Vol.4 No.2 2011
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Research paper : Thermoelectric hydrogen gas sensor (W. Shin et al.)−100−Synthesiology - English edition Vol.4 No.2 (2011) semiconductor types or catalytic combustion types. In principle these sensors detect only a narrow range of concentration of gas, and hydrogen sensor technology of selective gas detection has not yet been achieved. The semiconductor sensors have been most extensively studied up to date because of their ultra-sensitive gas detection performance, and the majority of papers on gas sensors contribute to this technology. When improving the sensitivity of semiconductor sensors, the reliability problems always occur because the sensor is also very sensitive to the environmental noise, such as humidity. In addition, the accuracy of the sensor signals has another problem of signal drift (variation of the indication) [1], showing lack of reliability.Catalytic combustion type sensors are widely used because they are robust with excellent stability. With this type of sensor, a slight rise in sensor temperature due to catalytic combustion of hydrogen gas increases the resistance of the platinum-coil heater, and this enables the detection of the hydrogen in the atmosphere. Catalytic combustion is a simple chemical reaction, and wrong signal outputs even in the presence of some environmental noise are extremely few. However, to achieve both reliability and sensitivity is difficult. The sensitivity of the detection signals of the catalytic combustion sensors which are the resistance rates of the sensor is significantly reduced in low concentration, and the practical range of concentration is in the detection range of a few percent from 1000 ppm[2].Figure 1 shows the concept of a multi-level security system required in the hydrogen stations. In hydrogen-related facilities, if a system is established which can detect hydrogen concentrations before a specified concentration of hydrogen or fraction of flammable limit is reached, it will allow for single and/or multilevel safety operations, such as nitrogen purging or ventilation and/or ways to avoid shut-downs. Furthermore, in the stations, the system should have a sensor element which is robust against poisoning, which is caused by any interferential gas that permanently affects the sensitivity of a sensor, and is hydrogen selective against other flammable gases. In actual hydrogen stations, catalytic combustion type sensors are used for high gas concentrations as an alarm sensor, and semiconductor type sensors are also used for low concentrations as a monitoring sensor, respectively. 3 Elemental technologies for the gas sensor synthesis3.1 Three elemental technologies for the three performances of the gas sensorWe have proposed a completely new principle of hydrogen gas sensor combining the catalytic combustion and thermoelectric conversion technology. This sensor uses the heat generated by combustion of gases within the catalyst in the same manner as conventional catalytic combustion type sensors. Rather than using a resistance change in temperature of the entire device, the changes in the local temperature inside the device are converted directly into voltage signals based on a thermoelectric conversion principle. We named this the thermoelectric sensor. Figure 2 shows three performances required for gas sensors or the 3S, correlating them with the elemental technologies: gas selectivity, high sensitivity, long-term stability.1) new principle, of “integration of knowledge” – The thermoelectric conversion principle is the most fundamental idea which enables the wide-range selective gas detection, which is not possible with conventional technology[3].2) microfabrication technology – For minimization of the heat capacity of the sensing element to catch a very small amount of heat for the detection of lower gas concentration, microfabrication techniques such as anisotropic etching of silicon was used. The thermoelectric sensor is a micro calorimeter with a catalyst combustor.3) catalyst technology – Mounting a high-performance ceramic catalyst combustor on a specific position of the pattern on the micro thermoelectric devices has been accomplished.By integrating these elemental technologies, a prototype of excellent hydrogen selectivity, of wide range detection up to 5 % hydrogen concentration from 0.5 ppm was realized and a year long stability field test proved that the same stability was realized as the current technology[4]. Typical characteristic of a high-sensitivity sensor is a trade-off between selectivity and stability, but thermoelectric gas sensor is able to satisfy both performances. Considering that the lower limit of catalytic combustion type sensor is about 500 ppm, the ppm-level detection performance of thermoelectric sensor is innovative. For the stability of the sensor, we have clarified with a scientific approach the factors causing degradation in the sensor response, and improved the stability of the sensor Fig. 1 Multi-level or multiple safety system for the operation of hydrogen facilitiesCurrent safety technology of hydrogen leak detection lacks lower level stability and hydrogen selectivity.Ventilation onPower offNot applicable to conventional technologyHydrogen detectorsSemiconductor-typeCatalyticcombustion-type20 ppm, H2 in human breath0.5 ppm, H2 in the atmosphereHydrogen detector of wide range detection is necessary for themulti-level safety operation.Safety operation5000 ppm, Lower alarm starting level1.0 %v/v, Alarm level for system power offHydrogen concentrationfor normal use1000 ppm, Lower limit of alarm level1.5 %v/v, Alarm level for station power off4.0 %v/v, Lower flammability limit

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