Role of Constituent Configuration on Shock-Induced Reactions in a Ni+Al Powder Mixture
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Role of Constituent Configuration on Shock-Induced Reactions in a Ni+Al Powder Mixture Daniel E. Eakins and Naresh N. Thadhani School of Materials Science and Engineering, 771 Ferst Drive, Georgia Institute of Technology Atlanta, GA 30332, U.S.A. ABSTRACT Ultra-fast reactions initiated within or immediately behind the shock front in powder mixtures are of importance in the synthesis of high-pressure phases and next-generation energetic materials. Reactions in nickel and aluminum powder mixtures and the establishment of a reaction threshold have been the source of many studies over the last 20 years. Prior work has suggested that the criterion for reaction is most probably mechanochemical in nature, in which shock loading environment plays a larger role than absolute shock energy input. The mechanisms responsible for intimate mixing of fresh reactants are however still unclear. In this work we are investigating the role of particle size and morphology on the loading, mixing, and their subsequent shock-induced reaction behavior, by performing shock-compressibility experiments on equi-volumetric mixtures of nickel and aluminum powders, with variations in nickel particle size (micron and nano-scale) and shape (spherical and flake). Determination of shock states is accomplished through time-resolved in situ PVDF gauge measurements of input shock stress and shock propagation speed obtained from transit time through the thickness of powder mixture. The reaction product shock-compressibility state is also being calculated based on constant pressure approximations to allow correlation with measured states for inference of the occurrence of shock-induced chemical reactions. The results of this study suggest that powder configuration in the nickel-aluminum system can be modified to encourage or discourage reaction.
INTRODUCTION Shock loading has been identified as a method of synthesizing novel materials for innovative applications [1]. Subjecting materials to high pressure at strain rates in excess of 106 s-1 can produce both physical and chemical changes unachievable through low pressure processes. Of particular interest is the initiation of ultra-fast chemical reactions for use in next generation energetic materials. Chemical reactions initiated by the incident shock wave during the microsecond time scale of pressure equilibration are known as shock-induced reactions. These reactions occur long before the effects of bulk temperature equilibration and mass diffusion. At this time scale, the influence of temperature is minimal, and the conversion of reactants to activated complexes and finally reaction products must be accomplished by mechanochemical processes. At the very least, the shock energy deposited into the material must perform a couple of key tasks. The first of these is the breaking of atomic bonds within the parent phase. Secondly, the free reactive species must be brought together and mixed. The failure to accomplish these tasks on the bulk scale prevents the initiation of shockinduced reaction in
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