Functionally Coated Faceted Aluminum Nanocrystals: Aerosol Synthesis and Reactivity
Kaplowitz, Daniel Alan
Zachariah, Michael R
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The demand for large scale manufacture of nanoaluminum for use in propellant applications has motivated research into development of an aerosol production scheme. In addition, the reactive nature of aluminum in the presence of oxygen has inspired investigation into functionally coating bare nanoaluminum prior to exposure to the atmosphere. Faceted aluminum crystals are fabricated in the aerosol phase via thermal pyrolysis of triisobutylaluminum, a low temperature gas-phase synthesis route, and combustion tests of oxygen passivated product in thermite combination show an increase in energy release compared to commercial nanoaluminum. Three different coatings on this bare nanoaluminum are developed: a decoration of Ni/Ni<sub>2</sub>O<sub>3</sub> particles by thermal decomposition of Ni(CO)<sub>4</sub>, a homogeneous layer of Fe<sub>3</sub>O<sub>4</sub> by thermal decomposition of Fe(CO)<sub>5</sub>, and a monolayer of perfluoropentanoic acid via bridge bonding between aluminum and carboxylate groups. X-ray photoelectron spectroscopy analysis indicates that the metal oxide coatings have facilitated formation of an expanded aluminum oxide layer during an air bleed, but perfluoropentanoic acid has successfully passivated aluminum. The protection from significant oxide formation for the perfluoropentanoic acid coating is evident in a 16% increase in active fuel content by thermogravimetric analysis compared to the untreated case. Subsequent temperature jump fine wire combustion tests show decreased ignition temperatures for all three coatings. Combustion chamber tests in thermite combinations display poor pressure output for the Ni/Ni<sub>2</sub>O<sub>3</sub> coated case, but reasonable response for the Fe<sub>3</sub>O<sub>4</sub> product. Flame ignition of perfluoropentanoic acid coated product is shown to produce AlF<sub>3</sub> by chemical analysis of char, indicating the passivation coating also functions in direct oxidizer delivery.