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.     

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.     

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.     

Phase Equilibria of the La2O3-SrO-CuO System at 950°C and 10 kbar

Phase equilibria of the La₂O₃-SrO-CuO system have been determined at 950°C and 10 kbar (1 GPa). Stable phases at the apices of the ternary phase diagram are CuO, La₂O₃, and SrO. Stable intermediate phases are La₂CuO₄ in the LaO_1.5-CuO binary and Sr₂CuO₃, SrCuO₂, and Sr₁₄Cu₂₄O₄₁ in the CuO-SrO binary. The La_2-xSr _x CuO_4-δ solid solution is stable where 0.0 ≤ x ≤ 1.3, the La_2-xSr_1+xCu₂O_6+δ solid solution is stable where 0.0 ≤ x ≤ 0.2, the La_8-xSr _x Cu₈O_20-δ solid solution is stable where 1.3 ≤ x ≤ 2.7, the La _x Sr_14-x-Cu₂₄O₄₁ solid solution is stable where 0 ≤ x ≤ 6, and the La_1+xSr_2-xCu₂O_5.5+δ phase is stable where 0.04 ≤ x ≤ 0.16. The La₂O₃-SrO-CuO phase diagram at 950°C and 10 kbar is almost identical to that determined by other authors at 950°C and 1 atm, in terms of phase stability and solid-solution ranges.     

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.     

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.     

30 kbar Phase Equilibria of the La2O3–SrO–CuO System at 950°C

Phase equilibria of the La₂O₃–SrO–CuO system have been determined at 950°C at 30 kbar (3 GPa). Stable phases at the apexes of the ternary phase diagram are CuO, La₂O₃, and SrO. Stable intermediate phases are La₂, CuO₄ and La₂Cu₂O₅ in the LaO_1.5–CuO binary and Sr₂CuO₃, SrCuO₂, and Sr₁₄Cu₂₄O₄₁ in the CuO–SrO binary. The La_{2– x} Sr _x -CuO_4–δ solid solution is stable for 0.00 is ≤ x ≤ 1.29, the La_{2– x} Sr_{1+ x} Cu₂O_6+δ solid solution is stable for 0.03 ≤ x ≤0.20, the La_{2– x} Sr _x Cu₂O_5–δ solid solution is stable for 0.00 ≤ x ≤1.08, and the La _x Sr_{14– x} Cu₂₄O₄₁ solid solution is stable for 0.00 ≤ x ≤ 6.15. The 30 kbar phase diagram differs from the 1 atm (0.1 MPa) and 10 kbar (1 GPa) results principally in the absence of La_{1– x} Sr_{2+ x} Cu₂O_5.5+δ as a stable phase and the extended range of the La_{2– x} Sr _x Cu₂O_5–δ solid solution at 30 kbar.     

Simultaneous Thermogravimetry and Evolved-Gas Analysis of YBa2Cu3O7 and LaBa2Cu3O7 Superconductors

Concurrent thermogravimetry (TG) and evolved-gas analysis (EGA) were done for YBa₂Cu₃O_7-z and LaBa₂Cu₃-O_7-z superconductors. The sample weights were monitored by thermobalance and the evolved O₂ and CO₂ species were monitored by quadruple mass spectrometer (QMS). No diffraction peak for the impurity phase containing a carbonate group was observed in the X-ray diffraction patterns for these samples, but the release of CO₂ was detected by EGA. CO₂ gas began to evolve from YBa₂Cu₃O_7-z at 543°C and from LaBa₂Cu₃O_7-z at 692°C. Preparation of high-quality YBa₂Cu₃O_7-z and LaBa₂Cu₃O_7-z superconductors is discussed on the basis of results of these thermal analyses.     

Crystal Chemistry and Phase Equilibrium Studies of the BaO(BaCO3)–½R2O3–CuOx Systems in Air: VI, R = Neodymium

Crystal chemistry and subsolidus phase equilibrium studies of the Ba-Nd-Cu-O system near the CuO and Nd₂O₃ corners have been carried cut at 950°C in air. Two solid-solution series have been identified in the Ba-Nd-Cu-O system. The first series involves the high- T _c superconductor phase, and has the formula Ba_2–xNd_1+xCu₃O_6+z, where × < ≅ 0.7. At the ideal compound stoichiometry of Ba₂NdCu₃O_6+z, the transformation from the high- T _c orthorhombic to tetragonal phase occurs at 550°–575°C in air. This temperature varies as a function of composition, and at x ≅ 0.2 to 0.3 it occurs at 950°C. The second solid solution is the non-superconducting "brown phase" represented by Ba_2+2x-Nd_4–2xCu_2–xO_10–2z 0 ≤ x ≤ 0.1. Preliminary phase diagrams of the BaO–Nd₂O₃ and Nd₂O₃–CuO_x systems are also presented. Standard X-ray diffraction patterns of BaNd₂–CuO₅ and (Nd_1.9Ca_0.1)CuO_4–z are provided.     

Superconducting Properties of Barium-Doped (Bi,Pb)2Sr2Ca2Cu3Ox Glass-Ceramics

The Ba-doped superconducting (Bi,Pb)₂Sr_{2- x} Ba _x Ca₂Cu₃O _y and (Bi,Pb)₂Sr₂Ca_{2- x} Ba _x Cu₃O _y (0 ≦ x ≦ 1.0) were prepared by using a melt-quenching method, and the effect of Ba additions on the glass-forming ability and the crystalline phase was examined. The glass-forming ability was not improved by substitution of Ba for Sr or Ca, and particularly BaPbO₃ as well as CaO was observed in the melt-quenched sample of (Bi,Pb)₂SrBaCa₂Cu₃O _y . BaPbO₃ crystals were precipitated in all glass-ceramics with Ba substituted for Sr or Ca. The partial substitution of Ba substituted for Sr was effective for the formation of the high- T _c phase, and (Bi,Pb)₂Sr_1.4Ba_0.6Ca₂Cu₃O _y glass-ceramics obtained by annealing at 830°C for 100 h exhibited superconductivity with a T _c of 103 K, although BaPbO₃ and the low- T _c phase were still largely present.     

Phase Diagram of the BaO(BaCO3)-CaO-CuO System

The phase diagram of the BaO(BaCO₃)-CaO-CuO system, especially in the barium-rich region at 900°C in air, was studied. Two new different oxycarbonates were observed: Ba₈Ca_16/15Cu_64/15O_11.20(CO₃)_2.66and a solid-solution series with a chemical composition of Ba₂Ca _{x + y} Cu_{1+( x /2)- y} O_2+delta(CO₃)_{1- z} (where 0 ≤ to x ≤ to 2/39 and 0 ≤ to y ≤ to 16 x /5). The oxycarbonate solid solution was formed in a region of the compositional triangle Ba:Ca:Cu (in moles) = (2:0:1)-(39:1:20)-(65:7:28). The solid-solution structure had P 4/ mmm symmetry, with lattice parameters a similar/congruent a _pand c similar/congruent 2 a _p, where a _prepresents the perovskite cell. The Ba₈Ca_16/15Cu_64/15O_11.20(CO₃)_2.66compound, which had Pm 3 m symmetry with a lattice parameter a = 0.8116(2) nm, had no chemical-solubility range.     

Hot Deformation of YBa2Cu3O7-δ and Composite YBa2Cu3O7-δ/Ag

The response of ceramic superconductors and ceramic composites to compressive stresses at high temperatures has been examined. Monolithic YBa₂Cu₃O_7-δ and composite YBa₂Cu₃O₇-δ₆/Ag were tested at constant true strain rates from 10^-6 to 10^-3 s^-1 at temperatures from 800° to 950°C. Fine-grained monolithic YBa₂Cu₃O_7-δ appears to have a regime of superplastic deformation between temperatures of 850° and 950°C at strain rates from 10^-6 to 10^-4 S^-1. The addition of 20 vol% Ag to a coarser-grained material enhances the ductility of the ceramic and lowers the flow stress by a factor of 3 to 10. However, there is no evidence of superplasticity in the composite material in the range of temperature and strain rate where it was tested.     

Stability of Bi2Sr2Ca1Cu2O8 ±δ Thick Films at Elevated Oxygen Pressures and Temperatures

Bi₂Sr₂Ca₂Cu₂O_8±δ-type compound thick films were exposed to oxygen-argon-gas mixtures (1% to 20% oxygen gas) at elevated pressures (up to 207 MPa) and temperatures (500° to 940°C) for times ranging from 5 to 96 h. At a sufficiently high oxygen fugacity and temperature, Bi₂Sr₂Ca₁Cu₂O_8±δ decomposed via a solid-state reaction. Room-temperature X-ray diffractometry and electron probe microanalysis of decomposed films revealed the presence of Bi₂(Sr,Ca)₂-Cu₁O_6±θ ro-type compound, Bi₂Sr₂,Ca₁O_8±δ-type compound, and CuO. Bi₂Sr₂Ca₁Cu₂O_8±δ decomposition was accompanied by a modest weight gain, which was consistent with an oxidation reaction. The solid-state decomposition reaction could be reversed by heat treatment of decomposed films at 860°C in pure, flowing oxygen at ambient pressure.     

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     

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.     

Synthesis of High-Tc Dual-Phase Ag–YBa2Cu3O7–x Superconductor Composite Powders by Sol–Gel Process

Fine, homogeneous, dual-phase Ag–YBa₂Cu₃O_{7– x} composite powders were prepared by a simple colloidal sol–gel coprecipitation technique that was previously used for synthesis of single-phase YBa₂Cu₃O_{7– x} . Samples containing up to 60 wt% silver were synthesized. Silver neither reacted with nor degraded the YBa₂Cu₃O_{7– x} . The presence of silver was found to aid the oxygenation of the precursor during calcination to form orthorhombic YBa₂Cu₃O_{7– x} . Measurements made by ac magnetic susceptibility showed no significant degradation in transition temperature for samples containing up to 40 wt% silver.     

Formation Mechanism of MoO3-Doped YSr2Cu3Oy Using the Citrate Process

The formation mechanism of the synthesis of MoO₃-doped YSr₂Cu₃O _y powders using the citrate process was investigated. It was shown that the precursor phase (Sr_{1- x} Y _x )₁₄Cu₂₄O₄₁ played a crucial role in forming the superconducting phase. It was found that the precursor phase (Sr_{1- x} Y _x )₁₄Cu₂₄O₄₁ interacted with water and decomposed when it was heavily milled and heated.     

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.     

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.     

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.