Study On ZnO Varistors With Low-voltage And High Surge Current Withstanding Capacity

ZnO varistors is a ceramic material with small amounts of other oxides doped. It is characterized by its excellent non-linear volt-ampere characteristic, huge surge absorption capacity and very low leakage current under steady-state voltage. With the development of home appliances, computer, communications technology and other aspects, to guarantee the normal operation of these systems, the demand of the low-voltage varistors is growing.However, the surge current withstanding capacity and leakage current value of domestic corresponding products are difficult to meet the requirements, so the purpose of this study is to prepare ZnO varistors with low-voltage and high surge current withstanding capacity.Firstly, the influence of doping in ZnO on the electrical performance was studied.When TiO2 was doped, the voltage-gradient of ZnO varistors was reduced. In this paper, in the doping range, when SiO2 was doped, the density was improved. When Al2O3 was doped, the large current characteristic was improved. Besides, it was found that with the addition of TiO2, the nonlinear coefficient was reduced and the leakage current was increased, Si O2 doping could increase the voltage-gradient, and the additive of Al2O3 could reduce the nonlinear coefficient and increase the leakage current. In this paper, by adjusting the doping of TiO2, SiO2 and Al2O3, a proper material formular with a better overall performance was achieved. In addition, the effects of the additives of Bi2O3 and Sb2O3 on the preparation of ZnO varistors with low-voltage and high surge current withstanding capacity was also studied.Secondly, the effect of cooling rate during the sintering process and the doping way of Al2O3 on the properties of ZnO varistors was studied after the composition was ascertained. It was found that an appropriate cooling rate could increase the grain boundary resistivity of ZnO varistors, resulted in the increased nonlinear coefficient and the reduced leakage current. In addition, It was also found that the way of Al2O3 doping had great effect on the properties of ZnO varistors, the pre-mixing of Al2O3 and ZnO andthen calcining could reduce the grain resistance of ZnO varistors,while having a high grain boundary resistance, so as to increase the nonlinear coefficient and reduce the leakage current.Finally, through improving the formular and the process for preparing the material,the voltage-gradient of the resulting ZnO varistors is 123.5 V/mm while the nonlinear coefficient is 58.5, and the leakage current is 1.2 ?A, residue voltage ratio K100 A is 1.56,variation of breakdown voltage after 8/20 ?s surge current of 5 kA is 7%.