Fluorescent compacts prepared by the entrapment of benzoxazole type dyes into a silica matrix at high pressure

We have produced silica-gel compacts doped with 2,5-Bis(benzoxazol-2^′-yl)-4-methoxyphenol dye using high-pressure processing of powders synthesized by the sol-gel technique. The high-pressure compaction of powders with three different dye concentrations was done at 4.5 GPa and room temperature. We have measured optical and mechanical properties of the obtained compacts. They were very stable, transparent, crack free, hard (3.56 ± 0.07 GPa) and dense (1.95 ± 0.03 g/cm³), being resistant to polishing and leaching, which enables its use in optical applications. The Stokes shift observed was higher than 100 nm indicating that the intramolecular proton-transfer in the electronically excited state (ESIPT) of this dye is maintained, even in an OH rich environment like silica. A shift to higher wavelength in the fluorescence spectra of the compacts, attributed to the increasing in the conjugation of the π system, was observed.     

The devitrification kinetics of silica powder heat-treated in different conditions

In the present paper, the devitrification kinetics of silica powder heat-treated in air and in nitrogen were investigated. The heat-treated powders were confirmed to partially crystallize into cristobalite by X-ray diffraction analysis. The devitrification kinetics data of silica powder fitted the Avrami equation very well. The measured value of n was ascertained to be 1.63 and 1.60 when the silica powder was heat-treated in air and in nitrogen, respectively, corresponding to activation energies of 408 kJ/mol and 529 kJ/mol, respectively. The effect of phenolic resin-derived carbon on crystallization of silica powder was also studied. By adding phenolic resin into silica powder, the devitrification of silica powder was fully restrained up to 1500 °C in flowing nitrogen. High concentrations of oxygen vacancies may retard the nucleation of cristobalite on the surface of silica powder. Phenolic resin-derived pyrolysis carbon restrained the nucleation of cristobalite, because it prevented the reactions of oxygen vacancies with H₂O and O₂ molecules.     

Structural evolutions of hydrothermally prepared mesostructured MCM-48 silica using differently manufactured amorphous silica powders

The hydrothermal synthesis of Si-MCM-48 mesoporous molecular sieves was carried out using a ternary SiO₂:CTAOH:H₂O system wherein differently manufactured amorphous silica powders such as fumed silica (FMDS), spray dried precipitated silica (SDPS) and flash dried precipitated silica (FDPS) were used as silica source materials. The changes in structural/textural properties were evaluated using powder XRD, N₂ adsorption-desorption and scanning electron microscopy techniques. Studies on the progressive development of MCM-48 mesophases revealed that, the reactivity of the silica source follow the trend: FMDS > SDPS > FDPS. MCM-48 synthesized using low cost FDPS has exhibited thicker pore walls but poorer orderness, while MCM-48 prepared from relatively expensive FMDS has thinner pore walls and more ordered structure. Moreover, the extent of contraction caused by calcinations, agglomerate size and structural stability were found to depend on the reactivity of the silica source used.     

Effect of method of preparation and drying time on photophysical properties of coumarin 1 laser dye embedded in HCl catalysed sol-gel glasses

Coumarin 1 (C1) dye is impregnated in transparent sol-gel glass samples prepared by sol-gel process using three methods - (I) using HCl as catalyst and glycerol as a drying control chemical additive (DCCA), (II) using HCl as catalyst at 60 °C subsequent drying at room temperature, and (III) using HCl as catalyst at 60 °C and heated at 600 °C for 3 h. The sol-gel matrices prepared by Methods I and II are given dip treatment with methanol/distilled water (50/50 volume) for 16 h before dipping into dye solution. The effect of method and drying time of matrix on spectroscopic properties of C1 dye doped glass samples has been studied. The optical density (OD) at absorption maximum wavelength and fluorescence intensity (FI) at fluorescence maximum wavelength of all C1 dye doped samples prepared by Methods I and II decrease, where as there is no change in photophysical properties (OD/FI) is observed in samples prepared by Method III with the time of drying of the sol-gel samples. These absorption/fluorescence properties of C1 dye in sol-gel glass matrices are compared with its respective properties in methanolic solution in acidic environment.     

Infrared and thermogravimetric study of high pressure consolidation in alkoxide silica gel powders

High density, transparent, crack-free and hard compacts of silica gel were produced by compaction under nearly hydrostatic environment at 4.5 GPa, at room temperature. The starting material was used three days after synthesis by hydrolysis of alkoxides without additional treatment. Fourier transform infrared spectroscopy (FTIR), using the KBr technique and a high vacuum cell at temperatures up to 450°C, and thermogravimetric analysis (TGA) up to 900°C was conducted. A reduction up to 60% in the adsorbed water content of the compacted silica gel was observed. Changes in the 3000 to 3800 cm⁻¹ region indicate that the surface silanol groups became bridged, which promotes condensation reactions of dehydroxylation. Those results suggest that the mechanism for consolidation under high-pressure, is ‘cold sintering’ process, where silanol groups at the surface of the nanoparticles condense to form siloxane bonds between the particles and water, resulting in a stiff body with closed nanopores containing trapped water.     

Grain size dependence of physico-optical properties of nanometallic silver in silica aerogel matrix

Synthesis of silver nanoparticles in silica aerogel matrix by sub-critical drying technique is reported in the present article. Physical characterization of silver/silica aerogel nanocomposites with 1, 5 and 25 wt% of silver has been discussed. Physico-optical properties of the composites have been evaluated as a function of the silver particle size crystallized within the silica matrix. The maximum size of the silver grains that could be accommodated in the amorphous matrix was observed to be 25 nm. Silver particles of diameter larger than 25 nm were found segregated out of the silica matrix; such silver particles were single crystalline with dendritic morphology. Optical absorption analysis confirmed the presence of both the oligomeric and nanometallic silver in the samples heat-treated up to 500 °C. With decreasing silver particle size, the surface plasmon resonance was found first to shift towards blue followed then by a red shift. The blue shift is attributed to the chemisorption occurring between the metallic core and the silica matrix. With larger grain size of 25-80 nm in 25 wt% Ag-silica aerogel sample, the diffuseness of electron cloud outside the potential well was observed to cause a red shift in the surface plasmon resonance.     

Energetics of CdSxSe1−x quantum dots in borosilicate glasses

The thermodynamics of CdSe quantum dots embedded in a glass matrix is of great interest because of the numerous applications as optical materials. In this study, the energetics and stability of CdSe quantum dots in a borosilicate glass matrix is investigated as a function of size using high-temperature oxide melt solution calorimetry. CdS_0.1Se_0.9 nanoparticles (1-40 nm) embedded in glass were analyzed by photoluminescence spectroscopy, electron microprobe, X-ray fluorescence, high-energy synchrotron X-ray diffraction, and (scanning) transmission electron microscopy using both electron energy loss and energy dispersive X-ray spectroscopy. As CdSe particles coarsen, their heat of formation becomes more exothermic. The interfacial energy of CdSe QDs embedded in a borosilicate glass, determined from the slope of enthalpy of drop solution versus calculated surface area, is 0.56 ± 0.01 J/m².     

Thermal and optical investigation of EuF3-doped lead fluorogermanate glasses

Europium-doped lead germanate and lead fluorogermanate glasses are studied by using differential thermal analysis, X-ray diffraction, photoluminescence and fluorescence lifetimes measurements of the ⁵D_j, j = 0, 1, 2 levels. PbF₂ addition increases the thermal stability of the lead germanate glass, while Eu³⁺ ions promote the crystallization of β-PbF₂:Eu³⁺ nano-crystals embedded in a glassy matrix. In the lead fluorogermanate glasses, Eu³⁺ ions exhibit a strong affinity for F⁻ ions although oxygen ions are much more numerous. It appears that luminescence concentration quenching is not important, while cross relaxation is very efficient in the glasses. The results allow to propose for these glasses a molecular model in which small fluorine rich island, incorporating the Eu³⁺ ions in low symmetry sites, are separated from each other by chains of germanate (GeO₄)⁴⁻ ions linked together.     

Effect of rogue particles on the sub-surface damage of fused silica during grinding/polishing

The distribution and characteristics of surface cracks (i.e., sub-surface damage or scratching) on fused silica formed during grinding/polishing resulting from the addition of rogue particles in the base slurry has been investigated. Fused silica samples (10 cm diameter × 1 cm thick) were: (1) ground by loose abrasive grinding (alumina particles 9-30 μm) on a glass lap with the addition of larger alumina particles at various concentrations with mean sizes ranging from 15 to 30 μm, or (2) polished (using 0.5 μm cerium oxide slurry) on various laps (polyurethane pads or pitch) with the addition of larger rogue particles (diamond (4-45 μm), pitch, dust, or dried Ceria slurry agglomerates) at various concentrations. For the resulting ground samples, the crack distributions of the as-prepared surfaces were determined using a polished taper technique. The crack depth was observed to: (1) increase at small concentrations (>10⁻⁴ fraction) of rogue particles; and (2) increase with rogue particle concentration to crack depths consistent with that observed when grinding with particles the size of the rogue particles alone. For the polished samples, which were subsequently etched in HF:NH₄F to expose the surface damage, the resulting scratch properties (type, number density, width, and length) were characterized. The number density of scratches increased exponentially with the size of the rogue diamond at a fixed rogue diamond concentration suggesting that larger particles are more likely to lead to scratching. The length of the scratch was found to increase with rogue particle size, increase with lap viscosity, and decrease with applied load. At high diamond concentrations, the type of scratch transitioned from brittle to ductile and the length of the scratches dramatically increased and extended to the edge of the optic. The observed trends can be explained semi-quantitatively in terms of the time needed for a rogue particle to penetrate into a viscoelastic lap. The results of this study provide useful insights and ‘rules-of-thumb’ relating scratch characteristics observed on surfaces during optical glass fabrication to the characteristics of the rogue particles causing them and their possible source.     

Morphology and microstructure in fused silica induced by high fluence ultraviolet 3ω (355 nm) laser pulses

The morphology and microstructure induced in high quality fused silica by UV (355 nm) laser pulses at high fluence (10-45 J/cm²) have been investigated using a suite of microscopic and spectroscopic tools. The laser beam has a near-Gaussian profile with a 1/e² diameter of ∼0.98 mm at the sample plane and a pulse length FWHM (full width at half maximum) of 7.5 ns. The damage craters consist of a molten core region (thermal explosion), surrounded by a near concentric region of fractured material. The latter arises from propagation of lateral cracks induced by the laser-generated shock waves, which also compact the crater wall, ∼10 μm thick and ∼20% higher in density. The size of the damage crater varies with laser fluence, number of pulses, and laser irradiation history. In the compaction layer, there is no detectable change in the Si/O stoichiometry to within ±1.6% and no crystalline nano-particles of Si were observed. Micro- (1-10 μm) and nano- (20-200 nm) cracks are found, however. A lower valence Si³⁺ species on the top 2-3 nm of the compaction layer is evident from the Si 2p XPS. The results are used to construct a physical model of the damage crater and to gain critical insight into laser damage process.     

Purification and mechanical nanosizing of Eu-doped GaN

Eu:GaN powder synthesized using a high temperature ammonothermal process is known to be dark in appearance due to presence of Eu-containing absorbing particles. Improvement of the visual quality of the Eu:GaN powder is achieved by rinsing in dilute acids. Acid-rinsed Eu:GaN has photoluminescence (PL) enhanced by a factor of 3 when compared to as-prepared Eu:GaN. Such visually clear powders are used for making Eu:GaN nanoparticles of sizes 30–50 nm using a soft ball-milling technique. The particle size was determined using X-ray diffraction, scanning electron microscopy and a dynamic light scattering system. Longer durations of a “soft” ball-milling technique results in particle size reduction. These nanopowders show significant photoluminescence intensity with no yellow luminescence, and have a reduced PL intensity with increasing ball-milling time. Eu:GaN nanopowder embedded in a KBr matrix shows at least a 10× improvement in transmittance when compared to as-prepared powders. The improvement of transmittance depends on both the concentration and particle size. This improved transmittance suggests that such a transparent matrix could be used as a laser gain medium.     

An in situ (W,Ti)C-Ni composite fabricated by SHS method

The combustion synthesis reaction was combined with liquid phase sintering to fabricate net-shape TiC-Ni and TiC-WC-Ni composites in a single processing operation. When Ti and C powders used for in situ production of TiC-Ni cermet, the final product was porous and had low strength. By the addition of fine W powder with W/Ti = 1 molar ratio, the product contained (Ti,W)C, W₂C and mix compounds of type Ni₂W₄C. Using W with W/Ti = 2 molar ratio results in a dense high strength product containing (Ti,W)C and WC.     

Size-controlled synthesis of LaAlO3 by reverse micelle method: Investigation of the effect of water-to-surfactant ratio on the particle size

Lanthanum monoaluminate (LaAlO₃) nanoparticles have been synthesized using microreactors made of poly(oxyethylene) nonylphenyl ether (Igepal CO-520)/water/cyclohexane microemulsions. The control of particle size was achieved by varying the water-to-surfactant molar ratio. The synthesized and calcined powders were characterized by thermogravimetry–differential thermal analysis (TGA-DTA), X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR). Differential thermal analysis showed that LaAlO₃ phase transformation decreases with increase in water/surfactant (R) value. Pure LaAlO₃ phase was synthesized by annealing at 800 °C for 2 h in air directly from amorphous precursors, without formation of intermediate phase. The average particle size was found to increase with increase in water-to-surfactant ratio (R). FTIR analysis was carried to monitor the elimination of residual oil and surfactant phases from the microemulsion-derived precursor and calcined powder.     

Photosensitive defects in silica layers implanted with germanium ions

Ge-implanted silica layers have been investigated by high-power pulsed synchrotron-photoluminescence (PL), photoluminescence excitation spectroscopy (PLE), and optically stimulated electron emission (OSEE) with respect to association of excitation and absorption bands to respective emission bands and lifetimes of excited defect states. In this way singlet-singlet (4.35 eV) and triplet-singlet (3.18 eV) radiative transitions from excited states of oxygen-deficient centers (ODC) in Ge-doped silica glass are characterized by their absorption and emission bands as well as their lifetimes. The main channel for non-radiative relaxation of photoexcitation is electron emission by the OSEE effect. The OSEE shows non-radiative transitions of surface and bulk E′-centers found with concentrations of (2.7-3.4) × 10¹² cm⁻² and (2-4) × 10¹⁶ cm⁻³, respectively.     

Microstructure and percolation threshold characteristics of reactive sputtered Au-TiO2 granular composite films

The Au/TiO₂ composite films were prepared by using reactive co-sputtering technique. The size and shape of the embedded Au particles and the absorption spectra of the composite films were investigated by using SEM, XRD, and UV-VIS-NIR spectrophotometer, respectively. The average size of Au particles and the electrical conductivity decrease as the sputtering pressure increases. The normalized conductivity of the films deposited at five different pressures with the Au concentration in the range of 0.15-0.91 were measured. The percolation threshold increases from 0.21 to 0.90 as the sputtering pressure increases from 2 × 10⁻² Torr to 9.5 × 10⁻² Torr.     

Effect of calcination temperature on the textural properties of 3 mol% yttria-stabilized zirconia powders

Zirconia-based ceramics have been commonly used in many different technological fields. This kind of materials have consequently received a great deal of attention. The main goal of this study was to investigate the effect of the calcination temperature on the morphological and textural properties of 3 mol% yttria-stabilized zirconia powders (3YSZ). Scanning electronic microscopy (SEM), nitrogen adsorption isotherms, thermogravimetry-differential scanning calorimetry (TG-DSC), X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy have been used with such an aim. Once the 3YSZ powders were dried at 100 °C they were subjected to calcination at 300, 500 or 700 °C. The calcination temperature noticeably influences the surface and textural properties of the 3YSZ powders. Textural parameters appear to be closely related to the removal of the alcohol used in the sol-gel synthesis procedure. The largest values of specific surface area and pore volumes correspond to samples prepared at 100 and 500 °C (i.e., 161 and 62 m²/g). In the first case, this fact is attributable to the removal of residual water and alcohol occluded within the powder particles whereas in the second case it may be due to the elimination of gases produced during the calcination stage.     

Systematic studies of Si and Ge hemispherical concave wafers prepared by plastic deformation

Si and Ge hemispherical concave wafers can be prepared by plastic deformation using Si and Ge single- and polycrystal wafers. Deformation regions in which such Si and Ge hemispherical wafers can be obtained by high-temperature plastic deformation were systematically studied. The deformation regions in which well-shaped Si concave wafers can be obtained were studied for (1 0 0), (1 1 1), and polycrystal Si. It was found that Si (1 1 1) crystal wafers can be more easily deformed to a perfect hemispherical shape than (1 0 0) wafers because of the crystallographic symmetry. Si hemispherical wafers with a small radius of 25 mm can be perfectly deformed when 0.5-mm-thick Si (1 1 1) crystal wafers are used. Ge hemispherical wafers with a radius of 100 mm can be perfectly deformed when 0.5-mm-thick Ge crystal wafers are used. Ge hemispherical concave wafers with a perfect shape can be more easily obtained using Ge (1 1 1) wafers than (1 0 0) wafers. According to these results, the deformation behavior of Ge wafers is very similar to that of Si wafers at a normalized pressing temperature. As both the radius and the load on Si and Ge hemispherical wafers increase, thicker Si and Ge wafers can be used to obtain hemispherical wafers with a perfect shape. The dislocation density in the shaped wafers markedly decreases as the pressing temperature and hemisphere radius increase. Thus, it is suggested that Si and Ge shaped wafers are of sufficient quality and have the high potential for use as several types of lens such as those in Si and Ge X-ray monochromators.     

Photoinduced degradation of fluorescence and formation of copper nanoparticles in sol-gel silica doped with flavins

Copper nanoparticles were formed by photoirradiation of doped sol-gel silica, which was prepared by mixing Cu²⁺ ions, ethylenediaminetetraacetic acid (EDTA), and riboflavin into sol-gel solutions of tetramethoxysilane. The doped silica exhibited broad absorption bands at 442 nm due to riboflavin and 740 nm due to Cu²⁺-EDTA complexes. After photoirradiation, the sol-gel silica showed a reddish brown color and the absorption peak around 580 nm due to the plasmon band of copper nanoparticles. Copper nanoparticles were also formed from other sol-gel silica doped with lumichrome or lumiflavin. The photostability of the flavins dyes obtained from the fluorescence intensities was in the order of lumichrome > lumiflavin > riboflavin in the sol-gel silica without Cu²⁺ ions. On the other hand, the fluorescence intensities were considerably reduced by photoirradiation of the sol-gel silica doped with Cu²⁺ ions, irrespective of the flavin dyes doped. Considering the absorption and fluorescence spectral changes during the photoirradiation, we concluded that copper nanoparticles are produced by the photoinduced electron transfer from the flavin dyes in the sol-gel silica.     

Characterization of nanocluster formation in Cu-implanted silica: Influence of the annealing atmosphere and the ion fluence

High-purity silica plates were implanted with 2 MeV Cu⁺ ions at various ion fluences: 0.7 × 10¹⁶, 3 × 10¹⁶ and 6 × 10¹⁶ ions/cm². After implantation, thermal treatments were performed at 400 °C and 900 °C in either an oxidizing (air) or a reducing (50% H₂ + 50% N₂) atmosphere for 1 h. All the samples were studied by electron paramagnetic resonance, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy (HRTEM), Rutherford backscattering spectrometry and optical absorption. The advantages of the reducing atmosphere (RA) over the oxidizing atmosphere (OA) are clearly observed. When annealed in a RA, the surface plasmon resonance is more intense and a narrower size distribution of the Cu nanoparticles is obtained. The existence of CuO nanoparticles was confirmed by HRTEM, and while both annealing atmospheres favor the formation of CuO nanoparticles, this process is strengthened when the sample is annealed in an OA.     

Synthesis of silica aerogel blanket by ambient drying method using water glass based precursor and glass wool modified by alumina sol

Silica aerogel blankets have been synthesized by ambient drying technique using cheap water glass as the silica source and glass wool modified by alumina sol. One step solvent exchange and surface modification were simultaneously conducted by immersing the wet hydrogel blanket in EtOH/TMCS/hexane solution. The synthesized silica aerogel blanket was light with the density of 0.143-0.104 g/cm³ and 89.4-95% porosity. The microstructure of silica aerogel blanket exhibits a porous structure consisting of glass fibers of diameter ∼2.5 μm interconnected with solid silica clusters (5-20 μm).