In ZnO-based varistors, during the liquid-phase sintering process using Bi2O3, contact points (“holes”) between ZnO grains inevitably occur within the grain boundary layer of the ZnO polycrystal. Considering these “holes” as microscopic electrical resistors in the polycrystalline ZnO and assuming that their size and the amount of oxygen adsorbed alter the depletion layer at the “hole” sites allows us to elucidate the mechanism behind a series of electrical properties. These include the electrical resistivity and its temperature dependence in the low-current region of the E-J characteristics, thermal runaway occurring in the electrical long-term leakage current porperties and improvements in long-term properties achieved through heat treatment. Furthermore, since trace additives also act at grain boundaries to improve the E-J characteristics, the electrical properties of the ZnO varistor can be interpreted as entirely attributable to chemical reactions and microstructural changes in the grain boundary layer. In contrast, the Schottky-type energy barrier model, which focuses primarily on the electronic structure within ZnO grains, cannot provide a unified interpretation of these phenomena. Based on these results, we propose a new conduction mechanism model, the “composite insulating layer tunnelling current model” (abbreviated as the “pinhole model”).
Okinaka et al. (Thu,) studied this question.