Structure of Mesoporous Aerogels
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Structure of Mesoporous Aerogels Dale W. Schaefer Introduction Several emerging technologies depend on the development of porous materials with pore dimensions in the nanometer range (1 run = 10 A). Based on the canonical classification scheme, such materials are defined as "mesoporous materials."1 Specialty uses, such as separation applications, require porosities engineered to meet stringent pore-size requirements, particularly for separating high-boiling-temperature organics. In addition, commodity products, such as high-performance thermal insulation, await credible manufacturing methods to produce fine-scale pores. Finally, information discovered during the development of mesoporous materials could be helpful in finding new strategies to generate void-free ceramic materials by reverse engineering. Although several methods exist to generate nanometer porosity, our ability to tailor porosity for specific applications is still primitive. Because we lack adequate structural data and models for pore formation, the relationship between synthetic protocol and pore structure remains enigmatic. To my knowledge, all mesopore synthetic strategies rely on extraction. Zeolites entail removal of pore-forming templates. Porous Vycor® glass is made by leaching a sacrificial borate phase. Organic foams with submicron porosity are made by thermally induced phase separation followed by solvent extraction.2-3 And until recently,4'5 aerogels have been formed by drying under supercritical conditions to avoid collapse of the solvent-imbibed gel precursor. Competing with gas-blown foams, extraction is a costly method that has limited the use of mesoporous materials to specialty applications.6 Therefore, the challenge is not just to engineer porosity, but simultaneously to develop cost-effective, manufacturable processes. This article reviews recent work on the
MRS BULLETIN/APRIL 1994
structure of mesoporous aerogels determined by small-angle scattering (SAS), a technique described by Smith, Hua, and Earl in this issue.7 With advances in instrumentation, small-angle x-ray (SAXS) and neutron scattering (SANS) studies are in progress in several laboratories,8"17 leading to structural information not readily extracted from electron microscopy. New perspectives on the characterization of disorder, based on fractal geometry,18'19 also contribute to this flurry of activity. Along
Fractal Analysis of Aerogels and Precursors Aerogels are porous solids made by extracting the solvent from a crosslinked polymer gel formed from high-functionality monomers such as TEOS, Si(OCH2CH3)4. Over the last decade, I have worked with synthetic chemists to collect an atlas of gel-derived structures determined by SAS. Fractal analysis of SAS data has proved useful in exposing previously unrecognized structural diversity. More recently, "chord analysis" proved equally useful in highlighting underlying universality. Depending on the synthetic protocol, a variety of microstructures are found, as shown in Figure 1 by SAXS data from silica aerogels synthesized
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