Effect of particle morphology on reactivity, ignition and combustion of boron powders

Spherical boron powders with 2-20 mu m sized particles were prepared by ball milling commercial boron in the presence of emulsion with hexane and acetonitrile serving as the continuous and droplet phases, respectively. A small amount of Fluorel (R), a fluorocarbon serving as binder was dissolved in the acetonitrile. For comparison, an irregularly-shaped boron powder was prepared by milling commercial boron with the same acetonitrile/Fluorel (R) solution. Reactivity of the spherical boron powders was compared to those of milled and as received commercial powders with irregularly-shaped particles. Thermo-gravimetric measurements in Ar/O-2 gas flow showed that milling boron with or without Fluorel (R) shifts the onset of boron oxidation to lower temperatures. The rate of initial oxidation was greater for the finer spherical powders compared to coarser spheres and to irregularly shaped milled powder. The milling did not appreciably change the activation energy of oxidation of boron and the observed accelerated oxidation was attributed to its increased specific surface area. Two of the finer prepared spherical powders could be ignited when coated on an electrically heated filament in the temperature range of ca. 600 - 1000 ?. Commercial powders, irregularly-shaped milled powder, and coarser spherical powders did not ignite. All boron powders aerosolized and ignited in an enclosed vessel in air generated similar pressures. In another experiment, blended boron and potassium nitrate powders were heated to ignition in room air using a laser beam. The ignition delays for the blends made with spherical powders were substantially shorter vs. similar blends with commercial powder. Combustion of the blends with spherical powders was more vigorous than with commercial boron. Particles were ejected more rapidly, and had shorter apparent burn times.