Phase changes in boron ignition and combustion

The ignition of electrically heated boron filaments in air and argon/oxygen mixtures has been studied. Boron filament resistance, temperature, and emi ssions from the BO and BO2 bands were monitored. Preliminary experimental d ata have also been obtained to characterize the phases formed inside burnin g boron particles produced and ignited from the same filament material by f eeding a vibrating boron filament into an oxygen-acetylene flame. The liqui d boron particles so formed and ignited burned in room air where their comb ustion was recorded using a high-speed Video system. Samples of both filame nts and particles quenched at different times during their combustion were analyzed using electron microscopy to characterize their internal structure s and compositions. The filaments "burned" in two distinct stages. The onse t of the first stage was characterized by a local minimum in the filament r esistance, a sharp spike in boron oxide radiation emission, and a rapid ris e in temperature. It occurred at a temperature of 1500 +/- 70 degrees C, in dependent of the preheating history and oxygen content (5-40%) in the gas e nvironment. These data and changes in the filament physical characteristics suggest that a phase transition occurs in the filaments at this temperatur e and triggers stage one combustion. A transition from a to b rhombohedral boron structures has been reported in this temperature range. The burning b oron particles exhibited periodic brightness oscillations that arise from a symmetric particle combustion associated with internal phase changes. Elect ron microprobe analyses indicated significant amounts of oxygen contained w ithin both quenched filaments and particles. Additionally, quenched filamen t samples collected during the second stage of combustion exhibited large s pherical voids. The present observations indicate that in-depth heterogeneo us processes play important roles in boron ignition and combustion and that their elucidation will result in a more complete description of ignition a nd combustion phenomena that previously have been only incompletely underst ood. (C) 1999 by The Combustion Institute.