Design of high-performance sensors by using specific sensing function of hetero-junction interfaces between ionic conductor and semiconductor

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The present study aimed to design, construct and control new hetero-junction interfaces between ionic conductor and oxide semiconductor and then to develop high-temperature sensing devices whose properties were drastically improved by giving specific gas-sensing function. Consequently, the following results were obtained.1) By using an oxide sensing electrode, a stabilized zirconia (YSZ)-based sensor was developed for the selective detection of hydrocarbons at high temperature. Among the 14 kinds of oxide tested, CdO was found to be best suited for the sensing electrode of a tubular device, giving selective and quick response to propylene in air at 600 ℃.2) An YSZ-based sensor using oxide sensing-electrode was also examined for NOx detection. Among spinel-type oxides tested ZnFe_2O_4 has the highest sensitivity to NO_2 in air at temperatures of 550-700 ℃.3) It was found that the combination of YSZ and SnO_2 sensing electrode gave a relatively high sensitivity to N_2O in air at 475 ℃. The addition of a foreign oxide (e.g. Bi_2O_3 and Sm_2O_3) to SnO_2 was effective to improve the N_2O sensitivity.4) A NASICON based CO_2 sensing device with In_2O_3 introduced in the sensing electrode was investigated. It was shown that the CO_2 sensing capability at room temperature was easily attained under humid condition. Some sorts of auxiliary phases such as NaHCO_3 and Na_2CO_3 were suspected to form spontaneously on the NASICON disk surface to give CO_2 sensitivity.5) A NASICON based amperometric sensor was designed for the detection of total NOx in air. To make the device sensitive to both NO_2 and NO, the sensing Au electrode was covered with NO oxidation catalysts. The device attached with double catalyst layers of WO_3 and Pt-loaded SiO_2 produced current response proportional to the concentration of NO_2 or NO up to 1 ppm at 150 ℃.