New tungstate fluorophosphate glasses

\A new class of tungstate fluorophosphate glasses was identified in the NaPO3-BaF2-WO3 ternary system. The variation of several physical properties was determined with respect to chemical composition. Characteristic temperatures, density and refractive index increase as tungsten oxide content increases. The optical transmission range and specially the energy bandgap depend of the WO3 amount. No crystallization could be observed for the Most WO3 concentrated vitreous samples (greater than or equal to20% molar). Color and optical properties of the glasses depend of the melting time because of the presence of reduced tungsten species like W5+ and W4+. In addition, photodarkening is observed in tungsten rich glass samples under UV laser illumination and this phenomenon can be reversible by heat treatment near the glass transition temperature. (C) 2004 Published by Elsevier B.V.

Physical properties of silicate glasses doped with SnO2

The presence of tin in the network of silicate glasses produces changes in several of their physico-chemical properties. Glasses with the composition (mol%) 22Na(2)O (.) 8CaO (.) 70SiO(2) containing up to 5 wt% of SnO2 were analyzed under several experimental techniques. Dilatometric measurements showed an increase of the glass transition temperature with increasing tin content, while the average thermal expansion coefficient is reduced. Vickers microhardness, density, and refractive index also increase with the tin content. Diffuse reflectance spectra in the infrared (DRIFT) showed that the presence of tin, even at low concentrations, is responsible for some structural changes since there is an increase of the bridging oxygen concentration. The doped glasses present a brown color and optical absorption spectra measurements are interpreted as being due to precipitation of tin in the form of colloidal particles during cooling of the melted glass. In the Na+ <-> K+ ion exchange process the presence of tin in the glass network hinders the diffusion of these ions. The diffusion coefficients of those ions were calculated by the Boltzmann-Matano technique, after concentration profiles obtained by EDS measurements. All results obtained present evidences that Sn4+ cation acts as a glass network former. (c) 2005 Elsevier B.V. All rights reserved.

Surface protection of fluoroindate glasses by sol-gel dip-coated SnO2 thin layers

SnO2 coatings were deposited by a sol-gel dip-coating process to shield fluoroindate glasses (40In-F-3:16BaF(2):20SrF(2):20ZnF(2):2NaF:2GaF(3)) against corrosion in aqueous environments. The effect of the number of coating applications and of the withdrawal speed on the thickness, density and roughness of tin oxide films was investigated by X-ray reflectivity. Film thickness increases both with the number of coating applications and the withdrawal speed. The aqueous leaching of uncoated and SnO2-coated fluoroindate glasses was studied by scanning electron microscopy (SEM) and infrared spectroscopy (FTIR), showing that the glass surface was protected against hydrolytic attack. (C) 1999 Elsevier B.V. B.V. All rights reserved.

Raman scattering, differential scanning calorimetry and Nd3+ spectroscopy in alkali niobium tellurite glasses

Alkali niobium tellurite glasses have been prepared and some of their properties measured by differential scanning calorimetry and Raman scattering. The vitreous domain was established in the pseudo ternary phases diagram for the system TeO2-Nb2O5-(0.5K(2)O-0.5Li(2)O). Raman scattering shows that for samples in the TeO2 rich part of the phase diagram the vitreous structure is composed essentially of (TeO4) units connected by the vertices, as in the alpha-TeO2 crystal. The addition of alkali and niobium oxides causes depolymerization to occur with structures composed essentially of (TeO3) and (NbO6) units. Samples with the composition (mol%) 80TeO(2)-10Nb(2)O(5)-5K(2)O-5Li(2)O, stable against crystallization, were prepared containing up to 10% mol Nd3+. The addition of this oxide increases the rigidity of the vitreous network shifting characteristic temperatures to higher temperatures. For the 10% Nd3+ sample amorphous phase separation is assumed to exist from the observation of two glass transition temperatures. Spectroscopic properties such as Judd-Ofelt Omega(lambda) intensity parameters, radiative emission probabilities, and induced emission cross sections were calculated. From these results and also from the emission quenching observed as a function of Nd3+ concentration, we suggest that these glasses could be utilized in optical amplifying devices. (C) 1999 Elsevier B.V. B.V. All rights reserved.

Nuclear magnetic relaxation of F-19 in fluoride glasses

Temperature and frequency dependence of the F-19 nuclear spin relaxation of the fluoroindate glass, 40InF(3)-20ZnF(2)- 20SrF(2)-2GaF(3)-2NaF-16BaF(2) and the fluorozirconate glass, 50ZrF(4)-20BaF(2)-21LiF-5LaF(3)-4AlF(3); are reported. Measurements were undertaken on pure and Gd3+ doped samples, in the temperature range of 185-1000 K, covering the region below and above the glass transition temperature, T-g. The temperature and frequency dependence of the spin-lattice relaxation rate, T-1(-1), measured in the glassy state at temperature <300 K, is less than the observed dependence at higher temperatures. At temperatures >T-g, the fluorine mobility increases, leading to a more efficient spins lattice relaxation process. Activation energies, for F- motion, are 0.8 eV for the fluoroindate glass and 1 eV for the fluorozirconate glass. The addition of Gd3+ paramagnetic impurities;at 0.1-wt%, does not alter the temperature and frequency dependence of T-1(-1), but increases its magnitude more than one order of magnitude. At temperatures <400 K, the spin-spin relaxation time, T-2(-1), measured for all samples, is determined by the rigid-lattice nuclear dipole-dipole coupling, and it is temperature independent within the accuracy of the measurements. Results obtained for the pure glass, at temperatures >400 K, show that T-2(-1) decreases monotonically as the temperature increases. This decrease is explained as a consequence of the motional narrowing effect caused by the onset of the diffusive motion of the F- ions, with an activation energy around 0.8 eV. For the doped samples, the hyperfine interaction with the paramagnetic impurities is most effective in the relaxation of the nuclear spin, causing an increase in the T(2)(-1)s observed at temperatures >600 K. (C) 1999 Elsevier B.V. B.V. All rights reserved.

Polymeric organic-inorganic hybrid material containing iron(III) porphyrin using sol-gel process

Here we describe the preparation of iron(II) porphyrinosilica in a simple one-pot reaction, where the -SO2Cl groups present in the phenyl rings of FeTDCSPP+ react with 3-aminopropyltriethoxysilane and tetraethoxysilane in the presence of a nitrogenous base, leading to iron(III) porphyrinosilica. In this same procedure, molecular cavities containing regularly spaced functional groups are created through the molecular imprinting technique, in which the nitrogenous base coordinated to the iron(III) porphyrin serves as a template. The removal of such template in a Soxhlet extractor leads to a cavity with the same shape and size as the nitrogenous base, enabling the construction of shape-selective catalysts mimicking cytochrome P-450. Five different imprinting molecules have been used: imidazole, 1-methylimidazole, 2-methylbenzimidazole, 4-phenylimidazole and miconazole and ultra-violet/visible absorption spectroscopy, thermogravimetric analysis and electron paramagnetic resonance carried out. (C) 1999 Elsevier B.V. B.V. All rights reserved.

A SAXS study of kinetics of aggregation of TEOS-derived sonogels at different temperatures

The kinetics of aggregation of tetraethoxysilane (TEOS)-derived silica sols, produced by acid-catalyzed and ultrasound-stimulated hydrolysis, were studied by 'in situ' measurements of small-angle X-ray scattering (SAXS) at the temperatures 40 degreesC, 60 degreesC and 70 degreesC. The results were analyzed in terms of the evolution with time (t) of the SAXS intensity probing the mass fractal characteristics of the system, the average radius of gyration (Rc,) of the clusters and the number of primary particles per cluster. The aggregation process yields mass fractal structures which exhibit a scattering exponent (alpha) practically equal to 2, in the probed length scale range (5.3 nm < 1/q < 0.22 nm), beneath and even far beyond the gel point. This suggests that a is a direct measure of the real mass fractal dimension (D) of the structure. The precursor sol (pH = 2) exhibits I nm mean sized clusters with mass fractal dimension D similar to 1.9. Increasing the pH to 4.5, the cluster mean size and the number of primary particles per cluster increase but the system keeps a more opened structure (D similar to 1.4). In the first aggregation stages, D increases up to similar to2 by incorporating primary particles to the clusters without changing their mean size. From this stage, the aggregation progresses following a thermally activated scaling law well described by R-G similar tot(1/D) in all cases. This is indicative of a diffusion-controlled cluster-cluster aggregation process. The activation energy of the process was found to be 91.7 kJ/mol. (C) 2001 Elsevier B.V. B.V. All rights reserved.

Phase separation in pyrex glass by hydrothermal treatment: evidence from micro-Raman spectroscopy

In this work, we investigated the formation of porous silica matrix obtained by hydrothermal treatment under saturated steam condition from Pyrex (R) glass. This investigation was carried out by scanning electron microscopy (SEM), infrared spectroscopy (IR), X-ray powder diffractometry (XRD) and Raman microscopy. We observed the presence of connected and homogeneously distributed pores in a non-crystalline silica phase and a detectable interface between silica and remnant glass phases resulting in a framework similar to asymmetric membranes. The results indicate that the process of phase separation takes place at lower temperature than that of glass-transition on the surface of the glass phase. Essential reaction between water and silica at supercritical condition together with the formation and leaching of soluble phase contribute to obtain porous silica matrix, (C) 2001 Elsevier B.V. B.V. All rights reserved.

Structure of SnO2 alcosols and films prepared by sol-gel dip coating

To obtain SnO2 films to be used for surface protection of fluoride glasses, a non-aqueous sol-gel route for the preparation was developed. An ethanolic SnO2 colloidal suspension was prepared by thermohydrolysis of SnCl4 solution at 70 degreesC. By using this procedure, redispersable powders with nanometer sized particles were obtained. Films were obtained by dip coating on glass and mica substrates. The structures of the ethanolic precursor suspension and films were compared to those of similar samples prepared by the classical aqueous sol-gel route. Comparative analyses performed by photon correlation spectroscopy demonstrated that the powders obtained by freeze-drying are fully redispersable either in aqueous or in alcoholic solutions at pH greater than or equal to 8. As prepared sols and redispersed colloidal suspensions have hydrodynamic radius distribution (2-14 nm) with an average size close to 7 nm. The variations in film structures with firing temperature were investigated by small-angle X-ray scattering and X-ray reflectometry. The experimental results show that the films have a two level porous structure composed of agglomerates of primary colloidal particles. The sintering of the primary particles leads to the densification of agglomerates and to the formation of inter-agglomerate spatially correlated pores. The volume fraction of intra-agglomerate pores is reduced from approximate to 50% to approximate to 30% by the precipitation of precursor salts partially hydrolyzed in ethanolic solution. (C) 2001 Elsevier B.V. B.V. All rights reserved.

Gelation of siloxane-poly(oxypropylene) composites

The viscoelastic properties of siloxane-poly(oxypropylene) (PPO) nanocomposites prepared by the sol-gel process has been analyzed during gelation by dynamic rheological measurements. The changes of storage and loss moduli, complex viscosity and phase angle has been measured as a function of time showing the newtonian viscosity of the sol in the initial step of gelation, and its progressive transformation to a viscoelastic gel. The rheologic properties have been correlated to mass fractal, nearly linear growth models and percolation theory. This study, completed by quasi-elastic light scattering and Si-29 solid state nuclear magnetic resonance measurements, shows that the mechanisms of gelation of siloxane-PPO hybrids depend on the molecular weight of the polymer and on the pH of the hybrid sol. For hybrids prepared in acid medium, a polymerization involving silicon reactive species located at the extremity of the polymer chains and presenting a functionality f = 2 occurs, forming a fractal structure during the first stage of sol-gel transition. For samples prepared under neutral pH, the fractal growth is only observed for hybrids containing short polymer chains (M-w similar to 130 gmol(-1)). The fractal dimensionality determined from the change in the rheological properties, indicates that the fractal growth mechanism changes from reaction-limited to diffusion-limited aggregation when the molecular weight of the PPO increases from 130 to 4000 gmol(-1) and as catalyst conditions change from acidic to neutral. Near the gel point, these hybrid gels have the typical scaling behavior expected from percolation theory. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Infrared-to-visible CW frequency upconversion in erbium activated silica-hafnia waveguides prepared by sol-gel route

70SiO(2)-30HfO(2) mol% planar waveguides, doped with Er3+ with concentrations ranging from 0.3 to 2 mol% were prepared by sol-gel route, using dip-coating deposition on vitreous-SiO2 substrates. Infrared-to-visible upconversion emission, upon excitation at 980 nm, has been observed for all the samples. The upconversion results in green, red and blue emissions. The investigation of the upconversion dynamic as a function of the Er3+ concentration and excitation power, show that processes such as excited state absorption and energy transfer upconversion are effective. (C) 2003 Elsevier B.V. All rights reserved.

Photoinduced structural changes in antimony polyphosphate based glasses

A photocontraction effect in amorphous films of the binary glass system 0.20 [Sb(PO3)(3)](n)-0.80 Sb2O3 has been observed after UV irradiation using the 350.7 nm Kr+ ion laser line with 5.0 W/cm(2). Good optical quality films up to 4.0 mum were deposited on silica substrates at room temperature in vacuum by electron beam physical vapor deposition (EB-PVD) and characterized using WDX, XRD, optical absorption, infrared reflectance, profilometry and atomic force microscopy (AFM) techniques. Very stable glasses were prepared by the melt quenching technique and used as evaporation source for the production of films. The photoinduced structural change (PSC) was observed as a variation of about 6% in the film thickness and this effect is accompanied by a photobleaching of the irradiated area with a blue shift of the optical absorption edge. Otherwise this photoinduced change in the film thickness is very sensitive to the variations in the shape and intensity of the laser beam; therefore several possibilities in optical recording arise from these results. (C) 2003 Published by Elsevier B.V.

Photo-induced transformations in chalcogenide composite layers

Investigations of photo-induced structural transformations (PST) and related changes of optical parameters in the light-sensitive amorphous chalcogenides were extended to composite layers, which consist of a wide band-gap material and an active material, Se60Te40 with a smaller band gap. Photo-stimulated interdiffusion and/or crystallization in layered Se0.6Tc0.4/As0.6Se0.94 and Se0.6Te0.4/SiOx were investigated with respect to their dependence on the compositional modulation of the multilayer at scale-dimensions (similar to3-10nm). It was established that PST due to the interdiffusion and crystallization can be efficiently operated by the composition of the adjacent layers of the multilayer which results in the change of the transformation rate and of the optical relief type (positive or negative). The comparison with a single Se0.6Te0.4 layer and with the known data for amorphous-Se/As2S3 multilayers supports the advantages of composite layers for amplitude-phase optical recording. (C) 2004 Published by Elsevier B.V.

Rheological study of the thermoreversible sol-gel transition of sulfate modified zirconyl chloride aqueous sol

The sols prepared by mixing a ZrOCl2 acidified solution to a hot H2SO4 aqueous solutions were studied in order to clarify the mechanism of thermoreversible sol-gel transition observed in this system. The viscoelastic properties of these suspensions were analyzed during the sol-gel transition by dynamic rheological measurements and quasi-elastic light scattering. The rheological properties were correlated to mass fractal and nearly linear growth models, and percolation theory. The results evidence that the thermoreversible sol-gel transition in this system is due to the formation of a network of physically linked aggregates having fractal structure. The decrease of the SO42- contents in the initial solution leads to the decrease of the fractal dimensionality from 2.3 to 1.8, indicating a change of the kinetic mechanism of aggregate growth. Near the gel point these samples have the typical scaling expected from percolation theory. (C) 2004 Elsevier B.V. All rights reserved.

Study of fluorine losses in oxyfluoride glasses

Glasses in PbGeO3-PbF-CdF2 and GeO2-PbO-PbF2-CdF2 systems were studied and the fluorine losses during synthesis were investigated. Samples were characterized by differential scanning calorimetry (DSC), X-ray diffraction, Fourier transform infrared spectroscopy (FTIR) and Raman scattering spectroscopy. The use of stoichiometric germanate glass, PbGeO3, instead of introducing individual oxides (GeO2 + PbO) lead to decreasing fluorine losses, as detected by a fluorine ion selective electrode. The main structural features obtained from vibrational spectroscopy could be described by a metagermanate basic structure permeating fluorine rich regions. (c) 2005 Elsevier B.V. All rights reserved.