Reactivity of Nanosize Aluminum with Metal Oxides and Water Vapor
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Reactivity of Nanosize Aluminum with Metal Oxides and Water Vapor Jan A. Puszynski Chemistry and Chemical Engineering Department South Dakota School of Mines and Technology Rapid City, SD 57701, U.S.A. ABSTRACT It has been well documented that the reactivity of nano-powders as well as bulk properties of materials derived from nano-particulates might be significantly different than those obtained from micron-sized grains. This paper addresses several aspects related to the characterization of coated and uncoated aluminum nanopowders, mixing of binary nanopowders, and the reactivity of nanosize aluminum with copper oxide under unconfined and confined conditions. It was found that the volumetric method is more versatile method than TGA or calorimetry for determination of reactive aluminum in coated aluminum nanopowders. It was shown that wet mixing in the presence of dispersants is essential to obtain mixtures with a high level of concentration uniformity. The combustion front velocity can be increased both by using an excess of aluminum in MIC systemand by coating of aluminum nanopowders or addition of dispersants during wet mixing. Protective organic coating with hydrophobic groups is essential to protect aluminum nanopowder from the reaction with moisture at higher relative humidity levels.
INTRODUCTION In recent years researchers have found that energetic materials that are produced on the nanoscale have shown significantly improved performance, especially in the area of sensitivity, mechanical properties, and energy release. Metastable Intermolecular Composites (MICs) represent one example of such materials. These systems consist of metal nanopowder (e.g. aluminum) and oxidizers [1]. The MIC formulations are based on intimate mixing of reactants on the nanometer length scale. As the specific surface area increases, the number of contact points between the reactants also increases and therefore the reaction rate increases [2]. Review of the recent literature on the combustion of pyrotechnic materials shows the relation between the reaction rate and an average particle size of reactants is significant [1-5]. The experimental studies have also shown that the reaction rate depends on other factors, including particle size distribution [6] and degree of intermixing [7]. Reaction rates between nanosize aluminum and metal oxides can be significantly greater than those observed with traditional micron-size thermite powders [1,8,9]. Reactions occurring between metal and metal oxide powders are accompanied by the generation of high temperatures (>3000 K) [10]. Super-thermites, formed by mixing aluminum and metal oxide nanopowders result in energy release rate by two orders of magnitude higher than similar mixtures consisting of micronsize reactants. These super-thermites may find an application in formulations of environmentally benign percussion primers, air bag initiators and inflators, as well as components of thermal batteries [11]. During the past few years, a significant research effort has been made in the
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