Synthesis and Characterization of Magnesium/Boron Solid Solutions for Energetic Applications

A major problem limiting boron (B) use as a fuel or fuel additive is the native oxide layer present at the surface, which acts as a diffusion barrier at the boron/oxidizer interface. A requirement for improving the reactivity and exothermicity during the oxidation of B particles is to reduce the thickness of the native oxide. This can be achieved by the addition of reactive metals with reasonable gravimetric energy density, such as Mg, in the form of a mechanical mixture or alloyed compounds, which can undergo an exothermic redox reaction to reduce native oxide and enrich metallic B. Herein, we develop an approach to synthesize Mg/B solid solutions through a self-propagating high-temperature synthesis (SHS) reaction at 500 °C leading to the formation of an outer shell comprised of Mg, MgO, and MgB2 around a B core as demonstrated by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and high-angle annular dark-field scanning transmission electron microscopy energy dispersive spectroscopy (HAADF-STEM-EDS). Particle size analysis by dynamic light scattering (DLS) shows a size distribution with an average size of 550 nm. The low synthesis temperature minimizes sintering and is inherently free of organic contamination compared to other available methods. The core–shell architecture offers two advantages: extension of the shelf life of B particles by the formation of a passivation shell and in addition to the exothermic oxidation of Mg, MgB2, and B, an exothermic redox reaction occurs between Mg and native B2O3 to produce additional B fuel, culminating in a synergistic interaction that leads to increased heat release. Thermal analysis shows the promotional effect of Mg on B oxidation with increased heat release (24% higher) and higher oxidative stability than Mg and B under identical conditions. Synthesized Mg/B solid solutions release 90% of the theoretical energy density of Mg/B and thus show promise as energetic materials.