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29Si Nmr Study of Silicon Alkoxides: from the Condensation Kinetics in Solution to the Determination of The Fractal Dimension in Aerogels

Published online by Cambridge University Press:  28 February 2011

F. Devreux ,
J.P. Boilot ,
F. Chaput  and
A. Lecomte
F. Devreux
Affiliation:
Laboratoire de Physique de la Matière Condensée, URA CNRS 1254, Ecole Polytechnique, 91128 Palaiseau Cédex (FRANCE)
J.P. Boilot
Affiliation:
Laboratoire de Physique de la Matière Condensée, URA CNRS 1254, Ecole Polytechnique, 91128 Palaiseau Cédex (FRANCE)
F. Chaput
Affiliation:
Laboratoire de Physique de la Matière Condensée, URA CNRS 1254, Ecole Polytechnique, 91128 Palaiseau Cédex (FRANCE)
A. Lecomte
Affiliation:
Ecole Nationale Supérieure de Céramique Industrielle, 87065 Limoges Cédex (FRANCE)
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Abstract

The complete condensation kinetics of three silicon alkoxides have been studied by 29 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.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 180: Symposium A – Better Ceramics Through Chemistry IV , 1990 , 211
Copyright
Copyright © Materials Research Society 1990

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References

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