When 10:30 AM - 11:30 AM Feb 19, 2016
Where 1670 Beyster Building
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Meso-Scale Simulations and Time-Resolved Diagnostics for Understanding the Shock Compression Response of Reactive Powder Mixtures

Naresh Thadhani
School of Materials Science and Engineering, Georgia Tech

Shock-compression of materials generates unique and non-equilibrium states that allow studies in thermodynamic regimes not easily accessible by other methods. Most intriguing is the shock-initiation of highly-exothermic chemical reactions in powder mixtures for possible applications as structural energetic materials. We are investigating shock-initiation of reactions in Ni+Al powder mixtures employing time-resolved gas-gun impact experiments. Piezoelectric PVDF stress gauges with nanosecond time resolution are used to measure the shock wave profiles and their propagation speeds, to obtain evidence of reactions occurring in the time scale of the high-pressure state, based on changes in pressure-volume compressibility. The diagnostics are however, unable to capture any spectroscopic/microstructural information which limits direct observation of transition states and the extent of reaction. Lack of spatial resolution also limits real time observations of localized changes in reactant configuration(s) and processes leading to reaction initiation. We are therefore employing, two-dimensional meso-scale numerical simulations, using actual micrographs of starting reactive powder mixtures imported into a multi-material hydrocode. The simulations, following validation of macroscopic properties through correlations with experiments, provide qualitative and semi-quantitative probing of the configurational changes and their effects on possible mechanisms of shock-induced intermetallic reactions. The discrete particle-level simulations also reveal effects of the highly-heterogeneous nature of shock-wave interactions with reactants of dissimilar properties and morphological characteristics. In the case of the Ni+Al powder mixtures, forced/turbulent flow of constituents, resulting in vortex formation and reactant mixing during void-collapse is the primary process which promotes reaction, which in turn is influenced by the starting reactant powder morphology and differences in reactant properties. To obtain experimental validation of such meso-scale effects, we are investigating the use of quantum dots and photonic crystals to obtain spectral signatures characteristic of local stresses and strains that can be correlated with the simulated reactant configuration changes and to ultimately establish the processes leading to and controlling shock-induced reactions in reactive powder mixtures. The understanding of the shock-compression response generated through such spatially and temporally-resolved diagnostics combined with meso-scale simulations can enable the design of performance-specific structural energetic materials.

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