Carbon Aerogels and Xerogels
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CARBON AEROGELS AND XEROGELS
RICHARD W. PEKALA AND CYNTHIA T. ALVISO Chemistry & Materials Science Department, Lawrence Livermore National Laboratory, Livermore, CA 94550
ABSTRACT The aqueous polycondensation of resorcinol with formaldehyde proceeds through a sol-gel transition and results in the formation of highly crosslinked, transparent gels. If the solvent is simply evaporated from the pores of these gels, large capillary forces are exerted and a collapsed structure known as a xerogel is formed. In order to preserve the gel skeleton and minimize shrinkage, the aforementioned solvent or its substitute must be removed under supercritical conditions. The microporous material that results from this operation is known as an aerogel. Because resorcinol-formaldehyde aerogels and xerogels consist of a highly crosslinked aromatic polymer, they can be pyrolyzed in an inert atmosphere to form vitreous carbon monoliths. The resultant porous materials are black in color and no longer transparent, yet they retain the ultrafine cell size (< 50 nm), high surface area (600-800 m2 /g), and the interconnected particle morphology of their organic precursors. The thermal, acoustic, mechanical, and electrical properties of carbon aerogels/xerogels primarily depend upon polymerization conditions and pyrolysis temperature. In this paper, the chemistry-structure-property relationships of these unique materials will be discussed in detail.
INTRODUCTION A number of methods are used to generate low density carbon foams: (1) phenolic microballoons are stabilized with an appropriate binder and pyrolyzed to form syntactic carbon foams, (2) the partial oxidation and pyrolysis of phase-separated polyacrylonitrile foams results in microcellular carbon foams, (3) reticulated vitreous carbon (RVC) foams are produced by pyrolyzing an open-cell polyurethane foam that has been infused with a reactive monomer such as furfuryl alcohol, and (4) replica carbon foams result from the pyrolysis of a phenolic resin within a sacrificial substrate that is subsequently removed [1-4]. In each of these cases, processing parameters control the morphology, density, and cell size distribution of the final foam. In general, it is difficult to achieve carbon foams that have both low density (< 0.1 g/cc) and small cell size (< 25 gt). In the past 5 years, our research has focused on sol-gel polymerizations that lead to organicbased aerogels that can be subsequently pyrolyzed into carbon aerogels. Aerogels are a special class of open-cell foams that have high porosity, ultrafine cell/pore sizes (< 50 nm), high surface area (400-1000 m 2/g), and a solid matrix composed of interconnected colloidal-like particles or fibrous chains with characteristic diameters of 10 nm. The densified version of an aerogel is known as a xerogel.
Mat. Res. Soc. Symp. Proc. Vol. 270. 01992 Materials Research Society
Traditional sol-gel polymerizations have involved the hydrolysis and condensation of metal alkoxides (e.g. tetramethoxy silane, tantalum ethoxide) to form xerogels that ar
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