Formation of Sm2+ lons in Sol-Gel-Derived Glasses of the System Na2 O-Al2O3-SiO2

Samarium ions (Sm²⁺) incorporated into aluminosilicate glasses by a sol-gel process showed persistent spectral hole burning at room temperature. Gels of the system Na₂O-Al₂O₃SiO₂ synthesized by the hydrolysis of Si(OC₂H₅)₄, Al(OC₄H₉)₃, CH₃ COONa, and SmCl₃·6H₂O were heated in air at 500°C, then reacted with H₂ gas to form Sm²⁺ ions. Whereas Al³⁺ ions effectively dispersed the Sm³⁺ ions in the glass structure, Na⁺ ions were not effective. The Al₂O₃-SiO₂ glasses proved appropriate for reacting the Sm³⁺ ions with H₂ gas and exhibited the intense photoluminescence of Sm²⁺ ions. The reaction of Sm³⁺ ions with H₂ in the Al₂O₂-SiO₂ glasses was determined by first-order kinetics, and the activation energy equaled 95 kJ/mol. At 800°C, the maximum photoluminescence of the Sm²⁺ ions was achieved within 20 min.     

Rapid Crystallization of SiO2-Al2O3 Glasses

The crystallization of Al₂O₃-rich glasses in the system SiO₂-Al₂O₃ which were prepared by flame-spraying and/or splat-cooling was studied by DTA, electron microscopy, and X-ray diffraction. Over a wide range of compositions, the crystallization temperature ( T_x ) remained near 1000°C, changing smoothly with composition. In all cases crystallization of mullite was detected by X-ray diffraction. In the low-Al₂O₃ region, coarsening of the microstructure during crystallization was observed by electron microscopy. In the high-Al₂O₃ region mullite and γ-Al₂O₃ cocrystallized; this behavior may be interpreted as evidence of a cooperative process of crystallization at the respective T_x 's. The crystallite size of the mullite immediately after rapid crystallization increased continuously with increasing Al₂O₃ content. In light of the T_x data, the adequacy of the evidence for the proposed metastable miscibility gap in the SiO₂-Al₂O₃ system is questioned.     

Crystallization of As2Se3-As2Te3 Glasses

Differential scanning calorimetry studies of As₄₀Se_xTe_60-x glasses with 0≤x≤60 show a 2-stage crystallization when 5≤x≤35, followed by a double-peaked melting endotherm. The enthalpy of crystallization for the low-temperature species is independent of composition and equals 11.6±0.9 cal/g for x≤25, whereas that for the high-temperature species decreases from 2.2±0.2 cal/g for x = 5 to 1.1±0.3 cal/g for x =15. The total enthalpy of melting also is independent of composition and equals 18.8±1.2 cal/g for x ≤25. No crystallization was observed for xe40. The high-temperature phase has the mono-clinic As₂Te₃ structure; that of the low-temperature phase was not identified. Annealing experiments for compositions with x = 15 or 20 gave time-temperature transformation maps. A kinetic analysis of the data was inconclusive.     

Structural Analysis of SiO2-Al2O3 Glasses

The structure of SiO₂-Al₂O₃ glasses with up to 60 wt% Al₂O₃ was investigated using the radial distribution function together with the correlation method based on X-ray scattering intensity data. Radial distribution curves are interpreted on the basis of glass-in-glass separation with the constituents of SiO₂-rich and Al₂O₃-rich glasses. The structure of the Al₂O₃-rich glass has a short-range ordering similar to the crystal structure of mullite. The calculated S- i (S) curve of this model gives good agreement with the observed one.     

Preparation of SiO2-TiO2-ZrO2 Glasses from Alkoxides

Relatively large compact glass samples (∼5 mm in diameter) of 65SiO₂·20TiO₂·15ZrO₂ and other binary and ternary compositions were prepared by the alkoxide gel method. Prolonged storage of the gels in water before they were dried reduced the residual carbon content of the resulting glasses considerably. Optimal preparation conditions are given, and the microstructure of the gels and glasses obtained is discussed.     

Formation and Properties of GeSe2–Ga2Se3–PbI2 Novel Chalcohalide Glasses

The glass-forming region of the GeSe₂–Ga₂Se₃–PbI₂ system was determined and homogeneous glasses were prepared. The maximum dissolvable PbI₂ can be up to 50 mol%. The structures of glasses were characterized by Raman spectroscopy. The thermal, optical, and some basic physical properties of the glasses were investigated. The results show that GeSe₂–Ga₂Se₃–PbI₂ glasses have a wide region of transmission window (0.7–16 μm) and high refractive index (∼2.5) with the addition of PbI₂. The glasses have good glass-forming ability and high glass transition temperatures. Consequently, these novel glasses may be promising candidate materials for infrared optics and nonlinear optical field.     

Rayleigh and Brillouin Scattering in K2O–SiO2 Glasses

The intensity and spectral distribution of light scattered by K₂O-SiO₂ glasses (K₂O content up to 40 mol%) were measured. The transverse and longitudinal sound-wave velocities and the photoelastic constants were evaluated from the results. The total intensity of the scattering (and therefore the attenuation caused by it) exhibited a minimum at a concentration of ∼25 mol% K₂O. For this composition the attenuation is ∼2/2 of that in pure SiO₂. This behavior results from the existence of anomalously small concentration fluctuations in the melt of K₂O·3SiO₂ glass. A qualitative explanation of this result, involving low-temperature immiscibility regions, is presented.     

Effects of Preparation History on TiO2· SiO2 Glasses

Differences in the Raman spectra of various heat-treated TiO₂· SiO₂ glasses could be related to their thermal and chemical histories. For instance, while rutile could be detected in batch-prepared glasses heated at 1100°C, only α-cristobalite could be detected in heat-treated devitrified flame-prepared glasses with comparative TiO₂-concentrations. Thermal expansion coefficients increased for batch-prepared glasses upon heat treatment due to exsolution of rutile from the glasses. Earlier work had noted similar behavior at lower temperatures due to exsolution of anatase.     

Correlation between Macro- and Microstructural Changes in Ge:SiO2 and SiO2 Glasses under Intense Ultraviolet Irradiation

The photochemical transformation mechanism of defects in germanosilicate and silica glasses under ultraviolet (UV) laser irradiation has been investigated based on the changes in Raman spectra before and after irradiation. Two types of silica glasses, fused silica (type I) and dry synthetic silica (type IV), and germanosilicate optical fiber preforms were irradiated by intense UV photons from excimer lasers. Spectral changes in optical absorption and Raman spectra were examined to clarify a correlation between the microscopic defect formation and the macroscopic structural changes causing a photorefractive effect. Successive generation of E' centers through divalent centers is closely correlated with changes in Raman spectra, indicating that large structural changes in the glass network involved in this process would be the origin of photon-induced densification of the glasses. In addition, it has been proposed that the successive generation of E' centers is mediated by transient divalent centers converted from relaxed cation homobondings.     

Equilibrium Compressibilities and Density Fluctuations in K2O–SiO2 Glasses

The density fluctuations contributing to light scattering in a glass are governed by the flctive temperature of the glass and the equilibrium compressibility of the melt. Using ultrasonic velocity data for K₂O–SiO₂ melts, these compressibilities were evaluated, and the magnitude of the density fluctuations were calculated. In this system, the mean–square amplitude of the fluctuations reaches a minimum value (about half that of pure SiO₂) for a composition of ∼20 mol% K₂O. By extrapolating the equilibrium compressibilities to zero K₂O content, the density fluctuations can be calculated for pure SiO₂ glass; this calculation agrees well with the result obtained from light–scattering measurements.     

Corrosion and Crack Growth in 33% Na2O-67% SiO2 and 33% Li2O-67% SiO2 Glasses

Environmentally enhanced crack growth data for 33% Na₂O-67% SiO₂ and 33% Li₂O-67% SiO₂ (mol%) were obtained. Corrosion data for the same glasses as determined by infrared reflection spectroscopy and atomic absorption spectroscopy were also measured. It was determined that, of the tested environments, those and only those environments which caused surface corrosion also enhanced crack growth. Crack growth curves were more complicated for these materials than for fused silica. Water-dominated regions occurred even in environments which are known to enhance crack growth.     

Formation of SiO2, Al2O3, and 3Al2O3. 2SiO2 Particles in a Counterflow Diffusion Flame

SiO₂, Al₂O₃, and 3Al₂O₃.2SiO₂ powders were synthesized by combustion of SiCl₄ or/and AlCl₃ using a counterflow diffusion flame. The SiO₂ and Al₂O₃ powders produced under various operation conditions were all amorphous and the particles were in the form of agglomerates of small particles (mostly 20 to 30 nm in diameter). The 3Al₂O₃.2SiO₂ powder produced with a low-temperature flame was also amorphous and had a similar morphology. However, those produced with high-temperature flames had poorly crystallized mullite and spinel structure, and the particles, in addition to agglomerates of small particles (20 to 30 nm in diameter), contained larger, spherical particles 150 to 130 nm in diameter). Laser light scattering and extinction measurements of the particle size and number density distributions in the flame suggested that rapid fusion leading to the formation of the larger, spherical particles occurred in a specific region of the flame.     

Energetics of La2O3–HfO2–SiO2 Glasses

A series of La₂O₃–HfO₂–SiO₂ glasses, approximately along the join 0.73SiO₂–0.27( x HfO₂–(1− x )La₂O₃), 0< x <0.3), was prepared using containerless processing techniques (aerodynamic levitation combined with laser heating in oxygen). The enthalpy of formation and enthalpy of vitrification at 25°C were obtained from drop solution calorimetry of these glasses and appropriate crystalline compounds in a molten lead borate (2PbO–B₂O₃) solvent at 702°C. The enthalpy of formation from crystalline oxides was exothermic and became less exothermic with increasing HfO₂ content. Heat contents were measured by transposed temperature drop calorimetry and depended linearly on the HfO₂ content. Differential scanning calorimetry showed that both the onset glass transition and the onset crystallization temperature of these glasses increased with increasing HfO₂ content. Upon slow cooling in air, the glasses crystallized to a mixture of baddeleyite, cristobalite, lanthanum disilicate, and hafnon.     

Reaction of CaTiO3 with PbO–B2O3 and PbO–SiO2 Glasses

Interfacial and powder reactions between CaTiO₃ and 90PbO–10B₂O₃ and 75PbO–25SiO₂ binary glasses were studied. The reaction has been analyzed as the effect of B₂O₃ and SiO₂ additions on the interaction between CaTiO₃ and PbO, and discussed from thermodynamic and kinetic points of view. For a fixed CaTiO₃/PbO ratio₂ the product perovskite phase became enriched with lead as the amount of additives increased, which is more pronounced with B₂O₃ addition. The reaction of CaTiO₃ with the lead–boron glass was controlled by a dissolution-precipitation mechanism, and that with the lead-silica glass by a diffusion mechanism.     

Fabrication of TiO2–SiO2 Photonic Crystals with Diamond Structure

Ceramic photonic crystals with diamond structure were fabricated using stereolithography and successive sintering. The green body of an epoxy resin incorporating 10 vol% TiO₂–SiO₂ was formed by stereolithography and then heated in air at 1100°–1400°C for 2 h. The sintered products maintained the diamond structure with a linear shrinkage ratio of about 57% and a porosity of 38%. The ceramic photonic crystal with eight unit cells showed a photonic band gap at the center frequency of 23.5 GHz. This fabrication method of three-dimensional (3D) ceramic photonic crystals is applicable to other 3D structural ceramics and does not require any molding techniques.     

Mechanical Damping Spectrum for Mixed-Alkali R2 O·Al2 O3·6SiO2 Glasses

The internal friction of R₂O·Al₂O₃·6SiO₂ glasses was measured from-180° to 700°C at 0.4 Hz. Glasses containing Li₂O or Na₂O exhibited only the one internal friction peak characteristic of the stress-induced movement of the alkali ions. Substitution of a second alkali resulted in two significant changes in the internal friction: (1) a rapid reduction in the magnitude of the original alkali peak and (2) the appearance of a new internal friction peak whose magnitude was especially sensitive to the concentration of the second alkali. Each combination of two alkali ions resulted in a new peak, with peaks being observed for the combinations Li-Na, Na-K, and Li-K. A mechanical damping spectrum is predicted for aluminosilicate glasses containing more than two alkali ions.     

Crystallization and Properties of Fe2O3—CaO—SiO2 Glasses

Nucleation and crystal growth rates and properties were studied in a two-stage heat treatment process for Fe₂O₃-CaO-SiO₂ glasses. Glass transition (T_g) and crystallization temperatures (T _c ) for the glasses lay between about 612.0° and 710.0°C, and 858.5° and 905.0°C, respectively, and magnetite was the main crystal phase. For a glass of 40Fe₂O₃. 20CaO·40SiO₂ (in wt%) the maximum nucleation rate was (68.6 ± 7) × 10⁶/mm³·s at 700°C, and the maximum crystal growth rate was 9.0 nm/min^1/2 at 1000°C. The mean crystal size of the magnetite increased from 30 to 140 nm with variation of nucleation and crystal growth conditions. The glass showed the maxima in saturation magnetization and coercive force, 212.1 × Wb/m² and 30.8 × 10³ A/m, when heat-treated for 4 h at 1000°C and 1050°C, respectively. The variation of the saturation magnetization could be quantitatively interpreted well in terms of the volume fraction of the magnetite, whereas that of the coercive forces could be explained only qualitatively in terms of the particle size of the magnetite. Hysteresis losses showed the maximum value of 1493 W/m³ when heat-treated at 1000°C for 4 h prenucleated at 700°C for 60 min, and increased linearly with increasing heat treatment time under a magnetic field up to 800 × 10³ A/m.     

Nanostructure of TiO2 Nano-Coated SiO2 Particles

We characterized SiO₂–TiO₂ nano-hybrid particles, prepared using the sol–gel method, using high-resolution transmission microscopy. A few nanometer-ordered TiO₂ anatase crystallites could be observed on the monodispersed SiO₂ nanoparticle surface. The quantum size effect of the TiO₂ anatase crystallites is attributed to the blue shift of the absorption band. The rough surface of the SiO₂–TiO₂ nano-hybrid particles was derived from the developed growth planes of the TiO₂ anatase crystallites, grown from fully hydrolyzed Ti alkoxide that did not react with acetic acid during the crystallization process at 600°C thermal annealing.