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

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., Bi₂O₃, Y₂O₃, and Co₂O₃) 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.