Shock-induced and shock-assisted solid-state chemical reactions in powder mixtures

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Abstract
Shock compression of powder mixtures can lead to chemical reactions, resulting in the formation of equilibrium as well as nonequilibrium compounds, and rapid increases in temperature. The reactions occur as manifestations of enhanced solid-state chemical reactivity of powders, caused by configurational changes and defect states introduced during shock compression. Two types of reactions are possible and can be distinguished on the basis of their respective process mechanisms and kinetics. Shock-induced chemical reactions occur during the shock-compression state, before unloading to ambient pressure, and in time scales of mechanical equilibrium. In contrast, shock-assisted reactions occur after unloading to ambient pressure, in an essentially shock-modified material, in time scales of temperature equilibration. The mechanisms of shock-assisted reactions include solid-state defect-enhanced diffusional processes. Shock-induced reactions, on the other hand, require mechanisms different from conventional solid-state nucleation and growth processes. The complex nature of deformation of powders has precluded a detailed understanding of the reaction mechanisms of such high-rate reaction processes. Results of controlled experiments, however, suggest that shock-induced chemical reactions involve nondiffusional processes giving rise to mechanochemical effects and solid-state structural rearrangements. Mechanistic concepts that distinguish between shock-induced and shock-assisted chemical reactions are described. The effects of configurational changes introduced during shock compression, and the influence of material properties and shock-loading characteristics on such effects, are analyzed to identify the mechanisms of complex processes leading to chemical reaction initiation and compound formation.