Mechanistic Approach to Identify Densification Kinetics and Mechanisms of Zinc Oxide Cold Sintering

Abstract

Cold sintering is an emerging process methodology that densifies ceramic powders at significantly reduced temperature. Among 4 densification process variables of the cold sintering (transient chemistry, sintering temperature, uniaxial pressure and time), this study extensively investigates the effects of temperature and time on sintering kinetics and mass transport mechanisms of zinc oxide nano powder. Uniaxial pressures of 175 - 350 MPa and sintering temperatures of 100 - 200 °C with dwell times up to 30 minutes are used to densify nano-powder compacts into a solid pellet with a relative density up to 98 ± 0.21%. Based on the experimental observations from non-isothermal and isothermal sintering methods, a 3-stage classification is proposed to identify physical models. During the initial stage of the cold sintering, liquid-assisted particle rearrangement accounts for up to 3% of the linear shrinkage. In the intermediate stage, pressure solution creep is employed as a governing mechanochemical mechanism. In the final stage, thin liquid films around grain boundaries may enable rapid mass and pore transport, leading to 4% of relative density increase. Surface area based sintering kinetics approach quantifies that the activation energy of the final stage is 29 ± 3.8 kJ mol−1, which is only 20% of the solid state sintering.