Behaviours of Nb~(5+) and Sr~(2+) in The Varistors Based on SrO-Nb_2O_5-TiO_2 Ceramics

By microstructure analysis, I - V characteristics and complex impedance spectra measurement, the microstructure and related electric properties for the varistors based on SrO-Nb2O5-TiO2 semiconducting ceramics with different doped amounts of Nb5+ and Sr2+ were investigated. The distribution and effects of Nb5+ and Sr2+ on electric properties for the materials were discussed. The results show that Nb5+ dissolves into TiO2 grains substitutes Ti4+ to enhance ceramic semiconducting behavior, and proper addition of Nb-doped amount can avail grain growing. The Sr2+ mainly distributes in grain boundary areas, which behaves as an interface acceptor state and so influences on the electric properties of varistor markedly. The experimental results on post heat-treatment in air show that the varistor voltage is increased observably with heat-treatment temperature above 800℃, but the nonlinear coefficient is ameliorated only at an appropriate temperature range.