Templated Synthesis of Aluminum Nanoparticles for Stable Energetic Materials

Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio

Reactive nanoparticles as energetic materials have received much recent attention for a variety of existing and/or potential applications. Among more extensively investigated are nanosized (sub-100-nm) aluminum (Al) particles. Their large specific surface area and energy density, when coupled or mixed with oxidative species, make them unique combustible additives in propellant formulations. Nanoscale Al particles are also studied as high-capacity hydrogen storage materials. Therefore, significant effort has been made in the development of synthetic methodologies for Al nanoparticles of desired properties.

Transmission electron microscopy (TEM) images of the Al-in-Nafion Film prepared with alane solution concentration of 0.5 M.

The chemical route based on thermal and/or catalytic decomposition of alane in the presence of a surface passivation agent for particle protection and stabilization has been identified as being particularly promising. The passivation agent for Al nanoparticles could be a metal coating or organic molecules such as perfluorinated carboxylic acids, which could also serve as an oxidant source under energetic conditions. This method has generally yielded Al particles of 50-200 nm in average size, though smaller particles have been obtained recently in a sonochemical environment with oleic acid as the surface passivation agent. Ideally, very small Al nanoparticles (thus, an extremely high surface area) of a narrow size distribution are desired for their distinctive advantages in energetic materials or for more effective hydrogen generation, but their bulk production in a consistent fashion and their protection for stability under ambient conditions present special challenges.

Nanoscale cavities in perfluorinated ionomer membrane were used as templates for the facile synthesis of small Al nanoparticles (diameters on the order of 10 nm) via catalytic decomposition of an alane precursor. While hosted in the cavities, for which the perfluorinated membrane structures should phenomenologically serve the same passivation function and also as a source of oxidant under energetic conditions, the Al nanoparticles were found to be mostly stable in ambient air. The effective hydrogen generation by the nanoparticles was used to determine the reactive Al content in the Al-in-membrane composite.

The results demonstrated that the structural cavities in ionomer membrane films could serve as ideal templates for facile production of well-dispersed small Al nanoparticles. The membrane structure could apparently protect the embedded Al nanoparticles from any significant oxidation, which made the reactive Al-in-Nafion composite films stable in ambient air. On the other hand, the Al nanoparticles could be used to produce hydrogen from water in a nearly quantitative fashion. The ability to incorporate a larger amount of reactive Al into Nafion membrane (up to more than 50% by weight in the resulting composite film) is fundamentally interesting and potentially technologically valuable, though a clear understanding of the structures in these composite films of very high Al loadings requires more investigation. The templated synthesis may represent a new route for stable Al nanoparticles and related energetic nanomaterials.

This work was done by Heting Li, Fushen Lu, and Ya-Ping Sun of Clemson University; Christopher E. Bunker of the Air Force Research Laboratory; and Elena A. Guliants of the University of Dayton Research Institute. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Materials category. AFRL-0146