Optimising Calcium Titanate Doping in Bi-Free ZnO Varistor Ceramics for Low-Voltage Protection

This study systematically investigates the effects of calcium titanate (CTO) concentration on the microstructure and electrical properties of ZnO-based varistor ceramics to identify optimal Bi-free formulations for low-voltage protection applications. CTO, a perovskite oxide, was incorporated to address the volatility and environmental limitations of conventional Bi₂O₃-based varistors. Analyses of microstructural and crystallographic data reveal that low CTO concentrations promote grain growth, while higher amounts induce moderate grain refinement and increased micro-strain. Electrical characterisation demonstrates enhanced nonlinearity in all CTO-doped ZnO ceramics, with the maximum nonlinearity coefficient (α = 3.850) and lowest leakage current density (425.11 μA/cm²) observed at 1.0 mol% CTO. The study identifies an optimal CTO concentration window that yields a balanced combination of large grain size, high potential barrier height, and stable non-ohmic response. These findings confirm CTO as an effective, thermally stable substitute for Bi2O3 in tailoring grain boundary properties and electrical performance, laying the groundwork for environmentally friendly ZnO varistor development. Future research should further refine doping strategies to maximise nonlinear characteristics and device reliability.