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.     

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.     

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.     

Influence of BaCuO2 on the Sintering and the Properties of YBa2Cu3O7−δ-Based Ceramics

The preparation of YBa₂Cu₃O_{7 - δ}-based ceramics was made from a mixture of oxides taken in the molar proportions α:1:2 BaCuO₂:Y₂BaCuO₅:CuO(0.95 α 3). The densification of the ceramics is strongly dependent on the initial amount of BaCuO₂. The highest density is obtained when α= 1. All the ceramics present a superconducting transition. A sintering mechanism is proposed in which the densification is mainly governed by the appearance of a metastable BaCuO₂-based liquid at around 900°C.     

Phase Composition and Compatibilities at 930° to 950°C in the System Europium(III) Oxide-Barium Oxide-Copper(II) Oxide in Air

Phase composition and compatibilites at 930° to 950°C were determined for the system Eu₂O₃–BaO–CuO in air. The binary compound Eu₂CuO₄ dissolves Ba to the extent 0 x 0.02 in Eu_2-xBa_xCuO₄, whereas the other binary compounds, Eu₂BaO₄ and BaCuO₂, do not exhibit solid solubility. Three ternary compounds were obtained, Eu₂BaCuO₅ and two solid solution phases. The first contains the 90 K '123' superconductor and has solubility limits represented by Eu_1+xBa_2-xCu₃O_7±y, where 0 x 0.5. The second has a solubility limit represented as Eu_1+xBa_8-xCu₄O_y, where 0 x 0.44. The limited solid solution range of this phase provides insight concerning the probable solid solution range of the analogous phase in the Y₂O₃-BaO-CuO system.     

Phase Stability of Ln2BaCuO5 (Ln = Dy, Er, Eu, Gd, Ho, Sm, Yb)

Phase stability of a series of Ln₂BaCuO₅ (Ln=Dy, Er, Eu, Gd, Ho, Sm, Yb) has been studied as a function of oxygen partial pressure and temperature by X-ray diffraction, thermal gravimetric, and differential thermal analyses. Above the critical temperature T_d, phase decomposition with a decrease in sample weight took place. In the case of Ln=Dy, Er, Ho, and Yb, the Ln₂BaCuO₅ was decomposed into Ln₂O₃BaCuO₃ and BaCuO₂. The critical temperature T_d (K) was changed depending heavily on the oxygen partial pressure P (Pa) and the effective ionic radius R (0.1-nm unit) of the Ln atom which was sevenfold coordinated by oxygen atoms. The observed T_d values for all of the systems, together with those of Y₂BaCuO₅ reported previously, closely followed the relation T_d=−17493R²+ 34582R − 15489 + 239.47 In P with a reliability index R_p=0.5%. [Key words: superconductors, barium, phases, X-ray diffraction, differential thermal analysis.]     

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.     

Phase Diagram of the BaO-CuO Binary System

The phase diagram of the BaO-CuO binary system has been investigated in air and in a mixed gas of Ar + 0.21 atm of O₂. The existence of two compounds, BaCuO₂ and Ba₂CuO₃, was confirmed. The phase transition of Ba₂CuO₃ from an orthorhombic to a tetragonal phase was observed to occur at 1083 K. The lattice constants of the tetragonal Ba₂CuO₃ phase were determined to be a = 1.2975 nm and b = 0.3992 nm. BaCuO₂ was shown to melt incongruently by a synthetic reaction at 1289 K. Furthermore, the existence of two eutectic reactions and a peritectic reaction in the present system was confirmed.     

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.     

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     

Formation, Transformation, and Decomposition of Y2TeO6

A metastable modification of Y₂TeO₆ crystallizes at 770°C to 870°C at a heating rate of 10°C·min⁻¹ in air from an amorphous material prepared by the simultaneous hydrolysis of yttrium and tellurium alkoxides. It has a tetragonal unit cell with a=1.0428 nm and c=1.7524 nm. The tetragonal-to-hexagonal phase transformation occurs at 920° to 960°C. Hexagonal Y₂TeO₆ decomposes into Y₆TeO₁₂ and TeO₃↑ at 1220° to 1320°C; only Y₆TeO₁₂ is obtained as the decomposition product. Both modifications of Y₂TeO₆ are built up by octahedral TeO₆ groups.     

High-Pressure Phase Transitions in Zirconia and Yttria-Doped Zirconia

Raman spectroscopy has been utilized to characterize the phase transformations and transition pressures in pure and doped zirconia containing 3, 4, and 5 wt% Y₂O₃. The pressure-induced transformations were investigated to over 6 GPa (at room temperature) using a diamond anvil pressure cell. Pure zirconia single-crystal samples transformed to a "new" tetragonal phase (different from the one obtained at high temperatures at atmospheric pressure) at about 4 GPa. The pressure transformation, like the temperature transition, was reversible and exhibited an approximately 0.45-GPa hysteresis at room temperature. The 3 and 4 wt% Y₂O₃ crystals underwent a monoclinic ( P 2_1/b) to tetragonal ( P 4_{2 nmc}) phase transition similar to that observed at high temperatures. This phase change was found to be irreversible on releasing the pressure. The 5 wt% Y₂O₃ at atmospheric pressure consists of a tetragonal modification in a disordered cubic matrix; a gradual, but reversible, disordering transformation of the tetragonal precipitate takes place with pressure.     

Grain Alignment in Bulk YBa2Cu3Ox Superconductors by Isothermal Peritectic Reaction Process

An isothermal peritectic reaction (IPR) process has been developed to fabricate YBa₂Cu₃O_x (123) superconductors with aligned grain structure in a relatively short time without a temperature gradient. The process consists of the preparation of Y₂BaCuO₅ (211) powder compact in wire form and infiltration of a Ba-Cu-O glass melt into the 211 compact below the peritectic temperature of ∼ 1010°C. During the infiltration at 965°C the 123 phase forms at the surface of the compact by the reaction between 211 grains and the glass, and the newly formed 123 grains grow directionally towards the center of the compact. The result of the infiltration is an aligned grain structure with a preferred orientation of {100} planes in the axial direction of wire-type compacts. These 123 superconductor specimens show the zero resistance ( T _c ) at 92 K and transport critical current density ( J _c ) of up to 1500 A/cm². By this process, superconductor wires can be produced at shorter times and lower temperatures than the melt-texturing methods.     

Alignment of YBa2Cu3O7-x and Ag─YBa2Cu3O7-x Composites at ∼930°C by Eutectic Formation

The present work describes a new technique to synthesize aligned YBa₂Cu₃O_{7- x} and Ag─YBa₂Cu₃O_{7- x} superconducting composites from Ba- and Cu-deficient compositions (relative to YBa₂Cu₃O_{7- x} ) plus BaCuO₂. For YBa₂Cu₃O_{7- x} , high transition temperature midpoint T_c (91 K), temperature of zero resistivity T ₀ (90 K), and critical current density J_c (>3000 A°Cm⁻² at 77 K) were achieved by using this technique. This procedure provides the potential for using a reliable and reproducible densification and alignment technique alternative to partial or full melting. The composite is highly aligned, with an average grain size of ∼1 to 2 mm and domains of width greater than 5 mm. The initial phase assemblage consists of YBa₂Cu₃O_{7- x} (123) as the major phase plus YBa₂CuO₅ (211) CuO as minor phases. The BaCuO₂ is added to the Ba- and Cu-deficient starting composition in order to assist in the formation of a CuO-rich liquid as well as to compensate for the Ba and Cu deficiences in 123. Since the liquid forms at ∼900°C and is compatible with 123, it can be used to facilitate alignment of 123 at ∼930°C. The addition of Ag to the system results in eutectic formation with the (solidified) liquid, substantial filling of the pores during sintering, and improved alignment.     

Grain Growth, Microstructure, and Superconducting Properties of Pure and Y2BaCuO5-Doped YBa2Cu3O7–x Ceramics

Superconducting properties of YBa₂Cu₃O_7–r ceramics have been investigated with the aim of avoiding the weak links that are the principal reasons for low critical current densities in this compound. An efficient grain growth process should be useful in this respect. Superconducting ceramics were prepared from a commercial precursor powder (Hoechst) with or without addition of Y₂BaCuO₅ and from a gel precursor powder. Grain growth kinetics and densification are first derived under such conditions that control the liquid-phase contribution. Grain growth in the solid-state regime is anisotropic, possibly due to anisotropic grain boundary energies. Grains in the fine powder (gel precursor) grow 10 times faster than in the commercial reference but grain size saturates because of steric hindrance. Addition of Y₂BaCuO₅ particles inhibits grain growth in the solid-state regime. 211 particles are incorporated into grains when a transient liquid phase is present. Superconducting properties were characterized by 50-Hz magnetization hysteresis cycles at 77 K under H = 0.2 to 12 mT to probe weak-link behavior. Additional studies were performed at 5 K using higher fields. Weak-link contribution is decreased when samples from the gel route are processed in the solid-state regime (below the peritectic temperature). This may be attributed to the propensity of gel powder to develop, naturally, preferential grain boundaries due to its intrinsic structural anisotropy.     

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.     

Traveling Reaction Zone Method for Preparation of Textured Ceramic Superconductor Thick Films

Textured thick films of superconducting YBa₂Cu₃O_7−x (YBCO) have been prepared on ceramic substrates using a traveling reaction zone method. The technique utilizes the rapid reaction between Y₂Cu₂O₅ and BaCuO₂ to form YBCO as the film passes through a steep temperature gradient furnance. The films consist of a single continuous superconducting phase with strong c -axis orientation normal to the translation direction of the film.     

Stability Diagram for the System Yba2Cu3O7–x

The dependence of the degree of nonstoichiometry of YBa₂Cu₃0_7–x (123) on temperature and oxygen pressure has been determined by thermogravimetric analysis (TGA) in the temperature range 400° to 950°C and the oxygen pressure range 10^–6 to 1 atm (1 atm = 10⁵ Pa). The nature of the decomposition of 123 in the temperature range 750° to 950°C and the oxygen pressure range 10^–6 to 10^–2 atm has been determined by TGA and X-ray diffractometry (XRD). As the oxygen pressure decreases, the decomposition of 123 follows the sequence 123→ Y₂BaCuo₅ (211) + BaCuO₂° Cu₂O→ 211 ° BaCuO₂° BaCu₂O₂→ 211 ° YBa₃Cu₂O_x (132) ° BaCu₂O₂→ 211 ° BaCu₂O₂°BaO. The incongruent melting temperatures have been determined in the oxygen pressure range 10^–6 to 1 atm by differential thermal analysis, and the phases formed on solidification have been identified by XRD. The stability diagram for the composition 123 has been constructed.     

Microstructural Analysis in Processing of the High-Tc Superconductor Ba2YCu3O7-x in Air

In the synthesis of the superconducting compound Ba₂YCu₃O_7-x from a stoichiometric mixture containing BaCO₃, Y₂O₃, and CuO In air, a low-melting liquid phase is formed at about 890°C. The liquid phase was identified as a ternary eutectic located within the compatibility triangle Ba₂YCu₃O_7-x–BaCuO₂–CuO. The implication of this finding for the processing of Ba₂YCu₃O_7-x is discussed.     

Transformation-Toughened ZrO2: Correlations between Grain Size Control and Composition in the System ZrO2-Y2O3

The average grain size of ZrO₂(+Y, o,) materials sintered at 1400°C was observed to depend significantly on the Y₂O₃ content. The average grain size decreased by a factor of 4 to 5 for Y₂O₃ contents between 0.8 and 1.4 mol% and increased at Y₂O₃ contents of 6.6 mol%. Grain growth control by a second phase is the concept used to interpret these data; compositions with a small grain size lie within the two-phase tetragonal + cubic phase field, and the size of the tetragonal grains is believed to be controlled by the cubic grains. This interpretation suggests that the Y₂O₃-rich boundary of the two-phase field lies between 0.8 and 1.4 mol% Y₂O₃. Transformation toughened materials fabricated in this binary system must have a composition that lies within the two-phase field to obtain the small grain size required, in part, to retain the tetragonal toughening agent.