Breakdown mechanism induced by edge pore defects in ZnO varistors and improvement of energy capacity by applying an in-situ repair medium

Abstract

Pores form inside the ZnO varistor during the sintering process, especially in the edge area. The pore defects frequently lead to breakdown of the varistor. In this study, a simulation model was developed to calculate the electric field and energy density within the varistor. The presence of edge pores resulted in the distortion of the electric field and uneven energy distribution, causing a decrease in the energy capacity of the varistor. Combining the simulations with the results of impact energy tolerance tests on varistors, the mechanism of varistor failure induced by edge pores was revealed. To enhance the energy capacity of varistors, a repair medium with high permeability was developed herein. By employing a secondary sintering method, new grains were grown within the open pores at the edge of the varistor, improving the energy capacity of the varistor from 600 to 1000 A for a 2 ms square wave. Moreover, the mismatch between coefficients of thermal expansion was addressed. This work helped clarify the effects of edge defects in varistors, and a strategy for enhancing their energy capacity was demonstrated.