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Thick-film hydrocarbon gas sensors
Sensors and Actuators, Bl (1990) 231-235
231
Thick-film Hydrocarbon Gas Sensors DUK-DONG LEE Department of Electronics, Kyungpook National University, Taegu 702- 701 (Korea) DONG-HAN CHOI Department of Semiconductor Engineering, Chongju University, Chongju, Choongchung buk-do (Korea)
Abstract Y-Fe,O, thick-film hydrocarbon gas sensors have been fabricated and their characteristics investigated. The sensitivity of a y-FezO, thick film without any other catalysts heat treated at 400 “C for 2 h is 70% in 1000 ppm C,Hlo ambient at an operating temperature of 300 “C. The addition of Pd( 1 wt.%) to y-FezO, enhances the sensitivity to butane gas. The sensitivity of y-Fe,OJ SnCl,*SH,O(O.Swt.%)/Pd( lwt.%) film heat treated at 400 “C for 2 h to 1000 ppm C,,HiO is 80% at an operating temperature of 300 “C. 1. Introduction Hydrocarbon gases have come into wide use as a fuel in industry and the home, and give us clean energy in burning. However, these gases are potentially hazardous because explosion accidents might be caused when they leak out accidently or by mistake. For this reason, there is is an everincreasing need for hydrocarbon gas sensors. For several decades, semiconductor gas sensors using SnOz, ZnO and FezOJ have been studied extensively by many authors [l-8]. Small amounts of noble metals such as Pd and Pt are added to base materials as activators or sensitizers. The addition of these metals has been known to be effective in promoting gas sensitivity and gas selectivity [g-lo]. In this paper, the gas-sensing characteristics of y-Fe,O, thick films with and without palladium catalyst have been investigated.
mixed powder was then calcined in air at 400 “C for 2 h. The calcined powder was crushed and made into a paste by adding DI water. The paste was then screen printed onto the surface of an alumina substrate and tied in air at various temperatures to form the gas-sensitive thick-film layer. A pair of gold electrodes was vacuum evaporated onto the thick fllrn and annealed at 400 “C. Lead wires were then attached onto the contact pads of the electrodes with a drop of Pt-Au paste. Figure 2 shows the structure of the fabricated thick-film device. The thickness of the fired thickf&n layer was about 1OOpm. Figure 3 shows a schematic diagram of the measurement set-up. The thick-film devices were set on a temperature-controlled hot plate in a stainless steel box, which was l&d with air with a relative humidity of 4040%. A load resistor R,_ was connected with the device and a d.c. voltage was applied to the circuit. Test gases were injected
Fig. 1. The process sequence diagram for the thick-film devices.
2. Experimental y-Fe,O, was chosen as the base material hydrocarbon gas-sensitive thick-film device. ure 1 shows the process sequence diagram preparation of the thick-film devices. y-Fe203 additives were weighed and mixed down. 09254005/90/$3.50
of a Figfor and The
i-C-_-
5mm+ 1Omm -I
Fig. 2. Cross-sectional view of the pnpared thick-film device.
0 Elsevier Sequoia/Printed
in The Netherlands
232
TC
Temperature controller
Fig. 3. Schematic diagram of the measurement set-up.
into the box with a syringe. device resistance were obtained output voltage across the chromel-alumel thermocouple device indicated the operating
The values of the by monitoring the load resistor. A placed on the temperature.
:(C)
100
80
60 28 (degree)
40
20
Fig. 5. XRD patterns of y-Fe,OB with various heat-treatment conditions. (a) Raw material; (b) heat treatment, 400 “C, 2 h; (c) heat treatment, 500 “C, 2 h.
3. Results and Discussion Figure 4 shows the DTA curve of the y-Fe203 powder that was used as base material in this experiment. In this Figure, an exothermic peak appears at about 495 “C, which means that a phase transition from spine1 structure (y-Fe,O,) to corundum structure (a-Fe,O,) (irreversible transition) occurs at this point. This result was confirmed by XRD analyses. Figure 5 shows the XRD patterns of Y-FezO, for various heat-treatment conditions. We can see that Y-Fe,O, changes phase to a-Fe,O, at 500 “C. y-Fe,O, and a-FezOJ coexist after firing at 400 “C, and above 500 “C, only a-Fe,O, patterns are exhibited. Figure 6 shows the sensitivity characteristics of a y-FelO, thick film for butane gas with various heat-treatment temperatures as an operating temperature of 300 “C. The sensitivity was defined as the value of AR/R, where AR means the resistance variation when gas was injected, and R is the initial resistance of the film. The thick film fired at 400 “C exhibited the highest sensitivity to butane gas. The higher the firing temperature, the lower the sensitivity to gas.
I 400
600
Temperature
600 I’C
1
Fig. 4. DTA curve of y-Fe,O, powder.
Operating
Heat
treatment
temp.
3OO'C
:
12 Fig. 6. Sensitivity characteristics of y-Fe,O, thick film for butane gas with various heat-treatment temperatures.
Figure 7 shows the sensitivity versus operating temperature of a y-Fe,O, thick film to butane gas with various heat-treatment temperatures. yFez03 thick films showed the highest sensitivity to butane gas at operating temperatures between 300 and 350 “C. Figure 8 shows the sensitivity characteristics of Y-Fe,O, thick films for various gases at an operating temperature of 300 “C. The sensitivities to butane, methane, propane and hydrogen (or CO) gas in 1000 ppm environments were 0.7,0.30, 0.18 and less than 0.08, respectively. Figure 9 shows the sensitivity characteristics of y
233
l-Fe203/Pd :99/l Gas
:
Heat
:
WX,
C&H10
: 4OO’C. 2 hrs.
treat.
10000ppm
Heat treatment
time
: Zhrs. t
0’ 200
I
300
250 Operating
350
temperature
Gas
temperature
2
4 Gas
6 8 concentration
10
12
( xlOC0ppm 1
(‘C J
Fig. 7. Sensitivity vs. operating temperature of y-Fe20, thick f?lm to butane gas with various heat-treatment temperatures.
Operating
0
400
Fig. 9. Sensitivity characteristics of y-Fe,OJPd( 1 wt.%) thick film fired at 400 “C for 2 h to butane gas with various operating temperatures.
: 300-C
:o
C4H10 n CH4 A C Hij
Gas concentration
1XlOOOppm 1
Fig. 8. Sensitivity characteristics of y-FezO, thick lihn to various gases at an operating temperature of 300 “C.
Figure 11 shows the energy band diagram at the grain boundary of y-Fe,O, when reducing gas is adsorbed between the grains. It is supposed that the adsorbed reducing gas lowers the potential barrier formed between the oxygen preadsorbed grains. The resistance change of the film is caused by a redox reaction between the injected gas and yFe,O, grains. The catalyst (Pd) is well known for reducing the activation energy of such a reaction [ 1l- 131. When oxygen is adsorbed on a Pd surface in air at elevated temperature, the potential barrier between y-Fe,O, grains might be raised (Fig. 12) [ 14- 151. As a result of this phenomenon, the resistance of the thick film will be increased
100
300 200 Temperature ( OC )
400
Fig. 10. Resistance vs. temperature for y-Fe,O, thick film. (a) y-Fe,OJPd( 1 wt.%); (b) y-Fe,O,; (c) y-Fe,03/SnCl,+H20.
a.
Reducing gas
3= I
///I/,
Sfvkmbc
‘I/////’
’ /
~~
Grain bcundary
Fig. 11. An energy band diagram at the grain boundary of y-Fe,03.
234
l-Fe203/SnC14.5H20/Pd Gas
w/c
: wlo
Heat treat. E
:98.5/O.Y1
AOO*C ,2 hrs.
Operating
:
temp.
3OO’C
I
350=x
Fig. 12. An energy band diagram at the interface between Pd and y-Fe,O,.
compared with that of a Y-FezO, thick film without any other catalysts. The Y-Fe203 thick film with 1 wt.% Pd as a catalyst showed the highest sensitivity to butane gas. At an operating temperature of 300 “C, the surface of the y-Fez03 grains would be surrounded by adsorbed oxygen species. The reaction between this adsorbed oxygen species and the injected gas decreases the oxygen coverage. In order to decrease the resistance of the yFe,O,/Pd thick film without impairing the sensitivity to gas, SnCl_,*SH,O was added in this study. Among the various compositions, y-Fe,0,(98.5)/ SnCld*5H,0( 0.5) /Pd( 1 wt. %) showed the highest sensitivity to butane gas and its resistance was similar to that of a +y-Fe,O, thick film. Figure 13 shows the resistance variations of y-Fe203 thick films with various SnCkSH,O contents at an operating temperature of 300 “C. As shown in this Figure, the higher the SnCkSH,O content, the lower the resistance of the Y-FezO, thick film for SnC1,*5Hz0 contents under 2 wt.%.
150°C
0
2
4
6
Gas
8
( x1000 pprn )
Figure 14 shows the sensitivity of a y-Fe,O,/ SnCl,*SH,O/Pd thick film (heat treated at 400 “C for 2 h) to butane gas with various operating temperatures. The thick film exhibits the highest sensitivity to butane gas at an operating temperature of 300 “C. The sensitivity of this film to 1000 ppm butane gas was about 80% at an operating temperature of 300 “C. Figure 15 shows the sensitivity of y-FezOJ SnC14*5H20(0.5 wt.%)/Pd( 1 wt.%) for various gases at an operating temperature of 300 “C. The sensitivities to butane, methane, propane, CO and hydrogen gas in 1000 ppm environments were 0.8, 0.4, 0.25, 0.3 and 0.35, respectively. Comparing
I
temp. : 3OO'C
Heat treatment:
4OO'C.
12
Fig. 14. Sensitivity characteristics of y-Fe,0,(98.5 wt.%)/ SnCl,.5H,0(0.5 wt.%)/Pd( 1 wt.%) thick film heat treated at 400 “C for 2 h to butane gas with various operating temperatures.
I-Fe20~/SnC14.5H20/Pd
Operating
10
concentration
Zhrs.
Operating
temperature:
:98.5iO.YlWC 3OO'C
60
SnCl4.5H20
contents
0
( w/o)
Fig. 13. Resistance variation of y-Fe,O, SnCl,.5H,O contents.
thick film with
2 Gas
12
ckentktion
(8xlOO$pm
)
Fig. 15. Sensitivity characteristics of y-Fe,O,/SnCMH,O(0.5 wt.%)/Pd( 1 wt.%) thick film for various gases.
235
these values with those obtained from Fig. 8, we see that the sensitivities to hydrocarbon gases are enhanced by adding Pd and SnCl,*SH,O to yFezO,. The devices started to respond to 1000 ppm hydrocabon gases at almost the same instant as the gases flowed over the devices at an operating temperature of 300 “C. We found that hydrocarbon gases can be detected by these devices in less than two seconds.
4. Conclusions
Thick-film hydrocarbon gas sensors were fabricated by a screen-printing method and their gassensing characteristics were investigated. Thick films of y-Fe,O, without any other catalysts showed a high sensitivity to hydrocarbon gases. Moreover, the addition of Pd catalyst to y-Fe,O, enhanced the sensitivity to hydrocarbon gases. SnC&.SH,O was also used as an additive in order to lower the device resistance, which was raised by Pd addition, without impairing the sensitivity to hydrocarbon gases. y-Fe,O,/ SnC1,*5HzO/Pd thick films with 0.5 wt.% SnCl,.SH,O and 1 wt.% Pd, fhed at 400 “C for 2 h, showed the highest sensitivity to hydrocarbon gases at an operating temperature of 300 “C. The sensitivities of this film to butane, methane, propane, CO and hydrogen gases in 1000 ppm environments were 0.8, 0.4, 0.25, 0.3 and 0.35, respectively, at an operating temperature of
300 “C, and the response time for 1000 ppm hydrocarbon gases was less than two seconds.
References 1 T. Seiyama, A. Kato, K. Fujiishi and M. Nagatanui, A new detector for gaseous components using semiconductive thin films, Anal. Chem., 34 (1962) 1502-1503. 2 P. J. Shaver, Activated tungsten oxide gas detectors, Appl. Phys. Len., II (1967) 255. 3 J. R. Mackintyre, A thin film hydrogen sensor, Instrum. Fe&&., 19 (i972) 29. _ M. Nitta, Thick 6hn CO gas sensor, IEEE Trans. Electron Devices, ED-24 (1979) 247-249. B. Morten, Thick 6lm technology and sensor, Sensors und Actuators, 4 (1983) 237-245. K. Dobos, Performance of carbon monoxide sensitive MOSFET’s with metal oxide semiconductor gates, IEEE Trans. Electron Devices, ED-32 (1985) 7. 7 Y. Nakatani, Some electrical properties of y-Fe,O, ceramics, Jpn. J. Appl. Phys., 2.2 (1983) 233. 8 Y. Nakatani, Microstructure of y-Fe,O, ceramics as a combustible gas sensor. Proc. In:. Meet. Chemical Sensors, Fukuoka, Japan, 1983, pp. 147. 9 K. Murata, Gas sensors, Denshi-Zairyo, (1977) 114. 10 T. Seiyama, Kagaku Sensors, Kodansha, Tokyo, 1982, pp. 49-50. 11 T. L. Poteat, Transition metal-gate MOS gaseous detectors, IEEE Trans. Electron Devices, ED-29 (1982) 124. 12 R. C. Baetxold, Interaction of saturated hydrocarbons with transition metal fihns, J. Am. Gem. SOL, (1983) 4271-4276. 13 J. F. McAleer, P. T. Moseley, B. C. Tofield and D. E. Williams. Proc. Br. Ceram. Sot.. 36(1985) 89-105. 14 H. Yam&da, Gas sensing characteristics’of porous zinc oxide ceramics with and without platinum catalyst, Proc. Int. Meet. Chemical Sensors, Fukuoka, Japan, 1983, pp. 95- 100. 15 N. Yamamoto, The effect of reducing gases on the conductivities of metal oxide semiconductors, Jpn. J. Appl. Phys., 20-4 (1981) 721-726.
231
Thick-film Hydrocarbon Gas Sensors DUK-DONG LEE Department of Electronics, Kyungpook National University, Taegu 702- 701 (Korea) DONG-HAN CHOI Department of Semiconductor Engineering, Chongju University, Chongju, Choongchung buk-do (Korea)
Abstract Y-Fe,O, thick-film hydrocarbon gas sensors have been fabricated and their characteristics investigated. The sensitivity of a y-FezO, thick film without any other catalysts heat treated at 400 “C for 2 h is 70% in 1000 ppm C,Hlo ambient at an operating temperature of 300 “C. The addition of Pd( 1 wt.%) to y-FezO, enhances the sensitivity to butane gas. The sensitivity of y-Fe,OJ SnCl,*SH,O(O.Swt.%)/Pd( lwt.%) film heat treated at 400 “C for 2 h to 1000 ppm C,,HiO is 80% at an operating temperature of 300 “C. 1. Introduction Hydrocarbon gases have come into wide use as a fuel in industry and the home, and give us clean energy in burning. However, these gases are potentially hazardous because explosion accidents might be caused when they leak out accidently or by mistake. For this reason, there is is an everincreasing need for hydrocarbon gas sensors. For several decades, semiconductor gas sensors using SnOz, ZnO and FezOJ have been studied extensively by many authors [l-8]. Small amounts of noble metals such as Pd and Pt are added to base materials as activators or sensitizers. The addition of these metals has been known to be effective in promoting gas sensitivity and gas selectivity [g-lo]. In this paper, the gas-sensing characteristics of y-Fe,O, thick films with and without palladium catalyst have been investigated.
mixed powder was then calcined in air at 400 “C for 2 h. The calcined powder was crushed and made into a paste by adding DI water. The paste was then screen printed onto the surface of an alumina substrate and tied in air at various temperatures to form the gas-sensitive thick-film layer. A pair of gold electrodes was vacuum evaporated onto the thick fllrn and annealed at 400 “C. Lead wires were then attached onto the contact pads of the electrodes with a drop of Pt-Au paste. Figure 2 shows the structure of the fabricated thick-film device. The thickness of the fired thickf&n layer was about 1OOpm. Figure 3 shows a schematic diagram of the measurement set-up. The thick-film devices were set on a temperature-controlled hot plate in a stainless steel box, which was l&d with air with a relative humidity of 4040%. A load resistor R,_ was connected with the device and a d.c. voltage was applied to the circuit. Test gases were injected
Fig. 1. The process sequence diagram for the thick-film devices.
2. Experimental y-Fe,O, was chosen as the base material hydrocarbon gas-sensitive thick-film device. ure 1 shows the process sequence diagram preparation of the thick-film devices. y-Fe203 additives were weighed and mixed down. 09254005/90/$3.50
of a Figfor and The
i-C-_-
5mm+ 1Omm -I
Fig. 2. Cross-sectional view of the pnpared thick-film device.
0 Elsevier Sequoia/Printed
in The Netherlands
232
TC
Temperature controller
Fig. 3. Schematic diagram of the measurement set-up.
into the box with a syringe. device resistance were obtained output voltage across the chromel-alumel thermocouple device indicated the operating
The values of the by monitoring the load resistor. A placed on the temperature.
:(C)
100
80
60 28 (degree)
40
20
Fig. 5. XRD patterns of y-Fe,OB with various heat-treatment conditions. (a) Raw material; (b) heat treatment, 400 “C, 2 h; (c) heat treatment, 500 “C, 2 h.
3. Results and Discussion Figure 4 shows the DTA curve of the y-Fe203 powder that was used as base material in this experiment. In this Figure, an exothermic peak appears at about 495 “C, which means that a phase transition from spine1 structure (y-Fe,O,) to corundum structure (a-Fe,O,) (irreversible transition) occurs at this point. This result was confirmed by XRD analyses. Figure 5 shows the XRD patterns of Y-FezO, for various heat-treatment conditions. We can see that Y-Fe,O, changes phase to a-Fe,O, at 500 “C. y-Fe,O, and a-FezOJ coexist after firing at 400 “C, and above 500 “C, only a-Fe,O, patterns are exhibited. Figure 6 shows the sensitivity characteristics of a y-FelO, thick film for butane gas with various heat-treatment temperatures as an operating temperature of 300 “C. The sensitivity was defined as the value of AR/R, where AR means the resistance variation when gas was injected, and R is the initial resistance of the film. The thick film fired at 400 “C exhibited the highest sensitivity to butane gas. The higher the firing temperature, the lower the sensitivity to gas.
I 400
600
Temperature
600 I’C
1
Fig. 4. DTA curve of y-Fe,O, powder.
Operating
Heat
treatment
temp.
3OO'C
:
12 Fig. 6. Sensitivity characteristics of y-Fe,O, thick film for butane gas with various heat-treatment temperatures.
Figure 7 shows the sensitivity versus operating temperature of a y-Fe,O, thick film to butane gas with various heat-treatment temperatures. yFez03 thick films showed the highest sensitivity to butane gas at operating temperatures between 300 and 350 “C. Figure 8 shows the sensitivity characteristics of Y-Fe,O, thick films for various gases at an operating temperature of 300 “C. The sensitivities to butane, methane, propane and hydrogen (or CO) gas in 1000 ppm environments were 0.7,0.30, 0.18 and less than 0.08, respectively. Figure 9 shows the sensitivity characteristics of y
233
l-Fe203/Pd :99/l Gas
:
Heat
:
WX,
C&H10
: 4OO’C. 2 hrs.
treat.
10000ppm
Heat treatment
time
: Zhrs. t
0’ 200
I
300
250 Operating
350
temperature
Gas
temperature
2
4 Gas
6 8 concentration
10
12
( xlOC0ppm 1
(‘C J
Fig. 7. Sensitivity vs. operating temperature of y-Fe20, thick f?lm to butane gas with various heat-treatment temperatures.
Operating
0
400
Fig. 9. Sensitivity characteristics of y-Fe,OJPd( 1 wt.%) thick film fired at 400 “C for 2 h to butane gas with various operating temperatures.
: 300-C
:o
C4H10 n CH4 A C Hij
Gas concentration
1XlOOOppm 1
Fig. 8. Sensitivity characteristics of y-FezO, thick lihn to various gases at an operating temperature of 300 “C.
Figure 11 shows the energy band diagram at the grain boundary of y-Fe,O, when reducing gas is adsorbed between the grains. It is supposed that the adsorbed reducing gas lowers the potential barrier formed between the oxygen preadsorbed grains. The resistance change of the film is caused by a redox reaction between the injected gas and yFe,O, grains. The catalyst (Pd) is well known for reducing the activation energy of such a reaction [ 1l- 131. When oxygen is adsorbed on a Pd surface in air at elevated temperature, the potential barrier between y-Fe,O, grains might be raised (Fig. 12) [ 14- 151. As a result of this phenomenon, the resistance of the thick film will be increased
100
300 200 Temperature ( OC )
400
Fig. 10. Resistance vs. temperature for y-Fe,O, thick film. (a) y-Fe,OJPd( 1 wt.%); (b) y-Fe,O,; (c) y-Fe,03/SnCl,+H20.
a.
Reducing gas
3= I
///I/,
Sfvkmbc
‘I/////’
’ /
~~
Grain bcundary
Fig. 11. An energy band diagram at the grain boundary of y-Fe,03.
234
l-Fe203/SnC14.5H20/Pd Gas
w/c
: wlo
Heat treat. E
:98.5/O.Y1
AOO*C ,2 hrs.
Operating
:
temp.
3OO’C
I
350=x
Fig. 12. An energy band diagram at the interface between Pd and y-Fe,O,.
compared with that of a Y-FezO, thick film without any other catalysts. The Y-Fe203 thick film with 1 wt.% Pd as a catalyst showed the highest sensitivity to butane gas. At an operating temperature of 300 “C, the surface of the y-Fez03 grains would be surrounded by adsorbed oxygen species. The reaction between this adsorbed oxygen species and the injected gas decreases the oxygen coverage. In order to decrease the resistance of the yFe,O,/Pd thick film without impairing the sensitivity to gas, SnCl_,*SH,O was added in this study. Among the various compositions, y-Fe,0,(98.5)/ SnCld*5H,0( 0.5) /Pd( 1 wt. %) showed the highest sensitivity to butane gas and its resistance was similar to that of a +y-Fe,O, thick film. Figure 13 shows the resistance variations of y-Fe203 thick films with various SnCkSH,O contents at an operating temperature of 300 “C. As shown in this Figure, the higher the SnCkSH,O content, the lower the resistance of the Y-FezO, thick film for SnC1,*5Hz0 contents under 2 wt.%.
150°C
0
2
4
6
Gas
8
( x1000 pprn )
Figure 14 shows the sensitivity of a y-Fe,O,/ SnCl,*SH,O/Pd thick film (heat treated at 400 “C for 2 h) to butane gas with various operating temperatures. The thick film exhibits the highest sensitivity to butane gas at an operating temperature of 300 “C. The sensitivity of this film to 1000 ppm butane gas was about 80% at an operating temperature of 300 “C. Figure 15 shows the sensitivity of y-FezOJ SnC14*5H20(0.5 wt.%)/Pd( 1 wt.%) for various gases at an operating temperature of 300 “C. The sensitivities to butane, methane, propane, CO and hydrogen gas in 1000 ppm environments were 0.8, 0.4, 0.25, 0.3 and 0.35, respectively. Comparing
I
temp. : 3OO'C
Heat treatment:
4OO'C.
12
Fig. 14. Sensitivity characteristics of y-Fe,0,(98.5 wt.%)/ SnCl,.5H,0(0.5 wt.%)/Pd( 1 wt.%) thick film heat treated at 400 “C for 2 h to butane gas with various operating temperatures.
I-Fe20~/SnC14.5H20/Pd
Operating
10
concentration
Zhrs.
Operating
temperature:
:98.5iO.YlWC 3OO'C
60
SnCl4.5H20
contents
0
( w/o)
Fig. 13. Resistance variation of y-Fe,O, SnCl,.5H,O contents.
thick film with
2 Gas
12
ckentktion
(8xlOO$pm
)
Fig. 15. Sensitivity characteristics of y-Fe,O,/SnCMH,O(0.5 wt.%)/Pd( 1 wt.%) thick film for various gases.
235
these values with those obtained from Fig. 8, we see that the sensitivities to hydrocarbon gases are enhanced by adding Pd and SnCl,*SH,O to yFezO,. The devices started to respond to 1000 ppm hydrocabon gases at almost the same instant as the gases flowed over the devices at an operating temperature of 300 “C. We found that hydrocarbon gases can be detected by these devices in less than two seconds.
4. Conclusions
Thick-film hydrocarbon gas sensors were fabricated by a screen-printing method and their gassensing characteristics were investigated. Thick films of y-Fe,O, without any other catalysts showed a high sensitivity to hydrocarbon gases. Moreover, the addition of Pd catalyst to y-Fe,O, enhanced the sensitivity to hydrocarbon gases. SnC&.SH,O was also used as an additive in order to lower the device resistance, which was raised by Pd addition, without impairing the sensitivity to hydrocarbon gases. y-Fe,O,/ SnC1,*5HzO/Pd thick films with 0.5 wt.% SnCl,.SH,O and 1 wt.% Pd, fhed at 400 “C for 2 h, showed the highest sensitivity to hydrocarbon gases at an operating temperature of 300 “C. The sensitivities of this film to butane, methane, propane, CO and hydrogen gases in 1000 ppm environments were 0.8, 0.4, 0.25, 0.3 and 0.35, respectively, at an operating temperature of
300 “C, and the response time for 1000 ppm hydrocarbon gases was less than two seconds.
References 1 T. Seiyama, A. Kato, K. Fujiishi and M. Nagatanui, A new detector for gaseous components using semiconductive thin films, Anal. Chem., 34 (1962) 1502-1503. 2 P. J. Shaver, Activated tungsten oxide gas detectors, Appl. Phys. Len., II (1967) 255. 3 J. R. Mackintyre, A thin film hydrogen sensor, Instrum. Fe&&., 19 (i972) 29. _ M. Nitta, Thick 6hn CO gas sensor, IEEE Trans. Electron Devices, ED-24 (1979) 247-249. B. Morten, Thick 6lm technology and sensor, Sensors und Actuators, 4 (1983) 237-245. K. Dobos, Performance of carbon monoxide sensitive MOSFET’s with metal oxide semiconductor gates, IEEE Trans. Electron Devices, ED-32 (1985) 7. 7 Y. Nakatani, Some electrical properties of y-Fe,O, ceramics, Jpn. J. Appl. Phys., 2.2 (1983) 233. 8 Y. Nakatani, Microstructure of y-Fe,O, ceramics as a combustible gas sensor. Proc. In:. Meet. Chemical Sensors, Fukuoka, Japan, 1983, pp. 147. 9 K. Murata, Gas sensors, Denshi-Zairyo, (1977) 114. 10 T. Seiyama, Kagaku Sensors, Kodansha, Tokyo, 1982, pp. 49-50. 11 T. L. Poteat, Transition metal-gate MOS gaseous detectors, IEEE Trans. Electron Devices, ED-29 (1982) 124. 12 R. C. Baetxold, Interaction of saturated hydrocarbons with transition metal fihns, J. Am. Gem. SOL, (1983) 4271-4276. 13 J. F. McAleer, P. T. Moseley, B. C. Tofield and D. E. Williams. Proc. Br. Ceram. Sot.. 36(1985) 89-105. 14 H. Yam&da, Gas sensing characteristics’of porous zinc oxide ceramics with and without platinum catalyst, Proc. Int. Meet. Chemical Sensors, Fukuoka, Japan, 1983, pp. 95- 100. 15 N. Yamamoto, The effect of reducing gases on the conductivities of metal oxide semiconductors, Jpn. J. Appl. Phys., 20-4 (1981) 721-726.