Boron doped with iron: Preparation and combustion in air

Combustion-relevant effects of iron as an additive to boron powders were examined. 95% pure boron powder was doped with iron by thermal decomposition of iron pentacarbonyl. Iron concentration was determined using inductively coupled plasma–mass spectrometry. Two doped boron samples with 1.6 and 3.1 wt-% Fe were compared to commercial boron powder for physical morphology and combustion characteristics. No significant morphological differences between the doped and undoped samples were detected by scanning electron microscopy. Elemental maps confirmed uniform distribution of iron in the coated powders. For combustion studies, individual particles of different powders were ignited in air by passing through a CO₂ laser beam. Burn times and combustion temperatures were determined from optical emissions of burning particles. Doped boron samples have shorter burn times than particles of the undoped commercial boron powder with the same size. The effect of doping boron with iron on burn times appears to be greater for larger particles. Combustion temperatures depend on the iron content. Lower iron concentrations showed temperatures near 3100 K, comparable to uncoated commercial boron. At higher Fe concentrations, the temperatures decreased to about 2500 K. A reaction mechanism is proposed explaining qualitatively a two-stage reaction pattern observed for the full-fledged combustion of boron; the approach uses a recently published binary B-O phase diagram. This combustion reactions are altered when boron is doped with iron, which is proposed to serve as a catalyst for boron oxidation.