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.     

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.     

Effect of Nd2O3 Concentration on the Defect Structure of CeO2–Nd2O3 Solid Solution

An extensive X-ray study of CeO₂–Nd₂O₃ solid solutions was performed, and the densities of solid solutions containing various concentrations of NdO_1.5 were measured using several techniques. Solid solutions containing 0–80 mol% NdO_1.5 were synthesized by coprecipitation from Ce(NO₃)₃ and Nd(NO₃)₃ aqueous solutions, and the coprecipitated samples were sintered at 1400°C. A fluorite structure was observed for CeO₂–NdO_1.5 solid solutions with 0–40 mol% NdO_1.5, which changed to a rare earth C-type structure at 45–75 mol% NdO_1.5. The change in the lattice parameters of CeO₂–NdO_1.5 solid solutions, when plotted with respect to the NdO_1.5 concentration, showed that the lattice parameters followed Vegard's law in both the fluorite and rare earth C-type regions. The maximum solubility limit for NdO_1.5 in CeO₂ solid solution was approximately 75 mol%. The relationship between the density and the Nd concentration indicated that the defect structure followed the anion vacancy model over the entire range (0–70 mol% NdO_1.5) of solid solution.     

High-Temperature Neutron Diffraction of the AlN–Al2O3–Y2O3 System

The importance of aluminum nitride (AlN) stems from its application in microelectronics as a substrate material due to high thermal conductivity, high electrical resistance, mechanical strength and hardness, thermal durability, and chemical stability. Yttria (Y₂O₃) is the best additive for AlN sintering. AlN densifies by a liquid-phase mechanism, where the surface oxide, Al₂O₃, reacts with Y₂O₃ to form an Y-Al-O-N liquid that promotes particle rearrangement and densification. Construction of the phase relations in this multicomponent system is essential for optimizing the properties of AlN. The ternary phase diagram of the AlN–Al₂O₃–Y₂O₃ was developed by Gibbs energy minimization using interpolation procedures based on modeling the binary subsystems. This paper aims at testing the resultant understanding experimentally at selected compositions using in situ high-temperature neutron diffractometry. These experimental results agree with the thermodynamic calculations of AlN–Al₂O₃–Y₂O₃. The ternary phase diagram has been constructed for the first time in this work. High-temperature neutron diffractometry has permitted real time measurement of the reactions involved in this ternary system, especially to determine the temperature range for each reaction, which would have been difficult to establish by other means.     

Phase Relations in the Garnet Region of the System Y2O3–Fe2O3–FeO–Al2O3

Liquidus equilibrium relations for the air isobaric section of the system Y₂O₃–Fe₂O₃–FeO–Al₂O₃ are presented. A Complete solid-solution series is found between yttrium iron garnet and yttrium aluminum garnet as well as extensive solid solutions in the spinel, hematite, orthoferrite, and corundum phases. Minimum melting temperatures are raised progressively with the addition of alumina from 1469°C in the system Y–Fe–O to a quaternary isobaric peritectic at 1547°C and composition Y _0.22 Fe _1.08 Al _0.70 O _2.83* Liquidus temperatures increase rapidly with alumina substitutions beyond this point. The thermal stability of the garnet phase is increased with alumina substitution to the extent that above composition Y _0.75 Fe _0.65 Al _0.60 O ₃ garnet melts directly to oxide liquid without the intrusion of the orthoferrite phase. Garnet solid solutions between Y _0.75 Fe _1.25 O ₃ and Y _0.75 Fe _0.32- Al _0.93 O ₃ can be crystallized from oxide liquids at minimum temperatures ranging from 1469° to 1547°C, respectively. During equilibrium crystallization of the garnet phase, large changes in composition occur through reaction with the liquid. Unless care is taken to minimize temperature fluctuations and unless growth proceeds very slowly, the crystals may show extensive compositional variation from core to exterior.     

Phase Equilibria in the System HfO2–Y2O3–CaO

Phase equilibria in the system HfO₂–Y₂O₃–CaO were studied in the temperature range 1250° to 2850°C by both experimental methods (X-ray phase analysis at 20° to 2000°C, petrography, annealing and quenching, differential thermal analysis in He at temperatures to 2500°C, thermal analysis in air using a solar furnace at temperatures to 3000°C, and electron microprobe X-ray analysis) and theoretical means (development of a mathematical model for the liquidus surface by means of the reduced polynomial method). Phase equilibria were determined by the structure of the restricting binary systems. No ternary compounds were found. The liquidus was characterized by the presence of six four-phase, invariant equilibria. Solid solutions were based on monoclinic (M), tetragonal (T), and cubic (F) modifications of HfO₂; C and H forms of Y₂O₃; CaO; and CaHfO₃ that crystallized in two polymorphous modifications, namely, the cubic and rhombic perovskite-type structure.     

High-Temperature Environmental Stability of the Compounds in the Al2O3–Y2O3 System

Heat treatments in several environments were performed on a series of compounds in the Al₂O₃ and Y₂O₃ system: Al₂O₃Y₃Al₅O₁₂ eutectic, Y₃Al₅O₁₂, YAlO₃, Y₄Al₂O₉, and Y₂O₃. The yttrium aluminates were found to be stable at high temperatures under vacuum and in air. However, when they were heat-treated under vacuum in proximity to SiC, degradation was observed. This was found to be primarily a result of carbothermal reduction. In a similarly reducing environment without Si, the yttrium aluminates, and Al₂O₃ and Y₂O₃, all exhibited degradation by carbothermal reduction. Based upon the experimental results, a degradation mechanism for yttrium aluminates was proposed.     

Phase Relations in the System UO2-UO3– Y2O3

The phase relations in the system U02-U03-Yz03, particularly in the Y203-rich region, were examined by X-ray and chemical analyses of reacted powders heated at temperatures up to 1700°C in H₂, CO₂-CO₂ and air. Four phases were identified in the system at temperatures between 1000° and 1700°C: U308, face-centered cubic solid solution, body-centered cubic solid solution, and a rhombohedral phase of composition (U,Y)₇O₂ ranging from 52.5 to 75 mole % Y₂O₃. The rhombohedral phase oxidized to a second rhombohedral phase with a nominal composition (U,Y), at temperatures below 1000°C. This phase transformed to a face-centered cubic phase after heating in air above 1000° C. The solubility of UO, in the body-centered cubic phase is about 14 mole % between 1000° and 1700°C but decreases to zero as the uranium approaches the hexavalent oxidation state. The solubility of Yz03 in the face-centered cubic solid solution ranges from 0 to 50 mole % Y2O3 under reducing conditions and from 33 to 60 mole % Y2O3 under oxidizing conditions at 1000°C. At temperatures above 1000° C, the face-centered cubic solid solution is limited by a filled fluorite lattice of composition (U,Y)O₂. For low-yttria content, oxidation at low temperatures (<300°C) permits additional oxygen to be retained in the structure to a composition approaching (U,Y)O_2.25 A tentative ternary phase diagram for the system UO₂-UO₃-Y₂O₃ is presented and the change in lattice parameter and in cell volume for the solid-solution phases is correlated with the composition.     

Thermodynamic Modeling of the MgO–Al2O3–CrO–Cr2O3 System

A complete critical evaluation and thermodynamic modeling of the phase diagrams and thermodynamic properties of the MgO–Al₂O₃–CrO–Cr₂O₃ system at 1 bar total pressure are presented. Optimized equations for the thermodynamic properties of all phases are obtained which 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 along with software for Gibbs energy minimization in order to calculate any type of phase diagram section.     

Quaternary Compounds Prepared in a CaO–Y2O3–SnO2 System

Compounds in a CaO–Y₂O₃–SnO₂ system were prepared by a solid-state reaction at 1673 K. The phase relation in this system was investigated by powder X-ray diffraction. Besides the previously reported ternary compounds, CaSnO₃, Ca₂SnO₄, Y₂Sn₂O₇, and a quaternary compound Ca_0.4Y_1.2Sn_0.4O₃, solid-solution series of Ca_{2− x} Y_{2 x} Sn_{1− x} O₄ with 0≤ x ≤0.5, and Ca_{1− y} Y_{2 y} Sn_{1− y} O₃ with 0≤ y ≤0.2 and 0.95≤ y ≤1.0 were found. The cell parameters of these solid-solution series were refined. The changes of rhombohedral cell parameters in the samples prepared in the range 0.565< y <0.714 of Ca_{1− y} Y_{2 y} Sn_{1− y} O₃ suggested the existence of solid solutions of Ca_0.4Y_1.2Sn_0.4O₃, although their single phases could not be prepared, except at y =0.6.     

Crack Resistance and Fatigue of Transforming Ceramics: I, Materials in the ZrO2–Y2O3–Al2O3 System

Crack resistance characteristics and fatigue properties have been studied in four types of Y₂O₃–TZP ceramics including one containing Al₂O₃. The largely linear-elastic behavior connected with the very small transformation zone (<5 μm) explains the absence of any resistance-curve behavior and the flaw-controlled strength. The crack resistance shows high sensitivity to environment-induced subcritical crack growth. This influence is also operative in both types of fatigue experiments, i.e., under static and cyclic stresses, leading to reduced fatigue thresholds compared with K _IC. While for static conditions a benefit is observed from enhanced t-m ZrO₂ transformation, cyclic stresses provoke an additional fatigue effect. However, if the cyclic stresses are restricted to subthreshold values, cyclic stress-induced effects in the process zone provide an improvement of the materials being visible as a strengthening effect.     

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.     

Physical Properties and Structure of rf-Sputtered Amorphous Films in the System Al2O3–Y2O3

Amorphous films in the system Al₂O₃–Y₂O₃ were prepared by the rf sputtering method in the range of 0–76 mol% Y₂O₃, and their density, refractive index, and elastic constants were measured. All of the physical properties of the amorphous Al₂O₃–Y₂O₃ films had a similar compositional dependence; that is, they increased continuously, but not linearly with increasing Y₂O₃ content. To confirm the coordination states of aluminum and yttrium ions in the amorphous Al₂O₃–Y₂O₃ films, the Al K α X-ray emission spectra and the X-ray absorption near edge structures (XANES) were measured. The average coordination number of aluminum ions in the amorphous films containing up to about 40 mol% Y₂O₃ content was 5, that is a mixture of 4-fold- and 6-fold-coordinated states. In the region of more than about 50 mol% Y₂O₃, the fraction of the 6-fold-coordinated aluminum ions increased with increasing Y₂O₃ content, while the results led to the conclusion that the coordination number of yttrium ions was always 6, regardless of composition. These results indicate that, in amorphous films in the system Al₂O₃–Y₂O₃, the change of the coordination state of aluminum ions has an important effect on physical properties.     

Low-Temperature High-Pressure Consolidation of Amorphous Al2O3–15 mol% Y2O3

This study examined pressure consolidation of amorphous Al₂O₃–15 mol% Y₂O₃ powders prepared by co-precipitation and spray pyrolysis. The two amorphous powders had similar true densities and crystallization sequences. Uniaxial hot pressing was carried out at 450°–600°C with a moderate pressure of 750 MPa. The co-precipitated powder could be hot pressed to a maximum relative density of 98% and remained amorphous. Pressure adversely affected the densification of the spray-pyrolyzed powder by favoring an early crystallization of γ-Al₂O₃ phase at 580°C. Plastic deformation of the amorphous phase is believed to be responsible for the large densification of the amorphous powders.     

Phase Stability and Physical Properties of Cubic and Tetragonal ZrO2 in the System ZrO2–Y2O3–Ta2O5

The cubic ( c -ZrO₂) and tetragonal zirconia ( t -ZrO₂) phase stability regions in the system ZrO₂–Y₂O₃–Ta₂O₅ were delineated. The c -ZrO₂ solid solutions are formed with the fluorite structure. The t -ZrO₂ solid solutions having a c/a axial ratio (tetragonality) smaller than 1.0203 display high fracture toughness (5 to 14 MPa · m^1/2), and their instability/transformability to monoclinic zirconia ( m -ZrO₂) increases with increasing tetragonality. On the other hand, the t -ZrO₂ solid solutions stabilized at room temperature with tetragonality greater than 1.0203 have low toughness values (2 to 5 MPa · m^1/2), and their transformability is not related to the tetragonality.     

Phase Transformation and Densification Behavior of Microwave-Sintered Si3N4–Y2O3–MgO–ZrO2 System

A 2.45 GHz microwave-sintered Si₃N₄–Y₂O₃–MgO system containing various amounts of ZrO₂ secondary additives have been studied with respect to phase transformation and densification behavior. The temperature dependent dielectric properties were measured from 25°C to 1400°C using a conventional cavity perturbation technique. Phase transformation behavior was studied using X-ray diffractometry. Microwave sintered results were compared with those of conventional sintered results. It has been found that α to β phase transformation was completed at a lower temperature in microwave-sintered samples than those of the conventionally sintered samples. Density of the microwave-sintered samples increased up to 2.5 wt% of ZrO₂ addition and thereafter it showed a tendency to decrease or remain constant. The decrease in density is attributed to the pore generation caused by decomposition due to the localized over heating.     

Tensile Ductility of Superplastic Al2O3–Y2O3–Si3N4/SiC Composites

Si₃N₄/SiC composites are ceramic materials that exhibit excellent performance for high-temperature applications. Prepared from an ultrafine amorphous Si-C-N powder, sintered materials are constituted mainly of a β -Si₃N₄ matrix with SiC inclusions and have a very small grain size (less than 1 μm). Such a microstructure is propitious for superplastic forming. Superplasticity has been studied in tension, from 1550° to 1650°C, under nitrogen atmosphere. Elongations over 100% have been achieved. In many cases, at the highest temperatures and slowest strain rates, materials are damaged by different processes, including microcracking, cavitation, and chemical decomposition. A map of the most suitable (strain-rate/temperature) domain has been established. It allows the prevention of any structural alteration by selecting carefully the testing conditions. Since specimens suffered considerable strain-induced hardening, sources for this phenomenon are examined. Although the experiments have involved high temperature and extensive strain, neither static nor dynamic grain growth has occurred. Crystallization of the amorphous grain-boundary phase, which is reported in most cases, may be invoked. However, based on microstructural observations, it is not the unique origin for flow hardening.