The controllable incorporation of multiple immiscible elements into a single nanoparticle merits untold scientific and technological potential, yet remains a challenge using conventional synthetic techniques. We present a general route for alloying up to eight dissimilar elements into single-phase solid-solution nanoparticles, referred to as high-entropy-alloy nanoparticles (HEA-NPs), by thermally shocking precursor metal salt mixtures loaded onto carbon supports [temperature ~2000 kelvin (K), 55-millisecond duration, rate of ~10 K per second]. We synthesized a wide range of multicomponent nanoparticles with a desired chemistry (composition), size, and phase (solid solution, phase-separated) by controlling the carbothermal shock (CTS) parameters (substrate, temperature, shock duration, and heating/cooling rate). To prove utility, we synthesized quinary HEA-NPs as ammonia oxidation catalysts with ~100% conversion and >99% nitrogen oxide selectivity over prolonged operations.

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Reactive nanolaminates are a high-energy-density configuration for energetics that have been widely studied for their tunable energy release rates. In this study, we characterized Al/CuO nanolaminate reactions with different fuel/oxidizer ratios and bilayer thicknesses using both macro- and microscale high-speed imaging/pyrometry. Under microscopic imaging, we observe significant corrugation (the ratio of the total geometrical length of the flame to the width of the sample in the direction perpendicular to propagation) of the flame, which can increase the reaction surface area by as much as a factor of 3. This in turn manifests itself as an increase in the global burn rate (total nanolaminate film length/total burn time). We find that the global burn rate can be predicted as the product of the microburn rate (local vector burn rate at the microscopic scale) and the corrugation. These corrugation effects primarily impact fuel-rich conditions, resulting in higher global burn rates. We find that the reaction zone has a thickness of ∼150 μm. Finally, we present a 3D rendering of what we believe the reaction zone looks like, based on the results from in-operando observation and SEM cross-sectional imaging.

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The reaction mechanism and ignition characteristics of the pyrotechnic composite of titanium nanoparticles and micron-sized potassium perchlorate was investigated under rapid heating conditions (∼5 × 105 K/s) by temperature jump (T-Jump) time-of-flight mass spectrometry. X-ray photoelectron spectroscopy surface analysis and transmission electron microscopy (TEM) characterization of titanium nanoparticles show a reactive oxide layer (∼6 nm) composed of amorphous TiO2 and roughly 20% crystalline TiN and titanium oxynitride. The T-Jump and thermogravimetric analysis reveals the oxide layer to be responsible for catalysis of oxygen release from KClO4, resulting in ignition temperatures as low as 720 K in atmospheric pressure argon. Fast and slow in situ heating TEM corroborate the findings of oxygen atmosphere ignition characteristics, which illustrate KClO4 melting and coating of titanium nanoparticles immediately before oxidizer decomposition and titanium oxidation. Unlike aluminum, which has been shown to have a rapid loss of surface area before combustion as a result of sintering, Ti retained its high surface area. A combination of a reactive shell and the preservation of titanium nanostructure under rapid heating may lead to enhanced oxygen diffusion and increased potential for transient energy release.

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A major challenge in formulating and manufacturing energetic materials lies in the balance between total energy density, energy release rate, and mechanical integrity. In this work, carbon fibers are embedded into ∼90 wt % loading Al/CuO nanothermite sticks through a simple extrusion direct writing technique. With only ∼2.5 wt % carbon fiber addition, the burn rate and heat flux were promoted >2×. In situ microscopic observation of combustion shows that the carbon fiber intercept ejected hot agglomerates near the burning surface and enhanced heat feedback to the unreacted material. This study outlines how these approaches may enhance the propagation and reduce the two-phase flow losses.

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