The sol–gel process represents a room‐temperature method of producing a wide range of ceramic materials through the formation of a glassy network by the polymerization of suitable monomers. The use of the technique to produce, for example, sonogels, organic‐inorganic composites, NLO glasses, and electrooptical materials such as gel‐glass dispersed liquid crystald (GDLCs) is reviewed. An example of a GDLC is shown on this issue's front cover, 4‐pentyl‐4‐biphenylcarbonitrile in the LC state being dispersed in a thin film of silica.

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Nowadays the sol–gel process is used by an increasing number of researchers for the preparation of various products, including bulk materials, films, membranes or fibers. The application of ultrasound (sonocatalysis) to the precursors gives rise to materials with new properties, known as sonogels. The absence of additional solvent and, mainly, the effects of ultrasonic cavitation create a unique environment for sol–gel reactions leading to particular features in the resulting gels: high density, fine texture, homogeneous structure etc. These properties determine the evolution of sonogels on further processing and the final material structure. In this sense, the full exploitation of sonocatalysis requires a thorough understanding of the processes involved and their sensitivity to reaction parameters. For this purpose, we have used diverse techniques to investigate the microstructural evolution during different steps in the sonogel process. The results of these studies are reported here. Finally, we present an overview of some applications for which the mechanical, textural and optical characteristics of sonogels are quite useful. Copyright © 1999 John Wiley & Sons, Ltd.

18The industry of phosphoric acid produces a calcium-rich by-product known as

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Several mixtures obtained by ultrasonic agitation of colloidal silica with a sol solution containing tetraethoxysilane (TEOS) were used to form crack-free monoliths. The dry gels that were submitted to mercury porosimetry showed differences in behavior according to the proportion of silica colloid in the gel. For low proportions, the mercury isostatic pressure exclusively induced an irreversible compaction of material, but intrusion occurred in gels with a high proportion of colloid. This permitted the evaluation of the textural properties of intruded gels, whereas the pressurization-depressurization paths, where intrusion was prevented, provided information about their compression behavior. Specimens exhibited elastic strain, followed by yield and plastic hardening. Bulk modulus was significantly reduced as the content of silica particles in the gel was increased. This was related to the increase in free volume of the network. The yield point, which characterizes the limit of the elastic region, was taken as a reference for estimating the maximum capillary pressure of drying; the inclusion of colloidal silica progressively reduced the maximum capillary pressure occurring during the drying for all samples except those containing the highest proportion of colloid (82%). This anomalous behavior was associated with low TEOS content, which complicates the formation of siloxane bonds. Textural properties determined by nitrogen adsorption confirmed the results obtained by porosimetry. Both porous volume and pore size were gradually increased in line with the colloid content. Therefore, we offer an effective means of tailoring the derived sol-gel porosity by increasing the proportion of colloidal silica, starting from a dense microporous material to achieve a mesoporous gel with a high pore volume and reduced capillary pressure.