens. Actuators B 16 (1993) 401±405.
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[13] T. Arakawa, Perovskite oxides as solid state chemical sensors, in: L.G. Tejuca, J.L. Fierro (Ed.), Properties and Applications of Perovskite-type Oxide, Marcel Dekker, New York, 1993, Chapter 18, pp. 361±377. [14] T.R. Ling, Z.B. Chen, M.D. Lee, Studies on catalytic and conductivity properties of LaNiO3 for oxidation of C2H5OH, CH3CHO, CH4, Catalysis Today 26 (1995) 79±86. [15] E. Connoily, V. Casey, J.B. McMonagle, A new thick-film ammonia gas sensors, in: Proceedings of the 12th European Conference on Solid-state Transducers and the 9th UK Conference on Sensors and Their Applications, Bristal, UK, 1998, pp. 495±500. [16] Z.-G. Zhou, G. Zhao, M. Wei, Z.T. Zhang, Temperature±humidity± gas multifunctional sensitive ceramics, Sens. Actuators 19 (1989) 497±500. [17] C.V.G. Reddy, S.V. Manorama, V.T. Rao, A. Lobo, S.K. Sulkarni, Noble metal additive modulation of gas, Thin Solid Films 384 (1999) 262±265. [18] C. Fushen, L. Yungi, X. Yu, L. Qu, Effects of Langmuir±Blodgettfilm gas sensors with integrate optical interferometers, Opt. Lett. 21 (1996) 1700±1702. [19] N. Yamazoe, Y. Kurokawa, T. Seiyama, Effects of additives on semiconductor gas sensors, Sens. Actuators 4 (1983) 283±289. [20] H.-X. Zhang, A. Kishimoto, O. Okada, H. Yamagida, Enhanced CO and CO2 gas sensitively of the CuO/ZnO heterol contact made by quenched CuO ceramics, J. Electrochem. Soc. 145 (1998) 632±638. [21] N. Yamazoe, H. Yamaura, N. Miura, Development of semiconductor sensors highly selective to carbon monoxide, in: Proceedings of Ceramic Sensor III, San Antonio, TX, USA, 6±11 October 1997. [22] U. Lampe, J. Gerblinger, H. Meixner, Carbon monoxide sensors based on thin films of BaSnO3, Sens. Actuators B 25 (1995) 657±660. [23] C.M. Chiu, Y.H. Chang, The structure, electrical and sensing properties for CO of the La0.8Sr0.2Co1 xNixO3 d system, Mater. Sci. Eng. A 266 (1999) 93±98. [24] C.K. Ong, W.T. Chong, W.H. Foon, H.C. Chua, S. Birlaseharen, Ambient temperature dependent heating power control for Motorola micro CO gas sensor, Int. J. Electron. 84 (1998) 675±684. [25] V. Ambrazeviciene, A. Galdikas, A. Mironas, A. Setkus, Cu-dopinglevel-controlled sensitivity and selectivity of tin-oxide based thin film, Sens. Mater. 7 (1995) 91±97. [26] L.A. Bashkirov, N.Y. Shishkin, O.A. Chebotar, I.M. Zharsky, Perspectives of phase-transition based sensors and the that yttriumdoped BaTiO3 semiconductor with posistor effect, Sens. Actuators B 55 (1999) 65±69. [27] X.-F. Tang, H.-L. Chen, J. Lou, Z.-G. Zhou, Low temperature sintering of PTCR ceramics, in: Jingkun Guo, Xi Yao (Eds.), The Relationship on Properties, Processing and Microstructure odd Advanced Ceramics, Shanghai Scientific and Technology Literature Publishing House, China, 1999, pp. 94±97. [28] W. Gopel, Chemisorption and charge transfer at ionic semiconductor surface, Prog. Surf. Sci. 20 (1985) 9±103. [29] M.J. Willett, Spectroscopy of surface reactions, in: P.T. Mosely, J.O.W. Norris, D.E. William (Eds.), Techniques and Mechanism in Gas Sensing, Adam Hilger, Bristol, 1991, pp. 61±107. [30] K. Takahashi, Introduction to Sensor Technique, Industrial Surveys Press, 1980, pp. 148±175 (in Japanese). [31] T. Kabayashi, M. Haruta, H. Sano, M. Nakane, A selective CO sensing Ti-doped a-Fe2O3 with coprecipitated ultrafine particles of gold, Sens. Actuators 13 (1988) 339±348. [32] V. Guidi, M.C. Caeota, M. Ferroni, G. Martinelli, L. Paglialoga, E. Comini, P. Nelli, G. Sberveglieri, Nanosized materials for gas sensing applications, in: S.D. Natale, A. D'Amico, G. Sberveglieri (Eds.), Sensors and Microsystems, World Scientific, Singapore, 1999, pp. 135±142. [33] S. Capone, G. Leo, R. Rella, P. Siliano, L. Vasanelli, M. Alvisi, L. Mirenghi, A. Rizzo, Hafnium oxide: a new sensitive material for detection of carbon monoxide, in: S.D. Natale, A. D'Amico, G. Sberveglieri (Eds.), Sensors and Microsystems, World Scientific, Singapore, 1999, pp. 159±164.
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Biographies Z.-G. Zhou is a prestige professor, Fellow of the IEEE and CIE, with the State Key Lab of New Ceramics and Fine Processing of Tsinghua University. He has chaired the 7th International Meeting on Chemical Sensors in Beijing, China on 27±30 July 1998. He was also a seniority director of Inorganic Nonmetallic Materials Division in the National Natural Science Foundation of China (NSFC) and an expert/consultant of the United Nations Industrial Development Organization (UNIDO). He holds four patents on ceramic sensors, one from the United States of America, one from the Europe and two from China since 1985. His field of interest is the ceramic sensors with emphasis on the perovskite-type chemical sensors.
Z.-L. Tang is a lecturer at the State Key Lab of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University. He received his PhD in 1997 in Materials Science and Engineering from Tsinghua University. During 1998±1999 Dr. Tang was engaged in thermoelectricity research for NEDO in Nagoya University, Japan. He now is interested in new energy conversion materials and ceramic sensors. Z.-T. Zhang is a professor at the State Key Lab of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University. He earned his PhD in 1987 from Tokyo University. During 1981± 1983 and 1986±1987 Prof. Zhang was a visiting scholar in Tokyo University, and worked as a senior visiting scholar in Nagoya University, Japan, during 1992±1993. He is interested in oxide sensors and defect chemistry.
Perovskite oxide of PTCR ceramics as chemical sensors Z.-G. Zhoua,*, Z.-L. Tanga, Z.-T. Zhanga, W. Wlodarskib a
State Key Laboratory of Fine Ceramics and New Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China b Department of Communication and Electronic Engineering, RMIT, GPO Box 2476V, Melbourne, Vic. 3001, Australia
Abstract A new CO gas sensor of perovskite-type oxide, semiconducting La-doped BaTiO3 PTCR ceramics was studied. It is capable of detecting P carbon monoxide gas in the concentration range of 1.0% in the high temperature NTCR region near and above the Tmax air (175±4008C) by P means of higher barrier potential of PTCR effect. However, there is only a little resistivity change below the Tmax air but above the Tc. The phenomenon is based on anionic adsorption, the extreme activity of the oxygen atoms at the grain boundaries, in the higher temperature P near and above the Tmax air The lattice oxygen in the surface layer of the PTCR ceramics participates in the sensing reaction, and the itinerant electron come from the conduction band of the n-type semiconducting ceramics bring about to a decrease in resistivity of the material. A rate of resistivity change of more than one decade on switching the atmosphere from air to 1.0% in low ¯ow rate was observed. We conclude that PTCR ceramics can be used as a new potential CO gas sensor, simple structure, middle/lower operating temperature and low cost, in ceramic chemical sensors development. # 2001 Elsevier Science B.V. All rights reserved. Keywords: CO gas sensor; BaTiO3; PTC effect; Perovskite oxide; Adsorption; Oxygen vacancy
1. Introduction It is well known that a number of perovskite oxides (ABO3) were used as gas sensor materials because of the stability in thermal and chemical atmospheres. So, over the last decade, the perovskite oxide ceramics has been created and promoted interest in new chemical sensors. The gas sensing perovskite oxide ceramics could be classi®ed into two groups: the semiconducting ceramics and electrochemical ceramics. And now the semiconducting perovskite oxide ceramics were widely used in the detection of reducing gases (such as CO [1±3], H2 [4], C2H5OH [5]), oxygenic gases (such as CO2 [6,7], NOx [8,9], O2 [10±12], CH3OH [13], CH4 [14]), offensive odorous gases (such as NH3 [15], H2S [20]), easy burning and explosion gases (such as C2H2 [16], C2H4 [16], C3H6 [16], C3H8 [14] and LPG [17]), and toxic gases (such as CO [1±3], H2S [18], Cl2 [18], NO2 [18]), because of low cost and high stability in thermal and atmospheres. Among these toxic gases, CO is the most interesting. It is caused by highly carbon monoxide gas can damage the human body. So, the metal oxide semiconductors as n-type SnO2 [19], ZnO2 [20], In2O3 [21], TiO2 [32], a-Fe2 O3 [31],
*
Corresponding author. Fax: 86-10-6277-1160. E-mail address: [email protected] (Z.-G. Zhou).
HfO2 [33], BaSnO3 [22], and p-type as (Ln M) BO3 [23] have been studied for a long time for detecting CO gas. Recently, for CO sensor development, a few of the researchers now are working on decreasing of working temperature of SnO2 based materials by means of promotion of the activity of CO reaction on the surface [24,25]. And a few of them are working on low price CO gas sensor by means of new application of doped BaTiO3 semiconductor with posistor effect [26]. As mentioned above, the perovskite-type semiconducting ceramics based on La doped BaTiO3 with PTCR effect was used for CO detection. The PTCR ceramics are widely used in industries and civilian as a control device. Now the PTCR ceramics have developed to be a new multifunctional semiconducting ferroelectric ceramics [27]. In this paper, the nature is revealed on CO gas response of PTCR ceramics in the low/middle temperature. The mechanism and the relation between the PTC effect and oxygen adsorption, the adsorption and CO sensitivity, and further work were described. 2. Experimental The n-type semiconducting BaTiO3 PTCR ceramics such as Ba0.92 La0.08 TiO3 was prepared by conventional ceramic semiconducting technology. The AP grade powders of
0925-4005/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 1 ) 0 0 6 6 7 - 0
Z.-G. Zhou et al. / Sensors and Actuators B 77 (2001) 22±26
commercial barium titanyl oxalates (i.e. BaTiO (C2O4)4H2O) as a starting material, La2O3, and MnO2 as donor and acceptor dopants. The starting materials were mixed with suitable properties, after wet-milled, dried and calcined, followed by pressing into pellets of 10 mm diameter and 1 mm thickness at pressure. The specimens were sintered at varied temperatures from 1050 to 12508C in air with soaking time of 0.5±2 h. This yielded sintered bodies with porosity of 15%. The resistivity measurements were conducted on a ceramic pellet with an In±Ga alloy rubbed on both end surfaces as electrodes. This two-probe method was used as in a dc bias of 10 V in varied concentration of CO gas in air, of course, air was used as a carrier gas with ¯ow rate of 30 ml min 1 during working. The measurements were performed on all samples, in the tubular ¯ow quartz-glass chamber with thermocouple, ®rst in air and then successively under the reducing CO gas. Switching the atmosphere from air to the 1.0 vol.% CO gas and return to air was conducted at appropriate temperature. The measurements were performed at 3008C in the 1.0 vol.% concentration of CO gas. The temperature was monitored and the resistance was measured automatically every 30 s. The sensitivity was de®ned by S Rgas Rair = Rair where Rgas and Rair are resistance in 1 vol.% CO and air, respectively.
23
Fig. 2. Sensitivity±temperature relations for concentrations of 1.0% CO gas and N2 in PTCR ceramics.
For easy of presentation, to evaluate the CO gas sensitivity of the PTCR ceramics, the ratio of the resistivity of Fig. 1 was taken as a function of temperature for respective samples, and the sensitivity are shown in Fig. 2. This ®gure indicates that the sensitivity increase with temperature increase to 1.0% CO gas and the sensitivities are 10, 20, 40, 70, 90 and 97% while the temperatures are 175, 185, 200, 225, 250 and 3008C, respectively.
3. Results and discussion
3.2. Response characteristics
3.1. Temperature dependence of the CO gas sensitivity Fig. 1 shows the resistivity±temperature characteristics obtained in air and in CO gas for samples. In this ®gure, remarkable decrease in the resistivity near the temperature P giving the maximum resistivity, Tmax air , are evidently, but whereas just a little changes in the resistivity±temperature P characteristics near the Tmax co were observed for all the samples.
The response characteristics of resistivity to CO gas is another important property to be examined for this PTCR ceramics being considered as a CO gas sensor. From this viewpoint, responses to reducing ambient gas containing 1.0 vol.% concentration of CO gas have been obtained for sample at 3008C. In Fig. 3, the response characteristic to N2 gas is also no changed. The data of Fig. 3 reveal that the resistivity changes on switching the ambient gas from air to the CO gas or N2 gas,
Fig. 1. Resistivity±temperature characteristics obtained in air and CO gas for PTCR ceramics.
Fig. 3. Response characteristics of resistivity to reducing gases containing concentration of 1.0% CO and N2 at 3008C for PTCR ceramics.
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Z.-G. Zhou et al. / Sensors and Actuators B 77 (2001) 22±26
vice versa, were reversible. The resistivity of the sample exhibits a gas sensitivity of decrease fast with an initial response rate of one decades per 10 min in low ¯ow rate of 30 ml min 1, but decrease slow in last. 3.3. Response mechanism It is well known that while La is adopted as a dopant, the La ions will occupy Ba site and behave as donors owing to the higher valence than Ba. As the La dopant concentration is at a low level, the semiconducting behavior is explained by the following impurity incorporation equation [27] in Kroger±Vink notation: 2La2 O3 4TiO2 ) 4LaBa 4TixTi 12OxO O2 4e (1) Since the resistivity of the PTCR ceramics near and above P Tmax air are con®rmed to be determined by the potential height of the blocking layers formed at the grain boundaries. Any way, no evidence for direct reaction of CO with bare cation sites has been obtained in this study. So, however, the decrease in resistivity of the ceramics seems to be attributed to the lowering of the potential height of the grain boundary blocking layers. It occurs after removal of oxygen atoms adsorbed on the grain surfaces by their reactions with CO molecules (Fig. 4). They are of the following types [28±32]. For oxygen adsorption: Oads CO ) CO2 e
(2)
COg ) COads ) COads e
(3)
COads O2 ) CO2 e
(4)
Forming and reaction with a surface vacancy: COg OxO ) CO2 e VxO CO
1 2 O2
VxO
e ) CO2
(5) OxO
(6)
were Oads and COads are the possible species of oxygen and carbon monoxide adsorbed on the grain surface of PTCR ceramics, e is an itinerant electron, OxO and VxO are the
Fig. 5. Schematic outline of the absorption possibilities of CO by a lattice oxygen ion.
lattice oxygen and oxygen vacancy in lattice of the ceramics, respectively. Eqs. (2)±(4) are the oxygen adsorption process P near but below the Tmax air and Eqs. (5)±(6) are the lattice oxygen reaction and forming a surface vacancy of oxygen P above the Tmax air . For the process of adsorption, vacancies to migrate from the bulk to the surface, are based on the forming of surface oxygen vacancies in the grain boundaries of the PTCR ceramics. A possible mechanism of oxidation of CO by lattice oxygen ions is as shown in Fig. 5. The anion oxygen can act as adsorption center for the CO molecules. In ®ne, it is of interest to note that the PTCR ceramics place in the atmosphere of CO gas, the CO molecule react on chemisorbed oxygen from the grain boundary of the semiconducting ceramics, free electrons as a driving force for this kind of CO gas sensor, it is caused by the transmission of the electron carriers in the conduction band, contribute to a decrease in resistivity of a material. 4. Conclusion
Fig. 4. A model of reaction processes for CO gas absorbed on the surface of grain boundary of PTCR ceramics.
The perovskite, n-type semiconducting PTCR ceramics, is a new perspective CO gas sensing material with their high temperature NTCR region. However, just a little change in P resistivity was observed at temperature below Tmax air but above Tc point. The sensing mechanism for the adsorption between the CO gas and PTCR ceramics are based on the higher potential barrier in grain boundaries of the semiconducting ceramics.
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The lattice oxygen in the surface layer of the grain boundaries is thought to be involved in the sensing reaction. As a result, the surface resistivity decreases with decreasing potential barrier, is caused by increased density of electron in the conduction band of the semiconducting ceramics. More generally, much work remains to be done in improving the properties of the sensor in the application level, such as increase the sensitivity and decrease the operating temperature, etc. Any way, it is expected that this new low/ middle operating temperature, simple structure and low cost perovskite oxide CO gas sensor will be appeared in the chemical sensor community soon. Acknowledgements We are grateful to the ®nancial support of the National Natural Science Foundation of China (NSFC) (Contract no. 59672012). We also thank many colleagues of the State Key Laboratory of New Ceramics and Fine Processing, Department of Materials science and Engineering of the Tsinghua University for their help. References [1] M. Kuwabara, T. Ide, Co gas sensitivity in porous semiconducting barium titanate, Am. Ceramic Soc. Bull. 66 (1987) 1401±1405. [2] C.M. Chiu, Y.H. Chang, The structure, electrical and sensing properties for CO of the La0.8Sr0.2CO1 xNixO3 d system, Mater. Sci. Eng. A 266 (1999) 93±98. [3] R. Sorita, T. Kawano, A highly selective CO sensor: screening of electrode materials, Sens. Actuators B 35/36 (1996) 274±277. [4] T. Inaba, K. Saji, H. Takahashi, Limiting current-type gas sensors using a high temperature-type conductor thin film, Electrochemistry 67 (1999) 458±462. [5] L.B. Kong, Y.S. Shen, Gas sensing property and mechanism of CaxLa1 xFeO3 ceramics, Sens. Actuators B 30 (1996) 217±221. [6] T. Ishihara, Y. Nishi, H. Nishiguchi, Y. Takita, Detection mechanism of CuO±BaTiO3 capacitive type CO sensor, in: Proceedings of Ceramic Sensors III, San Antonio, TX, USA, 6±11 October 1997. [7] Y.C. Zhang, H. Tagawa, S. Asakuwa, J. Mizusaki, H. Narita, Solid state electrochemical CO2 sensors by coupling lithium ion conductor (Li2CO3±Li3PO4±Al2O3) with oxide ion-electron mixed conductor (La0.9Sr0.1MnO3), Solid State Ionics 100 (1997) 275±281. [8] G. Martirelli, M.C. Carotta, M. Ferroni, Y. Sadaoka, E. Traversa, Screen-printed perovskite-type thick films as gas sensors for environmental monitoring, Sens. Actuators B 55 (1999) 99±110. [9] H. Yamaura, J. Tamaki, N. Miura, N. Yamazoe, NOx sensing properties of metal titanate based semiconductor sensor at elevated temperature, Engineering Science Reports, Vol. 17, 1995, Kyushu University, Japan, pp. 341±346. [10] Y. Noguchi, H. Kuroiwa, M. Takata, Sensing properties of oxygen sensor using hot sport on La0.8Sr0.8Co0.8Fe0.2O3 ceramic rod, Key Eng. Mater. 160±170 (1999) 79±82. [11] J.P. Lukaszewicz, N. Miura, N. Yamazoe, A LaF3 based oxygen sensor with perovskite-type oxide electrode operative at room temperature, Sens. Actuators B 1 (1990) 195±198. [12] P. Shuk, V. kharton, L. Tichonova, H.D. Wiemhofer, U. Guth, W. Gopel, Electrodes for oxygen sensors based on rare earth manganites or cobaltite, Sens. Actuators B 16 (1993) 401±405.
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Biographies Z.-G. Zhou is a prestige professor, Fellow of the IEEE and CIE, with the State Key Lab of New Ceramics and Fine Processing of Tsinghua University. He has chaired the 7th International Meeting on Chemical Sensors in Beijing, China on 27±30 July 1998. He was also a seniority director of Inorganic Nonmetallic Materials Division in the National Natural Science Foundation of China (NSFC) and an expert/consultant of the United Nations Industrial Development Organization (UNIDO). He holds four patents on ceramic sensors, one from the United States of America, one from the Europe and two from China since 1985. His field of interest is the ceramic sensors with emphasis on the perovskite-type chemical sensors.
Z.-L. Tang is a lecturer at the State Key Lab of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University. He received his PhD in 1997 in Materials Science and Engineering from Tsinghua University. During 1998±1999 Dr. Tang was engaged in thermoelectricity research for NEDO in Nagoya University, Japan. He now is interested in new energy conversion materials and ceramic sensors. Z.-T. Zhang is a professor at the State Key Lab of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University. He earned his PhD in 1987 from Tokyo University. During 1981± 1983 and 1986±1987 Prof. Zhang was a visiting scholar in Tokyo University, and worked as a senior visiting scholar in Nagoya University, Japan, during 1992±1993. He is interested in oxide sensors and defect chemistry.