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Reactions of (CH 3 O) 4 Si and RSi(OCH 3 ) 3 (R = CH 3 , C 2 H 5 , C 3 H 7 , C 4 H 9 , C 16 H 33 ) at a mole ratio of 1 to 1 were performed in methanol using base catalysis. Subsequent supercritical drying produced materials that have a wide range of surface areas and porosities. Spectroscopic characterization, revealed normal Si–O substitution as well as incorporation of the carbon chain into the Si framework. Heating of the stable forms of the materials in air at different temperatures yielded, depending upon oxidation conditions, several materials with much higher surface areas, typical of aerogels. Pore-size distribution measurements revealed mesopore features that differed from the non-oxidized materials for some R groups. Spectroscopic characterization revealed the disappearance of the R–Si substitution for all R groups and the appearance of an oxidized intermediate for R = C 16 H 33 .

The use of the R-substituted organosiloxanes in hydrolysis-condensation reactions with tetraalkoxysilanes is a flexible way to incorporate functional groups into silica gels and aerogels. When R = CH2CH2CF3, chemically and physically stable gels and aerogels that are hydrophobic can be readily made. These types of aerogels have been shown to be adsorbents for organics compounds from aqueous phase. In order to understand the role of the fluorinated group in making these materials hydrophobic, various formulations of CF3CH2CH2-modified aerogels have been synthesized and characterized. These materials were formed by the reaction of CF3CH2CH2Si(OCH3)3 and Si(OCH3)4 in CH3OH with NH4OH as the catalyst followed by supercritical drying in CO2. A 0.7 mole ratio of the reactants CF3CH2CH2Si(OCH3)3/Si(OCH3)4 is necessary to make hydrophobic aerogels that meet the 2.5% water adsorption limit. Lower reactant mole ratios yield materials that adsorb more water, reaching a saturation value in roughly 1 month, where the amount adsorbed is inversely proportional to the amount of CF3CH2CH2 group incorporated.

The hexadecyl substituted siloxane, C16H33Si(OCH3)3, was reacted with (CH3O)4Si at the mole ratio of 1–3 in methanol using HBF4 catalysis. Sol–gel materials were formed that have low surface areas (∼10 m2/g). Subsequent supercritical drying using CO2 at 40 °C produced materials that are very different than traditional aerogels, with surface areas around the same values as the corresponding sol–gels, as well as no detectable meso-pore features. In some cases, the aerogels even melted upon heating. Spectroscopic characterization, using IR, 29Si and 13C NMR revealed normal Si–O substitution as well as incorporation of the carbon substituent into the Si framework. Heating of the stable forms of the materials in air at different temperatures yielded, depending upon oxidation conditions, several materials with much higher surface areas, typical of aerogels. Pore size distribution measurements revealed meso-pore features with a narrow distribution of 37–39 A. Spectroscopic characterization revealed the disappearance of the R-Si substitution and the appearance of an oxidized intermediate. This paper describes the chemistry and characterization of these unusual sol–gels, aerogels, and oxidation products.

We have utilized a sol–gel synthetic approach in preparing nano-sized transition metal oxide components for new energetic nanocomposites. Nanocomposites of Fe 2 O 3 /Al(s), are readily produced from a solution of Fe(III) salt by adding an organic epoxide and a powder of the fuel metal. These materials can be processed to aerogel or xerogel monolithic composite solids. High resolution transmission electron microscopy (HRTEM) of the dried energetic nanocomposites reveal that the metal oxide component consists of small (3–10 nm) clusters of Fe 2 O 3 that are in intimate contact with ultra fine grain (UFG) ∼25 nm diameter Al metal particles. HRTEM results also indicate that the Al particles have an oxide coating ∼5 nm thick. This value agrees well with analysis of pristine UFG Al powder and indicates that the sol–gel synthetic method and processing does not significantly perturb the fuel metal. Both qualitative and quantitative characterization has shown that these materials are indeed energetic. The materials described here are relatively insensitive to standard impact, spark, and friction tests, results of which will be presented. Qualitatively, it does appear that these energetic nanocomposites burn faster and are more sensitive to thermal ignition than their conventional counterparts and that aerogel materials are more sensitive to ignition than xerogels. We believe that the sol–gel method will at the very least provide processing advantages over conventional methods in the areas of cost, purity, homogeneity, and safety and potentially yield energetic materials with interesting and special properties.

We have developed a new sol–gel route to synthesize several different transition and main-group metal oxide aerogels. The approach is straightforward, inexpensive, versatile, and it produces monolithic microporous materials with high surface areas. Specifically, we report the use of epoxides as gelation agents for the sol–gel synthesis of chromia aerogels and xerogels from simple Cr(III) inorganic salts. The dependence of both gel formation and its rate was studied by varying the solvent used, the Cr(III) precursor salt, the epoxide/Cr(III) ratio, as well as the type of epoxide employed. All of these variables were shown to affect the rate of gel formation and provide a convenient control of this parameter. Dried chromia aerogels were characterized by high-resolution transmission electron microscopy (HRTEM) and nitrogen adsorption/desorption analyses, results of which will be presented. The results presented here show that rigid monolithic metal oxide aerogels can be prepared from solutions of their respective metal ion salts (Fe3+, Al3+, In3+, Ga3+, Zr4+, Hf4+, Ta5+, Nb5+, and W6+), provided the formal oxidation state of the metal ion is greater than or equal to +3. Conversely, when di-valent transition metal salts are used precipitated solids are the products.

Traditional manufacturing of energetic materials involves processing of granular solids. One application is the production of detonators where powders of energetic material and a binder are typically mixed and compacted at high pressure to make pellets. Performance properties are strongly dependent on particle size distribution, surface area of its constituents, homogeneity of the mix, and void volume. The goal is to produce detonators with fast energy release rate that are insensitive to unintended initiation. Preparation of detonators from xerogel molding powders and aerogels, and comparison with materials produced by state-of-the-art technology are described.

Aqueous polycondensation of resorcinol with formaldehyde leads to organic gels the structure of which can be controlled by the reaction parameters. The amount of catalyst controls the size of the particles constituting the gel network. We could show that for very low catalyst concentrations in a high dilution of reactants, the particle growth can be further enhanced if the reaction temperature is kept low, and the gel time is prolonged. The resultant structure is largely affected by this treatment, producing particles with sizes of about 2 μm and average pore sizes of up to 7 μm. Due to these coarse structures it is possible to dry these RF gels subcritically with very little shrinkage. The structure has been studied by nitrogen sorption, small angle X-ray scattering and acoustic sound propagation. The change of the elastic modulus caused by pyrolysis at around 1000°C has been investigated. Mechanical properties of carbon aerogels are correlated with their electrical properties. The derived carbon aerogels have large specific surface areas, very little mesopore volume however, micron-sized macropores.

High resolution transmission electron microscopy (HRTEM) was used to study the morphology of ultralow-density transparent condensed-silica (CS) aerogels. Silica aerogels were synthesized by two slightly different two-step polymerization processes and they were supercritically dried in a high temperature autoclave. Aerogel CS1 had a density of 9 kg/m3 and a BET surface area of 574 m2/g; aerogel CS2 had a density of 30 kg/m3 and a specific surface area of 630 m2/g. Both samples were fractured, vertically replicated with 0.95 nm Pt-C and backed with approximately 12 nm of rotary evaporated carbon. The silica aerogel was then removed from the replica with dilute HF acid and the replicas were studied by HRTEM. The stereoscopic HRTEM images reveal that connectors in both CS aerogels are extended filaments which resemble bottlebrushes, having microporosity. This morphology results from side-chain formation on a nearly linear CS stem. The slightly different chemistry leads to different morphologies for the two aerogels. For CS1, the connectors between stems have diameters ranging from 1.7 to 14.2 nm with an average of 6.4 ± 0.5 nm and connector lengths averaged 62 ± 21 nm with some as long as 132 nm. Pore sizes ranged from 13 to 240 nm with an average of 74 ± 43 nm. The pores were slightly larger than those in CS2 which ranged from 12 to 277 nm and averaged 61 ± 56 nm. For CS2, the connectors had diameters ranging from 1.5 to 16.5 nm and averaging 9.7 ± 0.5 nm. The connector lengths in the CS2 aerogel averaged 58 ± 27 nm with some as long as 127 nm. The connector diameters in CS2 (9.7 ± 0.5 nm) were the only important aerogel feature significantly different and greater than those in CS1 (6.4 ± 0.5 nm). The CS1 and CS2 connectors had side-chain diameters of 2 ± 0.7 and 0.95 ± 0.5 nm, respectively, and a similar microporosity of approximately 2.0 ± 1 nm.

Commercialization of aerogels has been slow due to several factors including cost and manufacturability issues. The technology itself is well enough developed as a result of extensive work over the past decade by an international community of researchers. Several substantial markets appear to exist for aerogels as thermal and sound insulators, if production costs can keep prices in line with competing established materials. In the present paper, the elements identified as key cost drivers are discussed, and a prognosis for the evolution of the technology leading to reduced cost aerogel production is given.

Vanadium/silica and copper/silica aerogels have been prepared using the sol-gel method followed by CO{sub 2} exchange and supercritical extraction. Structural properties of samples supercritically dried, oxidized and used in reactions studies conducted with a feed representing the average composition of automobile exhaust from a lean burn engine were investigated using laser Raman spectroscopy and temperature-programmed reduction. No evidence of crystalline V{sub 2}O{sub 5} was found for the vanadium/silica aerogel, freshly extracted, oxidized or following exposure to reaction conditions using these techniques. However, results obtained for the copper/silica sample indicate that changes in the structure of the copper species had occurred as the sample was oxidized and exposed to reaction conditions.

Sol-gel synthesis and processing have the potential to create catalytic sites atomically isolated in the aerogel framework. V or Cu were incorporated during the solution chemistry into silica aerogels. Extended X-ray absorption fine structure and X-ray absorption near edge structure were employed to determine that copper is present as an oxide and probably isolated in the silica matrix, while vanadium is present as V 2 O 4 and V 2 O 5 . High resolution transmission electron microscopy and selected area diffraction were used to elucidate the characteristics of the aerogel structure itself, including the diameter of the chains, as well as to look for phase separation and crystallinity of the active metal oxide. It was found that the metals were highly dispersed in the silica aerogel, even after oxidation and reaction treatments.

In this paper we report on the preparation of mixed lanthanide-silicate and pure lanthanide aerogels from the chlorides of erbium, praseodymium, and neodymium. A two-step sol-gel method is described for preparing the mixed aerogels, using a sub-stoichiometric amount of water in the first step to prepare a partially condensed silica-lanthanide precursor. The pure lanthanide aerogels are prepared directly from the chlorides using propylene oxide as a scavenger for reaction generated hydrochloric acid. The aerogel microstructures vary from colloidal for the pure lanthanide and high weight percent lanthanide-silicate aerogels to polymeric for the low weight percent lanthanide-silicate aerogels. This change in microstructure is also indicated by the BET analyses, which show that the surface areas decrease with increasing lanthanide concentrations. In general, we measured reductions of lanthanide contents during the supercritical drying step due to insufficient linking and subsequent “washing out” of the lanthanides from the gels. Also, the retention efficiency for the lanthanide improves with higher silica concentrations, making quantitative doping by this method practical only for the lower lanthanide concentrations.

Conventional silica sol-gel chemistry is limited for the production of transparent ultralow-density aerogels because (1) gelation is either slow or unachievable, and (2) even when gelation is achieved, the large pore sizes result in loss of transparency for aerogels