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Current Issues in Sol-Gel Reaction Kinetics

  • Roger A. Assink (a1) and Bruce D. Kay (a1)

Abstract

This paper surveys a few of the current issues in sol-gel reaction kinetics. Many times seemingly modest changes in reactants or reaction conditions can lead to substantial differences in the overall reaction rates and pathways. For example, qualitative features of the reaction kinetics can depend on catalyst concentration. At very high acid-catalyst concentrations, reverse reactions are significant for TMOS solgels, while for moderate acid-catalyst concentrations, reverse reactions are substantially reduced. The reaction kinetics of two similar tetraalkoxysilanes: tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS), can be markedly different under identical reaction conditions. Under acid-catalyzed reaction conditions, a TMOS sol-gel undergoes both water- and alcoholproducing condensation reactions while a TEOS sol-gel undergoes only water-producing condensation. The early time hydrolysis and condensation reactions of a TMOS sol-gel are statistical in nature and can be quantitatively described by a few simple reaction rate constants while the reaction behavior of a TEOS sol-gel is markedly nonstatistical. A comprehensive theory of sol-gel kinetics must address these diverse experimental findings.

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1. Assink, R. A. and Kay, B. D. in Better Ceramics through Chemistry, edited by Brinker, C. J., Clark, D. E., and Ulrich, D. R. (Mater. Res. Soc. Proc. 32, Elsevier Science, NY, 1984) p. 301.
2. Kelts, L. W., Effinger, N. J., and Melpolder, S. M., J. Non- Cryst. Solids 83, 353 (1986).
3. Klemperer, W. G. and Ramamurthi, S. D. in Better Ceramics through Chemistry III, edited by Brinker, C. J., Clark, D. E., and Ulrich, D. R. (Mater. Res. Soc. Proc. 121, Mater. Res. Soc., Pittsburgh, Pennsylvania, 1988) p. 1.
4. Balfe, C. A., Ward, K. J., Tallant, D. R. and Martinez, S. L. in Better Ceramics through Chemistry II, edited by Brinker, C. J., Clark, D. E., and Ulrich, D. R. (Mater. Res. Soc. Proc. 73, Mater. Res. Soc., Pittsburgh, Pennsylvania, 1988) p. 619.
5. Assink, R. A. and Kay, B. D., J. Non-Cryst. Solids 107, 35 (1988).
6. Assink, R. A. and Brinker, C. J., unpublished results.
7. Assink, R. A. and Kay, B. D., J. Non-Cryst. Solids 99, 359 (1988).
8. Pouxviel, J. C. and Boilot, J. P., J. Non-Cryst. Solids 94, 374 (1987).
9. Kay, B. D. and Assink, R. A., J. Non-Cryst. Solids 104, 112 (1988).

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Current Issues in Sol-Gel Reaction Kinetics

  • Roger A. Assink (a1) and Bruce D. Kay (a1)

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