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

Abstract

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 MgB 2 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, MgB 2 , and B, an exothermic redox reaction occurs between Mg and native B 2 O 3 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.

Enhanced Energetic Performance of Aluminum Nanoparticles by Plasma Deposition of Perfluorinated Nanofilms

“…Metal-based energetic materials store large amounts of chemical energy and undergo highly exothermic reactions generating light, heat, and thrust. − These properties are important for a number of applications that include propellants, solid rocket fuels, and pyrotechnics. − Aluminum (Al)-based materials have emerged as prime candidates for propellants and fuel additives in civilian and military applications due to superior gravimetric energy density (31 kJ/g), high reactivity, and abundance on earth. ,− The size of the particles is an essential factor in the performance of Al as an energetic material. Nanosized particles exhibit higher reactivity, lower ignition temperature, and the ability to undergo faster and more complete oxidation, leading to enhanced heat release compared to micrometer-sized particles. − The surface of the Al nanoparticle (NP) is covered with a native oxide (Al 2 O 3 ) shell with an average thickness of 2–6 nm. ,,, This shell acts as a passivation coating, but under prolonged exposure to air and humidity it will further oxidize, thus depleting the metallic content of the particles. ,,, The oxide shell occupies 30–50% of the mass of the particles less than 100 nm. , Thus, a substantial fraction of the particle mass does not contribute to heat release under oxidation. , The high melting point of Al 2 O 3 (∼2100 °C) further hinders oxidation, leading to the slow kinetics and lesser heat release. − …”

Low-Temperature Cost-Effective Synthesis of MgB2 for Energetic Applications

“…Mg and B are chemically bonded, which leads to the unavailability of B and Mg atoms to form chemical bonds with O, preventing them from oxidation during storage in ambient conditions . Hence, MgB 2 displays extended shelf life, and therefore, it is possible to store them for a longer period without affecting their ability to store chemical energy. ,, The formation of metallic B due to redox reaction enriches the material in highly energetic boron and enhances the thermal output of the material. MgO, which is a byproduct in these reactions, is beneficial, as shown by Bello et al because it enhances the diffusive heat transfer when the sample is added as a secondary fuel to liquid propellants for the enhancement of overall combustion energy.…”

Nanoenergetic Materials: Enhanced Energy Release from Boron by Aluminum Nanoparticle Addition

“…This model can predict energetically favorable surface positions in metal blends and can be helpful in optimizing the process. Our recent work on Mg/B solid solutions 29 also exhibits the potential of more reactive Mg to enhance the oxidation heat release from B particles at lower temperatures. Hence, there is a need to study the effect of Al nanoparticles (NPs) on the energetic performance of B and to optimize the blend to extract the maximum chemical energy at lower temperatures.…”

“…Other studies include the investigation of Al–Mg alloys and Mg/B solid solutions. 27 − 29 When we add dopants to the metals, careful investigation of segregation dynamics is important. Saidi et al 27 , 28 studied the surface segregation dynamics in Al–Mg alloys by developing a robust atomistic potential based on machine learning principles.…”

“…Pivkina et al, with the help of thermochemical analysis, established that boron oxide on mixing with Al and Mg powders undergoes thermite reaction at temperatures between 620 and 860 °C to form refractory oxides and active B. Other studies include the investigation of Al–Mg alloys and Mg/B solid solutions. − When we add dopants to the metals, careful investigation of segregation dynamics is important. Saidi et al , studied the surface segregation dynamics in Al–Mg alloys by developing a robust atomistic potential based on machine learning principles.…”