- Email: [email protected]
Gas detection for automotive pollution control
Sensors and Actuators B 59 Ž1999. 195–202 www.elsevier.nlrlocatersensorb
Gas detection for automotive pollution control C. Pijolat ) , C. Pupier, M. Sauvan, G. Tournier, R. Lalauze Centre SPIN, ENSMSE, 158 cours Fauriel, Saint-Etienne 42023, France
Abstract The detection of the gases produced by the cars becomes an important objective for different applications as urban pollution control or for the development of car exhaust devices. The use of gas sensors can contribute to reach such objectives. At the moment, the performances of the gas sensors available on the market especially SnO 2 sensors are often not sufficient to satisfy these needs. The major limitations are dependent on their poor selectivity and stability. Some examples of such problems are presented through field experiments in both types of applications and some feasible improvements of the sensors are discussed. In urban pollution monitoring, it is necessary to take into account the irreversible action of SO 2 . The dual response to oxidising or reducing gases is a difficult problem to solve, especially for the NO x gases. Solutions with metallic filters above the sensing material are currently studied. In order to be able to use directly the sensors in the car exhausts, new types of sensors are developed mainly on the basis of electrochemical devices. An example of such new sensor is exposed with experimental results obtained on car exhausts. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Gas sensors; Automotive applications; CO and NO x pollution
1. Introduction The detection of gases produced by car pollution becomes more and more an inescapable need. On one hand, this is necessary for the control of the atmospheric pollution on urban sites which is, for the major part, related directly with car traffic. The different agencies which have the charge to control this pollution have installed in towns some measuring points equipped with analytical apparatus like infrared ŽIR. or UV spectrometers. These methods have a very good accuracy and allow, generally, to determine the exact concentrations of the different gases without major problem. Nevertheless, for financial reasons, it is difficult to multiply the number of such pollution control stations. In order to have a better understanding of pollution creation and of the moving of the pollutants, it becomes necessary to study the space distribution of the pollution and consequently to increase the number of measuring points. This can be obtained by the use of gas sensors. In the case of urban pollution, the pollutant gases are especially the gases produced by car, mainly CO, NO x ŽNO and NO 2 ., HC Žmixtures of burned and unburned hydrocarbons..
) Corresponding author. Tel.: q33-4-77-42-01-44; fax: q33-4-77-4200-00; E-mail: [email protected]
The challenge for the development of such gas sensors is moreover important with the requirement of the car manufacturers to use such sensors directly on the cars. Many applications are currently studied. On one hand, some of them concern the air quality in the passenger compartment with a control of the ventilation by filtration of the incoming air in case of pollution w1,2x. On the other hand, it is now necessary to measure the concentrations of gases directly in the exhausts. This point becomes more and more strategic for the car manufacturers because the need of gas sensors concerns the control of the motor Žoptimisation of the combustion rate after motor., the control of the different devices used for post-treatment Žthree-ways catalysts, DeNO x or NO x trap devices, etc.. and also diagnostics of the good efficiency of such catalysts Žcoming rules OBD II, etc... In both cases, urban pollution or for car control, the major gases to be detected are CO, NO, NO 2 , HC, but also some other gases as SO 2 , CO 2 , O 3 , etc. The required performances for the sensors, except a low price, are good stability, sensitivity and especially selectivity. This last property is generally difficult to satisfy and at the moment this constitutes the major limitation for the use of gas sensors. The semiconductor sensors are a good illustration of such problem. Actually, the tin oxide sensors are the only sensors Žexcept the oxygen sensors. which have been well-developed by various industrial companies and com-
0925-4005r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 9 9 . 0 0 2 2 0 - 8
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C. Pijolat et al.r Sensors and Actuators B 59 (1999) 195–202
Fig. 1. SnO 2 sensor ŽA5000 from Coreci.: response vs. time on an urban road.
mercialised for different applications, generally in the domestic field Žmethane detection, odour and air quality control, etc... Nevertheless, their use in the automotive field is very difficult because of their dual response to oxidising or reducing gases w3x. Consequently, the sensor development for automotive applications has been focused on electrochemical type sensors which can be more selective for major gases CO and NO x w4–6x. In this paper, some examples of experiments with the two types of sensors Ža SnO 2 semiconductor sensor and an electrochemical one. are presented for illustration of various problems encountered in automotive pollution control. Some solutions are proposed for future development of gas sensors.
2. Urban pollution control with a tin dioxide sensor SnO 2 sensors have been used by different laboratories to detect CO in combustion process w7x or for monitoring the pollution in town w8x. For many years, we have carried out such experiments in town. The used sensors are sintered sensors which have been developed in collaboration with a French company w9,10x. This sensor has been produced for various applications which require small quantities of sensors and not necessary very low cost sensors. An electronic unit allows to install easily these sensors on field for long-term experiments and with different possibilities to record the signal of the sensors on computers. A typical response of such a SnO 2 sensor is reported in Fig. 1. The sensor was placed directly along a downtown street and the signal was recorded in comparison with the response of an IR analyser used for CO measurement. Over a period of 1 day, two peaks of pollution are generally observed corresponding to the maximum traffic of cars in the morning and in the evening. After integration quarter by quarter for both signals, the responses are plotted in Fig. 2. The correlation is very good and this result points out the interest of such sensors for the moni-
toring of the urban pollution. On the basis of such correlation, the SnO 2 sensors used appear very selective to CO. In fact, it has been proved to that all the gases are produced by the cars with the same ratio. When the CO concentration increases, the concentrations of the others gases increase also in the same ratio. In consequence, even if the sensors are not completely selective for CO, their response constitutes a good measurement for the intensity of the urban pollution due to the traffic. Similar experiments have also been performed in a road tunnel with the same type of devices. The only difference is the range of CO concentrations which are higher in the tunnel: from 20 to 200 ppm in the tunnel against 1 to 20 ppm on the street. Concerning the interfering gases, it can be noticed that no differences are observed between the two sites: the two sets of measurements are in good agreement with a good junction near 20 ppm ŽFig. 3.. The experiments in the tunnel have proved that the automation of the ventilation can be easily achieved with such gas sensors. The best results have been obtained with palladium-doped SnO 2 sensors w11x with a special operating process involving temperature pulses between 1508 and 4508C and the CO concentration determination with the lower value of the
Fig. 2. Urban pollution control: correlation between SnO 2 sensor response and CO concentrations measured by IR analyser Žintegration quarter by quarter signals..
C. Pijolat et al.r Sensors and Actuators B 59 (1999) 195–202
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Fig. 3. SnO 2 sensor response vs. CO concentration for road Ž0–20 ppm. and tunnel Ž20–200 ppm. experiments.
sensor signal ŽFig. 4.. There is no problem to regulate the air ventilation in a range of 50–80 ppm of CO. The fluctuations observed on Fig. 4 for the period 14–18 h are related to the starts and the stops of the ventilation. During the night Ž20 to 24 h., the traffic is less important, the level of CO decreases under 50 ppm and the ventilation is always stopped.
3. Problem of stability of SnO 2 sensors The poor stability of the tin dioxide sensors is generally a great limitation Žas their poor selectivity. for their use in industrial applications. The drift observed can be the consequence of various phenomena as the textural evolution of the sensing material Žgrain growth. or chemical irreversible actions of some gases. During the experiments on urban pollution monitoring, such instabilities of the sensors have occurred. They appear not to be connected to the textural evolution because the sensors have been aged before their use during several weeks on laboratory testing bench. The drift has been observed in both applications Žstreets and tunnel. and the evolution was always an increase of the conductance of the sensors in the range of 20–50% for 1-month experiments. An example is reported in Fig. 5 for the tunnel experiments: after 20 days, the signal of the sensor is multiplied by a factor of 4.
Fig. 4. Monitoring of pollution control and ventilation Žstart at 80 ppm and stop at 50 ppm CO. in a tunnel with a SnO 2 sensor ŽPd-doped and pulse working..
Fig. 5. Instability Žincrease of signal. of a pure SnO 2 sensor during 1-month experiment in a tunnel.
After ageing in streets or tunnel, the sensing materials of the sensors have been analysed with different chemical analyses. The major results have been obtained with thermal-programmed desorption ŽTPD.: some sulphate species are present on the surface of the tin dioxide only for the aged sensors. These species are certainly the consequence of the irreversible action of SO 2 w12,13x which is always present is the atmospheric pollution, even if the concentrations are very low Žgenerally less than 1 ppm.. To confirm this irreversible effect, complementary tests have been realised on laboratory bench with SnO 2 . Even if the temperature of the sensors is set at a high value Ž5008C., the
Fig. 6. Stability of a pretreated Žby SO 2 . SnO 2 sensor: response during 1-month experiment in a tunnel.
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Fig. 7. Dual response of a thin film SnO 2 sensor to oxidizing ŽNO 2 . and reducing gases ŽCO, CH 4 .. Special behaviour of NO resulting to few parts per million NO 2 in the NO injection.
continuous pollution of the air by 0.5 ppm SO 2 produces an important irreversible increase of the sensor signal Ž50% after 5 days under a flow of 50 cm3rmin.. In order to avoid such an instability, the sensors have been pretreated with SO 2 before their use. Such a gaseous treatment has been used in the past to improve the stability Žand also the selectivity. of the sintered tin dioxide sensors, especially with regards to water vapour influence w14x. For the tunnel experiments, a typical result obtained with a pretreated sensor is reported in Fig. 6. Such sensors are very stable in comparison to the untreated sensors. It has also been verified that the effects of the treatment are stable and no drift was observable during a 6-month period with these sensors.
4. Problem of the dual response with NO x The detection of nitrous oxides with SnO 2 sensors is very difficult w15,16x. The problem is connected to the dual response of such metallic oxides to reducing or oxidising gases w17,18x: on one hand, reducing gases ŽCO, HC, NO, etc.. increase the conductance of SnO 2 Žn-type. and on the other hand, oxidising gases ŽNO 2 or O 3 , etc.. decrease the conductance. Consequently, in presence of both types of gases, the response of the sensor can be close to zero as a result of the addition of the opposite signals. Many authors have recently described this behaviour. This is especially the case in automotive applications for the detection of CO
Fig. 8. Comparison of the responses of pure and rhodium-covered Ž10 nm. SnO 2 thin films to CO Ž300 ppm. and NO 2 Ž100 ppm..
or HC in the presence of NO x and also for the detection of NO in presence of NO 2 Žor NO 2 in presence of NO.. This dual response constitutes certainly the main limitation of the use of the SnO 2 sensors in the automotive applications. In our case, we have observed the dual response in past experiments for the air quality control application. This application concerns the quality of the air incoming in the passenger compartment of cars. The goal is to filter the air when the pollution is too high and it needs gas sensors for the control of the device. As a consequence of the results exposed previously for the CO measurements, the SnO 2 sensors could be used with success for this application. Nevertheless, we have observed in past experiments that it is not the case when the car is directly behind a diesel car: the signal of the sensor is close to zero, resulting in the presence of NO 2 even if the concentration is very low. This behaviour is due to the very large influence of NO 2 on tin dioxide and whatever the type of material Žsintered, thin or thick films.. An example is reported in Fig. 7 in the case of SnO 2 thin film sensors. These sensors are realised by deposition of the film on an alumina substrate equipped with a platinum heater on the other side. The films are
Table 1 Sensitivities wŽ Ggas yGair .r Gair x of pure and covered with rhodium Ž10 nm. sensors ŽSnO 2 -reactive evaporated films. to CO, NO and NO 2 at 4508C. The sensitivity is expressed from the electrical conductance Žnoted G . measured under polluted gas Ž Ggas . and under pure air Ž Gair . Sensorrgas
CO Ž300 ppm.
NO Ž150 ppm.
NO 2 Ž100 ppm.
Pure SnO 2 qRH
q0.40 q1.22
y0.68 q0.1
y0.92 y0.43
Fig. 9. Response of an electrochemical b-alumina sensor Ž‘Econox’ sensor. at 5508C to increasing and decreasing steps of CO injections Ž150, 300, 475, 630, 780 and 945 ppm.. Comparison of the sensor response and the CO signal of the IR analyser.
C. Pijolat et al.r Sensors and Actuators B 59 (1999) 195–202
Fig. 10. Schemes of the sensing device Ža. and design of sensor prototypes Žb..
deposited by reactive evaporation of tin under low oxygen pressure w19x. The response of such sensors is positive Žincrease of conductance. under CO Ž150 ppm. or CH 4 Ž1000 ppm. and negative under NO 2 Ž100 ppm.. It can be noticed that under NO Ž30 ppm., the response is also negative contrary to the expected positive response. The reason is the presence of a few parts per million of NO 2 in the flux of NO which are very difficult to avoid in consequence of the very quick transformation of NO to NO 2 in air at ambient temperature ŽNO stable above 3008C.. In order to improve the selectivity of tin dioxide gas sensors, we have started a research from several years which consists of using filters above the sensing material
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w20x. With thin film gas sensors, it is possible to filter the gases near the surface by the use of porous films deposited directly onto the sensing material. These filtering films can be either metallic films with catalytic activity or porous ceramics ŽAl 2 O 3 , SiO 2 , Si 3 N4 , etc.. with permeation action. Some interesting results have been obtained with platinum or palladium films deposited above tin dioxide films to improve the selectivity for the detection of CO or CH 4 and especially to reduce the sensitivity to C 2 H 5 OH. Such type of solution can be used with success to solve the problem of the dual response to the NO x gases. The objective is to avoid or to reduce largely the negative response to NO 2 in order to be able to use the SnO 2 sensors in automotive applications to control the pollution, e.g., by measuring the CO and NO concentrations. The choice of the metal to use for the filter has been based on the catalytic metals generally added in catalytic converters to transform the gases and decrease the pollution after the exhausts. A part of these results has been recently reported w20x. They have been obtained with SnO 2 thin film sensors covered with very thin films of various metals. The experiments have been carried out on a laboratory testing bench equipped with IR and UV analysers for the measurement of the CO, NO and NO 2 concentrations. The metals are platinum, palladium, copper, molybdenum and rhodium. In fact, some of them are in the oxide form according to the working temperature ŽPd and Cu. and to the oxidation reaction ŽMo.. The best results are obtained with rhodium ŽTable 1.: the negative sensitivity to NO 2 is largely reduced and above all, the sensitivity to NO remains positive although the gaseous phase contains some parts per million NO 2 . To confirm such influence observed on sensors, additional experiments have been performed by electrical
Fig. 11. Sensitivities ŽEmf gas y Emf air . of the b-alumina sensor Ž‘Econox’. to CO and NO 2 at low Ž3308C. and high Ž5708C. temperatures. The sensitivity is expressed as the difference of the electrical potential recorded between the two electrodes under polluted gas ŽEmf gas . and under pure air ŽEmf air ..
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tests of the different materials Žthin films of SnO 2 : pure and covered with 10 nm rhodium. directly placed in a furnace without the sensor structure ŽFig. 8.. The optimisation of such filters and the applied tests for automotive applications and also for urban pollution monitoring are in progress.
5. Development of an electrochemical sensor Although some improvements of the SnO 2 sensors are possible as exposed Section 4, their direct use in automotive exhausts will perhaps always remain difficult in the future. To reply to the need of car suppliers to have selective sensors for CO, NO x and HC able to work directly in the exhausts, we have started, 5 years ago, a research on a new electrochemical sensor. It is well-known that the zirconia oxygen sensors are the only chemical sensors which are now usually used in exhausts w21x. This important development has largely pushed the market of
chemical sensors. Following this trend, new electrochemical sensors have been developed for the automotive applications. ZrO 2 materials have been recently used by different authors for NO x detection w4,5,22x. On the basis of such electrochemical principles, a new sensing device using SO 2 pre-treated b-alumina as sensing material has been recently proposed and patented w23x for the detection of CO. It uses the electrochemical properties of some solid electrolytes like b-alumina and sodium sulphate associated with two electrodes with different catalytic properties, for instance one made of platinum, the other of gold. Unlike conventional electrochemical devices like zirconia sensors which are based on two separate gas atmospheres, this sensor includes two electrodes which are in contact with the same gas mixture w24x. A part of the theoretical model is currently in press w25x. The main characteristic of this sensor is the dependency of its sensitivity to CO and NO x with the temperature which allows a selective detection of CO at high temperature Ž6008C. and NO 2 at low temperature Ž4008C.. A typical response of such a device Žat
Fig. 12. Responses of the Econox sensor in a car exhaust at low Ž3008C, a. and high Ž7008C, b. temperature. Comparison with the signal of an oxygen sensor to different motor accelerations ŽRPM..
C. Pijolat et al.r Sensors and Actuators B 59 (1999) 195–202
5508C. for increasing and decreasing steps of CO concentrations Ž150 to 950 ppm. is reported in Fig. 9. The sensitivity, the reversibility and also the stability appear sufficiently good for the considered automotive applications. Thick film technology has been chosen for the industrial development because of the strongly built quality required for the exhausts applications. The design of a prototype is reported in Fig. 10. A part of the performances of these sensors obtained on a laboratory testing bench is summarized in Fig. 11. On one hand, at high temperature Ž5708C., the sensitivity to NO 2 is really very weak with regard to the CO response and on the other hand, at low temperature Ž3308C., the sensitivity to CO is also very weak with regard to the large sensitivity to NO 2 . On the basis of these results, such sensors have been tested directly in car exhausts. An example of these experiments is reported in Fig. 12 for two setting points Ž7008 and 3008C.. The three signals simultaneously recorded vs. time are: our b-alumina sensor Žnamed Econox sensor because the industrial development is done in a EEC Brite project named Econox., a proportional oxygen sensor ŽUego sensor from Japan. and the motor rate ŽRPM.. It can be observed that the dual behaviour of the sensor with the temperature is in agreement with the bench tests: the response is positive when the temperature of the sensor is set at 7008C ŽFig. 12a. and, on the contrary, the response is negative at low working temperature of 3008C ŽFig. 12b.. An important point is the fact that the response times are very fast in both cases and comparable to that of the O 2 sensor. Considering the peaks after sudden engine accelerations, the sensor response can be estimated to about 0.5 s. The first conclusions of such experiments seem to point out the direct correlation between the responses of the sensor and the CO and NO 2 concentrations Žat, respectively, high and low temperatures.. Nevertheless, many complementary experiments must be performed especially on automotive motor benches equipped with gas analysers in order to confirm the real correlation of the sensor responses to the gas concentrations. Such automotive tests, laboratory tests under other gases Žwater and various hydrocarbon compounds. and technological improvements of the industrial prototypes and also progress on the model especially to take into account of the oxygen action, constitute the different parts of our current research. 6. Conclusions The automotive applications which need gas sensors are very large but also different. Consequently, the required performances depend largely on the type of application to be satisfied. Nevertheless, the long-term stability and the selectivity must be improved for all applications. For the urban pollution control, the semiconductor sensor as SnO 2 sensors can be used with success, especially for the measurement of CO concentrations which are a good estima-
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tion of the car traffic. On an other hand, the problem of the dual response to oxidising and reducing gases remains an important problem especially with NO x gases. The use of catalytic filters deposited directly above the sensing material appears as a future solution. The advantages of such improvements are connected to their general use with various sensing materials or devices not depending on a special application. Lastly, to put directly gas sensors in the car exhausts for emission control, the need concerns strongly built selective sensors. The current researches are mainly focused on the development of electrochemical sensors. This important effort will push certainly the future market of high quality gas sensors, but perhaps in a first step, these sensors will be not necessarily very low-cost sensors.
Acknowledgements The authors express their gratitude to Professors L. Montanaro and D. Negro from Politecnico di Torino for the b-alumina preparation and to the Centro Ricerch Fiat ŽTorino. for the technical support and the experiments on cars.
References w1x A. Zeppenfeld, K. Ingrish, M. Bauer, I. Denk, B. Ziegenbein, Chemical sensors for COrNO x detection in automotive climate control systems, Proceeding of Sensor ’97 ŽNuremberg ’97 May., Vol. B1, 1997, pp. 113–118. w2x G. Wiegleb, J. Hietbaum, Semiconductor gas sensor for detecting NO and CO traces in ambient air of road traffic, Sensors and Actuators B 17 Ž1994. 93–99. w3x M.A. Martin, J.P. Santos, H. Vasquez, J.A. Agapito, Study of the interferences of NO 2 and CO in solid state commercial sensors, Proceeding Eurosensors XII ŽSept. 1998, Southampton., pp. 1036– 1039, to be published in Sensors and Actuators. w4x N. Miura, G. Lu, N. Yamazoe, H. Kurosawa, M. Hasei, Mixed potential type NO x sensor based on stabilized zirconia and oxide electrode, J. Electrochem. Soc. 143 Ž2. Ž1996. 33–35. w5x Y. Nakanouchi, H. Kurosawa, M. Hasei, Y. Yan, A. Kunimoto, New type of NO x sensors for automobiles, SAE technical paper series, 961130 Ž1996. 11–18. w6x K. Satake, A. Katayama, H. Ohkoshi, T. Nakahara, T. Takeuchi, Titania NO x sensors for exhaust monitoring, Sensors and Actuators B 20 Ž1994. 111–117. w7x H. Torvela, A. Harkoma-Mattila, S. Leppavuori, Characterisation of a combustion process using catalysed tin oxide gas sensors to detect CO from emission gases, Sensors and Actuators B 1 Ž1990. 83–86. w8x G.B. Barbi, J.P. Santos, S. Serrini, P.N. Gibson, M.C. Horrillo, L. Manes, Ultra-fine grain-size tin-oxide films for carbon monoxide monitoring in urban environments, Sensors and Actuators B 25 Ž1995. 559–563. w9x C. Pijolat, Gas microsensors: types, principles, industrial systems, European market and perspectives of research, Ceramurgia, Ž5. Ž1995. 257–270. w10x R. Lalauze, C. Pijolat, J.P. Couput, Procede, ´ ´ capteur et dispositif de detection de traces de gaz dans un milieu gazeux, Patents: FR ´ 8119536 Ž1981., EC 4485667 Ž1984., US 824018722 Ž1984., JP 1753030 Ž1993..
202
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w11x G. Tournier, C. Pijolat, R. Lalauze, B. Patissier, Selective detection of CO and CH 4 with gas sensors using SnO 2 doped with palladium, Sensors and Actuators B 26–27 Ž1995. 24–28. w12x R. Romppainen, H. Torvela, J. Vaananen, S. Leppavuori, Effect of CH 4 , SO 2 and NO on the CO response of an SnO 2 -based thick-film gas sensor in combustion gases, Sensors and Actuators 8 Ž1985. 271–279. w13x A. Keshavaraja, B.S. Jayashri, A.V. Ramaswamy, K. Vijayamohanan, Effect of surface modifications due to superacid species in controlling the sensitivity and selectivity of SnO 2 gas sensors, Sensors and Actuators B 23 Ž1995. 75–83. w14x R. Lalauze, C. Pijolat, A new approach to selective detection of gas by an SnO 2 solid-state sensor, Sensors and Actuators 5 Ž1984. 55–63. w15x H.E. Endres, W. Gottler, R. Hartinger, S. Drost, W. Hellmich, G. Muller, C. Bosch-Braunmuhl, A. Krenkow, C. Perego, G. Sberveglieri, A thin film sensor system for simultaneous detection of CO and NO x with neural signal evaluation, Sensors and Actuators B 35–36 Ž1996. 353–357. w16x J. Santos, P. Serrini, B. O’Beirn, L. Manes, A thin film SnO 2 gas sensor selective to ultra-low NO 2 concentrations in air, Sensors and Actuators B 43 Ž1997. 154–160. w17x I. Sayago, J. Gutierrez, L. Ares, J.I. Robla, M.C. Horrillo, J. Getino, J.A. Agapito, The interaction of different oxidizing agents on doped tin dioxide, Sensors and Actuators B 24–25 Ž1995. 512–515. w18x G. Williams, G.S.V. Coles, The influence of deposition parameters
w19x w20x
w21x
w22x w23x
w24x
w25x
on the performance of tin dioxide NO 2 sensors prepared by radiofrequency magnetron sputtering, Sensors and Actuators B 24–25 Ž1995. 469–473. R. Lalauze, P. Breuil, C. Pijolat, Thin films gas sensors, Sensors and Actuators B 3 Ž1991. 175–182. M. Sauvan, C. Pijolat, Selectivity improvement of SnO 2 films by superficial metallic films, in: N.M. Whit ŽEd.., Proceeding Eurosensors XII ŽSept. 1998, Southampton., Vol. 1, 1998, pp. 625–628, to be published in Sensors and Actuators. E.M. Logothetis, Automotive oxygen sensors, in: N. Yamazoe ŽEd.., Chemical Sensor Technology, Vol. 3, Elsevier, Amsterdam, 1991, pp. 89–104. N. Kato, K. Nakagaki, N. Ina, Thick film ZrO 2 NO x sensors, SAE technical paper series 960334137-142 Ž1996. 11–18. E. Visconte-Facetta, C. Pijolat, R. Lalauze, Dispositif de detection ´ de la presence d’un gaz dans une atmosphere, Patents Ž1993.: Fr ´ 9207414, MI93 A 001319, P43 19 836.8. C. Pijolat, C. Pupier, C. Testud, J.C. Marchand, R. Lalauze, L. Montanaro, A. Negro, C. Malvicino, Electrochemical sensor for COrNO x detection in automotive applications, Proceedings of Sensor ’97 ŽNuremberg ’97 May., Vol. B1, 1997, pp. 119–124, to be published in J. Electroceram. C. Pupier, C. Pijolat, J.C. Marchand, R. Lalauze, Oxygen role on the electrochemical response of a gas sensor using polarisable electrodes, Proceedings 195 ECS Meeting Ž1998 May, San Diego., to be published in J. Electrochem. Soc.
Gas detection for automotive pollution control C. Pijolat ) , C. Pupier, M. Sauvan, G. Tournier, R. Lalauze Centre SPIN, ENSMSE, 158 cours Fauriel, Saint-Etienne 42023, France
Abstract The detection of the gases produced by the cars becomes an important objective for different applications as urban pollution control or for the development of car exhaust devices. The use of gas sensors can contribute to reach such objectives. At the moment, the performances of the gas sensors available on the market especially SnO 2 sensors are often not sufficient to satisfy these needs. The major limitations are dependent on their poor selectivity and stability. Some examples of such problems are presented through field experiments in both types of applications and some feasible improvements of the sensors are discussed. In urban pollution monitoring, it is necessary to take into account the irreversible action of SO 2 . The dual response to oxidising or reducing gases is a difficult problem to solve, especially for the NO x gases. Solutions with metallic filters above the sensing material are currently studied. In order to be able to use directly the sensors in the car exhausts, new types of sensors are developed mainly on the basis of electrochemical devices. An example of such new sensor is exposed with experimental results obtained on car exhausts. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Gas sensors; Automotive applications; CO and NO x pollution
1. Introduction The detection of gases produced by car pollution becomes more and more an inescapable need. On one hand, this is necessary for the control of the atmospheric pollution on urban sites which is, for the major part, related directly with car traffic. The different agencies which have the charge to control this pollution have installed in towns some measuring points equipped with analytical apparatus like infrared ŽIR. or UV spectrometers. These methods have a very good accuracy and allow, generally, to determine the exact concentrations of the different gases without major problem. Nevertheless, for financial reasons, it is difficult to multiply the number of such pollution control stations. In order to have a better understanding of pollution creation and of the moving of the pollutants, it becomes necessary to study the space distribution of the pollution and consequently to increase the number of measuring points. This can be obtained by the use of gas sensors. In the case of urban pollution, the pollutant gases are especially the gases produced by car, mainly CO, NO x ŽNO and NO 2 ., HC Žmixtures of burned and unburned hydrocarbons..
) Corresponding author. Tel.: q33-4-77-42-01-44; fax: q33-4-77-4200-00; E-mail: [email protected]
The challenge for the development of such gas sensors is moreover important with the requirement of the car manufacturers to use such sensors directly on the cars. Many applications are currently studied. On one hand, some of them concern the air quality in the passenger compartment with a control of the ventilation by filtration of the incoming air in case of pollution w1,2x. On the other hand, it is now necessary to measure the concentrations of gases directly in the exhausts. This point becomes more and more strategic for the car manufacturers because the need of gas sensors concerns the control of the motor Žoptimisation of the combustion rate after motor., the control of the different devices used for post-treatment Žthree-ways catalysts, DeNO x or NO x trap devices, etc.. and also diagnostics of the good efficiency of such catalysts Žcoming rules OBD II, etc... In both cases, urban pollution or for car control, the major gases to be detected are CO, NO, NO 2 , HC, but also some other gases as SO 2 , CO 2 , O 3 , etc. The required performances for the sensors, except a low price, are good stability, sensitivity and especially selectivity. This last property is generally difficult to satisfy and at the moment this constitutes the major limitation for the use of gas sensors. The semiconductor sensors are a good illustration of such problem. Actually, the tin oxide sensors are the only sensors Žexcept the oxygen sensors. which have been well-developed by various industrial companies and com-
0925-4005r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 9 9 . 0 0 2 2 0 - 8
196
C. Pijolat et al.r Sensors and Actuators B 59 (1999) 195–202
Fig. 1. SnO 2 sensor ŽA5000 from Coreci.: response vs. time on an urban road.
mercialised for different applications, generally in the domestic field Žmethane detection, odour and air quality control, etc... Nevertheless, their use in the automotive field is very difficult because of their dual response to oxidising or reducing gases w3x. Consequently, the sensor development for automotive applications has been focused on electrochemical type sensors which can be more selective for major gases CO and NO x w4–6x. In this paper, some examples of experiments with the two types of sensors Ža SnO 2 semiconductor sensor and an electrochemical one. are presented for illustration of various problems encountered in automotive pollution control. Some solutions are proposed for future development of gas sensors.
2. Urban pollution control with a tin dioxide sensor SnO 2 sensors have been used by different laboratories to detect CO in combustion process w7x or for monitoring the pollution in town w8x. For many years, we have carried out such experiments in town. The used sensors are sintered sensors which have been developed in collaboration with a French company w9,10x. This sensor has been produced for various applications which require small quantities of sensors and not necessary very low cost sensors. An electronic unit allows to install easily these sensors on field for long-term experiments and with different possibilities to record the signal of the sensors on computers. A typical response of such a SnO 2 sensor is reported in Fig. 1. The sensor was placed directly along a downtown street and the signal was recorded in comparison with the response of an IR analyser used for CO measurement. Over a period of 1 day, two peaks of pollution are generally observed corresponding to the maximum traffic of cars in the morning and in the evening. After integration quarter by quarter for both signals, the responses are plotted in Fig. 2. The correlation is very good and this result points out the interest of such sensors for the moni-
toring of the urban pollution. On the basis of such correlation, the SnO 2 sensors used appear very selective to CO. In fact, it has been proved to that all the gases are produced by the cars with the same ratio. When the CO concentration increases, the concentrations of the others gases increase also in the same ratio. In consequence, even if the sensors are not completely selective for CO, their response constitutes a good measurement for the intensity of the urban pollution due to the traffic. Similar experiments have also been performed in a road tunnel with the same type of devices. The only difference is the range of CO concentrations which are higher in the tunnel: from 20 to 200 ppm in the tunnel against 1 to 20 ppm on the street. Concerning the interfering gases, it can be noticed that no differences are observed between the two sites: the two sets of measurements are in good agreement with a good junction near 20 ppm ŽFig. 3.. The experiments in the tunnel have proved that the automation of the ventilation can be easily achieved with such gas sensors. The best results have been obtained with palladium-doped SnO 2 sensors w11x with a special operating process involving temperature pulses between 1508 and 4508C and the CO concentration determination with the lower value of the
Fig. 2. Urban pollution control: correlation between SnO 2 sensor response and CO concentrations measured by IR analyser Žintegration quarter by quarter signals..
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Fig. 3. SnO 2 sensor response vs. CO concentration for road Ž0–20 ppm. and tunnel Ž20–200 ppm. experiments.
sensor signal ŽFig. 4.. There is no problem to regulate the air ventilation in a range of 50–80 ppm of CO. The fluctuations observed on Fig. 4 for the period 14–18 h are related to the starts and the stops of the ventilation. During the night Ž20 to 24 h., the traffic is less important, the level of CO decreases under 50 ppm and the ventilation is always stopped.
3. Problem of stability of SnO 2 sensors The poor stability of the tin dioxide sensors is generally a great limitation Žas their poor selectivity. for their use in industrial applications. The drift observed can be the consequence of various phenomena as the textural evolution of the sensing material Žgrain growth. or chemical irreversible actions of some gases. During the experiments on urban pollution monitoring, such instabilities of the sensors have occurred. They appear not to be connected to the textural evolution because the sensors have been aged before their use during several weeks on laboratory testing bench. The drift has been observed in both applications Žstreets and tunnel. and the evolution was always an increase of the conductance of the sensors in the range of 20–50% for 1-month experiments. An example is reported in Fig. 5 for the tunnel experiments: after 20 days, the signal of the sensor is multiplied by a factor of 4.
Fig. 4. Monitoring of pollution control and ventilation Žstart at 80 ppm and stop at 50 ppm CO. in a tunnel with a SnO 2 sensor ŽPd-doped and pulse working..
Fig. 5. Instability Žincrease of signal. of a pure SnO 2 sensor during 1-month experiment in a tunnel.
After ageing in streets or tunnel, the sensing materials of the sensors have been analysed with different chemical analyses. The major results have been obtained with thermal-programmed desorption ŽTPD.: some sulphate species are present on the surface of the tin dioxide only for the aged sensors. These species are certainly the consequence of the irreversible action of SO 2 w12,13x which is always present is the atmospheric pollution, even if the concentrations are very low Žgenerally less than 1 ppm.. To confirm this irreversible effect, complementary tests have been realised on laboratory bench with SnO 2 . Even if the temperature of the sensors is set at a high value Ž5008C., the
Fig. 6. Stability of a pretreated Žby SO 2 . SnO 2 sensor: response during 1-month experiment in a tunnel.
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Fig. 7. Dual response of a thin film SnO 2 sensor to oxidizing ŽNO 2 . and reducing gases ŽCO, CH 4 .. Special behaviour of NO resulting to few parts per million NO 2 in the NO injection.
continuous pollution of the air by 0.5 ppm SO 2 produces an important irreversible increase of the sensor signal Ž50% after 5 days under a flow of 50 cm3rmin.. In order to avoid such an instability, the sensors have been pretreated with SO 2 before their use. Such a gaseous treatment has been used in the past to improve the stability Žand also the selectivity. of the sintered tin dioxide sensors, especially with regards to water vapour influence w14x. For the tunnel experiments, a typical result obtained with a pretreated sensor is reported in Fig. 6. Such sensors are very stable in comparison to the untreated sensors. It has also been verified that the effects of the treatment are stable and no drift was observable during a 6-month period with these sensors.
4. Problem of the dual response with NO x The detection of nitrous oxides with SnO 2 sensors is very difficult w15,16x. The problem is connected to the dual response of such metallic oxides to reducing or oxidising gases w17,18x: on one hand, reducing gases ŽCO, HC, NO, etc.. increase the conductance of SnO 2 Žn-type. and on the other hand, oxidising gases ŽNO 2 or O 3 , etc.. decrease the conductance. Consequently, in presence of both types of gases, the response of the sensor can be close to zero as a result of the addition of the opposite signals. Many authors have recently described this behaviour. This is especially the case in automotive applications for the detection of CO
Fig. 8. Comparison of the responses of pure and rhodium-covered Ž10 nm. SnO 2 thin films to CO Ž300 ppm. and NO 2 Ž100 ppm..
or HC in the presence of NO x and also for the detection of NO in presence of NO 2 Žor NO 2 in presence of NO.. This dual response constitutes certainly the main limitation of the use of the SnO 2 sensors in the automotive applications. In our case, we have observed the dual response in past experiments for the air quality control application. This application concerns the quality of the air incoming in the passenger compartment of cars. The goal is to filter the air when the pollution is too high and it needs gas sensors for the control of the device. As a consequence of the results exposed previously for the CO measurements, the SnO 2 sensors could be used with success for this application. Nevertheless, we have observed in past experiments that it is not the case when the car is directly behind a diesel car: the signal of the sensor is close to zero, resulting in the presence of NO 2 even if the concentration is very low. This behaviour is due to the very large influence of NO 2 on tin dioxide and whatever the type of material Žsintered, thin or thick films.. An example is reported in Fig. 7 in the case of SnO 2 thin film sensors. These sensors are realised by deposition of the film on an alumina substrate equipped with a platinum heater on the other side. The films are
Table 1 Sensitivities wŽ Ggas yGair .r Gair x of pure and covered with rhodium Ž10 nm. sensors ŽSnO 2 -reactive evaporated films. to CO, NO and NO 2 at 4508C. The sensitivity is expressed from the electrical conductance Žnoted G . measured under polluted gas Ž Ggas . and under pure air Ž Gair . Sensorrgas
CO Ž300 ppm.
NO Ž150 ppm.
NO 2 Ž100 ppm.
Pure SnO 2 qRH
q0.40 q1.22
y0.68 q0.1
y0.92 y0.43
Fig. 9. Response of an electrochemical b-alumina sensor Ž‘Econox’ sensor. at 5508C to increasing and decreasing steps of CO injections Ž150, 300, 475, 630, 780 and 945 ppm.. Comparison of the sensor response and the CO signal of the IR analyser.
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Fig. 10. Schemes of the sensing device Ža. and design of sensor prototypes Žb..
deposited by reactive evaporation of tin under low oxygen pressure w19x. The response of such sensors is positive Žincrease of conductance. under CO Ž150 ppm. or CH 4 Ž1000 ppm. and negative under NO 2 Ž100 ppm.. It can be noticed that under NO Ž30 ppm., the response is also negative contrary to the expected positive response. The reason is the presence of a few parts per million of NO 2 in the flux of NO which are very difficult to avoid in consequence of the very quick transformation of NO to NO 2 in air at ambient temperature ŽNO stable above 3008C.. In order to improve the selectivity of tin dioxide gas sensors, we have started a research from several years which consists of using filters above the sensing material
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w20x. With thin film gas sensors, it is possible to filter the gases near the surface by the use of porous films deposited directly onto the sensing material. These filtering films can be either metallic films with catalytic activity or porous ceramics ŽAl 2 O 3 , SiO 2 , Si 3 N4 , etc.. with permeation action. Some interesting results have been obtained with platinum or palladium films deposited above tin dioxide films to improve the selectivity for the detection of CO or CH 4 and especially to reduce the sensitivity to C 2 H 5 OH. Such type of solution can be used with success to solve the problem of the dual response to the NO x gases. The objective is to avoid or to reduce largely the negative response to NO 2 in order to be able to use the SnO 2 sensors in automotive applications to control the pollution, e.g., by measuring the CO and NO concentrations. The choice of the metal to use for the filter has been based on the catalytic metals generally added in catalytic converters to transform the gases and decrease the pollution after the exhausts. A part of these results has been recently reported w20x. They have been obtained with SnO 2 thin film sensors covered with very thin films of various metals. The experiments have been carried out on a laboratory testing bench equipped with IR and UV analysers for the measurement of the CO, NO and NO 2 concentrations. The metals are platinum, palladium, copper, molybdenum and rhodium. In fact, some of them are in the oxide form according to the working temperature ŽPd and Cu. and to the oxidation reaction ŽMo.. The best results are obtained with rhodium ŽTable 1.: the negative sensitivity to NO 2 is largely reduced and above all, the sensitivity to NO remains positive although the gaseous phase contains some parts per million NO 2 . To confirm such influence observed on sensors, additional experiments have been performed by electrical
Fig. 11. Sensitivities ŽEmf gas y Emf air . of the b-alumina sensor Ž‘Econox’. to CO and NO 2 at low Ž3308C. and high Ž5708C. temperatures. The sensitivity is expressed as the difference of the electrical potential recorded between the two electrodes under polluted gas ŽEmf gas . and under pure air ŽEmf air ..
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tests of the different materials Žthin films of SnO 2 : pure and covered with 10 nm rhodium. directly placed in a furnace without the sensor structure ŽFig. 8.. The optimisation of such filters and the applied tests for automotive applications and also for urban pollution monitoring are in progress.
5. Development of an electrochemical sensor Although some improvements of the SnO 2 sensors are possible as exposed Section 4, their direct use in automotive exhausts will perhaps always remain difficult in the future. To reply to the need of car suppliers to have selective sensors for CO, NO x and HC able to work directly in the exhausts, we have started, 5 years ago, a research on a new electrochemical sensor. It is well-known that the zirconia oxygen sensors are the only chemical sensors which are now usually used in exhausts w21x. This important development has largely pushed the market of
chemical sensors. Following this trend, new electrochemical sensors have been developed for the automotive applications. ZrO 2 materials have been recently used by different authors for NO x detection w4,5,22x. On the basis of such electrochemical principles, a new sensing device using SO 2 pre-treated b-alumina as sensing material has been recently proposed and patented w23x for the detection of CO. It uses the electrochemical properties of some solid electrolytes like b-alumina and sodium sulphate associated with two electrodes with different catalytic properties, for instance one made of platinum, the other of gold. Unlike conventional electrochemical devices like zirconia sensors which are based on two separate gas atmospheres, this sensor includes two electrodes which are in contact with the same gas mixture w24x. A part of the theoretical model is currently in press w25x. The main characteristic of this sensor is the dependency of its sensitivity to CO and NO x with the temperature which allows a selective detection of CO at high temperature Ž6008C. and NO 2 at low temperature Ž4008C.. A typical response of such a device Žat
Fig. 12. Responses of the Econox sensor in a car exhaust at low Ž3008C, a. and high Ž7008C, b. temperature. Comparison with the signal of an oxygen sensor to different motor accelerations ŽRPM..
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5508C. for increasing and decreasing steps of CO concentrations Ž150 to 950 ppm. is reported in Fig. 9. The sensitivity, the reversibility and also the stability appear sufficiently good for the considered automotive applications. Thick film technology has been chosen for the industrial development because of the strongly built quality required for the exhausts applications. The design of a prototype is reported in Fig. 10. A part of the performances of these sensors obtained on a laboratory testing bench is summarized in Fig. 11. On one hand, at high temperature Ž5708C., the sensitivity to NO 2 is really very weak with regard to the CO response and on the other hand, at low temperature Ž3308C., the sensitivity to CO is also very weak with regard to the large sensitivity to NO 2 . On the basis of these results, such sensors have been tested directly in car exhausts. An example of these experiments is reported in Fig. 12 for two setting points Ž7008 and 3008C.. The three signals simultaneously recorded vs. time are: our b-alumina sensor Žnamed Econox sensor because the industrial development is done in a EEC Brite project named Econox., a proportional oxygen sensor ŽUego sensor from Japan. and the motor rate ŽRPM.. It can be observed that the dual behaviour of the sensor with the temperature is in agreement with the bench tests: the response is positive when the temperature of the sensor is set at 7008C ŽFig. 12a. and, on the contrary, the response is negative at low working temperature of 3008C ŽFig. 12b.. An important point is the fact that the response times are very fast in both cases and comparable to that of the O 2 sensor. Considering the peaks after sudden engine accelerations, the sensor response can be estimated to about 0.5 s. The first conclusions of such experiments seem to point out the direct correlation between the responses of the sensor and the CO and NO 2 concentrations Žat, respectively, high and low temperatures.. Nevertheless, many complementary experiments must be performed especially on automotive motor benches equipped with gas analysers in order to confirm the real correlation of the sensor responses to the gas concentrations. Such automotive tests, laboratory tests under other gases Žwater and various hydrocarbon compounds. and technological improvements of the industrial prototypes and also progress on the model especially to take into account of the oxygen action, constitute the different parts of our current research. 6. Conclusions The automotive applications which need gas sensors are very large but also different. Consequently, the required performances depend largely on the type of application to be satisfied. Nevertheless, the long-term stability and the selectivity must be improved for all applications. For the urban pollution control, the semiconductor sensor as SnO 2 sensors can be used with success, especially for the measurement of CO concentrations which are a good estima-
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tion of the car traffic. On an other hand, the problem of the dual response to oxidising and reducing gases remains an important problem especially with NO x gases. The use of catalytic filters deposited directly above the sensing material appears as a future solution. The advantages of such improvements are connected to their general use with various sensing materials or devices not depending on a special application. Lastly, to put directly gas sensors in the car exhausts for emission control, the need concerns strongly built selective sensors. The current researches are mainly focused on the development of electrochemical sensors. This important effort will push certainly the future market of high quality gas sensors, but perhaps in a first step, these sensors will be not necessarily very low-cost sensors.
Acknowledgements The authors express their gratitude to Professors L. Montanaro and D. Negro from Politecnico di Torino for the b-alumina preparation and to the Centro Ricerch Fiat ŽTorino. for the technical support and the experiments on cars.
References w1x A. Zeppenfeld, K. Ingrish, M. Bauer, I. Denk, B. Ziegenbein, Chemical sensors for COrNO x detection in automotive climate control systems, Proceeding of Sensor ’97 ŽNuremberg ’97 May., Vol. B1, 1997, pp. 113–118. w2x G. Wiegleb, J. Hietbaum, Semiconductor gas sensor for detecting NO and CO traces in ambient air of road traffic, Sensors and Actuators B 17 Ž1994. 93–99. w3x M.A. Martin, J.P. Santos, H. Vasquez, J.A. Agapito, Study of the interferences of NO 2 and CO in solid state commercial sensors, Proceeding Eurosensors XII ŽSept. 1998, Southampton., pp. 1036– 1039, to be published in Sensors and Actuators. w4x N. Miura, G. Lu, N. Yamazoe, H. Kurosawa, M. Hasei, Mixed potential type NO x sensor based on stabilized zirconia and oxide electrode, J. Electrochem. Soc. 143 Ž2. Ž1996. 33–35. w5x Y. Nakanouchi, H. Kurosawa, M. Hasei, Y. Yan, A. Kunimoto, New type of NO x sensors for automobiles, SAE technical paper series, 961130 Ž1996. 11–18. w6x K. Satake, A. Katayama, H. Ohkoshi, T. Nakahara, T. Takeuchi, Titania NO x sensors for exhaust monitoring, Sensors and Actuators B 20 Ž1994. 111–117. w7x H. Torvela, A. Harkoma-Mattila, S. Leppavuori, Characterisation of a combustion process using catalysed tin oxide gas sensors to detect CO from emission gases, Sensors and Actuators B 1 Ž1990. 83–86. w8x G.B. Barbi, J.P. Santos, S. Serrini, P.N. Gibson, M.C. Horrillo, L. Manes, Ultra-fine grain-size tin-oxide films for carbon monoxide monitoring in urban environments, Sensors and Actuators B 25 Ž1995. 559–563. w9x C. Pijolat, Gas microsensors: types, principles, industrial systems, European market and perspectives of research, Ceramurgia, Ž5. Ž1995. 257–270. w10x R. Lalauze, C. Pijolat, J.P. Couput, Procede, ´ ´ capteur et dispositif de detection de traces de gaz dans un milieu gazeux, Patents: FR ´ 8119536 Ž1981., EC 4485667 Ž1984., US 824018722 Ž1984., JP 1753030 Ž1993..
202
C. Pijolat et al.r Sensors and Actuators B 59 (1999) 195–202
w11x G. Tournier, C. Pijolat, R. Lalauze, B. Patissier, Selective detection of CO and CH 4 with gas sensors using SnO 2 doped with palladium, Sensors and Actuators B 26–27 Ž1995. 24–28. w12x R. Romppainen, H. Torvela, J. Vaananen, S. Leppavuori, Effect of CH 4 , SO 2 and NO on the CO response of an SnO 2 -based thick-film gas sensor in combustion gases, Sensors and Actuators 8 Ž1985. 271–279. w13x A. Keshavaraja, B.S. Jayashri, A.V. Ramaswamy, K. Vijayamohanan, Effect of surface modifications due to superacid species in controlling the sensitivity and selectivity of SnO 2 gas sensors, Sensors and Actuators B 23 Ž1995. 75–83. w14x R. Lalauze, C. Pijolat, A new approach to selective detection of gas by an SnO 2 solid-state sensor, Sensors and Actuators 5 Ž1984. 55–63. w15x H.E. Endres, W. Gottler, R. Hartinger, S. Drost, W. Hellmich, G. Muller, C. Bosch-Braunmuhl, A. Krenkow, C. Perego, G. Sberveglieri, A thin film sensor system for simultaneous detection of CO and NO x with neural signal evaluation, Sensors and Actuators B 35–36 Ž1996. 353–357. w16x J. Santos, P. Serrini, B. O’Beirn, L. Manes, A thin film SnO 2 gas sensor selective to ultra-low NO 2 concentrations in air, Sensors and Actuators B 43 Ž1997. 154–160. w17x I. Sayago, J. Gutierrez, L. Ares, J.I. Robla, M.C. Horrillo, J. Getino, J.A. Agapito, The interaction of different oxidizing agents on doped tin dioxide, Sensors and Actuators B 24–25 Ž1995. 512–515. w18x G. Williams, G.S.V. Coles, The influence of deposition parameters
w19x w20x
w21x
w22x w23x
w24x
w25x
on the performance of tin dioxide NO 2 sensors prepared by radiofrequency magnetron sputtering, Sensors and Actuators B 24–25 Ž1995. 469–473. R. Lalauze, P. Breuil, C. Pijolat, Thin films gas sensors, Sensors and Actuators B 3 Ž1991. 175–182. M. Sauvan, C. Pijolat, Selectivity improvement of SnO 2 films by superficial metallic films, in: N.M. Whit ŽEd.., Proceeding Eurosensors XII ŽSept. 1998, Southampton., Vol. 1, 1998, pp. 625–628, to be published in Sensors and Actuators. E.M. Logothetis, Automotive oxygen sensors, in: N. Yamazoe ŽEd.., Chemical Sensor Technology, Vol. 3, Elsevier, Amsterdam, 1991, pp. 89–104. N. Kato, K. Nakagaki, N. Ina, Thick film ZrO 2 NO x sensors, SAE technical paper series 960334137-142 Ž1996. 11–18. E. Visconte-Facetta, C. Pijolat, R. Lalauze, Dispositif de detection ´ de la presence d’un gaz dans une atmosphere, Patents Ž1993.: Fr ´ 9207414, MI93 A 001319, P43 19 836.8. C. Pijolat, C. Pupier, C. Testud, J.C. Marchand, R. Lalauze, L. Montanaro, A. Negro, C. Malvicino, Electrochemical sensor for COrNO x detection in automotive applications, Proceedings of Sensor ’97 ŽNuremberg ’97 May., Vol. B1, 1997, pp. 119–124, to be published in J. Electroceram. C. Pupier, C. Pijolat, J.C. Marchand, R. Lalauze, Oxygen role on the electrochemical response of a gas sensor using polarisable electrodes, Proceedings 195 ECS Meeting Ž1998 May, San Diego., to be published in J. Electrochem. Soc.