Preparation And Electrical Properties Of TiO₂Based Low Voltage Varistor Material

TiO₂-based varistor ceramics, a kind of semiconductor with nonlinearcharacteristic, have low breakdown voltage and high dielectric constant. They canbe used as transient over-voltage protector and surge arrestor in low-voltage field,which cause more and more attention. In this paper, several dopants were addedinto TiO₂varistor ceramics in order to improve its electrical properties.To obtain nonlinear characteristic, the ceramic grain must be semi-conductive.Firstly, the microstructure and electrical properties of TiO₂varistor ceramicsdoped with Nb₂O₅and niobium oxalate (C₁₀H₅NbO₂₀) were investigatedrespectively. The result shows that both Nb₂O₅and niobium oxalate can enhancethe grain’s conductivity. The effect of niobium oxalate doped is better because ofits water-solubility resulting in better homogeneity. With the increasing amount ofNb₂O₅or niobium oxalate, both the breakdown voltage and the nonlinearcoefficient first decrease and then increase, but the dielectric constant is opposite.High nonlinear coefficient of varistor ceramics attribute to high grainboundary barrier. On the basis of niobium oxalate doped, we added CaCO₃as theacceptor. The result shows that small amount of Ca²⁺can promote grain growthand lower breakdown voltage. Most of Ca²⁺segregating in the grain boundary ishelpful to form the effective barrier, which increases nonlinear coefficient. Whenexcessive doping, the sample internal pores and defects increase and theperformance deteriorates. When the doping amount of CaCO₃is1.5wt%, thesample performance is best, the breakdown voltage about10.3V, the nonlinearcoefficient about5.5, the dielectric constant1.21×10⁵, the dielectric loss0.113.The sintering temperature is crucial. The sample properties get better whensintering temperature rises. The effect of adding CuO based on Nb₂O₅doped was investigated. It is foundthat CuO can hinder grain growth, so grain size reduces and breakdown voltageincreases. Besides, CuO doped enhances nonlinear coefficient significantly.Excessive doping makes the sample internal pores and defects increase, so thatdielectric constant decreases and dielectric loss increases. CuO can decomposeinto Cu₂O and O₂at high temperature. Lager ionic radius Cu+accumulates in grainboundary and hinders grain growth, resulting in the incease of breakdown voltage.O₂can adhere to grain boundary, increasing interface state density and grainboundary barrier, so the nonlinear coefficient is higher. When the doping amountof CuO is0.1wt%, the breakdown voltage is about40.7V, the nonlinear about8.03,the dielectric constant8.74×10³.