Shock initiation of crystalline boron in oxygen and fluorine compounds

Abstract

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. Particles ranging from submicron to 32 microns in diameter are ignited in the ambient conditions produced by a reflected shock wave in a shock tube. 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. Results indicate that pressure in the range studied does not affect the ignition delay or burn time. The additives, water vapor and sulfur hexafluoride, reduce the ignition delay time for amorphous and sub-micron crystalline boron when compared to oxygen. For 20 microns particles, H2O and SF6 reduce the ignition temperature limit from 2500 deg K in pure oxygen to 2200 deg K and 1900 deg K, respectively. Burn time is unaffected by the additives. Hydrogen fluoride did not show any change in ignition delay or burn time compared to pure oxygen. At the range of temperatures tested, very little (less than 2%) of HF is dissociated into H and F atoms. The report also presents reviews of previous chemical and physical models that have attempted to explain why boron powder is relatively difficult to ignite. Ignition of metal powder, Shock tube initiation.