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 voltage per grain in ZnO varistor ceramic and the breakdown voltage of a single grain boundary in this material is interesting and worth study for practical purposes. The contradiction is imputed to the existence of ineffective grain boundaries in this material. The ineffective grain boundary is demonstrated by elaborately designed experiments. Observation also shows that the presence of ineffective grain boundaries is related to deficiency of oxygen in the grain boundary region. Using computer-aided simulation, the influence of ineffective grain boundaries on the breakdown threshold of the component is discussed.