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.     

Water Corrosion of YBa2Cu3O7−δ Superconductors

Solid-state sintering was used to make YBa₂Cu₃O_7−δ superconducting bulk materials. Corrosion of the YBa₂Cu₃O_7−δ superconductor material was investigated in a humid environment. The superconducting materials exhibited significant corrosion after 4 h at 80° and 100% relative humidity. A grain-boundary phase was formed, and the percent superconducting phase in the material decreased by approximately 60%. The transition temperature (T_c) decreased with corrosion time. After 2 h of corrosion, T_c decreased from 87 to 81 K.     

Barium Rare-Earth Hafnates: Synthesis, Characterization, and Potential Use as Substrates for YBa2Cu3O7-delta Superconductor

A new group of complex perovskites Ba₂REHfO_5.5 (where RE = La, Pr, Nd, and Eu) has been synthesized and sintered as single-phase materials with high sintered density and stability using a solid-state reaction method for the first time. The structure of Ba₂REHfO_5.5 has been studied by X-ray diffactometry (XRD) and all of the perovskites are isostructural and have a cubic structure. The dielectric constant and loss factor values of these materials are in a range suitable for their use as substrates for YBa₂Cu₃O_7-delta superconductors. XRD and resistivity measurements show that there is no detectable reaction between YBa₂Cu₃O_7-delta and Ba₂REHfO_5.5, even when the two substances are mixed thoroughly and sintered at 950°C for 15 h. The addition of Ba₂REHfO_5.5 up to 20 vol% in YBa₂Cu₃O_7-delta-Ba₂REHfO_5.5 composite shows no detrimental effect on the superconducting transition temperature of YBa₂Cu₃O_7-delta. Thick films of YBa₂Cu₃O_7-delta fabricated on polycrystalline Ba₂REHfO_5.5 substrate have a superconducting zero resistivity transition of 92 K, indicating the suitability of these new materials as substrates for YBa₂Cu₃O_7-delta films.     

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.     

Local Dendrite Development during Seeded Peritectic Solidification of Large YBa2Cu3O7−δ Grains

The growth morphology of large YBa₂Cu₃O_7−δ grains during peritectic solidification has been reported to be responsible for the generation of processing defects, such as platelets, and an inhomogeneous distribution of 211 particles, both of which influence significantly the superconducting properties of the fully processed material. The present study demonstrates both experimentally and theoretically the formation of local dendrites at macroscopically planar YBa₂Cu₃O_7−δ growth fronts which propagate along different crystallographic directions and identifies these as key growth features of the peritectic solidification process.     

Liquid Formation at the Peritectic Temperature in Superconducting YBa2Cu3O7−x—Observation of a New Phase YBa4CuAIO8

In the system Y-Ba-Cu-O, partial melting of peritectically decomposing YBa₂Cu₃O_{7- x} (123) was used to produce a bulk material of high critical current density when the material was aligned. The liquid formation mechanism and its relation to reaction with alumina refractory was studied. A previously unreported phase of the approximate stoichiometric ratio Y:Ba:Cu:Al = 1:4:1:1 (YBa₄CuAlO₈) was detected. The crystal structure was determined to be tetragonal, with lattice parameters a ₀= b ₀= 1.651 nm, c ₀= 1.793 nm. The 1411 phase bears a close structural relationship to BaCuO₂.     

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.     

Preparation of Monodispersed Y-Ba-Cu-O Superconductor Particles via Sol—Gel Methods

The high-T _c superconducting material YBa₂Cu₃O_{7- x} was synthesized using a modified amorphous citrate or oxalate process. Phase-pure superconducting powders were obtained by firing the citrate precursor to 900°C and the oxalate precursor to 950°C inflowing O₂. The preparation is highly reproducible and leads to powders with excellent homogeneity and sinterability. The resulting material was examined by transmission electron microscopy, thermogravimetry, and X-ray diffraction. Particle sizes for the superconductors obtained varied from 75 to 3300 Å (7.5 to 330 nm).     

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.     

Characterization of a Large-Scale Nondoped YBa2Cu3O7−x Superconductor Prepared by Plastic Forming Without High-Pressure Molding

A slurry containing YBa₂Cu₃O_{7− x} particles and a fine YBa₂Cu₃(OH) _x colloid solution was prepared, and a large-scale bulk YBa₂Cu₃O_{7− x} superconductor (about 50 mm × 35 mm × 2 mm) was produced by plastic forming without high-pressure molding. The samples molded from the slurry were dried and then fired at 1223 K in air. X-ray diffraction data indicated that the samples had the characteristic orthorhombic YBa₂Cu₃O_{7− x} structure. Measurements of electrical resistance were carried out between 300 and 50 K by the standard four-probe DC electrical measurement. The samples began superconducting at an onset temperature around 92 K, and the full-transition temperature (critical temperature) ( T _c) was 88.7±1.4 K. The critical current density ( J _c) measured at 77 K was about 440 A/cm², the value of J _c was improved by the heat treatment under an oxygen atmosphere, and J _c=1.6 × 10³ A/cm² was observed. Under the magnetic field (B=1 T), the sample held its superconductivity, and demonstrated that this method can be used to produce the magnetic shielding used in magnetic resonance imaging diagnosis.     

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.     

Interaction of Chemically Vapor Deposited YBa2Cu3Ox with Yttria-Stabilized Zirconia Substrates

High-resolution transmission electron microscopy and optical diffractograms have revealed that chemically vapor deposited films of superconducting YBa₂Cu₃O _x react to form an interaction layer with single-crystal yttriastabilized zirconia. The approximately 5 nm thick interlayer was identified as BaZrO₃. Zirconium was also found to diffuse through the entire YBa₂Cu₃O _x film.     

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.     

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.     

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.     

Effect of Rigid Inclusions on the Densification and Constitutive Parameters of Liquid-Phase-Sintered YBa2Cu3O6+x Powder Compacts

The presence of rigid inclusions in a powder compact leads to a reduction in the densification rate of the compact and may also lead to processing defects. In this paper, the densification rate and the constitutive parameters of both homogeneous YBa₂Cu₃O_{6+ x} and composite powder compacts (YBa₂Cu₃O_{6+ x} powder with 10 vol% dense inclusions of YBa₂Cu₃O_{6+ x} ) are reported. A small amount of liquid phase, which formed during sintering, was present in the samples. However, even with the presence of a liquid phase, the addition of inclusions still reduces the densification rate of the composite and increases its viscosity. The results have been compared with a published analysis of the problem using measured values of the constitutive parameters. Both the viscosity and viscous Poisson's ratio of the porous body have been measured.     

Fabrication and Microstructure of Composite Metal-Clad Ceramic Superconducting Wire

Sheathed high- T _c superconducting ceramic wire composites have been fabricated by wire drawing. Superconducting powders of YBa₂Cu₃O_7-x (Y-123) and Bi₄(Sr, Ca)₆Cu₄O_16+x (Bi-464) have both been prepared and successfully fabricated into continuous wire form. Under certain circumstances, a displacement reaction occurs at the interface between the ceramic superconductor and the containment tube; the nature of this displacement reaction and its prevention are discussed. Microcracking of the ceramic core is attributable predominantly to relative shrinkage effects during sintering. Failure of the metal wall during wire fabrication is attributed to drawing conditions and the mechanical properties of core and cladding.     

Rare-Earth Barium Stannates: Synthesis, Characterization, and Potential Use as Substrates for YBa2Cu3O7-δ Superconductor

A new group of complex perovskites, REBa₂SnO_5.5 (where RE = Pr, Sm, Eu, and Gd) have been synthesized and sintered as single-phase materials with high sintered density and stability by the solid-state reaction method. The structure of REBa₂SnO_5.5 was studied by the X-ray diffraction technique, and all of them were found to be isostructural and have a cubic perovskite structure. The dielectric constant and loss factor values of these materials are in a range suitable for substrate applications. The X-ray diffraction and resistivity measurements have shown that there is no detectable reaction between YBa₂Cu₃O_7-δ and REBa₂SnO_5.5, even when the substances are mixed thoroughly and sintered at 950°C for 15 h. Thick films of YBa₂Cu₃O_7-δ fabricated on polycrystalline REBa₂SnO_5.5 substrates gave superconducting zero resistivity transition T _C(0)= 92 K, indicating the suitability of these new materials as substrates for YBa₂Cu₃O_7-δ films.     

Electronic Structure of High-Temperature Superconductors: Comparison of Theory and Experiment

The ab initio pseudofunction method has been used to compute electronic properties of superconductors in good agreement with experiment. La₂CuO₄ and CuO are computed to be antiferromagnetic and semiconducting in agreement with experiment. The 0.4-eV peak in the optical conductivity spectrum has been shown to be an interband transition for YBa₂Cu₃O₇. The bands for Bi₂Sr₂CaCu₂O₈ as well as YBa₂Cu₃O₇ agree well with angle-resolved photoemission near E _f. The shake-up peaks near 9 and 12 eV observed in photoemissio can be understood in terms of two-electron processes envolving the local densities of states of Cu and O. A mechanism for superconductivity based on these electronic properties will be advanced. All calculations use local density potentials with no parameters.     

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.