Oxidation kinetics and pyrotechnic cloud combustion of transition metal- enhanced boron particles

In an effort to enhance ignition and combustion of boron as a solid fuel ingredient, elemental boron was mechanically milled with 5 wt % of a metal additive (Ni, Co, Fe, Cu, Bi, Zr, or Hf). Oxidation at low heating rates, under thermal analysis conditions, showed a lower oxidation onset temperature for all milled materials, particularly for B-Bi and B-Co, at the expense of a lower overall degree of oxidation at higher temperatures. Kinetic modeling of the oxidation onset, at the stage where no significant amount of oxide had accumulated at the boron surface, was used to calculate particle burn times at combustion temperatures. Comparison with published burn times allowed prediction of combustion temperatures. For pure boron and B-Fe, the predicted combustion temperatures matched expected values, although for all other materials the predictions were too low. This work also evaluates the ignition and combustion behavior of selected boron-transition metal additive composites when used in a pyrotechnic mixture with potassium nitrate, KNO₃ in air. When mixed with KNO₃ as an oxidizer, and ignited by a CO₂ laser, all milled powders showed a shorter ignition delay compared with pure boron. Combustion temperatures of the milled powders were just below the boron melting point compared with elemental boron, which was found to burn above the boron melting point. Similarly, significant B-O gas phase products were observed spectroscopically for pure boron, but not for the milled composites. Captured combustion products were consistent with gas phase condensation for pure boron, while larger molten droplets and crystalline formations dominated for the milled composites. The combined results demonstrate that the studied metal additives cause boron to combust in the condensed phase, with temperatures and burn times that can be tailored by the choice of additives.