Preparation and optical properties of samarium doped sol-gel materials

Transparent crack-free glassy monoliths containing up to 0.2 Sm/Si are prepared at room temperature by sol-gel technique. The sol-gel preparation conditions are: acid catalyzed hydrolysis followed by basic catalyzed gelation and controlled drying. The prepared xerogels are characterized by X-ray diffraction, thermogravimetry with Fourier transforming infrared spectroscopy, scanning electron microscopy and optical spectroscopy. The values of the phenomenological Ω-intensity parameters of Sm(III) ions in the gels are (in 10⁻²⁰ cm²) Ω₂ = 76 ± 16.71, Ω₄ = 3.01 ± 0.21, Ω₆ = 2.99 ± 0.41. The oscillator strengths of the Sm(III) ions in the gels are calculated. The results show a linear concentration dependence of Sm(III) in UV/Vis absorption spectra and formation of a Sm(NO₃)₃ · 6H₂O complex at high samarium concentrations.     

Photo-initiation of intermolecular bonding and oxide deposition in Ti-based alkoxide solutions

Photoexposure of a water-stable, Ti-alkoxide, (OC₆H₆N)₂Ti(OC₆H₂(CH₂N(CH₃)₂)₃-2,4,6)₂, dissolved in a water/pyridine solution yielded an insoluble, solid phase, present in the form of suspended particles and a deposited material on the vertical side-wall of the solution container. Raman analysis of these photoproducts confirmed the UV-initiation of hydrolysis and condensation reactions in the alkoxide reagent. The excitation wavelength dependence of the photostructural effects observed provides insight into the mechanisms contributing to these photoinduced phenomena. The results are extended to enable the photopatterned deposition of physical relief structures directly from solution in this system.     

Sol-gel-derived organic-inorganic hybrid materials

Optically transparent organic-inorganic hybrid coating materials have been prepared by a sol-gel process. Four different types of the coating material produced by TWI in Cambridge, UK using the patented Vitresyn^® method, all identical in terms of the starting materials, but differing in terms of their relative proportions, have been examined. Tetraethoxysilane was used as the primary inorganic precursor and urethane acrylate was used as the source of the organic component. 3-(Trimethoxysilyl)propyl methacrylate was used as both a secondary inorganic source and a silane coupling agent to improve the compatibility of the organic and inorganic phases. The degree of chemical interaction of the organic and inorganic phases after processing was determined by ²⁹Si and ¹³C nuclear magnetic resonance and Fourier transform infrared spectroscopy. The effect of the relative amount of inorganic starting component in these hybrid materials on their thermal properties was investigated through differential scanning calorimetry and thermogravimetric analysis. Similar degrees of chemical interaction between the organic and inorganic phases were found in all four samples. T³, Q³ and Q⁴ are the main cross-linking network structures in these hybrid systems, the relative proportions of which are determined by the relative proportions of the starting materials.     

Synthesis and Judd-Ofelt analysis of a sol-gel material doped with a Ln2(Benzoate)6(Phen)2 (Ln = Er/Yb) complex

In this work, we have prepared a sol-gel derived hybrid material directly doped with Er_1.4Yb_0.6(Benzoate)₆(Phen)₂ (Phen = 1,10-phenanthroline) complex, which was reported with intramolecular Yb-Er energy-transfer process in our previous work. The infrared (IR) spectra of the pure complex and hybrid gel material were investigated. The NIR photoluminescence (PL) spectrum of hybrid gel material shows strong characteristic emission of Er³⁺ with broad full width at half-maximum (FWHM) of 70 nm. Judd-Ofelt theory was used in order to analyze the optical properties of Er³⁺ ions in the hybrid gel material.     

Characterization of HF-catalyzed silica gels doped with Degussa P25 titanium dioxide

SiO₂/TiO₂ composites were synthesized by adding Degussa P25 TiO₂ to a liquid sol that was catalyzed by HNO₃ and HF acids. Various composites were synthesized by altering the mass loading of TiO₂ and concentration of HF added to the liquid sol before gelation. The resulting materials were characterized by SEM, nitrogen adsorption-desorption, streaming potential, XRD, diffuse reflectance and TiO₂ surface area analyses. Approximate characteristics include an isoelectric point of 3, TiO₂ particle size of 30 nm, and a band gap energy of 3.2 eV. Small variations in these properties were noted for the different composites. Physical characteristics were largely affected by HF concentration and TiO₂ loading. Nitrogen adsorption-desorption isotherms were type IV for all materials and exhibited trends of decreased pore volume with an increase in TiO₂ loading and an increase in pore diameter with increased HF concentration. Surface areas of the composites ranged from 167 to 630 m²/g. Available TiO₂ surface area of the composite was also dependent upon TiO₂ loading and increased as the mass composition of TiO₂ increased but was not largely affected by HF concentration.     

Synthesis and characterization of SiO2-PEG hybrid materials

Sol-gel is a promising technique for the synthesis of organic-inorganic hybrid materials both of class I and II. In materials of class I organic molecules are physically entrapped in an inorganic matrix, while in those of class II organic and inorganic parts are connected by covalent bonds. In this paper a sol-gel procedure to obtain SiO₂-PEG hybrids of class I, in which PEG is simply mixed at the sol stage, is compared to a sol-gel procedure to obtain SiO₂-PEG hybrid materials of class II, where a particular sol-gel Si-C precursor is synthesized. XPS analyses showed the different distribution of the organic phase in the SiO₂ matrix and the bond between PEG and SiO₂ for hybrids of class II. The PEG molecule in hybrid of class II showed an enhanced thermal stability up to 350 °C. Doping with a lithium salt was performed on hybrids of class II, and the ionic conductivity was measured.     

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.     

Structure of Ni/SiO2 films prepared by sol-gel dip coating

The sol-gel dip coating technique has been used to deposit composite oxide films (NiO)_x(SiO₂)_1−x with x = 0.1 on silicon wafers. Single and multilayer coatings allowed a variation of the film thickness from 70 to 400 nm. Film morphology, atomic structure and atomic composition have been investigated by transmission electron microscopy (TEM) and Rutherford backscattering spectrometry (RBS). The local environment of the Ni atoms was characterized by extended X-ray absorption fine structure (EXAFS). The samples were studied in the as-prepared state and after annealing in H₂ at 600 °C for 1 h. The structural and chemical state evolution of clusters present inside the silica matrix is discussed in terms of out-of-equilibrium reaction processes specific to low-dimensional objects and superficial effects.     

Sol-gel deposition of silica films on silicate glasses: Influence of the presence of lead in the glass or in precursor solutions

Many lead silicate historical glasses suffer degradation phenomena often observed as color changes and iridescence caused by lead ions leaching from the outer layers of the glass. In order to repair and to prevent these phenomena, glasses with large amounts of lead (6.7 and 14.3 at.% of lead) have been coated with silica films at neutral pH by dipping them in a precursor solution of TEOS (tetraethyl orthosilicate), ethyl alcohol and deionized water without any other acid or basic catalyst. Experiments with long dipping times (24 h) and temperatures around 20 °C have been performed to evaluate the role of lead ions of the glass as a catalyst. Silica films of very good quality and optical transparency have been also obtained on lead-free, soda-lime glasses by adding catalytic amounts of Pb(NO₃)₂ instead of HCl to the precursor solution. The films have been characterized by optical microscopy, AFM (Atomic Force Microscopy), XPS (X-ray Photoelectron Spectroscopy) and SIMS (Secondary Ion Mass Spectrometry).     

Investigation of infrared emission and lifetime in Tm-doped 75TeO2:20ZnO:5Na2O (mol%) glasses: Effect of Ho and Yb co-doping

In this paper we describe fabrication and characterization of rare-earth-doped active tellurite glasses to be used as active laser media for fiber lasers emitting in the 2 μm region. The base composition is (mol%): 75TeO₂-20ZnO-5Na₂O with different concentrations of Tm³⁺, Yb³⁺ and Ho³⁺ as dopants or co-dopants. Optical properties of doped glasses were studied and pumping at 800 nm and at 980 nm were tested in order to compare the efficiency of two pumping mechanisms. Optical characterization carried out on glasses containing only Tm³⁺ ions indicated the optimum concentration of Tm₂O₃ in terms of emission efficiency as 1 wt%. The addition of 5 wt% of Yb₂O₃ to Tm³⁺-doped glasses led to the best results in terms of intensity of fluorescence emission and of lifetime values. Yb and Ho co-doped Tm-tellurite glass was measured in emission.     

Preparation and characterization of new fluorotellurite glasses for photonics application

Glasses based on (85 − x)TeO₂-xZnF₂-12PbO-3Nb₂O₅ (x = 0-40) system have been studied for the first time for fabricating mid-infrared optical fiber lasers. The thermal and optical properties including UV-Vis, Raman as well as FTIR spectra are reported. It is demonstrated that increasing the ZnF₂ concentration to 30 mol% significantly increased the thermal stability of the glass. Adding ZnF₂ also reduced the hydroxyl (OH) content of the glass resulting in lower optical absorption in the mid-infrared region, which is crucial for infrared laser applications. The glass absorption cut-off edge near 400 nm blue-shifts with increasing ZnF₂ addition. Raman spectra show a depolymerization of the glass network with increasing transformation of TeO₃₊₁ to TeO₃ structures.     

Spectroscopic investigation and optical characterization of novel highly thulium doped tellurite glasses

In this paper, optical properties of 75TeO₂-20ZnO-5Na₂O host glass doped with concentration of Tm³⁺ up to 10 %mol were studied in order to assess the most suitable rare earth content for short cavity fiber lasers. Raman spectroscopy revealed a change in the glass structure while increasing Tm³⁺ content, similar to the well known addition of alkali ions in a glass. Influence of the fabrication process on the OH⁻ content was determined by FTIR measurements. Refractive index of Tm³⁺ doped tellurite glasses was measured at five different wavelengths ranging from 533 nm to 1533 nm. Lifetime and emission spectra measurements of the Tm³⁺ doped tellurite glasses are reported.     

Characterization of low-density silica xerogel formed into microspheres

Low-density silica xerogels were formed into spherical pellets using a process in which a xerogel suspension containing sodium alginate was solidified through free fall in a solution of divalent ions that induce the droplet solidification. Pellet preparation was also carried out at laboratory scale by using a syringe instead of the material-consuming unit used for scaling up. When beads are made from a suspension of dried-and-calcined xerogel material, the textural properties such as porosity and surface area are significantly altered. Both properties are reduced by 30-50% compared to the starting reference material. Properties were improved when a suspension of dried xerogel that is less sensitive to contact with water was used: specific surface area was maintained at its initial level and porosity decreased by 25%.     

Synthesis and characterization of chemically cross-linked polyimide-siloxane hybrid films

Chemically cross-linked polyimide and silica hybrid films were prepared through the sol-gel processing. PI matrix was prepared by the reaction of pyromellitic dianhydride with a mixture of diamines e.g., oxydianiline and 2,5-diaminohydroquinon (2,5-DAHQ) to include pendant hydroxyl groups on the chain. These groups were reacted further with isocyanatopropyltriethoxysilane. An appropriate amount of tetraethoxysilane was then added and the sol-gel process was carried out to condense ethoxy groups from both types of silanes thus producing chemically bonded composite films. The films with different silica contents were evaluated by a variety of techniques including FTIR, ²⁹Si NMR, SEM, tensile, thermal, mechanical and thermogravimetric analyses. The chemical interaction between the phases brought about an intimate dispersion of the two phases, which resulted in the formation of nano-sized co-continuous domains. The tensile modulus of such films was higher and thermal expansion coefficient was much lower than those with similar silica contents without inter-phase bonding.     

Electrical conductivity of NbN-SiO2 films obtained by ammonolysis of Nb2O5-SiO2 sol-gel derived coatings

The studies of electrical conductivity of NbN-SiO₂ films are reported. To obtain these films, sol-gel derived xNb₂O₅-(100 − x)SiO₂ (where x = 100, 90, 80, 70, 60, 50 mol%) coatings were nitrided at 1200 °C. The nitridation process leads to the formation of some disordered structures, with NbN metallic grains dispersed in insulating SiO₂ matrix. The structure of the samples was studied using X-ray diffraction (XRD) and atomic force microscopy (AFM). The electrical conductivity was measured with the conventional four-terminal method in the temperature range from 5 to 280 K. The superconducting transition was not observed even for the sample that does not contain silica. All the samples exhibit negative temperature coefficient of resistivity. The results of conductivity versus temperature may be described on the grounds of a model proposed for a weakly disordered system.     

Characterization and optical properties of sol-gel processed PMMA/SiO2 hybrid monoliths

Organic-inorganic hybrid monoliths consisting of poly(methyl methacrylate) and SiO₂ were synthesized by the sol-gel process. Three PMMA/TEOS ratios by weight were prepared, namely PMMA/TEOS = 80/20, 50/50 and 20/80. The effects of the ratio of PMMA/TEOS on thermal stability, microstructure, morphology and optical properties were systematically studied. In particular, scanning electron microscopy (SEM) demonstrated morphological variation corresponding to the degree of miscibility between organic and inorganic components. Energy dispersive spectroscopy (EDS) clearly revealed that the SiO₂ was more uniformly distributed in the hybrids with increasing PMMA content. The refractive index could be adjustable in the range of 1.474-1.486. The 80/20 monolith had the similar transparency to pure PMMA in the visible region and lower absorption than PMMA in the near infrared region, suggesting its potential applications for optical devices.     

Investigation of silica particle structure containing metallocene immobilized by a sol-gel method

A series of homogeneous metallocenes (Cp₂ZrCl₂, (nBuCp)₂ZrCl₂, (iBuCp)₂ZrCl₂, (tBuCp)₂ZrCl₂, Cp₂TiCl₂, Cp₂HfCl₂, EtInd₂ZrCl₂ and Et(IndH₄)₂ZrCl₂) were immobilized within a silica matrix during its preparation using a non-hydrolytic sol-gel method. Supported metallocenes were characterized by small angle X-ray scattering (SAXS) and nitrogen adsorption. The results show that the silica particle was formed by the aggregation of primary structures having a diameter of about 160 nm. This was a result of the aggregation process, which was affected by the metallocene. This result suggests that the metallocene is surrounded by primary particles deep inside the silica matrix. The low polymer productivity exhibited by the less active metallocene catalysts could be related to the problems of particle fragmentation due to monomer mass transfer limitations along the partially fragmented particle, which would subsequently affect catalytic activity.     

Synthesis of ORMOSIL silica/rhodamine 6G: Powders and compacts

The sol-gel method was used to prepare a silica matrix with rhodamine 6G. This dye was chemically bonded to silica matrix by grafting reactions, obtaining ORMOSIL/R6G powder, at two different concentrations, 0.03 mmol of dye/g silica and 0.009 mmol of dye/g silica, called ORMOSIL1 and ORMOSIL2, respectively. Fluorescent compacts were also obtained, through the high-pressure processing of these powders, at 7.7 GPa and room temperature. The microstructure, chemical composition and thermal stability of the powders and compacts were comparatively studied, by N₂ adsorption/desorption isotherms, FTIR spectroscopy, thermogravimetric analyses (TGA) and elemental analyses for carbon, hydrogen, and nitrogen (CHN). The optical behavior was studied by fluorescence spectroscopy. The ORMOSIL1 compacts were completely opaque showing that the compaction was not effective, due to the high organic grade incorporated into the starting ORMOSIL1 powder. The ORMOSIL2 compacts were transparent showing an effective compaction, which are in agreement to the great surface area and porosity reduction. Additionally, it was observed in TGA results, the retention of the organics in these samples up to temperatures near 300 °C. The fluorescence emission showed that the dye was dispersed in molecular level in all samples.     

Microporous and mesoporous 8YSZ materials derived from polymeric acetylacetone-modified precursors for inorganic membrane applications

Both microporous and mesoporous 8% mol-yttria-stabilized zirconia (8YSZ) material can be prepared from the same 8YSZ sols. Stable 8YSZ sols with particle sizes smaller than 10 nm were prepared by using acetylacetone as a precursor modifier in combination with carboxylic acids. Nonanic and caproic acids are preferred due to their higher stability in combinations of highly concentrated sols. Cubic 8YSZ microporous material can be prepared by using acetylacetone as a precursor modifier with caproic acid as the catalyst. These materials can be transformed into mesoporous material composed of crystalline primary cubic YSZ nanoparticles by simply mixing with structure-directing agents. Use of the block copolymer F127 resulted in the largest specific surface area of 100 m²/g. Thin 8YSZ layers of these sols can be prepared on glass slips and on graded porous alumina supports with thicknesses between 20 and 100 nm.     

Ultrasonic refinement of magnesium by cavitation: Clarifying the role of wall crystals

Ultrasonic irradiation during solidification has emerged as an effective means of structural refinement for both metallic and non-metallic materials. A number of mechanisms have been proposed to explain the origin of such refinement in an attempt to both understand and exploit the refining potential. One such mechanism proposed is based on the hypothesis of formation and vibration-stimulated separation of wall crystals. This work examines the role of wall crystals in ultrasonic refinement of pure magnesium and Mg-3Al-1Zn alloy, where a titanium sonotrode is immersed into the melt for direct ultrasonication at an amplitude of 30 μm and a typical frequency of 20 kHz. Evidence is produced from different perspectives to show that the wall crystal effect has a negligible influence on the significant refinement observed under ultrasonic exposure. Instead, all of the experimental evidence produced consistently supports the hypothesis of microstructural refinement originating from cavitation-enhanced heterogeneous nucleation.