NOx Gas Sensing Properties of ZnO-based Varistor-type Gas Sensors (特集環境ガスセンサ)

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Current (I)-voltage (V) characteristics of ZnO-based oxides have been investigated in air as well as in 100 ppm NO2 and NO balanced with air at elevated temperatures. A pure ZnO exhibited a nonlinear I-V characteristic in air and its breakdown voltage shifted to a high electric filed upon exposure to NO2, but to a low electric field upon exposure to NO. Grain size of ZnO was increased slightly by the Bi2O3-based additive, which is essential for fabricating conventional varistors used as voltage-surge suppressors. In accordance with this change the sensitivities to NO2 and NO were decreased, especially at 300°C. The Bi2O3-based additive containing Y2O3 was effective for reducing the grain size slightly, increasing the breakdown voltage in air and enhancing sensitivities, especially to NO2 at 400°C and to NO at 300°C. The addition of Cr2O3 to the Bi2O3-based additive containing Y2O3 was more effective for reducing the grain size, but led to a deterioration of sensitivities to NO2 and NO.
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