Effects of Zr and Y Addition on the Electrical Degradation Characteristics of ZnO Varistors

Abstract

The effects of the addition of Y or both Y and Zr to Bi-Mn-Co-Sb-Si-Cr-Ni-added ZnO varistors (the same composition as one of commercial varistors) on the varistor voltage and the tolerance characteristics of electrical degradation were investigated. The tolerance characteristic of electrical degradation was deteriorated rapidly by addition of Y of more than approximately 1 mol%. The degree of deterioration for addition of both Zr and Y was smaller than that for addition of only Y. Moreover, the number of formed spinel (Zn2.33Sb0.67O4)-type particles monotonically increased with increasing amount of Y. On the other hand, the number of formed willemite (Zn2SiO4)-type particles decreased by addition of Y of more than approximately 1 mol%. It was suggested that the deterioration of the tolerance characteristics of electrical degradation by the addition of Y is probably caused by an increase in the number of willemite-type particles. Moreover, it was suggested that the reduction in the average ZnO grain size due to the addition of Y was a major factor in the increased varistor voltage and the ZnO grain growth was inhibited by the formation of an unknown compound, of which main ingredients are Y, Zn, and Sb or Y, Zr, Zn, and Sb, after adding Y. The varistor voltage of a varistor with 2 mol% added Y increased by approximately 50% compared to a varistor with no Y added. A ZnO varistor with a varistor voltage of approximately 600 V/mm and excellent tolerance characteristics of electrical degradation was fabricated by adding approximately 1.25 mol% Y and approximately 0.45 mol% Zr to a Bi-Mn-Co-Sb-Si-Cr-Ni-added ZnO varistor.