Improvement of electrical properties and aging characteristics of Pr–Co–Cr–Y-modified ZnO varistors by Al2O3 doping

The dependence of the microstructure, electrical properties, and DC accelerated aging characteristics of Zn–Pr–Co–Cr–Y–Al-based varistors on the doping levels of Al2O3 was studied. The Al2O3 doping led to a small increase in the sintered density and average grain size. The breakdown field decreased in a wide range from 6396 to 338 V/cm with increasing doping levels of Al2O3. The nonlinear coefficient increased from 38.4 for Al2O3-free samples to 47.1 for Al2O3-doped samples up to 0.005 mol%, whereas the further doping levels caused it to decrease. Al2O3 acted as a donor due to the electron concentration, which increases from 0.41 × 1018 to 1.80 × 1018 cm−3 with increasing doping levels of Al2O3. The sample doped with Al2O3 of 0.001 mol% exhibited the highest stability, in which the variation of electric field is −0.2% and the variation of nonlinear coefficient is −5.0% under DC accelerated aging stress condition of 0.95EB/150 °C/24 h.