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.

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Ammonia borane (NH3BH3, AB) represents a promising energy-dense material for hydrogen storage and propulsion; however, its energy release mechanisms on oxidation by solid-state oxidizers are not well understood. In this study, through in situ time-of-flight mass spectrometry supported by attenuated total reflection-Fourier transform infrared spectroscopy and density functional theory calculations, we investigate the fundamental reaction mechanisms involved in the energy release from solid-state AB with different chemical oxidizers. We show that the reaction of AB with oxidizers like KClO4 is mediated by [NH3BH2NH3]+[BH4]− (DADB) formation, resulting in its kinetic entrapment into low-energy BNH x clusters that are resistant to further oxidation, thus limiting complete energy extraction. In contrast, with an ammonium-based oxidizer such as NH4ClO4, the presence of NH4 + ions enables AB to follow an alternative reaction pathway forming [NH3BH2NH3]+[ClO4]− rather than DADB, thus inhibiting the formation of BNH x species and facilitating its complete oxidation. This alternative reaction route causes the AB/NH4ClO4 system to exhibit remarkably higher energy release rates over that of AB/KClO4 (∼27x) and the standard Al/NH4ClO4 propellant (∼7x).

Biocidal nanothermite composites show great potential in combating biological warfare threats because of their high-energy-release rates and rapid biocidal agent release. Despite their high reactivity and combustion performance, these composites suffer from low-energy density because of the voids formed due to inefficient packing of fuel and oxidizer particles. In this study, we explore the potential of plasma-synthesized ultrafine Si nanoparticles (nSi, ∼5 nm) as an energetic filler fuel to increase the energy density of Al/Ca­(IO3)2 energetic-biocidal composites by filling in the voids in the microstructure. Microscopic and elemental analyses show the partial filling of mesoparticle voids by nSi, resulting in an estimated energy density enhancement of ∼21%. In addition, constant-volume combustion cell results show that nSi addition leads to a ∼2–3-fold increase in reactivity and combustion performance, as compared to Al/Ca­(IO3)2 mesoparticles. Oxidation timescale analyses suggest that nSi addition can promote initiation due to faster oxygen transport through the oxide shell of Si nanoparticles. At nSi loadings higher than ∼8%, however, slower burning characteristics of nSi and sintering effects lead to an overall degradation of combustion behavior of the composites.

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