- Email: [email protected]
A model for the responses of Nb2O5 sensors to CO and NH3 gases
Sensors and Actuators B 43 (1997) 60 – 64
A model for the responses of Nb2O5 sensors to CO and NH3 gases L. Chambon a, A. Pauly a, J.P. Germain a,*, C. Maleysson a, V. Demarne b, A. Grisel b a
LASMEA, URA CNRS 1793, Uni6ersite´ Blaise Pascal Clermont II, 63177, Aubie`re Cedex, France b Microsens S.A. Rue Jaquet-Droz 1, 2007, Neuchaˆtel, Switzerland Accepted 3 April 1997
Abstract Through a study of Nb2O5 sensors in the presence of CO and NH3 gases, the optimal operating conditions of these sensors are defined in order to make a model of their responses according to relations based on elementary physico-chemical interaction processes. The association of such sensors with other metallic oxide sensors within a sensor array, and processing of their response signals, can lead to selective devices towards the experimentally tested gas species. © 1997 Elsevier Science S.A. Keywords: Nb2O5 sensors; Physico-chemical interaction; Metallic oxide sensors; CO; NH3
1. Introduction
2. Theory
Semiconductor gas sensors are not intrinsically selective. This problem can be solved by the association of such sensors within a sensor array associated to a data processing unit. The considered recognition methods use simple laws based on elementary physico-chemical interaction processes. These laws, that are shown to be valid in the case of SnO2 sensors, make it possible to relate the sensor signal to the test gas concentration by introducing specific coefficients for each sensor/gas pair. The discriminating power of the considered recognition methods is all the more efficient as these coefficients are distinct. For this purpose, we propose to associate SnO2 to other metallic oxides such as Nb2O5 that are likely to provide different or even complementary responses. First we present a study of the behavior of Nb2O5 sensors in the presence of CO and NH3 gases. After this first step, we can define the operating range of these sensors in view of making a model of their responses. In the last part, we compare the results of the response fits of SnO2 and Nb2O5 sensors in the presence of CO and NH3 gases.
Metallic oxides SnO2 and Nb2O5 exhibit anionic vacancies that make them N-type semiconductors. The detection principle in these materials is based on the reversible modulation of the conductance in the presence of oxidizing (O2) or reducing (NH3, CO) gases. A model makes it possible to define a simple relation between the conductivity of the oxide layer and the gas concentration. In a N-type semiconductor, acceptor or donor surface states are created by the chemisorption of oxidizing or reducing gases respectively. The presence of surface states below the Fermi level (acceptor surface states) results in an electronic transfer from the conduction band to these surface states and the building up of a depletion layer. Inversely, introduction of surface states above the Fermi level (donor surface states) sets up an enhancement zone. In a polycrystal such as SnO2 or Nb2O5, because of the presence of oxygen, the acceptor states at the surface of the crystallites produce potential barriers at grain boundaries that can be represented by two back-to-back Schottky barriers. In this kind of structure, if the grain size is large enough and if the grains touch each other but are not sintered, the contact resistance between the grains is assumed to overcome the sensor resistance [1]. In these conditions, an empirical law between the sensor relative resistance and the gas concentration [GAS] was formulated [2,3]:
* Corresponding 473407262.
author.
Tel.:
+33
473407240;
fax;
+ 33
0925-4005/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 5 - 4 0 0 5 ( 9 7 ) 0 0 1 3 6 - 6
L. Chambon et al. / Sensors and Actuators B 43 (1997) 60–64
61
Fig. 1. Nb2O5 sensor structure with interdigitated electrodes.
Y=
R =(1+a× [GAS])m R0
(1)
where R is the sensor resistance in the presence of the gas, R0 is its resistance in the absence of the gas, a and m are the specific coefficients of the sensor/gas pair under study. Combining the Schottky-barrier conduction mechanism and the Freundlich adsorption mechanism, it has been theoretically demonstrated that the conductance can be related to the gas concentration [GAS] according to the law [4]: Y=
G =exp(a × [GAS]m) G0
(2)
where G and G0 are the sensor conductance in the presence of the gas and in its absence respectively, a and m are the specific coefficients of the sensor/gas pair under study. It has been shown that these two relations can be applied to the SnO2 sensors behavior. A study of the Nb2O5 sensors is necessary to check the validity of the model assumptions.
3. Experimental results on Nb2O5 sensors Our study is centered on the detection of particular gases that rank among the major pollutants of ambient air (CO) or that take place in many industrial processes (NH3). To identify these gases, we use microelectronic sensor structures with SnO2 and Nb2O5 metallic oxide sensitive thin layers. The sensor structures are manufactured by Microsens. Because the Nb2O5 layers are highly resistive, interdigitated electrodes are used in order to decrease the resistance to be measured. The interelectrode spacing is only 10 mm long. Fig. 1 represents the Nb2O5 sensor structure. Such an electrode arrangement is not essential for the SnO2 sensors that are less resistive; in this case, the electrodes are made of two wide contacts 320 mm long
and 300 mm spaced. The average grain size is of the same order of magnitude for both oxides [5,6]. The sensitive layer thickness is 270 nm for SnO2 sensors [5] and 75 nm for Nb2O5 sensors [6]. CO and NH3 gases used for tests are diluted in synthetic air; the vector gas is synthetic air. The total gas flow of the doping gas/ vector gas mixture is 18 l h − 1. The same cycle made of increasing concentration steps, with 20 min-long steps, is applied for each gas. Eqs. (1) and (2) are based on surfacic conductivity. But in many oxides (SnO2, TiO2, Nb2O5...), bulk properties are thermodynamically equilibrated with oxygen partial pressure for temperatures above 600–700°C [7]. At lower temperatures, the conductivity variation is associated to a surfacic conductivity variation. In order to fit the data of SnO2 and Nb2O5 sensors in the presence of CO and NH3 gases by the laws Eqs. (1) and (2), our experiments were performed at operating temperatures below or equal to 500°C. The experimental results can obey the previous laws only in the case of ohmic contacts between the sensing layer and the electrodes. This assumption is checked for SnO2 sensors at every operating temperature and atmosphere, but it is not the case of the Nb2O5 sensors in which different conduction mechanisms are observed as a function of the polarization voltage. For various operating temperatures and gaseous atmospheres (pure synthetic air or presence of CO or NH3 gas), a non linear pattern (I=V n, with n\ 2) is displayed on the I–V characteristics of Nb2O5 sensors above a threshold voltage Vs. Rosenfeld et al. [6] showed that this feature is related to a decrease of the thickness of the space charge zone located at the metal/material contact that gives rise to tunneling charge carrier flow. At low voltages, n is close to one. The value of the threshold voltage Vs, that depends on the ambient environment, remains larger than 1 V for each test atmosphere. At 500°C and in the 200–1000 ppm CO concentration range, the threshold voltage is about 4 V. Fig. 2 shows the I–V characteristics of the Nb2O5 sensor at an operating temperature of
62
L. Chambon et al. / Sensors and Actuators B 43 (1997) 60–64
Fig. 2. I– V characteristics of the Nb2O5 sensor at 500°C for two concentrations of NH3 gas.
500°C in the presence of two concentrations of NH3 gas. This study emphasized an interesting behavior of the Nb2O5 sensors in the presence of CO gas as a function of the polarization voltage. Fig. 3 shows the influence of the polarization voltage on the sensor response to CO at 500°C. Almost no sensitivity to CO is observed at a 8 V polarization voltage, whereas a 1 V polarization voltage provides a good sensitivity to CO gas. In order to avoid any problem of non-ohmic metal/material contact, the polarization voltage of the Nb2O5 sensors is set to 1 V. At this voltage, Nb2O5 sensor responses to the action of CO and NH3 gases at 500°C are shown on Fig. 3a and Fig. 4 respectively. However, it must be noticed that the responses displayed on both figures result from the simultaneous contribution of NH3 or CO and of oxygen from synthetic air. For instance, in identical experimental conditions (T=500°C, U= 1 V), sensitivity of Nb2O5 to
Fig. 3. Influence of polarization voltage on the Nb2O5 response to CO at 500°C. a) × : U =1 V; b) : U =8 V.
Fig. 4. Nb2O5 response to NH3 diluted in synthetic air for T= 500°C, U =1 V.
NH3 is shown to be higher when NH3 is diluted in nitrogen (Fig. 5) than in synthetic air (i.e. 20.5% O2 in N2)(Fig. 4). This result points out that the current variations caused by increasing NH3 concentrations are attenuated by the presence of oxygen. But beyond this compensating action, oxygen may also be involved in reaction mechanisms with NH3. So, when the initial current I0 in nitrogen (0.5 · 10 − 9 A) is only slightly higher than in synthetic air (0.1 · 10 − 9 A), the value of the current during the ‘NH3 on’ step is much higher if NH3 is diluted in nitrogen than in synthetic air. For example, the value of the current at the end of 20 min
Fig. 5. Nb2O5 response to NH3 diluted in nitrogen for T= 500°C, U =1 V.
L. Chambon et al. / Sensors and Actuators B 43 (1997) 60–64
63
Table 1 fitting uncertainty for Nb2O5 Gas
Fig. 6. a) + R/R0: fitting according to law Eq. (1). b) × I/I0: fitting according to law Eq. (2).
exposure to 1000 ppm of NH3 is only 1.5 · 10 − 9 A for NH3 diluted in synthetic air against 0.38 · 10 − 5 A in nitrogen. The I– V characteristics of SnO2 sensors have also been measured; they exhibit an ohmic behavior at all tested operating temperatures and atmospheres. Consequently, the polarization voltage of SnO2 sensors is set to 1 V alike.
4. Estimation of the model validity Data fitted by relations Eq. (1) or Eq. (2) are the sensor responses to NH3 and CO diluted in synthetic air, at the end of 20 min exposure. Figs. 6 and 7 show compared fittings of the Nb2O5 sensor responses at 500°C to the action of NH3 and CO gas respectively, according to law Eq. (1) (Fig. 6a and Fig. 7a) or to law
T (°C)
Law Eq. (1) (%)
Eq. (2) (%)
NH3
500 450 400
2.12 1.40 1.34
0.39 0.53 1.24
CO
500 450 400
0.17 0.90 2.38
0.43 0.25 0.93
Eq. (2) (Fig. 6b and Fig. 7b). We compare fittings of Nb2O5 and SnO2 sensor data in the presence of NH3 and CO gases. For each temperature and gas, Tables 1 and 2 give the fitting uncertainty for K tested concentrations: INC(%)=
1 K 100 * Yi exp − Yi mod % K i=1 Yi mod
(Yi exp and Yi mod are the experimental relative data R/R0 or I/I0 and the fitted data respectively.) In order to get evidence of the type of gas, a classification rule has to extract only the gas-specific information. Tests on experimental data show that the higher accuracy of the fittings is obtained with relation Eq. (1) when the coefficient m is assigned an identical value for every sensor for a same gas. This condition makes it possible to define for each sensor i the quantity Pij, that is characteristic of gas j but independent of gas j concentration, as: Pij =
Yij N
% Yij i=1
=
j am ij
N
for aij [GASj ] 1
j % am ij
i=1
Then, setting T =500°C for the Nb2O5 sensor and T= 450°C for the SnO2 sensor, Fig. 8 shows the distribution of the Pij values derived from seven measurement series for each gas. The average fitting accuracy for these seven measurement series is 3.66% for Nb2O5 and 2.32% for SnO2 relatively to NH3, and 1.33% for Nb2O5 and 1.67% for SnO2 relatively to CO. Table 2 fitting uncertainty for SnO2 Gas
Fig. 7. a) + R/R0: fitting according to law Eq. (1). b) × I/I0: fitting according to law Eq. (2).
T (°C)
Law Eq. (1) (%)
Eq. (2) (%)
NH3
500 450
0.92 0.84
0.72 1.46
CO
500 450
0.34 0.33
0.36 0.70
L. Chambon et al. / Sensors and Actuators B 43 (1997) 60–64
64
Biographies L. Chambon took her master’s degree in electronics at the Blaise Pascal University and joined the LASMEA in order to prepare a Doctorat d’Universite´ on the design of a gas multisensor associating different sensitive metallic oxides and on data analysis. A. Pauly holds a Doctorat d’Universite´ from the Blaise Pascal University since 1993 and presently works at this university as a Maıˆtre de Confe´rences. His research activity in the LASMEA is devoted to gas sensors and to the interpretation of gas-interaction phenomena in various sensitive materials to optimize their gas-sensing properties.
Fig. 8. Distribution of the Pij values.
5. Conclusions With the operating conditions chosen for the Nb2O5 sensors (polarization voltage of 1 V and operating temperature below or equal to 500°C), the responses towards CO and NH3 gases are correctly fitted by the laws Eqs. (1) and (2) and the quality of these fittings is comparable to that of SnO2 sensors. This first approach was intended to check that recognition methods can be applied to a multisensor associating metallic oxides of different nature. It succeeds in showing that the association of Nb2O5 and SnO2 sensors can provide distinguishable signals for CO and NH3 gases while maintaining a low uncertainty on data fittings.
References [1] S.R. Morrison, Semiconductor gas sensors, Sensors and Actuators 2 (1982) 329 – 341. [2] P.K. Clifford, Homogeneous semiconducting gas sensors: a comprehensive model, in: Proceedings of the International Meeting on Chemical Sensors, Fukuoka, Japan, 1983. [3] S.R. Morrison, Mechanism of semiconducting gas sensor operation, Sensors and Actuators 11 (1987) 283–287. [4] R.K. Srivastava, P. Lal, R. Dwivedi, S.K. Srivastava, Sensing mechanism in tin oxide based thick-film gas sensors, Sensors and Actuators B 21 (1994) 213–218. [5] V. Demarne, A. Grisel, A new low temperature deposition technique for integrated sensors, Sensors and Actuators B 15 – 16 (1993) 63 – 67. [6] D. Rosenfeld, Proprie´te´s structurales et e´lectriques de couches minces de Nb2O5 lie´es au fonctionnement d’un capteur oxyge`ne, the`se No. 1313, Ecole Polytechnique de Lausanne, 1994. [7] D.E. Williams, Conduction and gas response of semiconductor gas sensors, in: P.T. Moseley, B.C. Tofield (Eds.), Solid Gas Sensors, Adam Hilger, Bristol, 1987.
J.P. Germain, Professor at the Blaise Pascal University, received his Doctorat d’Etat es Sciences in 1977 for his research in the field of liquid crystals. He is presently Head of the ‘Gas Sensors’ group in the LASMEA, with reseach interests in organic and inorganic sensitive materials, development of a multisensor and of gas recognition methods, and gas sensing devices miniaturization. C. Maleysson received her Docteur-Inge´nieur degree in 1984 from the Blaise Pascal University for her work on doping processes in electroactive polymers. As a Charge´e de Recherche of the C.N.R.S. in the LASMEA, she is presently studying the potential use of various semiconducting materials in a gas multisensor. Dr V. Demarne is Sensor Project Leader at Microsens S.A. since 1991. His main activity is focussed on the research and development in the field of integrated semiconductor gas sensors. Dr V. Demarne obtained a M.Sc. degree from ENSEEG Grenoble, France, in 1981 and a Ph.D in 1991 from the Federal Institute of Technology of Lausanne (EPFL), Switzerland. He concentrated his activity in the research and development of integrated semiconducting gas sensors between 1984 and 1990 in the departement of chemical sensors at the Swiss Center of Electronics and Microtechnology (CSEM) in Neuchaˆtel and from 1991 within Microsens S.A. Dr A. Grisel is President and Executive Director of the company Microsens S.A. located in Neuchaˆtel, Switzerland. He is leading research, development and production facilities within Microsens for integrated chemical sensor technology, such as integrated gas sensors and electrochemical ion sensors. Dr A. Grisel obtained a M.Sc. Degree in physics in 1976 and a Ph.D in solid state physics in 1981 from the Federal Institute of Technology of Lausanne (EPFL), Switzerland. He was leading the departement of R and D on integrated chemical sensors at the Swiss Center for Electronics and Microtechnology (CSEM) in Neuchaˆtel, from 1981 to 1990, as he created the company Microsens S.A.
A model for the responses of Nb2O5 sensors to CO and NH3 gases L. Chambon a, A. Pauly a, J.P. Germain a,*, C. Maleysson a, V. Demarne b, A. Grisel b a
LASMEA, URA CNRS 1793, Uni6ersite´ Blaise Pascal Clermont II, 63177, Aubie`re Cedex, France b Microsens S.A. Rue Jaquet-Droz 1, 2007, Neuchaˆtel, Switzerland Accepted 3 April 1997
Abstract Through a study of Nb2O5 sensors in the presence of CO and NH3 gases, the optimal operating conditions of these sensors are defined in order to make a model of their responses according to relations based on elementary physico-chemical interaction processes. The association of such sensors with other metallic oxide sensors within a sensor array, and processing of their response signals, can lead to selective devices towards the experimentally tested gas species. © 1997 Elsevier Science S.A. Keywords: Nb2O5 sensors; Physico-chemical interaction; Metallic oxide sensors; CO; NH3
1. Introduction
2. Theory
Semiconductor gas sensors are not intrinsically selective. This problem can be solved by the association of such sensors within a sensor array associated to a data processing unit. The considered recognition methods use simple laws based on elementary physico-chemical interaction processes. These laws, that are shown to be valid in the case of SnO2 sensors, make it possible to relate the sensor signal to the test gas concentration by introducing specific coefficients for each sensor/gas pair. The discriminating power of the considered recognition methods is all the more efficient as these coefficients are distinct. For this purpose, we propose to associate SnO2 to other metallic oxides such as Nb2O5 that are likely to provide different or even complementary responses. First we present a study of the behavior of Nb2O5 sensors in the presence of CO and NH3 gases. After this first step, we can define the operating range of these sensors in view of making a model of their responses. In the last part, we compare the results of the response fits of SnO2 and Nb2O5 sensors in the presence of CO and NH3 gases.
Metallic oxides SnO2 and Nb2O5 exhibit anionic vacancies that make them N-type semiconductors. The detection principle in these materials is based on the reversible modulation of the conductance in the presence of oxidizing (O2) or reducing (NH3, CO) gases. A model makes it possible to define a simple relation between the conductivity of the oxide layer and the gas concentration. In a N-type semiconductor, acceptor or donor surface states are created by the chemisorption of oxidizing or reducing gases respectively. The presence of surface states below the Fermi level (acceptor surface states) results in an electronic transfer from the conduction band to these surface states and the building up of a depletion layer. Inversely, introduction of surface states above the Fermi level (donor surface states) sets up an enhancement zone. In a polycrystal such as SnO2 or Nb2O5, because of the presence of oxygen, the acceptor states at the surface of the crystallites produce potential barriers at grain boundaries that can be represented by two back-to-back Schottky barriers. In this kind of structure, if the grain size is large enough and if the grains touch each other but are not sintered, the contact resistance between the grains is assumed to overcome the sensor resistance [1]. In these conditions, an empirical law between the sensor relative resistance and the gas concentration [GAS] was formulated [2,3]:
* Corresponding 473407262.
author.
Tel.:
+33
473407240;
fax;
+ 33
0925-4005/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 5 - 4 0 0 5 ( 9 7 ) 0 0 1 3 6 - 6
L. Chambon et al. / Sensors and Actuators B 43 (1997) 60–64
61
Fig. 1. Nb2O5 sensor structure with interdigitated electrodes.
Y=
R =(1+a× [GAS])m R0
(1)
where R is the sensor resistance in the presence of the gas, R0 is its resistance in the absence of the gas, a and m are the specific coefficients of the sensor/gas pair under study. Combining the Schottky-barrier conduction mechanism and the Freundlich adsorption mechanism, it has been theoretically demonstrated that the conductance can be related to the gas concentration [GAS] according to the law [4]: Y=
G =exp(a × [GAS]m) G0
(2)
where G and G0 are the sensor conductance in the presence of the gas and in its absence respectively, a and m are the specific coefficients of the sensor/gas pair under study. It has been shown that these two relations can be applied to the SnO2 sensors behavior. A study of the Nb2O5 sensors is necessary to check the validity of the model assumptions.
3. Experimental results on Nb2O5 sensors Our study is centered on the detection of particular gases that rank among the major pollutants of ambient air (CO) or that take place in many industrial processes (NH3). To identify these gases, we use microelectronic sensor structures with SnO2 and Nb2O5 metallic oxide sensitive thin layers. The sensor structures are manufactured by Microsens. Because the Nb2O5 layers are highly resistive, interdigitated electrodes are used in order to decrease the resistance to be measured. The interelectrode spacing is only 10 mm long. Fig. 1 represents the Nb2O5 sensor structure. Such an electrode arrangement is not essential for the SnO2 sensors that are less resistive; in this case, the electrodes are made of two wide contacts 320 mm long
and 300 mm spaced. The average grain size is of the same order of magnitude for both oxides [5,6]. The sensitive layer thickness is 270 nm for SnO2 sensors [5] and 75 nm for Nb2O5 sensors [6]. CO and NH3 gases used for tests are diluted in synthetic air; the vector gas is synthetic air. The total gas flow of the doping gas/ vector gas mixture is 18 l h − 1. The same cycle made of increasing concentration steps, with 20 min-long steps, is applied for each gas. Eqs. (1) and (2) are based on surfacic conductivity. But in many oxides (SnO2, TiO2, Nb2O5...), bulk properties are thermodynamically equilibrated with oxygen partial pressure for temperatures above 600–700°C [7]. At lower temperatures, the conductivity variation is associated to a surfacic conductivity variation. In order to fit the data of SnO2 and Nb2O5 sensors in the presence of CO and NH3 gases by the laws Eqs. (1) and (2), our experiments were performed at operating temperatures below or equal to 500°C. The experimental results can obey the previous laws only in the case of ohmic contacts between the sensing layer and the electrodes. This assumption is checked for SnO2 sensors at every operating temperature and atmosphere, but it is not the case of the Nb2O5 sensors in which different conduction mechanisms are observed as a function of the polarization voltage. For various operating temperatures and gaseous atmospheres (pure synthetic air or presence of CO or NH3 gas), a non linear pattern (I=V n, with n\ 2) is displayed on the I–V characteristics of Nb2O5 sensors above a threshold voltage Vs. Rosenfeld et al. [6] showed that this feature is related to a decrease of the thickness of the space charge zone located at the metal/material contact that gives rise to tunneling charge carrier flow. At low voltages, n is close to one. The value of the threshold voltage Vs, that depends on the ambient environment, remains larger than 1 V for each test atmosphere. At 500°C and in the 200–1000 ppm CO concentration range, the threshold voltage is about 4 V. Fig. 2 shows the I–V characteristics of the Nb2O5 sensor at an operating temperature of
62
L. Chambon et al. / Sensors and Actuators B 43 (1997) 60–64
Fig. 2. I– V characteristics of the Nb2O5 sensor at 500°C for two concentrations of NH3 gas.
500°C in the presence of two concentrations of NH3 gas. This study emphasized an interesting behavior of the Nb2O5 sensors in the presence of CO gas as a function of the polarization voltage. Fig. 3 shows the influence of the polarization voltage on the sensor response to CO at 500°C. Almost no sensitivity to CO is observed at a 8 V polarization voltage, whereas a 1 V polarization voltage provides a good sensitivity to CO gas. In order to avoid any problem of non-ohmic metal/material contact, the polarization voltage of the Nb2O5 sensors is set to 1 V. At this voltage, Nb2O5 sensor responses to the action of CO and NH3 gases at 500°C are shown on Fig. 3a and Fig. 4 respectively. However, it must be noticed that the responses displayed on both figures result from the simultaneous contribution of NH3 or CO and of oxygen from synthetic air. For instance, in identical experimental conditions (T=500°C, U= 1 V), sensitivity of Nb2O5 to
Fig. 3. Influence of polarization voltage on the Nb2O5 response to CO at 500°C. a) × : U =1 V; b) : U =8 V.
Fig. 4. Nb2O5 response to NH3 diluted in synthetic air for T= 500°C, U =1 V.
NH3 is shown to be higher when NH3 is diluted in nitrogen (Fig. 5) than in synthetic air (i.e. 20.5% O2 in N2)(Fig. 4). This result points out that the current variations caused by increasing NH3 concentrations are attenuated by the presence of oxygen. But beyond this compensating action, oxygen may also be involved in reaction mechanisms with NH3. So, when the initial current I0 in nitrogen (0.5 · 10 − 9 A) is only slightly higher than in synthetic air (0.1 · 10 − 9 A), the value of the current during the ‘NH3 on’ step is much higher if NH3 is diluted in nitrogen than in synthetic air. For example, the value of the current at the end of 20 min
Fig. 5. Nb2O5 response to NH3 diluted in nitrogen for T= 500°C, U =1 V.
L. Chambon et al. / Sensors and Actuators B 43 (1997) 60–64
63
Table 1 fitting uncertainty for Nb2O5 Gas
Fig. 6. a) + R/R0: fitting according to law Eq. (1). b) × I/I0: fitting according to law Eq. (2).
exposure to 1000 ppm of NH3 is only 1.5 · 10 − 9 A for NH3 diluted in synthetic air against 0.38 · 10 − 5 A in nitrogen. The I– V characteristics of SnO2 sensors have also been measured; they exhibit an ohmic behavior at all tested operating temperatures and atmospheres. Consequently, the polarization voltage of SnO2 sensors is set to 1 V alike.
4. Estimation of the model validity Data fitted by relations Eq. (1) or Eq. (2) are the sensor responses to NH3 and CO diluted in synthetic air, at the end of 20 min exposure. Figs. 6 and 7 show compared fittings of the Nb2O5 sensor responses at 500°C to the action of NH3 and CO gas respectively, according to law Eq. (1) (Fig. 6a and Fig. 7a) or to law
T (°C)
Law Eq. (1) (%)
Eq. (2) (%)
NH3
500 450 400
2.12 1.40 1.34
0.39 0.53 1.24
CO
500 450 400
0.17 0.90 2.38
0.43 0.25 0.93
Eq. (2) (Fig. 6b and Fig. 7b). We compare fittings of Nb2O5 and SnO2 sensor data in the presence of NH3 and CO gases. For each temperature and gas, Tables 1 and 2 give the fitting uncertainty for K tested concentrations: INC(%)=
1 K 100 * Yi exp − Yi mod % K i=1 Yi mod
(Yi exp and Yi mod are the experimental relative data R/R0 or I/I0 and the fitted data respectively.) In order to get evidence of the type of gas, a classification rule has to extract only the gas-specific information. Tests on experimental data show that the higher accuracy of the fittings is obtained with relation Eq. (1) when the coefficient m is assigned an identical value for every sensor for a same gas. This condition makes it possible to define for each sensor i the quantity Pij, that is characteristic of gas j but independent of gas j concentration, as: Pij =
Yij N
% Yij i=1
=
j am ij
N
for aij [GASj ] 1
j % am ij
i=1
Then, setting T =500°C for the Nb2O5 sensor and T= 450°C for the SnO2 sensor, Fig. 8 shows the distribution of the Pij values derived from seven measurement series for each gas. The average fitting accuracy for these seven measurement series is 3.66% for Nb2O5 and 2.32% for SnO2 relatively to NH3, and 1.33% for Nb2O5 and 1.67% for SnO2 relatively to CO. Table 2 fitting uncertainty for SnO2 Gas
Fig. 7. a) + R/R0: fitting according to law Eq. (1). b) × I/I0: fitting according to law Eq. (2).
T (°C)
Law Eq. (1) (%)
Eq. (2) (%)
NH3
500 450
0.92 0.84
0.72 1.46
CO
500 450
0.34 0.33
0.36 0.70
L. Chambon et al. / Sensors and Actuators B 43 (1997) 60–64
64
Biographies L. Chambon took her master’s degree in electronics at the Blaise Pascal University and joined the LASMEA in order to prepare a Doctorat d’Universite´ on the design of a gas multisensor associating different sensitive metallic oxides and on data analysis. A. Pauly holds a Doctorat d’Universite´ from the Blaise Pascal University since 1993 and presently works at this university as a Maıˆtre de Confe´rences. His research activity in the LASMEA is devoted to gas sensors and to the interpretation of gas-interaction phenomena in various sensitive materials to optimize their gas-sensing properties.
Fig. 8. Distribution of the Pij values.
5. Conclusions With the operating conditions chosen for the Nb2O5 sensors (polarization voltage of 1 V and operating temperature below or equal to 500°C), the responses towards CO and NH3 gases are correctly fitted by the laws Eqs. (1) and (2) and the quality of these fittings is comparable to that of SnO2 sensors. This first approach was intended to check that recognition methods can be applied to a multisensor associating metallic oxides of different nature. It succeeds in showing that the association of Nb2O5 and SnO2 sensors can provide distinguishable signals for CO and NH3 gases while maintaining a low uncertainty on data fittings.
References [1] S.R. Morrison, Semiconductor gas sensors, Sensors and Actuators 2 (1982) 329 – 341. [2] P.K. Clifford, Homogeneous semiconducting gas sensors: a comprehensive model, in: Proceedings of the International Meeting on Chemical Sensors, Fukuoka, Japan, 1983. [3] S.R. Morrison, Mechanism of semiconducting gas sensor operation, Sensors and Actuators 11 (1987) 283–287. [4] R.K. Srivastava, P. Lal, R. Dwivedi, S.K. Srivastava, Sensing mechanism in tin oxide based thick-film gas sensors, Sensors and Actuators B 21 (1994) 213–218. [5] V. Demarne, A. Grisel, A new low temperature deposition technique for integrated sensors, Sensors and Actuators B 15 – 16 (1993) 63 – 67. [6] D. Rosenfeld, Proprie´te´s structurales et e´lectriques de couches minces de Nb2O5 lie´es au fonctionnement d’un capteur oxyge`ne, the`se No. 1313, Ecole Polytechnique de Lausanne, 1994. [7] D.E. Williams, Conduction and gas response of semiconductor gas sensors, in: P.T. Moseley, B.C. Tofield (Eds.), Solid Gas Sensors, Adam Hilger, Bristol, 1987.
J.P. Germain, Professor at the Blaise Pascal University, received his Doctorat d’Etat es Sciences in 1977 for his research in the field of liquid crystals. He is presently Head of the ‘Gas Sensors’ group in the LASMEA, with reseach interests in organic and inorganic sensitive materials, development of a multisensor and of gas recognition methods, and gas sensing devices miniaturization. C. Maleysson received her Docteur-Inge´nieur degree in 1984 from the Blaise Pascal University for her work on doping processes in electroactive polymers. As a Charge´e de Recherche of the C.N.R.S. in the LASMEA, she is presently studying the potential use of various semiconducting materials in a gas multisensor. Dr V. Demarne is Sensor Project Leader at Microsens S.A. since 1991. His main activity is focussed on the research and development in the field of integrated semiconductor gas sensors. Dr V. Demarne obtained a M.Sc. degree from ENSEEG Grenoble, France, in 1981 and a Ph.D in 1991 from the Federal Institute of Technology of Lausanne (EPFL), Switzerland. He concentrated his activity in the research and development of integrated semiconducting gas sensors between 1984 and 1990 in the departement of chemical sensors at the Swiss Center of Electronics and Microtechnology (CSEM) in Neuchaˆtel and from 1991 within Microsens S.A. Dr A. Grisel is President and Executive Director of the company Microsens S.A. located in Neuchaˆtel, Switzerland. He is leading research, development and production facilities within Microsens for integrated chemical sensor technology, such as integrated gas sensors and electrochemical ion sensors. Dr A. Grisel obtained a M.Sc. Degree in physics in 1976 and a Ph.D in solid state physics in 1981 from the Federal Institute of Technology of Lausanne (EPFL), Switzerland. He was leading the departement of R and D on integrated chemical sensors at the Swiss Center for Electronics and Microtechnology (CSEM) in Neuchaˆtel, from 1981 to 1990, as he created the company Microsens S.A.