| The occurrence of shock-induced chemical reactions and the existence of a thermal reaction threshold for these reactions have been previously established. Observations of recovered post-shock specimens and a few time-resolved investigations have led to the possibility that these reactions may occur at the 100 nanosecond level in exothermic systems. But, complications and apparent conflicts between these investigations have restricted acceptance of these fast chemical reactions.;The reflected shock pressure-time profile was measured through exothermic mixtures and compared to the predicted inert values. If chemical reactions occur in the mixture during the shock wave, these values are expected to differ substantially. Material analysis was performed on the recovered post-shock specimens to corroborate the conclusions. For two Ni-Al mixtures the measured shock pressure substantially exceeded that predicted for an inert system, "excess pressure," beyond threshold impact velocities. These thresholds were corroborated by material analysis of the recovered specimens. For the larger particle size mixture, an experiment was conducted which showed that the reactions did not occur during the incident shock wave. So, the "excess pressure" was attributed to the energy released from an exothermic reaction occurring during the reflected shock front, i.e. at a 100 nanosecond rate.;For the smaller Ni-Al particle mixture, a thermal reaction threshold was observed in the recovered specimens below the ultrafast reaction threshold. The two Ni-Al mixtures were identical except for their constituent particle sizes. Yet, the ultrafast reaction threshold for the smaller Ni-Al mixture was at a significantly higher impact velocity than that of the larger particle mixture. This means that the ultrafast reaction threshold is inversely related to the particle size. This lends a hint to the mechanisms involved in the ultrafast reaction; in that, the interparticle shear is also inversely related to the particle size. If a relative particle velocity exists between the constituents during the shock front, then the interparticle shear will act to equilibrate this difference. After overcoming a critical value, this interparticle shear may expose fresh reaction surfaces and initiate a reaction within the shock front. A variety of Sn-S systems were also investigated by this method. Though reactions were observed in the recovered specimens, no fast reactions were recorded. |
