Effect of aluminum, iron, and zirconia additives on the combustion of boron

Spherical composite powders with particle sizes around 10 µm, combining boron and aluminum, were prepared by emulsion-assisted milling. Powders milled using the steel and zirconia milling media were contaminated by iron and zirconia, respectively. Some powders used small amounts of Fluorel® as a binder. The oxidation of the prepared powders heated in an Ar/O₂ gas flow was studied using thermal analysis. The ignition temperature in air was determined using an electrically heated filament. Prepared powders were blended with KNO₃ as an oxidizer and ignited using a CO₂ laser beam. The powders were also injected into a closed vessel and burned as an aerosol. The results show no effect of Fluorel® or zirconia contamination on the powder reactivity. The added iron causes a reduction in the oxidation onset temperature, whereas added aluminum increases that temperature. Both added iron and aluminum cause a reduction in the flame temperature and suppress the formation of the vapor-phase intermediate BO₂. A reduced flame temperature delays the aerosol flame propagation rate. The powder morphology achieved by emulsion-assisted milling enhances the powder reactivity, as is evidenced by reduced ignition temperatures and accelerated rate of flame propagation for the milled powders containing no iron or aluminum.