Rapid low-temperature densification of ZnO ceramics by cold sintering/spark plasma sintering with controlled transient liquid chemistry

Conventional ceramic sintering generally requires high sintering temperatures, high energy consumption, and long processing time. To overcome these limitations, a rapid and highly efficient cold sintering/spark plasma sintering (CSP/SPS) is employed, with ZnO serving as a model system to systematically investigate the effects of different transient liquid phases, including water, CH₃COOH and NaOH solution on the sintering behavior. ZnO shows appreciable solubility under both acidic and alkaline conditions. During CSP/SPS, the [Zn(CH₃COO)]+ complex promotes ZnO reprecipitation more effectively than [Zn(OH)₄]2-, leading to superior cold sintering performance in the CH₃COOH system. A relative density exceeding 95% is achieved at 100 MPa/150 °C in the CH₃COOH system, whereas the NaOH system requires more stringent conditions (150 MPa/200 °C) to reach comparable densification. Due to the limited solubility of ZnO in water, high densification is achieved only under more severe conditions (150 MPa/250 °C). The CSP/SPS approach leverages SPS-enabled rapid heating to preserve the transient liquid phase and thereby facilitate the dissolution-precipitation process during cold sintering. Systematic comparison of different transient liquid phases reveals that densification efficiency is governed by both solubility and precipitation kinetics. Enhanced electrical conductivity in wet compacts may enable localized charge transport, while electric-field-induced point defects and liquid-solid interfacial chemical defects enhance mass transport and accelerate densification. This work provides a mechanistically informed strategy for the low-temperature, energy-efficient fabrication of ceramic materials.