Defect-engineered ZnO-based varistor ceramics with ultrahigh breakdown strength via cold sintering processing

Abstract

The cold sintering process (CSP) has been successfully applied to fabricate ZnO-based varistor ceramics, but the relationship between the transient liquid phase and electrical properties is incomplete. In this study, ZnBiMnNbO varistor ceramics were selected as a model system, employing acetic acid as the transient liquid phase. By optimizing process parameters, samples with optimal varistor performance were obtained and compared with those prepared by conventional solid-state sintering (CSS). Results revealed that CH₃COO⁻ could coordinate with Zn²⁺ to suppress oxygen vacancy formation, thereby enabling the effective reduction of oxygen vacancy concentration through modulation of the CSP parameter. Under 250°C/250 MPa, the cold-sintered sample achieved ultrafine grains (0.29 μm) and high relative density (99.1%), yielding a breakdown strength (E_b = 2757 V/mm) significantly higher than the CSS-fabricated sample (526 V/mm). Microstructural and modulus spectrum analyses identified three mechanisms contributing to the enhanced E_b: ①Grain refinement effect of the cold-sintered sample; ②The coordination protection in cold-sintered sample; ③The ionization activation energy of oxygen vacancies (E_B = 0.32 eV) in cold-sintered sample exceeded that of CSS-fabricated sample (E_B = 0.28 eV), where higher E_B suppressed thermal excitation of carriers. Kelvin probe force microscopy (KPFM) further elucidated defect distribution, indicating that the ultralow sintering temperature of CSP induced heterogeneous dopant distribution, which hindered further improvement of varistor performance. This study demonstrates the regulation of oxygen vacancy concentration via CSP parameter optimization and provides a multi-perspective mechanistic analysis of high E_b formation mechanism. The above findings help establish the correlation between the transient liquid phase and electrical properties.