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.
Aerogel materials possess a wide variety of exceptional properties, hence a striking number of applications have developed for them. Many of the commercial applications of aerogels such as catalysts, thermal insulation, windows, and particle detectors are under development and new applications have been publicized since the ISA4 Conference in 1994: e.g., supercapacitors, insulation for heat storage in automobiles, electrodes for capacitive deionization, etc. More applications are evolving as the scientific and engineering community becomes familiar with the unusual and exceptional physical properties of aerogels. In addition to growing commercial applications of aerogels, there are also scientific and technical applications, as well. This paper discusses a variety of technical applications of aerogels. It reports current technical applications under development for which several types of aerogels are formed in custom sizes and shapes.
Aerogels are open-cell foams that have already been shown to be among the best thermal insulating solid materials known. This paper examines the three major contributions to thermal transport through porous materials, solid, gaseous, and radiative, to identify how to reduce the thermal conductivity of air-filled aerogels. We found that significant improvements in the thermal insulation property of aerogels are possible by (i) employing materials with a low intrinsic solid conductivity, (ii) reducing the average pore size within aerogels, and (iii) affecting an increase of the infrared extinction in aerogels. Theoretically, polystyrene is the best of the organic materials and zirconia is the best inorganic material to use for the lowest achievable conductivity. Significant reduction of the thermal conductivity for all aerogel varieties is predicted with only a modest decrease of the average pore size. This might be achieved by modifying the sol-gel chemistry leading to aerogels. For example, a thermal resistance value of R = 20 per inch would be possible for an air-filled resorcinol-formaldehyde aerogel at a density of 156 kg/m3, if the average pore size was less than 35 nm. An equation is included which facilitates the calculation of the optimum density for the minimum total thermal conductivity, for all varieties of aerogels.
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