Nano-structured Aluminum Powders with Modified Protective Surface Layers
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Nano-structured Aluminum Powders with Modified Protective Surface Layers Shasha Zhang1, Mirko Schoenitz1 and Edward L. Dreizin1 1 Otto H. York Dept. of Chemical, Biological and Pharmaceutical Engineering, University Heights, Newark, NJ 07102, ABSTRACT The advantage of aluminum powder as a fuel additive in energetic formulation includes its high volumetric combustion enthalpy and relatively low cost. However, the thermodynamically predicted benefits of aluminum combustion are rarely achieved because of extended ignition delays associated with heterogeneous reactions occurring at the alumina surface which surrounds the aluminum particle. In order to fully exploit aluminum’s high reaction energy, this effort focuses on adjusting its combustion dynamics by modifying its surface and structure. The modification is achieved by cryo-milling aluminum with cyclooctane, which is liquid at room temperature, but solid when cooled by liquid nitrogen. The prepared materials consist of micronsized, equiaxial, mostly Al particles with a small amount of cyclooctane. Aluminum surface in the prepared sample is coated with a cyclooctane-modified layer with properties significantly different from those of regular alumina. Its oxidation kinetics, as observed from thermoanalytical measurements, is different from that of pure aluminum. The powder ignites at substantially reduced temperatures, produces shorter ignition delays, and higher aerosol burn rates compared to a regular spherical Al powder with similar particle sizes. INTRODUCTION Aluminum is one of the most commonly used metal fuel additives for propellants, explosives, and pyrotechnics [1]. Its main advantages are a higher combustion enthalpy compared to organic energetic compounds, such as TNT, RDX and HMX. However, in many aluminized energetic formulations performance is not optimized because of the relatively low reaction rates and long ignition delays, which are associated with heterogeneous reactions during the onset of Al ignition. Such reactions are rate limited by relatively slow diffusion of reagents through a protective natural alumina layer that is always present on the Al surface. To fully exploit the combustion dynamics of aluminum, different approaches were described in the literature to modify aluminum surface. Most efforts focused on nanoscale aluminum due to its greater surface to volume ratio and thus most prominent role of the surface reactions in its ignition and combustion mechanisms. Jouet and co-workers [2] reported nano-Al stabilization with carboxylic acid coatings. Lewis and co-workers [3] used oleic acid as a capping agent on nano-Al. In both cases, alumina layers were replaced by new phases; however, such phases were relatively thick, reducing energy density of the prepared materials. In this work, mechanically milled micron-sized aluminum powder with highly developed grain boundaries and surface defects is used instead of nano-aluminum; its surface is protected by a very small amount of a hydrocarbon-based modifier to maintain high energy density of alu
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