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.     

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.     

Journal. Effects of Composition Changes on the Crystallization Behavior of MgO-CaO-SiO2-P2O5 Glass-Ceramics

The effects of composition changes on the crystallization behavior of apatite-containing glass-ceramics in the system MgO-CaO–SiO₂–P₂O₅ were studied. The eutectic composition of 40 wt% 3CaO-P₂O₅–60 wt% CaO MgO 2SiO₂ was selected as the base glass. Several parameters were quantitatively or qualitatively derived from thermal analysis, X-ray diffraction, and microstructural observations. These parameters can be divided into two groups according to their variation with glass composition. The parameters in the first group depend mainly on the apatite nucleation rate, which increases with an increase in CaO or PiO₅, and with a decrease in MgO or SiO₂. The parameters in the second group are closely related to the extent of deviation of the glass composition from the eutectic. The classical theory for nucleation employed to explain the observed phenomena is discussed.     

Effect of Neodymium:Yttrium Aluminum Garnet Laser Irradiation on Crystallization in Li2O–Al2O3–SiO2 Glass

A 355-nm neodymium:yttrium aluminum garnet laser, produced by a harmonic generator, was used for the nucleation process in photosensitive glass containing Ag⁺ and Ce³⁺ ions. The pulse width and frequency of the laser were 8 ns and 10 Hz, respectively. Heat treatment was conducted at 570°C for 1 h, following laser irradiation, to produce crystalline growth, after which a LiAlSi₃O₈ crystal phase appeared in the laser-irradiated Li₂O–Al₂O₃–SiO₂ glass. The present study compares the effect of laser-induced nucleation on glass crystallization with that of spontaneous nucleation by heat treatment.     

Effect of Alkali Metal Oxide Addition on the Sinterability of MgO–B2O3–Al2O3 Glass–Al2O3 Filler Composites

The sintering of a composite of MgO–B₂O₃–Al₂O₃ glass and Al₂O₃ filler is terminated due to the crystallization of Al₄B₂O₉ in the glass. The densification of a composite of MgO–B₂O₃–Al₂O₃ glass and Al₂O₃ filler using pressureless sintering was accomplished by lowering the sintering temperature of the composite. The sintering temperature was lowered by the addition of small amounts of alkali metal oxides to the MgO–B₂O₃–Al₂O₃ glass system. The resultant composite has a four-point bending strength of 280 MPa, a coefficient of thermal expansion (RT—200°C) of 4.4 × 10⁻⁶ K⁻¹, a dielectric constant of 6.0 at 1 MHz, porosity of approximately 1%, and moisture resistance.     

Enhanced Surface Crystallization of β-Barium Borate on Glass Due to Ultrasonic Treatment

β-Barium borate (BBO) surface crystallized glass composites were prepared for a glass of composition 40BaO·15Tio₂–45B₂O₃ (in mol%). Enhanced nucleation was attained by ultrasonic surface modification with an aqueous suspension of BBO particles, resulting in dense BBO thin films on the glass surfaces. Second harmonic generation signals were observed for samples subjected to ultrasonic treatment.     

Crystallization Kinetics and Phase Development of PbO–BaO–SrO–Nb2O5–B2O3–SiO2-Based Glass–Ceramics

The nucleation and crystallization kinetics of PbO–BaO–SrO–Nb₂O₅–B₂O₃–SiO₂-based glass–ceramics have been investigated. Strontium barium niobate (Sr_0.33Ba_0.67Nb₂O₆) with a tetragonal tungsten–bronze structure formed as the major crystalline phase, which nucleates and grows on the surface region of samples. The results of the present study showed an apparent activation energy of 193 kJ/mol for nucleation, which was controlled by the viscous flow of the glass. Quantitative X-ray analysis and differential thermal analysis showed that the rate-limiting mechanism of crystallization appeared to be a three-dimensional interfacial growth, which has an apparent activation energy of 386–430 kJ/mol, a value that is close to the dissociation of Si–O bonds in the glass system.     

Heterogeneous Plantinum Nucleation and Crystallization of a Heavy-Metal Fluoride Glass

The effect of controlled heterogeneous nucleation by platinum on the crystallization of a ZrF₄-BaF₂-LaF₃-AlF₃-NaF (ZBLAN) glass was studied. Various levels of platinum were incorporated into this glass by a combination of PtCl₂-doping and melting-atmosphere variation. The effect of doping levels and melting conditions on the incorporation of platinum and the subsequent nucleation of crystals was studied by optical microscopy, scanning electron microscopy, differential scanning calorimetry (DSC), and X-ray diffraction. Increased platinum in the glass resulted in an increased number of nucleation sites for the growth of β-ZrF₄-BaF₂ crystals. Analysis of isothermal and ramp-rate DSC measurements indicated that the crystallization of this glass changed from surface controlled to bulk controlled with an increased number of nuclei. This was confirmed by optical microscopy. In addition, Avrami analysis of the isothermal crystallization data gave an accurate approximation of the number of nuclei in the glass.     

Al2O3–ZrO2–SiO2 Ternary Glasses for Molybdenum Oxidation Barriers

The wettability of binary and ternary glasses belonging to SiO₂–Al₂O₃–ZrO₂ diagram has been studied using the sessile drop technique at 1750° and 1800°C. The ternary SiO₂–Al₂O₃–ZrO₂ (90–5–5 wt%) glass has proved to be well appropriated as a molybdenum oxidation barrier coating. The addition of 5 wt% of MoO₂ slightly improves its wettablity at higher temperatures without affecting its oxidation barrier properties. The Mo comes into the glass network as a mixture of Mo⁵⁺, Mo⁴⁺, and Mo⁶⁺. After oxidation at 1000°C in oxygen atmosphere, the molybdenum remains in the glass network as Mo⁶⁺.     

Major Influences on the Particle-Size–Transformation-Temperature Relation in Al2O3–ZrO2 Composites

The energetics of martensitic transformation in ZrO₂ is studied using a thermodynamic approach, with particular reference to Al₂O₃–ZrO₂ composites. The different characters of three types of transformation-toughened ceramics are analyzed, and several factors affecting the t → m transformation in Al₂O₃–ZrO₂ composites are discussed. The expression of transformation temperature dependence on particle size is derived and has good agreement with experimental results. The energetics concerned with nucleation of martensitic transformation is also discussed.     

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.     

Quantitative Identification of Phonon Modes Controlling the Multiphonon Relaxation in Heavy-Metal Oxide Glasses

The phonon mode(s) controlling the multiphonon relaxation (MPR) in PbO–Bi₂O₃–Ga₂O₃ glass was analyzed, and the effect of GeO₂ addition on the MPR process was investigated. MPR rates were obtained from the lifetimes of the Tm³⁺:³ H ₄ level in glasses over the temperature range 20–280 K. In PbO–Bi₂O₃–Ga₂O₃ glass, phonons from the bending vibration between GaO₄ tetrahedra (∼550 cm⁻¹) controlled the MPR process. On the addition of GeO₂, the phonon mode at ∼770 cm⁻¹ due to the stretching vibration of GeO₄ tetrahedra started to affect the MPR process. Phonon modes controlling the MPR process in PbO–Bi₂O₃–Ga₂O₃–GeO₂ glass were both 550 cm⁻¹ and 770 cm⁻¹.     

Devitrification Kinetics and Mechanism of K2O–CaO–SrO–BaO–B2O3–SiO2 Glass-Ceramic

The devitrification kinetics and mechanism of a low-dielectric, low-temperature, cofirable K₂O–CaO–SrO–BaO–B₂O₃–SiO₂ glass-ceramic have been investigated. Crystalline phases including cristobalite (SiO₂) and pseudowollastonite ((Ca,Ba,Sr) SiO₃) are formed during firing. Activation energy analysis shows that the nucleation of the crystalline phases is controlled by phase separation of the glass. The crystallization kinetics of both cristobalite and pseudowollastonite obey Avrami-like behavior, and the results show an apparent activation energy close to that of the diffusion of alkaline and alkali ions in the glass, suggesting that diffusion is rate limiting. The above conclusion is further supported by analysis of measured growth rates.     

Phase Separation Inducing Controlled Crystallization of GeSe2–Ga2Se3–CsI Glasses for Fabricating Infrared Transmitting Glass–Ceramics

Infrared transmitting glass–ceramics based on the selected glass of 65GeSe₂–25Ga₂Se₃–10CsI were obtained by a two-stage heat-treatment method. Results of X-ray diffraction and scanning electronic microscopy indicated that droplet-like nanoparticles containing cubic Ga_2−δGe_δSe₃ crystals are homogeneously generated in the glass–ceramics and that the whole glass–ceramic process is composed of phase separation, nucleation, and crystal growth. Evolutions of the optical and mechanical properties of glass–ceramics versus annealing time at the first-stage heat treatment were also investigated. Compared with the parent glass, the fabricated glass–ceramics show considerably enhanced fracture toughness, practicable infrared transparence, and microhardness, which confer them with considerable competitive advantages over currently used infrared materials.     

Thermal, Dielectric, and Optical Properties of Neodymium Borosilicate Glasses for Thick Films

Because of their thermal, dielectric, and optical properties, new glass compositions and thick-filmed transparent dielectrics containing neodymium oxide (Nd₂O₃) were studied as a source of purer images in plasma display panels. In the present study, PbO–B₂O₃–SiO₂ and PbO–B₂O₃–SiO₂–ZnO–Al₂O₃ were used as starting glass compositions, to which up to 25 wt% of Nd₂O₃ then was added. Increased amounts of Nd₂O₃ increased the glass transition temperature and dielectric constant of the bulk glasses and decreased the coefficient of thermal expansion. The fired thick films (around 30 μm) allowed selectively visible light to penetrate and showed deep absorption properties at 585 nm that were related to an extraneous gas from neon discharge.     

Effects of Doping Trivalent Ions in Bismuth Borate Glasses

Bismuth borate glasses from the system: 40Bi₂O₃–59B₂O₃–1Tv₂O₃ (where Tv=Al, Y, Nd, Sm, and Eu) and three glasses of composition: 40Bi₂O₃–60B₂O₃, 37.5Bi₂O₃–62.5B₂O₃ and 38Bi₂O₃–60B₂O₃–2Al₂O₃ were prepared by melt quenching and characterized by density, UV-visible absorption spectroscopy and differential thermal analysis (DTA) studies. Bismuth borate glasses exhibit a very strong optical absorption band just below their absorption edge. Glasses were devitrified by heat treatment at temperatures above their glass transition temperatures and the crystalline phases produced in them were characterized by Fourier transform infrared (FTIR) absorption spectroscopy and X-ray diffraction (XRD). Bi₃B₅O₁₂ was found to be the most abundant phase in all devitrified samples. DTA studies on glasses and FTIR and XRD analysis on crystallized samples revealed that very small amounts of trivalent ion doping causes significant changes in the devitrification properties of bismuth borate glasses; rare-earth ions promote the formation of metastable BiBO₃–I and BiBO₃–II phases during glass crystallization.     

Effect of Molybdenum on the Structure and on the Crystallization of SiO2–Na2O–CaO–B2O3 Glasses

Crystallization of the poorly durable Na₂MoO₄ phase able to incorporate radioactive cesium must be avoided in SiO₂–Al₂O₃–B₂O₃–Na₂O–CaO glasses developed for the immobilization of Mo-rich nuclear wastes. Increasing amounts of B₂O₃ and MoO₃ were added to a SiO₂–Na₂O–CaO glass, and crystallization tendency was studied. Na₂MoO₄ crystallization tendency decreased with the increase of B₂O₃ concentration whereas the tendency of CaMoO₄ to crystallize increased due to preferential charge compensation of BO₄⁻ entities by Na⁺ ions. ²⁹Si MAS NMR showed that molybdenum acts as a reticulating agent in glass structure. Trivalent actinides surrogate (Nd³⁺) were shown to enter into CaMoO₄ crystals formed in glasses.     

Interaction between Barium Titanate and Binary Glasses

Interactions between BaTiO₃, and three binary glasses were studied through the reaction of BaTiO₃, powder with glass powder. For PbO–B₂O₃ and PbO–SiO₂ glasses, the reaction led to stable compound formation, the substitution of Pb in the BaTiO₃ structure, and noticeable grain growth of BaTiO₃. The interaction phenomena for these two glass systems were very similar. The substitution of Pb into BaTiO₃ is assisted by chemical reactions in which BaB₂O₄ or Ba₂SiO₄ is formed. The substitution into BaTiO3 also seems to be closely related to the grain growth of BaTiO₃. On the other hand, only compound formation was observed during the processing of BaTiO₃ with Bi₂O₃–B₂O₃ glass. Neither BaTiO₃ grain growth nor Bi substitution took place with the Bi₂O₃–B₂O₃ glass system. Based on the observed reactions and the glass viscosity, several sintering aids for BaTiO₃ ceramic products are suggested in this paper.     

Influence of Carbon on SOx Emissions from Glass Processing

Carbon reacts with Na₂SO₄ in glass batches, influencing SO _x emissions. To study the role of carbon active surface area (ASA) in the decomposition phenomena, Na₂SO₄ is reacted with three carbons with a wide ASA range. The decomposition behavior was evaluated by thermogravimetric analysis (TGA) and evolved gas analysis via mass spectrometry and Fourier transform infrared spectrometry. Higher carbon ASA yields lower decomposition temperatures. SO _x emissions are realized at temperatures as low as 773 K. Isothermal Na₂SO₄–C decomposition initiates via the nucleation of Na₂S. The rate constants for the nucleation regime are normalized against ASA, yielding an activation energy of 275 kJ/mol for nucleation.     

High-Strength Mica-Containing Glass-Ceramics

Glass-ceramics containing barium–mica in the system Ba_0.5 Mg₃ (Si₃AIO₁₀)F₂–2MgO · 2Al₂O₃· 5SiO₂–Ca₃ (PO₄)₂ are two to three times stronger than conventional mica-containing glass-ceramics. Moreover, the barium-mica glass-ceramics are easier to machine, as confirmed by a drilling test using conventional steel tools. Such mechanical properties are attributable to the microstructure of the barium–mica glass-ceramics. Very fine, interlocking mica crystals are precipitated in the glass, and a crack-deflection mechanism is observed by scanning electron microscopy.