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

Abstract

Small crystalline boron particles (5–15 μm) are ignited in atmospheres consisting 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 and nitrogen on ignition delay time. A reflected shock is used to obtain pressures of ∼8.5 atm and temperatures of ∼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 μs 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 O 2 is increased from 0 to 0.25. For values of y F/ y O 2 greater than 0.5 there is little change of ignition burning time with y F/ y O 2 . Spectroscopic data taken in pure oxygen environment show residual BO 2 emission after particle combustion, while that taken in fluorine-containing environments show little or no emission from BO 2. This is consistent with predictions from theoretical modeling efforts of other researchers. However, these same models predict the presence of BF and BF 2 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 N 2 is increased from 0 to 0.8 with y O 2 held constant at 0.20.