NO_x Gas Sensing Properties of ZnO-based Varistor-type Gas Sensors

Abstract

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.