Magnesium-Induced Strain and Immobilized Radical Generation on the Boron Oxide Surface Enhances the Oxidation Rate of...

Despite their high gravimetric and volumetric energy densities, boron (B) particles suffer from poor oxidative energy release rates as the boron oxide (B₂O₃) shell impedes the diffusivity of O₂ to the particle interior. Recent experiemental studies have shown that the addition of metals with a lower free energy of oxidation, such as Mg, can reduce the oxide shell of B and enhance the energetic performance of B by ∼30-60%. However, the exact underlying mechanism behind the reactivity enhancement is unknown. Here, we performed DFTB-MD simulations to study the reaction of Mg vapor with a B₂O₃ surface. We found that the Mg becomes oxidized on the B₂O₃ surface, forming a MgBO phase, which induces a tensile strain in the B-O bond at the MgBO-B₂O₃ interface, simultaneously reducing the interfacial B and thereby developing dangling bonds. The interfacial bond straining creates an overall surface expansion, indicating the presence of a net tensile strain. The B with dangling bonds can act as active centers for gas-phase O₂ adsorption, thereby increasing the adsorption rate, and the overall tensile strain on the surface will increase the diffusion flux of adsorbed O through the surface to the particle core. As the overall B particle oxidation rate is dependent on both the O adsorption and diffusion rates, the enhancement in both of these rates increases the overall reactivity of B particles.