29 Si Nmr Study of Silicon Alkoxides: from the Condensation Kinetics in Solution to the Determination of The Fractal Dim
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FROM THE
Si NMR STUDY OF SILICON ALKOXIDES: CONDENSATION KINETICS IN SOLUTION TO THE DETERMINATION OF THE FRACTAL DIMENSION IN AEROGELS
F.DEVREUX*, J.P.BOILOT*, F.CHAPUT*,AND A.LECOMTE** *Laboratoire de Physique de la Matihre Condens~e, URA CNRS 1254, Ecole Polytechnique, 91128 Palaiseau C~dex (FRANCE) **Ecole Nationale Sup~rieure de C~ramique Industrielle, 87065 Limoges C~dex (FRANCE) ABSTRACT The complete condensation kinetics of three silicon alkoxides have been studied by 2 9 Si NMR in the conditions of rapid hydrolysis (acidic medium, water in excess). The gelation of the tetravalent TEOS takes several weeks, whereas the trivalent MTEOS and VTEOS do not form gels. From a quantitative data analysis, we deduce that the first steps of the condensation proceed by progressive formation of small organized units. This accounts for the occurence of highly condensed agglomerates, the absence of gelation in trivalent systems and the very slow kinetics (logarithmic function of time). For the tetravalent TEOS, this is followed by an aggregation phase, which leads to gelation. After the gel time, the transformation of the sol fraction into gel has been followed by observing the broadening of the static NMR spectra. Moreover, the structures of the aerogels obtained by hypercritical drying and of the densified materials resulting from further sintering are characterized by MAS NMR. In particular, measurements of the nuclear relaxation in samples doped with paramagnetic impurities give directly the fractal dimension D of the silicate lattice. The measured value is in agreement with that obtained by the SAXS technique in aerogels (D=2.2) and is close to D=3 for the densified materials.
INTRODUCTION Besides its possible interest for the preparation of homogeneous glasses and ceramics, sol-gel condensation of silicon alkoxides offers the unique possibility to build in a more or less controlled way randomized objects, which are believed to be a physical realization of fractal geometry [1,2]. In the present communication, we show that 29Si NMR can be used profitably in the whole condensation process and may provide information on both the polymerization kinetics and the structure of the condensed objects. EXPERIMENTAL Three silicon alkoxides have been studied: the well known tetraethoxy-silane (Si(OC2 H5 ) 4 or TEOS) to which a large number of studies have been already devoted and two new systems having three -instead of four- branching possibilities, methyl-triethoxysilane (CH3-Si(OC 2 H5 ) 3 or MTEOS) and vinyl-triethoxy-silane (CH 2 =CH-Si (OC H ) or VTEOS) . The polymerization process was initiated by adding acidic water to a solution of alkoxide and ethanol. The alkoxide:ethanol:water concentration ratios were Mat. Res. Soc. Symp. Proc. Vol. 180. 01990 Materials Research Society
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1:6:10 and the pH of the added water solution was fixed to 2.5. The chemical conditions (water in excess, acidic medium) were chosen in such a way that nearly complete hydrolysis of the alkoxides occurs in a few minutes. This allows to
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