Effects of Pr2O3 on the nonlinear electrical characteristics and dielectric properties of SnO2·Co2O3·Ta2O5 varistor systems

The effects of Pr2O3 on the SnO2·Co2O3·Ta2O5 varistor system sintered at 1300 °C for 80 min were investigated. It is found that Pr2O3 affects the grain size, electrical properties and the dielectric properties of the SnO2-based varistors. The average grain size decreases from 9.4 to 5.6 μm, the breakdown electrical field increases from 2460 to 11,346 V/cm, relative dielectric constant (at 1 kHz) falls from 1988 to 290 and the resistivity (at 1 kHz) rises from 39.3 to 178.3 kΩ cm with an increase in Pr2O3 concentration from 0.00 to 1.00 mol%. The sample with 0.10 mol% Pr2O3 has the best nonlinear electrical property and the highest nonlinear coefficient (α=23.9) among all samples. The reason that the grain size decreases with increasing Pr2O3 concentration is explained. A modified defect barrier model is introduced to illustrate the grain-boundary barrier formation of Pr2O3-doped SnO2-based varistors.