Preparation of CeO2-TiO2 coatings by the sol-gel process

Yellow coatings have been prepared from titanium tetraisopropoxide and cerium chloride by the sol-gel dip-coating process, and the transmittance, chromaticity, thickness and acidic durability of the coating films produced on glass or aluminum were studied. The effects of the pH of the solution on the preparation of the transparent and homogeneous coating films were studied by adding the alkali or acid, and as a result transparent and homogeneous yellow coating films were prepared by dipping the substrate in a solution containing acetic acidic solution. The acidic durability of the aluminum foil was improved by dip-coating the brilliant yellow CeO₂-TiO₂ coating three times.     

Colloidal sol/gel processing of ultra-low expansion TiO2/SiO2 glasses

A series of titania-silica glasses with 0–9% TiO₂ were fabricated using a sol/gel process. The sol was prepared by dispersing colloidal silica fume in an aqueous solution of titania which was synthesized through the acid-catalyzed hydrolysis of titanium isopropoxide. The sols gelled in 2–4 days, and then were dried for 6–8 days. The dry gels were sintered at 1450–1500°C to produce clear, dense, microstructure-free glasses. The gels underwent a total shrinkage of 50% to yield glass rods about 50 mm long and 5 mm in diameter, or glass discs about 4 cm in diameter and 5 mm thick. The drying step was most critical in the production of crack-free specimens.

In the gel, the transmission electron microscope (TEM) revealed the presence of 1–5 nm rutile microcrystallites uniformly distributed within a network of colloidal silica particles. After sintering to 1450–1500°C, though, a dense, transparent, microstructure-free glass was created. Fourier transform infrared spectroscopy (FTIR) verified the formation of an amorphous solid-solution of titania and silica after sintering.

The thermal expansion of the glasses was measured using a differential dilatometer. The average linear coefficients of thermal expansion (CTE @ 25–675°C) varied between +5 × 10⁻⁷ and −0.2 × 10⁻⁷°C⁻¹ in the range 0 to 9% TiO₂. The glass with 7.2% TiO₂ exhibited a zero thermal expansion coefficient at 150–210°C. The hysteresis in CTE on heating and cooling was of the order of 0.01–0.02 ppm.     

Semiconducting TiO2SiO2 glasses (II): Optical properties

The refractive index, optical absorption coefficient α and the thermomodulated absorption dα/dT have been measured on 70% TiO₂?30% SiO₂ glasses with up to 8% Ti³⁺. The direct absorption data show intense ligand field absorption at photon energies hν = 1.9 and 3.0 eV, arising from Ti³⁺ in a distorted octahedral environment. In the bandgap region at 3.5 eV α obey αhν ～ (hν ? E_g)²; it is qualitatively different from the bandgap absorption in crystalline TiO₂. The da/dT spectra show peaks in the bandgap region and at 1.1 eV in the near IR. This last peak is attributed to absorption by small polarons, and its line shape is compared with theoretical predictions.     

Phonons in TiO2---SiO2 glasses

We report the infrared and Raman studies of titanium doped vitreous silica glasses for a number of titanium concentrations. The vibrational modes associated with the randomly oriented chains of SiO₄ tetrahedra show broadenings and shifts. The LO-TO splitting of some Raman active modes decreases with increasing titanium concentration. This is attributed to the decrease in long-range coulomb fields associated with the chains of SiO₄ tetrahedra which are broken by the titanium atoms. The results are discussed in the context of random network models. An increase in the average intertetrahedral angle of the SiO₄ network is calculated from the data. This explains the anomalous decrease in the density of TiO₂---SiO₂ glasses with increasing titanium content. We identify two new modes associated with the distorted titanium tetrahedra. A polarized Raman mode at 1115 cm⁻¹ which is infrared inactive and an unpolarized Raman mode at 945 cm⁻¹ which is infrared active are observed.     

Interaction between colloidal particles in SiO2 and TiO2 sols

The effect of the interaction between colloidal particles in SiO₂ and TiO₂ sols on the viscosity of the sols has been examined. It was found that the interaction was significantly influenced by the zeta potential of electrical double layers and terminal bonds associated with particle surfaces. Water/alkoxide mole ratio and pH affect the zeta potential and the terminal bonds and, consequently, change the interaction between the particles and the viscosity of the sols. In addition, the relationship of viscosity with the volume fraction of the colloidal particles in SiO₂ sol has been described by a Dougherty-Kreiger equation when the viscosity reached a stable value with time.     

TiO2-SiO2 monolithic glass formation from sol-gel

This study demonstrates that hydrolysis should be carried out in a step manner in gel synthesis. The key to the increase in the amount of water added is the control of the hydrolysis rate of Ti(OC₄H₉)₄. The hydrolysis of Si(OC₂H₅)₄ can be carried out at about 75°C. The amount of added water (γ_WI), which varied with TiO₂ content (in mol%), was about 64–88% of the total amount of added water. The hydrolysis reaction should be performed at room temperature while Ti(OC₄H₉)₄ is added. The total amount of added water (γ_W) is related to the amount of solvent (R). For example, if TiO₂ is 40mol%, γ_W will vary from 3.2 to 8.0 when R varies from 0.8 to 2.0. The amount of added water was affected by the distribution of solvent in the metal alkoxides. The amount of added water can be increased when R_Si(OC2H5)4 = 1, R_Ti(OC4H9)4 > 1. The rate of rise in temperature of the thermolysis of the dried gel should be less than 10°C per hour, and the heat treatment temperatyre is related to the TiO₂ content (in mol%). Gel glasses without devitrification can only be obtained by thermolysis at 600°C from the gel with no less than 20 mol% TiO₂.     

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.     

Characterization and chemical durability of 70Ph2SiO·30TiO2 coating prepared by the sol-gel process

70Ph₂SiO·30TiO₂ coatings on slide glass and phosphate glass substrates have been prepared by the sol-gel process. Experimental results on the characterization and chemical durability of the coatings are presented. It is shown that the coatings heated at 150°C have reasonably good acid and water resistance while their alkali durability is very poor. The XPS results are used to verify the different corrosion behaviors of the coating in pure water and 1N HC1 solutions. The chemical durability of phosphate glass can be significantly improved by this coating.     

Crystallization of SiO2Al2O3MgO gel glasses

Basic glasses are prepared by chemical polymerization in a sol-gel process. Nucleation and crystallization of these glasses are analyzed in dependence of the composition of the basic glasses and “additions” of TiO₂ and LiO₂. A comparison of gel glasses with conventionally molten glasses is made. Gel glass-ceramics are prepared as bulk materials and thin coatings.     

Raman study of the structure of glasses along the join SiO2GeO2

In order to better understand the distribution of tetrahedra in multicomponent tetrahedral network structures of melts and glasses, we have investigated the Raman spectra of binary SiO₂GeO₂ glasses. We compare the Raman spectral features of the end-member glasses and discuss their vibrational origins. The mixing of GeO₂ and SiO₂ melts results in a continuous random network structure of TO₄ tetrahedra (T  Si, Ge) in the glass. Raman bands corresponding to the asymmetric stretch (v_as) of oxygen in GeOGe, SiOSi and SiOGe bonds are observed in the glasses having intermediate compositions along the SiO₂GeO₂ join. The presence of three distinct v_as (TOT) bands in the spectrum of a glass having Si/Ge one reveals that a considerable degree of SiGe disorder exists in the glass. The presence of a single symmetric oxygen stretching band in the spectra of binary SiO₂GeO₂ glasses indicates that the symmetric stretch modes (v_s) of oxygen in SiOSi, SiOGe and GeOGe bonds are strongly coupled. An observed decrease in the halfwidth of the v_s (TOT) band in the spectra of SiO₂GeO₂ glasses with increasing concentration of GeO₂ may be attributed to a decrease in the average TOT bond angle and a predominance of six-membered ring structures. Results of the present study support the assignment of the bands in the 900–1200 cm^?1 region of the alumino-silicate glasses, spectra to the v_as(AlOSi) and v_as(SiOSi) modes. In contrast to the alumino-silicate glasses, however, the SiO₂GeO₂ glasses have a much higher degree of disorder of the network-forming cations.     

Studying of optical and morphological properties of SiO2-MOx (M: Co/Cu) glasses prepared by the sol-gel method

The cobalt oxide-silica and copper oxide-silica glasses are prepared by the sol-gel method. Tetra-Ethyl-Ortho-Silicate (TEOS) is used as source of the silica and transition metal nitrate is used as the dopant. The morphological, structural and optical properties of the samples are investigated by Scanning Electron Microscope (SEM), Energy Dispersion Analysis by X-ray (EDAX), X-ray diffraction (XRD) and Ultra-Violet/Visible (UV/Vis) spectrophotometer. The influences of heat treatment temperature and withdrawal rate on absorption/transmission spectra are investigated in the range 400-800 nm. In the cobalt oxide-silica, the intensity and position of absorption bands (λ_max) are changed by heat treatment. These behaviors are explained through the Bathochromic and the Hyperochromic effects. In addition, the effect of the withdrawal rate on transmission curves is explained in terms of the Levich equation and Beer-Lambert law.     

Al2O3-TiO2 and Al2TiO5 ceramic materials by the sol-gel process

Ceramic materials with a very low thermal expansion coefficient are synthesized by the sol-gel process. The binary gel is obtained by hydrolysis and polycondensation reactions of organometallic compounds of aluminium and titanium. The thermal evolution of the amorphous powder is followed by DTA and TGA measurements. Structural evolution is followed using X-ray diffraction. The crystallization of the TiO₂ rutile and Al₂O₃ corindon starts at 700 and 900°C respectively. The transformation of Al₂O₃ and TiO₂ into Al₂TiO₅ appears between 1200 and 1300°C. The densification of the powder is performed by the hot pressing process. The shrinkage of the powder was previously followed by dilatometric measurements. The physical properties of the final material are studied as a function of pressing parameters.     

The structure of SiO2-GeO2 glasses: A spectroscopic study

Four glasses of the SiO₂-GeO₂ binary system have been synthesized via a sol-gel route followed by a heat treatment and a quench. Glass structure has been determined by Ge K-edge X-ray absorption spectroscopy (XAS) at low temperature and Raman spectroscopy. These mixed glasses present a continuous random network of interconnected GeO₄ and SiO₄ tetrahedra, with GeO₄ tetrahedra similar to the GeO₄ units in GeO₂ glass and continuous compositional variations from GeO₂-rich regions to SiO₂-rich regions. Such a random mixture is consistent with physical properties of these binary glasses as well as with the chemical dependence of their polyamorphism at high pressure. This EXAFS-derived mean Ge-O-Si angles are close to the Ge-O-Ge mean angle in GeO₂ glass, 134° and 130°, respectively. This misfit with the Si-O-Si angles might explain the ease of formation of isolated and pair defects centers, which are suspected to be at the origin of photo-induced modifications of optical properties in Ge-bearing SiO₂ glasses.     

Effect of Al2O3 on phase separation of SiO2-Nd2O3 glasses

The immiscibility boundary and the critical point of SiO₂-Nd₂O₃ system glass were determined as a function of Al₂O₃ addition. The critical temperature of the immiscibility boundary was observed to decrease with the addition of Al₂O₃. Using the regular solution model, the observed decrease of the immiscibility boundary was directly related to the decrease of the concentration fluctuation of Nd₂O₃ in SiO₂. It is concluded that the Al₂O₃ addition to Nd₂O₃ containing silica glass is beneficial in decreasing the concentration quenching effect, deterioration of the optical efficiency due to clustering of rare earth element, because Al₂O₃ addition diminishes the concentration fluctuation of Nd₂O₃ in silica glass.     

Structural analysis of Na2OGa2O3SiO2 glasses by the X-ray diffraction method

The structure of Na₂OGa₂O₃SiO₂ glasses of four different compositions containing up to 45 mol. % Ga₂O₃ has been determined by the X-ray diffraction method. The radial distribution function D(r) obtained indicates that Ga³⁺ ions are in fourfold coordination and GaO₄ tetrahedra are formed in these glasses. The reduced intensity function S·i(S) and radial distribution function D(r) which were calculated based on a structural model for A-25, 0.25(Ga₂O₃)·0.75(Na₂O·2SiO₂)/3 glass agreed well with the observed ones.     

Structure and crystallization of potassium titanium phosphate glasses containing B2O3 and SiO2

Transparent glasses composition of which can be expressed by the formula: (100−x) · (K₂O · 2TiO₂ · P₂O₅) · x(K₂O · 2B₂O₃ · 7SiO₂), where x=5, 10, 15 and 20 mol% (KTP-xKBS), were obtained by melt quenching technique. The structure and crystallization behavior of these glasses have been examined by Fourier transform infrared spectroscopy, differential thermal analysis and X-ray diffraction. In spite of their nominal composition, the studied glasses exhibit a similar oxygen polyhedra distribution. However, significant differences were found in the trigonal BO₃ units amount. During DTA runs all the examined glasses devitrify in two steps. In the former, very small crystals of an unknown crystalline phase are produced. In KTP-5KBS and KTP-10KBS glasses anatase phase was also detected. Attempts were made in order to identify the unknown phase (UTP) for which a AB₃(XO₄)₂(OH)₆ Crandallite-type structure was proposed where the A, B and X sites were occupied by K, Ti and/or Al, and P, respectively. In the second devitrification step the crystallization of the KTiOPO₄ phase occurs while the UTP phase previously formed disappears. Isothermal heat treatments performed at temperature just above T_g have allowed one to obtain transparent crystal-glass nanocomposites, formed by crystalline nanostructure of the UTP phase uniformly dispersed in the amorphous matrix.     

Synthesis, characterization and activity of Ru/SiO2 catalysts prepared by the sol-gel method

The preparation of ruthenium catalysts by the sol-gel method is described. The preparation consists of adding the active metal to a sol which, by hydrolysis and codensation reactions turns into a gel. The catalytic activity for benzene hydrogenation shown by these catalysts is higher than that of catalysts prepared by impregnation of a commercial silica. A superior resistance to catalytic autodeactivation is observed for catalysts prepared by this method.     

Synthesis of CuO---Al2O3---SiO2 glasses from the sol-gel method and properties

The synthesis of CuO---Al₂O₃---SiO₂ glasses by the sol-gel method was investigated and the thermal expansion of bulk glasses was measured. Phase separation was observed in these glasses and apparently consisted of a copper-rich dispersed phase and a network former-rich matrix phase. The low thermal expansion coefficient of these glasses was governed by the matrix phase.     

The effect of composition on UV-vis-NIR spectra of iron doped glasses in the systems Na2O/MgO/SiO2 and Na2O/MgO/Al2O3/SiO2

Glasses with the compositions xNa₂O · 10MgO · (90 − x)SiO₂, 10Na₂O · xMgO · (90 − x)SiO₂, 5Na₂O · 15MgO · xAl₂O₃ · (80 − x)SiO₂, xNa₂O · 10MgO · 10Al₂O₃ · (80 − x)SiO₂, 10Na₂O · 10MgO · xAl₂O₃ · (80 − x)SiO₂, 10Na₂O · 5MgO · 10Al₂O₃ · (80 − x)SiO₂ were melted and studied using UV-vis-NIR spectroscopy in the wavenumber range from 5000 to 30 000 cm⁻¹. At [Al₂O₃] > [Na₂O], the UV-cut off is strongly shifted to smaller wavenumbers and the NIR peak at around 10 000 cm⁻¹ attributed to Fe²⁺ in sixfold coordination gets narrower. Furthermore, the intensity of the NIR peak at 5500 cm⁻¹ increases. This is explained by the incorporation of iron in the respective glass structures.     

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.