- Email: [email protected]
CH4 thick-film gas sensors: Characterization method and theoretical explanation
191
Sensors and Acfuurors B, 3 (1991) 191-196
CH4 thick-film gas sensors: characterization explanation M. C. Carotta,
C. Dsllara,
G. Martinelli
Department of Physics, 12 t4a Pamdiso,
method and theoretical
and L. Passari
University of Ferram, I-44100 Fermra (Italy)
A. Camanzi Eniricerche S.pA., I5 Via Ramarini, I-00015 Montemtondo
(Rome) (Itab)
(Received August 9, 1990, in revised form February 5, 1991; accepted February 6, 1991)
Abstract The characterization of Sn02 thick-film gas sensors made from a homemade binderless paste is given. The Hall effect has been measured together with the conductance to determine the carrier concentration and the mobility. From temperature-stimulated conductance measurements at several fixed temperatures, the range where the intergranular conductance can be reasonably considered to be constant has been explored. The microstructure and morphology of the samples are analysed by TEM and SEM observations. The results confirm for these sensors the possibility of linking the electronic structure of the bulk and the chemical state of the surface by an energy barrier model. Moreover, we correlate the difference between the maximum and the minimum of the barrier energy, Al?, to the sensitivity to reducing gas.
1. Introduction In the last ten years great interest has been devoted to the research and development of gas sensors due to their potentiality to solve a wide range of problems, such as to control industrial processes, to sample environmental conditions and to warn of poisonous gas leakages. In this work we provide a characterization of thick-film gas sensors manufactured by printing on alumina substrates different thicknesses of a homemade paste based on SnO, powder. To avoid the effect of mineral binder at the grain boundaries, we prepared the paste with only organic solvents which have a maximum burnout temperature of approximately 500 “C; the samples were fired at 850 “C for half a hour. TEM and SEM micrographs were used to analyse the microstructural and morphological properties; the size of the grains ranges from 15 to ZOOnm. Our aim was to find suitable parameters relating to the sensitivity to reducing gas, in particular methane.
09254005/91/$3.50
Sensitivity curves for methane were obtained on samples without catalysts to prevent any variation of the physical and chemical properties at the surface and in the bulk of the devices.
2. Experimental
procedure
First we investigated the non-ohmic dependence of the current on the applied voltage by plotting the I-Vcurves in the temperature range 200 to 500 “C as suggested by other researchers [l]. The non-ohmic behaviour (Fig. 1) can be justified by an intergranular current controlled by the energy barriers at the contacting surfaces of the particles [2]. The consistency of the intergranular model for our samples has been pointed out by SEM and TEM observations (Figs. 2 and 3). On the basis of the barrier theory proposed by Clifford and Tuma [3], we analysed on our samples the link between the electronic structure of the bulk and the chemical state of the surface. The exponential term of the Morrison equation [2) ^
0 1991 -
Elsevier Sequoia, Lausanne
192
. I
1500 :_T 1000
z
2
2
.
15 10
.
2.
.
.
500
.
5
l
.
:
i
:’
.
.* -t..... 0 (a)
0
10
20 v
30
Fig. 2. SEM micrograph binderless paste based 850 “C.
5
as a function
15
:
of voltage
at (a) 200
of a typical sensor made from a on SnO, powder and sintered at
Fig. 3. TEM micrograph showing the sensor described in Fig. 2.
particles
G = G, exp[ - eVJk7’j is very sensitive to the potential which is related to the absorbed N, by the Schottky formula eV, = e2N,*/2c~,N,
10
v (v)
(b)
(VI
Fig. 1. Current values “C and (b) 550 “C.
OL 0
40
and
necks
in
(1) barrier eVS, oxygen ions
which is a direct consequence of the Poisson equation; EEL is the permittivity of the semiconductor and N, is the donor concentration. The bulk intergranular conductance is represented by Go=ge~n, where g is a geometrical factor (in our case the sample thickness), e is the electronic charge, I_Lis the electron mobility and nb the concentration of electrons in the bulk conduction band. G, is usually considered to be almost constant because the mobility and the other factors are expected to vary more slowly with temperature than the carrier concentration, which is controlled by the exponential factor 141. The conductivity is the product of the carrier concentration and mobility; to separate the two variables we performed two independent measurements: (a) Hall effect measurements at a convenient temperature, which allow us to determine the total carrier number; (b) Temperature-stimulated conductance measurements to determine the barrier energy at the same temperature as the Hall effect measurement. Moreover, from these two measurements it has been possible to verify the range of temperature where G, can actually be assumed to be almost constant. Hall measurement The thick-film sensors were placed inside a test chamber (300 cm3 in volume) of a suitable size to be located between the poles of a magnet. The gas (5 ppm of oxygen in nitrogen) flowed at a fixed flow rate of one litre per minute at a temperature of 350 “C. The Van der Pauw pattern with four symmetrical electrodes was used for the Hall effect measurement. The voltage across two electrodes was measured with and without a magnetic field B, with a constant current i across the two other electrodes. eV, and G,, measurements Using the same test chamber and oxygen concentration in nitrogen, the sensors were biased by a constant voltage of 9 V. The conductance was obtained from the voltage drop on a known resistor. The data were recorded and elaborated by an on-line computer.
193
of the sensors was The temperature changed by a heater printed on the back of the support and was directly measured by a Cr-Al thermocouple fixed on the samples. The barrier energy height versus T was obtained from the following three steps [3, 51: (a) The experimental measurement of the conductance versus T (Figs. 7-9). (b) The determination of G, from the barrier energy eV, corresponding to a convenient initial temperature Ti using temperature-stimulated conductance measurements. The values extrapolated to zero time, G,,, (Fig. 4) associated with each temperature T, give the conductance at T, but at the barrier value corresponding to Ti. In this way, if Go is constant, the only effect of the temperature variation is represented by
(a)
G, = G, exp[ -eV,(TJ/kT,]
w
T, =
1’5
2.5
2.0 103/T
(K-l)
1.5
1.2 103/T
(b)
1.8
(K-l)
1’5
2.0 lO’/T
1.4
1.2 Cd)
103/T
2.5
(K-l)
1.6
(K-l)
Fig. 5. (a)-(d) Logarithms of GJG, obtained from the values of Fig. 4 vs. 1000/T; from the slopes of the lines it has been possible to determine the barrier energy values for the four different temperatures.
From
we obtained eV, and then Go; Gi is the value of the conductance measured at Ti. The four straight lines obtained from the above equation (Fig. 5) describe the accuracy of the method and, at the same time, allow us to assume Go to be constant inside each temperature interval associated with the four Ti values. (c) The eV, curve as a function of T was derived from eqn. (1) and the experimental values (point (a)) of G. OOW
time (s)
(‘-‘)
time (5)
1000
Results and discussion
OOCc)
1000
time (5)
(d)
time (s)
Fig. 4. Conductance as a function of time after a fast change in the temperature from the initial temperatures of (a) 200 “C, (b) 300 “C. (c) 400 “C and (d) 480 “C. The extrapolation of these curves to zero time determines the conductances G,.
Starting from different Ti values, it is POSsible to obtain information about the range of temperature where G,, can be considered almost constant. The values of Go determined for Ti =200, 300, 400 and 480 “C are shown in Fig. 6. It is then easy to deduce that the suitable temperature for performing Hall effect measurements lies between 300 and 400 “C. The constancy of Go in this range is also justified by the linearity of the change of the absorbed oxygen from the 02- to O- form.
194
N,=4.11
,,,M 200
300
Temperature
400
500 (“C)
Fig. 6. Go values calculated from barrier energy values experimentally determined by temperature-stimulated conof 200, 300, 400 ductance measurements at temperatures and 480 “C.
The Hall effect measurements have been obtained for all the samples at 350 “C. Since the values of the carrier number measured on several samples showed a sufficient repetitiveness, we report here the mean value: n=
iB qtV,
=1.5x1015
RH = l/nq = 4.16 x 10P3 cm3/C The Fermi level of the bulk was then deduced from exp[ - eV,/kTJ
where NC is the effective state density in the conduction band at 350 “C; from the temperature-stimulated conductance measurement, we obtained at the same temperature and with an O2 partial pressure of 5 ppm in N2 a value for eV, of 0.41 eV. Then EF = - 0.064 and nb = 3.18 X 10” cme3. From the charge neutrality condition
Izb =
10” cmm3
which shows that at a temperature of 350 “C more than 70% of the donor levels are ionized. This means that the conductance has to be considered to be strongly dependent on the barrier energy height. Finally the mobility, which is p = GJgen, = 0.16 cm’/V s, is assumed to be very close to the Hall one. Values of the mobility reported in the literature [7, S] for single crystals differ by approximately two orders of magnitude from our result. The discrepancy can be explained by the granularity of our samples [7]. Our value is instead in good agreement with the Hall measurements performed by Ogawa et al. [8] on porous columnar samples where a modulating barrier effect is supposed. The scattering effects due to ionized donor impurities, interstitials and vacancies, which can affect the measurements by approximately 50%, have not been considered here.
cmh3
where i = 100 PA, B = 8000 G, t = 30 pm (sample thickness) and V, is the measured Hall voltage. The Hall constant is
n =NC exp( - E,IkT)
X
eV, dependence on temperature The conductance measurements have been performed at different oxygen partial pressures (2x 105 and 5 ppm in N2) between 150 and 550 “C at the heating and cooling rates of 6 K/min and 3 K/min. Figures 7, 8 and 9 show the dependence of the conductance on the heating or cooling run, on the rate of the run and finally on the thickness of the sample. In Fig. 9 it is clearly shown that the form of the conductance curves for sensors with different thicknesses differs greatly in the same heating run.
1 +g, exp[ - (EF - E&/kT]
+ 1 +a expE- 6% -Ed2)M’l with g, = 2 and g2 = l/2 as degeneracy factors, Edi = 0.034 eV and Ed2 = 0.145 eV the energies of native donor levels (experimentally determined by Samson and Fonstad [6] on SnO, crystals), we deduced
4i-w&?+ (a)
103/T
(K-l)
’
Fig. 7. Conductance as a function of inverse temperature measured at an oxygen partial pressure of 5 ppm in Nz at (a) a heating and (b) a cooling rate of 6 K/min.
,^
T-----l 1o-F-----l 7
.:i”,,,~~~~~~~,,~ 1.0 (a)
1.5
2.0
2.5
103/T (K-')
3.0
1.0
(b)
1.5
2.0
2.5
3.0
lO'/T (K-l)
(3)
lO'/T (K-l)
Fig. 8. Conductance as a function of inverse temperature measured at an oxygen partial pressure of 2~1~ ppm in Nr at a heating rate of (a) 6 K/min and (b) 3 K/min.
temperature (K)
W
aample E
0.3
(4
(2)
1 03/T
(K-l)
@I
103/T (K-l)
lO'/T (K-l)
‘(d)
4%nperBaOtre
(:)O
Fig. 10. (a)-(d) Conductance and energy barrier for sample A (10 pm thick) and sample B (30 pm thick); Oz partial pressures of 2X 105 ppm and 5 ppm in N2 are denoted by (A) and (0) respectively.
Fig. 9. Conductance as a function of inverse temperature measured at an oxygen partial pressure of 2~ 10r ppm in Nr at a heating rate of 6 K/min (a) for sample A (10 pm thick) and (b) for sample B (30 pm thick).
We will call the 10 pm thick sample type A and the 30 pm thick one type B. The form of the conductance curves of sample B corresponding to the runs with lower heating rate is closer to those of the sample A. It is interesting to note that the behaviour of sample A is closer to those of thick-film sensors reported by Lantto et al. [5], while the behaviour of sample B is more similar to that of the TGS sensor. Figure 10 shows the conductance curves in a heating run and the corresponding energy barrier values for sample A and sample B respectively. For both the samples the increase of the surface energy barrier starts around 200 “C and stops around 500 “C [9, lo]. Lowering the oxygen concentration in nitrogen is followed by an enlargement of the temperature interval whose extremes correspond to the minimum and maximum AE of the energy barrier. Moreover, the AE corresponding to sample B is significantly higher than the AE value associated with sample A. Starting from these
concentration (a)
(p.p.m.)
concentration
(p.p.m.)
0’)
Fig. 11. (a, b) Conductance of sensors A and B as a function of the concentration of methane. The energy barriers vs. T are reported respectively in Fig. 10(b) and (d).
observations, we formulated the hypothesis that AE, which is not only dependent on the thickness, could be taken as a parameter for controlling the sensitivity to gas. The curves in Fig. 11 representing the sensitivity to methane of the two types of sensors seem to confirm the above assumption: AE, is smaller than AE, and sample A shows a significantly lower sensitivity than sample B. Also the particle aggregation, and thus the reduction of the specific surface area, after
196
References
Fig. 12. SEM micrograph of a B type sample after one month of exposure to a high percentage of methane.
long exposure to a high methane concentration on sample B (Fig. 12) followed by a decrease of sensitivity is consistent with our hypothesis. Following these encouraging results, our research is now devoted to finding the most suitable catalyst for increasing the AE value.
Acknowledgements
This work was supported by Eniricerche S.p.A., Milan, Italy. The electron micrographs were obtained at the Centro di Microscopia Elettronica dell’Universit8 di Ferrara, Ferrara, Italy.
1 H. Torvela and S. Leppavuori, Microstructure and conductivity of short duration sintered tin oxide ceramics, Inr. J. High 7’echnul. Cerum., 3 (1987) 11. 2 S. R. Morrison, Semiconductor gas sensors, Sensors and Acruarors, 2 (1982) 329-341; S. R. Morrison, Selectivity in semiconductor gas sensors, Sensors and Actuators, 12 (1987) 425440. 3 P. K. Clifford and D. T. Tuma, Characteristics of semiconductor gas sensors: II. Transient response to temperature change, Sensors and Actuators, 3 (19821 83) 255-281. 4 M. J. Madou and S. R. Morrison, Chemical Sensing with Solid State Devices, Academic Press, New York, 1989, Ch. 3, p. 75. 5 V. J-antto, P. Romppainen and S. Leppavuori, A study of the temperature dependence of the barrier energy in porous tin dioxide, Sensors and Actuators, 14 (1988) 149-163. 6 S. Samson and C. G. Fonstad, Defect structure and electronic donor levels in stannic oxide crystals, J. Appl. Phys., 44 (1973) 4618-4621. 7 2. M. Jarzebski and J. P. Marton, Physical properties of SnOr materials: II, Electrical properties, J. Electrothem. Sot. Rev. News, 123 (1976) 299-310. 8 H. Ogawa, M. Nishikawa and A. Abe, Hall measurements studies and an electrical conduction model of tin oxide ultrafine particle films, J. A@. Phys., 53 (6) (1982) 4448-4457. 9 S. C. Chang, Sensing mechanisms in thin film tin oxide, in T. Seiyama et al. (eds.), Proc. Inf. Meet. Chemical Sensors, Fulcuoka, Japan, Kondansha_/Elsevier, Tokyo/ Amsterdam, 1983, pp. 78-83. 10 H. Chon and J. Pajares, Hall effect studies of oxygen chemisorption on zinc oxide,J. Catal., 14 (1969) 257-260.
Sensors and Acfuurors B, 3 (1991) 191-196
CH4 thick-film gas sensors: characterization explanation M. C. Carotta,
C. Dsllara,
G. Martinelli
Department of Physics, 12 t4a Pamdiso,
method and theoretical
and L. Passari
University of Ferram, I-44100 Fermra (Italy)
A. Camanzi Eniricerche S.pA., I5 Via Ramarini, I-00015 Montemtondo
(Rome) (Itab)
(Received August 9, 1990, in revised form February 5, 1991; accepted February 6, 1991)
Abstract The characterization of Sn02 thick-film gas sensors made from a homemade binderless paste is given. The Hall effect has been measured together with the conductance to determine the carrier concentration and the mobility. From temperature-stimulated conductance measurements at several fixed temperatures, the range where the intergranular conductance can be reasonably considered to be constant has been explored. The microstructure and morphology of the samples are analysed by TEM and SEM observations. The results confirm for these sensors the possibility of linking the electronic structure of the bulk and the chemical state of the surface by an energy barrier model. Moreover, we correlate the difference between the maximum and the minimum of the barrier energy, Al?, to the sensitivity to reducing gas.
1. Introduction In the last ten years great interest has been devoted to the research and development of gas sensors due to their potentiality to solve a wide range of problems, such as to control industrial processes, to sample environmental conditions and to warn of poisonous gas leakages. In this work we provide a characterization of thick-film gas sensors manufactured by printing on alumina substrates different thicknesses of a homemade paste based on SnO, powder. To avoid the effect of mineral binder at the grain boundaries, we prepared the paste with only organic solvents which have a maximum burnout temperature of approximately 500 “C; the samples were fired at 850 “C for half a hour. TEM and SEM micrographs were used to analyse the microstructural and morphological properties; the size of the grains ranges from 15 to ZOOnm. Our aim was to find suitable parameters relating to the sensitivity to reducing gas, in particular methane.
09254005/91/$3.50
Sensitivity curves for methane were obtained on samples without catalysts to prevent any variation of the physical and chemical properties at the surface and in the bulk of the devices.
2. Experimental
procedure
First we investigated the non-ohmic dependence of the current on the applied voltage by plotting the I-Vcurves in the temperature range 200 to 500 “C as suggested by other researchers [l]. The non-ohmic behaviour (Fig. 1) can be justified by an intergranular current controlled by the energy barriers at the contacting surfaces of the particles [2]. The consistency of the intergranular model for our samples has been pointed out by SEM and TEM observations (Figs. 2 and 3). On the basis of the barrier theory proposed by Clifford and Tuma [3], we analysed on our samples the link between the electronic structure of the bulk and the chemical state of the surface. The exponential term of the Morrison equation [2) ^
0 1991 -
Elsevier Sequoia, Lausanne
192
. I
1500 :_T 1000
z
2
2
.
15 10
.
2.
.
.
500
.
5
l
.
:
i
:’
.
.* -t..... 0 (a)
0
10
20 v
30
Fig. 2. SEM micrograph binderless paste based 850 “C.
5
as a function
15
:
of voltage
at (a) 200
of a typical sensor made from a on SnO, powder and sintered at
Fig. 3. TEM micrograph showing the sensor described in Fig. 2.
particles
G = G, exp[ - eVJk7’j is very sensitive to the potential which is related to the absorbed N, by the Schottky formula eV, = e2N,*/2c~,N,
10
v (v)
(b)
(VI
Fig. 1. Current values “C and (b) 550 “C.
OL 0
40
and
necks
in
(1) barrier eVS, oxygen ions
which is a direct consequence of the Poisson equation; EEL is the permittivity of the semiconductor and N, is the donor concentration. The bulk intergranular conductance is represented by Go=ge~n, where g is a geometrical factor (in our case the sample thickness), e is the electronic charge, I_Lis the electron mobility and nb the concentration of electrons in the bulk conduction band. G, is usually considered to be almost constant because the mobility and the other factors are expected to vary more slowly with temperature than the carrier concentration, which is controlled by the exponential factor 141. The conductivity is the product of the carrier concentration and mobility; to separate the two variables we performed two independent measurements: (a) Hall effect measurements at a convenient temperature, which allow us to determine the total carrier number; (b) Temperature-stimulated conductance measurements to determine the barrier energy at the same temperature as the Hall effect measurement. Moreover, from these two measurements it has been possible to verify the range of temperature where G, can actually be assumed to be almost constant. Hall measurement The thick-film sensors were placed inside a test chamber (300 cm3 in volume) of a suitable size to be located between the poles of a magnet. The gas (5 ppm of oxygen in nitrogen) flowed at a fixed flow rate of one litre per minute at a temperature of 350 “C. The Van der Pauw pattern with four symmetrical electrodes was used for the Hall effect measurement. The voltage across two electrodes was measured with and without a magnetic field B, with a constant current i across the two other electrodes. eV, and G,, measurements Using the same test chamber and oxygen concentration in nitrogen, the sensors were biased by a constant voltage of 9 V. The conductance was obtained from the voltage drop on a known resistor. The data were recorded and elaborated by an on-line computer.
193
of the sensors was The temperature changed by a heater printed on the back of the support and was directly measured by a Cr-Al thermocouple fixed on the samples. The barrier energy height versus T was obtained from the following three steps [3, 51: (a) The experimental measurement of the conductance versus T (Figs. 7-9). (b) The determination of G, from the barrier energy eV, corresponding to a convenient initial temperature Ti using temperature-stimulated conductance measurements. The values extrapolated to zero time, G,,, (Fig. 4) associated with each temperature T, give the conductance at T, but at the barrier value corresponding to Ti. In this way, if Go is constant, the only effect of the temperature variation is represented by
(a)
G, = G, exp[ -eV,(TJ/kT,]
w
T, =
1’5
2.5
2.0 103/T
(K-l)
1.5
1.2 103/T
(b)
1.8
(K-l)
1’5
2.0 lO’/T
1.4
1.2 Cd)
103/T
2.5
(K-l)
1.6
(K-l)
Fig. 5. (a)-(d) Logarithms of GJG, obtained from the values of Fig. 4 vs. 1000/T; from the slopes of the lines it has been possible to determine the barrier energy values for the four different temperatures.
From
we obtained eV, and then Go; Gi is the value of the conductance measured at Ti. The four straight lines obtained from the above equation (Fig. 5) describe the accuracy of the method and, at the same time, allow us to assume Go to be constant inside each temperature interval associated with the four Ti values. (c) The eV, curve as a function of T was derived from eqn. (1) and the experimental values (point (a)) of G. OOW
time (s)
(‘-‘)
time (5)
1000
Results and discussion
OOCc)
1000
time (5)
(d)
time (s)
Fig. 4. Conductance as a function of time after a fast change in the temperature from the initial temperatures of (a) 200 “C, (b) 300 “C. (c) 400 “C and (d) 480 “C. The extrapolation of these curves to zero time determines the conductances G,.
Starting from different Ti values, it is POSsible to obtain information about the range of temperature where G,, can be considered almost constant. The values of Go determined for Ti =200, 300, 400 and 480 “C are shown in Fig. 6. It is then easy to deduce that the suitable temperature for performing Hall effect measurements lies between 300 and 400 “C. The constancy of Go in this range is also justified by the linearity of the change of the absorbed oxygen from the 02- to O- form.
194
N,=4.11
,,,M 200
300
Temperature
400
500 (“C)
Fig. 6. Go values calculated from barrier energy values experimentally determined by temperature-stimulated conof 200, 300, 400 ductance measurements at temperatures and 480 “C.
The Hall effect measurements have been obtained for all the samples at 350 “C. Since the values of the carrier number measured on several samples showed a sufficient repetitiveness, we report here the mean value: n=
iB qtV,
=1.5x1015
RH = l/nq = 4.16 x 10P3 cm3/C The Fermi level of the bulk was then deduced from exp[ - eV,/kTJ
where NC is the effective state density in the conduction band at 350 “C; from the temperature-stimulated conductance measurement, we obtained at the same temperature and with an O2 partial pressure of 5 ppm in N2 a value for eV, of 0.41 eV. Then EF = - 0.064 and nb = 3.18 X 10” cme3. From the charge neutrality condition
Izb =
10” cmm3
which shows that at a temperature of 350 “C more than 70% of the donor levels are ionized. This means that the conductance has to be considered to be strongly dependent on the barrier energy height. Finally the mobility, which is p = GJgen, = 0.16 cm’/V s, is assumed to be very close to the Hall one. Values of the mobility reported in the literature [7, S] for single crystals differ by approximately two orders of magnitude from our result. The discrepancy can be explained by the granularity of our samples [7]. Our value is instead in good agreement with the Hall measurements performed by Ogawa et al. [8] on porous columnar samples where a modulating barrier effect is supposed. The scattering effects due to ionized donor impurities, interstitials and vacancies, which can affect the measurements by approximately 50%, have not been considered here.
cmh3
where i = 100 PA, B = 8000 G, t = 30 pm (sample thickness) and V, is the measured Hall voltage. The Hall constant is
n =NC exp( - E,IkT)
X
eV, dependence on temperature The conductance measurements have been performed at different oxygen partial pressures (2x 105 and 5 ppm in N2) between 150 and 550 “C at the heating and cooling rates of 6 K/min and 3 K/min. Figures 7, 8 and 9 show the dependence of the conductance on the heating or cooling run, on the rate of the run and finally on the thickness of the sample. In Fig. 9 it is clearly shown that the form of the conductance curves for sensors with different thicknesses differs greatly in the same heating run.
1 +g, exp[ - (EF - E&/kT]
+ 1 +a expE- 6% -Ed2)M’l with g, = 2 and g2 = l/2 as degeneracy factors, Edi = 0.034 eV and Ed2 = 0.145 eV the energies of native donor levels (experimentally determined by Samson and Fonstad [6] on SnO, crystals), we deduced
4i-w&?+ (a)
103/T
(K-l)
’
Fig. 7. Conductance as a function of inverse temperature measured at an oxygen partial pressure of 5 ppm in Nz at (a) a heating and (b) a cooling rate of 6 K/min.
,^
T-----l 1o-F-----l 7
.:i”,,,~~~~~~~,,~ 1.0 (a)
1.5
2.0
2.5
103/T (K-')
3.0
1.0
(b)
1.5
2.0
2.5
3.0
lO'/T (K-l)
(3)
lO'/T (K-l)
Fig. 8. Conductance as a function of inverse temperature measured at an oxygen partial pressure of 2~1~ ppm in Nr at a heating rate of (a) 6 K/min and (b) 3 K/min.
temperature (K)
W
aample E
0.3
(4
(2)
1 03/T
(K-l)
@I
103/T (K-l)
lO'/T (K-l)
‘(d)
4%nperBaOtre
(:)O
Fig. 10. (a)-(d) Conductance and energy barrier for sample A (10 pm thick) and sample B (30 pm thick); Oz partial pressures of 2X 105 ppm and 5 ppm in N2 are denoted by (A) and (0) respectively.
Fig. 9. Conductance as a function of inverse temperature measured at an oxygen partial pressure of 2~ 10r ppm in Nr at a heating rate of 6 K/min (a) for sample A (10 pm thick) and (b) for sample B (30 pm thick).
We will call the 10 pm thick sample type A and the 30 pm thick one type B. The form of the conductance curves of sample B corresponding to the runs with lower heating rate is closer to those of the sample A. It is interesting to note that the behaviour of sample A is closer to those of thick-film sensors reported by Lantto et al. [5], while the behaviour of sample B is more similar to that of the TGS sensor. Figure 10 shows the conductance curves in a heating run and the corresponding energy barrier values for sample A and sample B respectively. For both the samples the increase of the surface energy barrier starts around 200 “C and stops around 500 “C [9, lo]. Lowering the oxygen concentration in nitrogen is followed by an enlargement of the temperature interval whose extremes correspond to the minimum and maximum AE of the energy barrier. Moreover, the AE corresponding to sample B is significantly higher than the AE value associated with sample A. Starting from these
concentration (a)
(p.p.m.)
concentration
(p.p.m.)
0’)
Fig. 11. (a, b) Conductance of sensors A and B as a function of the concentration of methane. The energy barriers vs. T are reported respectively in Fig. 10(b) and (d).
observations, we formulated the hypothesis that AE, which is not only dependent on the thickness, could be taken as a parameter for controlling the sensitivity to gas. The curves in Fig. 11 representing the sensitivity to methane of the two types of sensors seem to confirm the above assumption: AE, is smaller than AE, and sample A shows a significantly lower sensitivity than sample B. Also the particle aggregation, and thus the reduction of the specific surface area, after
196
References
Fig. 12. SEM micrograph of a B type sample after one month of exposure to a high percentage of methane.
long exposure to a high methane concentration on sample B (Fig. 12) followed by a decrease of sensitivity is consistent with our hypothesis. Following these encouraging results, our research is now devoted to finding the most suitable catalyst for increasing the AE value.
Acknowledgements
This work was supported by Eniricerche S.p.A., Milan, Italy. The electron micrographs were obtained at the Centro di Microscopia Elettronica dell’Universit8 di Ferrara, Ferrara, Italy.
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