Spark plasma sintering of ZnO varistors: Parameter optimization via Taguchi method and correlation with microstructure and electrical characteristics

Spark plasma sintering (SPS) was employed to fabricate ten-component ZnO-based varistors with enhanced microstructural uniformity and electrical functionality. The rapid densification achieved under optimized SPS conditions (850 °C/10 min/50 MPa), followed by annealing at 650 °C for 1 h in air, ensured complete re-oxidation of boundary-forming phases and eliminated carbon-related artifacts. This processing route produced highly homogeneous grain-boundary networks enriched with Bi, Sb, Ni, and Co, leading to stable double-Schottky barriers across the microstructure. The resulting ceramics exhibited a marked improvement in nonlinear response. The SPS-850 °C specimen delivered the highest nonlinearity coefficient ( α ≈ 43), elevated breakdown field strength, and a significantly reduced leakage current (9.2 μA cm −2 ), outperforming counterparts processed through conventional sintering regimes. The superior electrical behavior is attributed to the combined effects of rapid, controlled densification and a carefully balanced multi-oxide additive system that governs grain-boundary chemistry and suppresses barrier inhomogeneity. Overall, the study demonstrates that SPS, when synergistically integrated with engineered additive chemistry and a post-sintering annealing step, provides a robust pathway for producing next-generation ZnO varistors with high reliability and advanced surge-protection capability.