Influence of processing parameters on microstructure and electrical properties of ZnO-based varistor ceramics

ZnO-based varistors are widely used in electrical equipment for surge protection. This research explores the influence of sintering parameters—such as sintering temperature, ramp rate and soaking time—on microstructure development and V-I characteristics of ZnO-based varistor ceramics doped with various oxide additives (Bi2O3, Sb2O3, MnO2, Co2O3 and Cr2O3). The experimental results show that changes in these parameters influence the phase concentration, microstructure and electrical properties. While altering soaking time at 1220 ̊C has a little effect on breakdown/varistor voltage, higher sintering temperatures for prolonged times have a pronounced effect on phase concentrations, especially those rich in Bi2O3. A reverse relationship was observed between varistor voltage and sintering temperature, while leakage current was remarkably reduced when ramp rate was slowed at temperatures exceeding 950 ̊C. Best results were obtained at slower heating rates and shorter soaking times, with a sintering temperature of 1220 ̊C. Moreover, this research also achieved much higher non-linearity coefficients (upto 55), indicating the development of more efficient varistors.