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61.
Wafa I. Abdel-Fattah M.SH. Fayed SH.R. Gooda W.F.F. Mekky 《Journal of Sol-Gel Science and Technology》1998,13(1-3):981-985
-Eucryptite solid solution (LAS) Li2O : Al 2O3 : 3SiO2(113) was prepared via a sol-gel route. Complex solutions of lithium or aluminum salts in ethanol/water mixtures were catalysed by inorganic acids (catalyst/alkoxide ratio 0.1). Different ratios of H2SO4 catalyst were also used.TGA showed that the maximum % yield of LAS 113 occurred using H2SO4 catalyst. Above 400°C exothermic peaks occurred in DSC followed by the glass transition. XRD and IR showed a high crystallinity of -eucryptite solid solution using all acids at the 0.1 ratio with the highest crystallinity for the 0.05 H2SO4 ratio. SEM revealed similar grain sizes in spite of higher acid concentrations. XRD data indicated a change to a tetragonal modification with higher catalyst ratio. 相似文献
62.
Vaz FA de Castro PM Molina C Ribeiro SJ Polachini FC Messaddeq Y Nunes AP de Oliveira MA 《Talanta》2008,76(1):226-229
Photopolymerized sol-gel monolithic columns for use in capillary electrochromatography were prepared in 125 microm i.d. polyacrylate-coated fused-silica capillaries. The polyacrylate-coating, unlike the polyimide one, is transparent to the radiation used (approximately 370 nm), and thus, no coating removal is necessary. This is a very important particularity since intrinsic capillary column characteristics, such as flexibility and mechanical resistance, are unchanged. A mixture containing metacryloxypropyltrimethoxysilane (MPTMS) as the polymeric precursor, hydrochloric acid as the catalyst, toluene as the porogen and bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide (Irgacure 819) as the photoinitiator was irradiated at 370 nm for 20 min inside the capillaries to prepare the columns through sol-gel approach. The versatility and viability of the use of polyacrilate as a new capillary external coating were shown through preparation of two columns under different conditions, which were tested in electrochromatography for separation of standard mixture containing thiourea (marker compound), propylbenzene, phenanthrene and pyrene. 相似文献
63.
Fernando A. Sigoli Younes Messaddeq Sidney J. L. Ribeiro 《Journal of Sol-Gel Science and Technology》2008,45(2):179-185
The sol–gel method combined with a spin-coating technique has been successfully applied for the preparation of rare-earth
doped silica:germania films used for the fabrication of erbium-doped waveguide amplifiers (EDWA), presenting several advantages
over other methods for the preparation of thin films. As with other methods, the sol–gel route also shows some drawbacks,
such as cracks related to the thickness of silica films and high hydrolysis rate of certain precursors such as germanium alkoxides.
This article describes the preparation and optical characterization of erbium and ytterbium co-doped SiO2:GeO2 crack-free thick films prepared by the sol–gel route combined with a spin-coating technique using a chemically stable non-aqueous
germanium oxide solution as an alternative precursor. The non-crystalline films obtained are planar waveguides exhibiting
a single mode at 1,550 nm with an average thickness of 3.9 μm presenting low percentages of porosity evaluated by the Lorentz–Lorenz
Effective Medium Approximation, and low stress, according to the refractive index values measured in both transversal electric
and magnetic polarizations. Weakly confining core layers (0.3% < Δn < 0.75%) were obtained according to the refractive index difference between the core and buffer layers, suggesting that low-loss
coupling EDWA may be obtained. The life time of the erbium 4I13/2 metastable state was measured as a function of erbium concentration in different systems and based on these values it is
possible to infer that the hydroxyl group was reduced and the formation of rare-earth clusters was avoided. 相似文献
64.
Rafael Leite de Oliveira Hernane da Silva Barud Rosana M. N. de Assunção Carla da Silva Meireles Geandre Oliveira Carvalho Guimes Rodrigues Filho Younes Messaddeq Sidney José Lima Ribeiro 《Journal of Thermal Analysis and Calorimetry》2011,106(3):703-709
In this study, microcrystalline cellulose (MCC) was prepared from the acid hydrolysis of bacterial cellulose (BC) produced
in culture medium of static Acetobacter xylinum. The MCC-BC produced an average particle size between 70 and 90 μm and a degree of polymerization (DP) of 250. The characterization of samples was performed by thermogravimetric analysis, X-ray diffraction, and scanning electron
microscopy (SEM). The MCC shows a lower thermal stability than the pristine cellulose, which was expected due to the decrease
in the DP during the hydrolysis process. In addition, from X-ray diffractograms, we observed a change in the crystalline structure.
The images of SEM for the BC and MCC show clear differences with modifications of BC fiber structure and production of particles
with characteristics similar to commercial MCC. 相似文献
65.
Cid B. de Araújo Glauco S. Maciel Leonardo de S. Menezes Nikifor Rakov Edilson L. Falco-Filho Vladimir A. Jerez Younes Messaddeq 《Comptes Rendus Chimie》2002,5(12)
We present recent results on frequency upconversion (UPC) obtained in fluoroindate glasses (FIG) doped with Ho3+, Tm3+ and Nd3+ ions and codoped with Pr3+/Nd3+ and Yb3+/Tb3+ ions. The results for the Ho3+-doped samples show strong evidence of energy transfer (ET) between Ho3+ ions resonantly excited at 640 nm. The origin of the blue-green upconverted fluorescence observed was identified and the dynamics of the signals revealed the pathways involved in the UPC process. In the case of Tm3+-doped FIG, the samples were resonantly excited at 650 nm and the main mechanism that contributes for the red-to-blue upconversion is excited-state absorption (ESA). The FIG samples codoped with Pr3+/Nd3+ were excited at 588 nm in resonance with transitions starting from the ground state of the Nd3+ and the Pr3+ ions. It was observed that the presence of Nd3+ ions enhanced the Pr3+ emission at 480 nm by two orders of magnitude. Multiphonon (MP)-assisted upconversion is also discussed for Nd3+-doped FIG pumped at 866 nm. Emission at 750 nm with a peculiar linear dependence with the laser intensity was observed and explained. A rate-equation model that includes MP absorption via thermally coupled electronic excited states of Nd3+ was developed and describes well the experimental results. The role played by effective phonon modes is clearly demonstrated. MP-assisted UPC process was also studied in Yb3+/Tb3+-codoped FIG samples excited at 1064 nm, which is off-resonance with electronic transitions starting from the ground state. It was determined that the mechanism leading to Tb3+ emission in the blue is due to ET from a pair of excited Yb3+ ions followed by ESA in the Tb3+ ions. 相似文献
66.
Glassy films of Ga10Ge25S65 with 4 μm thickness were deposited on quartz substrates by electron beam evaporation. Photoexpansion (PE) (photoinduced increase in volume) and photobleaching (PB) (blue shift of the bandgap) effects have been examined. The exposed areas have been analyzed using perfilometer and an expansion of 1.7 μm (ΔV/V ≈ 30%) is observed for composition Ga10Ge25S65 exposed during 180 min and 3 mW/cm2 power density. The optical absorption edge measured for the film Ge25Ga10S65 above and below the bandgap show that the blue shift of the gap by below bandgap photon illumination is considerable higher (ΔEg = 440 meV) than ΔEg induced by above bandgap illumination (ΔEg = 190 meV). The distribution of the refraction index profile showed a negative change of the refraction index in the irradiated samples (Δn = −0.6). The morphology was examined using a scanning electron microscopy (SEM). The chemical compositions measured using an energy dispersive analyzer (EDX) indicate an increase of the oxygen atoms into the irradiated area. Using a Lloyd's mirror setup for continuous wave holography it was possible to record holographic gratings using the photoinduced effects that occur in them. Diffraction efficiency up to 25% was achieved for the recorded gratings and atomic force microscopy images are presented. 相似文献
67.
Poirier G Ottoboni FS Cassanjes FC Remonte A Messaddeq Y Ribeiro SJ 《The journal of physical chemistry. B》2008,112(15):4481-4487
In this work, vitreous samples were prepared in the binary system (100 - x)NaPO3-xMO3 with M = Mo and W and x varying from 10 to 60. The transmittance properties in the UV, visible, and near-infrared were monitored as a function of MO3 concentration. In both cases, an increase in the amount of transition metal results in an intense and broad absorption band in the visible and near-infrared attributed to metal reduction under synthesis conditions. It was shown that this large absorption can be partially or totally removed using specific oxidizing agents or by improving synthesis parameters such as melting temperature or cooling rate of the melt. In addition, structural investigations by Raman and X-ray absorption spectroscopy suggest that reduction only occurs when the metal cation is in octahedral geometry and that the transmittance improvement is not related with any structural changes. These results were explained in terms of thermodynamic equilibrium of redox species in the melt and allowed to obtain for the first time transparent and chemically stable glasses containing high concentrations of MO3 with transition metals in octahedral geometry inside the glass network. 相似文献
68.
M. M. Heravi Y. SH. Beneshtiha R. Hekmat Shoar N. Nami 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1):285-289
Abstract The addition of dimethyl acetylenedicarboxylate (DMAD) to 6-menthy-1, 2,4-triazlne-3(2H)thione-5(4H)-one afforded 2-methoxylcarboxy-7-methyl-1,3-thiazino[3,2-b][1,2,4]triazine-4,8-dione. 相似文献
69.
In this paper we show that each factorization structure on a small category , satisfying certain conditions, yields a presheaf on and a morphism of presheaves . We then give connections, and set up one to one correspondences, between subclasses of the following classes: (a) closure operators on (b) subobjects of (c) morphisms from to (d) weak Lawvere–Tierney topologies (e) weak Grothendieck topologies (f) closure operators on . 相似文献
70.
Poirier G Nalin M Cescato L Messaddeq Y Ribeiro SJ 《The Journal of chemical physics》2006,125(16):161101
In this work, we present a new photochromic tungstate based glass which have both absorption coefficient and refractive index modified under laser exposure. The photosensitive effect is superficial under ultraviolet (UV) irradiation but occurs in the entire volume of the glass under visible irradiation. The effect can be obtained in any specific point inside the volume using an infrared femtosecond laser. In addition, the photosensitive phenomenon can be erased by specific heat treatment. This glass can be useful to substitute actual data storage supports and is a promising material for 3-dimensional (3D) and holographic optical storage. 相似文献