Aluminum Powder Oxidation in CO2 and Mixed CO2/O2 Environments

An experimental study of oxidation of micrometer-sized aluminum powder in CO2 and CO2/O-2 mixed oxidizers is described. The oxidation is studied using thermogravimetric (TG) measurements. Partially oxidized samples are recovered and their compositions are analyzed using X-ray diffraction. Oxidation of aluminum powders in CO2 occurs in several stages, similar to that in oxygen. Aluminum oxidation rates are effectively the same for O-2 and CO2 oxidizers for the low-temperature oxidation processes controlled by the growth of the initial amorphous oxide, its transformation to the gamma-Al2O3 polymorph, and initial growth of gamma-Al2O3. At higher temperatures, the growth of transition alumina in oxygen occurs faster than it does in CO2. The thermal stability of the transition alumina polymorphs, primarily for theta-Al2O3, is extended to higher temperatures in the presence of CO2 so that the denser alpha-Al2O3 Polymorph forms later than that in oxygen. Because of the lower diffusion resistance of transition alumina polymorphs as compared to alpha-Al2O3, for the samples heated to the same temperatures a higher degree of oxidation is achieved in CO2 as compared to the environments containing O-2 as the only oxidizer. In mixed CO2/O-2 oxidizers, the initial growth of gamma-Al2O3 proceeds as fast as that with oxygen only, while the region of stability of transition alumina polymorphs remains extended to higher temperatures. Activation energies for individual aluminum powder oxidation steps observed in CO2 are determined from processing the measured TG traces and compared to the activation energies reported earlier for aluminum oxidation in oxygen.