Oxidation of boron powders with distinct particle morphologies

Boron is a thermodynamically attractive fuel, and its heterogeneous oxidation is expected to determine its ignition delay in practical configurations. Boron powders are commonly micron-sized aggregates of submicron primary particles. In this work, oxidation is compared for four different commercial boron powders with broad particle size distributions and for the same powders processed using emulsion-assisted milling (EAM) to prepare spherical particles with narrow size distributions. The milled powders were more tightly packed. The observed differences in the oxidation behaviors of commercial powders were reduced after they were EAM-processed. The oxidation proceeded in one step for as-received powders and in two steps for all milled powders. For all powders, the oxidation was incomplete; it slowed down after a smaller mass gain for the milled powders compared to the as-received ones. This was attributed to a lower porosity of the milled powders, which were thus more readily clogged with the formed molten boron oxide, preventing further oxidation. Iron contamination introduced by milling caused the oxidation to begin at a lower temperature; however, it also led to an increase in the activation energy of oxidation. Contamination by zirconia did not affect the oxidation onset temperature or the activation energy.