The Effect of Nanopowder Attributes on Reaction Mechanism and Ignition Sensitivity of Nanothermites
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The Effect of Nanopowder Attributes on Reaction Mechanism and Ignition Sensitivity of Nanothermites Jan A. Puszynski, Chris J. Bulian and Jacek J. Swiatkiewicz Department of Chemical and Biological Engineering South Dakota School of Mines and Technology 501 East Saint Joseph Street Rapid City, SD 57701, USA ABSTRACT Nanothermite composites have several properties that are not typical of conventional thermites. The nanoscale size of individual reactants is responsible for the significant differences in these properties, especially the rate of energy release and mechanism of combustion front propagation. Several thermite mixtures were investigated, including Al-Fe2O3, Al-CuO, AlMoO3, and Al-Bi2O3. Previous studies have reported on the behavior of these mixtures during unconfined burning and on the characterization of particle attributes such as particle size, surface area, and reactive metal content. This study was focused on several other attributes, including mixing of nanoreactants in water and measurements of reaction kinetics and combustion front propagation characteristics under confined conditions. The nanoscale nature of the thermite components also has an effect on the kinetics of the reaction. Differential scanning calorimetry was used to determine activation energy of these reacting systems. Several experimental setups were used to monitor the nanothermite mixtures during combustion. The mixtures were monitored during combustion in small diameter tubes using high speed video technology and a pressure sensor system. These tests were used to characterize combustion propagation under confined conditions and to determine the effect of pressure and mixture density on propagation rate. Experiments were also performed using both a closed volume pressure cell and recoil force cell to measure the reactive energy of the mixtures. INTRODUCTION The scaling of thermite mixtures to the nanoscale level has allowed for many new capabilities and applications that were not previously possible with conventional micron-sized thermite mixtures. The nanoscale size of the individual reactant particles allows for much more intimate mixing between the fuel and oxide. This characteristic, coupled with much larger specific surface area in comparison with micron sized particles results in much faster reaction rates. In addition, this large surface area of nanoreactants results in significantly higher evaporation or decomposition rates of involved oxidizers1,2. The combustion propagation of nanothermite mixtures has been shown to be primarily convectively driven whereas the combustion front propagation in conventional thermites is primarily limited by conduction3-5. Along with significant benefits of nanothermite systems, such as a better control of energy output and in many cases environmentally benign nature of involved reactants, several new issues have to be addressed and solved. Processing of the mixtures became more difficult than systems involving micron sized particles. Mixing of nanoscale components has been
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