The Effect of DC degradation and heat-treatment on defects in ZnO varistor

Abstract

In this research, DC degradation for ZnO varistors at 3.2 kV/cm and 50 mA/cm2 for 115 hours was performed, and its effect on electrical properties and defects of ZnO varistors was investigated. It was found that the breakdown field and nonlinear coefficient drops sharply from 2845 V/cm to 51.6 V/cm and 38.3 to 1.1, respectively, when the DC degradaion time reaches 115 hours. For the degraded sample, the dielectric loss was dominated by the increase of conductivity so that some defect relaxation peaks cannot be observed is the DC degraded ZnO varistors. However, in electrical modulus plot, one relaxation peak can be observed. The conductivity in low frequency range increases greatly and the conductance activation energy drops from 0.84 to 0.083 eV. Additionally, the heat-treatment process of ZnO varistors at 800 ℃ for 24 hours was also performed. It is interesting to note that the electrical properties and the relaxation processes of ZnO varistor is restorable completely again after heat-treatment. The breakdown field and the nonlinear coefficient increase to 3085 V/cm and 50.8, respectively, and the activation energy of conductance increases to 0.88 eV. 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-retreating. Therefore, it is proposed that oxygen is likely to diffuse into the ZnO grain boundaries at the heat-treatment process, which can play an important role in restorability of the DC degraded ZnO varistor.