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

ZnO-based varistors of typical commercial compositions have been studi ed before and after DC electrical degradation. Impedance spectroscopy was used to provide information about the behaviour of the ZnO grain a nd grain boundary regions. Degradation caused an increase in leakage c urrent by a factor of 10, and a reduction in the Schottky barrier heig ht by a factor of 2. The activation energy for grain conduction increa sed 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 o bserved variation in the activation energies of bulk, grain and grain boundary regions are discussed in terms of positive interstitial Zn io n and negative oxygen ion migration in the grain boundary regions.