Role of Heterogeneity in the Chemical and Mechanical Shock-Response of Nickel and Aluminum Powder Mixtures
Eakins, Daniel Edward
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The design of non-classical materials, such as multifunctional energetic materials and/or the synthesis of high pressure phases rely on the understanding of the mechanisms responsible for shock-induced reactions in powder mixtures. The critical reactant powder configurational changes and mechanical mixing processes necessary for reaction initiation have yet to be determined. Consequently, shock-induced reactions have only been observed in select material systems under certain conditions, and remain an uncontrolled phenomenon. Shock-induced reactions in nickel and aluminum powder mixtures are investigated in this work through the use of instrumented gas-gun experiments performing time-resolved pressure and shock velocity measurements to determine the pressure-volume (P-V) shock compressibility (Hugoniot) of the mixture, from which evidence of reaction is inferred through deviations from the inert shock response calculated on the basis of mixture theory. The role of particle size and morphology on non-diffusional mixing and chemical reactivity is explored by conducting similar tests on micron-scale powders of spherical and plate-like (flake) shape. Recovery experiments performed just below the reaction threshold provide information about the densification and mixing behavior between reactants. Discrete-component numerical simulations of the shock-compression of powder mixtures are performed to reveal the micromechanics of particle deformation, and mechanisms of mass-flow and mixing that can lead to the formation of reaction products. The results obtained from time-resolved measurements, recovery experiments, and numerical simulations are coupled to model the linkages between starting powder configuration, mechanically-driven mixing, and chemical reactivity. The knowledge gained from this investigation will lead to understanding of reaction mechanisms, and the control over reaction initiation threshold, time and exothermicity, in addition to characteristics of reaction products formed. The scientific understanding attained will advance the design and application of multifunctional materials for next generation energetic applications, and/or the synthesis of novel materials.