Inorganic-Organic Hybrid Aerogels
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ULRICH SCHUBERT*, FRITZ SCHWERTFEGER, NICOLA HUSING* AND ELISABETH SEYFRIED Institut ftir Anorganische Chemie der Universitat Wiirzburg, Am Hubland, D-97074 Wiirzburg, Germany
ABSTRACT Organically modified silica aerogels were prepared by NH4 OH-catalyzed hydrolysis and condensation of RSi(OMe) 3 / Si(OMe) 4 mixtures, followed by supercritical drying of the alcogels with methanol or CO 2 . Terminal alkyl or aryl groups, bridging groups or functional organic (methacryloxypropyl or glycidoxypropyl) groups were employed for R. By the proper choice of the organic groups, the RSi(OMe) 3 / Si(OMe)4 ratio and the drying conditions, hydrophobic aerogels, being insensitive towards moisture, were obtained with no residual SiOH or Si-OMe groups left. The transparency and porosity of the organically modified aerogels was only slightly diminished relative to unmodified silica aerogels. The elastic constant of the aerogels was significantly influenced by the kind of organic groups. By pyrolysis of the phenyl-substituted aerogels, nanometer-sized carbon structures were generated. They partly coat the primary aerogel particles and provide a very high mass specific extinction in the wavelengths interval critical for thermal radiative transport.
INTRODUCTION Aerogels are extremely porous solids. Typical porosities of silica aerogels are in the range of 85 % to 98 %, and densities as low as 3 kg/m 3 have been reported. As a result of the high porosity, aerogels have very interesting physical properties, which can be utilized, inter alia, for thermal (super)insulations in windows or heat-storage systems, acoustic materials, luminescent solar concentrators, gas filters, catalysts or catalyst supports [1]. Aerogels of a variety of single or mixed oxides as well as carbon aerogels or organic aerogels have been made. However, silica aerogels are most thoroughly studied. In the first step of their preparation, sol-gel processing [2] of stabilized sols of metal oxides, or alcoholic solutions of hydrolyzable compounds, mostly alkoxides, gives "alcogels", which are then dried under supercritical conditions. By this kind of drying the filigreed texture of the gels is not destroyed by capillary forces. A major advantage of the sol-gel process for material syntheses is the low processing temperature. This allows one to prepare inorganic-organic hybrid materials, in which the properties of inorganic and organic compounds are to some extent combined. Organically * New address: Institut ftir Anorganische Chemie der Technischen Universitat Wien, Getreide-
markt 9, A-1060 Wien, Austria 151 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
substituted silicate materials are prepared by sol-gel processing of the alkoxysilanes RSi(OR') 3 or R2 Si(OR') 2 in which the (functional) organic group is directly bonded to silicon. This is possible because of the hydrolytic stability of Si-C bonds. The organic moieties retained in the final material can modify or functionalize the oxidic base material. Although the high potential of such
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