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.
Chen et al. (2026) studied this question.