Iron oxide-based porous solids were prepared by a sol−gel process using Fe(III) salts in various solvents. It was observed that the addition of propylene oxide to Fe(III) solutions resulted in the formation of transparent red-brown monolithic gels. The resulting gels were converted to either xerogels by atmospheric drying or aerogels by supercritical extraction with CO2(l). Some of the dried materials were characterized by nitrogen adsorption and desorption analysis and transmission electron microscopy (TEM). The results of those analyses indicate that the materials have high surface areas (∼300−400 m2/g), pore sizes with mesoporic dimensions (2−23 nm), and a microstructure made up of 5−10 nm diameter clusters of iron(III) oxide. The dependence of both gel formation and its rate was studied by varying the epoxide/Fe(III) ratio, the Fe(III) precursor salt, amount of water (H2O/Fe(III)) present, and the solvent employed. All of these variables were shown to affect the rate of gel formation and provide a convenient control of this parameter. Finally, an investigation of the mechanism of Fe2O3 gel formation was performed. Both pH and nuclear magnetic resonance (NMR) studies suggest that the added epoxide acts as an irreversible proton scavenger that induces the Fe(III) species to undergo hydrolysis and condensation to form an inorganic iron oxide framework. This method can be extended to prepare other transition- and main-group metal oxide materials.

Alumina aerogels were prepared through the addition of propylene oxide to aqueous or ethanolic solutions of hydrated aluminum salts, AlCl 3 •6H 2 O or Al(NO 3) 3 •9H 2 O, followed by drying with supercritical CO 2. This technique affords low-density (60-130 kg/m 3), high surface area (600-700 m 2 /g) alumina aerogel monoliths without the use of alkoxide precursors. The dried alumina aerogels were characterized using elemental analysis, highresolution transmission electron microscopy, powder X-ray diffraction, solid state NMR, acoustic measurements and nitrogen adsorption/desorption analysis. Powder X-ray diffraction and TEM analysis indicated that the aerogel prepared from hydrated AlCl 3 in water or ethanol possessed microstructures containing highly reticulated networks of pseudoboehmite fibers, 2-5 nm in diameter and of varying lengths, while the aerogels prepared from hydrated Al(NO 3) 3 in ethanol were amorphous with microstructures comprised of interconnected spherical particles with diameters in the 5-15 nm range. The difference in microstructure resulted in each type of aerogel displaying distinct physical and mechanical properties. In particular, the alumina aerogels with the weblike microstructure were far more mechanically robust than those with the colloidal network, based on acoustic measurements. Both types of alumina aerogels can be transformed to γ-Al 2 O 3 through calcination at 800 o C without a significant loss in surface area or monolithicity.

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The fundamental differences between energetic composites and energetic materials made from a monomolecular approach are the energy density attainable and the energy release rates. For the past 4 years, we have been exploiting sol-gel chemistry as a route to process energetic materials on a microstructural scale. At the last ISA conference, we described four specific sol-gel approaches to fabricating energetic materials and presented our early work and results on two methodssolution crystallization and powder addition. Here, we detail our work on a third approach, energetic nanocomposites. Synthesis of thermitic types of energetic nanocomposites are presented using transition and main group metal-oxide skeletons. Results on characterization of structure and performance will also be given.

The synthesis of iron(III) oxide aerogel monoliths was performed by adding any one of several different 1,2- and 1,3-epoxides to ethanolic Fe(III) salt solutions at room temperature. While all of the epoxides examined resulted in gel formation, robust low-density (∼30−40 kg/m3; 99% porous), high-surface-area (∼250−300 m2/g), aerogel monoliths were prepared by the addition of 1,3-epoxide derivatives to solutions of FeCl3·6H2O, followed by drying with supercritical CO2. Both types of iron(III) oxide aerogels (those made with 1,2- and 1,3-epoxides respectively) were characterized using elemental analysis, X-ray diffraction, thermal analysis, acoustic measurements, transmission electron microscopy, scanning electron microscopy, and N2 adsorption desorption analysis. Elemental analyses and powder X-ray diffraction indicated that the strong aerogel monoliths made with the 1,3-epoxides are made up predominately of polycrystalline β-FeOOH, akaganeite, and those made with the 1,2-epoxides are amorphous. To our knowledge, this is first known report of synthesis and characterization of akaganeite aerogel materials. Transmission electron microscopy analysis indicates that aerogels derived using 1,3-epoxides have a microstructure made up of a highly reticulated network of fibers with diameters from ∼5 to 35 nm and lengths several times that, whereas those resulting from the use of 1,2-epoxides consist of interconnected spherical particles, whose diameters are 5−15 nm. The difference in microstructure results in each type of aerogel displaying very distinct physical and mechanical properties. In particular, the stiffness of the β-FeOOH aerogels is remarkable for a transition metal oxide aerogel. Monolithic cylinders of β-FeOOH aerogel can be sintered at 515 °C, transforming to α-Fe2O3 without shattering.