Thermodynamic Modeling of Phase Diagrams in Binary Alkali Silicate Systems

A thermodynamic modeling of phase diagrams in binary alkali silicate systems is described by using the Gibbs free energy of the phases. Simple models employing regular, quasi-regular, and subregular solution models were applied to describe the liquid phase. The interaction parameters of the liquid phases were obtained by using the liquidus curves of silica (and subliquidus data) through a multiple linear regression method. The present calculated liquidus curves agree very well with Cs₂O—SiO₂ and Rb₂O—SiO₂ systems. In the K₂O—, Na₂O—, and Li₂O—SiO₂ systems, the calculations permitted discrimination between sets of data and, based upon the calculation choices, could be correctly made to select the most appropriate phase diagram. Also, the spinodals were calculated where phase separation can occur by a spinodal decomposition process.     

Phase Equilibria in the System Lithium Metasilicate–β-Eucryptite

A phase equilibrium study of the join lithium metasilicate–β–eucryptite in the system Li₂O–Al₂O₃–SiO₂ was made by the quench method. The phase diagram was found to be a simple binary with a eutectic at 1070°C and 57% eucryptite. The relationship of these data to the ternary system is discussed.     

Isothermal Section of a Cu2O–Al2O3–SiO2 Pseudo-Ternary System at 1150°C in Air

In this study, the isothermal section of a Cu₂O–Al₂O₃–SiO₂ pseudo-ternary phase diagram at 1150°C was analyzed by means of a scanning electronic microscope and powder X-ray diffraction of the quenched samples qualitatively, and the compositions of the tie-points of the tie-planes as well as their regions were determined by in situ high-temperature quantitative X-ray diffraction analysis and energy-dispersive X-ray spectroscopy. Then, the isothermal section of the Cu₂O–Al₂O₃–SiO₂ pseudo-ternary phase diagram at 1150°C was constructed; it was found that the isothermal section is composed of two single liquid-phase regions, five two-phase regions, and six three-phase regions.     

Corrosion of Mullite by Molten Salts

The interaction of molten salts of different Na₂O activities and mullite is examined with furnace and burner tests. The more-acidic molten salts form small amounts of Al₂O₃; the more-basic molten salts form various Na₂O–Al₂O₃–SiO₂ compounds. The results are interpreted using the Na₂O–Al₂O₃–SiO₂ ternary phase diagram, and some possible diffusion paths are discussed. The generally higher melting points of Na₂O–Al₂O₃–SiO₂ compounds lead to better behavior of mullite in molten salts, as compared to SiO₂-protected ceramics such as SiC. Mullite-coated SiC is discussed, and the corrosion behavior is evaluated.     

Thermodynamic Evaluation for the Y2O3–BaO–CuOx System

The thermodynamic data for the Y₂O₃–BaO–Cu₂O–CuO quaternary system were optimized from measured thermodynamic data. A two-sublattice model for ionic solution was used to express the Gibbs free energy of the liquid phase, and a two-sublattice regular solution model was used for the nonstoichiometric YBa₂Cu₃O_6+δ superconducting compound. The optimized thermodynamic data were used to calculate the phase diagrams of the Cu₂O–CuO binary system and the CuO _x –Y₂Cu₂O₅ and CuO _x –BaCuO₂ quasi-binary systems. The results were in good agreement with reported measured data. The liquidus projection and isothermal and vertical sections of the Y₂O₃–BaO-CuO _x quasi-ternary system were calculated. The effect of oxygen pressure on some reaction temperatures was predicted by calculating them at various oxygen pressures, and the oxygen contents (6 +δ) in YBa₂Cu₃O_6+δ were calculated at various temperatures and oxygen pressures. The results were compared with experimental data.     

The Ternary Systems Leucite—Corundum—Spinel and Leucite—Forsterite—Spinel

These ternary systems are limiting faces, respectively, of the volumes leucite-corundum-spinel-silica and leucite-forsterite-spinel-silica in the quaternary system K₂O–MgO–Al₂O₃–SiO₂. The results of quenching experiments on these two ternary systems and on the binary system spinel-leucite are given here. The relation of the system leucite-corundum-spinel to the volume leucite-corundum-spinel-K₂O·Al₂O₃·2SiO₂ is indicated. The data presented are of general interest to the geologist and provide basic information which can be applied to refractories and slags by the ceramist and metallurgist. Some observations are made on the refractoriness and changes in refractoriness of certain mixtures of ceramic materials.     

Mixed-Alkali Effect on Rayleigh Scattering in K2O–Na2O–MgO–SiO2 Glasses

The Rayleigh scattering of the mixed-alkali glass system K₂O–Na₂O–MgO–SiO₂ (KNMS) was investigated, both experimentally and theoretically. The lowest Rayleigh scattering coefficient (38% of that for pure SiO₂ glass) was obtained when the glass composition was 22K₂O–8Na₂O–10MgO–60SiO₂ (in mol%). These values are equal to or less than the minimum values reported for the ternary sodium silicate glass Na₂O–MgO–SiO₂. The Rayleigh scattering caused by concentration fluctuation was believed to have been reduced greatly in this KNMS glass, because the mobility of the alkali-metal ions was reduced by the mixed-alkali effect.     

Glass Formation and Recrystallization in the Lithium Metasilicate Region of the System Li2O–Al2O3–SiO2

The stability of the vitreous state in the lithium metasilicate region of the system Li₂O–Al₂O₃–SiO₂ was found to be a function of the concentration of lithia. The higher the lithia content, the less stable was the glass. The devitrification of glasses in this system was studied. In addition to the phases present at or near the liquidus, it was found that the β -eucryptite– β -quartz solid solution phase was metastable over most of the region. The Li₂O–SiO₂, β -Li₂O–Al₂O₃–4SiO₂ solid solution, β -Li₂O–Al₂O₃–2SiO₂ solid solution triple point was estimated to be near 62.5% SiO₂, 17% Al₂O₃, and 20.5% Li₂O (by weight). The thermal expansions of bodies in this region were measured and the values obtained are explained in terms of the phases present.     

Activity of Nickel(II) Oxide in Silicate Melts

Activity–composition relations of NiO have been determined at 1435°C in melts of the system CaO–NiO–SiO₂ and at 1400°C in melts of the systems CaO–MgO–NiO–SiO₂, CaO–MgO–NiO–Al₂O₃–SiO₂, and CaO–MgO–NiO–K₂O–SiO₂. In the CaO–NiO–SiO₂ and CaO–MgO–NiO–SiO₂ systems the activity coefficient of NiO (γ_NiO) decreases as the polymerization of the melt increases (basicity decrease). γ_NiO stays contant up to several weight percent NiO for melts with similar basicity, an indication of Henry's law behavior. Minima in the NiO activity coefficient are observed in melts of the CaO–MgO–NiO–Al₂O₃–SiO₂ and CaO–MgO–NiO–K₂O–SiO₂ systems at NBO/Si ratios between 1.0 and 1.5; i.e., γ_NiO decreases with decreasing basicity for NBO/Si ratios >1.5 and increases with decreasing basicity for melts with NBO/Si ratios <1.0 (NBO/Si; nonbridging oxygens per silicon). The addition of Al₂O₃ and K₂O to the CaO–MgO–NiO–SiO₂ system results in an increase in the activity coefficient of NiO.     

Thermochemical Modeling of Oxide Glasses

A modified associate species approach is used to model the liquid phase in oxide systems. The relatively simple technique treats oxide liquids as solutions of end-member and associate species. The model is extended to representing glasses by treating them as undercooled liquids. Equilibrium calculations using the model allow the determination of species activities, phase separation, precipitation of crystalline phases, and volatilization. In support of nuclear waste glass development, a model of the Na₂O–Al₂O₃–B₂O₃–SiO₂ system has been developed that accurately reproduces its phase equilibria. The technique has been applied to the CaO–SiO₂ system, which is used to demonstrate how two immiscible liquids can be treated.     

Phase Equilibrium Diagram CaO—CaF2—2CaO.SiO2

Differential thermal analysis and quenching experiments were used to establish the ternary phase diagram CaO-CaF₂-2CaO.SiO₂. Hermetically-closed platinum capsules were used to prevent fluorine loss in the form of HF, SiF₄, and CaF₂ by reaction of the CaF₂ with water vapor or SiO₂, or by evaporation. The melting point of pure CaF₂ was 1419°± 1°C. There is one binary eutectic in the system CaO-CaF₂ and there are two ternary eutectics in the system CaO-CaF₂-2CaO.SiO₂. The results of the present study were combined with literature data to construct the phase diagram CaO-CaF₂-SiO₂.     

Effects of Low-Pressure Oxidation on the Surface Composition of Single Crystal Silicon Carbide

We report on thermodynamic modeling and experimental studies of the reaction of oxygen with the 4H- and 6H-SiC surfaces at high temperatures T . It is observed that this reaction leads to the growth of passivating SiO₂ layers at high pressures P (O₂), etching of the surface at lower P (O₂), and enhancement of the surface segregation of carbon at still lower P (O₂). A unified P (O₂)– T phase diagram for the reaction of O₂ with SiC is presented and a thermodynamic model predicting these three distinct reaction regions is described. Evidence for the thermal decomposition of the SiO₂ layer due to its reaction with the SiC substrate is also presented.     

Thermodynamic Assessment of the Lithium-Borate System

The Li₂O–B₂O₃ quasi-binary system is assessed. A two-sublattice ionic solution model, (Li¹⁺) _P (O²⁻, BO₃³⁻, B₄O₇²⁻, B₃O_4.5) _Q , is adopted to describe the liquid phase. All solid phases are treated as stoichiometric compounds. A set of parameters consistent with most of the available experimental data on phase diagram and thermodynamic properties is obtained by using the CALPHAD technique.     

Studies in Lithium Oxide Systems: VII, Li2O–B2O2–SiO2

Phase relations in the system Li₂O–B₂O₃–SiO₂ were studied by quenching and solid-state reactions. No ternary compounds were detected in the portion of the system containing less than 53% Li₂O. Compatibility triangles were formed from the binary borate and silicate compounds. Liquidus data obtained by quenching are reported for four joins, Li₂O·2SiO₂–Li₂O·2B₂O₃, Li₂O·SiO₂-Li₂O·2B₂O₃, Li₂O·SiO₂-Li₂O·B₂O₃, and Li₂O·2B₂O₃-SiO₂. The last join cuts across the two-liquid region and is not a true binary system. Some probable ternary invariant points were located in the portion of the system which was quenchable to glass and adjacent to the two-liquid region. Further data on the previously reported immiscible liquid formation are given and the significance is discussed. Data on the thermal expansion behavior of certain glasses are presented.     

Equilibrium Compressibilities and Density Fluctuations in K2O–SiO2 Glasses

The density fluctuations contributing to light scattering in a glass are governed by the flctive temperature of the glass and the equilibrium compressibility of the melt. Using ultrasonic velocity data for K₂O–SiO₂ melts, these compressibilities were evaluated, and the magnitude of the density fluctuations were calculated. In this system, the mean–square amplitude of the fluctuations reaches a minimum value (about half that of pure SiO₂) for a composition of ∼20 mol% K₂O. By extrapolating the equilibrium compressibilities to zero K₂O content, the density fluctuations can be calculated for pure SiO₂ glass; this calculation agrees well with the result obtained from light–scattering measurements.     

Aqueous Solubility Diagrams for Cementitious Waste Stabilization Systems: II, End-Member Stoichiometries of Ideal Calcium Silicate Hydrate Solid Solutions

Solubility in the fully hydrated CaO–SiO₂–H₂O system can be best described using two ideal C-S-H-(I) and C-S-H-(II) binary solid solution phases. The most recent structural ideas about the C-S-H gel permit one to write stoichiometries of polymerized C-S-H-(II) end-members as hydrated precursors of the stable tobermorite and jennite minerals in the form of 5Ca(OH)₂·6SiO₂·5H₂O and 10Ca(OH)₂·6SiO₂·6H₂O, respectively. For thermodynamic modeling purposes, it is more convenient to express the number of basic silica and portlandite units in these stoichiometries using the coefficients n _Si and n _Ca. Thermodynamic solid-solution aqueous-solution equilibrium modeling by applying the Gibbs energy minimization (GEM) approach shows the best generic fits to the available experimental solubility data at solid 0.8 < Ca/Si < 2.0 if both stoichiometry and thermodynamic constants of the end-members are normalized to n _Si= 1.0 ± 0.3. Recommended stoichiometries and thermodynamic data for the C-S-H end-members provide a reliable basis for the subsequent multicomponent extension of the ideal C-S-H solid solution model by incorporation of end-members for the (radio)toxic elements or trace metals.     

Coordination of Zinc(II) in ZnO–K2O–SiO2 Glasses by X-ray Diffraction

The radial distribution functions of ZnO–K₂O–SiO₂ glasses with 7 and 10 wt% ZnO are compared with that of the corresponding K₂O–SiO₂ matrix leading to "difference distribution curves'representative of the zinc structural arrangement. Analysis of the curves indicates that Zn²⁺ ions are prevalent (65% to 80%) in the glasses in tetracoordinated form.     

Liquid–Solid Equilibria in the System NiO–TiO2–SiO2 in the Temperature Range 1430° to 1660°C

Equilibrium relations in the system NiO–TiO₂–SiO₂ in air have been investigated in the temperature range 1430° to 1660°C. The most conspicuous feature of the phase relations is the existence of a cation-excess spinel-type phase, in addition to NiO and NiTiO₃, on the liquidus surface and at subsolidus temperatures down to 1430°C. Three invariant points have been located on the liquidus. There is a peritectic at 1540°C characterized by coexisting NiO ( ss ), spinel( ss ), cristobalite, and liquid of composition 47 wt% NiO, 29 wt% TiO₂, and 24 wt% SiO₂. Two eutectics are present, one at 1480°C, with spinel( ss ), NiTiO₃, cristobalite, and liquid (42 wt% NiO, 43 wt% TiO₂, and 15 wt% SiO₂), as the coexisting phases. The other is at 1490°C with NiTiO₃, rutile, cristobalite, and liquid (32 wt% NiO, 56 wt% TiO₂, and 12 wt% SiO₂). A liquid miscibility gap extends across the diagram from the two bounding binary systems NiO–SiO₂ and TiO₂–SiO₂.