Determining the effect of bismuth oxide on the microstructure and electrical response of cold‐sintered ZnO varistors

Abstract Microstructure homogeneity and grain size reduction are needed to ensure the current uniformity and reliability required in metal oxide varistors for DC circuit breakers. Cold sintering (CS)–a low‐temperature digestive liquid and high‐pressure densification process–has previously shown reduced grain growth, but it is not clear how the lower processing temperatures will impact the bismuth intergranular phases that induce current nonlinearity. This study investigates the impact of CS on the microstructure and nonlinearity of bismuth added to ZnO varistors. ZnO powders with 0–1 mol% Bi 2 O 3 are either CS at 300°C and 300 MPa or high temperature sintered (HTS) at 1100°C. CS samples reach >97% of the theoretical density. The 1 mol% Bi 2 O 3 CS samples have limited grain growth with a final average grain size of 120 ± 85 nm, while the HTS sample shows over 1.991% increase to a grain size of 2.99 ± 1.17 µm. No Bi‐rich intergranular phase is observed in the CS samples, but bismuth clearly increases resistivity with increasing bismuth concentration, with a higher voltage onset for nonlinearity and an increase in nonlinearity with increasing bismuth. This work demonstrates the ability to use cold sintering to create dense ZnO ceramics with reduced grain growth and controlled nonlinearity.