Two Titanium Oxide-based Varistor Ceramics Preparation And Performance Study

TiO2 ceramic varistors, a kind of semiconductor with double functions, have low breakdown voltage, excellent non-linear coefficient and high dielectric constant. They have been widely utilized, mainly being used as transient high-voltage protector and surge arrestor. According to the development and recent status of TiO2 varistors, the present market prospect and the trend of development are overviewed in the paper. The electrical properties and technologies of TiO2 varistors are discussed in detail, the effect of surface layer on electrical properties of samples are also discussed briefly. As a whole, some useful experiment results have attained.(1) (Nb,Sr)-doped TiO2 varistors have double functions of varistor and capacitor, VlmA=7~22V/mm, a=3~4, IL~0.4mA, er>104, tan 0.5.(2) (Nb,Sr)-doped TiO2 varistors prepared by Sol-Gel method and electronic ceramic technology also have varistor and capacitor double functions, VlmA=8.4V/mm, a=3.4, IL=0.38mA, er>104, tan=0.78.(3)(Nb,La)-doped TiOa varistors have very excellent electrical properties, VlmA=8.5V/mm, a=5.5, IL=0.20mA, er=8.30x104, tan =0.53.(4) (Nb,La,Sr)-doped TiO2 varistors prepared by single-sintering method, The breakdown voltage is greatly influenced by surface layer, the thickness of surface layer is about 30um. With the reduction of thickness, VlmA is greatly reduced from 27~38V/mm to 8~15V/mm, but nonlinear coefficient, leakage current and dielectric constant hardly influenced by surface layer, which also contribute to the viewpoint that varistor effect of TiO2 ceramic is a kind of body effect.(5) The technology of TiO2 varistors prepared by single-sintering method has also studied. Samples sintered at the temperature of 1280C for 2 hours have best electrical properties, the grains have good developed and distributed evenly, the size of grain is about 4.5um; The grain boundary is very thin and enwrapped the grain equably. The results of Energy Spectra show that acceptors Sr are mainly segregated in the grain boundary, which greatly contribute to the form of varistor barrier. Samples sintered in air atmosphere, i.e. without any special reducing atmosphere,have full semiconducting, the resistivity of grain is about 20.m measured atfrequency of 104Hz.(6) The influence of resistivity of grain and grain boundary on breakdown voltage, the influence of grain size on breakdown voltage and dielectric constant are simply simulated. As a whole, almost computing data are consistent with experimental results.