Pseudobinary Phase Relations of Li2Ti3O7

The Li₂O-TiO₂ pseudobinary phase diagram was determined from 50 to 100 mol% TiO₂ by DTA, microscopy, and X-ray analysis; Li₂Ti₃O₇ effectively melts congruently at 1300° and decomposes eutectoidally at 940°C. A solid solution based on Li₂TlO₃ from 50 to ∼65 mol% TiO₃ was observed to exist at >930°C. A new metastable phase was discovered with a composition of ∼75 mol% TiO₂ and with a hexagonal unit cell (8.78 by 69.86 × 10⁻¹nm). Discrepancies in the literature regarding some of these phase equilibria are reconciled.     

Activities and Structure of Some Melts in the System Na2SiO3-Na2Si2O5

Different structural models for melts between Na₂SiO₃ and Na₂Si₂O₅ were tested by comparison of activities computed from the models with activities determined from the calorimetric heat of fusion. Data were used from the liquidus curve of the phase diagram for the system Na₂SiO₃-Na₂Si₂O₅ (Kracek and Morey and Bowen). A group model involving (SiO₃)_x^2x- rings or chains, with a random distribution of (SiO_2.5)₂ pairs which bridge between rings or chains, gives activities in good agreement with those determined from the calorimetric heat of fusion.     

Phase Relations in the System Na2O-Li2O-Al2O3

A subsolidus phase diagram for the system Li₂O-Na₂O-Al₂O₃ at 1500°C is presented. Unlike the analogous system MgO-Na₂O-Al₂O₃, β" was the only ternary compound observed in the section of the lithia-based system examined in this study. A fairly extensive region of β solid solution was found. In contrast, the range of compositions where β" exists is small and does not extend into the Na₂O-Al₂O₃ binary.     

Ba2Ti9O20 Phase Equilibria

The heterogeneous phase distribution found in Ba₂Ti₉O₂₀ ceramic resonators results from a temperature-dependent phase boundary and slow reaction kinetics. When sintered at 1350°C or higher in oxygen the Ba₂Ti₉O₂₀ phase becomes slightly reduced and barium-rich. Thus a stoichiometric composition forms rutile and "Ba₂Ti₉O₂₀'phase. On slow cooling the excess barium diffuses to the oxygen-rich surface where it reacts to form an envelope of rutile-free material surrounding a core containing a small amount of rutile.     

Phase Relations in the System NaF-CaF2-NaAlSiO4-CaAl2Si2O8

The phase diagram of the pseudoternary reciprocal system NaF–CaF₂–NaAlSiO₄–CaAl₂Si₂O₈ is reported in this paper. The phase relations in the system have been investigated by differential thermal analysis, quenching melts, X-ray diffractometry, and optical and electron microscopies. The stable diagonal CaF₂–NaAlSiO₄ divides the system in two pseudoternary systems. The solidus temperatures in the two subsystems NaF–CaF₂–NaAlSiO₄ and CaF₂–NaAlSiO₄–CaAl₂Si₂O₈ are 805°± 2°C and 1095°± 4°C, respectively. An extensive region of liquid–liquid immiscibility is evident in the NaF–CaF₂–NaAlSiO₄ subsystem. The compositions of the two liquids fall outside the compositional surface NaF–CaF₂–NaAlSiO₄–CaAl₂Si₂O₈, but only a small deviation from the ternary behavior is observed.     

The System TiO2-Bi2Ti4O11

The system TiO₂-Bi₂Ti₄O₁₁ was examined by Raman spectroscopy and X-ray diffraction to determine whether TiO₂ is soluble in Bi₂Ti₄O₁₁. The Raman spectral data obtained from preparations made at ∼ 1050°C and cooled to room temperature led us to conclude that TiO₂ is not soluble in the "high-temperature" form of Bi₂Ti₄O₁₁. It was also found that extensive grinding of the phase identified as the "high-temperature" form converts it to the "low-temperature" form, stable below 250°C.     

Phase Relations in the System Na2O-TiO2-SiO2

Liǵuid us and subsolidus phase relations were studied using the quenching technique. The system contains four ternary compounds stable to liquidus temperatures: Na₂TiSi₄O₁₁, Na₂TiSi₂O₇, Na₂TiSiO₅, and Na₂Ti₂Si₂O₉. Six eutectics, eight peritecties, and eight thermal maxima were located and a region of liquid immiscibility was delineated. X-ray powder data are given for the stable and metastable crystalline phases. Glass-transition temperatures were determined by thermal analysis. The relation between physical properties of melts and glasses and the configuration of the liquidus is discussed.     

Eutectic Composition in the Pseudobinary of Y4Al2O9 and Y2O3

The eutectic composition between Y₄Al₂O₉ and Y₂O₃ was determined using electron probe microanalysis (EPMA) on directionally solidified specimens with hypo- and hypereutectic compositions. The microstructures of the specimens as a function of composition differ considerably with small deviation from the eutectic composition (70.5 mol% Y₂O₃ and 29.5 mol% Al₂O₃). Based on the current results and other published data, the pseudobinary system between Al₂O₃ and Y₂O₃ is revised.     

Sol–Gel Preparation of BaTi4O9 and Ba2Ti9O20

BaTi₄O₉ and Ba₂Ti₉O₂₀ precursors were prepared via a sol–gel method, using ethylenediaminetetraacetic acid as a chelating agent. The sol–gel precursors were heated at 700°–1200°C in air, and X-ray diffractometry (XRD) was used to determine the phase transformations as a function of temperature. Single-phase BaTi₄O₉ could not be obtained, even after heating the precursors at 1200°C for 2 h, whereas single-phase Ba₂Ti₉O₂₀ (as determined via XRD) was obtained at 1200°C for 2 h. Details of the synthesis and characterization of the resultant products have been given.     

Phase Diagram of Na2O-B2O3-SiO2 System

The binary isopleth Na₂O.B₂O₃-SiO₂ of the Na₂O-B₂O₃ SiO₂ ternary system has been investigated. A phase diagram is presented based upon data from differential thermal analysis studies of prepared glasses and direct observation of the melting behavior using solid-state video imaging. Phase equilibria relations in the Na₂O-B₂O₃-SiO₂ ternary system have been reassessed by combining information from this study with existing data from the literature. A revised liquidus surface for the ternary is presented in which the form of the isotherms is updated.     

Effects of B2O3 on the Phase Stability of Ba2Ti9O20 Microwave Ceramic

Preparation of dense and phase-pure Ba₂Ti₉O₂₀ is generally difficult using solid-state reaction, since there are several thermodynamically stable compounds in the vicinity of the desired composition and a curvature of Ba₂Ti₉O₂₀ equilibrium phase boundary in the BaO–TiO₂ system at high temperatures. In this study, the effects of B₂O₃ on the densification, microstructural evolution, and phase stability of Ba₂Ti₉O₂₀ were investigated. It was found that the densification of Ba₂Ti₉O₂₀ sintered with B₂O₃ was promoted by the transient liquid phase formed at 840°C. At sintering temperatures higher than 1100°C, the solid-state sintering became dominant because of the evaporation of B₂O₃. With the addition of 5 wt% B₂O₃, the ceramic yielded a pure Ba₂Ti₉O₂₀ phase at sintering temperatures as low as 900°C, without any solid solution additive such as SnO₂ or ZrO₂. The facilities of B₂O₃ addition to the stability of Ba₂Ti₉O₂₀ are apparently due to the eutectic liquid phase which accelerates the migration of reactant species.     

Phase Equilibria in the System Na2O – Nb2O5

Phase equilibria data, obtained both by differential thermal analysis and by quenching, are presented for the system Na₂O-Nb₂O₅. Five compounds corresponding to the formulas 3Na₂O.1Nb₂0₆, lNa₂O. 1Nb₂O₅, lNa₂O 4Nb₂O₆, lNa_zO.7Nb₂O₅, and lNa₂O. 10Nb₂O₆ have been found. The compound 3Na_z0.lNb₂O₅ melts congruently at 992°C. The compounds 1Na₂O. 4Nb₂O₆, lNa₂O.7Nb₂O, and 1Na₂O. 1Onb₂O₅ melt incongruently at 1265°, 1275°, and 1290°C., respectively. The well-known perovskite structure phase NaNbO₃ was found to melt congruently at 1412°C. The transition temperatures in NaNbO₅ were checked by thermal analysis and only the major structural changes at 368° and 640°C. could be detected. A new disordered form of NaNbO₃ could be preserved to room temperature by very rapid quenching.     

Sol-Gel Process for the Preparation of Ba2Ti9O20 and BaTi5O11

Barium titanate precursors with Ba/Ti ratio 2:9 and 1:5 were prepared by first hydrolyzing titanium alkoxide and then mixing the resulting titania sol with a barium alkoxide-methanol solution. After drying, the xerogels of the precursors of barium titanates were sintered at temperatures from 700°C (4 h) to 1200°C (110 h or longer). Characterization of the product was performed using X-ray diffraction and laser Raman spectroscopy. At 700°C, BaTi₅O₁₁ was formed from the 1:5 precursor and a two-phase mixture of BaTi₂O₅ and BaTi₅O₁₁ was formed from the 2:9 precursor. After prolonged heating at 1200°C, the latter mixture converted to a single-phase material, Ba₂Ti₉O₂₀.     

High-Temperature Phase Relations in the System Gd2O3-Ta2O5

The Phase relations of the system Gd₂O₃-Ta₂O₅ in the composition range 50 to100 mol% Gd2O3 was studied by solidstate reactions at 1350°, 1500°, or 1700°C and by thermal analyses up to the melting temperatures. Weberite-type orthorhombic phase (W2 phase, space group C2221) with the composition of Gd3 TaO7 seems to melt incongruently; at about 2040°C, although this Gd₃TaO₇ Phase was previously reported to melt congruently. A new fluorite-type cubic phase (F phase, space group Fm3m ) was found for the first time above 1500°C in the system. It melts congruently with the composition of about 80mol% Gd2O3at 2318° 3°C. A phase diagram was proposed for the system Gd₂O3–Ta2O₅ in the Gd₂O₃–rich portion     

Phase Relations in the System CaO-Ta2O5-SiO2

The system CaO-Ta₂O₃-SiO₂ was studied using a combination of hot-stage microscopy and the quenching technique. Primary crystallization fields were defined for the various calcium silicates, and for the calcium tantalates: CaO·2Ta₂O₅, CaO·Ta₂O₅, 2CaO·Ta₂O₅, and 4CaO·Ta₂O₅. A congruently melting ternary compound 10CaO·Ta₂O₅·6SiO₂, isostructural with the mineral niocalite, is the only ternary phase in the system. A large twoliquid region extends across the system from the CaO-SiO₂ binary to within 1 wt% of the Ta₂O₃-SiO₂ binary but does not cut it, in marked contrast to the analogous CaO-Nb₂O₅-SiO₂ system. Other somewhat unexpected differences were noted between these systems.     

Kinetics of Hexacelsian-to-Celsian Phase Transformation in SrAl2Si2O8

The kinetics of hexacelsian-to-celsian phase transformation in SrAl₂Si₂O₈ have been investigated. Phase-pure hexacelsian was prepared by heat treatment of glass flakes at 990°C for 10 h. Hexacelsian flakes were isothermally heat-treated at 1026°, 1050°, 1100°, 1152°, and 1200°C for various times. The amounts of monoclinic celsian formed were determined using quantitative X-ray diffraction. Values of reaction rate constant, k , at various temperatures were evaluated from the Avrami equation. The Avrami parameter was determined to be 1.1, suggesting one-dimensional growth with the interface rather than a diffusion-controlled transformation mechanism. From the temperature dependence of k , the apparent activation energy for this reaction was evaluated to be 527 ± 50 kj/mol (126 ± 12 kcal/mol). This value is consistent with a mechanism involving the transformation of the layered hexacelsian structure to a three-dimensional network celsian structure which necessitates breaking of the strongest bonds, the Si─O bonds.     

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.