IOP Conference Series: Materials Science and Engineering
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Thermoelectric gas sensors of different catalyst
oxides and heater metals
To cite this article: W Shin et al 2011 IOP Conf. Ser.: Mater. Sci. Eng. 18 212010
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ICC3: Symposium 15: Advanced Engineering Ceramics and Composites
IOP Publishing
IOP Conf. Series: Materials Science and Engineering 18 (2011) 212010 doi:10.1088/1757-899X/18/21/212010
Thermoelectric gas sensors of different catalyst oxides and
heater metals
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W Shin , M Nishibori , N Izu , T Itoh , I Matsubara , N Watanabe , T Kasuga
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1
Electroceramics Processing Research Group, AIST, Nagoya 463-8560, Japan
Dept. Frontier Materials, Nagoya Institute of Technology, Nagoya 466-8555, Japan
2
E-mail: w.shin@aist.go.jp
Abstract. Thermoelectric hydrogen sensors with different catalyst oxides, Pt-Al2O3 and PtCeO2 have been prepared, and their gas sensing properties are investigated in the air and N2
flow. In air, the relationship between the sensor output and the gas concentration is a linear
indicating the combustion only depends on the gas concentration in air. In N2 atmosphere,
where the oxygen gas content of N2 source cylinder is below the industrial standard of 50 ppm,
the sensor output shows also a linear relationship with the gas concentration, but depressed
signal level. The microheater of the thermoelectric sensor has been prepared by the cosputtering of tungsten and platinum to enhance the high temperature stability. The temperature
coefficients of the Pt-W alloy multilayer heater were lowered to a half of the level of Pt.
1. Introduction
The thermoelectric (TE) gas sensor has unique and reliable working principle, a combination of
catalytic combustion of inflammable gas and thermoelectric conversion, and is one promising solution
to detect wide range gas concentration in air. The sensor devices, which can be produced in mass and
require less amplification electronics and less power consumption are also desirable, such as the
micro-hotplate type gas sensors. The gas sensitive area is located on the membrane, where a cavity is
etched through the whole substrate to release the membrane [1]. Recent development of the TE sensor
was to integrate a thick-film type ceramic Pt/alumina catalyst combustor to enhance the sensitivity,
detecting the hydrogen gas as low as 0.5 ppm in air [2]. Not only for safety sensor application, the
sensors can be used, for leak detection application. In this case, the sensors are used in low pressure or
low oxygen partial pressure atmospheres. As the TE sensor uses the reaction of gas combustion, the
low oxygen atmosphere is very difficult and challenging task. Ceria is of high oxygen storage capacity
(OSC), and used for the catalyst system in automobile exhaust gas purification [3]. Replacing or
adding high OSC oxide of ceria in the Pt/alumina catalyst of TE sensors may provide oxygen for
detecting hydrogen in low oxygen atmosphere.
In present report, the thermoelectric sensors with different catalyst oxides, Pt-Al2O3 and Pt-CeO2
have been prepared, and their gas sensing properties are investigated in the air flow and in the low
oxygen partial pressure, such as in N2 flow. Furthermore, the microheater of the thermoelectric sensor
has been prepared by the co-sputtering of tungsten and platinum to enhance the high temperature
stability. The sensor response also plotted to discuss how the gas adsorbed and oxidized on the catalyst
surface in different oxygen partial pressure. The temperature coefficients of the different heater metals
are also studied.
c 2011 Ceramic Society of Japan.
1
Published under licence by IOP Publishing Ltd
ICC3: Symposium 15: Advanced Engineering Ceramics and Composites
IOP Publishing
IOP Conf. Series: Materials Science and Engineering 18 (2011) 212010 doi:10.1088/1757-899X/18/21/212010
2. Experimental
2.1. Sensor Device with Thermoelectric and Heater patterns
Figure 1 shows a snap of the micro-fabricated TE sensor device which is composed of a catalyst of
circle shape, a thermoelectric SiGe line, Pt micro-heater meander patterns, contact electrodes and the
thin dielectric membranes are made of silicon nitride-silicon oxide multilayer. The unique feature of
the sensor in this study is that both thermoelectric and catalytic parts on a single membrane, hot-plates.
Normal heater for the sensor was 200-nm-thick Pt thin film and a new thin film heater material was
composed of a 35-nm-thick Pt-W layer, a 200-nm-thick Pt layer, and a second 35-nm-thick Pt-W layer,
deposited by sputtering and fabricated by the lift-off process.
Figure 1. Micro-fabricated TE sensor device
shows a circle-shape catalyst, a thermoelectric
SiGe line, Pt micro-heater meander patterns.
Figure 2. Schematic of ceramic catalyst
preparation process of impregnation of
oxide powder with PtCl4.
2.2. Catalyst Paste Preparation and Sensor Test Method
Pt-loaded on alumina (Pt/alumina) catalyst was prepared by impregnation of alumina with an aqueous
solution of platinum (IV) chloride pentahydrate. A commercially available α-Al2O3 (TM-10; TAIMEI
Chemicals, Tokyo, Japan) powder with an average particle size of 0.10 μm was used. CeO2 power was
prepared synthesized from a hydrazine or ammonium aqueous solution and cerium nitrate (Ce(NO3)3).
The precipitate was oxide power of 10 nm size, and well dispersed as reported previously [4]. The
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•
slurry was stirred at 100 C for 30 min, and then was dried at 120 C for 2 h in air. The concentration of
Pt chloride pentahydrate in water was adjusted to the weight of alumina powder to make the final
composition of 40wt% Pt in the catalyst. The process flow is summarized in Fig. 2 and the details of
the catalyst powder preparation were reported previously [5].
After deposition of the catalyst on the microdevice, the hydrogen combustion performance of the
catalyst was investigated using gas-flow-type chamber which has been used in the previous works.
With the gas flow 200 ccm of 0.01 to 1 1v/v% hydrogen in air and N2 in the test chamber, the surface
temperature of the catalyst and device have been monitored by IR camera and thermoelectric voltage
signal from the device was also monitored.
3. Results and Discussion
3.1. Multilayer Heater Performance
Figure 3 shows the power consumption of the Pt-W/Pt/Pt-W microheater meanders with respect to the
catalyst temperature, which was estimated by monitoring of the surface temperature of the Pt/Al2O3
•
catalyst. The microheater achieved high catalyst temperature over 500 C with the power consumption
of 0.37W, exhibiting good linearity between the temperature and power. The power consumption can
be reduced by the heater design as reported [6]. The temperature dependence of the heater resistance
•
was reduced to be 2360ppm/K up to 500 C by the multi-layer alloying as shown in Fig. 4.
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ICC3: Symposium 15: Advanced Engineering Ceramics and Composites
IOP Publishing
IOP Conf. Series: Materials Science and Engineering 18 (2011) 212010 doi:10.1088/1757-899X/18/21/212010
600.0
Temperature/ ℃
500.0
Catalyst
SiGe
400.0
300.0
200.0
100.0
0.0
0.0000
0.1000
0.2000
0.3000
0.4000
Heater power / W
Figure 4. Temperature dependence of the heater
resistance up to 500°C.(IR camera range changed at
Figure 3. Heater Power consumption for
heating the catalyst over 500 °C.
250℃ as indicated with different marks)
3.2. Gas Combustion and Sensing
The gas sensing performance the sensor device with the catalyst of 40wt% Pt/Al2O3 - CeO2 (1:2) in the
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catalyst temperature 25 to 160 C is studied as shown in Fig. 5. When hydrogen is introduced, the
temperature of the catalyst side, point A, indicated in Fig. 1, increases with the hydrogen combustion,
but the temperature of the point B hardly changed because of the good thermal insulation.
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At 100 C, the hydrogen flow in air effectively increases the catalyst temperature by resulting Vs =
9 mV and TA-B = 41 °C. A rough estimation of the Seebeck coefficient is then Vs/ TA-B = 0.22 mV/K.
As the Seebeck effect is a linear function of the temperature so that Vs increased linearly with TA-B
with respect to gas concentration.
Figure 5. Output voltage signal (left) and temperature differential of the sensor (right) with the
catalyst of Pt / Al2O3 - CeO2, for the 1v/v% H2 in air mixture gas flow 200 ccm.
Figure 6 shows the response of the sensor with three different ceramic catalysts, Pt /Al2O3-CeO2,
Pt /CeO2, Pt /Al2O3, for a wide range of H2 concentration in air and N2 atmosphere. In air, the
relationship between the sensor output and the gas concentration is a linear indicating the rate of
combustion heating, Qc, only depends on the gas concentration in air, cair, as expressed as,
-1
Qc = KHcs = KH cair /(KZ+1) = cair H/(Z+K )
∝
3
cair
(1),
ICC3: Symposium 15: Advanced Engineering Ceramics and Composites
IOP Publishing
IOP Conf. Series: Materials Science and Engineering 18 (2011) 212010 doi:10.1088/1757-899X/18/21/212010
3
where the Z (s/m ) is the flow resistance, H is the enthalpy of heat, K is the reaction constant, and , cair
and cs are hydrogen gas concentration in air and at the surface of the catalyst, respectively. At enough
high catalyst temperature, the reaction is temperature independent and K becomes constant. There is
no clear difference in the sensor performance between three different catalysts.
Figure 6. Response of the with three different ceramic catalysts, Pt /Al2O3-CeO2, Pt
/CeO2, Pt /Al2O3, at 100℃, for the 10 ppm to 1v/v% H2 in air and nitrogen .
In N2 atmosphere, where the oxygen gas content of N2 source cylinder is below the industrial
standard of 50 ppm, the sensor output shows still a linear relationship with the gas concentration, but
drastically depressed signal level. Two sensors of catalyst CeO2 showed the higher signal, i.e., higher
combustion rate, than that of Al2O3 for the 1v/v% H2 in N2. The signal for 100ppm gas was too low to
be measured but the sensor of the Pt /Al2O3-CeO2 catalyst showed a signal below 0.01mV. From this
result, it seems that the catalyst containing ceria is more active than alumina for the hydrogen
combustion in reduced oxygen pressure, and that the equation (1) should be modified to correct the
rate of combustion heating, Qc, also depends on the rate oxygen supply. At elevated catalyst
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temperature, such as 160 C, the signal increased twice for 0.1 and 1 v/v% H2 in N2, which may activate
the oxygen storage and release capacity of the ceria in the catalyst.
4. Summary
To investigate the sensor performance in different oxygen partial pressure can provide fundamental
data for the gas sensor using combustion reaction. We have fabricated thermoelectric hydrogen
sensors with different catalyst oxides, Pt-Al2O3 and Pt-CeO2 and investigated their gas sensing
properties in the air and N2 flow. The correlation between the catalyst oxides and their gas combustion
properties suggested a couple of idea how the improvement of gas combustion can be achieved.
References
[1] Shin W, Tajima K, Choi Y, Izu N, Matsubara I, Murayama N, 2006 Sens. Actuators, B 108 455
[2] Nishibori M, Shin W, Houlet L, Tajima K, Izu N, Itoh T, and Matsubara I, 2006 J. Ceram. Soc.
Japan 114 853
[3] Nakatani T, Wakita T, Ota R, Tanaka K, Wakasugi T, 2003 J. Ceram. Soc. Japan 111 0137
[4] Izu N, Itoh T, Shin W, Matsubara I, and Murayama N, 2006 J. Ceram. Soc. Japan 114 418
[5] Shin W, Nishibori M, Ohashi M, Izu N, Itoh T, and Matsubara I, 2009 J. Ceram. Soc. Japan
117 659
[6] Nishibori M, Shin W, Izu N, Itoh T, and Matsubara I, 2010 Sensor Letters 8 1
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