Effect of Chemistry on the Performance of Calcium Disilicide Primers

ABSTRACTRotary atomization was used to synthesize spheres of CaSi2-based compositions in order to understand issues relative to primer performance for military applications. Elemental silicon and calcium were used to synthesize the line compound CaSi2or the eutectic composition between CaSi2and Si. Fe was added to form FeSi2as a secondary phase in selected compositions. Rietveld analysis showed that CaSi2polytypes in the synthesized materials consisted primarily of 6R, with less 3R and some hexagonal material. Synthesized materials had low surface areas (≈0.1 m2/g), but short milling times increased the surface area by an order of magnitude. Peak pressures, pressure rise time, and ignition voltage showed no significant differences between experimentally prepared samples and existing commercial samples. Stoichiometric CaSi2performed as well as CaSi2-Si or CaSi2-FeSi2-Si mixtures. The military specification for calcium disilicide should be changed to reflect the broad chemistry which can be used for primer performance.

In-situ Remediation of Hydrogen Fluoride during a Detonation Event

ABSTRACTHydrogen fluoride (HF) is a known product from the combustion or detonation of explosives formulated with fluoropolymer binder systems. This presents the user with elevated risk levels, particularly during unintended combustion events or detonations in close combat situations. In an effort to remediate the production of HF, calcium disilicide was added to explosive formulations in an effort to decrease the amount of HF formed. First, VitonA/calcium disilicide mixtures were made and the thermal decomposition characteristics studied using thermal gravimetric/differential thermal analysis. The activation energy ranged from approximately 145-190 kJ/mol, indicative of C-F scission in the Viton binder prior to calcium fluoride formation. An energetic formulation was made which consisted of approximately a 5/3 mass ratio of Viton/CaSi2. Combustion calorimetry in oxygen and air and subsequent analysis of the residues using x-ray diffraction (XRD) and energy dispersive x-ray analysis (EDS) revealed that calcium fluoride formed. The amount of HF reduced was determined by trapping off gases in a cooling loop, rinsing into water, and analysis in anion exchange chromatography. Upon introduction of calcium disilicide into the formulation, a ~30% decrease in HF formation was observed along with appearance of CaF2 and free Si in the residue. Such products are consistent with the mechanism following a general decomposition path of 2F + CaSi2→CaF2+2Si. The same formulation was detonated, and upon product analysis it was determined the decomposition path followed nearly the same route with a net decrease in HF formation, but with a portion of the silicon oxidizing slightly further to SiO2.