Silica xerogel monoliths

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. 

Fields reported that continuous silica xerogels prepared from potassium silicate solutions could be used as highly permeable support media, and exhibit reasonable chromatographic efficiency in HPLC [23]. Minakuchi et al. reported the preparation and evaluation of continuous porous-silica columns that provide a much higher column efficiency in HPLC than do conventional columns packed with particles [13-16,18], The monolithic columns prepared in a capillary can also be used in CEC. 

A wide variety of approaches are currently being used in the fabrication and technology of columns for capillary electrochromatography (CEC). Continuous polymer bed, or monolithic columns (see Section 3.4), manufactured by in-situ polymerization within the columns, have been used in numerous application areas and have been shown to be highly efficient. In a second approach, a sol-gel process is employed to form a silica xerogel within the capillary, followed by bonding of the stationary-phase group alternatively, the separation medium itself may be polymerized in situ. 

Einasrud MA, Dahle M, Lima S, Haereid S (1995) Preparation and properties of monolithic silica xerogels from TEOS-based alcogels aged in silane solutions. J Non-Cryst Solids 186 96-103... 

Haereid S (1993) Preparation and characterization of transparent monolithic silica xerogels with low density, PhD thesis NTNU (Norway)... 

Figure 11-6. Monolithic silica xerogel before mercury porosimetry (A) the same sample after densification till a pressure just below Pt (B) and the same sample after pressurization till 200 MPa (C). Reprinted from Pirard (2002) with permission from Elsevier.

TABLE 19-4. MEASURED AND CALCULATED LIFETIMES OF THE < Do STATE, AT ROOM TEMPERATURE FOR MONOLITHIC SILICA XEROGELS TREATED AT DIFFERENT TEMPERATURES... 

Porcher P., Caro P. Influence of /-mixing on the phenomenological interpretation of the Eu + ion spectroscopic properties. J. Lumin. 1980 21 207-216 Pucker G., Parolin S., Moser E., Montagna M., Ferrari M., Del Longo L. Raman and luminescence smdies ofTb " doped monolithic silica xerogels. Spectrochimica Acta Part A 1998 54 2133-2142... 

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. 

As was earlier reported, flexible properties of sol-gel technology may be used for modification of PLE spectra of the incorporated lanthanides. In particular, appearance of the strong band at PLE spectrum at 360 nm for the emission wavelength 1.54 pm was observed for the erbium-doped monolith xerogel silica-titania or titania/opal structure [3,4], However, in ftie case of excitation terbium in composite YAG/titania xerogel/opal the obtained PLE... 

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