Effect of long-term electrical degradation on the distribution of donor impurities in ZnO varistor ceramics

The effect of long-term electrical degradation on the distribution of donor impurities in ZnO varistor ceramics and their electrical properties was examined. The results are consistent with the views that, in the initial stages of the process, the degradation is due mainly to local Joule heating at intergranular barriers, while at longer degradation times, the major mechanism of degradation is the redistribution of donors in the bulk of ZnO grains owing to their migration in the electric field of the barriers.