SHOCK-INDUCED AND SHOCK-ASSISTED SOLID-STATE CHEMICAL-REACTIONS IN POWDER MIXTURES

Shock compression of powder mixtures can lead to chemical reactions, r esulting in the formation of equilibrium as well as nonequilibrium com pounds, and rapid increases in temperature. The reactions occur as man ifestations of enhanced solid-state chemical reactivity of powders, ca used by configurational changes and defect states introduced during sh ock compression. Two types of reactions are possible and can be distin guished on the basis of their respective process mechanisms and kineti cs. Shock-induced chemical reactions occur during the shock-compressio n state, before unloading to ambient pressure, and in time scales of m echanical equilibrium. In contrast, shock-assisted reactions occur aft er unloading to ambient pressure, in an essentially shock-modified mat erial, in time scales of temperature equilibration. The mechanisms of shock-assisted reactions include solid-state defect-enhanced diffusion al processes. Shock-induced reactions, on the other hand, require mech anisms different from conventional solid-state nucleation and growth p rocesses. The complex nature of deformation of powders has precluded a detailed understanding of the reaction mechanisms of such high-rate r eaction processes. Results of controlled experiments, however, suggest that shock-induced chemical reactions involve nondiffusional processe s giving rise to mechanochemical effects and solid-state structural re arrangements. Mechanistic concepts that distinguish between shock-indu ced and shock-assisted chemical reactions are described. The effects o f 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 proc esses leading to chemical reaction initiation and compound formation.