Effects of annealing on the grain boundary potential barrier of ZnO varistor

Effects of post heat treatment on the potential barrier at the grain boundaries of ZnO varistors will be discussed from the viewpoints of the surface state density and the donor concentration. Leakage current of varistors at a low electric field increases by annealing at 750° C in air or nitrogen due to the lowered barrier height corresponding to the decrease of surface state density, which may be explained with the phase transformation of the bismuth-rich intergranular layer. It is also observed that theV-I non-linearity of the annealed ceramics is generally recovered as the annealing (in air) time is extended. This can be explained by the heightened barrier potential attributed to the decrease of donor concentration in ZnO grains, which was confirmed by aid ofC-V measurements. The decrease of donor concentration by the annealing in air can be considered to be responsible for the thermal oxidation of interstitial zinc ions or oxygen vacancies.