GRAIN AND GRAIN-BOUNDARY CONDUCTION IN ZINC-OXIDE VARISTORS BEFORE AND AFTER DC DEGRADATION

ZnO-based varistors of typical commercial compositions have been studied before and after DC electrical degradation. Impedance spectroscopy was used to provide information about the behaviour of the ZnO grain and grain boundary regions. Degradation caused an increase in leakage current by a factor of 10, and a reduction in the Schottky barrier height by a factor of 2. The activation energy for grain conduction increased from 0.023 eV to 0.104 eV as a result of degradation, whilst that for grain boundary conduction decreased from 0.55 eV to 0.25 eV. The observed variation in the activation energies of bulk, grain and grain boundary regions are discussed in terms of positive interstitial Zn ion and negative oxygen ion migration in the grain boundary regions.