The impulse current degradation of ZnO varistor ceramics

The effect of impulse current degradation on the property of ZnO varistor ceramics has been performed in this study. The breakdown field E_1mA (electric field at 1 mA/cm²) increases, keep stable and then decreases with increase of impulse degradation, while nonlinear coefficient α drops significantly over the whole degradation process. SEM results indicate that after the degradation of ZnO varistor ceramics, more voids and inhomogeneity of grain distribution can be observed. In addition, the critical thickness of dimensional effect of ZnO varistor ceramics increases with the impulse current degradation process. The relaxation processes of these samples were also studied by means of dielectric spectra measured in a wide range of temperature and frequency. The activation energy of intrinsic and extrinsic defects which exhibit structure of grain and grain boundary, were decreased obviously when degradation happens. It is suggested that the large impulse current will lead to the breakdown of some grain boundaries and consequently change the microstructure and defect structure of ZnO varistor ceramics.