Fine-grained ZnO ceramic fabricated by high-pressure cold sintering

High-density (∼99.34%), fine-grained (∼685 nm) ZnO ceramic was successfully fabricated by the high-pressure cold sintering. The influences of solvent (liquid phase) fraction and the applied pressure on densification in this process have been carefully studied. The excess solvent can result in grain refinement, but impedes the densification of ceramic. Significantly, the role of pressure is unlike that in the traditional pressure-assisted sintering, which can improve the densification and inhibit grain growth. There is a critical pressure (∼1 GPa) that can be associated with the transition of recrystallization from heterogeneous nucleation to homogeneous nucleation . When the pressure is lower than this critical value, the applied pressure can improve densification, but cannot inhibit grain growth. Above the critical value, the densification is dramatically weakened and many small crystals in the interstitial space between grains can be found.