Enhanced Energetic Performance of Aluminum Nanoparticles by Plasma Deposition of Perfluorinated Nanofilms

Morphology evolution of the aluminum surface in a fluorine-containing environment

The interaction between aluminum (Al) and F and O atoms is essential to understand the etching process of Al and alumina (Al2O3) by fluorine-containing gases. In addition, it also has an influence on the corrosion behavior of Al devices, e.g., the Al collector in lithium-ion batteries operates in fluorine-containing electrolytes. However, the understanding of the structural evolution of the Al surface by fluorination at the atomistic level still remains elusive. Here, the thermodynamic and kinetic behaviors of F adatoms as well as co-adsorbed F and O adatoms on typical Al surfaces have been systematically investigated by combining density functional theory (DFT) calculations, canonical Monte Carlo (CMC) simulations and reactive molecular dynamics (RMD) simulations. The results of DFT calculations indicate that there is a repulsion (about 0.07 eV on Al(111) and Al(110), and 0.7 eV on Al(100)) between the first nearest neighboring (1NN) F adatoms, while an attraction of 0.14 eV on Al(111) exists within a 1NN F-O pair. CMC simulations reveal that the configurations of co-adsorbed F and O adatoms on the Al(111) surface at medium to low temperature (<600 K) and low total coverage (<0.2 monolayer, ML) have F adatoms dispersed in the hexagonal islands of O adatoms due to the attraction within the O-O and F-O pairs and the repulsion between F adatoms. As the coverage increases, the surface undergoes serious deformation. The average 1NN coordination numbers (1st CN) of O-to-O, F-to-O and F-to-F are six, three and zero, respectively. As the temperature increases, the interactions among adsorbates begin to be disrupted: the 1st CNs of O-to-O and F-to-O decrease, while that of F-to-F increases. The O-F hexagonal pattern remains until above the Al melting temperature (>1200 K). For F adatoms, both their migration on the surface and the penetration into the subsurface are easier than those of O adatoms, confirmed by both the DFT and RMD simulations. Our study on the co-adsorbates with opposite lateral interactions is instructive for understanding the thermal etching of Al and Al2O3 by fluorine-containing compounds.

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. This cycle can be repeated as often as desired. The estimated energy penalty of the electrochemical activation process is only ∼4% of the total energy release. This approach presents a paradigm shift, offering the potential to make a "safe fuel" or alternatively a simple electrochemically driven fuel metering scheme.

Engineered Surface Chemistry and Enhanced Energetic Performance of Aluminum Nanoparticles by Nonthermal Hydrogen Plasma Treatment

Extracting the maximum chemical energy from aluminum nanoparticles (Al NPs) during oxidation is essential for their use in energetic applications. However, the shell of native Al₂O₃ limits the release of chemical energy by acting as a diffusion barrier and dead weight. Engineering the surface properties of Al NPs by modifying their shell chemistry can reduce the inhibiting effects of the oxide shell on the rate and heat release of oxidation. Here, we employ nonthermal hydrogen plasma at high power and a short time to alter the shell chemistry by doping it with Al-H, as examined and confirmed by HRTEM, FTIR, and XPS. Thermal analysis (TGA/DSC) shows that Al NPs with modified surfaces exhibit augmented oxidation and heat release (33% higher than those of untreated Al NPs). The results demonstrate the promising effect of nonthermal hydrogen plasma in engineering the shell chemistry of Al NPs to improve their overall energetic performance during oxidation.