Oxidation Mechanism of Aluminum Nanopowders
Résumé
Aluminum nanopowders, oxidized at different temperatures using thermogravimetric analyses performed in high resolution mode, are characterized in terms of morphology, structure and microstructure. The particle structure is modeled via geometrical considerations that enable the calculation of the variation of specific surface area during oxidation. A two-step oxidation scenario is proposed. In the early oxidation stage, that is, for temperatures up to 650 degrees C where a pseudoplateau is reached, the oxidation, which occurs by diffusion of oxygen or aluminum through the alumina layer, leads to a core shell structure. At higher temperatures, that is, above the melting point of aluminum, outward diffusion of aluminum through the oxide shell is controlling the reaction rate. The reaction interface is then located at the external surface and voids are formed inside the particles. This result is confirmed by energy filtered electron micrographs that allow distinguishing a thin metallic aluminum layer outside the alumina shell. This suggests that the migration of aluminum toward the surface of the particles is faster than the oxidation. Some insights on the nucleation process during the crystallization of liquid aluminum are also proposed which are related to the particle microstructure: heterogeneous nucleation is proposed to govern the crystallization of liquid aluminum and to give a signature of the alumina layer structural state.