Magnesium Boron Mixture: Overview
A magnesium/boron mechanical mixture is being developed for energy innovations. The combination of these two materials creates a strong and lightweight composite that has the potential to be used in a variety of energy applications.
Magnesium: A Promising Propellent Additive
Metals as fuel additives or solid fuels have become increasingly popular in recent years due to their high volumetric and gravimetric energy densities. Boron is one of the most commonly used metals due to its volumetric density of 140 kJ/mL and gravimetric density of 58 kJ/g. This makes it an ideal choice for propulsion systems where space is limited. In addition, boron-based propellants and rocket fuels emit fewer exhaust emissions and enhance combustion efficiency.
Aluminum (Al) and magnesium (Mg) are two of the most reactive metals that have been examined carefully for energetic applications due to their high gravimetric energy densities of 31 kJ/g and 25 kJ/g, respectively. On the other hand, a native oxide layer on top of these metals limits their performance as energetic materials. Even though the oxide layer behaves as a momentary passivation layer throughout storage, it also functions as a diffusion barrier at the interface of oxidizer/metal, preventing direct interaction between the metal and the oxidizer that reduces ignition kinetics and results in incomplete combustion.
Catalytic substances like aluminum and magnesium can remove the native B oxide coating via redox reactions and minimize the ignition delay. By limiting the B oxide to elemental B, these dopants can improve the volume of heat generated during B particle oxidation. Furthermore, dopant ignition raises reaction interface temperatures, enhancing B oxidation kinetics.
Magnesium is used as a propellant additive in aerospace propulsion systems and underwater due to its high energy density. The material has low melting and boiling points and good combustion efficiency that helps in the combustion and oxygenation of B. By improving the oxidation extent, magnesium is present in the form of a complex with B, including magnesium diboride (MgB2), enhances combustion efficiency even further. Metal borides also have a long lifespan with their high thermal stability.
Magnesium Affecting the Oxidative Stability of B Particles
Researchers used the self-propagating high-temperature synthesis (SHS) reaction technique among Mg and B to create energetic B/Mg solid solutions. They then matched the thermochemical behavior of B/Mg solid solutions to pure B particles that can evaluate the effect of magnesium addition on the oxidative stability of B at cold temperatures and gravimetric heat transfer of B particles during oxidation.
The core-shell B/Mg structures were built with high purity submicrometer-sized Mg and B particles, ensuring the solid solutions’ purity. Because of its autonomous nature in such initiation, the SHS reaction was chosen for the dry synthesis of solid solutions.
B and Mg powders were combined in a glass vial with the three dry-mixing techniques. The dry mixture was first stirred with a micro-spatula and then sonicated at 25°C for 15 minutes to scatter the aggregates. The as-prepared mixture was then applied to magnetic agitation to achieve a homogeneous mixture.
The prepared mixture was evenly spread across the surface of a glass slide, and another glass slide was added on top of it. The mixture’s sides were coated with Al foil. They were then forced between the glass slides to enhance close contact between the boron and magnesium particles and decrease particle exposure to ambient air, allowing for effective atom diffusion during heating.
Following that, the glass slides were put in a furnace at a temperature of 25°C and warmed at a rate of 30 °C per minute to 500 °C. After maintaining isothermal conditions at 500 °C for 90 minutes, the heated sample was cooled down for three hours.
Thermal analysis tests on various synthesized samples were carried out to optimize synthesis time and temperature process parameters.
The SHS reaction successfully synthesized energetic B/Mg solid solutions with a particle size of 550 nm. The synthesis requirements ensured maximum particle contact between the B and Mg particles while minimizing the sintering of the synthesized samples.
Because of the close interaction between the surface of the B particles and the liquid Mg, a commensurate MgB2/MgO layer developed around the B core. Observations of HAADF-STEM-EDS, XRD, and XPS reported the existence of an outer shell constituted of Magnesium diboride, Magnesium oxide, and Magnesium around the B core.
The B/Mg particles’ core-shell structure enabled the formation of a passivation shell, an exothermic redox reaction between both the native B oxide layer and Mg, and exothermic oxidation of B, MgB2, and Mg. Compared to the heat generated by uncontrolled B particles at temperatures up to 1000 °C, the exothermic heat release induced by B combustion in the B/Mg solid solutions rose by 24%. Furthermore, the oxidative stability of the B/Mg solid solutions was improved in comparison to pure B and Mg under similar circumstances.
It was found that B/Mg solid solutions are thermally stable and have a long shelf life. Additionally, the heat release from these solutions is almost 90% of the theoretical energy density of B/Mg systems. This makes B/Mg solid solutions a promising option for energetic applications.