Progress in Sol-Gel Production

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Authors: George W. Scherer, George S. Wheeler
Abstract: Consolidants are sols or solutions that are used to restore the strength of weathered stone. The liquids are drawn into the pores of stone by capillary suction, then they harden by gelation and/or drying. In this chapter, we discuss the requirements that such a material must fulfill, and demonstrate the effectiveness of consolidants based on alkoxysilanes and alkylalkoxysilanes.
Authors: Thierry Woignier, A. Hafidi Alaoui, Juan Primera, J. Phalippou
Abstract: Different sets of silica aerogels (classical aerogels, partially dense aerogels, composite aerogels) have been studied in the objective to understand the mechanical behaviour of these extremely porous solids. The mechanical behaviour of xerogels and aerogels is generally described in terms of brittle and elastic materials, like glasses or ceramics. The main difference compared to silica glass is the order of magnitude of the elastic and rupture modulus which are 104 times lower. However, if this analogy is pertinent when gels are under a tension stress (bending test) they exhibit a more complicated response when the structure is submitted to a compressive stress. The network is linearly elastic under small strains, then exhibits yield followed by densification and plastic hardening. As a consequence of the plastic shrinkage it is possible to compact and stiffen the gel at room temperature. These opposite behaviours (brittle and plastic) are surprisingly related to the same kinds of gel features: pore volume silanol content and the pore size. Both elastic modulus and plastic shrinkage depend strongly on the volume fraction of pores and on the condensation reaction between silanols. On the mechanical point of view (rupture modulus and toughness), it is shown that pores size plays likely an important role. Pores can be considered as flaws in the terms of fracture mechanics and the flaw size, calculated from rupture strength and toughness is related to the pore size distribution.
Authors: Nicolás de la Rosa-Fox, Victor Morales-Flórez, Manuel Piñero, Luis Maria Esquivias Fedriani
Abstract: Acoustic cavitation effects in sol-gel liquid processing permits to obtain nanostructured materials, with size-dependent properties. The so-called “hot spots” produce very high temperatures and pressures which act as nanoreactors. Ultrasounds force the dissolution and the reaction stars. The products (alcohol, water and silanol) help to continue the dissolution, being catalyst content, temperature bath and alkyl group length dependent. Popular choices used in the preparation of silica-based gels are tetramethoxysilane (TMOS), Si(OCH3)4, and tetraethoxysilane (TEOS), Si(OC2H5)4. The resultant “sonogels” are denser gels with finer and homogeneous porosity than those of classic ones. They have a high surface/volume ratio and are built by small particles (1 nm radius) and a high cross-linked network with low –OH surface coverage radicals. In this way a cluster model is presented based on randomly-packed spheres in several hierarchical levels that represent the real sonoaerogel. Organic modified silicates (ORMOSIL) were obtained by supercritical drying in ethanol of the corresponding alcogel producing a hybrid organic/inorganic aerogel. The new material takes the advantages of the organic polymers as flexibility, low density, toughness and formability whereas the inorganic part contributes with surface hardness, modulus strength, transparency and high refractive index. The sonocatalytic method has proven to be adequate to prepare silica matrices for fine and uniform dispersion of CdS and PbS quantum dots (QDs), which show exciton quantum confinement. We present results of characterization of these materials, such as nitrogen physisorption, small angle X-ray/neutrons scattering, electron microscopy, uniaxial compression and nanoindentation. Finally these materials find application as biomaterials for tissue engineering and for CO2 sequestration by means the carbonation reaction.
Authors: Quan Chen, Andrew M. Soutar
Abstract: Sol-gel technology has been proved as a very valuable tool for producing and processing ceramic materials for advanced applications especially when currently nanotechnology becomes as the dominant topic for most of the scientists and engineers. While many functional ceramic materials are of technological interest, this paper tries to give an over view of recent progress in synthesis of ceramic powders, mainly nano-scaled powders, by sol gel process and their applications.
Authors: Marcos Zayat, David Levy
Abstract: We emphasize in this chapter the Sol-Gel chemistry, which has gained a large number of researchers, developing interesting and sophisticated novel synthetic methods, offering a variety of approaches to new systems preparation, overcoming many of the synthetic difficulties of the past. A strong argument for using the Sol-Gel chemistry is found in the high flexibility of the method and the large choice of commercially available “dopants” that can be incorporated in the solid matrices, that might have a specific activity or reactivity to an external signal (i.e. light, magnetic, electrical, etc). From the point of view of nanotechnology applications, Sol-Gel materials are being required for critical components embedded in systems such as industrial equipment and scientific instrumentation, imaging and display, medical applications, aerospace and defense, etc. The rapidly developing sol-gel process has been used for the preparation of materials for a wide range of fields, adapting the chemistry and the novel synthetic routes to the specific systems, in order to achieve complicated developments oriented to nanotechnology applications. Clear examples can be found on Sol-Gel optics applications.
Authors: Plinio Innocenzi, Galo Soler Illia
Abstract: Self-assembly through supramolecular templates is an advanced process for preparing thin films with ordered mesostructure and tuned pore arrays; the overall process is a combination of sol-gel and supramolecular chemistry, while the organization is driven by solvent evaporation. Controlling of the process allows obtaining a nanomaterial whose ordered and open porosity can be exploited for applications in different fields. In the article we give a general overview of self-assembly during thin films deposition from a liquid phase and we present some possible fields of applications.
Authors: Dibyendu Ganguli, Raghavan Subasri, Ranganathan Varadharajan
Abstract: A brief update (2001 till the present) is presented on published records on nano- and microcomposites involving inorganic compounds as dispersed and matrix or support phase in powder or thin film/coating form. Properties and applications of the currently reported composite materials are also discussed. The status of this field of research is highlighted in a short analysis.
Authors: Antonio J. Salinas, Maria Vallet-Regí
Abstract: Sol–gel synthesis is used for the fabrication of new materials with technological applications including ceramics for implants manufacturing, usually termed bioceramics. Many bioactive and resorbable bioceramics, that is, calcium phosphates, glasses and glass–ceramics, have been improved by using the sol–gel synthesis. In addition, the soft thermal conditions of sol–gel methods made possible to synthesize more reactive materials than those synthesized by traditional methods. Moreover, new families of bioactive materials such as organic–inorganic hybrids and inorganic compounds with ordered mesostructure can be produced. In hybrid materials, the inorganic component ensures the bioactive response whereas the organic polymeric component allows modulating other properties of the resulting biomaterial such as mechanical properties, degradation, etc. On the other hand, the sol–gel processes also allow the synthesis of silica ordered mesoporous materials, which are bioactive and exhibit – as an added value – a possible application as matrices for the controlled release of biologically active molecules (drugs, peptides, hormones, etc.). Finally, by combining the bioactive glasses composition with synthesis strategies of mesoporous materials, template glasses with ordered mesoporosity can be obtained. In this chapter, the advances that sol–gel technology has brought to the silica-based bioactive bioceramics are presented.
Authors: Lisa C. Klein
Abstract: The sol-gel process has been used to modify the electrolyte membrane used in proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC). Recent progress is reported in the synthesis of hybrid membranes involving Nafion®. These membranes have been prepared by infiltration and recasting, and contain silicates, phosphosilicates, zirconium phosphosilicates, titanosilicates, or phosphotungstates.

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