Reactions of aluminum with halogen containing oxides



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Due to increasing threats of biological attacks, new methods for the neutralization of spore forming bacteria are currently being examined. Thermites may be an effective method to produce high temperature reactions, and some compositions such as aluminum (Al) and iodine pentoxide (I2O5) also have biocidal properties. This study examines the thermal degradation behavior of I2O5 mixed with micron and nanometer scale aluminum (Al) particles. Differential scanning calorimetry (DSC) and thermo-gravimetric (TG) analyses were performed in an argon environment on both particle scales revealing a non-reaction for micron Al and a complex multistep reaction for the nanometer scale Al. Results show that upon I2O5 decomposition, iodine ion adsorption into the alumina shell passivating Al particles is the rate controlling step of the Al-I2O5 reaction. This pre-ignition reaction is unique to nano-Al mixtures and attributed to the significantly higher specific surface area of the nanometric Al particles which provide increased sites for I- sorption. A similar pre-ignition reaction had previously been observed with fluoride ions and the alumina shell passivating Al particles. Composite energetic materials comprised of nanoparticle fuel and oxidizer can exhibit high flame propagation speeds on the order of 1000m/s when burning in an unconfined environment. In particular, halogen based formulations such as aluminum and iodine pentoxide have received significant attention due to both high flame speeds and biocidal properties. Studies have attributed high flame speeds to convective influences within a reaction driving the heat forward in a pressure wave accelerating ignition of unburned powders. This study examines factors contributing to convective flows such as gas and heat generation and their relationship to the measured flame speed as well as fundamental chemical kinetics influencing the observed flame speeds. The goal is to understand parameters directly related to high flame speeds in halogen containing composites. Results show a direct correlation between apparent activation energy and flame speed indicating that flame speed is directly influenced by chemical kinetics. For this reason, the intermediate chemistry associated with Al and iodine species was examined to identify chemical influences accelerating flame speeds. Ab initio quantum chemical calculations of gas-phase reactions resolve key exothermic intermediate reactions contributing toward the kinetics of the fastest burning mixtures. Condensed phase density functional theory calculations of Al2O3/I2O5 interfaces resolved adsorption/desorption properties. This study examines the effect of atmospheric oxygen concentration (4 or 93% oxygen) on energy propagation of nanometric aluminum with copper oxide (Al+CuO), iron oxide (Al+Fe2O3), calcium iodate (Al+Ca(IO3)2), and iodine pentoxide (I2O5). In all cases energy propagation was examined in terms of flame speed and higher in the high oxygen environments. However, the convectively driven mixtures showed a smaller percent increase in flame speeds. This behavior is attributed to the increased availability of oxygen as a result of solid oxidizer thermal decomposition at lower temperatures. The slower Al+Fe2O3 reaction showed greater increases in flame speed attributed to early stage reactions involving atmospheric oxygen that promote oxide decomposition and faster flame speeds. A predictor based on solid oxidizer thermal decomposition and flame speed was developed to evaluate the sensitivity of a reaction to atmospheric oxygen concentration.