(398k) Ignition and Combustion of Al?Mg Alloy Powders Prepared By Different Techniques

Aluminum is a high combustion enthalpy metal capable of boosting the energy density of energetic formulations used in propellants, explosives, and pyrotechnics.  A number of approaches have been investigated that shorten aluminum ignition delay, increase burn rate, and decrease the tendency of aluminum droplets to agglomerate.  Aluminum-based materials, including mechanically alloyed powders, are of interest as alternatives to aluminum in these formulations.  Mechanically alloyed Al∙Ti powders, in particular, have previously been shown to have higher burn rates compared to similarly sized Al powders.  Upon their heating, an intermetallic Al₃Ti forms causing a heat release accelerating their ignition.  A practical drawback of this material was that it could only be prepared as a powder with relatively coarse particle sizes, making it impossible to replace fine aluminum powders commonly used in the energetic formulations.  This work is aimed at preparing Al∙Ti powders with customized particle compositions and sizes.  Recent efforts showed a successful reduction in the particle sizes for Al∙Mg powders using a staged mechanical milling approach.  The first stage was aimed to prepare the mechanically alloyed powder and the second stage was used to adjust its particle size distribution.  A small amount of elemental iodine (I₂) was added during the final milling stage, helping to reduce the particle size distribution of the final product.  A conceptually similar approach is explored here for Al∙Ti alloys.  The milling protocol is modified using combinations of process control agents and staged milling to prepare equiaxial, micron-scale particles suitable for laboratory evaluations of their oxidation, ignition, and combustion characteristics.  Particle size distributions are measured using low-angle laser light scattering.  Electron microscopy and x-ray diffraction are used to examine particle morphology and phase makeup, respectively.  Combustion of aerosolized powder clouds is studied using a constant volume explosion apparatus.  Combustion of individual particles is studied using a laser ignition setup.  Oxidation and decomposition are studied using thermo-analytical measurements.  For all materials, ignition and combustion characteristics are compared to those of pure Al.  Additionally, ignition and oxidation characteristics are compared to determine which events lead to particle ignition.  Compositions with improved performance (i.e., shorter ignition delays, higher burn rates, and faster pressurization rates) compared to pure Al are identified.