Preparation And Electrical Properties Of Rare Earth Elements (Ho, Gd) And Nb, Mn, V Doped ZnO Varistor Ceramics

The typical crystal structure of ZnO is wurtzite in which the coordination number of Zn2+ or O2- is four, and that all the octahedral interstices and half of the tetrahedral interstices are empty. Therefore, additives can be easily doped into the crystal lattices of ZnO. The structural property of ZnO plays an important role in inherent defect and diffusion mechanism. Recently, researchers have found that the sintering temperature of new ZnO-V2O5-based varistor ceramics is as low as 900℃. This is important for multilayered chip component applications, because it can be co-sintered with a silver inner-electrode without using expensive Pt or Pd. However, the ZnO-V2O5-based varistors require specific additives and sintering process in order to exhibit higher nonlinear properties. In this article, the effect of several additives such as Mn, Nb, Ho, Gd on electrical properties of ZnO-V2O5-based varistors were investigated.Firstly, the binary system ZnO-V2O5 varistors were prepared. The effect of V2O5 content, sintering temperature, soaking time on electrical properties, sintering properties of ZnO-V2O5 varistors were investigated. According to research, the optimum properties were obtained when the doping content of V2O5 was 3.0mol% and varistor was sintered at 900℃ for 3 hours.Secondly, MnO2 was doped in ZnO-3.0mol%V2O5 varistors, the shrink rate of diameter and thickness, sintering densities and Ⅰ-Ⅴ characteristics of samples were tested, and the effect of MnO2 content, sintering temperature on electrical properties, sintering properties of samples were systematically investigated combining with microstructure analysis testified by means of XRD, SEM, EDX. The optimizational content of MnO2 was 2.0mol%, Nb2O5 was then added to ZnO-V2O5-MnO2 varistor. The effect of Nb2O5 content, sintering temperature on electrical properties, sintering properties of samples were also systematically investigated. The optimum doping content of Nb2O5 was 0.1mol%. The nonlinear properties were improved by doping transition elements of Mn and Nb.And then rare earth element such as Ho, Gd was respectively doped in the ZnO-3.0mol%V2O5-2.0mol%MnO2-0.1mol%Nb2O5 varistors. The sintering densities, nonlinear properties and dielectric characteristics of samples were tested, and the effect of Ho2O3 or Gd2O3 content, sintering temperature on electrical properties, sintering properties of samples were systematically investigated combining with microstructure analysed via XRD, SEM, EDX. The studying results indicate that the optimum properties were achieved when the doping content of Ho2O3 or Gd2O3 was 0.12mol% and sintering temperature arrived at 900℃. Compared with Ho or Gd undoped varistors, the electrical properties of new pentabasic system were further improved.At last, Ho2O3 was doped in ZnO-V2O5-MnO2-Nb2O5-Gd2O3 varistors in which the content of GdaO3 was the optimal content. The effect of Ho2O3 doped content, sintering temperature on sintering densities, nonlinear properties of hexabasic system ZnO-V2O5-based varistors were schematically investigated by means of XRD, SEM, EDX.In this study, new ZnO-3.0mol%V2O5-2.0mol%MnO2-0.1mol%Nb2O5-0.12mol%Ho2O3、 ZnO-3.0mol%V2O5-2.0mol%MnO2-0.1mol%Nb2O5-0.12mol%Gd2O3 varistors have been prepared by sintering at 900 ℃ for 3 hours. The nonlinear coefficients were about 30, and leakage current density was as low as 0.29mA/cm2. However, when Ho2O3 and Gd2O3 were co-doped, the electrical properties of hexabasic system ZnO-V2O5-based varistors had been weakened. It might be related to excessive segregation of RE elements in grain boundaries.