Nonlinear Electrical Properties of ZnO-V2O5 Based Rare Earth (Er2O3) Added Varistors

The densification, microstructure and nonlinear electrical properties of ZnO-V2O5-MnO2-Nb2O5 varistor ceramics have been investigated with different amounts (0–2 mol.%) of Er2O3 addition. The ball milled powder mixtures have been sintered at 900°C, 1100°C and 1300°C for 1 h. The microstructure shows the presence of ZnO grains as the primary phase with secondary phases like Zn3(VO4)2, ErVO4, Zn4V2O9, Er- and Mn-rich in the intergranular layers. Generation of ErVO4 and Er-rich secondary phases at triple points and grain boundaries is found to inhibit grain growth resulting in the significant reduction of ZnO grain size, although this slightly diminishes the densification process. At a particular sintering temperature, the size of ZnO grains gets reduced over ten times when Er2O3 addition is increased from 0 mol.% to 2 mol.%. The nonlinear electrical property of the varistors improves with Er2O3 addition up to 0.5 mol.% and further addition (1–2 mol.%) does not appear to be beneficial in the chosen sintering temperatures. For varistors sintered at 1100°C, average ZnO grain size reduces from 10.8 μm in Er2O3 free sample to 7.2 μm in 0.5 mol.% Er2O3 added sample; this, in turn, significantly improves nonlinear electrical properties. The breakdown field increases from 1588 V cm−1 to 2585 V cm−1 with concurrent enhancement of nonlinear exponent value from 16 to 26 when Er2O3 concentration is raised from 0 mol.% to 0.5 mol.%.