Development of a Catalytic Combustion Type Gas Sensor with Low Power Consumption

Abstract

Currently, gas leak alarms using AC power-supply are popular in Japan. However, there is pointed out, such as the hassle of installation property and power cord of the alarm, there was an increasing demand of the alarm of a battery drive system. The gas sensor of very low power consumption was required for realizing the battery operating system. Therefore, we focused on MEMS technology. The heating element occupying power most of for operating the sensor has been developed micro heater using a MEMS technology. The micro-heater has a rapid thermal response of several tens of milliseconds after the supply power. Furthermore, we developed a catalytic combustion type gas sensor reduced to 1/1000 of conventional power consumption by using the detection method of the intermittent operating. In general, miniaturization of the gas sensor would have a negative effect. Those effects were decreasing of the sensitivity / noise ratio and tending to susceptible to environmental factors. In this study, we report on improvement of the sensor for high sensitivity and high gas selectivity. The micro-heater was formed by sputtered platinum pattern on a silicon wafer. Formed platinum membrane was covered with a silicon nitride film. It is an air-bridge structure of the square of 90μm angle, suspended by etching the heater backside. The micro-heater has low power consumption because its thermal capacity is low. The catalytic combustion type micro-gas-sensor was prepared by sintering materials that Pd loaded γ-Al2O3 for a sensing element and only γ-Al2O3 for a reference element. The catalytic combustion type micro-sensor to give amplifies the potential difference generated both elements by a resistance change of the heater according to the catalytic combustion reaction of combustible gases. Measurements of gas sensitivity of the sensor which was heated to approximately 450°C， placed in a gas chamber, it was performed by injecting a test gas having a predetermined concentration. Response carve of the sensor when power was supplied to the micro-heater becomes detectable state at tens of milliseconds. Methane sensitivity was observed with good linearity. However, the hydrogen sensitivity was approximately three times the methane it, methane selectivity of the sensor was found to be low. Material of the reference element in order to improve this problem has been changed. The reference element materials have used γ-Al2O3 does not have catalytic combustion reaction with any combustible gases. The reference element material was to have sensitivity to miscellaneous gases such as hydrogen by changing from γ-Al2O3 in the Pt loaded γ-Al2O3. Miscellaneous gas sensitivity of each element cancelled each other, the selectivity of methane sensitivity was improved. As a result, the sensitivities of H2 / CH4 ratio have become possible to improve the methane selectivity to 1 times or less. The second improvement was to increase the methane gas sensitivity. By the small size of the device due to the low power consumption, the amount of catalyst could be applied would be very small. Further, since the planar structure, the detection target gas could react only one side of the catalyst, the sensitivity obtained by the combustion reaction was low. Therefore, we consider how to increase the gas contact point on the element. As a first method, it was optimize the amount of Pd supported on the catalyst. If the amount of Pd loaded ratio is substituted standard ratio for 1, the sensitivity of the methane was found to be a maximum value in the Pd ratio is 2. On the other hand, the specific surface area was decreased with an increase in the amount of Pd loaded in particular reduction of the specific surface area more than Pd ratio is 2 was significant. When the amount of Pd loaded increases gas contact points, gas sensitivity is improved to increase. However, the sensitivity is found to decrease by a metal agglomeration when Pd loaded content is too high may occur. As a second method, utilizing the characteristics of the sensor elements consists of an air-bridge, in order to encourage gas contact from the heater backside, some slit-shaped through-hole between the heater patterns. It becomes possible to wrap the catalyst heater backside by the structural modification. Then methane gas sensitivity of the sensor by gas contact point of the heater back side increases has been improved by 40% compared with the previous type. These improvements had the prospect of practical use of methane gas sensor of the catalytic combustion type with micro-heater using MEMS technology.