Shock initiation of crystalline boron in oxygen and fluorine compounds

The ignition delay and combustion of amorphous and crystalline boron particles is investigated at elevated temperatures and pressures for wet, dry, and fluorine-containing atmospheres. Micron-sized amorphous and sieved 20-μm crystalline particles are ignited in the reflected-shock ambient conditions produced at a shock-tube endwall. The ignition delay and combustion times are examined as a function of temperature for pressures of 8.5, 17, and 34 atm and for oxidizer mixtures of 100% oxygen, 30% water vapor, 1-3% sulfur hexafluoride, and 6-12% hydrogen fluoride. At 8.5 atm, SF₆ has little effect on the ignition delay or temperature limit for 20-μm particles, but at 34 atm the effect of SF₆ is to reduce both the ignition delay time and the ignition temperature limit, from 1900 to 1400 K. In addition, experiments conducted with 1- and 20-μm crystalline boron particles and with unsieved <200-μm B₂O₃ particles show that the first and second peaks observed in 20-μm particle combustion are associated with removal of the oxide layer.