Reaction Interface for Heterogeneous Oxidation of Aluminum Powders

Heterogeneous oxidation of aluminum is rate limited by diffusion through a growing aluminum oxide layer. If inward diffusion of oxygen ions is faster than outward diffusion of aluminum, the reaction will occur at the inner interface of the oxide. Conversely, the reaction will occur at the outer oxide surface if outward diffusion of aluminum is faster. The location of the heterogeneous reaction is identified processing results of thermogravimetric measurements for two oxidizing spherical aluminum powders with different but overlapping particle size distribution. For each experiment, the measured weight gain is distributed among particles of different sizes assuming that the rate of oxidation is proportional to the reactive interface area. Different models are considered to determine the interface area. For a ductile oxide shell, when there is no void between oxide and aluminum, two cases with the reaction occurring at both inner and outer surfaces of the shell were evaluated. In addition, a case with the reaction at the outer surface of a rigid oxide shell is considered, for which a void inside the particle forms when the aluminum core is shrinking. Oxidation weight gains for the same size particles present in different aluminum powders are expected to be identical to each other when the calculated reactive interface area reflects the true oxidation mechanism. It is concluded that the reaction at the outer surface of a rigid oxide shell describes the experiments most accurately. Thus, the outward diffusion of aluminum ions controls the rate of heterogeneous oxidation of aluminum in a wide range of temperatures of approximately 400–1500 °C. The conclusion is further supported by the electron microscopy of particles quenched at different temperatures, showing oxide surface features consistent with the identified reaction mechanism

Inactivation of aerosolized surrogates of <i>Bacillus anthracis</i> spores by combustion products of aluminum- and magnesium-based reactive materials: Effect of exposure time

<p>Targeting bioweapon facilities may release biothreat agents into the atmosphere. Bacterial spores such as <i>Bacillus anthracis</i> (Ba) escaping from direct exposure to the fireball potentially represent a high health risk. To mitigate it, reactive materials with biocidal properties are being developed. Aluminum-based iodine-containing compositions (e.g., Al·I₂ and Al·B·I₂) have been shown to inactivate aerosolized simulants of Ba effectively, i.e., by factors exceeding 10⁴ when the spores are exposed to their combustion products over a short time (∼0.33 s). This follow-up study aimed at establishing an association between the spore inactivation caused by exposure to combustion products of different materials and the exposure time. Powders of Al, Al·I₂, Al·B·I₂, Mg, Mg·S, and Mg·B·I₂ were combusted, and viable aerosolized endospores of <i>B. thuringiensis var kurstaki</i> (a well-established Ba simulant) were exposed to the released products for relatively short time periods: from ∼0.1 to ∼2 s. The tests were performed at two temperatures in the exposure chamber: ∼170°C and ∼260°C; both temperatures are lower than required for quick thermal inactivation of the spores. The higher temperature and exposure times above 0.33 s generated distinctively higher inactivation levels (as high as ∼10⁵) for iodine-containing materials. We also observed inactivation levels of up to ∼10³ at very short exposure times, 0.12s, in the presence of condensing MgO. However, the effect of MgO at longer exposure times became negligible. The biocidal effect of sulfur oxides was found to be weak. The study findings are crucial for establishing strategies and developing reaction models that target specific bioagent inactivation levels.</p> <p>Copyright © 2018 American Association for Aerosol Research</p