Effect of polymorphic phase transformations within an alumina layer on the ignition of aluminum particles
Department of Mechanical Engineering
Doctor of Philosophy
Dreizin, Edward L.
Dave, Rajesh N.
Experimental measurements of aluminum ignition temperature and models used to describe aluminum ignition are reviewed. It is shown that the current models cannot describe ignition of aluminum powders of different sizes and ignited under various experimental conditions. The properties of and phase changes occurring in the alumina scale existing on the surface of aluminum particles at different temperatures are systematically studied. The mechanism of aluminum oxidation is quantified and a new simplified ignition model is developed.
Thermogravimetry was used to study the oxidation of aluminum powders of various particle sizes and surface morphologies in oxygen at temperatures up to 1500°C. Partially oxidized samples were recovered from selected intermediate temperatures and the oxide phases present were analyzed by x-ray diffraction. Both micron- and nanosized aluminum powders were observed to exhibit characteristic stagewise oxidation in the temperature range from 300 to 1500°C. Kinetic parameters for both direct oxidative growth of alumina scale and phase transformations between different alumina polymorphs were determined from the thermal analysis data for a selected micron-sized powder. The observed oxidation trends for other micron- and nano-sized powders were well interpreted considering the established kinetics.
Melting of aluminum nanopowders was studied by differential scanning calorimetry in argon environment. No correlation was found between the melting and oxidation.
The developed aluminum ignition model describes ignition of a particle inserted in a hot oxygenated gas environment - scenario similar to the particle ignition in a reflected shock in a shock tube experiment. The model treats heterogeneous oxidation as an exothermic process leading to ignition. The ignition is assumed to occur when the particle's temperature exceeds the alumina melting point. The model analyzes processes of simultaneous growth and phase transformations in the oxide scale. Additional assumptions about oxidation rates are made to account for discontinuities produced in the oxide scale as a result of increase in its density caused by the polymorphic phase changes. The model predicts that particles of different sizes ignite at different environment temperatures. Generally, finer particles ignite at lower temperatures. The model consistently interprets a wide range of the previously published experimental data describing aluminum ignition.
njit-etd2006-086 (118 pages ~ 7,325 KB pdf)
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Created September 16, 2008