Regional microstructural and electrical variations in large-sized ZnO varistor ceramics processed by cold sintering assistance

The cold sintering process (CSP) is recognized as an emerging low-temperature densification strategy that provides a scalable and energy-efficient route for producing advanced ceramics and composite materials. However, its application has thus far been largely confined to small samples (diameter < 15 mm), leaving the behavior of large-sized electronic ceramics scarcely explored. Here, a multidoped ZnO-based varistor ceramic with a diameter of 40 mm was employed to investigate the regional heterogeneity induced by cold sintering and subsequent annealing. The synergistic effects of radial pressure attenuation, liquid-phase migration, Marangoni convection, and pore evacuation accelerated densification at the edges, leading to an intensified dissolution–precipitation process. Consequently, the edge region exhibited higher densification (relative density (ρ_r) = 94.3%), smaller pore volume (V_P = 0.215 cm³·g⁻¹), and larger grains (average grain size (G_aver) = 233 nm) compared with the center part (ρ_r = 91.6%, V_P = 0.385 cm^3·g−1, and G_aver = 189 nm). Furthermore, the regional heterogeneity originating from the CSP extended into the annealing stage, as reflected by a reduced leakage current (J_L = 0.3 μA·cm⁻²), enhanced nonlinearity (α = 66), and a high breakdown field (E_b = 1108 V·mm⁻¹) at the edge region. The superior electrical performance is attributed to the larger interface state density (N_S = 2.9×10¹⁵ m⁻²) and higher barrier height (φ_b = 1.92 eV). This work elucidates the regional effect of cold sintering and offers theoretical insight for its industrial application.