Defect engineering in reactive binary α-Al2O3 enables low-temperature pressureless densification of alumina ceramics

Alumina ceramics with full densification at low temperature are desirable owing to their enhanced mechanical properties and reduced energy consumption. However, resolving the inherent balance between density and reactivity remains a key challenge in pressureless sintering without additives. This work proposed a strategy for preparing low-temperature densified alumina ceramics via defect engineering and phase regulation in precipitated alumina trihydrate. Based on the pseudomorphic and topotactic transformations, highly reactive binary α-Al 2 O 3 powder was prepared by regulating the phase composition, structural defects and chemical coordination of the precursor alumina trihydrate via hydrothermal modification. Combining the high density of well crystallized gibbsite-derived α-Al 2 O 3 and the high reactivity of poorly developed boehmite-derived α-Al 2 O 3 , the binary α-Al 2 O 3 exhibited notably increased unsaturated Al coordination, enhanced microstrain, and decreased α-Al 2 O 3 content, enabling rapid sintering kinetics in grain boundary diffusion with a reduced activation energy of 302 kJ mol −1 . Meanwhile, sintering under nitrogen atmosphere facilitated the generation of oxygen vacancies and lattice distortion in the α-Al 2 O 3 powder. The lattice distortions of elongated Al-O bonds, increased interlayer spacing and expanded lattice volume further boosted the sintering activity of binary α-Al 2 O 3 powder by providing extensive diffusion tunnels for accelerated atomic migration. With remarkable grain refinement and improved microstructural homogeneity, the resultant binary alumina ceramic was fully densified at 1400 °C for 2 h with 99.9% relative density, 0.02% apparent porosity, 17.22 GPa Vickers hardness, 4.08 MPa m 1/2 fracture toughness, 8.34E + 12 Ω cm electrical resistivity and 34.35 W m −1  K −1 thermal conductivity. This study provides new insights into low-temperature densification of alumina ceramics, and improves the performance of alumina-bearing materials via structure-property modification of α-Al 2 O 3 powder and aluminum hydroxide precursors.