Breakdown phenomenon of ZnO varistors caused by non-uniform distribution of internal pores

Here, we analyzed the internal structure of ZnO varistors with X-ray micro-CT technology and showed that the porosity and pore volume at the radial edge is considerably higher than at other regions. Experimental results showed that the breakdown phenomenon usually occurs near the outer edge of circular varistors. The distribution of impulse currents applied to concentric ring electrodes indicated that the current ratio at the edge region increased as the total current increased. The pores are insulating under low electric fields and the current through the pores is almost zero. Hence, the voltage gradient at the edge region is higher than that elsewhere. Additionally, the simulation results were consistent with this assumption. The amounts of open pore defects, which showed a conductive state after break down, increased, and the proportion of the current flow through the pores increased accordingly, which lead to a local temperature rise and breakdown.