Unravelling the H_(2)O-assisted high-pressure sintering mechanism:A route to highly transparent cubic alumina ceramicsOA

Transparent cubic alumina(γ-Al_(2)O_(3))ceramics are promising optical materials but are challenging to densify without phase transformation or cracking.Herein,we report a novel H_(2)O-assisted highpressure sintering(HPS)strategy that enables the fabrication of highly transparent,crack-freeγ-Al_(2)O_(3)ceramics at temperatures as low as 300℃.By combining GPa-level pressures(0.8-5 GPa)with precisely controlled H_(2)O concentrations(5-75 wt%)as a transient solvent,we achieve neartheoretical densities(>99%)and exceptional optical transmittance(>80%in the visible range).Systematic investigation reveals a nonmonotonic dependence of densification and transparency on H_(2)O content,with an optimal concentration of~15 wt%under 5 GPa.Microstructural and spectroscopic analyses correlate superior optical quality with a pore-free,nanocrystalline microstructure and the absence of secondary phases.Crucially,through integrated first-principles calculations and ab initio molecular dynamics simulations,we unravel the atomic-scale mechanism.Namely,under high pressure,H_(2)O dissociates at the particle interface,with the resultant H+forming Al-H bonds and OH−incorporating into interstitial lattice sites.This process stabilizes a hydroxyl-cubic aluminate(HAl_(2)O_(4))-like structure,which not only facilitates stress-free densification via an enhanced dissolution-precipitation pathway but also effectively relieves internal stresses that typically cause cracking in pressure-only sintering.This work provides a fundamental mechanistic understanding of the synergistic role of H_(2)O and ultra-high pressure in low-temperature ceramic consolidation,establishing a generalizable route to transparent nanocrystalline ceramics that are otherwise inaccessible via conventional thermal sintering.