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.     

Prediction of Immiscibility Boundaries of the Systems K2O-SiO2, K2O-Li20-SiO2, K20-Na2O-Si02, and K2O-BaO-SiO2

In earlier work, a prediction method of the immiscibility boundary of a ternary silicate glass system was developed involving two known binary immiscibility boundaries and a measured immiscibility temperature of one ternary glass composition. In the present work, the method is extended to the case where one of the two binary immiscibility boundaries is not known and is applied as an example to ternary silicate systems containing K₂O. First, the immiscibility boundary of the system K₂O-SiO₂ is estimated by measuring the immiscibility temperatures of three glasses in the system K₂O-Li₂O (or Na₂O)-SiO₂. Using this result the immiscibility boundaries of the systems K₂O-Li₂O-SiO₂, K₂O-Na₂O-SiO₂, and K₂O-BaO-SiO₂ are estimated. The results agree reasonably well with the experimentally determined immiscibility temperatures at selected compositions.     

Phase Equilibria in the System CaO-TiO2–SiO2

Results of a study of phase equilibria in the system CaO–TiO₂–SiO₂ are presented. A prominent feature of the liquidus surface is a large two-liquid region which appears on the equilibrium diagram as a broad band extending from the SiO₂-CaO side to the SiO₂-TiO₂ side of the triangle. Evidence for the liquid immiscibility and the significance of the resulting large high-temperature liquidus region in silicate technology are discussed. Representative paths of crystallization of liquids in the system under equilibrium conditions are outlined. It is shown that solid solution in the system is virtually nonexistent except for the small-scale substitution of Ti⁴⁺ for Si⁴⁺ in wollastonite. Indices of refraction of glasses are given. Composition and temperature are listed for the twelve liquidus ternary invariant points.     

Effects of MoO3, on Phase Separation of Na2O-B2O3-SiO2 Glasses

Immiscibility temperatures of Na₂O-B₂O₃-SiO glasses, with andwithout 1 mol% MoO₃, additions, were determined and the effect of MoO₃ additions on the 65O°C immiscibility isotherms was established. In addition, immiscibility temperature and phase-separation morphology of an Na₂O-B₂O₃-SiO₂ glass with progressive additions of MoO₃, were investigated. It was found that the addition of small amounts of MoO₃ extends the immiscibility boundary of the system and raises the immiscibility temperature by ∼l8°C for each mol % MoO₃, addition. Analysis of phase-separation morphology suggests that the MoO₃, additions do not significantly alter the tie lines of phase separation in the system, although such additions cause a lowering of the viscosities and the glass-transition temperatures of these glasses.     

Phase Equilibria and Liquid Immiscibility in the System PbO-B2O3-SiO2

A detailed study of the system Pb0–B₂0₃–Si0₂ was undertaken because of the interest in lead borosilicate fluxes and the possibility of producing opaque low-melting enamels by controlled liquid-phase separations. Studies in the portion of the system containing greater than 60% PbO produced little change from previous work. The shape of the liquidus surface beneath the two-liquid region and the shape of the immiscibility dome were determined. Much of the observed immiscibility is metastable, particularly for compositions containing greater than 40% SiO₂. Using the electron microscope provided information on the way the rate of formation, stability, and texture of the immiscible liquids are affected by composition, time, and temperature. Finally, a high-temperature centrifuge was constructed to separate the immiscible liquids and thus define tie lines within the immiscibility dome.     

Phase Equilibrium Relationships at Liquidus Temperatures in the System FeO–Fe2O3–Al2O3–SiO2

Phase equilibrium data at liquidus temperatures are presented for mixtures in the system FeO–Fe₂O₃–Al₂O₃–SiO₂. The volume located between the 1 and 0.2 atm. O₂ isobaric surfaces of the tetrahedron representing this system was studied in detail. Scattered data were obtained at lower O₂ pressures. Results obtained in the present investigation were combined with data in the literature to construct a phase equilibrium diagram, at liquidus temperatures, for the entire system FeO–Fe₂O₃–Al 2 O₃–SiO₂. Methods for interpretation of the diagram are explained.     

The System Iron Oxide-TiO2-SiO2 in Air

Phase relations at liquidus temperatures in the system iron oxide-TiO₂-SiO₂ have been determined in air. The equilibrium existence of the crystalline phases magnetite (ss), hematite (ss), pseudobrookite(ss), rutile(ss), and silica (tridy-mite or cristobalite) has been established. Three isobaric eutectic points are formed by the intersections of quaternary liquidus univariant lines of the system Fe-Ti-Si-O and the 0.21 atmosphere isobaric surface. The liquid misci-bility gaps present in the bounding "binary" systems iron oxide-Si0₂ and TiO₂-SiO₂ extend across the composition triangle representing the "ternary" system iron oxide-TiO₂-SiO₂. Liquidus temperatures decrease within the two-liquid region from approximately l <57Q^a C. in the system iron oxide-SiO₂ and 1780°C. in the system Ti(VSiO₂ to 1540°C. at the cristobalite-rutile(ss) boundary curve which bisects the two-liquid region. Paths of equilibrium crystallization of representative mixtures are discussed with reference to a simplified projection into the plane FeOFe₂O₃-TiO₂-SiO₂ as well as in terms of the tetrahedron representing the system FeO-Fe₂O₃-TiO₂-SiO₂.     

Effect of Small Additions of Al2O3 and Ga2O3 on the Immiscibility Temperature of Na2O-SiO2 Glasses

Alumina and gallia were substituted separately for Na₂O in amounts of 0.2, 0.5, 1.0, 1.5, 2.0, and 3.0 wt% in three Na₂O-SiO₂ glass compositions (82, 84, and 86 wt% SiO₂) within the immiscibility region. The immiscibility regions for each system extend to ∼1.5 mol% of the added oxide. In general, the addition reduced the immiscibility temperature ( T _m), but at the edge of the immiscibility region (82% SiO₂) the Na₂O loss effect initially increased T _m. A structural model of the miscibility of Al₂O₃ added to silicate glasses is presented.     

Phase Equilibria in the System Li2O-Cr2O3-SiO2

The system Li₂O-Cr₂O₃–SiO₂ contains one previously reported ternary compound, LiCrSi₂O₆. Six subsolidus compatibility triangles and six ternary invariant points were located. The highest solidus, temperature is 1283°C, but liquidus temperatures are much higher for many compositions.     

Phase Equilibria in the System HfO2–Y2O3–CaO

Phase equilibria in the system HfO₂–Y₂O₃–CaO were studied in the temperature range 1250° to 2850°C by both experimental methods (X-ray phase analysis at 20° to 2000°C, petrography, annealing and quenching, differential thermal analysis in He at temperatures to 2500°C, thermal analysis in air using a solar furnace at temperatures to 3000°C, and electron microprobe X-ray analysis) and theoretical means (development of a mathematical model for the liquidus surface by means of the reduced polynomial method). Phase equilibria were determined by the structure of the restricting binary systems. No ternary compounds were found. The liquidus was characterized by the presence of six four-phase, invariant equilibria. Solid solutions were based on monoclinic (M), tetragonal (T), and cubic (F) modifications of HfO₂; C and H forms of Y₂O₃; CaO; and CaHfO₃ that crystallized in two polymorphous modifications, namely, the cubic and rhombic perovskite-type structure.     

Determination of 1600° and 1700°C. Liquidus Lines in CaO-2Al2O3 and AI2O3 Stability Fields of the System CaO—Al2O3—SiO2

The 1600° and 1700°C. liquidus lines in the CaO·2Al₂O₃ and A1₂O₃ stability fields of the system CaO-Al₂O₃-SiO₂ are determined from the chemical analyses of saturated slags at these temperatures.     

Liquidus Behavior in the Silica-Rich Region of the System PbO-SiO2

The liquidus for the system PbO-SiO₂ was determined from 0 to 38 mol% (0 to 70 wt%) PbO. The shape of the liquidus indicates a possible metastable immiscibility region, and the degree of flattening is comparable to that in the system BaO-SiO₂. Immiscibility was confirmed by electron microscopy. The densities and refractive indices of these glasses are reported.     

Phase Equilibria in the System CaO-Iron Oxide-SiO2, in Air

Phase equilibrium relations in the liquidus temperature region of the system CaO-iron oxide-SiO₂ in air were determined, using the quenching technique. The results are illustrated by means of a projection of the liquidus surface onto the composition triangle CaO-Fe₂O₃-SiO₂. A supplementary diagram is presented in order to indicate true compositions of liquids at liquidus temperatures. Data obtained in the present investigation are combined with literature data to construct diagrams showing stability relations among crystalline phases at subsolidus temperatures. A diagram is also presented to show phase relations along the join CaO SiO₂-2CaO Fe₂O₃.     

Thermodynamic Modeling of the Miscibility Gaps and the Metastable Liquidi in the MgO-SiO2, CaO-SiO2, and SrO-SiO2 Systems

A range of compositions and temperatures below the monotectic temperature exists where there are thermodynamic restrictions that prevent the equilibrium solid from forming directly from the undercooled homogeneous liquid. In this region, the solid can form only after liquid-liquid phase separation has occurred. As suggested by earlier research, the thermodynamic restrictions on the crystallization process may be useful to control the crystallized grain structure in glass-ceramic systems. Thus, understanding the thermodynamic limitations on the formation of the solid in monotectic systems could have commercial significance. In the present paper, the metastable liquidus boundaries, liquid miscibility gaps, and spinodal curves in binary MgO-SiO₂, CaO-SiO₂, and SrO-SiO₂ systems are calculated by using analytical expressions for the Gibbs free energies of the liquid phases. Calculating the metastable liquidus rather than using a simple extrapolation as originally proposed in the aforementioned previous research provides greater control of the heat-treatment processes and, thus, greater control over the resulting microstructure.     

Phase Equilibrium Studies in the System CaO-Cr2O3-SiO2

Results are presented of a study in air of mixtures in the system CaO-Cr₂O₃-SiO₂. The phase equilibrium diagram shows relations at liquidus temperatures for all but the high-lime part of the system. In this omitted part chromium in the mixtures oxidizes in air to higher valence forms. The compound Ca₃Cr₂Si₃O₁₂ (uvarovite) occurs at subsolidus temperatures, decomposing at 1370°C. to α-CaSiO₃ and Cr₂O₃. The inhibiting action of chromium oxide on the inversion of high-temperature forms of Ca₂SiO₄ to the low-temperature γ-Ca₂SiO₄ is discussed in the light of new data. Evidence is presented for the existence of a pentavalent chromium compound, Ca₃(CrO₄)₂, having solid-solution relations with Ca₃SiO₄.     

The System K2O-BaO-SiO2

Phase equilibrium studies of compound formation and liquidus and solidus surfaces of the system K₂O-BaO-SiO₂ are presented. The system contains 3 ternary compounds: K₄BaSi₃O₉, K₈BaSi₁₀O₂₅, and K₂Ba₃Si₈O₂₀. Both high and low polymorphs of the third have fields on the ternary liquidus surface. Solid solution with SiO₂ depresses the high-low inversion from 1030°C at K₂Ba₃Si₈O₂₀ to 835°C at 70.2 mol% SiO₂. Data for 20 liquidus invariant points were found; 8 are thermal maxima and 12 are eutectics or peritectics. The isofracts of quenched glasses were determined.     

The System HfO2-Y2O3-Er2O3

A mathematical model of the liquidus surface based on a reduced polynomial method was proposed for the system HfO₂-Y₂O₃-Er₂O₃. The results of calculations according to this model agree fairly well with the experimental data. Phase equilibria in the system HfO₂-Y₂O₃-Er₂O₃ were studied on melted (as-cast) and annealed samples using X-ray diffraction (at room and high temperatures) and micro-structural and petrographic analyses. The crystallization paths in the system HfO₂-Y₂O₃-Er₂O₃ were established. The system HfO₂-Y₂O₃-Er₂O₃ is characterized by the formation of extended solid solutions based on the fluorite-type (F) form of HfO₂ and cubic (C) and hexagonal (H) forms of Y₂O₃ and Er₂O₃. The boundary curves of these solid solutions have the minima at 2370°C (15. 5 mol% HfO₂, 49. 5 mol% Y₂O₃) and 2360°C (10. 5 mol% HfO₂, 45. 5 mol% Y₂O₃). No compounds were found to exist in the system investigated.     

Liquidus Temperatures in the System FeO—Fe2O3—TiO2

Liquidus temperatures in the system FeO–Fe₂O₃–TiO₂ have been estimated from data in the literature supplemented with experimental determinations of the liquidus temperature for about fifty different compositions within the system using the quenching method.     

Thermal Stability of Gallium Orthoferrite in the System Fe2O3-FeO-Ga2O3

Gallium orthoferrite (Ga_{2- x} Fe _x O₃) has a maximum thermal stability which coincides roughly with liquidus temperatures at oxygen pressures near atmospheric. As a result, changes in ambient oxygen pressure between 0.2 and 10 atm have a pronounced effect on equilibria. The compound exhibits a wide range in Ga:Fe ratio on both sides of the stoichiometric GaFeO₃ but is essentially invariant in oxygen content to 1500°C in air. The orthoferrite bears many similarities to the corresponding aluminum compound Al₂- _x Fe _x O₃.     

The System La2O3— TiO2; Phase Equilibria and Electrical Properties

Phase relations at liquidus temperatures in the system La₂O₃-TiO₂ were studied in air. The existence of two previously unreported compounds, La₂O₃. TiO₂ and 2La₂O₃-TiO₂, is postulated on the basis of X-ray and microscopic examination of crystalline samples, and in the case of La₂O₃.-TiO₂, on a maximum in the liquidus curve at that composition. Quenched liquids of the primary-phase field of rutile were found to be semiconducting. This property was related to oxygen loss from both liquid and crystalline phases and is discussed in the light of weight loss experiments, microscopic examination of quenched samples, and information obtained from the literature. Dielectric constant and loss factor of the compounds La₂O₃-TiO₂, La₂O₃-2TiO₂, and 2La₂O₃-9TiO₂ are reported at 1 Mc over the temperature range 25° to 500°C.