A novel two-step cold sintering process to fabricate dense ZnO nanocrystalline ceramics

The cold sintering process, a novel technique for low-temperature ceramic processing, effectively removes interparticle pores via dissolution and re-precipitation. However, it often triggers grain growth, especially in nanocrystalline ceramics. To address this issue, the present study proposed an innovative two-step cold sintering process with an initial step at 1.5 GPa/RT (room temperature) for 10 min, followed by a subsequent step at 300 MPa/250 degrees C for 45 min. This method yields ZnO nanocrystalline ceramics with 97.5 % relative density and a 39 nm average grain size (from 30 nm powder). The initial high-pressure step significantly enhances the pressure gradient and particle solubility, leading to very high supersaturation and the occurrence of homogenous nucleation during pressure release. Subsequently, the formation of the crystal nuclei in the interparticle voids can dramatically improve the density of the compact by the additional pore filling and avoid recrystallization-induced grain growth during the second-step cold sintering process.