In-Situ Thermochemical Shock-Induced Stress at the Metal/Oxide Interface Enhances Reactivity of Aluminum Nanoparticles

Abstract

Although aluminum (Al) nanoparticles have been widely explored as fuels in energetic applications, researchers are still exploring approaches for tuning their energy release profile via microstructural alteration. In this study, we show that a nanocomposite (∼70 nm) of a metal ammine complex, such as tetraamine copper nitrate (Cu(NH 3 ) 4 (NO 3 ) 2 /TACN), coated Al nanoparticles containing only 10 wt. % TACN, demonstrates a ∼200 K lower reaction initiation temperature coupled with an order of magnitude enhancement in the reaction rate. Through time/temperature-resolved mass spectrometry and ignition onset measurements at high heating rates, we show that the ignition occurs due to a condensed phase reaction between Al and copper oxide (CuO) crystallized on TACN decomposition. TEM and XRD analyses on the nanoparticles at an intermediate stage show that the rapid heat release from TACN decomposition in-situ enhances the strain on the Al core with induction of nonuniformities in the thickness of its AlO x shell. The thinner region of the nonuniform shell enables rapid mass transfer of Al ions to the crystallized CuO, enabling their condensed phase ignition. Hence, the thermochemical shock from TACN coating induces stresses at the Al/AlO x interface, which effectively switches the usual gas phase O 2 diffusion-limited ignition process of Al nanoparticles to become condensed phase Al ion transfer controlled, thereby enhancing their reactivity.

Pioneering insights into the superior performance of titanium as a fuel in energetic materials

“…For example, Williams and Pantoya [14] modified the Al particles reactivity by modifying the Al-Al2O3 core-shell microstructure through annealing and quenching of the powder, to vary interfacial strain and associated stress. Correlation between altering the Al microstructures by stress and lowering ignition point was also observed in [15,16]. It was also explored in multilayered Al/CuO thermite by Julien and Rossi [17] to tune the ignition point of micro-initiation chip.…”

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. − …”

“…To overcome these problems, one approach is to produce oxide-free Al NPs immediately followed by surface functionalization. This method is unsuitable for large-scale production due to the high reactivity of bare Al NPs, which can cause explosions and therefore has safety issues involved. , Alternatively, the native oxide may be removed after particle synthesis by reduction, followed by in situ surface passivation before exposure to air using secondary coatings that provide stability and extended shelf life to Al NPs. − Coatings with nonenergetic components, such as silanes, carboxylic acids, and glycols, inhibit the growth of native oxide by providing passivation, but their contribution to the heat released during oxidation is negligible. − They generally result in a decrease of the overall energy density due to the significant mass loadings (20–40%) required to provide adequate passivation. − Another category constitutes energetic components, such as metals (Fe, Ni) and nitrocellulose, that contribute to the heat release via intermetallic or thermite reactions between the coatings, the oxide, and the metal core. ,,− However, the energy contribution is relatively small because the energy density of these additives is low compared to that of Al.…”

Catalytic Cleavage of the Dative Bond of Ammonia Borane by Polymeric Carbonyl Groups for Enhanced Energy Generation

“…Chemical hydrides such as ammonia borane (AB/NH 3 BH 3 ) have been widely explored as hydrogen storage mediums for low-temperature energy conversion in fuel cells. − On the other hand, limited efforts have been made − on investigating the energy generation capability of these compounds for propulsion systems, which demand extremely high-energy release rates. − Elemental metals and metalloids, particularly boron (B), possess high oxidative energy density and are, therefore, attractive as fuel components in solid-state energy dense materials for propulsion systems . During the oxidation process of B nanoparticles, a molten oxide shell grows from the surface to the core of the particle, while core B is still in the solid state. , The liquid oxide shell presents a significant barrier to oxygen diffusion, which severely limits the reactivity of particulate B .…”