Cold sintering assisted ZrO2 composite engineering for high gradient ZnO-based varistor ceramics

Cold sintering bridges the ZnO–ZrO 2 co-sintering temperature mismatch. • ZrO 2 is retained as an intergranular pinning phase at 850 °C. • ZrO 2 pinning refines ZnO grains from 2.48 to 1.65 μm. • The optimized ceramic achieves α  = 65 and E b  = 992 V mm −1 . ZnO varistor ceramics are widely employed for overvoltage protection in electronic circuits and power grids owing to their excellent nonlinear non-ohmic characteristics. However, their insufficient voltage gradient limits further development toward miniaturized, lightweight, and highly reliable devices. Herein, ZrO 2 is introduced as a secondary pinning phase to refine ZnO grains and promote the formation of Bi-rich intergranular phases, enabling the synergistic enhancement of breakdown field ( E b ) and nonlinear response. More importantly, the low-temperature densification feature of cold sintering bridges the co-sintering temperature gap in ZnO-based composite ceramics, suppressing the solid solution of Zr 4+ ( r  = 0.80 Å) into the Zn 2+ ( r  = 0.74 Å) lattice and thus greatly enhancing the effectiveness of ceramic compositing. At the optimized ZrO 2 content of 2 wt%, the segregated ZrO 2 secondary phase pins grain-boundary migration and reduces the average ZnO grain size from 2.48 to 1.65 μm. Moreover, the increased formation of Bi-rich boundary phases provides additional interfacial sites for oxygen adsorption and electron trapping, raising interface state density ( N s ) from 4.7518 × 10 16  m −2 for the undoped ceramic to 7.4102 × 10 16  m −2 with 2 wt% ZrO 2 addition and barrier height ( φ b ) to 0.9245 eV. As a result, the ceramic achieves a high nonlinear coefficient ( α ) of 65 and an enhanced E b of 992 V mm −1 . Further ZrO 2 incorporation suppresses the grain size to 1.40 μm and increases E b beyond 1200 V mm −1 , highlighting the effectiveness of cold sintering in constructing high-gradient ZnO-based composite varistors. This finding demonstrates that cold sintering opens a new pathway for the fabrication of composite ceramics with improved co-sintering compatibility.