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 emissions from the BO and BO2 bands were monitored. Preliminary experimental data have also been obtained to characterize the phases formed inside burning boron particles produced and ignited from the same filament material by feeding a vibrating boron filament into an oxygen–acetylene flame. The liquid boron particles so formed and ignited burned in room air where their combustion was recorded using a high-speed video system. Samples of both filaments and particles quenched at different times during their combustion were analyzed using electron microscopy to characterize their internal structures and compositions. The filaments “burned” in two distinct stages. The onset of the first stage was characterized by a local minimum in the filament resistance, a sharp spike in boron oxide radiation emission, and a rapid rise in temperature. It occurred at a temperature of 1500 6 70°C, independent of the preheating history and oxygen content (5–40%) in the gas environment. These data and changes in the filament physical characteristics suggest that a phase transition occurs in the filaments at this temperature and triggers stage one combustion. A transition from a to b rhombohedral boron structures has been reported in this temperature range. The burning boron particles exhibited periodic brightness oscillations that arise from asymmetric particle combustion associated with internal phase changes. Electron microprobe analyses indicated significant amounts of oxygen contained within both quenched filaments and particles. Additionally, quenched filament samples collected during the second stage of combustion exhibited large spherical voids. The present observations indicate that in-depth heterogeneous processes play important roles in boron ignition and combustion and that their elucidation will result in a more complete description of ignition and combustion phenomena that previously have been only incompletely understood. © 1999 by The Combustion Institute