Characterization Of Partially Condensed Silica Formed From The Partial Hydrolysis Of Tmos
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CHARACTERIZATION OF PARTIALLY CONDENSED SILICA FORMED FROM THE PARTIAL HYDROLYSIS OF TMOS T.M. TILLOTSON AND L.W. HRUBESH, Department of Chemistry & Material Science, Lawrence Livermore National Laboratory, Livermore, CA 94550
ABSTRACT A stable mixture of partially condensed polysilicates is formed from the substoichiometric, acid catalysed hydrolysis of tetramethoxysilane [Si(OCH 3 )4] and the removal of the reaction- generated methanol by distillation. This mixture has been used as a precursor to make silica aerogels that show microstructural differences from base catalysed TMOS aerogels. In this paper, we report the analysis of this mixture for a determination of the type and distribution of the polysilicate species formed, and we include measurements of some physical properties of this fluid.
INTRODUCTION Aerogels were first prepared in the early 1930's by Kisfler in an effort to probe the nature of gels and gel struture[l]. Kistler's aerogels were time consuming to make and in the early 1960's a French graduate student working with Teichner used a silicon alkoxide instead of sodium silicate and the current "sol-gel" approach was born[2]. Silica aerogels are transparent solids with high surface areas, ultrafine cell/pore sizes, high melting points, low thermal conductivities, and unique acoustic properties. In recent years there has been a resurgence in aerogel research and where they were once just a scientific curiosity their unusual properties are beginning to find useful applications in the market place. One of the goals of aerogel makers is to control microstructure in an effort to tailor the aerogel properties. It is commonly believed that the molecular building blocks first formed in the initial hydrolysis of the alkoxide greatly influence the final microstructure. The evolution of these building blocks in acidic and basic mediums has been well studied[3,4,5,]. In this paper, we use NMR and GCUMS to describe a partially hydrolysed and condensed silica (C.S.) precursor.We also report on some of the physical properties of this fluid. This precursor has been successfully used to prepare uncracked, monolithic aerogel bricks with densities as low as .003 gms/cc[6]. Aerogels prepared with C.S. have compressive moduli 3-4 times higher than single step base catalysed aerogels[7].
EXPERIMENTAL Preparation of CS. TMOS supplied by Petrach Systems, Inc. was used as the starting material. To obtain consistent and reproducible results, purification of the alkoxide is necessary. This is accomplished by fractional distillation using a 4' silvered, vacuum jacketed fractionating Mat. Res. Soc. Symp. Proc. Vol. 180. 01990 Materials Research Society
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column filled with glass beads under a N2 purge. The purified TMOS is mixed with a substoichiometric amount of water (molar ratio: 1 TMOS: 1.3 H 20) and refluxed under acidic (10-5 moles) conditions for 16.0 hours. This amount of water is 65% of that required to form SiO 2 according to the net hydrolysis/condensation reaction: Si(OCH 3)4 + 2H 20 . SiO 2 + 4CH 3OH Suffi
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