Effect of Pr6O11 doping on the microstructure and electrical properties of ZnO varistors

Abstract ZnO-Pr6O11 based varistors have anticipated application prospect in electrical system, but the study of the detailed microstructure is still poor. In this paper, the effect of Pr6O11 doping on the microstructure (such as element distributions and grain boundary characteristics) and electrical properties of the ZnO-Pr6O11–Co2O3–Cr2O3–Y2O3 varistors manufactured by the conventional solid-state sintering procedure was studied detailedly. Without Pr6O11 doping, the elements have the similar distributions in the interior and grain boundaries of ZnO phase, and the double Schottky barrier cannot be formed. With Pr6O11 doping, Pr6O11 liquid phase dissolving a large number of Zn, Cr, and Y forms during sintering and Pr6O11 solid phase forms in the ZnO grain boundaries during cooling, so the double Schottky barriers form. The voltage gradients E1 mA and nonlinearity coefficients α of the samples with Pr6O11 contents of 0.25–1 mol% are much larger than those of the sample without Pr6O11 doping, and the values of α and E1 mA increase with Pr6O11 content increasing from 0 to 1 mol%. The values of α and E1 mA simultaneously depend on the double Schottky barrier height ϕb and the CPE exponent αCPE. Finally, the ZnO varistor with a voltage gradient of 333.6 V/mm and a nonlinearity coefficient of 24.4 was obtained by Pr6O11 doping of 1 mol%. The detailed study of the microstructure and related mechanism is of great importance for preparing ZnO-Pr6O11 based varistors with excellent electrical properties.