Characterization Of Ultralow-Density Silica Aerogels Made From A Condensed Silica Precursor
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CHARACTERIZATION OF ULTRALOW-DENSITY SILICA AEROGELS MADE FROM A CONDENSED SILICA PRECURSOR
LAWRENCE W.HRUBESA4, THOMAS M.TILLOTSON, AND JOHN F. POCO Chemistry and Material Science Department, Lawrence Livermore National Laboratory, Livermore, CA 94550
ABSTRACT Silica aerogels having densities in the range of 0.003 to 0.05 g/cm 3 , have been prepared from a condensed silica formulation involving a two-step hydrolysis/condensation process with tetramethoxysilane (TMOS). This work describes the preparation of these unique ultralow-density aerogels, and compares several of their properties with those of aerogels made by a single step, base catalysed process.
INTRODUCTION The production of silica aerogels having less than 1% solids content using single step hydrolysis/condensation of silicon alkoxides is limited by the time necessary to gel an extremely dilute solution, and ultimately by the alcoholysis equilibrium reactions that reverse the hydrolysis/condensation process. One means to bypass this limitation is to use two steps; the first is to prepare a partially hydrolysed, partially condensed, polysilicate mixture from which all the alcohol is removed; the second is to add water to the polysilicate mixture in order to complete the hydrolysis, but this is done in the presence of a nonalcohol diluent. Thus, the minimal amount of alcohol that is present as a reaction product does not strongly influence the polycondensation that leads to gelation. Others have reported similar two-step methods in which the second step includes an alcohol diluent [1,2]. We have successfully used the above method to prepare gels that produced aerogels having as little as 0.13% solids content. The aerogels appear to have a microstructure that differs from the connected colloidal particle models that are reported for aerogels made from the single step, base catalysed TMOS process [3,4].
EXPERIMENTAL Preparation
The method of preparation for the ultralow-density "alcogels" first involves the formation of a condensed silica oil (CS), as described previously [5]. The oil is prepared by reacting TMOS with a sub-stoichiometric amount of water in the presence of methanol, under acidic conditions (molar ratios: 1TMOS: 1.3H 2 0: 2.4MeOH: 10- 5 HCl). This mixture is refluxed for 16 hours before all the methanol is removed by distillation, leaving the condensed silica oil. The oil is then further reacted with sufficient water to complete the Mat. Res. Soc. Symp. Proc. Vol. 180. @1990 Materials Research Society
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hydrolysis of the original amount of TMOS, with 100% excess, (ie. ITMOS: 4.0H2 0). The reactions are done in a pyrex glass mold, in the presence of a non-alcohol diluent (eg. acetone or acetonitrile), under basic conditions (NH4OH). The amount of diluent is chosen to give the target density, and the catalyst concentration (typically 10-4 molar) is selected to achieve a gel time of less than 72 hours. The silica aerogel is obtained from the "alcogel" by the direct super-critical point extraction of the solvent liquid, by placing the
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