Abstract

• ZnO based varistors with 5 additive oxides Bi2O3, Cr2O3, Co3O4, MnO2 and Sb2O3 are prepared and characterized in this research work. Standard varistor processes, known as weighting, ball milling, mixing with distilled water, pre heat treatment, secondary ball milling, seiving, sintering, pressing into circular shape disc, electroding and reheating processes are carried out to obtain the varistors with desired stoichiometric compositions. XRD technique is used to examine the structural features of prepared varistors. It is obvious that a little change of lattice dimensions is obtained. But there are no significant change of hexagonal, wurtzite ZnO structure after addition of additive oxides. Microstructural features of prepared varistors are characterized with the help of SEM. From the SEM images, it is noticed that irregular micro grains are uniformly distributed and there are no cracks. Further, average grain size slightly increases with increasing composition " x " in prepared varistors. Non - ohmic behaviour of prepared varistors are studied by using the high voltage DC power supply. From the ln V vs. ln I variations, non linearity factor, threshold voltage and leakage current are estimated. It is observed that non linearity factor slightly increases as composition " x " increases, on the other hand, threshold voltage and leakage current decrease when the composition " x " is raised. The transition metal oxides are involved in formation of interface states and deep traps in host matrix ZnO and both of which are contributed to high - non ohmic behaviour. Additive oxides, used in this study are known to be non - linearity enhancers and inducers. Threshold voltage is affected by the numbers of grain boundaries across the series between the two electrodes and inversely proportional to grain size. In this study, average grain size slightly increases with composition " x " in prepared varistors and that leads to lower the threshold voltage. The higher the non - linearity factor affects the lower the leakage current and the better the performance of varistor. It can be concluded that, prepared varistors can be used as transient surge voltage protection device.