Variations in aluminum particle surface energy and reactivity induced by annealing and quenching

Surface interface properties of fuel particles play an important role in their reactivity. Since diffusion-controlled reactions occur at the interface between a solid fuel and oxidizer, variations in surface energy alter diffusion rates in ways that can be measured macroscopically. In this study, surface energy was purposefully altered by annealing and quenching aluminum (Al) (3–4.5 µm particle diameter) powder. Bulk aluminum exhibits reduced surface energy resulting from annealing and quenching. In the current work, annealing and quenching was extended to powder media to assess the magnitude of reduced surface energy. An inverse gas chromatography (iGC) technique was used to energetically characterize the particle surface for both dispersive and polar properties. Results show up to a 40% decrease in surface energy with annealing and quenching. Both untreated and thermally processed powder were reacted by launching projectiles into a chamber where the ensuing powder explosions were examined. Experiments were performed in ambient air and repeated in argon environments at projectile velocities of 1200 m/s. Overall, higher surface energy resulted in a more pronounced system performance, i.e., in stronger reactions with higher probability of ignition and combustion. A theory explaining the differences in reactivity links surface energy with a mechanism for particle reaction.