Crystallization Hierarchy of CaO-P2O5-SiO2-Al2O3-TiO2 Glass-Ceramics

Glasses in the tricalcium phosphate-anorthite (Ca₃(PO₄)₂-CaAl₂Si₂O₈ or TCP-CAS₂) system with additional TiO₂ were melted. Crystallization was investigated using thermal analysis, X-ray diffraction, and transmission optical and electron microscopies. During the heat treatment, as the growth temperature increases, successive crystalline phases separate from the glass matrix. The phases present in the fully heat-treated glass-ceramic are ß-TCP, anor-thite (CAS₂), and rutile (TiO₂). Apart from TiO₂, these phases evolve from other polymorphs during the heat treatment. The metastable phases, pseudo-orthorhombic CAS₂ and alpha-TCP, appear first around 880°C and transform into the stable phases, triclinic anorthite CAS₂ and ß-TCP, around 940° and 1000°C, respectively. The material crystallizes in stages. The first stage is the separation from the glass matrix of rutile and what is believed to be a calcium phosphate phase, with crystal sizes varying from 20 to 200 nm. This is followed by the appearance of larger crystals (1-2 µm) of the metastable pseudo-orthorhombic CAS₂, surrounding the previously crystallized phases. Finally, this pseudo-orthorhombic CAS₂ phase transforms to anorthite (15-20 µm) spherulites. TiO₂ does not act as a direct nucleating agent in the glass composition studied: no sign of heterogeneous nucleation and growth from TiO₂ crystals has been found, and moreover, TiO₂-free base glass crystallizes in a manner similar to that of the glass containing TiO₂.     

High-Temperature Energy Relations in the Alkali Borates: Binary Alkali Borate Compounds and Their Glasses

The energy relations existing between the congruently melting compounds Li₂O -2B₂O₃, Na₂O–2B₂O₃, Na₂O-4B₂O₃, and K₂O-4B₂O₃ and their glasses have been determined for the temperature range 25° to 1100°C. High-temperature heat-content, entropy, and heat of solution data are given for both the glasses and the corresponding devitrified materials. A comparison of the heats of fusion of the alkali borates on a gram atom of oxygen basis shows that they follow the order Li > Na > K. The entropy differences between the glass and the corresponding crystalline material have been determined at 25°C. The free-energy change at 25°C. for the reaction crystal → glass has been calculated for the four compounds.     

Superconducting Glass-Ceramics in the Bi-Sr-Ca-Cu-O System

Bi_1.5SrCaCu₂O _z was prepared in the glassy state by rapid quenching of the melt. The recrystallization of the glass during various heat treatments was studied by differential thermal analysis, X-ray diffraction, scanning electron microscopy, and resistivity measurements. Activation energies and frequency factors for the crystallization events below 600°C were determined by nonisothermal differential scanning calorimetry. Heating at 450°C formed mainly the Bi_{2+ x} Sr_{2− x} -CuO _z solid solution ("R"). Between 765° and 845°C, R reacted slowly with the glass to form the 80 K superconductor Bi₂(Sr,Ca)₃Cu₂O _z and CuO. Heating for 7 d at 845°C, followed by slow cooling, eliminated a low-temperature resistive "tail" and raised the temperature of zero resistance to 77 K. Heating at 867°C caused partial melting, with segregation of a Bi- and Cu-rich liquid, and loss of the superconducting phases.     

Crystallization of Li2O · Al2O3· 6SiO2 Glasses Containing Niobium Pentoxide as Nucleating Dopant

Niobium pentoxide (T form, orthorhombic system) was utilized to promote devitrification in Li₂O · Al₂O₃· 6SiO₂ glasses. Two or more mole percentage of this nucleating dopant enhanced crystallization in these glasses. Glasses containing 4.0 and 8.0 mol% T-Nb₂O₅ exhibited a high tendency to form dispersed TT-Nb₂O₅ (monoclinic system) precipitates during the glass quenching process. The crystallization process in glasses containing 2.0 or 4.0 mol% T-Nb₂O₅ occurred as microphase separation, followed by the formation of dispersed TT-Nb₂O₅ crystalline precipitates (760°C), followed by β-quartz solid-solution ( ss ) formation (850° to 900°C) heterogeneously nucleated from the precipitates. β-quartz( ss ) transformed to β-spodumene( ss ), along with a polymorphic transition from the TT-Nb₂O₅ to M-Nb₂O₅ (tetragonal system) crystalline phase.     

Chemical Reactions of Ultraphosphate Glasses with Water at Various Temperatures

The chemical reactions between P₂O₅-ZnO-H₂O ultraphosphate glasses and water were characterized between room temperature and 500°C, using thermogravimetry, differential scanning calorimetry, X-ray diffraction, and ³¹P nuclear magnetic resonance. Water adsorption and hydrolysis reactions of the glass leads to the formation of H₃PO₄ and crystalline ZnH₂P₂O₇ below 200°C. The rate of water adsorption increases, owing to the hygroscopicity of the hydrolysis products of the glass. Devitrification occurs at 250°C via surface reactions. The microstructure of the devitrified glass consists of crystalline Zn₂P₄O₁₂ and a liquid phase containing hydrolysis products of P₂O₅ like metaphosphoric acid (HPO₃) _n. Devitrification is finally followed by water desorption at higher temperatures.     

Phase Equilibria of Nuclear Waste Ceramics: The Effect of Oxygen Fugacity

Wasteform phase assemblages and radionuclide immobilization are described for a Synroc-D bulk composition processed at 1200°C under oxygen fugacities ( f O₂) ranging from air to those approximating graphite-CO-CO₂ buffer. All processing conditions produce assemblages of titanate and aluminate crystalline phases plus alkali aluminosilicate glass. In all cases the important radionuclides are immobilized in crystalline phases which have been shown to be highly leach-resistant.     

Crystallization of Barium Osumilite Glass

The crystallization of stoichiometric barium osumilite (BaMg₂Al₆Si₉O₃₀) glass manufactured by melting or by sol-gel processing using an all-alkoxide route (AAR) or a partial-alkoxide route (PAR) using mixed alkoxides and salts has been characterized by thermal methods, phase analysis, and microstructural analysis. In all glasses, crystallization with heat treatment at a finite heating rate (1 or 10 K/min) occurs via an initial phase separation, leading to regions devoid of and enriched in BaO. In PAR gel glass and melt glass, the BaO-deficient regions crystallize as µ-cordierite, but they appear to remain amorphous in AAR gel glass, in which hexacelsian is the first phase, to crystallize at the interface between the phase-separated regions. With further heat treatment, barium osumilite becomes the dominant phase. Crystallization with isothermal heat treatment at 1250°C leads to direct crystallization of barium osumilite in bulk and powder samples of melt glass, but not in bulk and powder samples of sol-gel glass, which again form precursor phases, such as µ-cordierite, alpha-cordierite, hexacelsian, and mullite. The crystallization mechanisms and morphologies are discussed here.     

Intermediate Phases in Mullite Synthesis Via Aluminum- and Alumina-Filled Polymethylsiloxane

Active filler pyrolysis of polymethylsiloxane/aluminum (PMS/Al) mixtures leads to the formation of mullite at 1400°C, and by 1700°C mullite is the only crystalline phase in an amorphous matrix. These mixtures show a complex series of oxidation and reduction reactions prior to the formation of mullite. Initial oxidation of PMS at low temperatures results in the formation of an amorphous product before any significant oxidation of the Al. After melting of the Al phase, redox reactions lead to the formation of elemental Si and SiC as intermediate crystalline phases along with Al₂O₃ both as the crystalline α-phase and dissolved in the amorphous matrix. Mullite formation occurs by precipitation from the matrix at temperatures above 1400°C. In contrast, PMS/Al₂O₃ mixtures did not show mullite formation until heat treatment at 1700°C. In this case no significant intermediate crystalline phases occur, except for some limited cristobalite crystallization from the matrix.     

Crystallization Kinetics of Lithium Orthosilicate Glasses

The formation of crystalline Li₄SiO₄ and Li₂SiO₃ from lithium orthosilicate glasses has been studied by means of in situ high-temperature X-ray diffraction. The first phase that crystallizes from the glass could not be identified, but was followed by the transformation to crystalline orthosilicate and metasilicate. Orthosilicate formation was tracked at temperatures between 600° and 650°C, whereas at higher temperatures, the formation of a small amount of crystalline metasilicate occurs. The kinetics of the initial phase of both crystallization processes are described by the Avrami–Erofeev equation, resulting in activation energies of 90 kJ/mol for the formation of the unidentified phase, and 68 kJ/mol for the formation of Li₄SiO₄. The rate constant for the crystallization of the unidentified phase is 0.014 s⁻¹ at 510°C, equaling that of the orthosilicate formation at 630°C. After isothermal heat treatment for 100–800 s, depending on temperature, 80%–95% of the sample is crystallized and further crystallization is controlled by diffusion in both cases.     

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.     

High-Temperature Phase Relations in the Sc2O3-Ta2O5 System

The phase relations for the Sc₂O₃-Ta₂O₅ system in the composition range of 50-100 mol% Sc₂O₃ have been studied by using solid-state reactions at 1350°, 1500°, or 1700°C and by using thermal analyses up to the melting temperatures. The Sc_5.5Ta_1.5O₁₂ phase, defect-fluorite-type cubic phase (F-phase, space group Fm 3 m ), ScTaO₄, and Sc₂O₃ were found in the system. The Sc_5.5Ta_1.5O₁₂ phase formed in 78 mol% Sc₂O₃ at <1700°C and seemed to melt incongruently. The F-phase formed in ∼75 mol% Sc₂O₃ and decomposed to Sc_5.5Ta_1.5O₁₂ and ScTaO₄ at <1700°C. The F-phase melted congruently at 2344°± 2°C in 80 mol% Sc₂O₃. The eutectic point seemed to exist at ∼2300°C in 90 mol% Sc₂O₃. A phase diagram that includes the four above-described phases has been proposed, instead of the previous diagram in which those phases were not identified.     

Stability Diagram for the System Yba2Cu3O7–x

The dependence of the degree of nonstoichiometry of YBa₂Cu₃0_7–x (123) on temperature and oxygen pressure has been determined by thermogravimetric analysis (TGA) in the temperature range 400° to 950°C and the oxygen pressure range 10^–6 to 1 atm (1 atm = 10⁵ Pa). The nature of the decomposition of 123 in the temperature range 750° to 950°C and the oxygen pressure range 10^–6 to 10^–2 atm has been determined by TGA and X-ray diffractometry (XRD). As the oxygen pressure decreases, the decomposition of 123 follows the sequence 123→ Y₂BaCuo₅ (211) + BaCuO₂° Cu₂O→ 211 ° BaCuO₂° BaCu₂O₂→ 211 ° YBa₃Cu₂O_x (132) ° BaCu₂O₂→ 211 ° BaCu₂O₂°BaO. The incongruent melting temperatures have been determined in the oxygen pressure range 10^–6 to 1 atm by differential thermal analysis, and the phases formed on solidification have been identified by XRD. The stability diagram for the composition 123 has been constructed.     

Sol–Gel Preparation and Characterization of Lithium Disilicate Glass–Ceramic

A sol–gel process is described for preparation of crystalline lithium disilicate (Li₂Si₂O₅) from tetraethylorthosilicate and lithium ethoxide. The glass network structure and crystallinity resulting from heat treatment at temperatures from 150° to 900°C were investigated by nuclear magnetic resonance, X-ray diffraction, and differential scanning calorimetry/thermogravimetric analysis. Q³ structural units (SiO₄ tetrahedra with three bridging oxygen atoms) formed in the amorphous gel at a low temperature (≤150°C) persist to elevated temperature (≤500°C) and directly transform to crystalline Li₂Si₂O₅ at about 550°C. The heating schedule slightly affects the crystalline phase transformation.     

Raman and 11B Nuclear Magnetic Resonance Spectroscopic Studies of Alkaline-Earth Lanthanoborate Glasses

Glasses from the RO.La₂O₃.B₂O₃ (R = Mg, Ca, and Ba) systems have been examined. Glass formation is centered along the metaborate tie line, from La(BO₂)₃ to R(BO₂)₂. Glasses generally have transition temperatures >60°C and expansion coefficients between 60 × 10^-7/°C and 100 × 10^-7/°C. Raman and solid-state nuclear magnetic resonance spectroscopies reveal changes in the metaborate network that depend on both the [R]:[La] ratio and the type of alkaline-earth ion. The fraction of tetrahedral sites is generally reduced in alkaline-earth-rich glasses, with magnesium glasses possessing the lowest concentration of B[4]. Raman spectra indicate that, with increasing [R]:[La] ratio, the preferred metaborate anion changes from a double-chain structure associated with crystalline La(BO₂)₃ to the singlechain and ring metaborate anions found in crystalline R(BO₂)₂ phases. In addition, disproportionation of the metaborate anions leads to the formation of a variety of other species, including pyroborates with terminal oxygens and more-polymerized species, such as diborates, with tetrahedral borons. Such structural changes are related to the ease of glass formation and some of the glass properties.     

High-Temperature Phase Relations in the System Y2O3-Ta2O5

The phase relations for the system y₂o₃–Ta₂o₅ in the composition range 50 to 100 mol% Y₂O₃ have been studied by solid-state reactions at 1350°, 1500°, or 17000C and by thermal analyses up to the melting temperatures. Weberite-type orthorhombic phases (W2 phase, space group C222₁), fluorite-type cubic phases (F phase, space group Fm3m )and another orthorhombic phase (O phase, space group Cmmm )are found in the system. The W2 phase forms in 75 mol% Y₂O₃ under 17000C and O phase in 70 mol% Y₂O₃ up to 1700°C These phases seem to melt incongruently. The F phase forms in about 80 mol% Y₂O₃ and melts congruently at 2454° 3°C. Two eutectic points seem to exist at about 2220°C 90 mol% Y²O₃, and at about 1990°C, 62 mol% Y₂O₃. A Phase diagram including the above three phases were not identified with each other.     

Crystallization of Bismuth Borosilicate-Based Dielectric Thick Films on a LiZn Ferrite Substrate

The compatibility and crystallization of dielectric thick films, consisting of a bismuth borosilicate glass and crystalline cordierite, on a LiZn ferrite substrate were investigated by focusing on phase development and microstructural changes. Significant diffusion of Li and Fe from the substrate to the dielectric was confirmed as unexpected crystalline phases such as Li₂Al₂Si₃O₁₀ and Fe₂O₃ were found in the thick films fired at 850°C. The crystallization was believed to be initiated from the film interface and developed further toward the film surface as evidenced from cross-sectional microstructures of the films with additional firings. The degree of crystallization and the relative contents of the observed phases were dependent on the ratio between the glass and cordierite and the number of refirings.     

Phase Relations in the System Iron Oxide—Cr2O3 in Air

The quenching method has been used to determine approximate phase relations in the system iron oxide-Cr₂O₃ in air. Only two crystalline phases, a sesquioxide solid solution (Fe₂O₃–Cr₂O₃) with corundum structure and a spinel solid solution (approximately FeO ·Fe₂O₃–FeO – Cr₂O₃), occur in this system at conditions of temperature and O₂ partial pressure (0.21 atm.) used in this investigation. Liquidus temperatures increase rapidly as Cr₂O₃ is added to iron oxide, from 1591°C. for the pure iron oxide end member to a maximum of approximately 2265°C. for Cr₂O₃. Spinel(ss) is the primary crystalline phase in iron oxide-rich mixtures and sesquioxide (ss) in Cr₂O₃–rich mixtures. These two crystalline phases are present together in equilibrium with a liquid and gas (po₂= 0.21 atm.) at approximately 2075°C.     

Thermal Analysis of Reactions in Soda–Lime Silicate Glass Batches Containing Melting Accelerants: II, Multicomponent Systems

The glass melting reactions in a multicomponent system (sand–soda ash–calcite–dolomite–feldspar) were studied using data from DTA, TGA, and XRD interactively. The first-formed liquid phase occurred at 700°C from eutectic melting among CaCO₃, Na₂CO₃, and MgO. Further liquid phase formed at the CaCO₃–Na₂CO₃, eutectic at 785°C and a fusion reaction among SiO₂, CaO, and the molten phase at 812°C. Reactions between molten soda ash and silica grains to form a sodium disilicate coating also occurred in this temperature range. The effects of reaction accelerant additions (Na₂SO₄, NaNO₃, NaCI) on batch fusion were analyzed. Sodium chloride was found to be the most effective melting accelerant due to the formation of a NaCI–Na₂CO₃ eutectic liquid phase at ∼636°C, which effectively attacked the silica relic. CO₂ gas release terminated ∼80°C earlier with 1 wt% NaCI additions to the base glass.     

Influences of Zirconia and Silicon Nucleating Agents on the Devitrification of Li2O · Al2O3· 6SiO2 Glasses

The effects of Si and ZrO₂ dopants on the crystallization and phase transformation process in Li₂O · Al₂O₃· 6SiO₂ glasses were investigated using differential thermal analysis, X-ray powder diffractometry (XRD), and high-resolution transmission electron microscopy (TEM) interactively. Phase separation was observed in the studied glasses prior to substantial crystallization. Elemental Si (1 mol%) significantly aided in glass devitrification. Dropletlike phase-separated regions in the as-quenched or heat-treated glass devitrified at ∼760°C, which in turn provided sites for the heterogeneous nucleation and growth of β-quartz(ss) (solid solution), which transformed to β-spodumene(ss) at higher temperature. Low-temperature surface crystallization in these glasses occurred as low as 760°C. ZrO₂ has limited solubility in this glass system. Small ZrO₂ crystallites (·5 nm) in the as-quenched glass acted as sites for the heterogeneous nucleation and subsequent growth of large (<5 μm) β-quartz(ss) crystals in glasses containing 1.0 mol% or more ZrO₂. The transformation from β-quartz(ss) to β-spodumene(ss) was increasingly inhibited with ZrO₂ additions. The nucleating efficiency of Si was significantly greater than that of ZrO₂ in this glass system.     

Sintering of Si3N4-Y2O3-Al2O3

Sintering kinetics of the system Si₃N4-Y2O₃-Al₂O3 were determined from measurements of the linear shrinkage of pressed disks sintered isothermally at 1500° to 1700°C. Amorphous and crystalline Si₃N₄ were studied with additions of 4 to 17 wt% Y₂O₃ and 4 wt% A1₂O₃. Sintering occurs by a liquid-phase mechanism in which the kinetics exhibit the three stages predicted by Kingery's model. However, the rates during the second stage of the process are higher for all compositions than predicted by the model. X-ray data show the presence of several transient phases which, with sufficient heating, disappear leaving mixtures of β ' -Si₃N₄ and glass or β '-Si₃N₄, α '-Si₃N₄, and glass. The compositions and amounts of the residual glassy phases are estimated.