Improvement on the degradation of microwave sintered ZnO varistors by postannealing

Abstract

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 degrees C/min after sintering, followed by 600 degrees 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 N-2 (or air) or loss of Zn species in these regions when annealed at 750 degrees C (900 degrees C). By contrast, the degradation behavior of these materials can be improved by the annealing process regardless of the annealing atmosphere or temperature.