Influence of Activating Compounds on Oxidation Mechanisms of Aluminum Powders

This study involves an investigation of the mechanism underlying activating effects of inorganic additives on the oxidation of aluminum. The oxidation of nano aluminum powders in air was characterized by variable temperature x-ray powder diffraction and thermogravimetric analysis. For a 33-nm aluminum powder the aluminum oxide produced by air oxidation was poorly crystallized until the sample was heated to above 1050°C, where the alpha-alumina phase crystallized. For a mixture of the aluminum with cryolite, crystallization of oxide phases is enhanced, with Na-Al-O phases evident at 550°C and above. Fluorine is lost from the sample between 550 and 850°C, presumably due to reaction with moisture to produce HF. In a similar experiment with aluminum and silver molybdate, the only crystalline product was alpha-alumina, which was observed at 550°C and higher. The general result is that alpha-alumina, the ultimate product in most cases, crystallized at a lower temperature when activating agents were present.