Improvement on the degradation of microwave sintered ZnO varistors by postannealing

The microwave sintering process not only densified the ZnO materials in a higher rate, but also resulted in significantly better varistor characteristics. Large nonlinear coefficient and low leakage current density were attained by cooling the samples under a rate of 80 °C/min after sintering, followed by 600 °C postannealing for 60 min under oxygen atmosphere. Inappropriate annealing deteriorated the varistor characteristics that can either be attributed to the insufficient reoxidation along grain boundaries when annealed in N2 (or air) or loss of Zn species in these regions when annealed at 750 °C (900 °C). By contrast, the degradation behavior of these materials can be improved by the annealing process regardless of the annealing atmosphere or temperature.