Abstract
Varistor ceramics are typical electronic ceramics, which are widely used in circuits of overvoltage protection, high voltage stabilization and high energy surge absorption. TiO2 varistor ceramics has the advantages of low varistor voltage and good dielectric properties, but their low nonlinearity limited the application. The influence of annealing on the varistor properties of TiO2-Nb2O5-SrCO3 ceramics was investigated in this paper. TiO2-Nb2O5-SrCO3 varistor ceramics were prepared by the traditional method of ball grinding-forming-sintering and they were annealed in oxygen and nitrogen, respectively. The nonlinear coefficient α and the breakdown voltage EB of the samples were tested using the varistor dc parameter meter. The microstructure of samples was analyzed by XRD, SEM, STEM-EDAX and SAEDP. The results show that during annealing, Sr2+ ions with a larger radius obtain the kinetic energy and are segregated to grain boundaries, which increases the acceptor density of the grain boundaries and improves α. Annealing in an oxygen atmosphere, the enrichment of oxygen at grain boundaries is also helpful to increase the density of acceptor states and the height of the potential barrier, so as to further increase α. Meanwhile, annealing makes crystalline grains grow properly, which results in even grain size, reduces the porosity and increases the density of grains. So EB tends to reduce. As the doping concentration of Nb2O5 and SrCO3 is 0.15 mol%, respectively, and sintering temperature is 1300°C, TiO2-Nb2O5-SrCO3 varistor ceramics annealed in oxygen for 3 h at 750°C achieved the highest nonlinear coefficient α = 8.9 and the lowest breakdown voltage EB = 19.1 V·mm-1, which annealed in nitrogen achieved α = 8.4, EB = 20.6 V·mm-1, both superior to unannealed samples.
Highlights
Varistor ceramics are typical electronic ceramics, which are widely used in circuits of overvoltage protection, high voltage stabilization and high energy surge absorption, because of their non-ohmic electrical properties and sensitivity to instantaneous voltage fluctuations [1]
As the doping concentration of Nb2O5 and SrCO3 is 0.15 mol%, respectively, and sintering temperature is 1300 ̊C, TiO2-Nb2O5-SrCO3 varistor ceramics annealed in oxygen for 3 h at 750 ̊C achieved the highest nonlinear coefficient α = 8.9 and the lowest breakdown voltage EB = 19.1 V∙mm−1, which annealed in nitrogen achieved α = 8.4, EB = 20.6 V∙mm−1, both superior to unannealed samples
The results indicate that the addition of Sb2O3 effectively decreases the breakdown voltage EB, while having less impact on the nonlinear coefficient α [17]
Summary
Varistor ceramics are typical electronic ceramics, which are widely used in circuits of overvoltage protection, high voltage stabilization and high energy surge absorption, because of their non-ohmic electrical properties and sensitivity to instantaneous voltage fluctuations [1]. A simple preparation process makes it low cost It has a good application prospect [3]-[11]. Ekaphan et al studied dielectric and nonlinear properties of Ca1−1.5xPrxCu3Ti4O12/TiO2 ceramics. Yang et al studied TiO2-based varistor-capacitor ceramics co-doped with fixed Nb2O5, MnO2, Sm2O3 and various contents of Sb2O3. The results show that GeO2 doping changes the microstructures of TiO2-Ta2O5-CaCO3 ceramics, resulting in increased α and decreased EB [19]. These methods of doping in the literature have a certain effect on improving the nonlinear coefficient α, it seems difficult to improve further α value by doping.
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