Subsolidus Phase Relationship in the CaO–CuO–TiO2 Ternary System at 950°C in Air

The subsolidus phase relationship in the CaO–CuO–TiO₂ ternary system at 950°C in air was investigated. Total 26 samples having various nominal compositions were prepared by the solid‐state reaction at 950°C in air, and their equilibrium phases were analyzed by powder X‐ray diffraction (XRD). The CaCu₃Ti₄O₁₂ phase exhibits variable stoichiometry and forms as the Ca_1?xCu_3+xTi₄O₁₂‐type (?0.019 ≤x ≤0.048) solid solution at 950°C in air. On the basis of our results and previous reports on the binary phase diagrams, the subsolidus phase diagram of the CaO–CuO–TiO₂ ternary system could be constructed at 950°C in air.     

Isothermal phase diagram of CaO–SiO2–Nb2O5-5wt% Fe2O3–TiO2 system at 1200 °C

The lack of thermodynamic data such as the phase diagram of CaO–SiO₂–Nb₂O₅–Fe₂O₃–TiO₂ system has seriously hampered the comprehensive utilization of niobium, titanium and other resources in the Bayan Obo tailing. In this study, phase equilibria of the CaO–SiO₂–Nb₂O₅–Fe₂O₃–TiO₂ system at 1200 °C were investigated using high temperature equilibrium experiment for the first time, and the CaTiO₃–Ca₁₀Nb₂Si₆O₂₇–Ca₂SiO₄–Ca₂Nb₂O₇ solid phase coexistence region was determined. Afterwards, based on the high temperature equilibrium experiments, the liquidus surfaces of the liquid + CaTiO₃ and liquid + SiO₂ equilibrium coexistence regions in CaO–SiO₂–Nb₂O₅-5wt% Fe₂O₃–TiO₂ system at 1200 °C were calculated using mathematical methods of interpolation and fitting. Finally, the 1200 °C isothermal phase diagrams of CaO–SiO₂–Nb₂O₅-5wt%Fe₂O₃–TiO₂ system were plotted. The results of the study can provide theoretical guidance for the enrichment and extraction processes of niobium and titanium resources in the Bayan Obo tailing.     

Phase composition of sintered lime refractories with TiO2 additions

Conclusions A study of mixtures of CaO and TiO₂ fired at 1770 K with various heating rates confirmed the presence of three binary compounds in the system CaO-TiO₂: CaTiO₃, Ca₃Ti₂O₇, and Ca₄Ti₃O₁₀.Mixtures of CaO and TiO₂ of the same composition but fired at different rates develop calcium titanates that are stoichiometrically different: with a heating rate of 50 K/h Ca₃TiO₇ and Ca₄Ti₃O₁₀ are formed; and at 300 K/h — -mainly calcium monotitanate CaTiO₃, with a small amount of Ca₃Ti₂O₇.The features of the crystallochemical structure of calcium titanate indicating that the structure of Ca₃Ti₂O₇ and Ca₄Ti₃O₁₀ can be perceived as alternating layers of CaTiO₃ and CaO mean that we can assume that the first crystal phase developing in the solid-phase sintering in mixtures CaO-TiO₂ is CaTiO₃. Subsequently, during slow heating gradual ordering of the structure of calcium titanates occurs, leading to the formation of equilibrium with a high concentration of CaO (> 88%) in the association of Ca₃Ti₂O₇ and Ca₄Ti₃O₁₀.With increase in the CaO content in calcium titanate there should be an increase in the resistance of lime refractories to atmospheric hydrolysis. The sintering schedule of lime refractories and especially lime clinker should therefore ensure the formation of titanates and solid solutions most highly enriched with CaO.Translated from Ogneupory, No. 2, pp. 15–17, February, 1993.     

Experimental determination of the liquidus line in the high Bi2O3 region in the TiO2–Bi2O3 system

Liquidus line in the high-Bi₂O₃-containing region of the TiO₂–Bi₂O₃ system was determined experimentally. The equilibrating and quenching technique with subsequent electron probe microanalyser (SEM-EDS) microanalysis were employed. Based on the data, liquidus line was constructed between 60 and 92 mol% Bi₂O₃. The current results showed a higher solubility of Bi₂O₃ in the liquid phase in equilibrium with the Bi₄Ti₃O₁₂ compound compared with the existing phase diagram. In addition, differential scanning calorimetry (DSC) was used to estimate the transformations covering the composition range from 60 to 95 mol% Bi₂O₃. Further, the phase diagram of the TiO₂–Bi₂O₃ system was calculated using a quasichemical model for the liquid phase. The thermodynamic properties of the intermediate compounds were estimated from the data of TiO₂ and Bi₂O₃ pure solids.     

Phase equilibria of CaO–SiO2–CaF2–P2O5–Ce2O3 system and formation mechanism of britholite

Recently, the sustainable utilization of REE-bearing slag for the recovery and application of rare-earth elements (REEs) has attracted considerable attention. However, a limited amount of thermodynamic data and crystal information for REE systems has been reported, which greatly limits the utilization of REE-bearing slag. In this study, the isothermal phase diagram of the CaO–SiO₂–CaF₂(30 wt%)-P₂O₅(10 wt%)-Ce₂O₃ system was constructed to provide phase equilibria data for the REEs in REE-bearing slag. The formation mechanism of britholite (Ca_5-xCe_x[(Si,P)O₄]₃F) in the quinary system was found: it evolved from Ca₅(PO₄)₃F, when a Ce³⁺ replaced a Ca²⁺, there would be a SiO₄^4? instead of a PO₄^3?. The phase equilibria and formation mechanism of REEs in the CaO–SiO₂–CaF₂–P₂O₅–Ce₂O₃ system are supplied to provide the data required for sustainable utilization of REE-bearing slag.     

Phase equilibria studies in the CaO–SiO2–Al2O3–MgO system with CaO/SiO2 ratio of 0.9

Phase equilibria and liquidus temperatures in the CaO–SiO₂–Al₂O₃–MgO system at a CaO/SiO₂ weight ratio of 0.9 in the liquid phase have been experimentally determined employing high-temperature equilibration and quenching technique followed by electron probe X-ray microanalysis. Isotherms at 1573, 1623, 1673, and 1773 K were determined and the primary phase fields of wollastonite, melilite, olivine, periclase, spinel, and corundum have been located. Compositions of the olivine and melilite solid solutions were analyzed and discussed. Comparisons between the newly constructed diagram, existing data, and FactSage predicted phase diagrams were performed and differences were discussed. The present study will be useful for guidance of industrial practices and further development of thermodynamic modeling.     

Thermodynamics and crystallization kinetics of REEs in CaO–SiO2–Ce2O3 system

Rare earth elements (REEs) have become increasingly important as ceramic materials. The RE-bearing slags contain massive REEs resources, whereas the lack of thermodynamic and kinetic data of REEs has brought great difficulties to efficient recovery of REEs from RE-bearing slags and the application in ceramics. According to the compositions of the RE-bearing slags in industrial production, the isothermal phase equilibria of CaO–SiO₂–Ce₂O₃ system at 1500°C and 1300°C were constructed by means of liquid-quenching method combined with a series of analyses, which provides the thermodynamic data for the equilibria of REEs. On this basis, the crystallization behaviors of the RE phase (Ce_9.33−xCa_x(SiO₄)₆O_2−0.5x) was investigated, and the temperature range in which the RE phase crystallized singly in RE-bearing slags with a selected compositions was acquired. CCT and TTT diagrams for CaO–SiO₂–Ce₂O₃ system were established to characterize the crystallization kinetics of the RE phase, and the favorable conditions for its crystallization and growth in RE-bearing slags were determined. In this study, the complete thermodynamic and kinetic basic data of REEs in CaO–SiO₂–Ce₂O₃ system are provided for RE-bearing slags.     

Phase equilibria in the CaO-TaO2.5-YO1.5 system and its implication for YTaO4 reactivity with CMAS

Phase equilibria in the CaO-TaO_2.5-YO_1.5 system were experimentally investigated and the isothermal section at 1400 °C was constructed. Ten three-phase equilibrium fields were determined, and the solid solution regions of the binary compounds were analyzed. The Ca₄Ta₂O₉ in the ternary system is marked as (YO_3/2)_x(Ca_2/3Ta_1/3O_3/2)_1?x. Its maximum solubility is the formula of Ca₂YTaO₆. The solubilities of CaO in the M′-YTaO₄ and fluorite phases reach up to about 2.0 mol% and 8.9 mol%, respectively. A ternary pyrochlore-type phase was found, which was expressed as the chemical formula of Ca_0.5–0.5xY_0.75xTa_0.5–0.25xO_1.75. The morphology of pyrochlore was polygonal, and it was the only reaction product when the YTaO₄ oxides were corroded by the molten silicate (CMAS). Since the CaO is the main reactant, the CaO-TaO_2.5-YO_1.5 phase diagram was used to successfully explain the corrosion behavior of YTaO₄. The current experimental phase diagram is important to understand the CMAS degradation of the thermal barrier coatings.     

1250°C liquidus for the CaO–MgO–SiO2–Al2O3–TiO2 system in air

Ti-bearing blast furnace slags have been regarded as an important secondary material in modern society, and the efficient recycling of Ti oxides from it is of key interest. For this reason, more thermodynamic data is needed regarding the phase relations in different composition ranges and sections. Therefore, the equilibrium phase relations of CaO–MgO–SiO₂–Al₂O₃–TiO₂ system in a low w(CaO)/w(SiO₂) ratio of 0.6–0.8 at 1250 °C in air and fixed concentrations of MgO and Al₂O₃, were investigated experimentally using a high temperature equilibration and quenching method followed by SEM-EDS (Scanning Electron Microscope and Energy Dispersive X-ray Spectrometer) analyses. The equilibrium solid phases of perovskite (CaO·TiO₂), a pseudo-brookite solid solution (MgO·2TiO₂, Al₂O₃·TiO₂)_ss, and anorthite (CaO·Al₂O₃·2SiO₂) were found to coexist with the liquid phase at 1250 °C. The calculated results of Factsage and MTDATA were used for comparisons, and significant discrepancies were found between predictions and the experimental results. The 1250 °C isotherm has been constructed and projected on the CaO–SiO₂–TiO₂-8 wt.% MgO-14 wt% Al₂O₃ quasi-ternary plane of the phase diagram. The obtained results provide new fundamental data for Ti-bearing slag recycling processes, and they add new experimental features for thermodynamic modeling of the high-order titanium oxide-containing systems.     

Phase equilibria in the system CaO–SiO2–Nb2O5–La2O3 at 1473 K with pO2=10−15.47 atm

Phase equilibria in the CaO–SiO₂–La₂O₃–Nb₂O₅ system are important for the utilization of Nb and Rare Earth resources. In the present work, phase equilibria in the CaO–SiO₂–La₂O₃–Nb₂O₅ system at 1473 K in H₂ (pO₂ = 10^−15.47atm) were studied by the equilibrium experiment. SEM, EDS and XRD analysis were employed to identify the equilibrium phase relations. The isothermal spatial phase diagram of the CaO–SiO₂–La₂O₃–Nb₂O₅ system and the isothermal section of CaO–SiO₂–Nb₂O₅-(5%, 10%) La₂O₃ pseudo-ternary system were constructed. A total of eight equilibrium phase regions were determined, including Liquid + CaNb₂O₆+CaSiO₃+LaNbO₄ region, Liquid + CaNb₂O₆+SiO₂+CaSiO₃ region, Liquid + CaSiO₃+CaNb₂O₆ region, Liquid + CaSiO₃+SiO₂ region, Liquid + CaNb₂O₆+SiO₂ region, Liquid + CaSiO₃ region, Liquid + SiO₂ region and Liquid + CaNb₂O₆ region. Additionally, the influence of pO₂ on equilibrium composition and temperature was also investigated considering its significant effect on phase equilibria in the slag system with variable valence elements. The results indicated that higher external pO₂ can increase the equilibrium O content in a slag system, while higher equilibrium temperature corresponds to higher equilibrium pO₂. Besides, the relationship between phase equilibria shown in the traditional slag system and in the related generalized alloy system was also revealed. The present experimental data can help further study on Nb-bearing and RE-bearing slag systems, and the theoretical conclusions will help the development of thermodynamic models.     

Thermochemistry of BaSm2O4 and thermodynamic assessment of the BaO–Sm2O3 system

The BaO–Sm₂O₃ system is of interest for the optimization of synthesis of electroceramics. The only systematic experimental study of phase equilibria in the system was performed more than 40 years ago. The reported experimental values of the enthalpy of formation of BaSm₂O₄ are in conflict, and the reported compound Ba₃Sm₄O₉ has never been confirmed. In this work we synthesized BaSm₂O₄ by solid‐state reaction and determined its heat capacity, enthalpy of formation, and phase transitions by differential scanning calorimetry, high‐temperature oxide melt solution calorimetry and ultra‐high‐temperature differential thermal analysis, respectively. We confirmed the existence of Ba₃Sm₄O₉ and its apparent stability from 1873 to 2273 K by X‐ray diffraction on quenched laser‐melted samples but were not able to obtain single‐phase material for calorimetric measurements. The CALPHAD method was used to assess phase equilibria in the BaO–Sm₂O₃ system, using both available literature data and our new measurements. A self‐consistent thermodynamic database and the calculated phase diagram of the BaO–Sm₂O₃ system are provided. This work can be used to model and thus to understand the relationships among composition, temperature, and microstructure for multicomponent systems with BaO and Sm₂O₃.     

Phase equilibria and liquidus surface of CaO-SiO2–5 wt%MgO-Al2O3-TiO2 slag system

The lack of phase equilibria relations and liquidus surface thermodynamic information for CaO-SiO₂–5?wt%MgO-Al₂O₃-TiO₂ system has seriously restricted the comprehensive utilization of the titanium resources. In present study, the phase equilibrium relationships were investigated for CaO-SiO₂–5?wt%MgO-Al₂O₃-TiO₂ phase diagram system at 1300?°C and 1400?°C using the high temperature equilibrium technique followed by X-Ray Fluoroscopy (XRF), X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM) and Energy Dispersive X-ray spectroscope (EDX) analysis. In the composition range studied, the liquid phase, melilite solid solution phase and CaO·TiO₂ phase were found. The two-phase equilibrium of liquid coexisting with CaO·TiO₂ phase was intensively dicussed, and the spatial liquidus surfaces at 1300?°C, 1400?°C and 1450?°C (data from literatures only) for liquid coexisting with CaO·TiO₂ phase were first constructed in CaO-SiO₂–5?wt%MgO-Al₂O₃-TiO₂ tetrahedral phase diagram, which can realize a visual understanding of the phase relation change trendency in 3-dimensional space.     

Thermochemistry of the ZrO2–SrO System: From enthalpies of formation and heat capacities of the compounds to the phase diagram

The thermodynamics of the ZrO₂–SrO system is of interest for the optimization of synthesis and applications of functional materials and high-temperature structural ceramics. Earlier data on phase relationships and thermodynamic properties of the system are unfortunately scattered and inconsistent. In this study, the compounds Sr_n+1Zr_nO_3n+1 (n = 3, 2, and 1) were prepared by solid-state reaction. Their heat capacities from 573 to 1273 K were measured by differential scanning calorimetry and their enthalpies of formation from component oxides at 298 K were determined by high-temperature oxide melt solution calorimetry. The CALPHAD method was used to assess the ZrO₂–SrO system, using both available literature data and our new measurements. A self-consistent thermodynamic database and the calculated phase diagram of the ZrO₂–SrO system are provided. This work is a prerequisite for accurate predictions of the relationships among the composition, temperature, and microstructure of complex functional and structural materials containing ZrO₂ and SrO.     

Phase stability and equilibria in the La2O3–TiO2 system

XRD, electron probe wavelength and energy dispersive X-ray analyses were used to reexamine the phase relations in the La₂O₃–TiO₂ system. The diagram was redrawn to include the compound La₄Ti₃O₁₂ in addition to La₄Ti₉O₂₄, La₂Ti₂O₇ and La₂TiO₅. Above 1455°C a cation deficient perovskite La_2/3TiO₃ is stabilized by a small number of Ti³⁺ ions and remains stable on cooling in air. The proposed diagram represents a section through the system at the normal oxygen pressure in air at 1 atm, compositions being expressed in terms of the oxide components stable at room temperature.     

Thermodynamics of the CaO–SiO2–VOx system at 1873 K under the oxygen partial pressure of 6.9×10−11 atm

Based on the phase equilibria of the CaO–SiO₂–VO_x system determined experimentally at 1873 K and the oxygen partial pressure of 6.9×10^?11 atm, the isothermal section diagram of the system was constructed. Aside from the simple oxides SiO₂ and V₂O₃, three complex crystal phases CaV₂O₄, Ca₂SiO₄ dissolving V₂O₃, and CaVO₃ dissolving SiO₂ were found in the present investigated composition range. The solubility of V₂O₃ in Ca₂SiO₄ phase can reach 5 mass %, and the CaVO₃ phase dissolves about 5% mass SiO₂. Furthermore, the thermodynamic activities of VO_x and SiO₂ in the determined single liquid region were measured by equilibrating the melts with liquid copper. The activity coefficient of SiO₂ decreases linearly with the increase in the basicity of the melts and is almost irrelevant to the total content of vanadium oxides. The activities of vanadium oxides increase slightly with the increase in the basicity but are mainly determined by the total content of vanadium oxides in the melts. With the increase in the basicity, the activity coefficient of VO_1.5 increases almost linearly, whereas those of VO₂ and VO_2.5 decrease gradually.     

Experimental determination of the 1300 °C and 1400 °C isotherms for CaO–SiO2–TiO2-10 wt% Al2O3 system in air

Phase equilibrium relations for the CaO–SiO₂–TiO₂-10 wt%Al₂O₃ system were studied at 1300 °C and 1400 °C by a high temperature equilibration and quenching method, with the purpose of extending the equilibrium phase information for the TiO₂-containing slag systems. Equilibrium phase assemblies and phase compositions were analyzed by Scanning Electron Microscopy (SEM) equipped with an X-ray Energy Dispersive Spectroscopy (EDS). The equilibrium solid phases of perovskite CaO·TiO₂, wollastonite CaO·SiO₂, rutile TiO₂, silica SiO₂ and sphene CaO·SiO₂·TiO₂ were confirmed to coexist with a liquid phase. Based on the experimental results, the 1300 °C and 1400 °C isotherms were constructed in the CaO–SiO₂–TiO₂-10 wt% Al₂O₃ quasi-ternary phase diagram. Significant discrepancies were found between the present results and the simulated results by FactSage and MTDATA databases, as well as the results from previous literature.     

Phase relations of CaO-Al2O3-Sc2O3 ternary system

We investigated the isothermal section of the CaO-Al₂O₃-Sc₂O₃ ternary system at 1773 and 1873 K for 24 hours in Ar, and quenched in water to determine the operative phase equilibrate. The composition of the phases in equilibrium was determined by electron probe microanalysis. The isothermal section phase diagram of two temperature points (1773 and 1873 K) is obtained. The 1773 K isothermal section consists of one liquid compound (L), six binary compounds (CaO+L, Ca₂Sc₆Al₆O₂₀+L C₃A+L, CaO.Sc₂O₃+L, CA+L, Ca₂Sc₆Al₆O₂₀+Sc₂O₃) and seven ternary compounds (Ca₂Sc₆Al₆O₂₀+Sc₂O₃+CA₆, Ca₂Sc₆Al₆O₂₀+Sc₂O₃+L, Ca₂Sc₆Al₆O₂₀+CA+L, Ca₂Sc₆Al₆O₂₀+CA₂+CA, Ca₂Sc₆Al₆O₂₀+CA₂+CA₆, CaO.Sc₂O₃+L+Sc₂O₃, C₃A+CaO+L). At 1873 K, we found one liquid compound (L), five binary compounds (CaO+L, Ca₂Sc₆Al₆O₂₀+L, CaO.Sc₂O₃+L, CA+L, Ca₂Sc₆Al₆O₂₀+Sc₂O₃₎ and six ternary compounds (Ca₂Sc₆Al₆O₂₀+Sc₂O₃+CA₆, Ca₂Sc₆Al₆O₂₀+Sc₂O₃+L, Ca₂Sc₆Al₆O₂₀+CA+L, Ca₂Sc₆Al₆O₂₀+CA₂+CA, Ca₂Sc₆Al₆O₂₀+CA₂+CA₆, CaO.Sc₂O₃+L+Sc₂O₃) to exist at the isothermal section. The experimental information obtained in the present work not only is essential for the thermodynamic assessment of the CaO-Al₂O₃-Sc₂O₃ ternary system, but also is important for further investigation on separation of rare earths from metallurgical slags and rare-earth recovery.     

Selected Phase Equilibria Studies in the Al2O3–CaO–SiO2 System

The Al₂O₃–CaO–SiO₂ system provides the basis for describing many important chemical processes. Although the system has previously been extensively studied, recent advances in experimental technique have provided the opportunity to obtain accurate liquidus measurements in the low‐silica region at fixed temperatures. The experimental procedures involve equilibration of high‐purity oxide powder mixtures at selected temperatures, rapid quenching, and accurate measurement of phase compositions using electron probe X‐ray microanalysis. The liquidus isotherms have been determined at selected temperatures between 1503 and 1873 K in the anorthite, gehlenite, pseudowollastonite, corundum, CaAl₁₂O₁₉, CaAl₂O₆, lime, Ca₃SiO₃, and Ca₂SiO₄ primary phase fields. The results are compared with currently available thermodynamic model predictions of the phase chemistry.