The Effect of Point Defects on DC Degradation of ZnO Varistors

Lowered power loss and asymmetrical reference voltage are reported in the DC degradation of ZnO varistors in this paper. Based on the frequency domain dielectric responses of the pristine and degraded samples, the present study explores the roles of point defects in the degradation process via dielectric relaxations and their activation energies. It is found that the degradation leads to the decrease of the activation energies for the two relaxations under high temperature. Given the lowest migration barrier for Zn
i
̈ (0.57 eV) and high conduction of oxygen ion in Bi-rich phase, it is speculated that Zn
i
̈ and O
ad
″ migrate under DC bias, and then change the defect structure and the double Schottky barrier (DSB) at grain boundaries, during which the reverse-biased barrier height gradually increases, leading to the lowering of power loss.