Effect of Quenching on Dielectric Properties of ZnO Varistor Ceramics

ZnO varistor ceramics quenched under different temperatures were investigated from the aspect of dielectric responses. Nonlinear coefficient α and electrical breakdown field E_1mA were significantly different when quenching temperature was around 600 ℃. From dielectric responses under low temperatures, it was found that densities of both zinc interstitials and oxygen vacancies increased at first and then decreased when quenching temperature decreased from 1200 ℃ to room temperature. Noticeably, the quenching temperature where zinc interstitial density reaches its peak was lower than that for oxygen vacancy, suggesting that oxygen vacancies were more sensitive to the ambient temperature. For dielectric responses under high temperatures, DC conductance was so intense that relaxations with long relaxation times were covered and not easily characterized via traditional dielectric spectra. Therefore, an improved dielectric spectroscopy free of DC conductance was employed. In samples quenched below 600 ℃, two distinct relaxations originated from interfacial polarization and interface states were found. The interface polarization only appears when quenching temperature is below 600 ℃, accompanied by notably improved resistance in small-current region. With quenching temperature increased, a new low frequency dielectric relaxation appeared, whose origin still needs further investigation.