(753b) Heterogeneous Oxidation of Mechanically Alloyed Al-Mg Powders By Oxygen and Water

Alloys of aluminum and magnesium are widely used in pyrotechnics as well as in other energetic formulations.  They were also explored recently as metal additives in oxygen generators and as materials capable of reacting with water for hydrogen production.  Thus, mechanisms of low-temperature, heterogeneous oxidation of such alloys by both oxygen and water are of interest.  Understanding such mechanisms enables one to model both their aging and initiation in various devices.  In this work, mechanically alloyed Al-Mg powders are prepared and their oxidation in dry oxygen and in steam is compared to that of commercial atomized alloy powders.

Shapes of mechanically alloyed powders are poorly suited for detailed kinetics analysis. In this project, mechanically alloyed powder particles are deformed to become more spherical by rolling them with the spherical glass beads for 1-2 hours. Such rounded, mechanically alloyed powders are oxidized in a controlled environment using thermo-gravimetry and microcalorimetry. Following our previous work, the measured weigh gain is distributed among all particles of the sample taking into account the measured powder particle size distribution.  During the oxidation experiment, the weight gain distribution is adjusted continuously accounting for the changes in the particle sizes and respective surface area at which reaction occurs.  Different reaction models are considered, resulting in a different predicted evolution of the oxide film thickness and respectively differently varied calculated reactive surface area.  The locations of the reaction interfaces for the alloyed powders oxidized in different environments are identified by finding the oxidation model that gives the best match between oxidation dynamics for particles with the same initial sizes but belonging to powders with different initial particle size distributions. Kinetics of the low-temperature oxidation reactions are quantified for both oxidizing environments by processing results of thermo-analytical measurements using different methods. Fully and partially reacted powders are recovered and characterized using electron microscopy and x-ray diffraction. The goal of this effort is development of a simplified, diffusion reaction model for oxidation of alloyed powders with water and oxygen and for both atomized and mechanically alloyed materials.