Surface Passivation of Bare Aluminum Nanoparticles using Perfluoroalkyl Carboxylic Acids
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AA2.4.1
SURFACE PASSIVATION OF BARE ALUMINUM NANOPARTICLES USING PERFLUOROALKYL CARBOXYLIC ACIDS R. JASON JOUET*, ANDREA D. WARREN, DAVID M. ROSENBERG, AND VICTOR J. BELLITTO Naval Surface Warfare Center – Indian Head Division Research and Technology Department 101 Strauss Avenue, Indian Head, MD 20640
Abstract. Surface passivation of unpassivated Al nanoparticles has been realized using self assembled monolayers (SAMs). Nanoscale Al particles were prepared in solution by catalytic decomposition of H3Al•NMe3 or H3Al•N(Me)Pyr by Ti(OiPr)4 and coated in situ using a perfluoroalkyl carboxylic acid SAM. Because the Al particles are prepared using wet chemistry techniques and coated in solution, they are free of oxygen passivation. This SAM coating passivates the aluminum and seems to prevent the oxidation of the particles in air and renders the composite material, to some extent, soluble in polar organic solvents such as diethyl ether. Characterization data including SEM, TEM, TGA, and ATR-FTIR of prepared materials is presented. Introduction. Metallization of energetics, the addition of combustible metal powders to formulations, is a technique used to increase the energy output of explosives and propellants. The metal of choice is aluminum because of its density and the high relative heat of formation of the oxide, Al2O3. The addition of Al to a material typically requires the addition of an oxidizer, such as ammonium perchlorate (AP). The reaction of the metal with the oxidizer is an intermolecular reaction, which is to some extent diffusion limited. In attempt to reduce this diffusion limitation and increase the rate of Al oxidation – thereby contributing to the detonation pressure and velocity – smaller and smaller Al particles are being employed. There are, however, significant drawbacks associated with the use of nanoscale aluminum particles for energetics applications. Fundamentally the issue associated with all types of metallized energetic formulations is limited realization of the energy potential of the fuel. The two primary causes associated with this are incomplete Al combustion and excessive Al oxidation prior to combustion. Incomplete combustion results principally from the formation of a thick oxide coating on the particle during the combustion event. In the case of conventional micron– sized Al, the particles combust from the outside inward and eventually the oxide coating becomes so thick so as to prevent oxygen migration to the Al core, thus stopping the combustion process. This problem is somewhat mitigated with the use of nanoscale Al particles as they are small enough to combust completely before the oxide layer can prevent oxygen diffusion to the core.
AA2.4.2
Excessive oxidation prior to the combustion event is a concern when nanoscale or ultra-fine Al (UFAl) is employed. The typical oxide coating on an aluminum surface is between 1.7 and 2.2 nm. In the case of micron–sized aluminum this corresponds to approximately ≤0.5% of the total particle mass and is therefore not a significant reductio
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