Shock initiated chemical reaction experiments have been performed on a 1:l atomic ratio mixture of 20-to 45,um nickel and -325 mesh crystalline silicon powders. It has been observed that no detectable or only minor surface reactions occur between the constituents until a thermal energy threshold is reached, above which the reaction goes to completion. The experiments show the energy difference between virtually no and full reaction is on the order of 5 percent. Differential scanning calorimetery (DSC) of statically pressed powders shows an exothermic reaction beginning at a temperature which decreases with decreasing porosity. Powder, shock compressed to just below the threshold energy, starts to react in the DSC at 621 "C while powder statically pressed to 23% porosity starts to react at about 30 "C higher. Tap density powder starts to react at 891 "C. The DSC reaction initiation temperature of the shock compressed but unreacted powder corresponds to a thermal energy in the powder of 382 J/g which agrees well with the thermal energy produced by a shock wave with the threshold energy (between 384 and 396 J/g). (Thermal energies referenced to 20 "C.) A sharp energy threshold and a direct correlation with DSC results indicates that the mean thermal energy determines whether or not the reaction will propagate in the elemental Ni + Si powder mixture rather than local, particle level conditions. From this it may be concluded that the reaction occurs on a time scale greater than the time constant for thermal diffusion into the particle interiors.
Elemental powder mixes were subjected to plane-wave shock processing which reduced the initial porosity to essentially zero. Two powder mixes in a 5:3 Ti:Si atomic ratio were used: Ϫ325 mesh Ti and Si ͑Ͻ45 m͒, and Ϫ100 mesh Ti and Si ͑Ͻ150 m͒ with shock pressures up to 7.3 GPa and shock energies up to 671 J/g. Shock pressures were calculated using hugoniot parameters for porous elemental powder mixtures and shock energies were taken to be the work done by the shock ( P⌬V/2). Shock energy thresholds for complete reaction of the elemental powders were found which depend upon powder particle size and the initial porosity of the powder. The threshold energy for the larger powder mix was found to be ϳ80% larger than that for the smaller powder. A decrease in initial porosity from 0.49 to 0.40 caused an increase in threshold shock energy of about 75% for both powders. At shock energies slightly below the threshold energy, evidence for the reaction of solid Ti and liquid Si was observed in small isolated regions. These regions contained spherical micronodules with the composition of TiSi 2 in Si. The results are compared to those of previous studies reported in the literature, and mechanisms for reaction initiation and the observed threshold values are proposed.
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