Acceptor-doped SnO ₂ -Co ₂ O ₃ -Nb ₂ O _{5 Preparation And Properties Of The Varistor}

Varistors are available to protect electronic and electrical equipments against transient voltage surges because of its highly nonlinear current-voltage characteristics and good energy absorption capability.Contrary to the multi phase structure of Zno varistors, SnO2varistor material is composed of only one phase. It can be deduced that SnO2varistors are much better than ZnO varistors in thermal stability, moreover, their nonlinear electrical properties exhibit a usage value, and thus became a kind of potential varistor material in application.It was found that the pure SnO2exhibits no electrical nonlinearity for lacking of carriers, donor Nb2O5-doped SnO2varistor exhibits electrical nonlinearity but its grains can not integrate.Therefore, the density of Nb2O5-doped SnO2is low, which lead to a small electrical nonlinearity. Acceptor Co2O3-doped SnO2varistor is very dense, but exhibits no electrical nonlinearity for lacking of carriers.The SnO2varistor doped with Nb2O5and Co2O3exhibits high electrical nonlinearity and density. In this paper, several dopants and sintering processes were introduced into SnO2varistor in order to enhance its electrical nonlinearity and density.Firstly in this paper, the ZnO was introduced as the acceptor into SnO2varistor and adjusted the sintering process to study on the effect of SnO2varistor materials property. The results show that the ZnO and the higher sintering temperature can promote the semi-conduction and the density of the SnO2varistor. It is found that the sample doped with1.0mol%ZnO sintered at1300℃shows the best density, minimum resistivity and largest relative dielectric constant. But meanwhile the nonlinear coefficient is small and the breakdown voltage is not more than300V/mm. The increase of SnO2grain size is the main reason for the decrease of breakdown electrical field and the increase of relative dielectric constant.On the basis of the previous studies, the BaCO3was doped in the SnO2varistor materials to improve nonlinear characteristics. The radius of Ba2+is much bigger than Sn4+, so that the Ba2+will segregate to grain boundary and hinder the grain growth which lead to the SnO2-Nb2O5-Co2O3varistor system exhibits excellent varistor properties with the large nonlinear coefficient (a>11) and high breakdown voltage (1110.5V/mrn).The high varistor electrical field indicates that these materials are good candidates for application in the fields of high voltage portection.Finally, ZnO and BaCO3were co-doped to SnO2varistor materials. It is found that the relative density of the sample reaches to94.26%, the resistivity of the sample is the same order of magnitude with single-doped BaCO3. At the same time, the breakdown voltage of SnO2varistor materials with ZnO and BaC03co-doped is high while the relative dielectric constant is small.