M. Piñero

Sonogels and derived materials

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

Larnite powders and larnite/silica aerogel composites as effective agents for CO2 sequestration by carbonation

This paper presents the results of the carbonation reaction of two sample types: larnite (Ca(2)SiO(4)) powders and larnite/silica aerogel composites, the larnite acting as an active phase in a process of direct mineral carbonation. First, larnite powders were synthesized by the reaction of colloidal silica and calcium nitrate in the presence of ethylene glycol. Then, to synthesize the composites, the surface of the larnite powders was chemically modified with 3-aminopropyltriethoxysilane (APTES), and later this mixture was added to a silica sol previously prepared from tetraethylorthosilicate (TEOS). The resulting humid gel was dried in an autoclave under supercritical conditions for the ethanol. The textures and chemical compositions of the powders and composites were characterized.The carbonation reaction of both types of samples was evaluated by means of X-ray diffraction and thermogravimetric analysis. Both techniques confirm the high efficiency of the reaction at room temperature and atmospheric pressure. A complete transformation of the silicate into carbonate resulted after submitting the samples to a flow of pure CO(2) for 15 min. This indicates that for this reaction time, 1t of larnite could eliminate about 550 kg of CO(2). The grain size, porosity, and specific surface area are the factors controlling the reaction.

Control growth of PbS quantum dots doped sono-ormosil

Semiconductor PbS quantum dots doped-SiO 2 organically modified silicate (ormosil) gels were synthesised via sol-gel by using high-power ultrasounds (sonogel). The effect of PbS crystal concentration and the addition of (3-mercaptopropyl)trimethoxysilane acting as surface capping agent (SCA) were investigated. By adjustment of the SCA to lead ratio, PbS nanoparticles of different sizes and morphologies were obtained. Textural parameters were calculated from N 2 physisorption isotherms. The PbS galena phase was identified by x-ray diffraction, the crystal size by high-resolution transmission electron microscopy, and the exciton confinement by ultraviolet–visible–near-infrared spectrophotometry. Crystallite mean sizes of spheres and cubes ranging from 6.5 to 10.5 nm and needles 7-nm wide and 15–20 nm long, for different PbS and SCA concentrations, were obtained. These results differ from those predicted by the effective mass approximation corroborating the band gap modifications in the smallest nanocrystals. The method allows the control of the crystal size and improves the stabilization of the PbS nanocrystals.

Aerogels Synthesis by Sonocatalysis: Sonogels

High power ultrasound applied to liquids produces cavities that attain very high temperatures and pressures (“hot spots”). When an alkoxide/water mixture is sonicated, the cavities act as nanoreactors, where the hydrolysis reaction starts. The products (alcohol, water, and silanol) help continue the dissolution of that immiscible mixture. The reactions depend on catalyst content, temperature bath, and alkyl group length. When the resultant sonosol gels, it produces a sonogel; it is denser, with finer and more homogeneous porosity than that of a classic counterpart. Thus, acoustic cavitation makes it possible to obtain nanostructured materials. Sono-aerogels have a high surface-to-volume ratio and are built by small particles (~1 nm radius) and a highly cross-linked network with low surface coverage of –OH radicals. The processing as well as their short-range order at an atomic scale and at a micrometric scale is presented in this chapter. Finally, these materials find application, among others as biomaterials for tissue engineering and for CO2 sequestration.

Creep and Stress Relaxation of Hybrid Organic-Inorganic Aerogels

Organic/inorganic hybrids silica aerogels were synthesized by the classical sol-gel method with application of high power ultrasounds to the liquid mixture. Precursors were tetraethoxysilane (TEOS), as inorganic phase, and polydimethyl siloxane (PDMS), as organic one. These hybrid organic-inorganic materials are known as ORMOSIL (ORganically MOdified SILicates). Monolithic aerogels were obtained by supercritical drying in ethanol. Failure tests by uniaxial compression shows an increase of the rupture modulus as well as a decrease of the Young’s modulus with the polymer content, tuning from a brittle solid to a rubbery-like one. These hybrid aerogels behave as elastomers showing a decrease in the relaxation viscoelastic modulus. Nanoindentation tests have been performed in these hybrid aerogels: load/unload cycles about 1.5 mN of maximum load have shown a decreasing value of the reduced modulus, as well as both plastic and elastic work with the organic content, while hardness remains almost constant. Elastic recovery parameter rised with the increasing organic content. Results from creep tests made with uniaxial compression configuration are discussed and compared with nanoindentation. Viscoelastic behavior of these hybrids materials can be described by a rheological model.

Photoluminescence from CdS Quantum Dots in Silica Gel

CdS semiconductor nanocrystals were grown as quantum dots (QDs) inside a silica matrix obtained by the sol-gelmethod and assisted in the mother liquid by high powerultrasounds. Small-angle neutron scattering (SANS) accountsfor a 3.6 nm crystal size homogeneously distributed. Optical excitation from the third harmonic of a Nd:YAG ns laser wasfocused on the sample to study the photoluminescence (PL) atroom temperature. The PL spectrum shows radiative processfrom intrinsic transitions and a broad band corresponding tothe traps. Variable stripe length (VSL) method was used to measure the optical gain spectra by the growth of theamplified luminescence. A broad optical gain spectrumproduced by the biexciton-exciton transitions revealing thestimulated emission from the CdS QDs. It is also observed ared-shift of the PL emission crystal size-dependent.

CdS semiconductor nanoparticles in silica sonogel matrices: Code: F12

SiO2 gels obtained by sonocatalytic method combined with DCCA were used as host-matrices for extremely fine dispersions of CdS semiconductor particles. Small crystallites were produced “in situ” by H2S gas diffusion method. The particles were characterized by TEM and HRTEM, EXAFS, UV-Vis and Raman spectroscopies. The size of crystallites ranged from 5 to 10 nm. The optical transmission spectra showed the characteristic blue shift as a function of the particles size, as predicted by the theory. The optical and mechanical qualities of the samples were substantially improved by an infiltration method using a sono-sol which sealed the superficial pores thus ensuring greater longevity and the possibility of obtaining transparent gels by polishing.

5YSZ Powders from Gels: Densification and Microstructure Characterization

Five mol% yttria doped zirconia powders were prepared by controlled hydrolysis of alkoxides. Their sintering behavior was monitored by dilatometry to optimize the sintering schedule. The compact bodies were characterized by SEM and quantitative analysis of their X-ray patterns. Re-sintering under axial pressure slightly improved the final relative density of the zirconia powders at the expense of a grain size increase that, however, for most of them remains submicrometric.

Mechanical Properties of Organic-Inorganic Hybrid Black Coating of Silica and Polyurethane

The mechanical properties of black thin film on glass substrates and metallic, obtained from the synthesis of organic-inorganic silica hybrids via sol-gel are described. Hybrid silica-based coatings with tetraethoxysilane (TEOS), methyltrietoxysilane (MTES), polyurethane (PU) and commercial silica colloidal solution (Ludox HS30) as precursors, were used. The procedure is divided in the following three steps: 1) obtaining homogenous and stable solutions of different organic silica-polymer composition, 2) aggregation of the black pigment with the formulate FeMnOx, 3) Coating substrates by both dip-coating and aerograph airbrush, at room temperature. The films obtained were characterized by SEM, AFM and nano-indentation. The coatings of the Ludox-MTES-PU/FeMnOx system showed the better characteristics concerning the morphology and the adhesion to the glass and metallic copper substrates

Bioactive organic-inorganic hybrid aerogels

We have prepared organic-inorganic hybrid materials (OIHM), incorporating an organic phase in the inorganic precursor sol in combination with the assisted high power ultrasounds. A sonoormosil, results after gelation. Colloidal silica particles have been added to these hybrids to enable network porous volume and pore radius to be tailored. Finally, in vitro bioactivity of this material has been promoted by adding calcium to the initial sol. The structure and bioactivity of these materials have been studied as preliminary step that includes their mechanical behaviour. These materials have a very fine structure especially when colloidal silica particles are included. When are immersed in a solution simulating blood plasma they are bioactive, presenting a better in vitro behaviour the sample with colloid particles