INEFFECTIVE GRAIN-BOUNDARIES AND BREAKDOWN THRESHOLD OF ZINC-OXIDE VARISTORS

In order to evaluate the breakdown threshold of ZnO varistor ceramic, the geometry-independent average breakdown voltage per grain boundary, or V-gb was examined. The discrepancy between the average breakdown v oltage per grain in ZnO varistor ceramic and the breakdown voltage of a single grain boundary in this material is interesting and worth stud y for practical purposes. The contradiction is imputed to the existenc e of ineffective grain boundaries in this material. The ineffective gr ain boundary is demonstrated by elaborately designed experiments. Obse rvation also shows that the presence of ineffective grain boundaries i s related to deficiency of oxygen in the grain boundary region. Using computer-aided simulation, the influence of ineffective grain boundari es on the breakdown threshold of the component is discussed.