The mechanism of combustion of superfine aluminum powders

An experimental study of the combustion of superfine aluminum powders (average particle diameter as ∼ 0.1 μm) in air is reported. The formation of aluminum nitride during the combustion of aluminum in air and the influence of combustion conditions on the structures and compositions of the final products are addressed. The experiments were conducted in static air at 1 atm. Superfine aluminum powders were produced by exploding an electrically heated wire. Such a superfine aluminum powder is stable in air, but once ignited can burn in a self-sustaining way due to its low bulk density (∼0.1 g/cm3) and low thermal conductivity. During combustion, the temperature and radiation were measured; also the burning was recorded by a video camera. Scanning electron microscopy, X-ray diffraction and chemical analysis were performed on both the initial powders and final products. It was found that the powders ignited by local heating and burned in a two-stage self-propagating regime. The products of the first stage consisted of unreacted aluminum (∼70 mass %) and amorphous oxides with traces of AlN. After the second stage, the AlN content exceeded 50% and the residual Al content decreased to ∼10%. A qualitative discussion is given of the kinetic limitations for the oxidation of AlN due to rapid condensation and encapsulation of gaseous AlN. [Copyright &y& Elsevier]