Fluorocarbon-Capped Aluminum Nanoparticles: Synthesis, Characterization, Combustion, and Comparison to Hydrocarbon-Capped Aluminum Nanoparticles

Abstract

ABSTRACT Aluminum nanoparticles (NPs) are attractive as fuel additives in propulsion or pyrotechnic applications, but their combustion is inhibited by native oxide layer formation, which also reduces the energy content. Capping Al NPs with fluorocarbon ligands may inhibit native oxide formation and create an intimate mixture of fuel with an oxidizer capable of reacting exothermically with both Al and the passivating oxide layer. We present an approach to producing Al NPs capped by perfluorotetradecanoic acid (PFTDA), based on rapid mechanical attrition of NPs followed by PFTDA capping under mild conditions. The materials were characterized by dynamic light scattering, electron microscopy with energy dispersive spectroscopy, elemental analysis, X-ray photoelectron spectroscopy, X-ray diffraction, and infrared spectroscopy. Ignition and combustion properties were studied using bomb calorimetry, thermogravimetric analysis with differential scanning calorimetry, and photographic analysis of both flame and laser ignition of particle samples. For comparison, Al NPs were produced identically but capped with a hydrocarbon (oleic acid). PFTDA-capped NPs were found to ignite at lower laser power and burn far more vigorously than oleic acid-capped NPs, despite the observation that oleic acid capping resulted in a thinner native oxide layer on the air-exposed NPs. PFTDA-capped NPs were also found to have shorter ignition delay and lower ignition energy compared to commercially available Al NPs. XPS and XRD were used to probe the combustion products for the PFTDA-capped NPs, confirming formation of AlF3 and other products. Density functional theory was used to calculate the heat of formation of PFTDA, which was then used to calculate reaction enthalpies for reactions in both inert and oxidizing environments.