Microstructural Dependence Of Aerogel Mechanical Properties
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MICROSTRUCTURAL DEPENDENCE OF AEROGEL MECHANICAL PROPERTIES J.D. LEMAY, T.M. TILLOTSON, L.W. HRUBESH, AND R.W. PEKALA, Chemistry and Materials Science Department, Lawrence Livermore National Laboratory, Livermore, CA 94550 ABSTRACT Aerogels are highly porous solids derived from the hypercritical drying of covalently crosslinked gels. SEM, TEM and scattering studies reveal that these materials have an open-cell morphology with a solid matrix composed of interconnected colloidal-like particles (30-200 A) Processing variables and synthetic conditions largely determine the microstructure and morphology. In this study, the structure-mechanical property relationships of silica, resorcinolformaldehyde, and carbon aerogels are examined. INTRODUCTION At Lawrence Livermore National Laboratory we have been involved in the research and development of aerogels for a number of years. We particularly are interested in aerogels of lowdensity in the range of 0.030-0.300 g/cc. Our applications for these materials include high energy physics experiments like inertial confinement fusion (ICF). In addition to studying more common aerogels based on metal alkoxides (e.g., TMOS or tetramethoxysilane), we have developed new aerogels made of organic polymers and carbon [1-4]. We also have developed new technologies for making ultra-low density silica aerogels (0.005-0.020 g/cc) [5,6]. Low density aerogels are not robust materials, yet our applications require that we handle and machine precisely dimensioned parts from these delicate materials. Consequently we find it useful to evaluate their response to mechanical deformation. Uke other workers [7-9] we also are interested in the general scientific characterization of these unique materials (4,10]. In this work we report on the compressive mechanical properties of silica, polymeric, and carbon aerogels. EXPERIMENTAL Synthesis The precursors for the silica aerogels were "alcogels" prepared via the hydrolysis and polycondensation of tetramethoxysilane (TMOS) in aqueous methanol. Gels were prepared under acidic (HCI) and basic (NH4 OH) conditions using a TMOS:H 2 0 mole ratio of 1:4. The density of the alcogel was adjusted by the quantity of methanol diluent. We prepared a third type of aerogel by replacing the TMOS monomer with a syrup of partially hydrolyzed TMOS. To make the syrup we reacted TMOS with a sub-stoichiometric amount of water (molar ratio; 1TMOS:1.3H 2 0) under acidic conditions, then isolated the product [4]. Alcogels were made from the resulting syrup under basic conditions [4,5]. The method of preparation for the polymeric aerogel is detailed elsewhere [1,2]. Briefly, It involved the polymerization of an aqueous solution of resorcinol and formaldehyde (mole ratio 1:2) in the presence of a basic catalyst (sodium carbonate). The density was adjusted through the concentration of the diluent (water). The carbon aerogels were made by pyrolyzing the polymeric resorcinol-formaldehyde (RF) aerogels. The procedure is described elsewhere [3,4]. The RF polymer aerogel is readily pyrolyz
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