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TITLE Development of Super-Miniaturized Heat-Resistant Ceramic Oxygen Sensors
(Joint Program to Promote Technological Development with the Private Sectors)



We have developed a high-precision ceramic oxygen sensor for combustion control. The approach to this development was made by two methods. One was a method for the development of an improved type sensor, and the other was a method for the development of a new type oxygen sensor. Directing our attention to the structure of conventional zirconia oxygen sensors, we have established a technique to provide a gas diffusion hole in the zirconia solid electrolyte and seal the cathode side with glass. This technique made it feasible to attain reduction in size and weight and, at the same time, mass production. In the functional aspect, the output of the improved oxygen sensor during normal combustion in a water heater was confirmed to be stable, following up sufficiently changes in the operation of the gas control valve and applied voltage of the fan motor. At the time of abnormal combustion, however, an increase in the oxygen concentration by the decomposition of CO2 and H2O coexisting in exhaust gases due to interference of unburned gases was observed. To cope with this phenomenon, a second sensor method was adopted which stacks one sensor element for detection of unburned gases. As a result, it became possible to make correction for the abnormal output. Moreover, this sensor was subjected to a heat cycle test in actual exhaust gases. In the test, some decrease in the output was observed at the end of 10,000 cycles. Even in this condition, it was possible to measure the oxygen concentration in the air.

On the other hand, in the development of a new type oxygen sensor, we have confirmed that an 8YSZ thin film of excellent crystallinity can be obtained by annealing an RF sputtered film. The new type oxygen sensor element (0.7mPt/10m8YSZ/0.7mPt) using this thin film exhibited the specified V-I performance in actual exhaust gases and is applicable to combustion control as well as the improved type sensor. Compared with the improved ty pe, the new type oxygen sensor is less affected by unburned gases and has higher sensitivity to oxygen concentration measurement in the region of low air ratio. Therefore, the new type ones can probably be very excellent as the oxygen sensor for combustion control in the fully premixed combustion system. As mentioned in the foregoing, we have developed two types of oxygen sensors.