Controlled Synthesis of Alumina Nanopowders Using Supercritical Fluids for Ceramics Applications

Alumina is the most widely used oxide ceramic material. It exists in many metastable forms which lead upon severe thermal treatment towards the thermodynamically stable phase alpha. Sintering of alumina generally takes part in a minimum of two stages: i) phase transition(s) towards the stable phase alpha and ii) densification of this phase alpha. The former step(s), linked to transition phase(s) is (are) strongly dependent on the crystallinity of the initial powders. Using this parameter, it is therefore possible to optimise the sintering properties, in particular to lower the transition temperature. Such effect has been studied for aluminas elaborated in suband supercritical fluid media. This work highlights the possibilities to obtain, according to the nature of the fluid, different kind of transition aluminas: boehmite AlO(OH) or amorphous Al2O3. Sintering these powders led both to alpha-alumina but with different microstructures and densities. We could also observe a significant shift towards lower temperature for the gamma-Al2O3→alpha-Al2O3 transition in the case of amorphous alumina compare to the boehmite one. The transfer to this know-how to the design of core-shell nanoparticles with alumina as a shell is investigated to develop nanostructured ceramics for telecommunications.