Polarized Raman Scattering in Rare-Earth Fluoride Glasses

The polarized Raman spectra of rare-earth fluoride glasses based on BaF₂ and ThF₄, in combination with ZnF₂, YF₃, YbF₃, LuF₃, andlor AlF₃, are reported and compared with those of other heavy-metal fluoride glasses. The spectra of rare-earth fluoride glasses, especially those containing large amounts of AlF₃, have relatively broad and mostly depolarized features, suggesting that these glasses are more highly disordered than fluorozirconate and transition-metal fluoride glasses.     

Surface -OH Profile from Reaction of a Heavy-Metal Fluoride Glass with Atmospheric Water

The surface -OH profile in a 15BaF₂-28.3ZnF₂-28.3YbF₂-28.3ThF₄ (mol%) glass resulting from reaction with atmospheric water for 22.5 h at 344°C was determined by monitoring the 2.9-pm absorption band as material was removed from the surface by polishing. The -OH penetrated -10 pm into the surface. The-OH concentration profile agreed with that predicted from a diffusional analysis, allowing determination of the surface -OH concentration; this corresponded to replacement of one of seven F⁻ ions with OH⁻.     

Effect of Aluminum Content on Infrared Characteristics of BaF2/ThQ-Containing Glasses

The incorporation of large amounts of aluminum substantially alters the ir characteristics of BaF₂/ThF₄-based glasses. In particular, a distinct peak associated with aluminum is observed in the fundamental reflectivity spectrum at ∼ 625 cm⁻¹, with a concomitant shift of the ir absorption edge to higher frequencies. Although beneficial for glass formation, the addition of aluminurn has a deleterious effect on the ir transparency range of heavy-metal, fluoride glasses.     

Extension of Glass-Forming Region in a ZrF4-Based System by Rapid Melt Quenching

It was shown that rapid melt quenching can extend the glass-forming region of a ZrF₄–BaF₂–YF₃–AlF₃ composition considerably. Cooling at a rate of ∼10² K · s⁻¹ prevented glass formation at compositions less than about 25 mol% BaF₂, whereas quenching at a rate of ∼2×10⁶ K · s⁻¹ resulted in glasses with a BaF₂ content as low as about 13 mol%. This high quenching rate was achieved by a specially designed twin-roller apparatus.     

Controlled Crystallization and Properties of Zirconium Fluoride-Based Glass-Ceramics

51ZrF₄16BaF₂, 5LaF₃.3AlF₂,.20Li.F5PbF₂, glasses were pre-pared with CdF, additions of 0, 2, 5, and 7.5 mol%. The glasses with 2 and 5 mol% exhibited controllable nucleation upon heat treatment above the glass transformation tem-perature. The nucleation and crystallization of glasses with 5 mol% CdF, were characterized using bulk heat treat-ments, differential scanning calorimetry, and scanning elec-tron microscopy. The isothermal crystallization of nucleated glasses resulted in the formation of a glass-ceramic that was transparent in the infrared (>70% transmission) and more resistant to fracture than typical fluoride glasses.     

Photoluminescence Properties of Zinc Oxide in Barium and Fluorine Silicate Glasses

Zinc oxide (ZnO) was embedded in barium and fluorine silicate glass matrices, and luminescence properties of as-prepared glass samples were studied. For the fluorine-free glasses, a blue emission was observed. With partial substitution of BaF₂ for BaO in glass compositions, emission spectra of glasses show an obvious blue shift to the near-UV region. For the BaO-free glasses, the dominant UV emission shifts to the near-band-edge exciton emission position of ZnO and also shows a red shift with increasing BaF₂. At the same time, a visible emission shoulder appears in the blue to green region and becomes weak with introduction of increasing BaF₂.     

Positron Lifetime Study of the Crystal Evolution and Defect Formation Processes in a Scintillating Glass

The crystal evolution and defect formation in scintillating glasses as a consequence of thermal annealing were studied by annihilation lifetime spectroscopy and UV-Vis absorption spectroscopy. The annihilation lifetime spectra and UV-Vis spectra were recorded on glass 50SiO₂–45ZnO–5BaF₂ before and after annealing at 580°C for 16, 32, and 48 h, respectively. The results show that the three lifetime components (τ₁, τ₂, and τ₃) and the corresponding intensities ( I ₁, I ₂, and I ₃) change systematically with increasing annealing time. This reflects the crystal evolution and defect formation in the glass matrix. The continued crystal evolution was also revealed by the UV-Vis spectra, as the absorption edge of the material shifted to a lower energy with prolonged annealing.     

Sol-Gel Synthesis of High-Quality Heavy-Metal Fluoride Glasses

Fluoride glasses are excellent laser hosts and are very well suited for a broad spectrum of optical applications. However, current fluoride glass synthesis is expensive. The solgel method is an affordable alternative for producing highperformance, optical-quality heavy-metal fluoride glasses. The method involves forming a hydrous oxide gel of the constituent metal alkoxides and salts, polymerizing the solution to form a gel, fluorinating the gel with anhydrous HF, melting the amorphous material in an oxidizing atmosphere of SF₆, and casting the melt into desired shapes. ZBLA (57ZrF₄·36BaF₂·4LaF₃·3AlF₃, in mol%) and Nd-doped (0.3 mol%) ZBLA glass rods were prepared by this process and their properties were measured. The solgel-based glasses had thermal and optical properties similar to those found in the literature for conventionally prepared fluorides.     

Crystallization of ZBLAN Glass

The glass transition, onset crystallization, solidus, and liquidus temperatures of a ZBLAN glass have been measured by differential thermal analysis in a sealed Pt ampoule. The onset crystallization was observed at 335°C for a powdered glass, while for a monolithic glass, prepared by in situ quenching in the Pt capsule, the onset crystallization was observed at 395°C, 135°C higher than T _g. Surface-induced heterogeneous nucleation is therefore proposed to be important for the crystallization of ZBLAN glasses. The solidus temperature was observed at 452°C, about 10°C lower than reported for the subsystem ZrF₄–BaF₂–NaF.     

Melting Apparatus for the Synthesis of Bulk ZrF4-Based Fluoride Glass

Heavy-metal fluoride glasses are currently prepared by melting of the solid fluoride precursors. We have constructed a melting facility for the synthesis of ZrF₄-based fluoride glass cullet. This stand-alone system provides a controlled (dry and reactive) atmosphere and in situ quenching of the melts. The glass is melted in a vitreous carbon crucible, which is mounted in a silica tube flow reactor. The reactor is resistively heated with a tube furnace and purged with He and SF₆ or Cl₂. It was found that of the latter two halogenating species only Cl₂ prevents effectively the formation of black inclusions in the melt (ZrF₃ and ZrF₂ particles). With this melting apparatus we have routinely melted 250-g batches of optically clear ZrF₄–BaF₂–LaF₃–AlF₃–NaF glass samples.     

Vacuum Plasma Spray Deposition of Titanium Particle/Glass-Ceramic Matrix Biocomposites

The vacuum plasma spray technique (VPS) has been successfully employed to coat Ti-6A1-4V substrates with bioactive glasses and Ti-particle/glass-ceramic matrix biocomposites. The composites were prepared by sintering, under an Ar flow, green bars of bioactive glass powders and 30% volume Ti particles. The bioactive glasses have the two following compositions: SCB (48.8SiO₂−48.8CaO−2.4B₂O₃) and TSCB (46.6SiO₂−48.7CaO−2.5B₂O₃−2.2TiO₂) (mol%). The VPS bioactive coatings were characterized by means of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and mechanical tests (Vickers indentations and tensile and shear tests). Their bioactivity was tested by soaking the samples in a simulated body fluid (SBF) and by analyzing the growth of hydroxylapatite (HA) by SEM, EDS, and XRD. Leaching tests of Ca, Si, and P in SBF were made by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Perkin-Elmer 5000) to study the in vitro bioactivity of the samples versus time. Each coating was found to be bioactive and well bonded to the substrate; the composites showed better mechanical properties than the pure glass matrices and the hydroxylapatite coatings deposited by the same VPS technique.     

Infrared Spectroscopic Studies of the Hydrolysis Reaction During Leaching of Heavy-Metal Fluoride Glasses

In this work, the interfacial reactions of heavy-metal fluoride glasses with water have been investigated by Fourier transform infrared spectroscopy. High dilution and stirring conditions were maintained to prevent any influence of pH drift and reprecipitation. From comparisons of the integrated absorption peak areas for the O-H stretching and H-O-H bending vibrations in the glass with those for molecular water, the contributions of metal hydroxyls formed in the corrosion layer can be separated from the total O-H band. The comparison between the infrared spectra for leached samples and hydroxides of the glass components show the presence of Zr-OH and La-OH groups in ZrF₄-based glasses after leaching and drying and of Th-OH groups in ThF₄ glass, indicating the existence of F.^-/OH^- ion exchange between the glass surface and water.     

Effect of Yb3+-Codoping on the Upconversion Emission Intensity in Er3+-Doped ZBLAN Fluoride Glasses under 800-nm Excitation

The effect of Yb³⁺-codoping on the upconversion emission intensity in Er³⁺-doped ZBLAN fluoride glasses is investigated. The codoping of Yb enhanced the emission intensity for the samples excited by an 800-nm laser diode beam. The enhancement in a constant YbF₃ content (2 mol%) increased with increasing ErF₃ content was about 70% of the initial value at 550 nm for the glasses containing 8 mol% of ErF₃. The emission intensity at 550 nm in a constant ErF₃ content (5 mol%) increased remarkably with the addition of YbF₃ and was maximized around 7 mol% of YbF₃ content, giving an increased ratio of about 200% of the initial value. The reason for the enhancement is discussed and it is derived mainly from two-step excitation of Er³⁺ assisted by Yb³⁺ excited through the energy transfer from Er³⁺.     

Phase Equilibria in the System NaF-ThF4-UF4

Phase equilibria in the condensed system NaF-ThF₄-UF₄ were determined by cooling-curve and thermal-gradient quenching experiments from the liquidus to ∼450°C. A phase diagram of the ternary system was constructed, showing three singular points and eleven invariant points, including two eutectics, occurring in association with the primary phases of ten solid solutions and two pure solids. The eutectic mixture having the composition 75.5NaF-10.5ThF₄-14UF₄ (mole %) melts at 575°C, the lowest liquidus temperature in the system. Except for NaF, NaF-ThF₄, and ThF₄-UF₄ solid solutions, phases crystallizing from molten NaF-ThF₄-UF₄ mixtures are solid solutions formed by interchange of U⁴⁺ and Th⁴⁺ ions in NaF-ThF₄ and NaF-UF₄ compounds. No ternary compounds were found. Phase diagrams of the systems NaF-ThF₄ and NaF-UF₄, revised in the course of this study to include the newly identified phases, 7NaF.2ThF₄ and 7NaF.2UF₄, are presented. X-ray diffraction and optical data are given for the pure crystal phases, 4NaF.ThF₄, 7NaF.2ThF₄, 7NaF.2UF₄, 3NaF.UF₄, and for 3NaF.2ThF₄-5NaF.3UF₄ solid solutions. The system is unique among the fluoride systems reported in containing so wide a variety of substitutional solid solutions and in containing a continuous solid solution formed of binary compound pairs 3NaF.2ThF₄ and 5NaF.3UF₄ which are isomorphous but of different stoichiometry.     

Eu2+-Doped Glass Ceramics Containing BaF2 Nanocrystals as a Potential Blue Phosphor for UV-LED

The Eu²⁺-doped glass ceramics containing BaF₂ nanocrystals were prepared and their luminescence properties were investigated. The excitation spectra of Eu²⁺-doped glass ceramics showed an excellent overlap with the main emission region of an ultraviolet light-emitting diode (UV-LED) centered at 380 nm. The 450 nm emission of Eu²⁺ in glass ceramics under the 385 nm excitation was much stronger than that in glass. The Eu²⁺-doped glass ceramics containing BaF₂ nanocrystals may be used as a potential blue-emitting phosphor for UV-LED.     

Nucleation in ZBLAN Glasses

Nucleation rates were measured in a ZrF₄–BaF₂–NaF–LaF₃–AlF₃ glass (ZBLAN) using an optical technique. The results were compared with a similar glass having a slightly different composition. The difference in the nucleation rate is explained by classical nucleation theory using calculated free-energy differences between the ZBLAN liquid and the BaZrF₆ crystal, which is found as the precipitating phase.     

Surface Tension of ZrF4-BaF2-LaF3 Glass

The surface tension of a 62ZrF₄.33BaF₂.5LaF₃ (mol%) glass was measured by a solidified pendant drop technique. The surface tension at 550°C is 0.174 ± 0.005 J/m².     

Preparation by a Sol-Gel Process of Borosilicate Glasses Containing Microcrystals of Tellurium and Zinc Telluride

Borosilicate glasses, 5B₂O₃· 95SiO₂ (mol%), containing TeO₂ and ZnO nominally equivalent to 10 wt% Te and ZnTe were prepared by a solgel method from Si(OC₂H₅)₄, B(OCH₃)₃, H₆TeO₆, and Zn(NO₃)₂. A study by electron spectroscopy for chemical analysis (ESCA) showed that glasses heated at high temperature (450°C) in air contained both Te⁶⁺ and Te⁴⁺ ions on the surface layer, but that mainly Te⁴⁺ ions occurred inside the bulk glass. When solgel-derived borosilicate glasses containing the TeO₂ compound were reduced at elevated temperature in a hydrogen atmosphere, Te crystallites ranging in size from 4 to 15 nm were produced at a lower temperature, between 200° and 250°C. The absorption edge moved from the infrared to the visible wavelength region as the particle size decreased to about 4 nm. For glasses containing both TeO₂ and ZnO, ZnTe crystallites formed at high temperature—over 300°C—and existed along with the Te phase.     

Structure, Thermal Behavior, Chemical Durability, and Optical Properties of the Na2O–Al2O3–TiO2–Nb2O5–P2O5 Glass System

The FTIR, Raman, UV-Vis, ³¹P MAS-NMR, DTA, and refractive index measurements have been combined to investigate a series of glasses with the general formula 20Na₂O–5Al₂O₃− x TiO₂–(45− x )Nb₂O₅–30P₂O₅, 15≤ x ≤45. The glass structure, as well as thermal, optical, and chemical durability properties, were then described as functions of the f _Nb/ f _Ti ratio. An increase of the f _Nb/ f _Ti ratio correlates with a decrease in length of the average phosphate chains linked through Nb–O–P and Ti–O–P bonds, with an increase in the glass stability and with increase in the linear refractive indices at 632.8 nm from 1.79 to 1.89. Furthermore, niobium is more effective than titanium in improving chemical durability.     

Electrical Resistivity of Titanium-Containing Glasses

Titanium-containing glasses were prepared by fusion of a base glass (BaO·B₂O₃SiO₂) and TiO₂ and/or Ti₂O₃ in Ar. Their resistivities did not vary with melting time and temperature. Interaction of Ti⁴⁺ and Ti³⁺ in the glasses was deduced by spectroscopy, but the valence states in the batch compositions were preserved in the glasses, according to the chemical analysis. Glasses containing either Ti⁴⁺ or Ti³⁺ had very high resistivities, whereas the glass prepared by melting a mixture of a Ti⁴⁺-containing and a Ti³⁺-containing glass had much lower resistivity. All results confirmed the possibility of controlling the resistivity by batch composition for these glasses.