Oxidation, ignition and combustion behaviors of differently prepared boron-magnesium composites

Effect of magnesium on ignition and combustion characteristics of boron was explored for three materials containing ca. 23 wt% of magnesium. Boron was sintered with elemental Mg, leading to the formation of MgB₂. This material showed the most homogeneous distribution of Mg. Boron was also mechanically milled with elemental Mg, as well as with commercial MgB₂, respectively. The presence of Mg delayed the oxidation of boron in all materials. The delay was nearly the same for both milled powders and it was somewhat greater for the sintered material. The observed oxidation rate of all materials, however, was greater than for pure boron. All materials could be ignited by a heated wire at nearly the same temperature, although boron could not. Particles of all materials, including elemental boron, could be ignited by a CO₂ laser with the lowest power required for the sintered material. The addition of Mg had a weak effect on particle burn times. Boron milled with elemental Mg showed distinct two-stage combustion patterns interpreted as delayed, and separate, Mg combustion. A small increase in the maximum pressure and rate of pressure rise is observed for the Mg-bearing powders in constant volume explosion experiments.