Processing high critical current density Bi-2223 wires and tapes

Considerable progress has been made in fabricating (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ in high-T_c superconductor wires or tapes with high critical current densities that are attractive for electric power and high-field magnet applications. The powder-in-tube technique appears to be useful for making silver-clad Bi-2223 composites. This article discusses the processing and the excellent superconducting properties of the resulting wires and tapes.     

Effect of water intercalation on the structure and electrophysical properties of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.9</Subscript>

The influence of water vapors and plastic deformation on the structure and electrophysical properties of YBa₂Cu₃O_6.9 (123) has been studied. It has been established that, at T = 200°C, the introduction of water into the structure of YBa₂Cu₃O_6.9 leads to its transition into a defect tetragonal phase of the 124 type as a result of the formation of planar stacking faults. After annealing at T = 930°C, these defects are partially retained and are efficient centers of pinning in the magnetic fields applied perpendicularly to the c axis, which makes it possible to increase (by an order of magnitude) the critical current density in the high-textured ceramics at 77 K in the external magnetic field of 5–10 T. The plastic deformation of the hydrated ceramics favors the reverse transition of the arising 124 phase to the 123 phase at T = 930°C and is accompanied by a recrystallization of the material, which leads to the appearance of a texture and an increase the critical current density.     

Advances in the processing and properties of YBa2Cu4O8

High-filament-count, silver-sheathed composite wires of YBa₂Cu₄O₈ were prepared by a metallic precursor route. The ductility of the metallic precursor enabled the manufacture of tapes containing up to 962,407 filaments, with filament dimensions as fine as 0.25 μm thick and 1 μm wide. The combination of thermal-mechanical treatment and fine filament dimensions resulted in significant biaxial crystallographic texture. Transport critical current densities in the oxide filaments of 69.5 kA/cm² at 4.2 Kin self-field with reduced weak-link behavior in an applied field were obtained. Critical current retention during bending was shown to increase as filament count increased, with the highest filament-count material showing close to one percent critical bending strain. These wires exhibited some of the best properties for a polycrystalline, sintered wire of YBCO in an applied magnetic field.     

Cube textured nickel alloy tapes as substrates for YBa2Cu3O7−δ-coated conductors

Suitable substrates for biaxially textured high temperature superconducting YBa₂Cu₃O_7−δ-thin films, which are necessary to carry high currents, are highly cube ({001}〈100〉) textured Ni-tapes that can be produced by heavy cold rolling and succeeding recrystallization annealing. At high temperatures which are necessary for the deposition of the YBa₂Cu₃O_7−δ-film such tapes often tend to abnormal grain growth destroying the cube texture. It was found that this can be avoided by microalloying Ni with 0.1 at.% of different elements each leading to a texture improvement with increasing annealing temperature. YBa₂Cu₃O_7−δ-films deposited on these microalloyed tapes reach critical current densities up to 0.5×10⁶ A/cm². Also non-magnetic Ni-alloys, namely Ni–13 at.%Cr and Ni–9 at.%V, were successfully processed to highly cube textured tapes. Tapes made of these alloys are not only an improvement because magnetic losses are avoided but also because of a three times enhanced yield strength which is due to solid solution hardening.     

Fabricating Ag2O-stabilized superconductive wires

Fabrication of metal-shielded YBa₂Cu₃O_x superconductive wires provides a possible route to improve the poor mechanical properties and the low critical current densities associated with the ceramic nature of high-T_c superconductive oxide materials. However, results show that wires produced by the powder-in-tube method have low critical current densities and a wide superconducting transition range. A possible reason for these phenomena is the presence of a considerable amount of oxygen-deficient phase in the wire core due to insufficient oxygen diffusion through the metallic shield in the final annealing process. Silver oxide (Ag₂O) was introduced as an in-situ oxygen donor to stabilize the oxygen stoichiometry of the core material. Traditional powder and mechanical metallurgy methods have been used to fabricate these metal-shielded YBa₂Cu₃O_x superconductive wires for bulk electricity applications.     

Influence of water vapor on the formation of pinning centers in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> upon low-temperature annealing

The influence of the double heat treatment (T = 300 and 930°С) on the critical parameters of highly textured YBa₂Cu₃O_6.96 and YBa₂Cu₃O_6.8 ceramics has been investigated. It has been shown that, upon low-temperature annealing in humid air, planar stacking faults are formed in these ceramics. These defects are partly retained after reduction annealing (at T = 930°С) and are efficient pinning centers in magnetic fields applied parallel and perpendicular to the c axis. Due to the absorption of water, the oxygen content is increased in the ceramics, which is accompanied by an increase in the critical temperature of superconducting transition up to 94 K for YBa₂Cu₃O_6.96 and up to 90 K for YBa₂Cu₃O_6.8. Optimal conditions of the double annealing have been established, after which the critical-current density increased to j _c ≥ 10⁴ А/сm² in an external magnetic field of up to 6 T. The low-temperature treatment in the neutral atmosphere saturated by water vapors deteriorates the current-carrying capacity of the highly textured ceramics, which is connected with the disappearance of texture due to the copper reduction and the precipitation of impurity phases.     

Critical issues in the OPIT processing of high-Jc BSCCO superconductors

The oxide-powder-in-tube (OPIT) method has produced superconducting wires or tapes that exhibit high critical current densities in high magnetic fields when the superconducting oxide is the two-layer compound Bi₂Sr₂Ca₁Cu₂O_y or the three-layer compound Bi_2?xPb_xSr₂Ca₂Cu₃O_y. For good properties to be obtained, however, numerous critical processing issues must be addressed.     

Models for long-range current flow in bulk oxide superconductors

Mitigating the weak-link behavior at grain boundaries, which limits critical current density (J_c) in polycrystalline high-transition-temperature (high-T_c) superconducting materials, is required for all applications in magnetic fields. Although in general oxide superconductors in polycrystalline form have very poor in-field J_c, conductors containing the bismuth-based compounds Bi₂Sr₂CaCu₂O₈ (Bi-2212)andBi₂Sr₂Ca₂Cu₃O₁₀ (Bi-2223) are very important exceptions. Long-range, strongly linked conduction has been demonstrated in just two additional polycrystalline materials: TlBa₂Ca₂Cu₃O₁₀ (Tl-1223) deposits prepared by spray pyrolysis, and YBa₂Cu₄O₈ (Y-124) conductor made by the oxidation of metallic precursor method. Determining the characteristics of these materials that permit strongly linked conduction is of critical importance to the development of better conductors. Models of strongly linked polycrystalline conductors are reviewed. Recent studies reveal that small-angle boundaries are present in much higher numbers than expected from calculations based on macroscopic texture, indicating a strong influence of grain boundary energy.It is proposed that strongly linked current flows through a connected network of low energy grain boundaries. Evidence has been obtained to support this model in Bi-2223, Y-124 and Tl-1223.     

Percolative current flow in high-Jc,polycrystalline high-Tc superconductors

In this article, experiments designed to ascertain the percolative nature of current flow in high critical-current density (J_c) polycrystalline superconductors are reviewed. A direct correlation between the grain-orientation texture with current transport in high-J_c TlB₂Ca₂Cu₃O_x thick films and Bi₂Sr₂Ca₂Cu₃O_x powder-in-tube tapes is obtained. Magnetooptical visualization of the current flow in the same regions confirms the percolative nature of current flow. Furthermore, numerical modeling of the current flow based on the observed grain boundary character distribution and the effects of the grain boundary misorientation angle on (J_c) was found to predict the percolative nature of current flow. These studies indicate that in order to increase the (J_c) further, increased fractions of small-angle boundaries are required and, hence, provide a research direction for the next generation of high-temperature superconducting wire.     

Effect of hydrogen intercalation on the critical parameters of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

The effect of hydrogenation at T = 150 and 200°C on the electrophysical properties of highly textured YBa₂Cu₃O _y ceramics with different oxygen content has been investigated. Like hydration, hydrogenation results in the deterioration of these properties. However, in samples with high oxygen contents (y = 6.96) hydrogenated at T = 150°C after oxidation (400°C) or recovery annealing with subsequent oxidation, the critical current density and first critical field increase compared to the initial state. The improvement of the properties occurs mainly in a magnetic field applied perpendicularly to the c axis. As after hydration, this is connected with the formation of planar defects in the course of low-temperature annealing. In addition, in the process of the hydrogenation, the partial reduction of copper occurs with the formation of microinclusions of Cu₂O and other products of chemical decomposition, which are extra pinning centers of magnetic vortices.     

Progress in the melt texturing of RE-123 superconductors

Steady progress continues in improving the critical current density and flux trapping capability of bulk RE-Ba₂Cu₃O_x (RE = rare earth) superconductors. In this article, advances in processing, the understanding of the role defects play in flux pinning, and promising new developments are discussed. In particular, reviewed are advances in melt-textured rare earth superconductors (particularly YBa₂Cu₃O_x) and avenues for property enhancement.     

Pseudo-gap and the nonlinearity of the magnetization of textured polycrystals of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 − <Emphasis Type="Italic">x</Emphasis></Subscript> above the temperature of transition into the superconducting state

Temperature dependences of the third and other higher harmonics of the magnetization of textured polycrystalline samples of YBa₂Cu₃O_{7 ? x} have been studied in the temperature range of 77–120 K. It has been revealed that the nonlinearity of magnetization of YBa₂Cu₃O_{7 ? x} is observed up to temperatures that considerably exceed the temperature of transition into the superconducting state. The observed nonlinearity of magnetization of YBa₂Cu₃O_{7 ? x} is ascribed to the appearance of a pseudogap state at T ～ 102 K in this compound. A method of determining the temperature of the appearance of the pseudogap T * in the high-T _c compounds is suggested, which is based on the measurement of magnetization harmonics.     

Preparation and characterization of microcrack-free thick YBa2Cu3O7-δ films

High quality epitaxial YBa2Cu3O7-δ （YBCO） superconducting films were fabricated on （00l） LaAlO3 substrates using the direct-current sputtering method. The attainment of an unusually high film thickness （up to 2.0 μm） without mi-crocracking was attributed in part to the presence of pores correlated with yttrium-rich composition in the films. The influ-ence of the film thickness on the microstructure was investigated by X-ray diffraction conventional scan （θ-2θ,ω-scan,pole figure） and high-resolution reciprocal space mapping. The films were c-axis oriented with no a-axis-oriented grains up to the thickness of 2 μm. The surface morphology and the critical current density （Jc） strongly depended on the film thickness. Furthermore,the reasons for these thickness dependences were elucidated in detail.     

Critical current and microstructure in oxide superconductors

Critical current density (J_c) in the presence of a magnetic field is the property that currently limits many applications of the high-critical-temperature oxide superconductors. Poor current transmission at grain boundaries and weak flux pinning are responsible for the low critical current densities observed in bulk materials. Since 1986, substantial progress has been made both in understanding the microstructural factors affecting J_c and in developing practical fabrication methods. Much of the fundamental understanding has been obtained from studies of YBa₂Cu₃O_7?x while the bismuth and thallium-based compounds appear to offer greater potential as the basis for the first practical conductors.     

Niobium aluminide as a source of high-current superconductors

The development of high-critical-current superconducting A15 conductors able to carry current in very high magnetic fields (25–30 T) is recognised as an enabling technology for the construction of second-generation NMR magnets operating at frequencies well above 1 GHz. This paper highlights the current status of development of the niobium-aluminide intermetallics with special attention to Nb₃Al, and Nb₃(Al_1-x,Ge_x). Discussion is focused on the materials science aspects of conductor manufacture, such as β-phase (A15) formation, with particular emphasis on the maximisation of the superconducting parameters, such as critical current density, J_c, critical temperature, T_c, and upper critical field, H_c2. Many successful manufacturing techniques of the potential niobium-aluminide intermetallic superconducting conductors, such as solid-state processing, liquid-solid processing, rapid heating/cooling processes, are described, compared and assessed. Special emphasis has been laid on conditions under which the J_c(B) peak effect occurs in some of the Nb₃Al wires. The case is made that mechanical alloying during final wire preparation plays an important role in diminishing the peak effect, except in the case of A15 formation from a mixture of ‘σ-phase’ and Nb. Further study of the influence of mechanical alloying on the maximisation of the critical current density at high magnetic fields is crucial to the understanding of the peak effect formation in tapes formed from ‘σ-phase’. Additionally, many aspects of conductor design requirements such as stress, strain, a.c. losses, thermal and electromagnetic stabilisation, are discussed with reference to literature sources.     

Effect of Plastic Deformation on the Formation of the YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> Structure

The structure of the YBa₂Cu₃O _y (123) compound has been investigated after severe plastic deformation by high-pressure torsion performed at room temperature. Distinctive structural features of the samples prepared by standard synthesis and the samples (with increased critical current density) after treatment at t = 200°C in water-saturated atmosphere have been revealed. It has been shown that a lamellar textured structure formed during the annealing of hydrated and deformed samples includes a superconducting orthorhombic phase with a high oxygen index and residual defects that can serve as pinning centers.     

Microstructure of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> subjected to severe plastic deformation by high pressure torsion

The influence of plastic deformation carried out by high pressure torsion at room temperature on the microstructure of the YBa₂Cu₃O _y (123) compound prepared by standard ceramic technology and annealed at low temperature (200°C) in a water-saturated atmosphere has been studied. It has been shown that the directional growth of recrystallized lamellar-shaped grains initiated by the 124–123 phase transformation takes place upon recovery (after deformation) annealing at 930°C in ceramics subjected to additional low-temperature annealing, which leads to the formation of the texture. A rodlike structure has been observed in samples prepared by standard technology, after deformation and recovery annealing (930°C).     

Tensile superplastic deformation of YBa2Cu3O7−x high-TC superconductors

The superplastic behavior of polycrystalline YBa₂Cu₃O_7−x was investigated. Tensile elongations above 85% were achieved for fine-grained (<1 μm) microstructures tested in the temperature range of 800–875°C and strain rates varying from 6×10⁻⁶ to 10⁻³/s. It is suggested that the dominant superplastic deformation mechanism is grain boundary sliding accommodated and controlled by interface reaction, characterized by a stress exponent of n=2, a grain size exponent of p=1.5, and an activation energy of Q_sp=515±104 kJ/mol. A Langdon–Mohamed deformation mechanism map was constructed. Overlaying the available experimental data onto the map, including the results from creep studies, gives an insight into the deformation mechanisms involved in high-temperature deformation of YBa₂Cu₃O_7−x.     

A method for measuring optical twin spacings in the superconducting material,YBa2Cu3Ox

The optical twins in grains of the high temperature superconducting material, YBa₂Cu₃O_x, lie along {110} planes, and the long boundaries of elongated grains are basal planes. Equations are presented utilizing these facts, which allow the apparent twin spacing to be converted to true twin spacing by measurement of the angles between twin sets and the basal plane boundary on the polish surface. The true thickness of the elongated grains in their C axis direction may also be determined from the analysis.     

Structure of buffer CeO2 and LaNiO3 layers prepared by method of pulsed laser deposition on textured Ni-Cr-W substrates

The results of an investigation of the structure, composition, and topography of the surface of buffer layers of CeO₂, LaNiO₃, and CeO₂ + LaNiO₃ on nonmagnetic textured NiCr_9.2W_2.4 substrates with a cube texture, which were prepared by pulsed laser deposition, are presented. The substrates with a buffer layer of this composition can be of interest for growing thin films and heterostructures of a wide class of oxide compounds, including superconductors of composition YBa₂Cu₃O_{7 ? δ} (123).