Densification mechanism of ZnO ceramics prepared by cold sintering with molten zinc acetate dihydrate as the transient liquid medium

The transportation of Zn 2+ in acetic acid (HOAc) or Zn(OAc) 2 water solution was considered crucial for the successful cold sintering of ZnO ceramics. However, our work shows that ZnO ceramics can even be densified by cold sintering with the aid of solid Zn(OAc) 2 ·2 H 2 O without introducing extra free water. DSC-TG analysis reveals that pure Zn(OAc) 2 ·2 H 2 O is dehydrated completely into solid Zn(OAc) 2 during heating in an open environment, while it melts in ZnO-Zn(OAc) 2 ·2 H 2 O mixture or a high-pressure and semi-closed circumstance due to the strongly suppressed dehydration. Consequently, molten Zn(OAc) 2 ·2 H 2 O acts as the transient liquid medium for the dissolution-precipitation process at over 100 °C, and plays a dominant role in the densification, microstructural evolution , and mechanical robustness of the cold-sintered ZnO ceramics. The densification mechanism is confirmed by the pressure-time curves, and reveals a novel strategy for cold sintering of water-insoluble ceramics by introducing molten hydrated salt or alkali as the transient liquid medium.