Evolution of Electron Trapping at the Grain Boundary During Long-Term Stability Transition of Stable ZnO Varistor Ceramics

The aging mechanism of stable ZnO varistor ceramics with anomalous steadily decreasing power loss has emerged as a major future challenge for the varistor community, as underscored by CIGRE. The electrical properties of ZnO varistor ceramics originate from the electron trapping at the grain boundary. In this paper, stable ZnO varistor ceramics are subjected to accelerated DC aging at an elevated temperature, during which the changes in those electron traps are measured by in-situ and offline dielectric spectra for investigating the aging mechanism. The stable ZnO varistor ceramics are found to transition to a mixed stable type at 150°C, with power loss first decreasing then increasing. Their nonlinear characteristics and electrical parameters degrade severely and are found to be difficult to recover. The high-temperature relaxation associated with intergranular phase shifts to higher frequencies after the transition. The activation energies of the relaxation and the relaxation related to interface states both decrease to some extent. Moreover, low-temperature relaxations associated with intrinsic point defects Znr’ and Vo decrease continuously during the aging. An anomalous elevation in the low-frequency part is also observed after the transition. Overall, evolutions of these electron trapping behaviors are suggested to be closely associated with the consumption of interface states.