Control of Varistor Voltage by Grain-size Control of Bi-added ZnO Varistors

Relationship between the grain size of ZnO and the amounts of Bi and Sb for Bi-Ba-Sb-Co-Mn-added ZnO was investigated. The amount of Bi at which the grain size of ZnO became the maximum shifted to the notably larger one by adding small amount of Sb such as 0.01 mol%. The maximum value of the grain size when adding Sb was approximately 1.3 times larger than that when not adding Sb. Because Ba3Mn2O8 was not formed when Sb is added and the much amount of Ba forms the coexistence phase with Bi at grain boundaries of ZnO, the grain size of ZnO becomes large. The tolerance characteristic to the electrical degradation was improved by addition of Sb, because Ba3Mn2O8 was not formed and Mn distributed more uniformly at grain boundaries.