Development of a Catalytic Combustion Type Gas Sensor with MEMS Heater for Methane

Since low power consumption gas sensors are required for battery-driven gas alarm, we developed a catalytic combustion type gas sensor with low power consumption. This sensor is equipped with a micro heater and intermittent-drive system that we produced with the use of MEMS (Micro Electro Mechanical Systems) technology and which provides high-speed response times of several dozen milliseconds. The combustion type gas sensor consists in a detection element and a compensation element. In our company, these element materials mainly employ Pd- and Pt-loaded γ-alumina (Pd/Al 2 O 3 and Pt/Al 2 O 3 ) catalysts, respectively. The Pt/Al 2 O 3 catalyst was used for cancelling the hydrogen sensitivity. However, the methane output of the sensor was low because the amounts of these elements with micro heater have quite small. In this study, the improvement of the heater structure for increasing the catalyst amount and the performance for the Pd/Al 2 O 3 catalyst on the output were examined. The configuration of the micro heater is shown below in Figure 1. The micro heater was produced by forming an insulated layer consisting of silicon nitride film, etc., on a silicon wafer and then patterning a platinum heater. The rear surface was then etched to suspend a heating element reducing the amount of heat generated, which produces high speed response times of several dozen milliseconds. The Pt heater consists of a 90 μm rectangular air bridge structure. The Pd/Al 2 O 3 catalyst as the detection element was prepared by using an impregnation method. The loaded amount of Pd was 30wt%. As the Pd precursor, two different materials were used as the active metal source: (CH 3 COO) 2 Pd (Pd(OAc) 2 ) and Pd(NO 3 ) 2 aq.. Each precursor and an γ-alumina were mixed and evaporated in vacuum using an evaporator. These catalysts were then dry and calcined. By using a dispensing machine, The Pd/Al 2 O 3 and the Pt/Al 2 O 3 catalysts were coated as each element. The catalytic combustion type micro sensor was assembled onto the bridge circuit. These elements were heated to 400－450°C by the micro heater and then placed in a chamber so that the sample gas could be injected at the prescribed concentration. The output was measured by amplifying the electric potential difference between both elements generated by the fluctuations in the micro heater resistance, which were caused by a catalytic combustion reaction of flammable gases on the surface of the catalyst. Firstly, we examined to add slit-shaped vents between the heater wires to encourage contact with gas on the rear of the heater by exploiting the characteristics of the sensor elements that made up the air bridge. The heater structure is shown in Figure 2(a). Moreover, we investigated to add arch-shape structure around the heater for increasing the catalyst amount, as shown in Figure 2(b). The structure leads to laminate larger the amounts of these catalysts than that without the arch part because the catalyst was coated on not only the heater part but also the arch part. The output properties of these sensors having the heater with each structure are shown in Figure 3. In this figure, the previous structure indicates the heater without slit-shaped vents and arch part in Figure 1. Comparing to the previous structure, the structure with the vents showed higher output for 40%. Furthermore, by adding the arch part around the heater, the output increased for about 60%. Therefore, it is possible to increase the laminated amount of the catalyst by changing the heater structure, which leads to improve the output. Figure 4 shows the output ratio of the sensors using Pd(OAc) 2 and Pd(NO 3 ) 2 aq. as the Pd precursor to methane concentration . In this result, the sensor using Pd(OAc) 2 exhibited higher output than that using Pd(NO 3 ) 2 aq. . Figure 5 indicates the TEM images of the catalyst using these precursors. Pd particle sizes using Pd(OAc) 2 and Pd(NO 3 ) 2 aq. were 5.7 and 24.4 nm, respectively. These results implied that particle size have an affect on the output property. Figure 6 shows the change ratio of the output of methane 3000ppm for 150 days at various conditions: at 20°C/65% relatively humidity (rh), 45°C/80%rh, and 5°C/Free rh. In this figure, the ratios were almost unchanged. In other words, the sensor showed the high stability at various conditions. The sensor having the heater with slit-shaped vents and arch structure indicated higher output than that without the structure. In the detection element, The Pd/Al 2 O 3 catalyst using (CH 3 COO) 2 Pd showed higher performance, comparing with that using Pd(NO 3 ) 2 aq. . The Pd catalyst with smaller particle leads to show higher output. In addition, the sensor exhibited high stability at various conditions. Figure 1