Role of Surface Tension on Heat Feedback and Power from Energetic Composites.

Heat feedback to the unburned reaction interface is an important controlling factor of the velocity of the reaction front and power delivery. In this paper, we investigate the effect of agglomerate surface tension and its relationship to surface residence time and heat feedback on the combustion characteristics by Si addition to an Al/KClO composite. Macroscopic imaging demonstrates a significant increase in burn rate with the addition of Si despite the fact that Si/KClO has a slightly lower energy density than Al/KClO.

Controlled Release of Diborane from Alkali Metal Borohydride using Ionic Liquid-Based Lewis Acids.

Alkali metal borohydrides present a rich source of energy dense materials of boron and hydrogen, however their potential in propellants has been hitherto untapped. Potassium borohydride is a promising fuel with high gravimetric energy density and relatively low sensitivity to air and moisture. Problems arise due to the dehydrogenation of the borohydride on heating with minimal energy release.

Enhancing the Combustion of Magnesium Nanoparticles via Low-Temperature Plasma-Induced Hydrogenation.

The hydrogenation of metal nanoparticles provides a pathway toward tuning their combustion characteristics. Metal hydrides have been employed as solid-fuel additives for rocket propellants, pyrotechnics, and explosives. Gas generation during combustion is beneficial to prevent aggregation and sintering of particles, enabling a more complete fuel utilization.

Magnesium-Induced Strain and Immobilized Radical Generation on the Boron Oxide Surface Enhances the Oxidation Rate of Boron Particles: A DFTB-MD Study.

Despite their high gravimetric and volumetric energy densities, boron (B) particles suffer from poor oxidative energy release rates as the boron oxide (BO) shell impedes the diffusivity of O to the particle interior. Recent experiemental studies have shown that the addition of metals with a lower free energy of oxidation, such as Mg, can reduce the oxide shell of B and enhance the energetic performance of B by ∼30-60%. However, the exact underlying mechanism behind the reactivity enhancement is unknown.

Electrochemical Modulation of the Flammability of Ionic Liquid Fuels.

Flammability and combustion of high energy density liquid propellants are controlled by their volatility. We demonstrate a new concept through which the volatility of a high energy density ionic liquid propellant can be dynamically manipulated enabling one to (a) store a thermally insensitive oxidation resistant nonflammable fuel, (b) generate flammable vapor phase species electrochemically by applying a direct-current voltage bias, and (c) extinguish its flame by removing the voltage bias, which stops its volatilization. We show that a thermally stable imidazolium-based energy dense ionic liquid can be made flammable or nonflammable simply by application or withdrawal of a direct-current bias.

Enhanced Decomposition of Ammonium Perchlorate-Ethyl Cellulose-Molybdenum Disulfide Nanocomposites.

Ammonium perchlorate (AP) is commonly used in propulsion technology. Recent studies have demonstrated that two-dimensional (2D) nanomaterials such as graphene (Gr) and hexagonal boron nitride (hBN) dispersed with nitrocellulose (NC) can conformally coat the surface of AP particles and enhance the reactivity of AP. In this work, the effectiveness of ethyl cellulose (EC) as an alternative to NC was studied.

Microscopic Studies on the Interaction of Multi-Principal Element Nanoparticles and Bacteria.

Multi-principal element nanoparticles are an emerging class of materials with potential applications in medicine and biology. However, it is not known how such nanoparticles interact with bacteria at nanoscale. In the present work, we evaluated the interaction of multi-principal elemental alloy (FeNiCu) nanoparticles with () bacteria using the graphene liquid cell (GLC) scanning transmission electron microscopy (STEM) approach.

Perfluoroalkyl-Functionalized Graphene Oxide as a Multifunctional Additive for Promoting the Energetic Performance of Aluminum.

Aluminum (Al) is a widely used metal fuel for energetic applications ranging from space propulsion and exploration, and materials processing, to power generation for nano- and microdevices due to its high energy density and earth abundance. Recently, the ignition and combustion performance of Al particles were found to be improved by graphene-based additives, such as graphene oxide (GO) and graphene fluoride (GF), as their reactions provide heat to accelerate Al oxidation, gas to reduce particle agglomeration, and fluorine-containing species to remove AlO. However, GF is not only expensive but also hydrophobic with poor mixing compatibility with Al particles.

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

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)(NO)/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.

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

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.

High-Temperature Interactions of Metal Oxides and a PVDF Binder.

Interactions between energetic material relevant nanoscale metal oxides (SiO, TiO, MgO, AlO, CuO, BiO) and poly(vinylidene fluoride) (PVDF) at high temperature were investigated by temperature-jump/time-of-flight mass spectrometry (T-jump/TOFMS) and thermogravimetric-differential scanning calorimetry (TGA-DSC). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the morphology of the compositions, while X-ray diffraction (XRD) was utilized to analyze the condensed phase crystalline species at temperatures of interest. The exergonicity and exothermicity of HF gas with hydroxyl-terminated metal oxide surfaces make HF the likely fluorine-bearing moiety and primary mode of the fluorinating reactions, where terminal OH configurations are replaced by F in the formation of a stronger metal-fluorine bond.

Understanding Dimethyl Methylphosphonate Adsorption and Decomposition on Mesoporous CeO.

The increased risk of chemical warfare agent usage around the world has intensified the search for high-surface-area materials that can strongly adsorb and actively decompose chemical warfare agents. Dimethyl methylphosphonate (DMMP) is a widely used simulant molecule in laboratory studies for the investigation of the adsorption and decomposition behavior of sarin (GB) gas. In this paper, we explore how DMMP interacts with the as-synthesized mesoporous CeO.

Carbon Fibers Enhance the Propagation of High Loading Nanothermites: In Situ Observation of Microscopic Combustion.

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.

Magnetic-Field Directed Vapor-Phase Assembly of Low Fractal Dimension Metal Nanostructures: Experiment and Theory.

While gas-phase synthesis techniques offer a scalable approach to production of metal nanoparticles, directed assembly is challenging due to fast particle diffusion rates that lead to random Brownian aggregation. This work explores an electromagnetic-levitation technique to generate metal nanoparticle aggregates with fractal dimension () below that of diffusion limited assembly. We demonstrate that in addition to levitation and induction heating, the external magnetic field is sufficient to compete with random Brownian forces, which enables the formation of altered fractals.

Modelling and simulation of field directed linear assembly of aerosol particles.

Unlike liquid phase colloidal assembly, significantly changing the structure of fractal aggregates in the aerosol phase, is considered impractical. In this study, we discuss the possibility of applying external magnetic and electric fields, to tune the structure and fractal dimension (D) of aggregates grown in the aerosol phase. We show that external fields can be used to induce dipole moments in primary nanoparticles.

Elucidating the dominant mechanisms in burn rate increase of thermite nanolaminates incorporating nanoparticle inclusions.

It was experimentally found that silica and gold particles can modify the combustion properties of nanothermites but the exact role of the thermal properties of these additives on the propagating combustion front relative to other potential contributions remains unknown. Gold and silica particles of different sizes and volume loadings were added into aluminum/copper oxide thermites. Their effects on the flame front dynamics were investigated experimentally using microscopic dynamic imaging techniques and theoretically via a reaction model coupling mass and heat diffusion processes.

Revealing High-Temperature Reduction Dynamics of High-Entropy Alloy Nanoparticles Transmission Electron Microscopy.

Understanding the behavior of high-entropy alloy (HEA) materials under hydrogen (H) environment is of utmost importance for their promising applications in structural materials, catalysis, and energy-related reactions. Herein, the reduction behavior of oxidized FeCoNiCuPt HEA nanoparticles (NPs) in atmospheric pressure H environment was investigated by gas-cell transmission electron microscopy (TEM). The reduction reaction front was maintained at the external surface of the oxide.

Silicon Nanoparticles for the Reactivity and Energetic Density Enhancement of Energetic-Biocidal Mesoparticle Composites.

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(IO) energetic-biocidal composites by filling in the voids in the microstructure.

Oxidation Studies of High-Entropy Alloy Nanoparticles.

Although high-entropy alloys (HEAs) have shown tremendous potential for elevated temperature, anticorrosion, and catalysis applications, little is known on how HEA materials behave under complex service environments. Herein, we studied the high-temperature oxidation behavior of FeCoNiCuPtHEA nanoparticles (NPs) in an atmospheric pressure dry air environment by gas-cell transmission electron microscopy. It is found that the oxidation of HEA NPs is governed by Kirkendall effects with logarithmic oxidation rates rather than parabolic as predicted by Wagner's theory.

High-Temperature Pulse Method for Nanoparticle Redispersion.

Nanoparticles suffer from aggregation and poisoning issues (e.g., oxidation) that severely hinder their long-term applications.

Aerosol Synthesis of High Entropy Alloy Nanoparticles.

Homogeneously mixing multiple metal elements within a single particle may offer new material property functionalities. High entropy alloys (HEAs), nominally defined as structures containing five or more well-mixed metal elements, are being explored at the nanoscale, but the scale-up to enable their industrial application is an extremely challenging problem. Here, we report an aerosol droplet-mediated technique toward scalable synthesis of HEA nanoparticles with atomic-level mixing of immiscible metal elements.

Synergistically Chemical and Thermal Coupling between Graphene Oxide and Graphene Fluoride for Enhancing Aluminum Combustion.

Metal combustion reaction is highly exothermic and is used in energetic applications, such as propulsion, pyrotechnics, powering micro- and nano-devices, and nanomaterials synthesis. Aluminum (Al) is attracting great interest in those applications because of its high energy density, earth abundance, and low toxicity. Nevertheless, Al combustion is hard to initiate and progresses slowly and incompletely.

Quantifying protein aggregation kinetics using electrospray differential mobility analysis.

Protein aggregation is a critical concern in bioprocessing, where its presence can result in serious adverse interactions in clinical end-use applications. In this study, an aerosol-based technique, electrospray differential mobility analysis (ES-DMA), was used to quantify thermally-induced protein aggregation kinetics for bovine serum albumin (BSA) and α-chymotrypsinogen A (α-chymo), employing a new methodology to modify the solution for compatibility with the electrospray process. Results are compared orthogonally with asymmetrical-flow field-flow fractionation (AF4), a hydrodynamic separation technique with UV detection.

High temperature shockwave stabilized single atoms.

The stability of single-atom catalysts is critical for their practical applications. Although a high temperature can promote the bond formation between metal atoms and the substrate with an enhanced stability, it often causes atom agglomeration and is incompatible with many temperature-sensitive substrates. Here, we report using controllable high-temperature shockwaves to synthesize and stabilize single atoms at very high temperatures (1,500-2,000 K), achieved by a periodic on-off heating that features a short on state (55 ms) and a ten-times longer off state.

Ultrafast, Controllable Synthesis of Sub-Nano Metallic Clusters through Defect Engineering.

Supported metallic nanoclusters (NCs, < 2 nm) are of great interests in various catalytic reactions with enhanced activities and selectivities, yet it is still challenging to efficiently and controllably synthesize ultrasmall NCs with a high-dispersal density. Here we report the in situ synthesis of surfactant-free, ultrasmall, and uniform NCs via a rapid thermal shock on defective substrates. This is achieved by using high-temperature synthesis with extremely fast kinetics while limiting the synthesis time down to milliseconds (e.