Crystallization of Titanosilicate Glasses for Nuclear Waste Immobilization

The effects of different cooling and reheating rates on the phase constitutions of Na₂O–Al₂O₃–TiO₂–SiO₂ glass-ceramics containing up to 20 wt% of simulated nuclear fuel recycle waste have been studied using X-ray diffraction, differential thermal analysis, and scanning and transmission electron microscopy. The metastable formation of a perovskite-structured phase (nominally CaTiO₃, but containing ionic substituents) was observed in samples with up to 15 wt% of simulated waste after cooling from the melt at rates between 0.25 and 50°C/min. When the partially devitrified glass was reheated to 1050°C, incomplete conversion of this phase to sphene (nominally CaTiSiO₅) occurred by reaction with the silica-rich glass matrix. The conversion was completed by heating further to 1150°C. Waste loadings ⋝10 wt% produced crystallization of powellite (nominally CaMoO₄) in addition to sphene and perovskite, whereas metastable perrierite (a rare-earth titanosilicate) was also crystallized at waste loadings ⋝15 wt%. New data on elemental partitioning between the crystalline and vitreous phases confirmed earlier results obtained in different atmospheres and with simplified waste compositions and were largely in accord with crystal-chemical predictions.     

Prominent Nanocrystallization of 25K2O·25Nb2O5·50GeO2 Glass

Some K₂O-Nb₂O₅-GeO₂ glasses are prepared, and their crystallization behaviors are examined. 25K₂O·25Nb₂O₅·50GeO₂ glass with the glass transition temperature T _g= 622°3C and crystallization onset temperature T _x= 668°3C shows a prominent nanocrystallization. The crystalline phase is K_3,8Nb₅Ge₃O_20,4 with an orthorhombic structure. The sizes of crystals in the crystallized glasses heat-treated at 630° and 720°3C for 1 h are °10 and 20–30 nm, respectively, and the crystallized glasses obtained by heat treatments at 620°-850°3C for 1 h maintain good transparency. The density of crystallized glasses increases gradually with increasing heat-treatment temperature, and the volume fraction of crystals in the sample heat-treated at 630°3C for 1 h is estimated to be ∼35%. The usual Vickers hardness and Martens hardness (estimated by nanoindentation) of 25K₂O·25Nb₂O₅·50GeO₂ glass change steeply by heat treatment at T _g, i.e., at around 35% volume fraction of nanocrystals. The present study demonstrates that the composite of nanocrystals and the glassy phase has a strong resistance against deformation during Vickers indenter loading in crystallized glasses.     

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.     

Red Color GeSe2-Based Chalcohalide Glasses for Infrared Optics

GeSe₂–Ga₂Se₃–CsI chalcohalide glasses are synthesized, and their optical properties and thermo-mechanical properties are studied. A typical characteristic of the glasses is their excellent transparency in the red-light region in addition to the 8–14 μm atmospheric window, which is of vital importance to the quality control of infrared systems. The short-wavelength absorption edge λ_s of the glass system has a distinct blue shift with increasing CsI content, and the physicochemical interpretations are suggested and formulated. These glasses present a glass transition temperature ( T _g) around 300°C and good thermal stability. Consequently, they can be promising candidate materials for infrared optics, although their hardness is relatively weak.     

The Influence of Sb2O3 Addition on the Properties of Low-melting ZnO–P2O5 Glasses

The influence of 0–16 mol% Sb₂O₃ substitution for P₂O₅ on the properties of ZnO–P₂O₅ glasses has been investigated. It was shown that Sb₂O₃ could participate in the glass network and thermal stability of the glasses decreased with increasing Sb₂O₃ content. Glass transition temperature T _g, softening temperature T _s, and water durability all decreased firstly (up to 6 mol% Sb₂O₃ added) and then increased. Substitution of 12 mol% Sb₂O₃ led to a 16°C decrease in T _g and 30°C decrease in T _s, and weight loss of the glass was only 0.42 mg/cm², which is ∼11 times lower than that of the glass without Sb₂O₃ after immersion in deionized water at 90°C for 1 day. The glass containing 12 mol% Sb₂O₃ might be a substitute for Pb-based glasses in some applications.     

Characterization and Crystallization of Y-Si-Al-O-N Glass

Glasses were prepared from sintered powders ofSi₃N₄, Al₂O₃, Y₂O₃ AlN, andSiO₂ to study their crystallization on subsequent heat treatment. Appreciable crystallization was efected only after the glasses were doped with up to 5 wt% ZrO₂. Electron microscopy and microanalysis showed that the crystalline phase was Y₂O₃·2SiO₂ without detectable Zr. The sofening temperature is in the range 850° to 1020°C. In-situ heating in a high-voltage electron microscope at ∼12OO°C produced renucleation and growth of the crystalline phase; at higher temperatures, however, the glass phase volatilized.     

Thermal Expansions and Chemical Durabilities of Yttria–Aluminosilicate Glasses Containing Na2O and ZrO2

Properties of glasses in the system Y₂O₃–Al₂O₃–SiO₂ containing Na₂O and ZrO₂ were investigated. The difference between the thermal expansion coefficients (Δα) at temperatures above T _g and those below T _g, microhardness, density, and chemical durability were measured in relation to the Al₂O₃/Y₂O₃ molar ratio. These glasses were found to have a smaller value of Δα than that of a commercial coating glass.     

Effect of Nature of Surfaces on Wetting of Sapphire by Liquid Aluminum

Contact angles of aluminum drops on sapphire measured under vacuum conditions from 660° to 1250°C generally fell into three ranges. Large obtuse contact angles indicating interfacial energies greater than either of the two surface energies were obtained up to about 900°C; van der Waals bonding then existed at a compound interface. In the intermediate range, contact angles were 90° or slightly greater indicating a common interface with an energy, _sγ_l, greater than _sγ_g but less than lγ_g. Acute contact angles indicating a _sγ_l less than _sγ_g and greater than _lγ_g occurred above about 950°C because of the formation of a high temperature complex surface structure with _sγ_g > _lγ_g. A hydroxylated sapphire surface has a lower _sγ_g which increases with gradual dehydroxylation and conversion to the high temperature surface structure with a corresponding change in contact angle through the three ranges. Chemical bonding existed in the latter two ranges. Reactions occurred between Al and the sapphire surface to form volatile species at contact angles less than 90°. Molten Al normally has an oxide coating the effect of which appears to be removed at about 870° C.     

Control of Radioactive Waste Glass Melters: I, Preliminary General Limits at Savannah River

A ceramic, Joule-heated, slurry-fed melter will be used to vitrify Savannah River Site high-level waste at the Defense Waste Processing Facility (DWPF) in Aiken, SC. The entire vitrification process must be performed remotely and the borosilicate host glass must accommodate a variety of waste compositions. The control bases for such a melter are unique and complex. DWPF operations are tightly constrained by processing parameters and by product specifications. Part I discusses the melter control considerations which have been identified to facilitate compliance with these constraints. These considerations include melter temperature, glass composition, product durability, waste loading limits, glass redox control, and glass cooling requirements.     

Preparation of Beryllium Fluoride Glasses Containing Xenon(VI) and/or Thallium (I)

The dissolution of CsXeF₇ in molten alkali fluoroberyllate glasses between 250° and 500°C is described. The apparent small solubility of Xe(VI) in these ionic glass-forming melts (about 1 to 2 wt% or less) may be related to retention of a molecular configuration by XeF₆ that is caused by its strong fluoride ion donor properties and/or to the nonmelting behavior of numerous MF-XeF₆ salts when they are heated. Rather low-softening (about 100°C) glasses were also prepared in the TIHF₂-NaHF₂-BeF₂ system, whereas TlHF₂ has a relatively low melting point (about 85°C). Contrary to expectations, Xe(VI) appears to oxidize TI⁺ in these molten glass environments at 250°C.     

Effect of Silicon Carbide Additions on the Crystallization Behavior of a Magnesia-Lithia-Alumina-Silica Glass

Glass in the MgO-Li₂O-A1₂O₃-SiO₂ system was observed to crystallize readily at temperatures from 700° to 900°C. The primary crystalline phase evolved was Li₂Si₂O₅, and the secondary phase evolved was Li₂SiO₃. The glass was amorphous after heating in air at 1050°C for 30 min. The addition of 0.5 wt% SiC powder resulted in the crystallization of Li₂SiO₃ during heating in air at 1050°C for 30 min. It was suggested that the difference in crystallization behavior with Sic addition was due to dissolution of Sic into the oxide glass.     

Structure and Properties of Low Phonon Antimony Glasses in the K2O–B2O3–Sb2O3–ZnO System

The monolithic glass-forming region of the low phonon and low softening point antimony glasses containing high Sb₂O₃ (40–75 mol%) in the novel quaternary K₂O–B₂O₃–Sb₂O₃–ZnO system has been found with the help of X-ray diffraction (XRD) analysis. The structure of a series of glasses with the general composition of (mol%) 15K₂O–15B₂O₃–(70− x )Sb₂O₃– x ZnO (where x =5–25) has been evaluated by infrared reflection spectral (FT-IRRS) analyses. All the glasses are found to possess a low phonon energy of around 600 cm⁻¹, as revealed by FT-IRRS. Their softening point ( T _s), glass transition temperature ( T _g), and coefficient of thermal expansion (CTE) have been found to vary in the ranges of 351°–379°C, 252°–273°C, and 195–218 × 10⁻⁷ K⁻¹, respectively. These properties are found to be controlled by their fundamental property, like the covalent character of the glasses, which is found to increase with an increase in Sb₂O₃ content. In addition, the devitrified glasses have been characterized by XRD and field emission scanning electron microscopy, which manifests the presence of nanozinc antimony oxide crystals with sizes of 21–43 nm. The exhibited properties have revealed that they are a new class of versatile materials.     

Visible Transparent GeSe2–Ga2Se3–KX (X=I, Br, or Cl) Glasses for Infrared Optics

GeSe₂–Ga₂Se₃–KX (X=I, Br, or Cl) chalcohalide glasses are synthesized, and their optical properties and thermo-mechanical properties are investigated. A structural model is put forward to elucidate the interesting compositional dependences of the short-wavelength absorption edge (λ_s) and glass transition temperature ( T _g). These glasses are transparent in the red-light region in addition to the 3–5 and 8–14 μm atmospheric windows. Most of their T _g exceed 300°C, and they also present good thermal stability. These properties make them attractive materials for infrared optics.     

Structural Similarities Between a Glass and Its Melt

It is suggested that the large expansion coefficient increase that occurs near T _g, for mixed oxide glasses is related to cooperative thermal displacements involving polyanionic clusters in the liquid state. This is illustrated by the striking relation between melt isoexpansion coefficient contours and structure for these ternary oxide systems. It is also shown that the molar volume dependence on composition for a series of oxide glasses can be quite similar to that observed for their liquid state analogs. The adjusted melt partial molar volume models can be used to develop realistic glass models, suggesting structural similarities between an oxide glass and its corresponding high-temperature melt. Recently acquired evidence for alkali borogermanate compositions indicates a shift toward the right for the GeO₆⇄GeO₄ equilibrium in GeO₂₋rich melts at relatively high temperatures. Thus, an exact structural correlation between an oxide melt and its corresponding glass may, in some cases, be limited to temperatures below T _g+Δ T (where ΔT ~300° to 400°C).     

Phase Separation and Structural Differences between Alkali Silicate Glasses Prepared by the Sol-Gel and Melt-Quench Methods

Glasses of composition x M₂O:(1- x )SiO₂ [M = Na, Li] with x = 0.15 and 0.2 have been prepared both by the melt-quench and the sol-gel methods. X-ray photoelectron spectroscopy shows that with increasing sintering temperature the structure of gels approaches that of the melt-quench glass but does not become identical even on heating above the glass transition temperature ( T _g). Phase separation occurs in gel much below T _g, and before it evolves into the structure of melt-quench glass. Thus phase separation is achieved more readily in sintered gels than in the corresponding melt-quench glass.     

Crystallization in 70Ga2S3.30La2S3(mol%) Glasses as a Function of Oxide/Hydroxide Concentration

The crystallization of 70Ga₂S₃^.30La₂S₃(mol%) glasses has been studied using X-ray diffraction and transmission electron microscopy. Two of the glasses were prepared from raw materials with nominally different oxide concentrations. The third was prepared from raw materials aged in an oxygen-depleted, argon-flushed glove box for more than 1 yr. Their oxide/hydroxide impurity content was qualitatively ranked using Fourier transform infrared spectroscopy. The lowest oxide content composition (≤0.5 wt%, supplied information) devitrified readily close to the glass transition temperature, T _g, forming crystallites of a new (GLS) phase with a monoclinic Bravais lattice and a lathlike internal structure. Ga₂S₃was observed in small quantities between the laths. Samples prepared from nonaged, high oxide (1–3 wt%) content precursors produced the most stable glass. On crystallization, these samples exhibited spherulites composed of intergrown laths of melilite-structured La_3.33Ga₆S₁₄and the new monoclinic GLS phase. Whiskers of Ga₂S₃were found in the residual glass between crystallites. Samples prepared from aged raw materials produced spherulites of La_3.33Ga₆S₁₄on crystallization with no identifiable regions of the new GLS phase.     

Physical Aging of Arsenic Trisulfide Thick Films and Bulk Materials

Differential scanning calorimetry (DSC) measurements were performed at different heating rates under nonisothermal conditions to probe the glass transition kinetics of As₂S₃ bulk glass and thick films. We found that, for films, the glass transition temperature ( T _g) increases with increasing annealing temperature, and sharp crystalline-like melting endothermic peaks appear in samples annealed at 160°C. Such sharp endothermic peaks could also be found for samples annealed at 140°C for 300 h, indicating a very slow crystallization process. The activation energy for the molecular motions and rearrangements around T _g could be estimated as 2.12 eV for films and 2.3 eV for the bulk, respectively.     

Magnetic Properties of Barium Ferrite Formed by Crystallization of a Glass

A glass with composition 0.265 B₂O₃-0.405 BaO-0.33 Fe₂O₃ (mole ratio) was prepared by a fast-quenching technique. When it is heat-treated, this glass exsolves up to ∼45 wt% BaFe₁₂O₁₉ as the only magnetic phase. Magnetic measurements of glasses heated at various temperatures show that superparamagnetic, single-domain, or multidomain magnetic behavior is present, depending on the thermal history. The volume of a typical superparamagnetic particle (calculated from the magnetic data) is equivalent to that of a sphere 47 Å in diameter. The intrinsic coercive forces of two heat-treated glasses were independent of temperature at high levels of H_ci (2600 and 2900 Oe) from 77° to 300°K. Another heat-treated glass has an H_ci of 5350 Oe at 300°K. Apparently, the coherent rotation model of Stoner and Wohlfarth describes the magnetic behavior of BaFe₁₂O₁₉ very well. The single-domain critical size for BaFe₁₂O₁₉ was ∼0.5 μm. An attractive feature of this system is that the BaFe₁₂O₁₉ powder can be recovered from the barium-borate-rich matrix by leaching with a weak acid.     

Infrared Spectroscopic Method for Determination of Thermal Properties of Fluorophosphate Glasses

Fluorophosphate glasses of the Ba(PO₃)₂–MgF₂–CaF₂–AlF₃ system exhibit anomalous behavior, in regard to the glass-transition temperature ( T _g) and the coefficient of thermal expansion (α_CTE), at a fluoride concentration of ∼36 mol%. A similar trend in the infrared band absorption coefficient (α_IR) at 2170 cm⁻¹ of the overtone of 2ν_ss(OPO) and/or a combination of ν_as(OPO)+ν_as(POP) fundamental vibrations of PO₄ tetrahedra also has been observed, relative to changes in the glass composition. Statistical analyses have shown that the T _g and α_CTE values are correlated with α_IR at 2170 cm⁻¹ by linear equations. The thermal properties of some of the glasses have been calculated using these equations, to test their validity and utility. The experimental property values of the glasses present a very good fit of data within a 95% confidence interval of the calculated mean values.     

Crystallization of Anorthite-Seeded Albite Glass by Solid-State Epitaxy

Stoichiometric albite glass (NaAlSi₃O₈) was seeded with 5 wt% crystalline anorthite (CaAl₂Si₂O₈) to make albite glass-ceramics. The epitaxial crystallization of the albite glass to the glass-ceramics was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectrometry (EDS). High albite was observed as the major crystallization product over the temperature range of 800–1200°C. No crystalline albite could be crystallized from pure albite glass without seeds. Small amounts of nepheline (NaAlSiO₄), however, crystallized along with albite after heat treatments at temperatures lower than 1000°C. The platelike microstructure of albite crystals was revealed in the seeded glasses. The albite blades grew epitaxially from the anorthite seeds, and the Ca content decreased in the direction away from the seeds. The degree of crystallization and the grain size were dependent upon the heat treatment conditions. By increasing the particle size of the seed, the crystallization process was retarded and the resultant microstructure was degraded. The seeding efficiency was also lowered by adding nonisostructural hexagonal anorthite seeds which produced less albite but more nepheline crystals. Crystallization of albite glass by seeding with 5 wt% anorthite is much greater than with the surface nucleation which takes place in a homogeneous 95 wt% albite + 5 wt% anorthite glass.