Understanding of DC Degradation of ZnO Varistor Ceramics from the Aspect of High-temperature Relaxation

A high-temperature dielectric relaxation which neatly correlated with the behavior of dc degradation in ZnO varistor ceramics was released from intense dc conduction in traditional dielectric spectroscopy using an improved methodology. High-temperature dielectric relaxation, which was commonly covered by intense dc conduction in traditional dielectric spectroscopy, was clearly exhibited in an improved dielectric spectroscopy correlating with the behavior of dc degradation in ZnO varistor ceramics. It was suggested that the newly detected dielectric relaxation arose from the capture and emission of electrons by interface states in Schottky barriers at grain boundaries. Its activation energy, which represented the energy level under the bottom of the conduction band, was found to gradually decrease from 1.01 eV to 0.89 eV with dc degradation. Neutralization of negatively charged interface states with positively charged migrating zinc interstitials and desorption of oxygen at grain boundaries were responsible for the continuous reduction of its activation energy.