Xerogel Monoliths

Xerogel monoliths are semiporous monoliths (usually 30-70% porosity). Yoldas [40] in the 1970s was the first to demonstrate fabrication of monoliths via a sol-gel process. Since then numerous routes for fabrication of xerogel silica monoliths and nonsilica monoliths were suggested and demonstrated. In this section a few examples of xerogel monolith febrication will be presented. 

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EDX-analyses reveal observation of stretched xerogel clusters which fill almost entirely the channels of the PAA after sequential spin-on deposition followed by heat treatment. Unlike to all our earlier works with titania or alumina xerogel, the proposed technology does not allow fabrication of YAG xerogel monolith, whereas only occasional clusters are observed inside the mesoscopic channels of PAA-Fig. 2. 

Figure 25.12. Photograph of a xerogel monolith of a Fesorcinol-formaldehyde/ammonium nitrate (3 7 w/w) nanostructured energetic composite illustrating the capability for sol-gel energetic materials to be cast and dried to well-defined shapes.

Job, N., Sabatier, F., Pirard, J.-P., Crine, M., Leonard, A., 2006b. Towards the production of carbon xerogel monoliths by optimizing convective drying conditions. Carbon 44 2534-2542. 

Leonard, A., Crine, M., Jomaa, W., 2007. Production of cylindrical carbon xerogel monoliths efiect of sample diameter on drying-induced cracks. Extended Abstracts of 6 European Congress of Chemical Engineering (ECCE6), Copenhagen, Denmark, II (T2-7b). 

In this chapter we review the principles of monolith fabrication, routes for febrication of monoUths such as xerogel monoliths, nanocomposite monoliths, ORMOSIL monohths, hybrid monoliths, and aerogel monohths. In addition, we will present several characterization methods of monohths, encapsulation or doping in monohths, and examples of apphcations of monohths. 

Silica xerogel monoliths are the major class of sol-gel process materials and their fabrication procedures were described extensively in many reviews and papers [54-58]. The principal concepts of fabrication of silica xerogel monoliths have already been described in Section 10.2, and the effects of parameters such as metal alkoxide precursor types, type of catalyst used, H2O Si molar ratio R), steric effect of precursor ligand groups, and functionality of organically modified silanes have also been discussed. Important pioneering research on the influence of these parameters on xerogel fabrication was presented in Refs [9,59-63]. 

Here are some examples of fabrication of silica xerogel monoliths by one- and two-step catalyst procedures. 

Fabrication procedures of silica monoliths by a one-step catalyst procedure are presented in Ref [20]. A basic procedure to prepare a silica xerogel monolith is by following the stoichiometry ratio (H2O Si) R = 4, while using a similar molar amount of alcohol as a common solvent for achieving homogeneous solution. In fabrication of sol-gel monoliths, the catalyst used has crucial influence. The common route for preparing silica monoliths is to use a recipe based on a one-step catalyst reaction. 

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