Alumina is one of the most widely used oxide ceramic material. It exists in many metastable forms, among which is the thermodynamically stable α phase, obtained upon severe thermal treatment. Sintering of alumina is generally performed in several stages: first, phase transitions towards the stable α phase followed by its densification. The first step is strongly dependent on the crystallinity of initial powders. By controlling this parameter, it is possible to optimize the sintering properties, in particular by decreasing the phase transition temperature. This effect has been studied for alumina elaborated in sub- and supercritical fluid media. This work highlights the possibilities to obtain, according to the nature of the fluid, different kinds of transition alumina: boehmite AlO(OH) or amorphous Al2O3. The sintering processes of these powders all lead to α-alumina, however, different microstructures and densities can be obtained. A significant shift towards lower λ/α phase transition temperature is also observed when amorphous alumina is considered, compared to boehmite. The transfer of this know-how to the design of core-shell nanoparticles and film deposition onto copper heat sinks is investigated to develop nanostructured ceramics for telecommunications and electronics.