In-Situ Pore Structure Analysis During Aging and Drying of Gels
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IN-SITU PORE STRUCTURE ANALYSIS DURING AGING AND DRYING OF GELS
Douglas M. Smith*, Pamela J. Davis*, C. Jeffrey Brinker" *UNM/NSF CENTER FOR MICRO-ENGINEERED CERAMICS, University of New Mexico, Albuquerque, NM, 87131, **Division 1846, Sandia National Laboratories, Albuquerque, NM, 87185
ABSTRACT
The use of NMR relaxation measurements for the in-situ study of pore structure evolution during gel aging and drying is illustrated. The change in the pore size distribution and surface area of both wet and dried gels is examined as a function of aging conditions including temporal aging, thermal aging, changing pH, and changing pore fluid. The effect of pore fluid pH on dissolution/reprecipitation in ordered packings of monodisperse silica spheres is also examined as a model system for particulate gels. As expected, the pore size distribution narrows with increasing time of treatment in high pH pore fluids. Interpretation of high pH results for the wet state is complicated by a microporous layer which forms on colloidal silica resulting in significantly larger wet surface area as compared to the final dried material. Narrowing of the pore size distribution, which is of interest for maximizing drying rates, is maximized in the least time by using either high pH or repeated ethanol washes for the base-catalyzed gel (B2) used.
INTRODUCTION
A major feature of sol-gel processing is the large degree of variation in pore structure (i.e., mean pore size, pore size distribution, surface area, porosity, etc.) that occurs both during processing and in the final product (coating, fiber, aerogel, or xerogel). Pore structure control at all processing stages is of great practical interest since pore structure plays an important role in fixing the highest drying rate that can be achieved without cracking (xerogels), sintering temperatures (dense glasses), and physical properties such as the refractive index or adsorption capacity. By varying process parameters such as temperature, pore solvent, pH, aging time, etc., large changes in the final structure may be realized. However, the relationship between these parameters and pore structure is unclear and may be only elucidated with in-situ characterization techniques. Mat. Res. Soc. Symp. Proc. Vol. 180. @1990 Materials Research Society
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BACKGROUND-PORE STRUCTURE CHARACTERIZATION
The ability to monitor pore structure changes during aging and drying is complicated by the nature of sol-gel processing. In general, changes during processing have been inferred from the final dried gel since for traditional pore structure analysis (nitrogen adsorption/condensation and mercury porosimetry), the sample must be dried before analysis. However, chemistry and structure continue to evolve during aging and drying and the interpretation of how a parameter affects the final pore structure is not straightforward. The few studies of pore structure
evolution in gels du ring
esing use either scattering (SAXS, SANS),
thermoporometry, NMR relaxation, or magnetic resonance imaging (MRI). Scattering
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