Combustion of boron particles with different morphologies

Spherical composite boron powders with different particle sizes and consisting of densely aggregated nano-sized primary particles were prepared using emulsion-assisted milling. They were compared to commercial boron consisting of irregularly shaped, finer, and less densely packed primary particle aggregates. Powder particles were injected into an oxygen-acetylene burner. Their combustion was characterized using recorded optical emission pulses. For each particle pulse, characteristic brightness, duration, and color temperature were determined. Measured color temperatures of the burning commercial boron particles were close to the melting point of boron. The composite milled powders exhibited higher combustion temperatures and greater emission intensities than commercial boron, despite comparable burn times. Characteristic BO 2 emission was observed and used for additional temperature measurements. Recovered combustion products did not coalesce into single molten droplets, retaining, at least partially, their initial aggregate structure. This was attributed to the production of volatile suboxides (e.g., BO 2 ) at the surfaces of individual primary particles. Staged combustion was observed at high temperatures and distinctly after ignition. The present results emphasize the importance of vapor-phase reactions and challenge previous models that assigned the first combustion stage solely to the removal of molten B 2 O 3 . Bulk burn rates of boron particles were consistent with those estimated using the reaction kinetics quantified from thermoanalytical experiments on boron oxidation at much lower temperatures.