Cold sintering of large‐sized ZnO varistor ceramics: unlocking superior non‐ohmic behavior and breakdown strength via targeted thermal treatment

Abstract Cold sintering process (CSP) provides an effective method for controlling grain size through low‐temperature densification in hundreds of ceramic materials, including ZnO‐based ceramics. However, the interfacial layers, residual amorphous phases, and insufficient ionic solubility left by CSP hinder the overall performance of cold‐sintered ZnO varistor ceramics. Here, thermal treatment is employed to assist cold‐sintered ZnO varistor ceramics in addressing the defects arising from the CSP. Using CSP at 300 °C, large‐size ZnO varistor samples (40 mm in diameter) with a relative density ( ρ r ) of ~ 90% were achieved. Following the thermal treatment, the amorphous phase (e.g., Bi 2 O 3 , Y 2 O 3 , and Co 2 O 3 ) was transformed into Bi‐rich phases, which effectively filled pores and bonded loose grains, resulting in a notable increase in ρ r to ~ 98% and an impressive Vickers hardness of 252 HV. Meanwhile, the interfacial layer surrounding the ZnO grains transformed into high‐resistance grain boundaries that are essential for high nonlinear ohmic properties. Specifically, the sample annealed at 850 °C exhibited a high Schottky barrier ( Φ B ) of 0.97 eV and an ultra‐low leakage current of 2.1 μA cm −2 , leading to a remarkable breakdown electric field ( E b ) of 1573 V mm −1 and a high nonlinear coefficient ( α ) of 96. This work thus shows that combining CSP and thermal treatment provides a significant guideline for designing ZnO varistor ceramics with superior mechanical and electrical performances.