Synthesis and Microstructure of Porous Aluminum and Intermetallic Nanomaterials
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Synthesis and Microstructure of Porous Aluminum and Intermetallic Nanomaterials Andrew P. Purdy and Joel B. Miller Chemistry Division, Code 6120, Naval Research Laboratory, 4555 Overlook Av, SW, Washington DC 20375 ABSTRACT A series of porous aluminum-based materials are prepared by the reduction of solutions of metal chlorides with lithium powder in diethyl ether under dry argon. The reactants must be combined slowly, but either order of addition is used. The reduction of AlCl3 produces hollow Al balls composed of ~100 nm aluminum particles in nearly quantitative yield after the LiCl byproduct is washed away with dry tetrahydrofuran. Similar structures are formed when mixtures of AlCl3 and SiCl4, are reduced, except that the second component has the effect of reducing the Al nanoparticle size. Mixtures of AlCl3 with FeCl3 reduce to similar ball-like porous structures that are composed of Al, Fe, and Fe-Al intermetallic nanoparticles. When AlCl3 and ZnCl2 are co-reduced, flake-like nanoporous structures are obtained, and solutions of AlCl3 + VCl3 produce more compact nanoporous structures. Some side reactions involving ether cleavage that produces aluminum alkoxides and alkyls do occur, and the amount of side reaction is dependent on the identity of the second metal. The reduction of AlCl3 with excess (4 eq) Li powder produces LiAl nanomaterials. NMR shows the intermetallic compound LiAl to be the only Li-Al intermetallic present. INTRODUCTION Chemical synthesis of reactive metal nanomaterials is an emerging area of chemistry that has many challenges.[1-5] Aluminum, its alloys, and its intermetallic compounds are important for a number of purposes including energetic formulations, hydrogen storage, and structural alloys. Alkali metal reduction is an established method of preparing metal nanoparticles, most notably those of magnesium.[6-9] Earlier, we explored the synthesis of Al nanomaterials reduction of halides with sodium-potassium alloy.[10] While the liquid Na/K alloy is a very effective reducing agent for Al nanoparticle synthesis, it does have the drawback of by-product salts that are not very soluble in anything other than water and alcohols. As part of a broader project in reactive intermetallic nanomaterials, we have been exploring the synthesis of Al-based intermetallics by co-reduction of Al and other metal halides with lithium powder. The unique advantage of Li is the high solubility of its halides in many non-protic solvents. This work is an account of the result of lithium reduction of ether solutions of AlCl3 and several of the more common metal chlorides that are also soluble in this medium. EXPERIMENTAL All manipulations were performed in an argon filled drybox or on a vacuum line. Dry ether was obtained from Aldrich in sure-seal bottles. Tetrahydrofuran (THF) was dried over Na+OCPh2- and distilled under vacuum. The oil was washed out of a commercial lithium dispersion (Alfa) repeatedly with pentane by decanting off the bottom layer, as the lithium floats on top. The washed dispersion was then pump
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