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.     

Stability of YBa2Cu3O7-x in Molten Chloride Salts

The decomposition products of YBa₂Cu₃O_7-x depend on the composition of the molten chloride salt for exposure at 1173 K in air. The presence of dichloride salts such as CuCl₂, CaCl₂, or MgCl₂ promote formation of CuO, Cu₂Y₂O₅, and loss of barium to the chloride salt as BaCl₂. Salts based on BaCl₂ or containing LiCl result in YBa₂Cu₃O_7-x decomposition products of Y₂BaCuO₅, CuO, and BaCl₂. High barium activity in the salt supports formation of the Y₂BaCuO₅ phase and reaction of CO₂ with the salt producing BaCO₃. Decomposition is most sluggish in binary NaCl-KCl salts where minimal amounts of reaction or decomposition products are observed.     

Intrinsic Kinetics of Carbon Dioxide Degradation of YBa2Cu3O7-x

The intrinsic kinetics, unaffected by diffusional and masstransfer effects, of the CO₂ degradation of superconducting particles have been determined using a nonisothermal technique. Below 900°C, the carbonization of YBa₂Cu₃O_{7- x} leads to formation of BaCO₃, Y₂Cu₂O₅, CuO, and Cu₂O. A further increase in temperature results in formation of BaCuO₂ from BaCO₃ and CuO. The carbonization rate shows the 1.5th-order dependence on the amount of unreacted YBa₂Cu₃O_{7- x} for the temperature range of 550° to 815°C. The activation energy of carbonization was determined to be 95.1 kJ · mol⁻¹.     

Phase Equilibria of the Calcium Oxide-Copper Oxide System in Oxygen at 1 atm

Phase equilibria in the CaO-CuO system have been determined at 1 atm pressure in oxygen over the temperature range 800° to 1300^deg;C. CaO is the stable phase at the calcium-rich end of the system. Two intermediate crystalline phases, namely Ca₂CuO₃ and Ca₃Cu₇O₁₀, form. Ca_zCuO₃ is stable up to 1085°± 3^deg;C, where it melts incongruently to CaO + liquid (peritectic liquid has ε82% CuO). Ca₃Cu₇O₁₀ becomes stable at 977°± 3^deg;C by reaction between Ca₂CuO₃ and CuO. Ca₃Cu₇O₁₀ melts incongruently at 1046°± 3^deg;C to Ca₂CuO₃+ liquid (peritectic liquid has ε83% CuO). CuO is the stable copper oxide phase up to 1061°± 3^deg;C at the copper-rich end of the system; at higher temperatures, Cu₂O is stable until the liquidus is reached at 1121^deg;C. The binary eutectic is at 1045°± 5^deg;C, in which a liquid with 83% CuO coexists with Ca₃Cu₇O₁₀ and CuO.     

10 kbar Phase Equilibria in the Y-Ba-Cu Oxide System at 950°C

Phase equilibria in the YO_1.5BaO-CuO system have been determined at 950°C at 10 kbar using a piston-cylinder apparatus. The oxide phases stable under these conditions are Y₂O₃, Y₂Cu₂O₅, CuO, Y₂BaCuO₅, YBa₂Cu₃O_6.5, BaCuO₂, Y₂BaO₄, Y₂Ba₃O₆, and YBa₄Cu₂O_7.5. The phase stabilities observed at 950°C at 10 kbar are identical to those observed at 950°C in air or oxygen at 1 atm for compositions with <40% Ba of the cations. In more Ba-rich portions of the phase diagram, carbonates and oxycarbonates are stabilized and a systematic determination of the phase equilibria has not been successful.     

Effect of Praseodymium on Superconductivity and Magnetism in Y1–xPrxBa2Cu4O8 Prepared by Nitrite Pyrolysis Method

X-ray diffraction patterns show that most samples of Y_1-x Pr_xBa₂Cu₄O₈ examined in the present study contained a single YBa₂ Cu₄O₈ (1-2-4) superconductive phase for x<0.7.Lattice parameters a and b increased with Pr concentration, suggesting that most of the Pr is trivalent in Y_1-x Pr_x-Ba₂Cu₄O₈. The zero-resistance temperature, T _co, decreases monotonically from 80 K at x=0 to 12 K at x=0.65, and superconducting transition widths tend to broaden for x>0. The room-temperature resistivity changes linearly until x=0.7 and increases abruptly at x=-0.75. The critical concentration, x_cr, thus was estimated to be 0.7. The effective magnetic moments of Pr in Y _1-x Pr_xBa₂Cu₄O₈ were 3.63., 3.35, and 3.23, μ_B for x=0.2, 0.4 and 0.6, respectively. In the R_0.8 Pr_0.2Ba₂Cu₄O₈ system, the depression of T_c weakly depends on the ionic radius of rare-earth elements. Similarities and differences between Y _1-x Pr_xBa₂Cu₄O₈ and Y_1-xPr_x-Ba₂Cu₃O_7-y also were noted and are discussed in this paper.     

Low-Temperature Synthesis and Phase Equilibria in the Y-Cu-O Binary System

The low-temperature phase relations in the Y-Cu-O system were examined using ultrasonically prepared, freeze-dried nitrate precursors. Under 1 atm of oxygen, Y₂Cu₂O₅ was found to be stable above 955 ± 5 K; below this temperature, the component binary oxides CuO and Y₂O₃ are the only stable phases in the system. The entropy of formation of Y₂Cu₂O₅, with respect to the component oxides, was calculated to be 5.3 ± 2.3 J/(mol.K) at 955 K. Bond valence analyses indicate that the entropy stabilization of this compound is most likely caused by the underbonded character of the copper ions.     

Hot Isostatic Pressing of YBa2Cu3O7-δ and YBa2Cu3O7–δ/Ag Composites

Hot isostatic pressing (HIP) can be used to produce fully dense shapes of high-temperature ceramic superconductors. Densification modeling of monolithic YBa₂Cu₃O_7-δ and the composite YBa₂Cu₃O_7-δ/Ag systems allows an understanding of the HIP process and has led to the development of successful protocols for HIP of these materials. Ag metal is the best encapsulation material found for both systems. HIP of monolithic YBa₂Cu₃O_7-δ requires a slow ramp of pressure in order to prevent decomposition into more basic oxides such as Y₂BaCuO₅ and CuO. HIP of composite YBa₂Cu₃O_7-δ/Ag requires careful powder processing to obtain dense material with a fine dispersion of Ag.     

Thermodynamics of CuAlO2 and CuAl2O4 and Phase Equilibria in the System Cu2O-CuO-Al2O3

The standard Gibbs free energies of formation of CuAlO₂ and CuAl₂O₄ were determined in the range 700° to 1100°C, using emf measurements on the galvanic cells (1) Pt,CuO +] Cu₂O/CaO-ZrO₂/O₂,Pt; (2) Pt,Cu +] CuAlO₂+] Al₂O₃/CaO-ZrO₂/ Cu +] Cu₂O,Pt; and (3) Pt,CuAl₂O₄+] CuAlO₂+]Al₂O₃/CaO-ZrO₂/O₂,Pt. The results are compared with published information on the stability of these compounds. The entropy of transformation of CuO from tenorite to the rock-salt structure is evaluated from the present results and from earlier studies on the entropy of formation of spinels from oxides of the rock-salt and corundum structures. The temperatures corresponding to 3-phase equilibria in the system Cu₂O-CuO-Al₂O₃ at specified O₂ pressures calculated from the present results are discussed in reference to available phase diagrams.     

Low-Temperature Phase Equilibria in the Y-Ba-Cu-O System

Ultrasonically prepared freeze-dried nitrate precursors and high-precision solution calorimetry were used to investigate the low-temperature thermodynamic stabilities of compounds in the Y-Cu-O, Ba-Cu-O, Y-Ba-O, and Y-Ba-Cu-O pseudobinary and pseudoternary systems at 1 atm of oxygen. Y₂Cu₂O₅, Y₂BaCuO_s, and BaCuO₂ were found to be metastable below 682°, 728°, and 710°± 5°C, respectively. The only stable phases in the Y-Ba-Cu-O system at 298 K and 1 atm of oxygen are Ba₂Cu₃O₆, CuO, BaO₂, and Y₂O₃. By compiling the calorimetric and phase equilibria data, a series of Y-Ba-Cu-O isothermal phase diagrams were constructed between 25° and 900°C at 1 atm of oxygen.     

Gibbs Energy of Formation of CuGd2O4 and Phase Relations in the Cu-Gd-O System

Oxygen potentials for the coexistence of Cu₂O + Gd₂O₃+ CuGd₂O₄, Cu₂O + CuO + CuGd₂O₄, and Cu + Cu₂O + Gd₂O₃ three-phase equilibrium have been measured by employing the solid-state galvanic cells
between 960 and 1290 K, respectively. The results obtained in this study indicate that the ternary phase, CuGd₂O₄, is nearly stoichiometric. The measured oxygen potentials for the coexistence of various three-phase assemblages are given by the expressions
between 960 and 1290 K, respectively. Combining the oxygen potentials for the coexistence of Cu₂O + Gd₂O₃+ CuGd₂O₄ with that of Cu₂O + CuO gives for the reaction
The results obtained in the present investigation suggest that the formation of CuGd₂O₄ from component oxides is slightly endothermic and the phase CuGd₂O₄ is an entropystabilized compound. The enthalpy and entropy of formation of CuGd₂O₄ from component oxides obtained in this study are in excellent agreement with the recent calorimetric data. Phase relations and oxygen-potential diagram for the Cu-Gd-O ternary system have been constructed at 1273 K by combining the results obtained in this study and the auxiliary thermodynamic data available in the literature.     

Formation of Pores and Y2BaCuO5-Free Regions during Melt Processing of Y1.6 Ba2.3Cu3.3Ox

The formation of spherical pores and regions free of Y₂BaCuO₅ (2-1-1) has been studied by melt processing Y_1.6Ba_2.3Cu_3.3O _x: in two different atmospheres (air and oxygen). When the sintered Y_1.6Ba_2.3Cu_3.3O _x specimens are melted at 1050°C, many spherical pores form in the melted specimens. During the subsequent cooling, the pores are filled by liquid flow and finally solidified to Y₂BaCuO₅-free regions. Melt processing in an oxygen atmosphere produces more pores and regions free of 2-1-1 than in air. Because peritectic melting of YBa₂Cu₃O_7-y in an oxygen atmosphere produces more oxygen gas than that in air, the formation of the pores and Y₂BaCuO₅-free regions is suggested to be attributed to the oxygen evolution during the peritectic melting of YBa₂Cu₃O_7−y     

Phase Equilibria and Crystal Chemistry in the Quaternary System Ba-Sr-Y-Cu-O in Air

Studies in the Sr-Y-Cu-O and Ba-Sr-Y-Cu-O systems have revealed that Sr will substitute for Ba in (Ba,Sr)₂YCu₃O_{6+ x} up to about 60%. There are no ternary compounds in the Sr-Y-Cu-O system equivalent to the three ternary phases in the Ba system. A new binary phase, "Sr₁₄Cu₂₄O₄₁"(CuO ∼ 63.158 mol%), was found which forms a solid solution with Y₂O₃ to a Sr:Y ratio of approximately 2:1. This phase can also incorporate considerable amounts of Ba and Ca and many other large ions.     

Rapid Calcination and Sintering of YBa2Cu3Ox Superconductor Powder Mixture in Inert Atmosphere

The rates of forming the superconducting YBa₂Cu₃O_x phase during the calcination of the Y₂O₃, BaCO₃, and CuO powder mixture at 790° and 850°C are considerably enhanced when an inert atmosphere of N₂ or He is used instead of O₂. Sintering in an inert atmosphere also produces higher density and larger grain size than in O₂. These results are consistent with the possibility of rapid atomic diffusion in tetragonal YBa₂Cu₃O_x due to either high oxygen vacancy concentration or expanded lattice in an inert atmosphere.     

Influence of the Oxide Precursors on the Reaction Sequencing and Kinetics in the Yttrium-Barium-Copper-Oxygen System

The reactions of stoichiometric Y₂O₃, CuO, and different barium salts (BaCO₃, Ba(NO₃)₂, BaO₂, BaCuO₂) for forming various compounds in the yttrium-barium-copper-oxygen system (i.e., YBa₂Cu₃O_7–δ, BaCuO₂, Y₂BaCuO₅, and Y₂Cu₂O₅) were systematically investigated by thermal analysis and X-ray diffractometry. In a few cases, the relevant activation energies were calculated. The reaction pathway and kinetics were significantly dependent on the physicochemical and thermal stability of the barium precursors, as well as on the crystalline size of the reagent. Binary BaO-CuO phases formed at low temperature (650°–700°C) when in the presence of easy-to-decompose barium precursors, and then slowly transformed to ternary compounds; in contrast, when barium ions were released at temperatures of >900°C, ternary phases formed directly from the components.     

Quantitative Analysis of Coexisting Byproducts in Orthorhombic YBa2Cu3Ox by X-ray Diffraction

BaCuO₂, Y₂Cu₂O₅, and Y₂BaCuO₅ as well as YBa₂Cu₃O_x were synthesized and nonlinear calibration curves for mole fraction versus integrated intensity ratio of X-ray diffraction peaks were obtained for BaCuO_2-, Y₂Cu₂O_5-, and Y₂BaCuO_5-YBa₂Cu₃O_x systems. It is shown that the amount of BaCuO₂ contained in orthorhombic YBa₂Cu₃O_x is not negligible, even if the relative intensity of the strongest diffraction peak of BaCuO₂ is negligibly weak.     

Theoretical Studies of the Electronic Properties of Ceramic Materials

The first-principles orthogonalized linear combination of atomic orbitals (OLCAO) method for electronic structure studies has been applied to a variety of complex inorganic crystals. The theory and the practice of the OLCAO method in the local density approximation are discussed in detail. Recent progress in the study of electronic and optical properties of a large list of ceramic systems are summarized. Eight selected topics on different ceramic crystals focusing on specific points of interest are presented as examples. The materials discussed are AIN, Cu₂O, β-Si₃N₄, Y₂O₃, LiB₃O₅, ferroelectric crystals, Fe-B compounds, and the YBa₂Cu₃O₇ superconductor. The results include the band structure, density of states, charge density distribution, spin density distribution, effective charges, total energy and totalenergy-derived results, optical absorption, positron annihilation spectra, and more. Extension of the band theoretical approach to the study of other areas of fundamental ceramic science is also discussed.     

Subsolidus Relations in the La2O3─CuO─CaO Phase Diagram and the La2O3─CuO Binary Join

The phase diagram for the CuO-rich part of the La₂O₃─CuO join was redetermined. La₂Cu₂O₅ was found to have a lower limit of stability at 1002°± 5°C and an incongruent melting temperature of ∼1035°C. LagCu₇O₁₉ had both a lower (1012°± 5°C) and an upper (1027°± 5°C) limit of stability. Subsolidus phase relations were studied in the La₂O₃─CuO─CaO system at 1000°, 1020°, and 1050°C in air. Two ternary phases, La_1.9Ca_1.1Cu₂O_5.9 and LaCa₂Cu₃O_8.6, were stable at these temperatures, with three binary phases, Ca₂CuO₃, CaCu₂O₃, and La₂CuO₄. La₂Cu₂O₅ and La₈Cu₇O₁₉ were stable only at 1020°C, and did not support solid-solution formation.     

Effect of Ta2O5, Nb2O5, and HfO2 Alloying on the Transformability of Y2O3-Stabilized Tetragonal ZrO2

The addition of Ta₂O₅, Nb₂O₅, and HfO₂ enhanced the transformability of Y₂O₃-stabilized tetragonal ZrO₂ polycrystal (Y-TZP), which was indicated by an increase in phase transformation temperatures and fracture toughness of Y-TZP. Comparison of the alloying effects of these oxides on the transformability and crystal structure of Y-TZP suggested that an alloying oxide which increases the c/a axial ratio (tetragonality) of TZP also increases the transformability. Empirical equations to predict the tetragonality are proposed. Calculated tetragonalities showed good agreement with measured values in the systems ZrO₂-Y₂O₃-Ta₂O₅, -Nb₂O₅, and -HfO₂.     

A Simple Way to Synthesize Yttrium Aluminum Garnet by Dissolving Yttria Powder in Alumina Sol

A precursor for Y₃Al₅O₁₂ was synthesized as a YAG sol by simply dissolving Y₂O₃ powder in an alumina sol. Phase-pure Y₃Al₅O₁₂ powder was obtained by precipitating the YAG sol with an aqueous dilute ammonia solution followed by calcination at 1100°C. TG/DTA analysis showed an exotherm at 938°C attributed to formation of YAG phase and weight loss of 44% at 1000°C. XRD and FT-IR analysis showed that phase-pure YAG can be formed through noncrystalline and metastable hexagonal YAlO₃ without forming either yttrium or aluminum formate intermediate.