DC degradation of ZnO varistor and its restorability by heat-treatment

ZnO varistors are used extensively as the surge suppressor in the form of surge arresters in the power protection against lightning, temporary over-voltages, switching surges, transients, etc. In this research, DC degradation process at 50 mA/cm² for 115 hours and restorability by heat-treatment at 800 oC for 24 hours for ZnO varistors were performed. The breakdown field E_1mA (electric field at 1 mA/cm²) and nonlinear coefficient a drops sharply when the DC degraded time reach 115 hours. Additionally, the dielectric property of ZnO varistors was also investigated. Four relaxation processes were found in the original ZnO sample, which exhibit structure of grain and grain boundary. However, three of the relaxation processes can not be observed after DC degradation. It is interesting to note that the electrical property and the relaxation processes of ZnO varistor can be restorable completely again after heat-treatment. It is also found that the defect relaxation peak, which is shown in dielectric spectra corresponding to oxygen vacancy defect, is suppressed evidently by heat-retreat. It is proposed that oxygen is likely to diffuse into the ZnO grain boundaries at the heat-treatment process, which exert an important role in restorability of the DC degraded ZnO varistor.