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

Abstract 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 (ρr) = 94.3%), smaller pore volume (VP = 0.215 cm3·g−1), and larger grains (average grain size (Gaver) = 233 nm) compared with the center part (ρr = 91.6%, VP = 0.385 cm3·g−1, and Gaver = 189 nm). Furthermore, the regional heterogeneity originating from the CSP extended into the annealing stage, as reflected by a reduced leakage current (JL = 0.3 μA·cm−2), enhanced nonlinearity (α = 66), and a high breakdown field (Eb = 1108 V·mm−1) at the edge region. The superior electrical performance is attributed to the larger interface state density (NS = 2.9×1015 m−2) and higher barrier height (φb = 1.92 eV). This work elucidates the regional effect of cold sintering and offers theoretical insight for its industrial application.