Influence of SnO2 on ZnO–Bi2O3–Co2O3 based varistor ceramics

Abstract

ZnO–Bi2O3–Co2O3 based varistors doped with SnO2, from 0 to 0.3mol% were prepared by conventional ceramic processing. The effect of SnO2 on the microstructure and the electrical characteristics of varistor samples were investigated. Results showed that the addition of small amounts of SnO2 inhibited the grain growth and induced a change of grain morphology. With increasing SnO2 doping level up to 0.3mol%, Bi2O3 reacted with SnO2 to form a cubic crystalline phase Bi2(Sn2O7) at 950°C and a pyrochlore phase Bi2Sn2O7 at 1100°C. Moreover, small amounts of SnO2 produced higher nonlinear coefficients and reduced leakage currents of ZnO–Bi2O3–Co2O3 based varistors. Meanwhile, the breakdown fields increased noticeably with the addition of SnO2, which was due to the decrease of average grain sizes. It was found that the addition of SnO2 as well as the sintering temperature had an obvious effect on the insulating level at grain boundary region, which was indicated from the changes of relative dielectric constants.