Thermal oxidation of fine aluminum powders in carbon dioxide gas

The thermal oxidation of fine aluminum (Al) powders in carbon dioxide (CO2) gas was investigated for the purpose of metal-based propulsion fuel development. The thermal behavior and reaction energy was studied using simultaneous thermogravimetric (TG) analysis and differential scanning calorimetry (DSC). The reactivities of Al powders with nanometer and micrometer-scale average particle sizes were compared. The particle morphology was examined at different stages of the process using field emission gun scanning electron microscopy (FEG-SEM) and transmission electron microscopy (TEM). The corresponding chemical changes were analyzed by X-ray diffraction spectrometry (XRD) and energy dispersion X-ray spectrometry (EDS).; Nano- and micrometer-scale Al powders exhibited different calorimetric behaviour. The primary oxidation occurred at around 533°C for the nanopowders and at 1045°C for the micropowders. The mechanism of the oxide growth, particle deformations, and hollow oxide shells are discussed. A low-temperature transformation of the amorphous Al2O3 coating to crystalline gamma-Al 2O3 is identified as a trigger to the exothermic reaction in the case of the Al nanopowders. Carbon was also shown to be involved in the reaction.