Self-Diffusion Coefficients of Sol-Gel Intermediates
Abstract
Over the past few decades, the sol-gel technique has shown great potential for producing ceramic materials of designed composition and properties. A typical sol-gel process involves the hydrolysis and polymerization of one or more metal alkoxides in an alcohol solution in a batch reactor to form a homogeneous gel, which is then further treated to provide the type of ceramic desired (aerogel, glass, crystal, coating, etc.). Several authors have suggested that diffusion effects may influence the development of the gel microstructure. To investigate if diffusion effects are important, we begin by determining what governs the diffusivity of the precursors using the pulsed-gradient-spin-echo NMR technique. This report focuses on the early stages of the batch reaction. The system chosen is the acid catalyzed hydrolysis and condensation of tetraethyl orthosilicate (TEOS) in ethanol. Our results from 1H PGSE NMR and 29Si NMR show that the diffusivities of the silicate precursors are not just a function of the molecular weight. Hydrogen bonding between the polar functional groups and the solvent also decreases the mobility of the reactants. Moreover, since hydrolysis increases and polymerization reduces the concentration of these polar groups, the mobility of the reactive oligomers changes in a complex manner. Combining our results from PGSE NMR and gas chromatography, we estimate the extent of hydrogen bonding effect on the mobility of various oligomers, providing a model to predict diffusivity trends throughout the batch reaction.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jul 12, 1993
- Accession Number
- ADA266957
Entities
People
- A. V. Mccormick
- L. Ng
Organizations
- University of Minnesota