Abstract
Current (I)-voltage (V) characteristics of porous ZnO varistors with and without Bi2O3-based, Cr2O3 and Y2O3 additives have been investigated in air as well as in NO2 and NO at elevated temperatures. All the varistors exhibited nonlinear IV characteristics under every atmosphere, whereas the nonlinearity tended to deteriorate with a rise in temperature. The breakdown voltage of all the varistors shifted to a high electric field upon exposure to NO2, and the magnitude of the shift, i.e. the sensitivity to NO2, was increased by the simultaneous addition of Bi2O3-based additive and Cr2O3 (or Cr2O3+Y2O3) at 300°C. However, the sensing behavior of NO depended markedly on both the compositions of the varistors and the operating temperature. The breakdown voltage of pure ZnO and ZnO added only with the Bi2O3-based additive shifted to a low electric field upon exposure to NO at 300°C, but it shifted to a high electric at higher temperatures. This suggested different chemisorption states of NO on the surface of ZnO, i.e., NO+ (nitrosyl-type) at 300°C and ONO_??_ (nitrito-type) at higher temperatures. Further addition of Cr2O3 or Cr2O3 plus Y2O3 resulted in a shift to a lower breakdown voltage upon exposure to NO at every temperature, suggesting the stabilization of NO+ adsorbates even at higher temperatures. Especially at 300°C, the further addition led to enhancement in NO sensitivity. A. c. impedance measurements have revealed that the variation in the potential height induced by the chemisorption of NO2 and NO determines the resistance of the grain boundary and hence controls the breakdown voltage of the varistors.
