Thermal-assisted cold sintering of ZnO varistor ceramics to realize high breakdown field: Roles of hydrated metal salts

ZnO varistors with high breakdown strengths are essential for compact overvoltage protection devices, yet achieving high breakdown strength ( E _b), high nonlinear coefficient ( α ), and low leakage current density ( J _L) simultaneously remains challenging. Herein, ZnO varistor ceramics are fabricated via cold sintering process (CSP) using hydrated metal salts as both transient liquid media and dopants. This approach enables homogeneous dopant distribution along grain boundaries through a dehydration-assisted CSP at 350 °C, attaining a relative density of ∼94%. Subsequent annealing at 850 °C stabilizes grain boundaries and forms Bi-rich phases, constructing a high Schottky barrier of 3.36 eV. This combined approach of CSP and annealing suppresses ZnO grain growth (average grain size of 1.42 μm) and enhances densification to 98.8%, yielding an ultrahigh E _b of 2777.52 V/mm, a high α of 147.53, and a low J _L of 0.07 μA/cm². This work provides an energy-efficient route to obtain superior ZnO varistor ceramics by cold sintering using hydrated metal salts, which was achieved by post-annealing.