Consolidation, dried gels

The Raman spectra (0-1400 cm l) shown in Fig re 6 illustrate the structural changes which accompany the consolidation of silica gels. The 1100°C sample is fully dense, whereas the 50 and 600°C samples have high surface areas (1050 and 890 m2/g), respectively. The important features of the Raman spectra attributable to siloxane bond formation are the broad band at about 430 cm 1 and the sharp bands at 490 and 608 cm 1(which in the literature have been ascribed to defects denoted as D1 and D2, respectively). The D2 band is absent in the dried gel. It appears at about 200°C and becomes very intense at intermediate temperatures, 600-800°C. Its relative intensity in the fully consolidated gel is low and comparable to that in conventional vitreous silica. By comparison the intensities of the 430 and 490 cm 1 bands are much more constant. Both bands are present at each temperature, and the relative intensity of the 430 cm 1 band increases only slightly with respect to D1 as the temperature is increased. Figure 7 shows that in addition to elevated temperatures the relative intensity of D2 also decreases upon exposure to water vapor. 

Thermodynamic and Kinetic Aspects of Gel Consolidation points out that dry gels have high free energies and that consolidation kinetics depend on the competition between condensation and structural relaxation. 

The solgel process uses a liquid reactive precursor material that is converted to the final product by chemical and thermal means. This precursor is prepared to form a colloidal suspension or solution (sol) which goes through a gelling stage (gel) followed by drying and consolidation. The process requires only moderate temperatures, in many cases less than half the conventional glass or ceramics... 

The structures of sol-gel-derived inorganic polymers evolve continually as products of successive hydrolysis, condensation and restructuring (reverse of Equations 1-3) reactions. Therefore, to understand structural evolution in detail, we must understand the physical and chemical mechanisms which control the sequence and pattern of these reactions during gelation, drying, and consolidation. Although it is known that gel structure is affected by many factors including catalytic conditions, solvent composition and water to alkoxide ratio (13-141, we will show that many of the observed trends can be explained on the basis of the stability of the M-O-M condensation product in its synthesis environment. 

Figure 6. Raman spectra of silicates silica gel dried at 50°C, heated to 600°C, consolidated at 1100°C, and conventionally prepared fused silica.

Figure 8. 29Si MASS and CPMASS NMR spectra of silica gels dried at 50°C, heated to 200 and 600°C, and consolidated at 1100°C. Hydrated samples were exposed to 100% RH at 25°C for 24 hours prior to analysis.

The presence of sulfate ions markedly affects the nanopore structure of titania-sulfate aerogels. In Ti02-S042 materials, unlike in zirconia-sulfate aerogels, the larger sulfate load stimulates formation of a more consolidated structure. The XRD analysis shows that even a crystalline phase (anatase) may be present in fresh, dry aerogels, which, perhaps, is the first observation of this phase in sol-gel titania obtained from the low temperature drying process. 

Aminosilanes contain the catalyzing amine function in the organic chain. The reaction of aminosilanes with silica gel in dry conditions is therefore self-catalyzed. They show direct condensation, even in completely dry conditions. Upon addition of the aminosilane to the silica substrate, the amine group may form hydrogen bonds or proton transfer complexes with the surface silanols. This results in a very fast adsorption, followed by direct condensation. This reaction mechanism of APTS with silica gel in dry conditions, is displayed in figure 8.9. After liquid phase reaction, the filtered substrate is cured, in order to consolidate the modification layer. 

Immediately following the Sol 2 gel transition in the dry process, a primary gel structure is obtained. This gel is seldom Isolated because continued evaporation produces the completely consolidated membrane, known as the secondary gel. The secondary gel is ordinarily the only product of interest. However, this is not usually the case for the wet process. After the viscous solution has been gelled by Immersion and the solvent system removed, a stable primary gel is obtained. Such a membrane is easily distinguished from the secondary gel which results after the primary structure has been subjected to various postformation treatments. 

In other work silica based helical and twisted nanoribbons of controlled chirality were synthesized by sol-gel processing in acidic conditions using organic self-assembly as a template (Fig. 19). The authors have demonstrated that nanohelices can be successfully fragmented into individualized chiral helical and twisted silica ribbons of several hundred nanometers by a sonication technique. It was found that the power of sonication and nature of the solvent are crucial parameters for achieving narrow size distribution of the fragmented helices, and the better the dispersion. In addition it was shown that freeze-drying of the helices clearly consolidated the Si-O-Si bonds. The sonication of helices in water or in ethanol directly after the transcription destroyed the local chiral structures, whereas the helices which were freeze-dried first and then dispersed in these solvents preserved their local chiral structure after sonication. 

Hgure6.7 Raman spectra of Si02 gels dried at 50 °C, heated to 200 °C or 600 °C, or fully consolidated at 1100 °C (From Reference 19.)... 

By using the sol—gel process, porous bulk oxide materials can be sinply obtained by drying and calcination of fresh gels prepared by hydrolysis and condensation of precursors either in aqueous or organic solvents. On the contrary a three-step process is needed for membrane top-layer synthesis which consists first in the preparation of formulated sols, then in the deposition of supported gel layers from these sols and finally in the thermal treatment of the gel layers in order to obtain porous or almost dense consolidated top-layer materials with specific separation properties. 

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