Study on the Decrease of Power Loss during DC Degradation of Zinc Oxide Varistor Ceramics

Zinc oxide (ZnO) varistor ceramics present excellent nonlinear current-voltage characteristics so that they are widely used as core materials in overvoltage protection. Generally, the nonlinear characteristics of traditional ZnO varistor ceramics would deteriorate under constant DC bias voltage, manifested as the continuous increase of power loss. However, it has been reported that some commercial samples experience a decrease in power loss during DC degradation, which makes them more stable in operation. The physical mechanism behind this unusual phenomenon is still unclear yet. In this paper, commercial ZnO varistor ceramics with this unique characteristic were employed for DC degradation and accelerated recovery. Current-voltage curves were measured in the ageing and recovery process. The results show that the forward barrier height declines and the nonlinearity becomes better, in contrast, the reverse nonlinearity falls off. But after enough recovery, the electrical properties of the ageing samples almost restored to the initial levels. To sum up, the opposite changes occurred between the forward and reverse characteristics in the ZnO varistors ceramics, but both are invertible.