Thermodynamic modeling of the CaO-SiO2-ZrO2 system coupled with key phase diagram experiments

Thermodynamic modeling coupled with key phase diagram experiments of the ternary CaO-SiO₂-ZrO₂ system was carried out. The isothermal phase diagram of the CaO-SiO₂-ZrO₂ system at 1873 K was established by using a classical phase equilibration and quenching technique followed by EPMA phase analysis. The Gibbs energy of each phase was optimized through critical evaluation of all available thermodynamic properties and phase diagram data in literature with new experimental data. The discrepancies between experimental phase diagram data and possible errors in the existing thermodynamic data were resolved in this study.     

Isothermal section of CaO-SiO2-La2O3 system within specific region at 1673–1473 K

So far, the relevant phase equilibrium relations and the type of equilibrium phase fields in the CaO-SiO₂-La₂O₃ basic slag system phase diagram are still unknown, which restricts the smelting process and application in materials of rare earth elements. In the current work, phase equilibrium relations within specific region of CaO-SiO₂-La₂O₃ system at 1673–1473?K were studied experimentally by using the thermodynamic equilibrium experiment followed by X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive spectrometer (EDS). According to the experimental results, the existence of ternary compound CaO·3SiO₂·2La₂O₃ was determined, it is confirmed to be a solution solid phase. The sub-solidus phase relations between different solid phases were also determined. Finally, the isothermal sections of CaO-SiO₂-La₂O₃ system within specific region at 1673?K, 1573?K and 1473?K were obtained, respectively. The experimental results can not only enrich the phase diagram information of silicate system, but also have practical significance for the application of rare earth in materials.     

Phase equilibria in the La2O3-Sm2O3-ZrO2 system: Experimental studies and thermodynamic modeling

A series of ceramic samples were prepared to experimentally investigate sub-solidus phase relations in the La₂O₃-Sm₂O₃-ZrO₂ system at 1873 K and 1673 K. No ternary compounds have been observed, while the binary La₂Zr₂O₇ and Sm₂Zr₂O₇ pyrochlore phases form a continuous solid solution La_2?xSm_xZr₂O₇ in the ternary system at the selected temperatures. X-ray diffraction and microstructure results demonstrated that the pyrochlore phase is stable in the ZrO₂-rich corner. The homogeneity range of the pyrochlore phase was carefully determined and the phase boundary of the cubic ZrO₂ (fluorite phase) which extends into the ternary system was also constructed via electron probe microanalysis. The as-obtained data were adopted to determine the mixing parameters for the pyrochlore and fluorite phases in the present thermodynamic modeling. A self-consistent database of the La₂O₃-Sm₂O₃-ZrO₂ system was accordingly established for the first time and the calculations agree well with the experimental data in the current work.     

Thermodynamic assessments of ZrO2-YO1.5-TiO2 system

The ZrO₂–TiO₂ and ZrO₂-YO_1.5 binary systems have been reassessed based on the latest literature information, then the thermodynamic parameters of the two systems combine with that of the YO_1.5-TiO₂ system have been used to extrapolate the thermodynamic database of ZrO₂-YO_1.5-TiO₂ ternary system. Based on the available experimental information, the ternary interaction parameters of liquid, F (fluorite), and P (pyrochlore) phases are introduced to fix this ternary system. Especially, the calculated solid solubility limit of Tss (tetragonal) and F phases are first considered in this work, which are consistent with the reported experiments. Finally, the isothermal sections calculated at 1573 K, 1773 K, 1823K 1873K and 1923K (1300 °C, 1500 °C, 1550 °C, 1600 °C and 1650 °C) are plotted, which are more consistent with the experimental results than other calculations.     

Phase equilibria of SiO2–Ce2O3–CaO-25 wt %Al2O3 system at 1673 K–1773 K

The utilization of rare earth resources, especially secondary resources (e.g., RE-oxide system slag), has been limited by the lack of thermodynamic information. In order to supplement and improve the thermodynamic data related to rare earth, the equilibrium experiments of SiO₂–Ce₂O₃–CaO-25 wt %Al₂O₃ system phase diagram was carried out at 1673 K and 1773 K by the high-temperature isothermal equilibration/quenching technique in current paper. The composition of seven phase regions were determined by FE-SEM, XRD, EPMA and XRF analysis on the samples obtained by high temperature equilibrium technology at 1673 K and 1773 K, including the primary crystal regions of three compounds (C₂AS, 2CaO·SiO₂, CaO·2Ce₂O₃·3SiO₂) and three three-phase coexistence regions (L + C₂AS + 2CaO·SiO₂, L + C₂AS + CaO·2Ce₂O₃·3SiO₂, L + CaO·2Ce₂O₃·3SiO₂+CeAl₁₁O₁₈) and a liquid region. The phase relations and isotherms of SiO₂–Ce₂O₃–CaO-25 wt %Al₂O₃ system obtained in current work are beneficial to the recycling of rare earth resources containing cerium.     

Phase diagram study and thermodynamic modeling of the MgO-Y2O3-MgF2-YF3 system

The phase diagram of the MgO-Y₂O₃-MgF₂-YF₃ system (Mg,Y//O,F reciprocal system) at 1273–1773 K was investigated for the very first time using a classical equilibrium/quenching and differential thermal analysis (DTA) experiments followed by electron probe microanalysis (EPMA) and X-ray diffraction (XRD) phase analyses. No ternary or quaternary crystalline phase was found, and the eutectic reactions in the reciprocal system were identified. The overall phase diagram of the reciprocal system was also calculated based on the thermodynamic modeling using the CALculation of PHAse Diagram (CALPHAD) method.     

Fabrication of transparent ceramics through melt solidification of near eutectic compositions in HfO2–Al2O3–GdAlO3 system

Solidification of eutectic melts in multiple oxide systems can produce directionally solidified eutectic composites by slow cooling, while rapid cooling would give the formation of amorphous phases as super cooled liquids. We have successfully fabricated an amorphous bulk ceramics in the ternary system HfO₂–Al₂O₃–GdAlO₃ for the first time. It has the near eutectic composition of HfO₂ (14 mol%), Al₂O₃ (63 mol%) and Gd₂O₃ (23 mol%) and highly transparent, >85%, in the visible region after the cooling of around 200–500 K/s for 2–5 mm Ø globules. The sample had kept amorphous up to 1073 K but crystallized above 1273 K then lost the transparency. The formation of an amorphous phase could be discussed by the equilibrated temperature (T₀) lines in meta-stable phase diagram. The present study suggests possible formation of transparent bulk ceramics by the melt-solidification of eutectic melts in various ternary or multiple phase systems.     

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.     

Thermodynamic Properties of LaCrO4, La2CrO6, and La2Cr3O12, and Subsolidus Phase Relations in the System Lanthanum–Chromium–Oxygen

In the system La–Cr–O, there are three ternary oxides (LaCrO₄, La₂Cr₃O₁₂, and La₂CrO₆) that contain Cr in higher valence states (V or VI). On heating, LaCrO₄ decomposes to LaCrO₃, La₂Cr₃O₁₂ to a mixture of LaCrO₄ and Cr₂O₃, and La₂CrO₆ to LaCrO₃ and La₂O₃ with loss of oxygen. The oxygen potentials corresponding to these decomposition reactions are determined as a function of temperature using solid‐state cells incorporating yttria‐stabilized zirconia as the electrolyte. Measurements are made from 840 K to the decomposition temperature of the ternary oxides in pure oxygen. The standard Gibbs energies of formation of the three ternary oxides are derived from the reversible electromotive force (EMF) of the three cells. The standard enthalpy of formation and standard entropy of the three ternary oxides at 298.15 K are estimated. Subsolidus phase relations in the system La–Cr–O are computed from thermodynamic data and displayed as isothermal sections at several temperature intervals. The decomposition temperatures in air are 880 (±3) K for La₂Cr₃O₁₂, 936 (±3) K for LaCrO₄, and 1056 (±4) K for La₂CrO₆.     

Phase diagram of the Al2O3–ZrO2–La2O3 system

The phase diagram of the Al₂O₃–ZrO₂–La₂O₃ system was constructed in the temperature range 1250–2800 °C. The liquidus surface of the phase diagram reflects the preferentially eutectic interaction in the system. Three new ternary and two new binary eutectics were found. The minimum melting temperature is 1665 °C and it corresponds to the ternary eutectic LaAlO₃ + T-ZrO₂ +  La₂O₃·11Al₂O₃. The solidus surface projection and the schematic of the alloy crystallization path confirm the preferentially congruent character of phase interaction in the ternary system. The polythermal sections present the complete phase diagram of the Al₂O₃–ZrO₂–La₂O₃ system. No ternary compounds or regions of remarkable solid solution were found in the components or binaries in this ternary system. The latter fact is the theoretical basis for creating new composite ceramics with favorable properties in the Al₂O₃–ZrO₂–La₂O₃ system.     

Equilibrium phases in the Bi2O3–TiO2–WO3 system

Phase relations in the Bi₂O₃–TiO₂–WO₃ ternary system were evaluated for different compositions calcined in air at 850 °C by means of XRD techniques. A solid solution area was observed for Bi₄Ti₃O₁₂ compositions with a small amount of WO₃. The experimental results suggest the existence of a new phase with a nominal composition close to Bi₃Ti_2.5W_0.5O₁₁. This phase allows the definition of four triangles of compatibility at 850 °C in the ternary system. The new phase was characterized by XRD, SEM and EDS.     

Effects of oxygen partial pressure on the thermodynamics of CaO–SiO2–VOx system at 1873 K

In this study, two aspects concerning with the thermodynamics of CaO–SiO₂–VO_x system were investigated. One aspect is about the effects of oxygen partial pressure on the phase relationships in CaO–SiO₂–VO_x system at 1873 K, and the other is the measurement of the standard Gibbs energy of formation of the vanadium compounds. In the first aspect, the phase relationships in CaO–SiO₂–VO_x system at 1873 K under oxygen partial pressure of 1.7 × 10⁻⁹ atm were determined, and the isothermal section diagram was constructed. Furthermore, by comparing this diagram with that under 6.9 × 10⁻¹¹ atm, the effects caused by oxygen partial pressure on this system were elucidated. With the increase in oxygen partial pressure, the compound CaV₂O₄ that exists stably under 6.9 × 10⁻¹¹ atm is oxidized to V₂O₃ and CaVO₃. The solubility limit of V₂O₃ in liquid phase also increases remarkably, which results in the enlargement of the single liquid area. In the second aspect, the standard Gibbs energy of formation of CaV₂O₄, CaVO₃, V₂O₃, and Ca₂Si_1−δV_δO₄ (0 < δ < 0.1) at 1873 K were measured. The determined values for CaV₂O₄ and CaVO₃ are −1 259 468.9 ± 5090.5 J/mol and −963 479.8 ± 3298.6 J/mol, respectively. Then, the limit of oxygen partial pressure for CaV₂O₄ existing stably at 1873 K is determined to be 4.4 × 10⁻¹⁰ atm.     

Subsolidus Phase Relations of the BaO–Y2O3–MnO2 System in Air

The subsolidus phase relations of the BaO–Y₂O₃–MnO₂ system have been investigated in air. There are eight binary compounds, a new ternary compound and 11 three‐phase regions in this system. The ternary compound with the BaO:YO_1.5:MnO₂ molar ratio of 8:2:5 was indexed by a rhombohedral lattice with a = 5.7929 (6) and c = 28.586 (4) Å. The synthesized compounds were determined to be stoichiometric except for the Ba_1?xY_xMnO₃ phase (x ≤ 0.01).     

Adjacent relations of primary phase fields and invariant reactions of the system CaO-SiO2-Nb2O5-La2O3

The adjacent relation of primary phase fields and corresponding invariant reactions of the system CaO-SiO₂-Nb₂O₅-La₂O₃ are of great importance for the study on its phase diagram. In the present work, the phase equilibrium in the high w(La₂O₃) region of CaO-SiO₂-Nb₂O₅-La₂O₃ system was studied by thermodynamic equilibrium experiment. The adjacent relation of primary phase fields was determined and represented in the form of adjacent tetrahedrons. The Alkemade Rule applicable to quaternary phase diagram was deduced, which can be used to infer the liquidus temperature trend on univariant curves. The rule was then used to determine the possible temperature range of invariant reactions corresponding to the adjacent tetrahedron in CaO-SiO₂-Nb₂O₅-La₂O₃ system, and the result was shown in the form of Schairer diagram. Finally, the reaction types of five invariant points were determined according to the Lever Rule for quaternary phase diagram, including: ① L₁+CaO·3SiO₂·2La₂O₃→CaO·SiO₂+SiO₂+La₂O₃·Nb₂O₅, ② L₂→CaO·SiO₂+La₂O₃·Nb₂O₅+SiO₂+CaO·Nb₂O₅, ③ L₃+CaO·3SiO₂·2La₂O₃+2CaO·Nb₂O₅→10CaO·6SiO₂·Nb₂O₅+La₂O₃·Nb₂O₅, ④ L₄+10CaO·6SiO₂·Nb₂O₅→CaO·SiO₂+2CaO·Nb₂O₅+La₂O₃·Nb₂O₅, ⑤ L₅+2CaO·Nb₂O₅→CaO·Nb₂O₅+La₂O₃·Nb₂O₅+CaO·SiO₂.     

The MgO–TiO2–SiO2 system: Experiments and thermodynamic assessment

Phase relations in the MgO–TiO₂–SiO₂ system have been investigated in air over a wide temperature range using the equilibration method. X-ray powder diffraction, scanning electron microscopy combined with wave length X-ray spectroscopy (SEM/EPMA), and differential thermal analysis (DTA) have been used for sample characterization. Based on the obtained experimental results, isothermal sections of the system at 1523, 1673, and 1773 K have been established. The solid-state invariant reaction MgTi₂O₅ + T-SiO₂⇋P-MgSiO₃ + TiO₂ has been detected at 1625 ± 8 K by step-wise heat treatment. A partial liquidus projection has been suggested, and the temperatures and compositions of three eutectic invariant reactions have been experimentally measured by DTA and ex-situ analysis of the sample microstructures after melting using SEM/EPMA. Considering the newly obtained experimental data, thermodynamic parameters describing the system have been thermodynamically evaluated within the CALPHAD approach.