Effects of Temperature and Oxygen Partial Pressure on Nonstoichiometry of YBa2Cu3O6+δ

The oxygen content (6 +δ) in YBa₂Cu₃O_6+δ varies with temperature and oxygen partial pressure. An equation for the content as a function of temperature and oxygen partial pressure has been obtained based on the two-sublattice model of YBa₂Cu₃O₆( V ₀, O)₁. The results calculated using the equation are in very good agreement with published measured data.     

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.     

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.     

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.     

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 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.     

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.     

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.     

Calculation of Phase Diagrams for the YO1.5—BaO—CuOx System

The thermodynamic data for the YO_1.5–BaO, BaO-CuO_x, and YO_1.5–CuO_x quasi-binary systems were optimized from experimental phase diagrams. They were used to calculate tentative phase diagrams for the YO_1.5–BaO—CuO_x quasi-ternary system. The equilibrim liquidus surface and the isothermal sections of the ternary system at 900°, 925°, 950°, 975°, and 1000°C were calculated. The isopleths containing YBa₂Cu₃O_7-δ were also calculated.     

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     

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.     

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.     

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.     

Isothermal Section of a Cu2O–Al2O3–SiO2 Pseudo-Ternary System at 1150°C in Air

In this study, the isothermal section of a Cu₂O–Al₂O₃–SiO₂ pseudo-ternary phase diagram at 1150°C was analyzed by means of a scanning electronic microscope and powder X-ray diffraction of the quenched samples qualitatively, and the compositions of the tie-points of the tie-planes as well as their regions were determined by in situ high-temperature quantitative X-ray diffraction analysis and energy-dispersive X-ray spectroscopy. Then, the isothermal section of the Cu₂O–Al₂O₃–SiO₂ pseudo-ternary phase diagram at 1150°C was constructed; it was found that the isothermal section is composed of two single liquid-phase regions, five two-phase regions, and six three-phase regions.     

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.     

Superconductivity in the (Yba2Cu3)1−xNaxO7−δ System

The (YBa₂Cu₃)_1−xNa_xO_7–δ system in the range of x = 0–0.8 was investigated. Experimental data suggest that the sodium doping with x 0.26 does not affect the critical transition temperature T_c, and the crystal structure maintains the orthorhombic lattice with a slightly smaller unit cell. However, sodium doping increases the sintering and grain growth kinetics, resulting in a higher superconducting phase volume and an enhanced Meissner effect. It also lowers the processing temperaturel. The experimental data also suggest that the sodium atoms diffuse into the superconducting YBa₂Cu₃O_7−δ crystallites, which stabilizes the orthorhombic phase. The transition temperature (ortho-rhombic to tetragonal) in sodium-doped materials increases with the increasing concentration of sodium.     

Synthesis and Irreversibility of Oxygen Weight Loss in 80 K YBa2Cu4O8 Superconductor

Synthesis of YBa₂Cu₄O₈ superconductor usually requires oxygen pressure greater than 20 MPa and very long heating schedules at high temperatures. Here we report synthesis of YBa₂Cu₄O₈ superconductor by the standard solid-state reaction between the precursors BaCu₂O₃ and Y₂O₃ in the presence of alkali nitrate. NaNO₃, under ambient O₂ pressure. The resulting sample showed zero resistance at 78 K. The presence of 124 was confirmed by high-resolution X-ray diffraction studies. Irreversibility of oxygen weight loss was studied by thermogravimetric analysis and differential thermal analysis.     

Characteristic Spherulitic Microstructure in Partial-Melt-Processed YBa2Cu3Ox/HfO2

The microstructure of partial-melt-processed YBa₂Cu₃O _x /HfO₂ has been studied by transmission electron microscopy. A characteristic spherulitic microstructure is formed in the system. A model for the growth mechanism has been proposed. The critical heterogeneous nucleation of the YBa₂Cu₃O _x phase appears to occur from the melt in an epitaxially controlled manner on CuO particles. Subsequent growth of YBa₂Cu₃O _x platelets from the nucleus region is repeatedly interrupted by the nucleation of hafnium-rich phases in the liquid at the solid/liquid interface in a manner that again appears to be epitaxially controlled and that promotes the splay of the c orientation of the YBaCuO grain.     

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.     

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.