Boron suboxides measured during ignition and combustion of boron in shocked Ar/F/O-2 and Ar/N2/O2 mixtures

Small crystalline boron particles (5-15 mu m) are ignited in atmospheres consisting of Ar and O-2 mixed with varying amounts of F (from dissociated SF6) or N-2 at the endwall of a shock tube to study the effect of fluorine and nitrogen on ignition delay time. A reflected shock is used to obtain pressures of similar to 8.5 atm and temperatures of similar to 2600 K. Visible wavelength emission spectra are recorded using a spectrometer coupled to a streak camera and two photodetectors record intensity versus time at a wavelength of 546.1 nm. The streak camera allows recording of multiple time-resolved spectra at rates of approximately 100 mu s per spectrum. Boron particles ignited in Ar/F/O-2 mixtures show a rapid decrease by a factor of 4 in ignition and burning times as the mole fraction ratio y(F)/y(O2) is increased from 0 to 0.25. For values of y(F)/y(O2) greater than 0.5 there is little change of ignition burning time with y(F)/y(O2). Spectroscopic data taken in pure oxygen environment show residual BO2 emission after particle combustion, while that taken in fluorine-containing environments show little or no emission from BO2. This is consistent with predictions from theoretical modeling efforts of other researchers. However, these same models predict the presence of BF and BF2 molecules, which are not observed in emission. When boron particles are burned in Ar/N-2/O-2 atmospheres, there is a decrease of over 60% in ignition delay times as y(N2) is increased from 0 to 0.8 with y(O2) held constant at 0.20. (C) 1999 by The Combustion Institute.