Indication of initial degradation of ZnO varistor ceramics

The interior distributions of electrical and dielectric properties of ZnO varistor at initial aging stage are investigated. Two commercial varistors at different aging stages were compared. These two varistors were cut into slices along the axial direction, and then the variations of varistor slices' electrical and dielectric performances were obtained. The results show that the interior performance of intact varistor is nearly the same. But, at initial aging stage, there exists a gradient varying tendency of electrical and dielectric performance in varistors' interior, indicating a growing degradation level. In addition, the leakage current I
L
and nonlinear coefficient α are better indicators of degradation than the threshold electric field E
1mA
. Moreover, two loss peaks originating from electronic relaxation of two intrinsic defects (oxygen vacancy and zinc interstitial) are observed in the ε″-f curve in the wide temperature range. By fitting with Debye function, it can be seen that for slightly aged varistor, the concentration of intrinsic defect oxygen vacancy rather than zinc interstitial increases in axial direction. The deterioration mechanism of ZnO varistor ceramic is attributed to the interaction and cooperation of the ion electric migration and ion thermal diffusion, and the initial degradation is mainly owing to oxygen desorption at grain boundaries.