Coupled Experimental and Thermodynamic Optimization of the Na2O–FeO–Fe2O3–Al2O3 System: Part 1. Phase Diagram Experiments

As part of the complete thermodynamic modeling of the Na₂O–FeO–Fe₂O₃–Al₂O₃ system, the Na₂O–FeO–Fe₂O₃–Al₂O₃ phase diagrams in air (1583 and 1698 K) and at Fe saturation (1573 and 1673 K) were investigated using the quenching method followed by Electron Probe Micro‐Analyzer (EPMA) and X‐ray Diffraction (XRD) phase analysis. General features of the phase diagrams in this system were well revealed for the first time. A complete meta‐oxide solid solution between NaAlO₂ and NaFeO₂ was observed. An extensive solid solution of Na₂(Al,Fe)₁₂O₁₉ Na‐β?‐alumina was found and the existence of a miscibility gap in this solution was confirmed. Several compatibility triangles of three‐phase assemblages were also identified in air and at Fe saturation.     

Experimental Phase Diagram Determination and Thermodynamic Assessment of the CeO2‐Gd2O3‐CoO System

New phase diagram data and a thermodynamic assessment of the CeO‐Gd₂O₃‐CoO system using the CALPHAD approach are presented. This information is needed to understand the surprisingly low sintering temperature (950°C–1050°C) of CeO₂‐based materials doped with small amounts of transition metal oxide (e.g., CoO). Experimental phase equilibria between 1100°C and 1300°C are reported based on the analysis of annealed and molten samples. No isolated compound exists in the ternary. At 1300°C the Co solubility in the ternary compounds Ce_1?x?yGd_xCo_yO_2?x/2?y (fluorite) is 2.7 mol% and is less than 1 mol% in the Gd_2?xCe_xO_3+x/2 (bixbyite). The Ce solubility in the perovskite GdCoO_3?δ was found to be 1 mol%. The lowest temperature eutectic melt in the ternary has a composition of 57.2 mol% Co and 41.1 mol% Gd melting at an onset temperature of 1303 ± 5°C, which is close to the binary eutectic in the Gd₂O₃‐CoO system at 60 ± 2 mol% Co and 1348 ± 1°C.     

Critical Evaluation and Thermodynamic Modeling of the MgO–MnO–Mn2O3–SiO2 System

A complete literature review, critical evaluation, and thermodynamic optimization of phase diagrams and thermodynamic properties of the MgO–MnO–Mn₂O₃–SiO₂ system at 1 atm pressure are presented. The molten oxide phase was described by the Modified Quasichemical Model considering the short‐range ordering in molten oxide, and the Gibbs energies of solid solutions were described using the Compound Energy Formalism considering the crystal structure of each solid solution. A set of optimized model parameters of all phases was obtained which reproduces all available and reliable thermodynamic data and phase diagrams within experimental error limits from 25°C to above the liquidus temperatures over the entire range of composition under the oxygen partial pressures from metallic saturation to 1 atm. The database of the model parameters can be used along with software for the Gibbs energy minimization to calculate any phase diagram section and thermodynamic property within the present system.     

High‐Temperature Phase Relations in the Lu2O3–Ta2O5 System

The compounds formed from the Lu₂O₃–Ta₂O₅ system in the composition range 25–60 mol% Ta₂O₅ were prepared by solid‐state reaction from 1350°C to 2058°C, and the phase transitions were investigated by X‐ray diffraction (XRD). Cubic Lu₃TaO₇, M′‐LuTaO₄, M‐LuTaO₄, O‐Ta₂O₅, and T‐Ta₂O₅ are observed. With the temperature increase, there is an irreversible phase transition from M′ to M‐LuTaO₄ near 1770°C in the composition of 30–52 mol% Ta₂O₅, and another phase transition from T‐Ta₂O₅ to O‐Ta₂O₅ at about 1685°C when the ratio of Ta₂O₅ is >52 and ≤60 mol%. A phase diagram of the Lu₂O₃–Ta₂O₅ system in the range 0–100 mol% Ta₂O₅ was constructed. These results are helpful to explain the phase transition of Lu₂O₃–Ta₂O₅ system and design the preparation technique of LuTaO₄ single crystal or ceramic scintillator, which may be applied in the fields of nuclear medicine and high‐energy physics.     

Thermodynamic assessment of the pseudoternary Na2O–Al2O3–SiO2 system

Vitrified high‐level radioactive waste that contains high concentrations of Na₂O and Al₂O₃, such as the waste stored at the Hanford site, can cause nepheline to precipitate in the glass upon cooling in the canisters. Nepheline formation removes oxides such as Al₂O₃ and SiO₂ from the host glass, which can reduce its chemical durability. Uncertainty in the extent of precipitated nepheline necessitates operating at an enhanced waste loading margin, which increases operational costs by extending the vitrification mission as well as increasing waste storage requirements. A thermodynamic evaluation of the Na₂O–Al₂O₃–SiO₂ system that forms nepheline was conducted by utilizing the compound energy formalism and ionic liquid model to represent the solid solution and liquid phases, respectively. These were optimized with experimental data and used to extrapolate phase boundaries into regions of temperature and composition where measurements are unavailable. The intent is to import the determined Gibbs energies into a phase field model to more accurately predict nepheline phase formation and morphology evolution in waste glasses to allow for the design of formulations with maximum loading.     

Thermodynamic Assessment of the Pr–O System

The Calphad method was used to perform a thermodynamic assessment of the Pr–O system. Compound energy formalism representations were developed for the fluorite α‐PrO_2–x and bixbyite σ‐Pr₃O_{5 ± x} solid solutions while the two‐sublattice liquid model was used to describe the binary melt. The series of phases between Pr₂O₃ and PrO₂ were taken to be stoichiometric. Equilibrium oxygen pressure, phase equilibria, and enthalpy data were used to optimize the adjustable parameters of the models for a self‐consistent representation of the thermodynamic behavior of the Pr–O system from 298 K to melting.     

Critical evaluation and thermodynamic optimization of the Na2O–FeO–Fe2O3–Al2O3–SiO2 system

A critical assessment and thermodynamic optimization of phase diagrams and thermodynamic properties of the entire Na₂O–FeO–Fe₂O₃–Al₂O₃–SiO₂ system were carried out at 1 atm total pressure. A set of optimized model parameters obtained for all phases present in this system reproduces available and reliable thermodynamic property and phase equilibrium data within experimental error limits from 298 K to above liquidus temperatures for all compositions and oxygen partial pressures from metallic Fe saturation to 1 atm. The Gibbs energy of liquid solution was described based on the Modified Quasichemical Model considering the possible formation of NaAlO₂ and NaFeO₂ associates in the liquid state. The solid solutions wüstite, spinel, feldspar, nepheline, carnegieite, mullite, corundum, clino-pyroxene, meta-oxides and Na-β″-alumina were treated within the framework of Compound Energy Formalism. The database of model parameters can be used to calculate any thermodynamic property and phase diagram section of the present system.     

Critical Evaluation and Thermodynamic Modeling of the Mg–Mn–O (MgO–MnO–MnO2) System

All phase diagram and thermodynamic data of the MgO–MnO and MgO–MnO–MnO₂ systems were critically evaluated and thermodynamically optimized to obtain one set of thermodynamic model parameters. The MgMn₂O₄–Mn₃O₄ spinel solutions were modeled with two sublattices Compound Energy Formalism by considering the cation distribution between tetrahedral and octahedral sites. The slag phase and monoxide solid solution were also described using the Modified Quasichemical Model and Bragg–Williams random mixing model, respectively. The optimized thermodynamic model parameters can be used with a Gibbs energy minimization routine to calculate the phase diagram and thermodynamic properties of the system at oxygen partial pressures between metal saturation and 1 atm.     

Thermodynamic Modeling of the HfO2–La2O3–Al2O3 System

Based on phase equilibria, thermodynamic, and crystal structure data, the thermodynamic modeling of HfO₂–La₂O₃–Al₂O₃ system is presented. Liquid phase is described by the modified quasichemical model considering the short‐range ordering in liquid solution. Solid solutions are described by the ionic sublattice model considering respective crystal structure. The model (La³⁺, Hf⁴⁺)₂(Hf⁴⁺, La³⁺)₂(O^2?, Va)₆(O^2?)₁(Va, O^2?)₁ successfully describes the structure defect, homogeneity range, and thermodynamic property of pyrochlore solid solution. A set of optimized model parameters is obtained which reproduces most experimental data well. Isothermal sections, liquidus and solidus projections, and Scheil reaction scheme are constructed.     

Thermal Conductivity of Molten B2O3, B2O3–SiO2, Na2O–B2O3, and Na2O–SiO2 Systems

With the aid of the transient hot‐wire method, the thermal conductivity of molten B₂O₃, B₂O₃–SiO₂, Na₂O–SiO₂, and Na₂O–B₂O₃ systems was measured along with their temperature and composition. It was observed that the thermal conductivity of pure B₂O₃ increased with temperature, until about 1400 K, and then decreased subsequently. Using the MAS‐NMR, 3Q‐MAS, and Raman spectroscopy, the structure of B₂O₃ and SiO₂ in the B₂O₃–SiO₂ system was confirmed. Findings show that an addition of B₂O₃ into the pure SiO₂ system causes a significant decrease in thermal conductivity, due to the formation of boroxol rings. The thermal conductivity of the Na₂O–SiO₂ system was measured and its phonon mean free path was calculated. In addition, a positive linear relation between viscosity and thermal conductivity was observed. In the Na₂O–B₂O₃ system, it was found that a change in the relative fraction of 4‐coordinated boron has an influence on the thermal conductivity when the concentration of Na₂O is between 10 and 30 mol%, in which case the tetraborate unit is dominant.     

New calcium‐free Na2O–Al2O3–P2O5 bioactive glasses with potential applications in bone tissue engineering

Sodium aluminophosphate glasses were evaluated for their bone repair ability. The glasses belonging to the system 45Na₂O–xAl₂O₃‐(55‐x)P₂O₅, with x = (3, 5, 7, 10 mol%) were prepared by a melt‐quenching method. We assessed the effect of Al₂O₃ content on the properties of Na₂O–Al₂O₃–P₂O₅ (NAP) glasses, which were characterized by density measurements, DSC analyses, solubility, bioactivity in simulated body fluid and cytocompatibility with MG‐63 cells. To the best of our knowledge, this is the first investigation of calcium‐free Na₂O–Al₂O₃–P₂O₅ system glasses as bioactive materials for bone tissue engineering.     

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.     

Control of Oxidation State of Eu Ions in Na2O–Al2O3–SiO2 Glasses

Glasses doped with well‐controlled Eu³⁺ and Eu²⁺ ions have attracted considerable interest due to the possibility of tuning the wavelength range of the emitted light from violet to red by using their ⁵D₀→⁷F_j and 5d–4f electron transitions. Glasses were prepared to dope Eu³⁺ ions in a Na₂O–Al₂O₃–SiO₂ system, and the changes in the valence state of Eu³⁺ ions and the glass structure surrounding the Eu atoms during heating under H₂ atmosphere were investigated using fluorescence spectroscopy, X‐ray absorption fine‐structure spectroscopy, and ²⁷Al magic‐angle spinning solid‐state nuclear magnetic resonance spectroscopy. The reduction behavior of Eu³⁺ ions was dependent on the Al/Na molar ratio of the glass. For Al/Na < 1, the Al³⁺ ions formed the AlO₄ network structure accompanied by the Na⁺ ions as charge compensators; the Eu³⁺ ions occupied the interstitial positions in the SiO₄ network structure and were not reduced even under heating in H₂ gas. On the other hand, in the glasses containing Al₂O₃ with the Al/Na ratio exceeding unity, the Eu³⁺ ions commenced to be coordinated by the AlO₄ units in addition to the SiO₄ network structure. When heated in H₂ gas, H₂ gas molecules reacted with the AlO₄ units surrounding Eu³⁺ ions to form AlO₆ units terminated with OH bonds, and reduced Eu³⁺ ions to Eu²⁺ via the extracted electrons.     

Structure Property Relationship in BaTiO3–Na0.5Bi0.5TiO3–Nb2O5–NiO X8R System

A novel BaTiO₃–Na_0.5Bi_0.5TiO₃–Nb₂O₅–NiO (BT‐NBT‐Nb‐Ni) system that meets the X8R specification (?55°C–150°C, ΔC/C≤±15%) of multilayer ceramic capacitors (MLCCs) was fabricated, with a maximum dielectric constant of 2350 at room temperature (25°C). Core–shell microstructure was observed by transmission electron microscopy (TEM), accounting for the good dielectric temperature stability. The role of Ni on the formation of core–shell structure and phase structure, and the subsequent relationship between structure and dielectric/ionic conduction properties were investigated. It was observed that the addition of Ni could adjust the ratio of core/shell, and significantly reduces the dielectric loss over the studied temperature range. A new Ba₁₁(Ni, Ti)₂₈O_66+x phase with a 10‐layer close‐packed structure was identified by X‐ray diffraction (XRD), serving as a source of oxygen vacancies for ionic conduction in addition to Ba(Ni,Ti)O₃. Furthermore, the impedance spectroscopy measurements demonstrated the remarkable impact of these Ni‐induced oxygen vacancies on both the grain and grain‐boundary conductivities.     

Experimental phase equilibria studies of the PbO–SiO2 system

Phase equilibria of the PbO–SiO₂ system have been established for a wide range of compositions: (i) liquid in equilibrium with silica polymorphs (quartz, tridymite, and cristobalite) between 740°C and 1580°C, at 60‐90 mol% SiO₂; (ii) with lead silicates (PbSiO₃, Pb₂SiO₄, and Pb₁₁Si₃O₁₇) and lead oxide (PbO) between 700°C and 810°C. A high‐temperature equilibration/quenching/electron probe X‐ray microanalysis (EPMA) technique has been used to accurately determine the compositions of the phases in equilibrium in the system. Significantly, no liquid immiscibility has been found in the high‐silica range, and the liquidus in this high‐silica region has been accurately measured. The phase equilibria information in the PbO–SiO₂ system is of practical importance for the improvement of the existing thermodynamic database of lead‐containing slag systems (Pb–Zn–Fe–Cu–Si–Ca–Al–Mg–O).     

Thermal Conductivity of Molten Li2O–B2O3 and K2O–B2O3 Systems

Using the transient hot‐wire method, the thermal conductivity properties of the molten Li₂O–B₂O₃ and K₂O–B₂O₃ systems were measured. The thermal conductivity increases with decreasing the temperature due to the borate structure change. In addition, calculations of the one‐dimensional Debye temperature and the phonon mean free paths as a function of temperature of the alkali borate systems were made. At a fixed temperature of 1273 K, the effect of the alkali oxide concentration on the thermal conductivity was evaluated. Within a range of 10–30 mol% Li₂O (or K₂O), a positive relationship between the thermal conductivity and 4‐coordinate boron was obtained. However, below 10 mol% Li₂O (or K₂O), the change in the intermediate‐range order of the borate structure had a more dominant effect on the thermal conductivity. Finally, the effect of cations on the thermal conductivity in the various molten R₂O–B₂O₃ (R=Li, Na and K) systems was considered. Depending on the type of cation, the change in the ionization potential had an effect on the thermal conductivity and also resulted in a change in the bond strength.     

High‐temperature calorimetric study of oxide component dissolution in a CaO–MgO–Al2O3–SiO2 slag at 1450°C

Blast‐furnace slags are formed, as iron ore is reduced to metal, as a molten a mixture of refractory and not easily reducible oxides, largely silica, alumina, lime, and magnesia. Their relatively low silica content makes them basic and poor glass formers. Their thermodynamic properties, though important for modeling their formation and reactivity, as well as furnace heat balance, are poorly known. Solution calorimetry of small amounts of solid oxides in a molten oxide solvent at high temperature (up to about 1500°C) permits direct assessment of energetics of dissolution. The enthalpies of solution of slag forming oxides: CaO, SiO₂, Al₂O₃, MgO, and Fe₂O₃ in a simplified model slag of composition: CaO (45.9 mol%), SiO₂ (35.1 mol%), Al₂O₃ (8.3 mol%), MgO (10.7 mol%) were measured by high‐temperature drop solution calorimetry at 1450°C. For this slag composition, enthalpies of solution become more exothermic in the order: Fe₂O₃ (279.3 ± 20.8 kJ/mol), MgO (56.7 ± 9.1 kJ/mol), Al₂O, (41.6 ± 11.3 kJ/mol), CaO (?4.3 ± 2.3 kJ/mol), and SiO₂, (?20.4 ± 4.4 kJ/mol), reflecting the relatively basic character of this low‐silica melt. Within these fairly large experimental errors, characteristic of calorimetry at this high temperature, there is little or no discernible concentration dependence for these heats of solution. The trends seen for these five solutes parallel those seen for heats of solution of the same oxides in other melts at various temperatures, with changes in magnitude reflecting the differences in acid‐base character of the melts. The new data for quartz show systematic behavior which extends the range of basicity studied for the enthalpy of dissolution of silica. The results provide reliable data for future modeling of the thermal balance of steel‐making furnaces and geologic and ceramic systems.     

Phase relation studies in the CeO2–La2O3–Er2O3 system at 1500°C

Materials based on CeO₂–La₂O₃–Er₂O₃ system are promising candidates for a wide of applications, but the phase relationship has not been studied systematically previously. To address this challenge, the isothermal section of the phase diagram for 1500 °C was investigated. The phase relations in the CeO₂–La₂O₃–Er₂O₃ ternary system at 1500 °C were studied by X-ray diffraction and scanning electron microscopy in the overall concentration range. To study phase relationships at 1500 °C the as-repared samples were thermally treated in two stages: at 1100 °C (for 300 in air) and then at 1500 °C (for 70 h in air) in the furnaces with heating elements based on Fecral (H23U5T) and Superkanthal (MoSi2), respectively. The solid solutions based on various polymorphous forms of constituent phases and with perovskite-type structure of LaErO₃ (R) with orthorhombic distortions were revealed in the system. No new phases were found. The isothermal section of the phase diagram for the CeO₂–La₂O₃–Er₂O₃ system has been constructed. It was established that in the ternary CeO₂–La₂O₃–Er₂O₃ system there exist fields of solid solutions based on hexagonal (A) modification of La₂O₃, cubic modification of CeO₂ with fluorite-type structure (F), cubic modification Er₂O₃ and with perovskite-type structure of LaErO₃ (R) with orthorhombic distortions. The maximal solubility of ceria in LaErO₃ was found to be around ∼ 2 mol% CeO₂ along the section CeO₂–(50 mol % La₂O₃ –50 mol% Er₂O₃).     

Oxygen‐ion conduction in scandia‐stabilized zirconia‐ceria solid electrolyte (xSc2O3–1CeO2–(99−x)ZrO2, 5 ≤ x ≤ 11)

Solid oxide fuel cells (SOFCs) operating at intermediate temperature (500°C‐700°C) provide advantages of better durability, lower cost, and wider target application market. In this work, we have studied Sc₂O₃ (5‐11 mol%) stabilized ZrO₂–CeO₂ as a potential solid electrolyte for application in IT‐SOFCs. Lower Sc₂O₃ doping range than the traditional 11 mol% Sc₂O₃‐stabilized ZrO₂ is an interesting research topic as it could potentially lead to an electrolyte with reduced oxygen vacancy ordering, lower cost, and higher mechanical strength. XRD and Raman spectroscopy was used to study the phase equilibrium in ZrO₂–CeO₂–Sc₂O₃ system and impedance spectroscopy was done to estimate the grain, grain boundary, and total ionic conductivities. Maximum for the grain and grain‐boundary conductivities as well as the tetragonal‐cubic phase boundary was found at 8‐9 Sc₂O₃ mol% in ZrO₂‐1 mol% CeO₂ system. It is suggested that the addition of 1 mol% CeO₂ in the ZrO₂ host lattice has improved the phase stability of high‐conductivity cubic and tetragonal phases at the expense of low‐conductivity t′‐ and β‐phases.     

Phase Relationship and Ionic Conductivity in Na–SrSiO3 Ionic Conductor

The Na–SrSiO₃ as a potential high‐conductivity ionic conductor for intermediate temperature solid oxide electrochemical cells (SOECs) has drawn much attention recently. Some of these studies questioned the feasibility of Na doping and therefore the creation of oxygen vacancies, while others suggested an alternative phase responsible for the ionic conduction. In this study, a systematic investigation was carried out to understand the ionic conduction in Na–SrSiO₃. Through in situ high‐temperature X‐ray diffraction, thermal analysis, microstructural characterization, and electrical conductivity measurement, Na–SrSiO₃ was shown as a two‐phase material, one being slightly Na‐doped SrSiO₃ and another being amorphous Na₂Si₂O₅. The former was an electrical insulator whereas the latter was a good ionic conductor. It was also found that the amorphous Na₂Si₂O₅ phase was unstable at the temperature ≥500°C, crystallizing into the insulating polycrystalline Na₂Si₂O₅ which causes conductivity to “bend‐over” at higher temperatures. A preliminary Ab Initio Molecular Dynamics (AIMD) simulation suggested that the amorphous Na₂Si₂O₅ be predominantly a Na⁺ conductor.