Characterization of fine nickel-coated aluminum powder as potential fuel additive

Oxidation, ignition, and combustion processes are studied and compared for a fine nickel-coated aluminum powder and reference uncoated aluminum powder with a similar particle size distribution. Oxidation is studied by thermogravimetry in argon-oxygen mixtures. Ignition processes are studied for powders coated on an electrically heated metal filament. Combustion is characterized in constant volume explosion tests. Both ignition and combustion experiments were performed in air. Thermogravimetric measurements showed selective oxidation of Ni at low temperatures, at which the oxidation of Al remains undetected. At higher temperatures, oxidation for both the nickel-coated and uncoated powders occurs in a characteristic stepwise process with individual oxidation steps associated with polymorphic phase changes in the growing alumina layer and with the growth of individual alumina polymorphs. The activation energies for individual oxidation steps appear to be unaffected by the Ni coating; however the oxidation occurs somewhat faster for the coated powder, indicating an increase in the preexponential coefficients in Arrhenius formulations describing the respective oxidation processes. Ignition kinetics for both coated and uncoated powders are similar; however, ignition is more readily detected and appears to be more violent for the coated powders. Finally, powder combustion experiments showed substantially reduced ignition delays and somewhat increased overall burn rates for the coated powders.