Influence of Annealing Atmosphere on Varistor Property of TiO2-Nb2O5-SrCO3 Ceramics

Abstract

Varistor ceramics are typical electronic ceramics, which are widely used in circuits of overvoltage protection, high voltage stabilization and high energy surge absorption. TiO 2 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 TiO 2 -Nb 2 O 5 -SrCO 3 ceramics was investigated in this paper. TiO 2 -Nb 2 O 5 -SrCO 3 varistor ceramics were prepared by the traditional method of ball grind-ing-forming-sintering and they were annealed in oxygen and nitrogen, respec-tively. The nonlinear coefficient α and the breakdown voltage E B 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, Sr 2+ 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 E B tends to reduce. As the doping concentration of Nb 2 O 5 and SrCO 3 is 0.15 mol%, respectively, and sintering temperature is 1300˚C, TiO 2 -Nb 2 O 5 -SrCO 3 varistor ceramics annealed in oxygen for 3 h at 750˚C achieved the highest nonlinear coefficient α = 8.9 and the lowest breakdown voltage E B = 19.1 V∙mm −1 , which annealed in nitrogen achieved α = 8.4, E B = 20.6 V∙mm −1 , both superior to unannealed samples.