Non-uniform ageing behavior of individual grain boundaries in ZnO varistor ceramics

In this paper, the ageing characteristics of individual grain boundaries in ZnO varistor ceramics were directly measured for the first time, which were carried out based on the microcontact technique in combination with accelerated AC voltage ageing tests. The current–voltage (I–V) characteristics of individual grain boundaries were measured on a microprobe station after the sample was degraded under different ageing time. The results indicated that the I–V characteristics of individual grain boundaries varied non-uniformly during ageing process, which could be roughly classified into two categories as monotonic and non-monotonic. It is revealed that degradation and recovery of electrical properties coexisted in the ageing process, which could be explained with ion migration and oxygen desorption mechanisms.