Effect of MgO doping on electrical characteristics of ZnO–0.5 mol% V₂O₅–2 mol% MnO varistors

Most of commercial ZnO varistors containing Bi₂O₃ exhibit excellent properties but they have few drawbacks due to Bi₂O₃ having high volatility, high reactivity, and high sintering temperature. In this study, V₂O₅ is added as the varistor forming oxide to lower down the sintering temperature of ZnO varistors. It was found that addition of V₂O₅ to ZnO leads to lower down the sintering temperature to 600 °C. However, the addition of V₂O₅ has no improvement in the electrical characteristics, and they need many additives to obtain high performance. For that, the non-linear properties in these varistors can be improved by incorporation of some oxide additives. The MnO and MgO were used as minor oxide additives. The effect of MgO (0–1 mol%) on the microstructure and electrical properties have been studied in ZnO–0.5 mol%–V₂O₅–2 mol% MnO system. The phase composition of samples consists mainly ZnO grains, while Zn₃(VO₄)₂ and ZnV₂O₄ are the secondary phases. MgO is found to be effective as a grain growth inhibitor, and MnO is founded to be an electrical performance enhancer. Better varistor performance has been achieved for 97.4 mol% ZnO, 0.5 mol% V₂O₅, 2.0 mol% MnO and 0.1 mol% MgO with non-linear coefficient α = 16, breakdown voltage 251.3 V/mm and leakage current J_L = 7.7 × 10⁻⁵ A/cm². SEM morphology shows the average grain size depends on Mg and Mn content.