Structural characteristics of silica sonogels prepared with different proportions of TEOS and TMOS

Sonohydrolysis of mixtures of tetraethoxysilane (TEOS) and tetramethoxysilane (TMOS) with different TMOS/(TMOS + TEOS) molar ratio R was carried out to obtain similar to 2.0 x 10(-3) mol SiO2/cm(3) and similar to 86%-volume liquid phase wet gels. Aerogels were obtained by supercritical CO2 extraction in autoclave. The samples were analyzed by small-angle X-ray scattering (SAXS) and nitrogen adsorption. The structure of the wet gels can be described as a mass fractal structure with fractal dimension D similar to 2.2 and characteristic length increasing from similar to 4.6 nm for pure TEOS to similar to 6.4 nm for pure TMOS. A fraction of the porosity is eliminated with the supercritical process. The fundamental role of the TMOS/(TMOS + TEOS) molar ratio on the structure of the aerogels is to increase the porosity and the pore mean size as R changes from pure TEOS to pure TMOS. The supercritical process increases the mass fractal dimension and shortens the fractality domain in the mesopore region. A secondary structure appearing in the micropore region of the aerogels can be described as a mass/surface fractal structure with correlated mass fractal dimension D-m similar to 2.6 and surface fractal dimension D-s similar to 2.3. (C) 2007 Elsevier B.V. All rights reserved.

Microwave-hydrothermal synthesis of perovskite bismuth ferrite nanoparticles

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Microwave-hydrothermal synthesis of barium strontium titanate nanoparticles

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

The role of the Eu3+ ions in structure and photoluminescence properties of SrBi2Nb2O9 powders

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

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..

Effect of addition of MTES on the structure of siloxane-PPG nanocomposites

Hybrid transparent and flexible siloxane-polypropyleneglycol (PPG) materials with covalent bonds between the inorganic (siloxane) and organic (polymeric) phases were prepared by sol-gel process. In order to improve the quality of the mechanical properties of these materials, different amounts of methyltriethoxysilane (MTES) were added to the initial sol. The effect of MTES addition on the structure of the composites was studied by Small-Angle X-Ray Scattering (SAXS) and Si-29 Nuclear Magnetic Resonance (Si-29 NMR). In absence of MTES, SAXS spectra exhibit a peak that is assigned to spatial correlation due to short range order between the siloxane clusters embedded in the polymeric phase. The experimental results indicate that, for low MTES concentrations ([MTES]/[O] less than or equal to 0.8, O: ether-type oxygen of PPG), the silicon species resulting from hydrolysis and condensation of MTES fill the open spaces between polymeric chains, interacting with the ether-type oxygens. For larger MTES content ([MTES]/[O] greater than or equal to 0.8), the number of free ether-type oxygen sites avalaible for reaction with such silicon species is not large enough. Consequently, a fraction of silicon species resulting from MTES addition graft to siloxane clusters formed by hydrolysis and condensation of the hybrid precursor. For all MTES concentrations the condensation degree of the siloxane phase, determined from Si-29 NMR spectroscopy, is high (> 69%), as expected under neutral pH synthesis conditions.

Lead-cadmium oxyfluoride glasses and glass-ceramics

Glasses and glass-ceramics have been obtained in oxyfluoride systems involving lead and cadmium fluorides and one of the well-known glass former oxides SiO2, B2O3 and TeO2. Vitreous domains were established and a wide range of compositions including high heavy metal contents lead to stable glasses. Amorphous structures have been studied by short-range order spectroscopy techniques (Raman scattering and x-ray absorption) and molecular basic structures have been identified. Besides the usual oxides, the role of glass former could also be proposed for cadmium ions. Special attention has been paid for crystallization process. Cubic lead fluoride, cubic lead tellurite, tetragonal tellurium oxide and a solid solution of the type Pb1-xCdxF2 are obtained as crystallization products depending on the composition and temperature of heat treatments. Pb1-xCdxF2 solid solutions are well known superionic materials and obtaining this solid solution as a crystal phase could be very interesting for applications concerning ionic electrical conduction properties. The addition of rare earth ions led to the control of the crystallization process. In the presence of the nucleating ion only the cubic form beta-PbF2 was identified. Rare earth ions are present in the crystal phase and crystal-like spectroscopic properties were observed suggesting interesting applications for these perfectly transparent glass ceramics in photonics.

Room-temperature photoluminescence in structurally disordered SrWO4

Intense and broad visible photoluminescent (PL) band in structurally disordered SrWO4 compounds was observed at room temperature. The polycrystalline scheelite strontium tungstate (SrWO4) samples prepared by the polymeric precursor method at different temperatures of annealing were structurally characterized by x-ray diffraction and Fourier transform Raman spectroscopy measurements. Quantum-mechanical calculations showed that the local disorder in the cluster of the network modifiers Sr has a very important role in the charge transfer. The experimental and theoretical results are in good agreement, indicating that the generation of the intense visible PL band can be related to short-range order-disorder degree in the scheelite structure. (c) 2006 American Institute of Physics.

Structural studies on TeO2-PbO glasses

Binary tellurite-based glasses in the TeO2-PbO system were prepared and its structure investigated by means of Raman Scattering and X-ray Absorption Spectroscopy. Both spectroscopies indicate strong modifications of the first coordination shell around tellurium atoms when the PbO content increases revealing for lead its glassy network modifier role. Also, Pb L-3-edge EXAFS measurements reveal this structural role played by lead atoms, but the presence of a medium range order contribution indicates that lead also participates to the glassy network formation. (C) 2001 Elsevier B.V. Ltd. All rights reserved.

Structural evolution of Eu-doped hydroxyapatite nanorods monitored by photoluminescence emission

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

Reply to Comment on 'Pb1-xCaxTiO3 solid solution (x=0.0, 0.25, 0.50, and 0.75): A theoretical and experimental approach'

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

EXAFS and XRD analyses of the structural evolutions involved during drying of SnO2 hydrogels

In order to determine structural changes during drying of inorganic gels, the local and long-range order structure of SnO2-x(OH)2x xerogels resulting from drying hydrogels with different concentrations of electrolyte (Cl- and NH+ 4) have been measured by extended X-ray absorption fine structure (EXAFS), X-ray diffraction (XRD) and N2 adsorption techniques. EXAFS measurements performed at the Sn K edge on the hydrogels and xerogels show the existence of microcrystallites with the cassiterite structure. Two drying modes have been used: freeze drying and drying by evaporation at 45°C. It is shown that the microcrystallite size determined by XRD and EXAFS techniques on the drying mode and on the electrolyte concentrations. The microcrystallite size measured on the freeze dried xerogels is similar to that of their parent hydrogels, whatever the concentration of electrolyte; however, during drying by evaporation, a preferential growth of microcrystallites along the c-axis of the cassiterite structure is observed. The size of these crystallites is enhanced with a decrease of the electrolyte concentration. Specific surface areas calculated by the Brunauer-Emmett-Teller method indicate that this preferential growth is related to the improvement of the network connectivity. The comparison of both drying processes indicates that crystallization and polycondensation are independent phenomena. © 1995 Elsevier Science B.V. All rights reserved.

Fe K-edge X-ray absorption spectroscopy study of Pb(Fe2/3W 1/3)O3-PbTiO3 multiferroic ceramics

The present paper is a comprehensive study concerning Fe K-edge X-ray absorption spectroscopy (XAS) measurements, which were performed to characterize the local structure of (1 - x)Pb(Fe2/3W1/3)O 3-xPbTiO3 samples as a function of temperature and PbTiO3 content. Results obtained by the fits of extended X-ray absorption fine structure consist with rhombohedral symmetry for Pb(Fe 2/3W1/3)O3 composition at temperatures lower than room temperature. This result is in apparent disagreement with X-ray and neutron diffraction characterization which have been reported. This apparent disagreement is related to the fact that XAS probes the short-range order, whereas X-ray diffraction provides structural information about the average structure. Moreover, as the PbTiO3 content increases, a disorder has been detected at local structure of the FeO6 octahedron. Analysis of X-ray absorption near edge structure spectra did not show modifications in intensity nor energy of transitions. © 2013 American Institute of Physics.

New methodology for a faster synthesis of SrSnO3 by the modified Pechini method

SrSnO3, a perovskite-type complex oxide, was synthesized by the modified Pechini method using two different precursors, tin chloride and metallic tin. The first one is already traditional in the literature and it claims about 30 days, only for the cleaning of tin citrate aiming at the elimination of the chloride. The second route was developed by our research group and saves time, taking 6 h to complete the synthesis of the resin. The results show that SrSnO3 obtained from the metallic tin show a higher short range order, leading to a band gap value higher than those reported in the literature, besides a meaningful reduction in the formation of SrCO 3, as compared to the one obtained from tin chloride.

Estudo no grau de ordem-desordem no 'SR''BI IND 2''NB IND 2''O IND.9 pela influência do íon 'EU IND.3'/' e tratamento térmico por microondas

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