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Tin oxide (SnOX) carbon monoxide sensor fabricated by thick-film methods
ELSEVIER
Sensors and Actuators B 24-25 (1995) 537-539
Tin oxide (SnOx) carbon monoxide sensor fabricated film methods
by thick-
Ping Ping Tsai, I-Cherng Chen, Ming Hann Tzeng Industrial Technology Research Institute, Materials Research Labomton’es, Bldg. 77, 195-5 Chung-hsing Rd, Sec. 4, Chutun~ Hsinchu 31015, Taiwan
Abstract A new way to fabricate thick-film semiconductor-type tin oxide (SnOx) carbon monoxide sensors is reported in this paper. The sensing material is made by metallo-organic decomposition and is composed of calcium oxide- and niobium oxide-doped tin oxide with platinum as a catalyst; the electrodes and the heater are made by screen printing. Sensing measurements are done by cycling the sensor temperature with high and low heating powers. The CO response curve is almost linear at CO concentrations below 400 ppm. The sensor can detect CO concentrations be.low 30 ppm. The CO response with time, CO sensitivity, operation temperatures, power consumption, reliability of the sensor and also the selectivity of the sensor to various gases have been investigated. Keywomb: Carbon monoxide sensors; Thick-film sensors; Tin oxide
1. Introduction
Semiconductor gas sensors can be classified according to the fabrication method into the following types: sintered bulk type, thick-film type and thin-film type [l-7]. Most commercial sensors now in use are the sintered bulk type and the thick-film type. In the history of semiconductor-type carbon monoxide (CO) sensors, the bulk-type sensor was first developed by Figaro Engineering Inc. in 1974; later (in 1983) the method of measurement was modified by temperature. Further, in 1991 Figaro announced the development of a miniaturized, pulse-driven thick-film CO sensor [8]. The cycling of sensor temperature could help in purging the sensor and improving the selectivity [9]. Although the necessity to detect carbon monoxide in domestic and industrial applications can never be overemphasized and the demand for CO sensor is now surging, the semiconductor-type CO sensor is underdeveloped both marketwise and technologywise. Some reasons for this are the low threshold-limit value (TLV), which is only 50 ppm CO for 8 h weighted average concentration, and severe problems in the cross-sensitivity. Following our previous development of a thick-film alcohol sensor, a unique sensor with short recovery time and low detecting limit for sensing CO is described here [lO-121. 0925-4005/95/$09.50 0 1995 Elsevier Science SA. All rights reserved SSDI 0925-4005(94)01412-B
2. Experimental Thick-film carbon monoxide sensors were fabricated according to the basic method described in our previous publications [N-12]. The sensor was made by spincasting of metal organics on the alumina substrate, followed by metallo-organic decomposition (MOD) and screen printing of electrodes and heating elements. Fine crystalline tin oxide doped with calcium oxide and niobium oxide was formed as the sensing layer. The estimated composition of the sensing material in wt.% was CaO:Nb,O,:SnO,=5:5:90. The catalyst was pre-
1e
s
61
8
ti
*“ICI
S”
-
l-l
Fig. 1. The testing circuit and thermal cycles.
P.P.Tsai et al. I Sensors and Actuators B 24-25 (1995) 537-539
538
ra 2
! -
m
-
;
10;
-
: -
1-I 0
Fig. 2. Collective appearance of both sides of 15 sensors made by the unique thick-filmmethod
air-1 air-2 w-60-1 m-60-2 co-llc-1 m-110-2 100
200
IO
TIME (oec) Fig. 5. The response profile for carbon/monoxide sensing in air (60 and 110 ppm CO) from the beginning of the sensing cycle up to 250 s.
600 100I500 80 u 0
400
60
L"
2
300 -
e3 ZL
E
40 200 -
;
20
lW-
01
-
UP
-
down
aII
.,.,.,.,.,.I’,
0.2
0.4
0.6
0.8
1.0
1.2
1.4
CO
Power(watt) Fig. 3. The relationship behveen power consumption and temperature of the sensor.
iooool
0
0
--
180% 100%
conc.(ppm)
Fig. 6. The CO sensitivity of the sensor from 30 to 2800 ppm CO. The inset shows that below 400 ppm CO the sensitivity vs. cnocentration curve was close to linear.
pared from a platinum or palladium organic solution diluted with acetyl acetone to a 5 wt.% solution. The testing circuit and thermal cycles are shown in Fig. 1. During sensing, 2 V d.c. voltage was applied to the heater for 60 s, followed by purging at 5 V for 10 s. 3. Results and discussion
101 10
100
1000
10000
CO conc.(ppm) Fig. 4. The resistance of the sensing element vs. concentration of CO from 40 to 3000 ppm at 100, 180 and 230 “C, respectively.
The collective appearance of both sides of 15 sensors made by the unique thick-film method is shown in Fig. 2. Each sensor has the dimensions 0.42 cm x0.25 cm X 0.038 cm. The relationship between the power consumption and temperature of a sensor is shown in Fig. 3, which indicates that at a temperature of about 300 “C the element consumed 0.6 W. The resistance of the sensing element versus CO concentration from 40 to 3000 ppm followed almost parallel curves at 100, 180
P.P. Tsai et al. I Sensors and A&atom B 24-25 (1995) 537-539
-
co HZ
conc.(ppm)
-
co
-
HZ
539
and 230 “C, respectively (Fig. 4). The resistance decreased as the temperature increased. The response profile for carbon monoxide sensing in air with 60 and 110 ppm CO from the beginning of the sensing cycle up to 2.50 s. is shown in Fig. 5, in which the resistance continued to decrease with time but was reproducible in two consecutive measurements. The CO sensitivity of the sensors from 30 to 2800 ppm carbon monoxide was quite good (Fig. 6), and below 400 ppm carbon monoxide the sensitivity versus concentration curve was close to linear. The sensing element did detect CO concentrations below 30 ppm. However at about 150 “C (2 V) the cross-sensitivity of hydrogen and ethanol to carbon-monoxide sensing was serious (Fig. 7(a)). It was possible to decrease the interference by applying an active carbon filter (Fig. 7(b)). Efforts to solve this problem were made by lowering the operation temperature to about 50 “C (0.5 V, 25 mW) using a sensor with fine Pd-black particles as a catalyst; the crosssensitivity of ethanol, methane, butane and hydrogen became negligible (Fig. 8). Further work is needed to establish the long-term stability of this sensor. Acknowledgements Support for this research from the Ministry of Economic Affairs, Republic of China (contract No. 3612321) is gratefully acknowledged. References Patent No. 3631436 (1971). 121 M.J. Madou and S.R. Morrison, Chemical Sensing with Solid State Devices, Academic Press, New York, 1989. [31 C. Xu, J. Tam&, N. Miura and N. Yamazoe, Grain-size effects on gas sensitivity of porous SnO,-based elements, Sensors and Actuators a, 3 (1991) 147. 141 N. Yamazoe, New approaches for improving semiconductor gas sensors, Sensors and Achutors B, 5 (1991) 7. [51 T. Oyabu, T. Osawa and T. Kurobe, Sensing characteristics of tin oxide thick film gas sensor, J. Appl. Phys., 53 (11) (1982) 7125. El T. Oyabu, Sensing characteristics of tin oxide thick film gas sensor, J. Appl Phys., 53 (4) (1982) 2785. PI P. Van Geloven, J. Moons, M. Honore and J. Roggen, Tin(IV) oxide gas sensors: thick-film versus metallo-organic based se.nsors, Senrors and A&atom, 17 (1989) 361. PI S. Maatsuura, New developments and applications of gas sensors in Japan, Sensors and Actuators B, 13-14 (1993) 7-11. [91 W.M. Sears, K. Colbow and F. Consadori, Algorithms to improve the selectivity of thermally-cycled tin oxide gas sensors, Sensors and Achratois, 19 (1989) 333-349. WI P.P. Tsai, I.-C. Chen and M.H. Tzeng, An innovative tin oxide (SnOx) alcohol sensor fabricated by thick fdm techniques, MRL BUN. Res. Develop, 6 (2) (1992) 67. Pll P.P. Tsai, I.-C. Chen and M.H. Tzeng, Tin oxide (SnOx) alcohol sensor from metal organic decomposed (MOD) thick film, Senrors and Actuators B, 13-14 (1993) 610-612. WI P.P. Tsai, I.-C. Chen, M.H. Tzeng, CF. Liaw and J.C.H. Ku, US Patent No. 5 273 779 (1993).
Ul N. Taguchi, Gas detecting device, US
160
lOi0
conc.(ppm) Fig. 7. The cross-sensitivity of hydrogen and ethanol to carbon monoxide sensing at about 150 “C with: (a) sensor without any filter; (b) sensor together with an active carbon filter.
501
I oco IEICH
400
*I u
30-
.
cH4 C4HiO
n
H2
l
*
0
0
i? pe
zo-
.
0
. . .
10 1 1
0
x
.
I*
a*
=
: .!.T.?.,...,...,... l
100
x
x
I
._ l
200
300
400
500
conc.(ppm) Fig. 8. The cross-sensitivity of ethanol, methane, butane and hydrogen to carbon monoxide sensing at about 50 “C using a sensor with Pd as a catalyst.
Sensors and Actuators B 24-25 (1995) 537-539
Tin oxide (SnOx) carbon monoxide sensor fabricated film methods
by thick-
Ping Ping Tsai, I-Cherng Chen, Ming Hann Tzeng Industrial Technology Research Institute, Materials Research Labomton’es, Bldg. 77, 195-5 Chung-hsing Rd, Sec. 4, Chutun~ Hsinchu 31015, Taiwan
Abstract A new way to fabricate thick-film semiconductor-type tin oxide (SnOx) carbon monoxide sensors is reported in this paper. The sensing material is made by metallo-organic decomposition and is composed of calcium oxide- and niobium oxide-doped tin oxide with platinum as a catalyst; the electrodes and the heater are made by screen printing. Sensing measurements are done by cycling the sensor temperature with high and low heating powers. The CO response curve is almost linear at CO concentrations below 400 ppm. The sensor can detect CO concentrations be.low 30 ppm. The CO response with time, CO sensitivity, operation temperatures, power consumption, reliability of the sensor and also the selectivity of the sensor to various gases have been investigated. Keywomb: Carbon monoxide sensors; Thick-film sensors; Tin oxide
1. Introduction
Semiconductor gas sensors can be classified according to the fabrication method into the following types: sintered bulk type, thick-film type and thin-film type [l-7]. Most commercial sensors now in use are the sintered bulk type and the thick-film type. In the history of semiconductor-type carbon monoxide (CO) sensors, the bulk-type sensor was first developed by Figaro Engineering Inc. in 1974; later (in 1983) the method of measurement was modified by temperature. Further, in 1991 Figaro announced the development of a miniaturized, pulse-driven thick-film CO sensor [8]. The cycling of sensor temperature could help in purging the sensor and improving the selectivity [9]. Although the necessity to detect carbon monoxide in domestic and industrial applications can never be overemphasized and the demand for CO sensor is now surging, the semiconductor-type CO sensor is underdeveloped both marketwise and technologywise. Some reasons for this are the low threshold-limit value (TLV), which is only 50 ppm CO for 8 h weighted average concentration, and severe problems in the cross-sensitivity. Following our previous development of a thick-film alcohol sensor, a unique sensor with short recovery time and low detecting limit for sensing CO is described here [lO-121. 0925-4005/95/$09.50 0 1995 Elsevier Science SA. All rights reserved SSDI 0925-4005(94)01412-B
2. Experimental Thick-film carbon monoxide sensors were fabricated according to the basic method described in our previous publications [N-12]. The sensor was made by spincasting of metal organics on the alumina substrate, followed by metallo-organic decomposition (MOD) and screen printing of electrodes and heating elements. Fine crystalline tin oxide doped with calcium oxide and niobium oxide was formed as the sensing layer. The estimated composition of the sensing material in wt.% was CaO:Nb,O,:SnO,=5:5:90. The catalyst was pre-
1e
s
61
8
ti
*“ICI
S”
-
l-l
Fig. 1. The testing circuit and thermal cycles.
P.P.Tsai et al. I Sensors and Actuators B 24-25 (1995) 537-539
538
ra 2
! -
m
-
;
10;
-
: -
1-I 0
Fig. 2. Collective appearance of both sides of 15 sensors made by the unique thick-filmmethod
air-1 air-2 w-60-1 m-60-2 co-llc-1 m-110-2 100
200
IO
TIME (oec) Fig. 5. The response profile for carbon/monoxide sensing in air (60 and 110 ppm CO) from the beginning of the sensing cycle up to 250 s.
600 100I500 80 u 0
400
60
L"
2
300 -
e3 ZL
E
40 200 -
;
20
lW-
01
-
UP
-
down
aII
.,.,.,.,.,.I’,
0.2
0.4
0.6
0.8
1.0
1.2
1.4
CO
Power(watt) Fig. 3. The relationship behveen power consumption and temperature of the sensor.
iooool
0
0
--
180% 100%
conc.(ppm)
Fig. 6. The CO sensitivity of the sensor from 30 to 2800 ppm CO. The inset shows that below 400 ppm CO the sensitivity vs. cnocentration curve was close to linear.
pared from a platinum or palladium organic solution diluted with acetyl acetone to a 5 wt.% solution. The testing circuit and thermal cycles are shown in Fig. 1. During sensing, 2 V d.c. voltage was applied to the heater for 60 s, followed by purging at 5 V for 10 s. 3. Results and discussion
101 10
100
1000
10000
CO conc.(ppm) Fig. 4. The resistance of the sensing element vs. concentration of CO from 40 to 3000 ppm at 100, 180 and 230 “C, respectively.
The collective appearance of both sides of 15 sensors made by the unique thick-film method is shown in Fig. 2. Each sensor has the dimensions 0.42 cm x0.25 cm X 0.038 cm. The relationship between the power consumption and temperature of a sensor is shown in Fig. 3, which indicates that at a temperature of about 300 “C the element consumed 0.6 W. The resistance of the sensing element versus CO concentration from 40 to 3000 ppm followed almost parallel curves at 100, 180
P.P. Tsai et al. I Sensors and A&atom B 24-25 (1995) 537-539
-
co HZ
conc.(ppm)
-
co
-
HZ
539
and 230 “C, respectively (Fig. 4). The resistance decreased as the temperature increased. The response profile for carbon monoxide sensing in air with 60 and 110 ppm CO from the beginning of the sensing cycle up to 2.50 s. is shown in Fig. 5, in which the resistance continued to decrease with time but was reproducible in two consecutive measurements. The CO sensitivity of the sensors from 30 to 2800 ppm carbon monoxide was quite good (Fig. 6), and below 400 ppm carbon monoxide the sensitivity versus concentration curve was close to linear. The sensing element did detect CO concentrations below 30 ppm. However at about 150 “C (2 V) the cross-sensitivity of hydrogen and ethanol to carbon-monoxide sensing was serious (Fig. 7(a)). It was possible to decrease the interference by applying an active carbon filter (Fig. 7(b)). Efforts to solve this problem were made by lowering the operation temperature to about 50 “C (0.5 V, 25 mW) using a sensor with fine Pd-black particles as a catalyst; the crosssensitivity of ethanol, methane, butane and hydrogen became negligible (Fig. 8). Further work is needed to establish the long-term stability of this sensor. Acknowledgements Support for this research from the Ministry of Economic Affairs, Republic of China (contract No. 3612321) is gratefully acknowledged. References Patent No. 3631436 (1971). 121 M.J. Madou and S.R. Morrison, Chemical Sensing with Solid State Devices, Academic Press, New York, 1989. [31 C. Xu, J. Tam&, N. Miura and N. Yamazoe, Grain-size effects on gas sensitivity of porous SnO,-based elements, Sensors and Actuators a, 3 (1991) 147. 141 N. Yamazoe, New approaches for improving semiconductor gas sensors, Sensors and Achutors B, 5 (1991) 7. [51 T. Oyabu, T. Osawa and T. Kurobe, Sensing characteristics of tin oxide thick film gas sensor, J. Appl. Phys., 53 (11) (1982) 7125. El T. Oyabu, Sensing characteristics of tin oxide thick film gas sensor, J. Appl Phys., 53 (4) (1982) 2785. PI P. Van Geloven, J. Moons, M. Honore and J. Roggen, Tin(IV) oxide gas sensors: thick-film versus metallo-organic based se.nsors, Senrors and A&atom, 17 (1989) 361. PI S. Maatsuura, New developments and applications of gas sensors in Japan, Sensors and Actuators B, 13-14 (1993) 7-11. [91 W.M. Sears, K. Colbow and F. Consadori, Algorithms to improve the selectivity of thermally-cycled tin oxide gas sensors, Sensors and Achratois, 19 (1989) 333-349. WI P.P. Tsai, I.-C. Chen and M.H. Tzeng, An innovative tin oxide (SnOx) alcohol sensor fabricated by thick fdm techniques, MRL BUN. Res. Develop, 6 (2) (1992) 67. Pll P.P. Tsai, I.-C. Chen and M.H. Tzeng, Tin oxide (SnOx) alcohol sensor from metal organic decomposed (MOD) thick film, Senrors and Actuators B, 13-14 (1993) 610-612. WI P.P. Tsai, I.-C. Chen, M.H. Tzeng, CF. Liaw and J.C.H. Ku, US Patent No. 5 273 779 (1993).
Ul N. Taguchi, Gas detecting device, US
160
lOi0
conc.(ppm) Fig. 7. The cross-sensitivity of hydrogen and ethanol to carbon monoxide sensing at about 150 “C with: (a) sensor without any filter; (b) sensor together with an active carbon filter.
501
I oco IEICH
400
*I u
30-
.
cH4 C4HiO
n
H2
l
*
0
0
i? pe
zo-
.
0
. . .
10 1 1
0
x
.
I*
a*
=
: .!.T.?.,...,...,... l
100
x
x
I
._ l
200
300
400
500
conc.(ppm) Fig. 8. The cross-sensitivity of ethanol, methane, butane and hydrogen to carbon monoxide sensing at about 50 “C using a sensor with Pd as a catalyst.