Homogeneity of ZnO Varistors. Distribution of Local Electric Parameters before and after Degradation

Abstract

By means of a grid of point like electrodes the homogeneity of ZnO varistors is investigated. The local distribution of voltage drops on a varistor disk at a fixed local current can be described by a normal distribution, whereas the local distribution of current densities at constant voltage along the whole varistor disk (usual operating case) is given by a log‐normal distribution. Deviations from these distributions are caused by strong inhomogeneities. Degradation of ZnO varistors by electric load leads to initially increasing inhomogeneity of the local current and voltage distributions. Proportionality between degradation rate and passed charge rate is also valid in microscopic ranges in the case of small charge rates. For higher current densities and longer loading time saturation phenomena are observed.

Percolation effects in dc degradation ofZnOvaristors

“…25,26 However, at such a direct transference of this concept to whole varistor sample its electric inhomogeneity usually is not taken into account. Meanwhile, numerous microcontact measurements, [27][28][29] the inhomogeneous space distribution of electroluminescence accompanying highly nonlinear current, [30][31][32] the distribution of the relaxation time constant revealed by ac measurements 9 and by isothermal depolarization current spectroscopy, 33 the size effect in the leakage current, 34,35 the inhomogeneous space distribution of temperature 36,37 and current 38,39 over cross section of ZnO varistors, give reliable experimental evidences that ZnO ceramic varistor is electrically inhomogeneous system (disordered system) due to a random variation in the barrier height throughout a sample.…”