Vaporization-Controlled Energy Release Mechanisms Underlying the Exceptional Reactivity of Magnesium Nanoparticles

Abstract

Magnesium nanoparticles (NPs) offer the potential of high-performance reactive materials from both thermodynamic and kinetic perspectives. However, the fundamental energy release mechanisms and kinetics have not been explored due to the lack of facile synthetic routes to high-purity Mg NPs. Here, a vapor-phase route to surface-pure, core−shell nanoscale Mg particles is presented, whereby controlled evaporation and growth are utilized to tune particle sizes (40−500 nm), and their size-dependent reactivity and energetic characteristics are evaluated. Extensive in situ characterizations shed light on the fundamental reaction mechanisms governing the energy release of Mg NP-based energetic composites across particle sizes and oxidizer chemistries. Direct observations from in situ transmission electron microscopy and high-speed temperature-jump/time-of-flight mass spectrometry coupled with ignition characterization reveal that the remarkably high reactivity of Mg NPs is a direct consequence of enhanced vaporization and Mg release from their high-energy surfaces that result in the accelerated energy release kinetics from their composites. Mg NP composites also demonstrate mitigated agglomeration and sintering during reaction due to rapid gasification, enabling complete energy extraction from their oxidation. This work expands the compositional possibilities of nanoscale solid fuels by highlighting the critical relationships between metal volatilization and oxidative energy release from Mg NPs, thus opening new opportunities for strategic design of functional Mg-based nanoenergetic materials for tunable energy release.

Magnesium-Enhanced Reactivity of Boron Particles: Role of Mg/B2O3 Exothermic Surface Reactions

“…The composition of the formulations fabricated have been tabulated in Table S1 in section S4 of the Supporting Information. Constant-volume combustion cell (∼20 cm –3 ) characterization was performed on 25.0 mg of the samples using the setup described in prior works. , …”

“…Alternatively, exploring binary fuel systems by incorporating energy-dense metal additives, such as Al, Mg, and Ti, in boron powders has been demonstrated as a promising strategy to accelerate B oxidation while maintaining a high energetic content. − Employing similar dual fuel systems has been used to control and modulate the transport processes and ignition in thermite mixtures. , Among the metal additives explored thus far, Mg is particularly attractive as a result of its high volatility and reactivity. In our previous work on Mg nanoparticle (NP)-based thermites, we have shown that reducing the particle size of Mg NPs results in their rapid vaporization and a fast vapor-phase Mg release (release time scale of ∼100 μs at a heating rate of 10 5 °C/s) . Additionally, as a result of the highly negative formation energies of MgO compared to B 2 O 3 (Mg lies lower than B in the Ellingham diagram), the reaction between Mg and B 2 O 3 is thermodynamically feasible with a reaction enthalpy (Δ H r ) of ∼ –420 kJ/mol or −2.9 kJ/g.…”

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

“…MgB 2 has been regarded as an interesting candidate for energetic materials as it can store and release a large amount of chemical energy (40 kJ/g), − which lies between the gravimetric energy density of Mg (25 kJ/g) and B (58 kJ/g). At the same time, MgB 2 is thermally and chemically more stable than Mg or B; − hence, it can help store energy for extended periods. Unlike metal particles, a lower concentration of native oxide is present on the surface of MgB 2 because of the unavailability of Mg or B to form bonds with oxygen.…”

“…MgB 2 decomposes into Mg and B at elevated temperatures, which subsequently react with oxygen to form ternary oxides of Mg and B. This avoids the accumulation of B 2 O 3 , which, as a low-temperature melting oxide (450 °C), forms a molten layer that clogs pores and promotes aggregation and sintering during the oxidation of B, thereby delaying the process and reducing its energy output. − ,− …”

“…Metal-based energetic materials are quite attractive due to their ability to store a significant amount of chemical energy that can be released by oxidation and can be used in several applications, including batteries, fuel cells, rocket propellants, and solid ramjet fuels. − The combination of metals such as aluminum and magnesium with boron in the form of their borides can be very effective because of the high energy density of boron and the rich chemistry and chemically tunable properties of such materials. In March 2001, Cava, in his article “Genie in a bottle,” quoted“an overlooked compound (MgB 2 ) has a surprise in store for the physicists” after the discovery of the superconducting nature of MgB 2 by Akimitsu and colleagues that could solve the mysteries of high-temperature superconducting materials.…”