The Processing and Characterization of DCCA Modified Gel-Derived Silica
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THE PROCESSING AND CHARACTERIZATION OF DCCA MODIFIED GEL-DERIVED SILICA S. WALLACE AND L. L. HENCH Department of Materials Science Gainesville, FL 32611
and Engineering,
University
of Florida,
INTRODUCTION The production of small silica gel monoliths by the hydrolysis and polycondensation of tetramethyl orthosilicate (TMS) in a methanol solution is now a common procedure. Drying is generally done slowly in a methanolic atmosphere [1] or under hypercritical conditions [2]. The potential application of sol-gel technology in the production of materials for large structures requires a rapid processing time, which causes a problem due to cracking. For large scale space structures the materials produced also require a low molecular weight, a low densification temperature, control of
devitrification and a wide range of physical properties and gel densities. These requirements are potentially met by multicomponent silica based gels [3].
To decrease the drying time, which is the longest part of
gel processing,
the gel strength needs to be increased to resist cracking
and
the
drying
stress
reduced.
The
gel
strength
can
be
improved
by
optimizing the variables in the gel manufacturing process. During the initial stages of drying, the capillary force caused by the evaporation of solvents from the micropores in the gel creates an overall drying stress and local differential stresses due to non-uniform pore size distributions [2]. The capillary force depends on the rate of liquor evaporation which is a function of solvent vapor pressure, and is inversely proportional to the pore size. During the final stages of drying, cracking is the result of non-uniform shrinkage of the drying body [2]. This can be due to temperature gradients, compositional inhomogeneities and different local rates of reaction. It has been observed that for a given volume of TMS, there is a critical ratio S* (S = Volume of Solvents/Volume of TMS) above which cracking will not occur during the final stages of drying. The
drying stress is therefore a function of the pore size and the rate of evaporation of the liquor, which depends on the liquor vapor pressure. The pore size can be controlled by the processing variables and the vapor pressure can be controlled by adding an organic solvent, called a drying control chemical additive (DCCA), to the sol. In this study we examine the role of formamide (NH2 CHO) as a DCCA in TMS gel-derived silica, as a basis for the further investigation of multicomponent silica gels. The vapor pressure of NH2 CHO is 0.1 Torr at 40°C, compared to 100 Torr at 21°C for CH3OH. The combination of the two liquids reduces the rate of evaporation during the initial stages of drying, thus reducing the drying stress. GEL PROCESSING OPTIMIZATION Base
catalyzed
gels
gelate
quickly
due
to
OH-
catalysis
of
the
polycondensation reaction leading to a particulate structure with many unreacted SiOCH3 radicals. The addition of an acid causes rapid hydrolysis of the TMS, catalyzed by H+ ions [4], as shown by the instan
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