Effect of Er2O3 addition on the microstructure, electrical properties, and stability of Pr6O11-based ZnO ceramic varistors

The microstructure, electrical properties, and stability of Pr₆O₁₁-based ZnO varistors, which are composed of ZnO-Pr₆O₁₁-CoO-Er₂O₃ systems, were investigated with Er₂O₃ additive content. The density of ceramics was in the range of 84–88% of TD at 1300 °C and 93–98% of TD at 1350 °C, and greatly affected the stability. Most of the added-Er₂O₃ were segregated at nodal points. The varistors with 0.5 mol% Er₂O₃ sintered at 1300 °C exhibited the best nonlinear current-voltage characteristics, which the nonlinear exponent is 52.8 and the leakage current is 9.8 A. All the varistors sintered at 1300 °C, even under relatively weak stress, exhibited the thermal runaway within short time in order of high leakage current. On the contrary, the stability of varistors sintered at 1350 °C exhibited far higher stability than that at 1300 °C. Particularly, the varistors with 0.5 mol% Er₂O₃ exhibited not only relatively good nonlinear current-voltage characteristics, which the nonlinear exponent is 34.8 and the leakage current is 7.4 A, but also excellent stability, which the variation rates of varistor voltage, nonlinear exponent, and leakage current are below 1%, 3%, and 3%, respectively, even under more severe stress such as (0.80 V _{1 mA}/90 °C/12 h) + (0.85 V _{1 mA}/115 °C/12 h) + (0.90 V _{1 mA}/120 °C/12 h).