Preparation, ignition, and combustion of mechanically alloyed Al-Mg powders with customized particle sizes
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Preparation, ignition, and combustion of mechanically alloyed Al-Mg powders with customized particle sizes Yasmine Aly, Vern K. Hoffman, Mirko Schoenitz, and Edward L. Dreizin Otto H. York Department of Chemical, Biological, and Pharmaceutical Engineering, New Jersey Institute of Technology, Newark, NJ 07102, U.S.A. ABSTRACT Adding aluminum to propellants, pyrotechnics, and explosives is a common way to boost their energy density. A number of approaches have been investigated that shorten aluminum ignition delay, increase combustion rate, and decrease the tendency of aluminum droplets to agglomerate. Previous work showed that particles of mechanically alloyed Al-Mg powders burn faster than similarly sized particles of pure aluminum. However, preparation of mechanically alloyed powders with particle sizes matching those of fine aluminum used in energetic formulations was not achieved. This work is focused on preparation of mechanically alloyed, composite Al-Mg powders in which both internal structures and particle size distributions are adjusted. Binary powders with compositions in the range of 50 - 90 at. % Al were prepared and characterized. Milling protocol is optimized to prepare equiaxial, micron-scale particles suitable for laboratory evaluations of their oxidation, ignition, and combustion characteristics. Quantitative particle size analyses are done 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 setup. For all materials, ignition and combustion characteristics are compared to each other and to those of pure Al. Compositions with improved performance (i.e., shorter ignition delay and faster pressurization rate) compared to pure Al are identified. INTRODUCTION Metal-based, binary mechanically alloyed and nanocomposite materials offer advantageous performance as fuel additives in energetic formulations for propellants, explosives, and pyrotechnics [1-5]. Research has been active to produce nano-sized powders of reactive metals [6-8], which achieve high burn rates due to highly developed reactive surface. Alternatively, micron-sized aluminum-based powders prepared by mechanical milling, including alloys or composites of Al-Mg, Al-Ti, etc., [9-14] with enhanced reactivity compared to pure Al powders have also been explored. These materials are desirable because of the high combustion enthalpies (typical of Al), with tailored density, high energy density, reduced ignition delays, and faster burning rates. In order for these materials to be practically useful, their particle size distribution should be adjusted to make them compatible with the existing protocols used for preparation of energetic formulations. However, cold welding that often occurs due to the ductile nature of Al makes it difficult to prepare mechanically alloyed powders with useful size distributions. Process control agents (PCAs) have been used previously to
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