Rheology of the gelation of fluorine-doped silica sols

Dynamic oscillatory shear measurements are used to probe the gelation kinetics of aqueous sols composed of either particulate silica or silicon alkoxide solution. Unlike steady shear measurements, these dynamic tests do not alter the structure and kinetics of the sol-gel process. The dynamic storage moduli of both systems show sharp transitions at the onset of gelation. However, the gelation kinetics of the two systems are very different; the modulus of alkoxide system remains unchanged until the gel point is reached, whereas that of the particulate system increases with time even below the gel point as the colloid cluster grow. Unlike the alkoxide gel, the particulate gel reverses to a sol upon shearing. The gelation kinetics of a resheared sample is slower than that of a fresh sample. The structures associated with the sol-gel transition for this particulate system have been monitored using freeze fracture microscopy and correlate with rheological observations. The gelation times for both sols are varied by changing the fluoride ion content.     

Spinnablity and structure characterization of mullite fibers via sol–gel-ceramic route

Viscosity and rheology behaviors of the mullite sols prepared from aqueous solution of aluminum nitrate, aluminum isopropoxide and tetraethylorthosilicate has been investigated. Rheological measurement suggested that mullite sols exhibited good spinniabilty when the evaporation of the solvent is allowed during sol–gel process. Spinnable sols showed shear-thinning flow behavior with high viscosity to the time of gelation. By adjusting temperature, the gelation degree and viscosity of the sol could be stabilized at a certain value and the sol–gel transition could be transferred to the spinning line of a laboratory dry-spinning apparatus. Continuous mullite fibers were obtained from such sols using sol gel dry-spinning method. The final ceramic fibers had smooth surfaces with an average diameter of 50 μm. Structure evolution of mullite ceramic fibers were characterized by MAS NMR and specific surface area analyses.     

Photochromic behavior of Li-stabilized MoO3 sol–gels

Photochromic behavior of Li-doped MoO₃ sol–gels prepared by the peroxo sol–gel method was studied in details using various experimental techniques, including UV–vis, XRD, Raman, EPR, and XPS. The lithium doping has drastically enhanced the stability of the MoO₃ sol–gels. Upon UV-light irradiation from a high-pressure mercury lamp, the Li-doped MoO₃ sol turned from yellowish into deep blue, and the sol in ethanol exhibited much more intense color change than the sol in water. The UV-light exposure has the effect of re-arranging the structural units of MoO₃ and building up short-range order inside the solid. The formation of a blue colored bronze in the photo-irradiated sols because of the reduction of Mo⁶⁺ to Mo⁵⁺ is also evidenced by the experimental data. Based on these data, a mechanism for the photochromic behavior is proposed.     

Gain cross-section of 1.06 μm emission in Nd-doped SiO2-LaF3 glass-ceramics prepared by sol-gel method

The room temperature near-infrared emission at 1.06 μm corresponding to the ⁴F_3/2 → ⁴I_11/2 laser transition in Nd³⁺-doped SiO₂-LaF₃ transparent glass-ceramics prepared by sol-gel method has been characterized in terms of the product of emission cross-section and fluorescence lifetime. Stimulated emission cross-sections have been calculated from experimental emission spectra using the Füchtbauer-Ladenburg equation. The precipitation of LaF₃ nanocrystals after heat-treatment, confirmed by X-ray diffraction, induces a significant increment of the 1.06 μm near-infrared emission compared with the as made sample. An enhancement of emission cross-section of about 10% has been achieved with increasing temperature of heat-treatment of the nano-ceramming process from 800 to 1000 °C, which is related to the growth of LaF₃ nanocrystals.     

Effects of Er3+ doping concentration and calcination on fluorescence properties of La3Ga5.5Nb0.5O14 nanocrystals

The effects of Er³⁺ doping concentration and calcination were examined on the fluorescence properties of La₃Ga_5.5Nb_0.5O₁₄ (Er:LGN) nanoparticles for the first time. High quality Er:LGN nanoparticles were synthesized by sol‐gel method. The room temperature fluorescence spectra showed a green emission, which can be attributed to ²H_11/2‐⁴I_15/2 and ⁴S_3/2‐⁴I_15/2 transition. The relationship between the relative emission intensity and the doping concentration was investigated. The maximum of the Er³⁺ doping concentration in LGN nanopowders is 2.0%. The fluorescent lifetime of 2.0% Er:LGN nanoparticles is 1.45ns. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystal growth and optical properties of LuYAG:Ce single crystal

Crystals of Ce³⁺-doped (Lu_xY_1?x)₃Al₅O₁₂ (LuYAG) have been grown and studied by X-ray powder diffraction, emission spectroscopy, excitation spectroscopy and X-ray excited fluorescence spectroscopy. The X-ray powder diffraction pattern revealed that the as-grown LuYAG:Ce crystal possessed the garnet structure. Compared with Ce³⁺-doped Y₃Al₅O₁₂ (YAG), the absorption bands associated with the 4f–5d transition shifted to shorter wavelengths, the emission band that originated from a transition from the lowest 5d level to the ²F ground state of the Ce³⁺ ions shifted to the blue, which was probably due to a larger Stokes shift of the emission, and the reduction of relative intensity of antisite defect emission in the X-ray excited fluorescence spectra revealed that introducing Lu ions into YAG could reduce the antisite defect.     

Effects of rehydration on Tb3+ spectroscopy in sol–gel glasses

Spectroscopy of sol–gel silicate glasses containing Tb³⁺ is used to investigate the diffusion of water from the atmosphere into annealed glass samples. In materials that are annealed to 900 °C the blue–violet ⁵D₃ → ⁷F_J fluorescence is comparable in intensity to green emission from the ⁵D₄ level. However, the ⁵D₃ emission is reduced to ∼5% of its original strength in several hours if glasses are exposed to ambient conditions. Re-annealing restores the original intensity of the ⁵D₃ fluorescence. The degree of fluorescence quenching is shown to depend strongly on initial preparation of the sol–gel materials and on annealing conditions.     

Investigation of inorganic network formation in photopatternable hybrid sol-gel films by Si liquid NMR

Liquid ²⁹Si NMR spectroscopy was used to investigate the sol-gel process of methacryloxypropyltrimethoxysilane. This hybrid precursor was involved in the realization of optical elements in laminated crack-free thick films (up to 100 μm), through spatially controlled photopolymerization. Understanding the formation of the inorganic network was of first importance to insure the creation of crack-free photopatterned thick films in a laminated configuration. Material preparation required evaporation of the volatile solvents released during the sol-gel process and limitation of the condensation degree. Both conditions were achieved by a drying process at room temperature. The structure and the composition of the sols were investigated and compared to non-dried sols. Evolution of inorganic species distribution was also studied under increasing aging time or storage temperature. NMR peak fitting of T1 species gave fruitful information on the sol structure evolution during the sol-gel process. It pointed out the presence of a large variety of oligomers in the sol. The study also allowed the identification of more constraint cyclic species in dried sols stored at room temperature. Their amount significantly decreased when increasing the storage temperature and was attributed to ring opening of cyclic species. Consequently, the structure of the dried sol will depend both of the aging time and of the storage temperature. All these results have to be taken into account when the degree of condensation has to be limited to achieved photopatternable hybrid layers for specific optical applications.     

Cocrystallization of chelating-energetic-material and metal-organic-framework with the nitrogen-rich ligand bis(tetrazole)methane

ABSTRACT

In this contribution, the emission response of 4-(2,5-dimethylthiophen-3-yl)-2-(2-hydroxy- phenyl)benzothiazole (THBT) was investigated in detail. The experimental shows that Zn²⁺ could induce fluorescence increasing in tetrahydrofuran (THF). Especially, the quenched emission of THBT was significantly enhanced with 0.76 quantum yield. Due to the coordination of Zn²⁺, the excited state intramolecular proto transfer (ESIPT) was inhibited and leads to emission quenching correspondingly. The strong emission could ascribe to the Zn-THBT complex. As well, the addition of BF₃·Et₂O also could form fluorine-boron complex and inhibit ESIPT effect. Additionally, the BF complex is also a strong blue emitter. Therefore, strong blue emission upon BF₃·Et₂O addition to THBT in dioxane was found. The possible coordinating mechanisms of THBT coordinate with Zn²⁺ and BF₃·Et₂O were given.     

Porous silicon grains in SiO2 matrix: Ultrafast photoluminescence and optical gain

We search for the presence of stimulated emission in samples of porous silicon embedded in the sol–gel derived SiO₂ matrix. By modifying the etching conditions of the porous silicon using hydrogen peroxide, we decrease substantially the nanocrystal size and produce a significant blue shift of the PL emission. Femtosecond variable-stripe length experiments combined with the shifting-excited spot technique demonstrates positive optical gain (modal gain ∼25 cm⁻¹) in the range 550–730 nm. Ultrafast photoluminescence dynamics indicates the origin of the stimulated emission as possibly due to recombination of excitonic states inside silicon nanocrystals.     

Vegetal nanoclusters in hybrid silica films prepared by sol-gel spin coating technique

Hybrid sol-gel films containing Rosemary extract nanoclusters embedded into hybrid silica network have been successfully synthesized using the sol-gel procedure by exploiting the template route, in association with an adequate spin-coating method. Formation of film precursor sols and effect of selected amounts of octyl trimethoxysilane and ethyl oleate succinic anhydride into the starting acid sols on the sol-gel product size have been evaluated by dynamic light scattering technique. The spectral characteristics of hybrid organic-inorganic films have shown that the multiple functional groups from Rosemary extract associated with residual Si-OH groups can cause the increase in the degree of physical interaction. The effect of hybrid sols meaning the silica precursors molar ratio and template concentration on the fluorescence of hybrid films has been also investigated. The fluorescence properties of synthesized films were found to be dependent on template and natural extract concentration. A higher amount of template resulted in doubling the fluorescence intensity in the 400-480 nm domain. The microstructural characteristics of the hybrid films revealed by atomic force microscopy have shown a homogeneous surface morphology with cluster-like structure. Hybrid silica films exhibit a periodic structure with cluster size less than 150 nm.     

Spectroscopic properties of Yb3+ ions in halogeno-sulfide glasses

In this work, we report the optical properties of Yb³⁺ ions in halogeno-sulfide glasses of composition (75 − x)GeS₂–25Ga₂S₃–xCsCl (x = 5%, 10%, 15%, 20%, and 25% CsCl). This study includes an analysis of the influence of halide concentration on the absorption and emission cross-sections and lifetimes of Yb³⁺ ions. A blue shift of the absorption and emission bands and a decrease of the absorption and emission cross-sections and transition probability are observed as the halide concentration increases in the glass.     

Optical properties of fluorine-substituted zinc bismuth phosphate glasses

The effects of the substitution of fluoride ions for oxide ions on the thermal and optical properties of ternary ZnO-Bi₂O₃-P₂O₅ glass with low-P₂O₅ content (20-25 mol%) were investigated. Fluoride ions were introduced into the glass up to about 12 mol% as ZnF₂. Raman spectra indicated that fluoride ions were substituted for oxide ions connected with bismuth ions. Deformation and glass transition temperatures decreased monotonically with fluorine concentration. The absorption edge shifted toward higher energies with increasing fluorine concentration by about 0.3 eV for 12 mol% ZnF₂ substitution. The blue shift of the absorption edge is attributable to two effects. One was a blue shift of an absorption band which was observed as a peak at 4.7 eV in the reflection spectra and was attributed to the spin forbidden 6s-6p interband transition in Bi³⁺ ions. The blue shift originates from a change in electron-donating ability through anions as expected from electronegativity or optical basicity. Another is a disappearance of a shoulder at around 4.3 eV in the reflection spectra. The latter was the major reason for the large blue shift of the absorption edge energy, because the band relating to the 4.3 eV shoulder is close to the absorption edge.     

Optical properties of Er3+-singly doped and Er3+/Yb3+-codoped novel oxyfluoride glasses

Novel oxyfluoride glasses SiO₂–Al₂O₃–Na₂O–ZnF₂ doped with Er³⁺ and Er³⁺/Yb³⁺ were fabricated. The optical properties of the synthesized glasses were experimentally and theoretically investigated in detail. The experimental and calculated oscillator strengths of Er³⁺ were determined by measurement of the absorption spectrum of Er³⁺-singly doped glass. According to the Judd–Ofelt theory, the experimental intensity parameters were calculated, from which the radiative transition probabilities, fluorescence branching ratios and radiative lifetimes were obtained. The fluorescence lifetime and quantum efficiency for the near-infrared emission of Er³⁺-singly doped glass were determined to be 3.0 ms, and 42%, respectively. Visible upconversion luminescence was observed under 980 nm diode laser excitation. The dependence of the upconversion emission intensity upon the excitation power was examined, and the upconversion mechanisms are discussed.     

Photochemistry of azobenzene in sol-gel systems

The photophysical and photochemical behavior of azobenzene incorporated into sol-gel systems was studied. Sols doped with azobenzene were prepared by the hydrolysis of tetramethoxysilane. The absorption spectra of azobenzene in sols with and without HCl as a catalyst showed photochromism; UV irradiation changes trans-azobenzene to cis-azobenzene, which returns to trans-azobenzene by successive visible light irradiation. The results indicate that azobenzene is not protonated in the sol prepared by the present conditions. The absorption spectra showed that the azobenzene-doped xerogel prepared without HCl is mostly adsorbed on silica surfaces by the hydrogen bonding between the azo groups and silanol and/or water molecules. The adsorption did not affect photochromic behavior and photo-reversible changes were observed in the xerogels. UV photolysis of the azobenzene-doped xerogel prepared with HCl so produced protonated benzo[c]cinnoline that photochromic behavior was deteriorated. Surface modified xerogels were prepared from the mixture of tetraethoxysilane and methyltriethoxysilane in order to make clear the effect of the surface silanol groups. It was shown that the formation of the protonated benzo[c]cinnoline is suppressed by the introduction of Si-CH₃. These present results confirm that the acidic sites of Si-OH₂⁺ of xerogels play an important role in the photochemical reaction of azobenezene in silica xerogels.     

Investigations on sol–gel process and structural characterization of SiO2-P2O5 powders

The purpose of the study is to investigate the influence of the precursors, pH of the solution and temperature on the gelation and structure evolution of the samples from the SiO₂-P₂O₅ system. Tetraethoxysilane (TEOS) was used as precursor for SiO₂ and triethylphosphate (TEP) or phosphoric acid for P₂O₅, together with water as reagent for hydrolysis reaction and ethylic alcohol as solvent. The pH of the sols was modified by adding hydrochloric acid, in the case of TEP and by adding ammonia, in the case of H₃PO₄. The samples have been prepared starting from P₂O₅/SiO₂ = 1/10 and 1/5 molar ratio, H₂O/TEOS = 1; 2; 3 mass ratios and C₂H₅OH/TEOS = 1 mass ratio. We prepared silico-phosphate samples in the 1.5–5 pH domain and we observed that in all the cases, the lowest gelation time was found in the 3.5–4.5 pH range. We found that for the same pH value samples prepared with H₃PO₄ had a lower gelation time (few days) by comparison with the samples prepared with TEP (weeks), explainable by the low rate of the hydrolysis and condensation reactions of TEP. When the amount of water was increased, the gelation time increased in the case of samples prepared with H₃PO₄ and it was not significantly changed in the case of the samples prepared with TEP. The increasing of the solution temperature up to 40–41 °C yielded a decreasing of the gelation time (hours), especially for the samples prepared with H₃PO₄ by comparison with those prepared using TEP. In all the cases, the increased amount of water resulted in an increasing of the gelation time, even the temperature was raised. FTIR and Raman spectroscopy characterization aimed at getting information about the structural changes in the case of the samples dried in air and also for those heated at 100 °C, 300 °C, 600 °C and 900 °C. Vibration modes specific for SiOEt, SiOH, hydrogen bonds, H₂O and combined vibrations have been observed, which are in agreement with those revealed in literature data. ³¹P and ²⁹Si MAS NMR spectra gave interesting information about first surrounding of P and Si ions meaning the type and proportion of Q species and their evolution starting from the room temperature up to 900 °C.     

Photoluminescence properties of Sm3+-doped SFB glasses for efficient visible lasers

The photoluminescence properties of different concentrations of Sm³⁺ ions doped sodium fluoroborate (SFB) glasses of composition Na₂O–LaF₃–CaF₂–AlF₃–B₂O₃–SmF₃ have been investigated. The energy level analysis is carried out using free-ion Hamiltonian model. The Judd–Ofelt intensity parameters are used to evaluate the laser characteristic parameters such as spontaneous transition probability, radiative lifetime and fluorescence branching ratio for ⁴G_5/2 → ⁶H_J (J = 5/2, 7/2, 9/2 and 11/2) emission transitions of Sm³⁺ ion. The principal photoluminescence transitions of interest are identified by recording the emission spectra for different Sm³⁺ ion concentrations and measuring their emission cross-sections, integrated absorption cross-sections and optical gain parameters. The decay profiles from the ⁴G_7/2 excited manifold state to its lower lying energy levels have been recorded by monitoring the excitation and emission wavelengths at 402 and 600 nm, respectively. The dependence of effective fluorescence lifetime on the Sm³⁺ concentration is also discussed.     

Blue emission of ZrO2:Tm nanocrystals with different crystal structure under UV excitation

In this paper, thulium doped ZrO₂ nanocrystals with different phases were prepared by Pechini sol-gel complex method. The effects of host structure, phases and concentration on luminescence properties of Tm³⁺ ions were investigated. With the increase of the concentration of Tm³⁺ ions, the phosphors phase transformed from monoclinic to tetragonal structure and all the phosphors emitted blue light. However, because the environment symmetry around Tm³⁺ in m-ZrO₂ (C_2h) is worse than that of t-ZrO₂ (D_4h), there were still obvious differences in the emission and excitation. The reason was investigated with the help of energy level diagram. In addition, the quenching concentration of Tm³⁺ in ZrO₂ host is 1 m/m%.     

Influence of Nature of Silica Source on Template‐Free Synthesis of ZSM‐5

Synthesis of ZSM‐5 from template‐free batches which preceded the preparation of template‐free ZSM‐5 layers on porous supports was studied to ascertain the effect of nature of silica source on the purity of template‐free ZSM‐5. Silicic acid and two colloidal silica sols were used as silica sources to prepare the template‐free batches with a molar composition of 6.5Na₂O:Al₂O₃:80SiO₂:3196H₂O. One of the colloidal silica sols contained methanol as stabilizer while the other did not. The product purity and rate of crystallization increased when colloidal silica sols were used as silica source, however, use of silicic acid led to low purity and slow crystallization rate. The methanol in the colloidal silica sol appeared to act as template to promote the crystallization and was occluded in the resultant ZSM‐5 pores. The dissolution of the meta‐stable ZSM‐5 phase and formation of quartz was observed regardless of the nature of the silica source in case of prolonging the crystallization time more than 90 h at 200°C.     

Preparation and characterization of chemically deposited (Cd0.7‐Zn0.3)S:CdCl2,Gd nanocrystalline films

Nanocrystalline films of (Cd_0.7‐Zn_0.3)S:CdCl₂,Gd are prepared by chemical deposition technique on microscopic glass substrates with varied concentrations of capping agents thiophenol and methanol. The deposition is based on the principle of precipitation followed by condensation. Results of XRD, SEM, optical absorption spectra and photoluminescence emission spectra are presented. The crystalline nature of (Cd_0.7‐Zn_0.3)S:CdCl₂,Gd was found by XRD studies in which different diffraction peaks of CdS and ZnS are observed. The SEM micrographs show long tubes and irregular distributions of particles in presence of capping agents. The blue shift observed from optical absorption spectra in the present case indicates a quantization effect. Band gaps are also found to increase due to particle size reduction. The photoluminescence emission spectra show shift of peaks in the lower wavelength side due to reduction in particle size. The particle sizes are determined from optical absorption and XRD studies. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)