Rayleigh Scattering in Silica Glasses

Rayleigh-scattering properties in commercial silica glasses is clarified with relation to their fabrication methods. Fused silica and synthetic silica made by the flame hydrolysis of SiCl₄ (VAD) method exhibit similar magnitudes of scattering, regardless of fabrication method. Synthetic silica made by flame fusion of SiCl₄ exhibits ∼10% lower scattering than other silica glasses. The factors affecting scattering are evaluated based on refractive index dispersion and glass transition temperature (T_g). The results suggest that the difference in Rayleigh scattering is predominantly attributable to the difference in T_g , which is affected by impurities, such as OH ions.     

Infrared Absorption Coefficients of Silica Glasses

The absorption coefficient values of several silicate glasses in the IR radiation range were determined. Four methods were employed in this study: (1) direct transmission by CO₂ laser, (2) direct transmission through ultrathin sections using an IR spectrometer, (3) glass particle dispersions in KBr pressed pellets, and (4) IR reflection spectral analysis. Wide variations in values were observed for some of these techniques. The reasons for the variations were explored, and evidence is presented to support the conclusion that the reflection measurements and the low-power transmission technique yield accurate values. The pellet technique was found to possess too many experimental variables which could not be controlled and gave erroneous data. The damage produced by direct transmission of high-power radiation was severe and introduced effects which gave spurious results.     

Characterization of Invert Soda Lime Silica Glasses Containing High Titania Content Together with their Glass Ceramics

Silicon - Collective structural, optical and thermal properties were investigated for some prepared soda lime silicate glasses containing high TiO2 contents (35, 40, 45%). X-ray diffraction and...     

Spectroscopic Features of Silica Glasses Doped with Tin

The absorption, photoluminescence, and photoluminescence excitation spectra are investigated for tin-doped silica glasses synthesized by different methods. In all the glasses studied, two new centers belonging to the tin dopant in the SiO₂ network are revealed in addition to the well-known oxygen-deficient center with the absorption band at 4.9 eV. One new center is associated with the absorption band at 5.9 eV and the photoluminescence bands at 2.7 and 3.6 eV, whereas the other center is characterized by the absorption band in the range of 4.56 eV and the photoluminescence band at 2.95 eV. Both new centers are identified as oxygen-deficient centers. The latter center is an analog of the centers observed in pure silica glasses synthesized in a nitrogen atmosphere and in germanosilicate glasses prepared by the MCVD and SPCVD methods. It is revealed that the formation of the oxygen-deficient center is one of the main channels of incorporating tin into the SiO₂ network in all tin-containing silica glasses. Consideration is given to the reasons for the lower efficiency of the photodecay of oxygen-deficient centers in the SiO₂ network in tin-containing glasses as compared to that in germanosilicate glasses and for the relatively high efficiency in the formation of the photoinduced refractive index in optical fibers with a core consisting of tin-containing silica glass.     

Crystallization of Yttria–Alumina–Silica Glasses

Bulk nucleation has been studied for glasses within the Y₂O₃–Al₂O₃–SiO₂ system, using a two-stage nucleation and growth heat treatment. The crystalline phases formed have been identified. Annealing in an inert atmosphere is required in order to prevent surface nucleation from dominating the results. Despite this, the phases observed are in general agreement with those observed in previous studies done in air. The nucleation kinetics of the y -Y₂Si₂O₇ phase have been measured and the optimum nucleation temperatures have been identified. The kinetic data have been analyzed using two existing models based on classical nucleation theory. The results of this study have been compared with crystallization results of a previous study of glass of similar composition within sintered silicon nitride samples. A two-stage heat treatment is suggested as a process that may lead to improved devitrification of grain boundary glass in such materials.     

Water Absorption and Mechanical Properties of Silica Porous Glasses

We have estimated the mechanical properties of silica porous glasses (SPG) using special two-layered structures consisting of a SPG layer on a solid glass substrate. These structures turned curved after keeping them in moist atmosphere. The linear dimensions of dry free-standing SPG wafers with the same pore size distribution increased significantly (up to 1.5%) after water vapour soaking. Bending angles of two-layered structures and the values of linear expansions were used to estimate SPG elastic constants and the mechanical stresses arising in porous glasses as the result of water absorption.     

Sol–Gel Synthesis of Cadmium Telluride-Microcrystal-Doped Silica Glasses

The sol–gel process has been applied to the preparation of small-sized CdTe-doped silica glasses. Gels synthesized by the hydrolysis of a complex solution of Si(OC₂H₅)₄, Cd(CH₃COO)₂· 2H₂O, and Te were heated from 350° to 600°C in a H₂─N₂ atmosphere to form fine cubic CdTe crystals. The size of CdTe crystals, determined from the line broadening of X-ray diffraction pattern, increases from 4 to 9 nm in diameter with increasing heat-treatment temperature. The optical absorption edge shifts to the higher-energy side as the size of the CdTe crystals decreases. This phenomenon is interpreted in terms of a quantum confinement effect of electron and hole in the CdTe microcrystals.     

Thermooptic Coefficients of Soda–Lime–Silica Glasses

The thermooptic coefficients, d n /d T , of two series of soda–lime–silica glasses, 25Na₂O· x CaO·(75 – x )SiO₂ and (25 – y )Na₂O· y CaO·75SiO₂, have been measured. When CaO is substituted for SiO₂, the magnitude of d n /d T passes through a maximum negative value. When CaO is substituted for Na₂O, there is a monotonic change from negative to positive d n /d T . These effects are accounted for by the changes in relative values of the coefficient of thermal expansion and the temperature coefficient of electronic polarizability, Θ. The effect of composition on Θ is explained by the changes in density of various nonbridging and bridging oxygens in the glass.     

Shrinkage Behavior of Knoop Indentations in Silica and Soda–Lime–Silica Glasses

The annealing characteristics of Knoop-indented silica and soda–lime–silica glasses were investigated. These glasses were indented using a Knoop indenter in water, and they were annealed at various temperatures below the glass transition temperature. The major diagonal length of the Knoop indentation was measured before and after annealing, and the change of the diagonal length was determined. The change of diagonal length in silica glass was much larger than that in soda–lime–silica glass. This was attributed to the occurrence of more densification around the Knoop indentation in the silica glass. The activation energy of the shrinkage of the Knoop indentation in the silica glass, estimated from the temperature dependence of the relaxation time, was 46 kJ/mol, which was much less than that of viscous flow in silica glass. This suggested that the shrinkage of the Knoop indentation was caused by the structural relaxation of densified glass around the Knoop indentation.     

Mechanochemical Treatment of Amorphous Silica Powder for Glasses Without Firing

An amorphous silica powder was subjected to mechanical and hydrothermal treatment. Silanol surface groups, which play an active role in the chemical modification of silica, were produced by these two treatments. The surface activity of raw and treated powders was measured as the amount of dissolved Si⁴⁺ ion in ammonia (NH₃) and potassium hydroxide (KOH) solutions at different concentrations. KOH solution was more effective than NH₃ solution in activating the surface of the silica powder. Nonfiring glass was successfully fabricated using mechanochemically treated silica powder. Glass properties such as morphology and transparency were analyzed using scanning electron microscope and UV–vis–NIR techniques. Possible reaction mechanisms of powder surface activation occurring during hydrothermal and condensation processes are also discussed.     

Structural Role of Zinc Oxide in Silica and Soda-Silica Glasses

The structures of amorphous zinc silicate and sodium zinc silicate have been studied using the molecular dynamics computer simulation technique. Results showed that bond lengths and coordination numbers for the individual atomic species fit within the range of results from a number of crystalline and glassy silicates. Discrete ZnO₄ tetrahedra were seen to exist to a higher degree in the sodium zinc silicate system due to charge compensation from neighboring sodium atoms. Simulation results are also compared to EXAFS data of low concentrations of zinc oxide in silica and sodium silicate glasses. The local environment of oxygen in the simulated systems is also discussed.     

Preparation and Characterization of Li-Si-Al-O-N Glasses

The glass-forming region in the AIN-Li₂O–SiO₂ ternary was defined, and glass batches up to 15 g were prepared and characterized. Thermogravimetric data on thermal decomposition were interpreted in terms of probable decomposition reactions. Crystalline phases identified in partly crystalline and devitrified glasses include α'- and β'-sialons, eucryptite, and τ-LiAlO₂, all of which may contain some substituted N. Micro-hardness and elastic modulus of a glass of approximate composition Li₂Si₂AlO₅N are both considerably higher than values for similar oxide glasses.     

Dispersion of Thermooptic Coefficients of Soda–Lime–Silica Glasses

The thermooptic coefficients, i.e., the variation of refractive index with temperature (d n /d T ), are analyzed in a physically meaningful model for two series of soda–lime–silica glasses, 25Na₂O· x CaO·(75 – x )SiO₂ and (25 – x )Na₂O· x CaO·75SiO₂. This model is based on three physical parameters—the thermal expansion coefficient and excitonic and isentropic optical bands that are in the vacuum ultraviolet region—instead of on consideration of the temperature coefficient of electronic polarizability, as suggested in 1960. This model is capable of predicting and analyzing the thermooptic coefficients throughout the transmission region of the optical glasses at any operating temperature.     

Effect of Aluminum and Neodymium Admixtures on Devitrification of Silica Glasses

This investigation concerned the devitrification kinetics (in the temperature range 1100° to 1500°C) of silica glasses doped with aluminum oxide (0.1 and 1.0 mol%), neodymium oxide (0.08 and 0.25 mol%), and with aluminum and neodymium oxides simultaneously, at their mutual molar ratio close to 4. The glasses were obtained by electric melting of quartz under vacuum and by the sol/gel method. On the basis of the obtained results, conclusions have been drawn concerning the mode in which the admixtures become incorporated into the silica glasses as well as the structural differences between the melted and sol/gel glasses.     

Properties of Nitrogen-Containing Yttria—Alumina—Silica Melts and Glasses

The viscosity and solubility of nitrogen in Y₂O₃–Al₂O₃–SiO₂ melts have been systematically examined. The effects of nitrogen content on viscosity for Y-Al-Si-O-N melts and on Vickers hardness of oxynitride glasses also have been examined. Although the viscosity of Y₂O₃–Al₂O₃–SiO₂ melts was decreased, the solubility of nitrogen into the melts was increased with increased Y₂O₃ content. These results indicated that the yttrium ion behaved as a network modifier. Therefore, the structural units for viscous flow became small, and the amount of nonbridging oxygen increased in the melts when the Y₂O₃ content increased. The viscosity of Y-Al-Si-O-N melts and Vickers hardness of oxynitride glasses remarkably increased with increased nitrogen content. These results suggested that the substitution of nitrogen for oxygen in the melts may have led to a high average coordination of nonmetal atoms and that the increased cross-linking produced a more rigid glass network.     

Formation of Radiation-Induced Defects in Silica Glasses at High Irradiation Temperatures

The mechanisms of formation of radiation-induced defects in silica glasses are investigated under both simultaneous and sequential exposure to -radiation and high temperatures. It is demonstrated that silica glasses contain E"(T) centers (T = Si and Al) in impurity-free regions andE"_M (T) centers in the vicinity of alkali metal impurities. These centers can undergo transformations. The origin of a new center responsible for the absorption band at 360 nm is elucidated. This center is identified as a twofold-coordinated aluminum atom with a trapped electron.