Coupled regulation by sintering and post-sintering heat treatment for improved current-carrying stability of ZnO varistor ceramics

Abstract

A processing window that simultaneously preserves nonlinear electrical properties and high-current stability is required for conventional ZnO varistor ceramics. Here, six sintering temperatures (1050–1200 °C) were evaluated using phase composition, microstructure, E–J characteristics, 2 ms square-wave loading, and 4/10 μs and repeated 8/20 μs impulse tests. The sample sintered at 1150 °C was selected by a multi-criterion assessment that prioritized impulse endurance and energy absorption while maintaining acceptable voltage gradient, leakage-current density, nonlinear coefficient, and relative density. Post-sintering treatment was then mapped independently over 425–725 °C and cooling rates of 5–90 °C/h. Treatment at 525 °C for 2 h followed by cooling at 15 °C/h produced the smallest measured E_1mA variation (4.43%) after two 100 kA, 4/10 μs impulses. XRD, SEM, temperature-dependent conductivity, and dielectric-relaxation analyses show that changes in the bismuth-rich grain-boundary phase state are accompanied by systematic variations in the apparent grain-boundary barrier, relaxation response, and impulse stability. The resulting processing-stability map provides a practical basis for optimizing conventionally sintered ZnO varistors for high-current service.