Structural changes and thermal properties of aluminium micro- and nano-powders

Structural and chemical properties of aluminium powders with size distribution in the micrometer and nanometer range are studied. The particles can be described by an aluminium core surrounded with an alumina layer which, depending on the passivation method, can be amorphous or constituted with small crystallites of γ-alumina. Using calorimetric measurements and in situ neutron diffraction experiments, the crystallization of the amorphous shell is evidenced and a tetragonal structure for the crystallized γ-alumina is proposed. The thermal expansion of the aluminium core is obtained by following the variation of the cell parameter with temperature. It is shown that, whatever is the size of particles, the thermal expansion of the metallic core is not affected by an alumina shell with a 3 ± 2 nm thickness. In contrast, when the thickness of the crystalline alumina shell exceeds 20 nm, strong strain effects are evidenced.