Aluminum powder oxidation in CO₂ and mixed CO₂/O ₂ environments

An experimental study of oxidation of micrometer-sized aluminum powder in CO₂ and CO₂/O₂ 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 O₂ and CO₂ oxidizers for the low-temperature oxidation processes controlled by the growth of the initial amorphous oxide, its transformation to the γ-Al₂O₃ polymorph, and initial growth of γ-Al₂O₃. At higher temperatures, the growth of transition alumina in oxygen occurs faster than it does in CO ₂. The thermal stability of the transition alumina polymorphs, primarily for θ-Al₂O₃, is extended to higher temperatures in the presence of CO₂ so that the denser α-Al₂O₃ polymorph forms later than that in oxygen. Because of the lower diffusion resistance of transition alumina polymorphs as compared to α-Al₂O₃, for the samples heated to the same temperatures a higher degree of oxidation is achieved in CO₂ as compared to the environments containing O₂ as the only oxidizer. In mixed CO₂/O₂ oxidizers, the initial growth of γ-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.