Influência da catálise ácida e básica na preparação da sílica funcionalizada pelo método sol-gel

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Estudo da secagem de materiais híbridos siloxano-PMMA preparados pelo processo sol-gel

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Método sol-gel modificado para obtenção de alumina nanoencapsulada com terras raras

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Sol phase and sol-gel transition in SnO2 colloidal suspensions

The effect of concentration on the structure of SnO2 colloids in aqueous suspension, on their spatial correlation and on the gelation process was studied by small angle x-ray scattering (SAXS). The shape of the experimental SAXS curves varies with suspension concentration. For diluted suspensions ([SnO2] less than or equal to 0.13 mol L-1), SAXS results indicate the presence of colloidal fractal aggregates with an internal correlation length xi congruent to 20 Angstrom, without any noticeable spatial correlation between them. This suggests that the aggregates are spatially arranged without any significant interaction like in ideal gas structures. For higher concentrations ([SnO2] = 0.16, 0.32, and 0.64 mol L-1), the colloidal aggregates are larger (xi = 24 Angstrom) and exhibit a certain degree of spatial correlation between them. The pair correlation function corresponding to the sol with the highest concentration (0.92 mol L-1) reveals a rather strong short range order between aggregates, characteristic of a fluid-like structure, with an average nearest-neighbor distance between aggregates d(1) = 125 Angstrom and an average second-neighbor distance d(2) = 283 Angstrom. The pair distribution function remains essentially invariant during the sol-gel transition, suggesting that gelation involves the formation of a few points of connection between the aggregates resulting in a gel network constituted by essentially linear chains of clusters..

Sol-gel synthesis of titania-alumina catalyst supports

Titanium oxide is a good candidate as new support for hydrotreating (HDT) catalysts, but has the inconvenience of presenting small surface area and poor thermal stability. To overcome these handicaps TiO2-Al2O3 mixed oxides were proposed as catalyst support. Here, the results concerning the preparation, characterization and testing of molybdenum catalyst supported on titania-alumina are presented. The support was prepared by sol-gel route using titanium and aluminum isopropoxides, chelated with acetylacetone (acac) to promote similar hydrolysis ratio for both the alcoxides. The effect of nominal complexing ratios [acac]/[Ti] and of sol aging temperature on the structural features of nanometric particles was analyzed by quasi-elastic light scattering (QELS) and N-2 adsorption isotherm measurements. These characterizations have shown that the addition of acac and the increase of aging temperature favor the full dispersion of primary nanoparticles in mother acid solution. The dried powder presents a monomodal distribution of slit-shaped micropores, formed by irregular packing of platelet primary particles, surface area superior to 200 m(2) g(-1) and mean pore size of about 1 nm. These characteristics of porous texture are preserved after firing at 673 K. The diffraction patterns of sample fired above 973 K show only the presence of anatase crystalline phase. The crystalline structure of the support remained unaltered after molybdenum adsorption, but the surface area and the micropore volume were drastically reduced. (C) 2002 Published by Elsevier B.V. B.V.

Photoelectrochemical properties of sol-gel Nb2O5 films

Structural, optical, electro and photoelectrochemical properties of amorphous and crystalline sol-gel Nb2O5 coatings have been determined. The coatings are n-type semiconductor with indirect allowed transition and present an overall low quantum efficiency (phi < 4%) for UV light to electric conversion. The photoconducting behavior of the coatings is discussed within the framework of the Gartner and Sodergren models. Improvement can be foreseen if Nb2O5 coatings can be made of 10-20 nm size nanoparticles.

Effect of aging on the stability of ceramic foams prepared by thermostimulated sol-gel process

This work presents a new route of preparation of zirconium ceramic foams based on the thermostimulated sol-gel process. This method produces gelled bodies with up to 90% of porosity in the wet gel and can be used to make complex-shaped components. Unfortunately, the shrinkage during the drying step allows to a catastrophic reduction (50%) of the foam porosity. To improve the foam stability we carried out a systematic study of the effect of gel foam aging on the drying process. Samples were aged in closed vessel at 25 C during different time period (from 6 to 240 h). The shrinkage and the mass loss during drying at 50 C were measured in situ, using a non-contact technique performed with a special apparatus. The results show that the total linear shrinkage decreases from 46% to 8% as the aging period increase from 6 to 240 h. This behavior is followed by a small change of total mass loss, from 42 to 54%. It indicates that by aging the structural stiffness of the foams increases due to secondary condensation reactions. Thus, by controlling the aging period, the porosity can be increased from 67 to 75% and the average size of mesopores of dried foams can be screened from 0.3 to 0.9 mum. Finally, these results demonstrate that the thermostimulated sol-gel transition provides a potential route to ceramic foams manufacture.

Small-angle X-ray scattering study of sol-gel-derived siloxane-PEG and siloxane-PPG hybrid materials

Hybrid organic-inorganic two-phase nanocomposites of siloxane-poly(ethylene glycol) (SiO3/2-PEG) and siloxane-poly(propylene glycol) (SiO3/2-PPG) have been obtained by the sol-gel process. In these composites, nanometric siloxane heterogeneities are embedded in a polymeric matrix with covalent bonds in the interfaces. The structure of these materials was investigated in samples with different molecular weights of the polymer using the smalt-angle X-ray scattering (SAXS) technique. The SAXS spectra exhibit a well-defined peak that was attributed to the existence of a strong spatial correlation of siloxane clusters. LiClO4-doped siloxane-PEG and siloxane-PPG hybrids, which exhibit good ionic conduction properties, have also been studied as a function of the lithium concentration [O]/[Li], O being the oxygens of ether type. SAXS results allowed us to establish a structural model for these materials for different basic compositions and a varying [Li] content. The conclusion is consistent with that deduced from ionic conductivity measurements that exhibit a maximum for [O]/[Li] =15.

Formation of zirconia foams using the thermostimulated sol-gel transition

In this paper we describe the production of zirconia-based foams by a novel thermostimulated sol-gel route, that employs the foaming of colloidal suspensions prior to the sol-gel transition promoted by small increase of temperature (congruent to3 degreesC). This method produces gelled bodies having porosity >70% in the wet stage, and can be used to produce complex-shaped components. The effect of a foaming agent (Freon11 or CCl3F) and surfactant content on the formation and stability of the foams was analyzed. The rheologic measurements demonstrate that by increasing the surfactant concentration, the gelation time decreases increasing foam stability. As the surfactant concentration and quantity of foaming agent increase, the density decreases and the porosity increases. Hg porosimetry results show that the dry foam presents a bimodal pore size distribution. The family of sub-micrometer pores was attributed to the formation of a microemulsion between Freon11 and water. Scanning electron microscopy analysis shows that the foam structure consists of a three-dimensional network of spherical pores, which may be open and interconnected or closed, at larger or smaller porosities, respectively. Finally these results show that the thermostimulated sol-gel transition provides a potential route for ceramic foam manufacture. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Ultraviolet excitation of photoconductivity in thin films of sol-gel SnO2

Tin dioxide (SnO2) thin film photoconductivity spectra were measured for a large temperature range using a deuterium source, the intensity of photocurrent spectra in the range 200-400 nm is temperature dependent, and the photocurrent increases in the ultraviolet even for illumination with photon energies much higher than the bandgap transition. This behavior is related to recombination of photogenerated electron-hole pairs with oxygen adsorbed at grain boundaries, which is consistent with nanoscopic crystallite size of sol-gel deposited films. (c) 2005 Elsevier Ltd. All rights reserved.

O processo sol-gel: uma visão físico-química

The aim of this work was to revise the main concepts involved in the physical-chemistry of the sol-gel process. First, the preparation and its influence on the gel's structure are focalized; the sol-gel transition is revised under the thermodynamics of crytical phenomena point of view and by considering the kinetic models of aggregation. Second, the experimental methods usually used to characterize the sol-gel transition are discussed.

Catalysis and temperature dependence on the formation of ZnO nanoparticles and of zinc acetate derivatives prepared by the sol-gel route

The chemical and structural nature of powders prepared from the zinc acetate-derived precursor using the sol-gel route is discussed. The influence of the synthesis temperature and of the hydrolytic catalyst on the structural features of the powder is focused on the basis of X-ray powder diffraction (XRPD) and extended X-ray absorption fine structure (EXAFS) measurements and complemented with density and thermoanalysis (TG-DTA) results. EXAFS and XRPD results show that no-washed nanoparticulate powders are composed of a mixture of ZnO (wurtzite), zinc acetate, and zinc hydroxyacetate. The latter has a layered structure typical of hydroxy double salts (HDS). The main component of no-washed powders is always unreacted zinc acetate solid but the relative amount of the zinc-based compounds depends on the nature of the hydrolytic catalyst, hydrolysis ratio, and of synthesis temperature. According to the proportion of the three zinc-based compounds, three families of powders could be distinguished. The amount of ZnO nanoparticles (1.6 +/- 0.6 nm) decreases as the synthesis temperature increases, as the hydrolysis ratio decreases, or by changing from basic to acid catalysis. This finding suggests that the formation of zinc compounds is controlled by the equilibrium between hydrolysis-condensation and complexation-reprecipitation reactions.

Small-angle X-ray scattering study of gelation and aging of Eu3+-doped sol-gel-derived siloxane-poly(oxyethylene) nanocomposites

The aggregation, gelation, and aging of urea-cross-linked siloxane-poly(oxyethylene) nanohybrids [(U600)-n] containing two different amounts of europium triflate initially dissolved in an ethanol-water mixture were investigated by in situ small-angle X-ray scattering (SAXS). For both low (n = [O]/[Eu] = 80) and high (n = 25) europium contents, the SAXS intensity was attributed to the formation of siloxane clusters of about 8-11 Angstrom in size. Siloxane cluster formation and growth is a rapid process in hybrids with low Eu contents and slow in Eu-rich hybrids. An additional contribution to the scattering intensity at very low angles was attributed to the formation of a coarse structure level. At this secondary level, the structure can be described as a set of dense domains containing siloxane clusters embedded in a depleted matrix composed of unfolded polymer chains and solvent. By fitting a theoretical function for this model to the experimental SAXS curves, relevant structural parameters were determined as functions of time during the sol-gel transition and gel aging. For hybrids with low europium contents (n = 80), the size of the siloxane clusters remains essentially invariant, whereas the dense segregation domains progressively grow. In hybrids with high doping contents (n = 25), the preponderant structure variation during the first stages of the sol-gel transformation is the slow growth of siloxane clusters. For these hybrids, the segregation of siloxane clusters forming dense domains occurs only during advanced stages of the process.

Characterization of the porosity developed in a new titania-alumina catalyst support prepared by the sol gel route

Titanium oxide (TiO2) is a good candidate for support of hydrotreating catalysts but has the disadvantage of presenting a low surface area and a poor thermal stability when compared with Al2O3. A mixed TiO2-Al2O3 support was proposed as an alternative that is expected to be free from these drawbacks. The variation during firing of the nanoporous texture of supports composed of TiO2-Al2O3, TiO2 and Al2O3 was studied by small angle X-ray scattering (SAXS). The supports were prepared by the sol-gel route using Ti and Al isopropoxides. We have particularly analyzed the effects of acid and basic hydrolysis on the nanostructural features of catalyst supports fired at different temperatures. The nanopore radius distribution functions were determined from SAXS results assuming a simple model of spherical nanopores embedded in a homogeneous solid matrix. The modal pore radius in both pure TiO2 and pure Al2O3 supports grows from 1.3 to 2.2 nm as the firing temperature increases from 673 to 973 K. on the other hand, the modal pore radius in the mixed TiO2-Al2O3 support remains below 1.2 nm over the same range of firing temperatures. These results demonstrate the good thermal stability of the nanoporous texture of mixed TiO2-Al2O3 supports.

Spectroscopic characterization of BPSCCO thin films grown by dip-coating technique

This work describes the growth of Bi2-xPbxSr2Can-1CunO2n+4 thin films by the dip-coating technique for 0.4 less than or equal to x less than or equal to 1. X-ray and Raman spectroscopic techniques were carried out in order to characterize the films at room temperature. From X-ray data it is observed that the films are multi-phased presenting phases 2201, 2212 and 2223 along with the undesirable Ca2PbO4 phase. It is also observed that phase 2212 becomes dominant when Pb content increases. The Raman modes observed agree with the overall features expected for these compounds. (C) 2003 Elsevier B.V. B.V. All rights reserved.