(388aa) Oxidation Mechanism of Metal Particles Derived From Thermal Analysis

Thermal analysis is widely used to establish rates of oxidation of metal powders, which can be correlated with their ignition mechanisms.  Measurements are performed with polydisperse powders and are commonly processed considering a representative average particle size.  However, particles of different sizes may react with different rates; understanding and quantifying these differences is of interest for developing a comprehensive oxidation mechanism.  As a simple example, reaction between metal and oxidizer may occur at the surface of the growing oxide layer, or inside this layer, at the metal-oxide interface, depending on whether metal or oxidizer diffuse faster through the growing oxide.  As the oxide layers grow, the changes in reactive surface to volume ratio will change differently depending on the location of the reactive surface.  This work describes a methodology of processing thermo-analytical measurements aimed to distinguish such differences in oxidation rates of particles of different sizes.  Thermo-gravimetric (TG) measurements describing oxidation of different aluminum powders are processed.  For each powder, particle size distribution is measured and used to process the experimental data. For each time step, the measured weight gain is distributed among particles proportionally to the available surface of the reactive interface.  This distribution depends on the selected reaction model, and thus, the interface location. The surface area of the reactive interface is recalculated for each experimental data point.  Note that the selected reaction model can include effects of the growing oxide thickness and changes in diffusion rates as a function of temperature and other parameters.  As a result of this processing, for each TG run, individual weight gain curves are obtained for particles of different sizes.  The appropriate reaction model is validated comparing weight gain curves obtained for particles of the same size for powders with different, but overlapping particle size distributions.  These weight gain curves must be identical, assuming that the oxidized particles do not interact with one another.  Results and conclusions for oxidation of spherical aluminum powders will be presented and discussed.