Influence of Processing Parameters on Microstructure and Electrical Properties of ZnO-Based Varistor Ceramics

ZnO-based are the most common amongst varistors, used for surge protection of electrical equipment. The effect 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 (Bi ₂ O ₃ , MnO ₃ , Co ₂ O ₃ .etc) was studied. The results from the experiments exhibit that phase concentration, microstructure and electrical properties can be varied upon change in above said parameters. Soaking time at the sintering temperature has a little effect on microstructure and properties, however, higher sintering temperatures for prolong times widely affect phase concentrations, especially the Bi ₂ O ₃ -rich phases. Varistor voltage exhibited a reverse relation with sintering temperature while leakage current was reduced remarkably on slowing down the ramp rate at temperatures higher than 950 ̊C. Best results were obtained on slower heating rates and lower soaking times with sintering temperature of 1220 ̊C.