Influence of sintering temperature on microstructure and electrical properties of Er2O3 added ZnO-V2O5-MnO2-Nb2O5 varistor ceramics

Highlights: • Effect of sintering temperature on Er₂O₃ modified ZnO-based varistor was studied. • Varistor sintered at 950 °C exhibited marked improvement of electrical properties. • High nonlinear exponent (150) and breakdown voltage (10286 V cm⁻¹) were obtained. • Coarsening of ZnO grain during sintering was retarded by Er-rich spinel phases. • Donor concentration and depletion layer width is reported with sintering temperature. The effect of sintering temperature (950–1100 °C) on the densification, microstructure and nonlinear electrical properties of 0.5 mol. % Er₂O₃ added ZnO–V₂O₅–MnO₂–Nb₂O₅ ceramics powder mixture was investigated. Microstructural characterizations by XRD and SEM coupled with EDS microanalyses confirm the development of equiaxed ZnO grains surrounded by different secondary phases, mainly Er-rich spinel phases in the intergranular layers and triple points. Image analyses reveal that the average size of ZnO grain increases from 1.8 to 7.2 μm with increasing sintering temperature from 950 to 1100 °C. Coarsening of grain structure is found to reduce the breakdown field (10286–2584 V cm⁻¹), and nonlinear exponent (150–26). The donor concentration, N_D increases from 2.2 × 10¹⁷ to 4.9 × 10¹⁷ cm⁻³ with increasing sintering temperature from 950 to 1100 °C, whereas, the depletion layer width, ω decreases from 56.4 nm to 37.8 nm. Typically, a fine-grained (1.8 μm) and dense (95% TD) varistor sample possessing a high value of nonlinear exponent (α = 150); enhanced breakdown field (10286 V cm^{− 1}) and leakage current density of 0.18 mA cm⁻² could be made by conventional sintering technique at a reasonably lower temperature of 950 °C from the chosen multicomponent ZnO varistor system.