Percolation effects in dc degradation of ZnO varistors

For quantitative estimation of the degree of electrical disorder (electrical inhomogeneity) in ZnO varistor ceramics caused by a variation in the barrier height at different grain boundaries in a sample, the comparison of threshold electric fields (onsets of highly nonlinear current-voltage characteristics) in ceramics and single grain boundary (GB) is suggested and approved. At dc degradation similar behavior of the current-voltage characteristics of ZnO varistor ceramics and single GB is observed. The percolation model of Shklovskii-De Gennes is applicable for the description of a disorder in ZnO varistor ceramics. The degree of the disorder in ZnO varistor ceramics is not dependent on the duration of dc degradation at least at degradation time below 60 h. At voltages close to the onset of a highly nonlinear region of current-voltage characteristic the correlation radius of infinite cluster is similar to 5 times greater than the average grain size.