Thermodynamic Modeling of the Isomorphous Phase Diagrams in the Al2O3–Cr2O3 and V2O3–Cr2O3 Systems

The phase diagrams in the Al₂O₃–Cr₂O₃ and V₂O₃–Cr₂O₃ systems have been assessed by thermodynamic modeling with existing data from the literature. While the regular and subregular solution models were used in the Al₂O₃–Cr₂O₃ system to represent the Gibbs free energies of the liquid and solid phases, respectively, the regular solution model was applied to both phases in the V₂O₃–Cr₂O₃ system. By using the liquidus, solidus, and/or miscibility gap data, the interaction parameters of the liquid and solid phases were optimized through a multiple linear regression method. The phase diagrams calculated from these models are in good agreement with experimental data. Also, the solid miscibility gap and chemical spinodal in the V₂O₃–Cr₂O₃ system were estimated.     

Thermodynamic Assessment of the CaO–MgO–Al2O3 System

The system CaO–MgO–Al₃O₃ has been assessed with the Calphad technique using a computerized optimization procedure called parrot . The rather meager experimental information, mainly on liquidus relations, is described reasonably well, but the lack of data, especially on solid-phase relations, implies that the present assessment should be regarded as provisional. The system contains one stable ternary phase with the stoichiometry 3CaO·2Al₂O₃·MgO.     

Thermodynamic Modeling of the FeO–Fe2O3–MgO–SiO2 System

The liquid and solid phases in the FeO–Fe₂O₃–MgO–SiO₂ system are of importance in ceramics, metallurgy, and petrology. A complete critical evaluation and thermodynamic modeling of the phase diagrams and thermodynamic properties of this system are presented. Optimized equations for the thermodynamic properties of all phases are obtained that reproduce all available thermodynamic and phase equilibrium data within experimental error limits from 25°C to above the liquidus temperatures at all compositions and oxygen partial pressures. The optimized thermodynamic properties and phase diagrams are believed to be the best estimates presently available. The database of the model parameters can be used with software for Gibbs energy minimization to calculate any type of phase diagram section.     

Quaternary System Al2O3–CaO–MgO–SiO2: I, Study of the Crystallization Volume of Al2O3

Compatibility relations of Al₂O₃ in the quaternary system Al₂O₃–CaO–MgO–SiO₂ were studied by firing and quenching followed by microstructural and energy-dispersive X-ray examination. A projection of the liquidus surface of the primary phase volume of Al₂O₃ was constructed in terms of the CaO, SiO₂, and MgO contents of the mixtures recalculated to 100 wt%. Two invariant points, where four solids coexist with a liquid phase, were defined, and the positions of the isotherms were tentatively established. The effect of SiO₂, MgO, and CaO impurities on Al₂O₃ growth also was studied.     

Determination of CaO–SiO2–MgO–Al2O3–CrOx Liquidus

In this work, the liquidus of synthetic CaO–SiO₂–MgO–Al₂O₃–CrO _x slags is evaluated in the industrially relevant compositional domain. Equilibrium experiments are carried out at 1500°C and partial oxygen pressure ( p _O2) 10^−11.04 atm, and at 1600°C and p _O2=10^−10.16 and 10^−9.36 atm. The studied basicities (CaO/SiO₂) are 1.2 and 0.5. Al₂O₃ levels range from 0 to 30 wt%. Oversaturated liquid is sampled and phase relations are measured with quantitative electron probe microanalysis–wavelength dispersive spectroscopy (EPMA–WDS). The results are compared with the commercially available FactSage thermodynamic databases. Qualitative agreement is always obtained. Also a good quantitative agreement is found at the higher basicity, especially for the spinel liquidus. A minor but systematic deviation can be observed for the eskolaite liquidus. At the lower basicity, the calculated phase diagram deviates strongly from the experimental results, probably due to missing ternary interactions in the database.     

Effect of Cr2O3 on Slag Resistance of Al2O3–SiO2 Refractories

Refractory bodies of 65 wt% Al₂O₃ were prepared from a mixture of calcined alumina and raw kaolin with the addition of Cr₂O₃ up to 15 wt%. The Cr₂O₃ addition effectively enhances slag resistance and reduces mullite formation. Petrographic analysis of the refractories after the slag test suggests that Cr₂O₃ increases the viscosity of both the glassy phase in the refractory as well as the slag, thereby retarding slag penetration and reaction at elevated temperature.     

Phase Equilibria in High-Alumina Part of the System CaO–MgO–Al2O3–SiO2

Results are presented of a study of phase equilibria among crystalline and liquid phases in the quaternary system CaO–MgO-Al₂O₃–SiO₂ at Al₂O₃ contents greater than 35%. Equilibrium diagrams shown are for the five triangular joins CaAl₂Si₂O₃-Ca₂Al₂SiO₇-MgAl₂O₄, Ca₂Al₂SiO₇-MgAl₂O₄-Al₂O₃, CaAl₂Si₂O₈-MgO-Al₂O₃, CaAl₂Si₂O₈-Mg₂SiO₄-MgAl₂O₄, and CaAl₂Si₂O₈-MgO-Mg₂SiO₄. The composition and nature of the four quaternary peritectic points and the relationships of univariant lines and primary phase volumes are discussed.     

Effect of Alkali Metal Oxide Addition on the Sinterability of MgO–B2O3–Al2O3 Glass–Al2O3 Filler Composites

The sintering of a composite of MgO–B₂O₃–Al₂O₃ glass and Al₂O₃ filler is terminated due to the crystallization of Al₄B₂O₉ in the glass. The densification of a composite of MgO–B₂O₃–Al₂O₃ glass and Al₂O₃ filler using pressureless sintering was accomplished by lowering the sintering temperature of the composite. The sintering temperature was lowered by the addition of small amounts of alkali metal oxides to the MgO–B₂O₃–Al₂O₃ glass system. The resultant composite has a four-point bending strength of 280 MPa, a coefficient of thermal expansion (RT—200°C) of 4.4 × 10⁻⁶ K⁻¹, a dielectric constant of 6.0 at 1 MHz, porosity of approximately 1%, and moisture resistance.     

Effect of Composition on Microstructural Development in MgO–Al2O3–SiO2 Glass-Ceramics

The transformation kinetics and microstructures of glass-ceramics, which deviate from those of a stoichiometric cordierite compound, are greatly dependent on the compositions of the starting glasses. Compositions richer in (MgO,SiO₂) than the stoichiometric cordierite compound suppress the formation of μ-cordierite, yet enhance the crystallization of α-cordierite, resulting in a higher content of α-cordierite. In contrast, compositions richer in Al₂O₃ than the stoichiometric cordierite compound have no effect on the crystallization of α-cordierite. Thus, most of the glass crystallizes to μ-cordierite in the initial stage, followed by the slow transformation of μ-cordierite into an α-phase, which results in a low content of α-cordierite.     

Phase Equilibrium Relationships in a Portion of the System MgO–Al2O3–2CaO·SiO2

The system MgO–Al₂O₃–2CaO·SiO₂ comprises a plane through the tetrahedron CaO–MgO–Al₂O₃–SiO₂. A total of 108 compositions were prepared having an alumina content below the line joining 2CaO·Al₂O₃SiO₂ (gehlenite) and MgO·Al₂O₃ (spinel). Quenching experiments were carried out on 96 of these compositions at temperatures up to 1590°C. Three binary eutectic systems and two ternary eutectic systems are described. Compositions on this plane are of significance in an investigation of the constitution of basic refractory clinkers made from Canadian dolomitic magnesites. They also concern the compositions of certain blast furnace slags.     

Phase Relations in the System Al2O3–B2O3–Nd2O3

The phase relations at a temperature below "subsolidus" in the system Al₂O₃–B₂O₃–Nd₂O₃ are reported. Specimens were prepared from various compositions of Al₂O₃, B₂O₃, and Nd₂O₃ of purity 99.5%, 99.99%, and 99.9%, respectively, and fired at 1100°C. There are six binary compounds and one ternary compound in this system. The ternary compound, NdAl₃(BO₃)₄ (NAB), has a phase transition at 950°C ± 15°C. The high-temperature form of NAB has a second harmonic generation (SHG) efficiency of KH₂PO₄ (KDP) of the order of magnitude of the form which has been used as a good self-activated laser material, and the low-temperature form of NAB has no SHG efficiency.     

Subsolidus Phase Relationships in the Ga2O3–Al2O3–TiO2 System

Subsolidus phase relationships in the Ga₂O₃–Al₂O₃–TiO₂ system at 1400°C were studied using X-ray diffraction. Phases present in the pseudoternary system include TiO₂ (rutile), Ga_{2−2 x} Al_{2 x} O₃ ( x ≤0.78 β-gallia structure), Al_{2−2 y} Ga_{2 y} O₃ ( y ≤0.12 corundum structure), Ga_{2−2 x} Al_{2 x} TiO₅ (0≤ x ≤1 pseudobrookite structure), and several β-gallia rutile intergrowths that can be expressed as Ga_{4−4 x} Al_{4 x} Ti _{n −4}O_{2 n −2} ( x ≤0.3, 15≤ n ≤33). This study showed no evidence to confirm that aluminum substitution of gallium stabilizes the n =7 β-gallia–rutile intergrowth as has been mentioned in previous work.