CORRELATION OF SHOCK INITIATED AND THERMALLY INITIATED CHEMICAL-REACTIONS IN A 1-1 ATOMIC RATIO NICKEL-SILICON MIXTURE

Shock initiated chemical reaction experiments have been performed on a 1:1 atomic ratio mixture of 20- to 45-mu-m 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-degrees-C while powder statically pressed to 23% porosity starts to react at about 30-degrees-C higher. Tap density powder starts to react at 891-degrees-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-degrees-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.