Studies on the degradation properties of the ZnO-based ceramic film varistors

The accelerating-degraded method with ac and dc fields was used to study the degradation properties of the ZnO-based ceramic film varistors. When acted on the dc bias, the J–V curve of the ZnO-based ceramic film varistors shows distinct dissymmetry, and the nonlinearity characteristics of the reverse side of the grain boundaries go severely bad after the degradation .When subjected to the ac bias, however, the J–V curve shows symmetrical metamorphosis and the changes of the nonlinearity characteristics of both sides of the grain boundaries are identical on the whole. The ZnO-based ceramic film varistors possess the properties of higher leakage current density, lower nonlinear voltage and smaller nonlinearity coefficient after the change of the grain boundaries took place caused by the ac or dc bias. It is proposed that the degradation of the ZnO-based film varistors is influenced by both the diffusion of interstitial zinc ion in the depletion layer and the oxygen desorption in the grain boundaries.