Direct cold sintering of translucent gamma-Al2O3 ceramics

Gamma-Al2O3 ceramics can hardly be densified by conventional sintering methods because of the low surface energy of gamma-Al2O3 powder and phase transformation at elevated temperature. Here, we report the preparation of translucent gamma-Al2O3 ceramics by cold sintering process at merely 350 oC via using the gamma-Al2O3 as starting powder. Due to the enhanced solubility and reaction of gamma-Al2O3 in alkaline solution, the nanocrystalline particles can be densified to ~92% of full density via adding 10 M NaOH solution as the transient liquid. It is proved that the high concentration of alkaline solution can greatly accelerate the dissolution-precipitation process, leading to the activation energy as low as 16.9 kJ/mol. Consequently, a translucent ceramic with in-line transmittance reaching 16% around 800 nm can be obtained, despite the residual porosity. Considering the stable hardness and translucency after water leaching or heating to 1000 oC, the cold sintered gamma-Al2O3 ceramic may have the potential application as optical windows under extreme environments.