Unravelling the H₂O-assisted high-pressure sintering mechanism: a route to highly transparent cubic alumina ceramics

Transparent cubic alumina (γ‑Al₂O₃) ceramics are promising optical materials but challenging to densify without phase transformation or cracking. Herein, we report a novel H₂O‑assisted high‑pressure sintering (HPS) strategy that enables the fabrication of highly transparent, crack‑free γ‑Al₂O₃ ceramics at temperatures as low as 300 °C. By combining GPa‑level pressures (0.8-5 GPa) with precisely controlled H₂O concentrations (5–75 wt.%) as a transient solvent, we achieve near‑theoretical densities (>99%) and exceptional optical transmittance (>80% in the visible range). Systematic investigation reveals a non‑monotonic dependence of densification and transparency on H₂O 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, H₂O 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 (HAl₂O₄)‑ 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 H₂O 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.