Effect of polymorphic phase transformations in Al2O3 film on oxidation kinetics of aluminum powders

Thermogravimetry was used to study the oxidation of aluminum powders at elevated temperatures. Aluminum powders of various particle sizes and surface morphologies were heated in oxygen up to 1500 C at different heating rates. Partially oxidized samples were recovered from selected intermediate temperatures and the oxide phases present were analyzed by X-ray diffraction. The experimental data were related to current information on stabilities and phase changes of Al2O3 polymorphs. Aluminum powders were observed to oxidize in four distinct stages in the temperature range from 300 to 1500 C. During stage I, from 300 to about 550 C, the thickness of the natural amorphous alumina layer on the particle surface increases. The rate of this process is controlled by the outward diffusion of Al cations. At about 550 C, when the oxide layer thickness exceeds the critical thickness of amorphous alumina of about 4 nm, the oxide transforms into g-Al2O3. The specific volume of g-Al2O3 is less than that of amorphous alumina; therefore, the newly formed g-Al2O3 only partially covers the aluminum surface. The oxidation rate increases rapidly at the onset of stage II, but it decreases when the g-Al2O3 layer becomes continuous. During stage III oxidation, the g-Al2O3 layer grows and partially transforms into the structurally similar f-Al2O3 polymorph. Finally, oxidation stage IV is observed after the transition to stable a-Al2O3 results in an abrupt reduction of oxidation rate. Qualitative analysis of the rates of oxidation at the different stages enables one to understand the wide range of aluminum ignition temperatures observed for particles of different sizes.