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 co nsisting of Ar and O-2 mixed with varying amounts of F (from dissociated SF 6) or N-2 at the endwall of a shock tube to study the effect of fluorine an d nitrogen on ignition delay time. A reflected shock is used to obtain pres sures 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 wave length of 546.1 nm. The streak camera allows recording of multiple time-res olved spectra at rates of approximately 100 mu s per spectrum. Boron partic les 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 increas ed from 0 to 0.25. For values of y(F)/y(O2) greater than 0.5 there is littl e change of ignition burning time with y(F)/y(O2). Spectroscopic data taken in pure oxygen environment show residual BO2 emission after particle combu stion, 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 t he presence of BF and BF2 molecules, which are not observed in emission. Wh en boron particles are burned in Ar/N-2/O-2 atmospheres, there is a decreas e of over 60% in ignition delay times as y(N2) is increased from 0 to 0.8 w ith y(O2) held constant at 0.20. (C) 1999 by The Combustion Institute.