Effects of addition of Al on electrical degradation of ZnO varistors

Effects of addition of Al on the electrical degradation of the Bi, Mn, and Al added ZnO varistors and the Bi, Co, and Al added ZnO varistors were investigated by V-I, C-V, and ICTS methods. From V-I method, it is found that the optimum quantity of Al additive to prevent the electrical degradation is approximately 3ppm in the case of Bi-Mn-Al added ZnO varistor and approximately 13ppm in the case of Bi-Co-Al added ZnO varistor. The degree of the electrical degradation of Bi-Co-Al added ZnO varistors was smaller than that of Bi-Mn-Al added ZnO varistors for the optimum content of Al. The electrical degradation was hindered with increasing the depth of the trap levels. From the results of C-V method, it was found that, the height of phi(B) became lower for Bi-Mn-Al added ZnO varistors and became higher for Bi-Co-Al added ZnO varistors. The density N-is of the interface state shows the minimum value for the optimum content of Al for both Bi-Co added and Bi-Mn added ZnO varistors.