Preparation and Characterization of a Large-Scale YBa2Cu3O7−x Superconductor Prepared by Plastic Forming without a High-Pressure Molding: Effect of Polyvinyl Alcohol (PVA) Addition on the Superconducting Properties

The preparation of large-scale YBa₂Cu₃O_{7− x} superconductor samples was investigated. This method is based on plastic forming using a slurry consisting of YBa₂Cu₃O_{7− x} particles and a sol solution made up of multimetallic hydroxide particles (YBa₂Cu₃(OH) _x colloidal particles) and poly(vinyl) alcohol (PVA). The effects of adding PVA on the product, the crystallinity, and the superconducting properties of the sample were investigated. It was found that PVA acted as a protective colloid in the sol solution and stabilized YBa₂Cu₃(OH) _x colloidal particles, and that the role of PVA changed from a thickener to a flocculant during drying so that the formability/workability of the green sheet sample was improved and large samples (about 80 mm × 80 mm × 3 mm) without large cracks were obtained after firing. The samples became superconducting at 91.5±0.5 K ( T _con) and the full transition temperature ( T _coff) was 88.5±1.5 K. The critical current density ( J _c) of the sample prepared from the slurry containing 1 wt% PVA was 713±150 A/cm² at 77 K. This J _c value was improved to 2300 A/cm² by heat treatment at 773 K under an oxygen 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.     

Processing Dependence of Texture, and Critical Properties of YBa2Cu3O7−δ Films on RABiTS Substrates by a Non-Fluorine MOD Method

YBa₂Cu₃O_{7− δ} (YBCO or Y123) films on rolling-assisted biaxially textured substrates (RABiTS) were prepared via a fluorine-free metallorganic deposition (MOD) through spin coating, burnout, and high temperature anneal. The effects of substrate texture and surface energy of the CeO₂ cap layer were investigated. Except for the commonly accepted key factors, such as the textures of substrate and buffer layers, we found some other factors, for example, the deposition temperature of the cap layer, are also critical to the epitaxial growth of Y123 phase. With the CeO₂ cap layer deposited at relative high temperature of 700°C, a critical current density, J _c, over 1 MA/cm² has been demonstrated for the first time on Ni-RABiTS by a fluorine-free MOD method. Whereas for samples with CeO₂ cap layers deposited at a lower temperature of 600°C, even though XRD data showed a better texture on these buffer layers, texture degradations of YBCO grains under the optimized processing conditions were observed and a lower oxygen partial pressure around 40 ppm was necessary for the epitaxial growth of Y123 phase. As a result, J _c fell to 0.45 MA/cm² at 77 K. The observed phenomena points to the change of surface energy and reactivity of the CeO₂ cap layer with respect to the CeO₂ deposition temperature. In this paper, the YBCO phase diagram was also summarized.     

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.     

Epitaxial Solution Deposition of YBa2Cu3O7−δ-Coated Conductors

A variety of solution deposition routes have been reported for processing complex perovskite-based materials such as ferroelectric oxides and conductive electrode oxides, due to ease of incorporating multiple elements, control of chemical stoichiometry, and feasibility for large area deposition. Here, we report an extension of these methods toward long length, epitaxial film solution deposition routes to enable biaxially oriented YBa₂Cu₃O_7−δ (YBCO)-coated conductors for superconducting transmission wires. Recent results are presented detailing an all-solution deposition approach to YBCO-coated conductors with critical current densities J _c (77 K)>1 MA/cm² on rolling-assisted, biaxially textured, (200)-oriented Ni–W alloy tapes. Solution-deposition methods such as this approach and those of other research groups appear to have promise to compete with vapor phase methods for superconductor electrical properties, with potential advantages for large area deposition and low cost/kA·m of wire.     

High Critical Current YBa2Cu3O7−δ Thick Films on Improved Rolling-Assisted Biaxially Textured Substrates (RABiTS™)

YBa₂Cu₃O_7−δ (YBCO) films with thicknesses ranging from 1.0 to 6.4 μm were deposited by pulsed laser deposition on rolling-assisted biaxially textured substrates (RABiTS). The RABiTS were of the configuration CeO₂/YSZ/Y₂O₃/Ni–3 at.% W. As the YBCO film thickness increased, I _c continued to increase and reached ∼300 A/cm width for a 4.3 μm-thick YBCO film. Commonly observed mechanisms for J _c decrease with increasing YBCO film thickness were not observed. Homogeneous microstructures obtained in even the thickest YBCO films, suggest that the I _c/width can still be enhanced considerably.     

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.     

Thermal Expansion of YBa2Cu3O7_xas Determined by High-Temperature X-ray Diffraction under Controlled Oxygen Partial Pressures

The high-temperature phase relationship and thermal expansion coefficient of YBa₂Cu₃O_7-x under constant oxygen nonstoichiometry, x, were determined by high-temperature powder X-ray diffraction analysis under controlled oxygen partial pressure at temperatures up to 800^deg;C. The results are discussed based on reported nonstoichiometry data. The present study showed an orthorhombic-to-tetragonal transition near the composition x = 0.5. The lattice parameter c, perpendicular to the Cu-O plane, showed a maximum at around x = 0.7 to 0.8. In the ortho-rhombic phase, the lattice parameters a and b along the Cu-O plane were essentially constant for x < 0.2. For 0.2 < x < ∼ 0.5, a increased and b decreased with x. In the tetragonal phase, with x < ∼ 0.5, the lattice parameter a decreased with x. The thermal expansion coefficient, α, along the c-axis ranged from 19 × 10^-6 to 25 × 10^-6-K^-1, whereas a along the a- and b-axes ranged from 12 × 10^-6 to 22 × 10^-6-K^-1 at 400° to 800^deg;C, and these values were very small below 400^deg;C. It was found that a, b, and α along the a- and b-axes are smaller when the oxygen content along the respective axes is less, while the area of the ab plane and its thermal expansion coefficient are larger when the deviation of the oxygen content from the stoichiometric compositions of YBa₂Cu₃O₇ or YBa₂Cu₃O₆ is larger. Changes of x and temperature affected c more strongly than a and b.     

Effect of Extrusion on the Critical Current Density of Bi1.4Pb0.6Sr2Ca2Cu3Ox Superconducting Wires

The effect of extrusion on improving the critical current density ( J _c) of Bi_1.4Pb_0.6Sr₂Ca₂Cu₃O _x superconducting wires is investigated. Calcined powders (Bi_1.4Pb_0.6Sr₂Ca₂Cu₃O _x ) are first mixed with a forming aid—a thermoplastic polymer (polyethylene)—for workability, and then extruded, using a capillary rheometer, to form wires 2 mm in diameter. The J _c value, measured by the four-probe method in liquid nitrogen at 77 K, is improved substantially by the following process: (1) the superconducting precursors are extruded at high viscosity with a forming aid, to align the platelike particles unidirectionally; (2) the forming aid alone is carefully burned out, without destroying the extruded configuration; and (3) the extruded wires are annealed at 850°C in air for more than 96 h.     

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.     

Preparation of YBCO Films on CeO2-Buffered (001) YSZ Substrates by a Non-Fluorine MOD Method

YBa₂Cu₃O_7−δ (YBCO) films were fabricated via a fluorine-free metal organic deposition (MOD) method followed by high-temperature, low oxygen partial pressure annealing. Trimethyl acetate salts of copper, yttrium, and barium hydroxide were used as the precursors, which were dissolved in proponic acid- and amine-based solvents. After spin-coating and burnout, samples were annealed at 740°C in 180 ppm oxygen partial pressure and exposed to humid atmosphere for different times. A critical transition temperature, T _c( R =0) of 90.2 K and a transport critical current density ( J _c) of 0.55 MA/cm² (77 K and self-field) were obtained for 0.2 μm YBCO films on CeO₂-buffered yttria-stabilized zirconia (YSZ) substrates. X-ray studies shows that the YBCO films have sharp in-plane and out-of-plane texture for all samples; however, the porosity of the YBCO film varies with the time of exposure to the humid atmosphere. A reaction between YBCO and CeO₂ during the high-temperature anneals and formation of the reaction product BaCeO₃ was confirmed by X-ray diffraction (XRD) studies. The XRD and transmission electron microscopy analysis indicated that the epitaxial relations in the film were YBCO (00 l )//CeO₂ (00 l )//YSZ (00 l ) and YBCO [100]//CeO₂ [110]//YSZ [110].     

Superconductive Properties of the YBa2Cu4O8 Superconductor

A bulk density of 85% of the theoretical density was achieved by sintering a powder compact of YBa₂Cu₄O₈ (124) at 850°C in flowing oxygen at 1 atm (≅10⁵ Pa). This value is very close to that obtained by the hot isostatic pressure technique (90%). The superconducting properties of the sample were characterized by magnetization and ac susceptibility techniques. The magnetization critical current density at 20 K in zero field was determined to be ∼5 × 10⁴ A/cm², and the superconducting transition temperatures were found to be 77 K for the bulk material and 82 K for the granular phase. The powder X-ray diffraction and ac susceptibility studies revealed the sintered 124 material to be single phase.     

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.     

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.     

A Review of the Influence of Grain Boundary Geometry on the Electromagnetic Properties of Polycrystalline YBa2Cu3O7−x Films

Shortly after the discovery of high-temperature superconducting (HTS) materials in the late 1980s, it was revealed that grain boundaries in these complex oxides are strong barriers to current flow. This fact has remained one of the most significant challenges to a viable HTS conductor, and necessitated the development of technologies capable of producing biaxially textured substrates in long lengths. Multiple studies have reported that the critical current density ( J _c) across grain boundaries in the perovskite-like superconductor YBa₂Cu₃O_{7− x} (YBCO) falls off exponentially below the intragrain J _c beyond a critical misorientation angle θ_c of only ≈2°–3°. Here we review our recent work demonstrating that certain grain boundary geometries permit significant enhancements of J _c well beyond the conventional J _c(θ) limit, and also that the grain boundary structure in YBCO films is tied closely to the films' deposition technique. Pulsed laser deposition, a physical vapor deposition technique, results in a columnar grain structure and planar grain boundaries that exhibit the typical J _c(θ) dependence. Ex situ growth processes, where the YBCO film is converted from a previously deposited precursor, can result in laminar grain growth with highly meandered grain boundaries. These latter grain boundary structures are directly correlated to greatly improved J _c values over a wide range of applied magnetic fields. Consequently, very high J _c values are possible in polycrystalline HTS wire even when significant misorientations between grains are present.     

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.     

Ag2O-Doped Bi2Sr2Ca1Cu2Ox Superconductors Prepared via Melt-Quenching

Ag₂O-doped superconducting Bi₂Sr₂Ca₁Cu₂O _x ceramics were prepared by a melt-quenching–reheating method. It is found that the Ag₂O-doped, as-cast specimens exhibit superconductivity ( T _c= around 80 K) by heat treatment at temperatures around 800°C even in an evacuated and sealed silica glass tube, while the undoped specimens do not and vaporize by the corresponding heat treatment. Conversion of the Ag₂O-doped, as-cast specimens into superconducting ceramics when heated in an evacuated vessel is explained in terms of the oxygen donor of Ag₂O in the specimen. This finding enables us to fabricate a desired shape of superconducting Bi₂Sr₂Ca₁Cu₂O _x ceramics sealed in metals or glasses. The addition of Ag₂O to Bi₂Sr₂Ca₁Cu₂O _x melt, however, had deleterious influences on the superconducting properties ( T _c and J _c) of the resultant ceramics when obtained by heat treatment in air.     

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.     

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.     

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.