Grain growth kinetics of Er2O3 doped ZnO-V2O5 based varistor ceramics

Abstract

Control of grain coarsening during sintering of oxide mixtures remains a challenge to manufacture high-performance varistors. This article reports microstructural evolution, grain growth kinetics and nonlinear electrical properties of Er2O3 (0–1.0 mol%) doped ZnO-V2O5 based varistors sintered at 900–1300 °C for a duration ranging from 30 to 480 min. Microstructural characterizations revealed that raise in sintering temperature and time enhanced coarsening of ZnO grains while Er2O3 doping inhibited the grain growth substantially with marginal slowdown of the densification process. Phenomenological grain growth kinetics analyses inferred that the magnitudes of grain growth exponent (3–5), as well as apparent activation energy (263–774 kJ mol-1), increased monotonically with Er2O3 concentration establishing its effectiveness as a strong grain growth inhibitor. In Er2O3 doped sintered pellets, the development of Er-rich and ErVO4 spinel phases along the intergranular layers of ZnO was confirmed by microstructural characterizations like FESEM, EDS and XRD studies. These phases found to exert pinning effects causing retardation of grain coarsening. Judicial selection of Er2O3 doping concentration (0.5 mol%) and sintering parameters (900 °C and 120 min) generated ZnO-V2O5 varistor ceramic with excellent electrical (nonlinear exponent: 80 and breakdown field: 8715 V cm−1) properties.