High-temperature Superconductor Bi Research Articles

Josephson plasma resonance in Bi 2Sr 2CaCu 2O 8+ δ in vortex liquid and solid states

Both vortex liquid and solid states of high-temperature superconductor Bi 2Sr 2CaCu 2O 8+ δ have been studied by means of Josephson plasma resonance (JPR). It is found that although the JPR frequency changes at the first-order vortex lattice melting transition (VLMT), T M, by 20–30% at low temperatures, it crosses T M almost continuously at high temperatures near T c. Such a nearly continuous change from liquid to solid state strongly implies that the phase transition is so weak that the disappearance of the superconducting phase correlations is nearly continuous. This situation may be realized by considering the strong 2D superconducting fluctuations, resulting in the strong softening of the proper plasma mode in the solid phase.

Properties of Ag–Mg alloy sheathed Bi-2223 tapes

The mechanical properties of 37 filament, pure Ag and Ag alloy (0.2 and 0.5 wt% Mg) sheathed Bi-2223 high temperature superconductor tapes were investigated. The use of Ag–Mg alloys enabled strengthening of the sheaths by an oxide dispersion strengthening (ODS) mechanism. The sheath hardness, tape tensile strength and the tolerance of the superconducting critical current to bending and axial strains increased with the concentration of the alloy addition. The sheath hardness results revealed evidence of overaging. Excessive hardening of the sheath, however, lead to the formation of rolling defects during the intermediate deformation stage of tape processing.

X-ray, resistivity and AC susceptibility studies of 100 MeV oxygen ion irradiated polycrystalline Bi-2223

Changes in the physical properties of Pb-doped and Pb–Sn doped polycrystalline Bi-2223 high temperature superconductors irradiated by 100 MeV O 7+ were studied. All the samples irradiated were prepared by usual solid state reactions. The parent compound was single phase with a T c (0) value of 106 K for the Pb-doped batch and 107 K for the Pb–Sn batch. The samples were irradiated at various fluences in the range of 10 12–10 14 ions/cm 2. For both the cases it was observed that maximum damage has been produced at an optimal fluence value varying between 10 13 and 5 × 10 13 ions/cm 2. The phase transitions probably due to oxygen disordering are evident from X-ray analysis which is supported by ρ– T measurements. AC susceptibility studies show changes in the real part of χ. A shift in the peak temperature in the imaginary part of the AC susceptibility with various fluences is also observed.

Third-order elastic constants of the high temperature superconductor Bi 2Sr 2CaCu 2O 8

The complete set of non-vanishing third-order elastic moduli of high temperature superconductor Bi 2Sr 2CaCu 2O 8 (Bi-2:2:1:2) is obtained theoretically. The interactions of nine nearest neighbours of each atom in the unit cell of Bi-2:2:1:2 are taken into account, while estimating the strain energy density. Here the finite strain elasticity theory is used to obtain the strain energy density of the Bi-2:2:1:2 system. The energy density φ is compared with the strain dependent lattice energy from the continuum model approximation. The second-order parameter of the potential function φ is fixed by making use of easily and more accurately measurable principal axis C ij . The third-order potential parameter is estimated by assuming a Mie–Grüneisen type interatomic potential. The interlattice displacements as well as the second-order elastic constants are evaluated along with the 12 third-order elastic constants of Bi 2Sr 2CaCu 2O 8. The second-order elastic constants are in good agreement with the reported experimental data. In the case of third-order elastic moduli no such experimental data are available for comparison. All the 12 third-order elastic constants are negative; and the absolute magnitude of third-order elastic constants are generally one order higher than second-order elastic constants as generally expected of crystalline solids. The third-order elastic moduli of different modes show anisotropy.

Gapless superconductivity in Raman studies of cation substituted Bi 2Sr 2CaCu 2O 8+ δ

The electronic Raman scattering (ERS) spectrum of the high-temperature superconductor Bi 2Sr 2CaCu 2O 8+ δ is studied as a function of cation substitution and oxygen content. In unsubstituted crystals with δ near and above that for the optimal T c, the ERS spectra show a redistribution of spectral weight below T c consistent with the opening of a superconducting energy gap at 2 Δ. At low oxygen concentrations and in cation substituted samples with optimal oxygen content, the pair-breaking peak is not observed, displaying instead “gapless” or “pseudogap” spectra. When cation substituted samples are post-annealed in high pressure oxygen, the 2 Δ peak remarkably appears.

Ion beam switching magnet employing HTS coils

This paper describes the design, construction, and testing of an ion beam switching high-temperature superconducting (HTS) magnet installed at Institute of Geological and Nuclear Science (IGNS) in New Zealand. It was designed by a consortium comprising American Superconductor Corporation (ASC), ISYS, Applied Engineering Technologies (AET), the New Zealand Institute for Industrial Research and Development (IRL), and Alphatech. The work was also supported in part by New Zealand Foundation for Research and Technology-Technology for Business Growth Programme. The magnet generates 0.72 T in the airgap between two 410/spl times/700 mm warm iron poles. The Bi-2223 HTS coils are conduction-cooled with a single stage Gifford-McMahon (G-M) type cryocooler for steady-state operation. The magnet was fully tested at ASC during the fall of 1996. This represents the first large-scale fully operational HTS physics magnet announced so far. The successful operation of this magnet has verified maturation of HTS magnet technology employing conduction cooling techniques with G-M type cryocoolers. Long term operation of this magnet in continuous use will prove the reliability of HTS magnet systems in critical applications and is expected to open future opportunities for HTS in other related areas.

Isolation techniques and electrical characterization of single grain boundaries of Bi 2Sr 2CaCu 2O 2 high-temperature superconductor

We have isolated single grain boundaries of a bulk material of polycrystalline high-temperature superconductor Bi 2Sr 2CaCu 2O 2 (Bi-2212) tapes. The tapes were grown on a single crystal of MgO or Ag-foil by partial melting of the superconductor. The isolation techniques consisted of fabricating metal micro-contact pads, wire bonding, and laser patterning of the Bi-2212 superconductor. Results of electrical and magnetic measurements of single grain boundaries suggested a possible existence of Josephson-like junctions coexisting with flux flow junctions. In contrast to bulk samples, T c of a single grain boundary did not change by applying magnetic field. J c and T c were found to be greatly dependent on heat treatment and growth conditions. The current–voltage characteristics of a single patterned bridge followed a relation of the form V∼( I− I c) n for I> I c with n∼1.2.

Interlayer resistance and thermoelectric power as a probe of the Josephson coupling in layered superconductors

We present an experimental study of the interlayer resistance and thermoelectric power (TEP) in the mixed state of the high-temperature superconductor Bi 2Sr 2CaCu 2O 8+ x and organic superconductor κ-( d 8,BEDT-TTF) 2Cu(NCS) 2 in the vicinity of T c. The behaviour of these quantities is discussed in terms of the Josephson coupling in these layered superconductors. In contrast to the tunnel origin of the Josephson coupling in Bi 2Sr 2CaCu 2O 8+ x , the superconductor-normal metal-superconductor coupling is suggested to take place in κ-( d 8,BEDT-TTF) 2Cu(NCS) 2. The observed difference in the temperature dependencies of the TEP and the resistance in the latter compound is related to the significant charge imbalance at the multiple NS boundaries.

Development and fabrication of a Bi-2223 racetrack coil for generator applications

The development and fabrication of a layer-wound, epoxy-impregnated Bi-2223 high-temperature superconducting (HTS) racetrack coil which generates 40000 ampere-turns of magnetomotive force (MMF) at 25 K is described. The coil was wound using Ag-sheathed Bi-2223 tape conductor laminated with copper foils for strength enhancement and insulated using a paper-wrap method. After epoxy impregnation, the coil was tested over a range of 16-25 K in a vacuum dewar using a closed-cycle helium refrigeration system. Descriptions of the tape lamination and insulation processing, the coil winding and impregnation, and the experimental test setup are given.

Mechanical behavior of MgO-whisker reinforced (Bi, Pb)2Sr2Ca2Cu3Oy high-temperature superconducting composite

The inherently weak mechanical properties of bulk monolithic high-temperature superconductors (HTS) have been a concern. Properly selected reinforcements in fiber and whisker forms have been introduced to the HTS ceramics to improve their mechanical properties. In this paper, mechanical behavior of a MgO-whisker reinforced Pb-doped Bi-2223 (BPSCCO) HTS composite fabricated by a solid-state processing method is studied. The (MgO)w/BPSCCO HTS composite has been shown to exhibit excellent superconducting properties. Elastic properties, strengths, and notched fracture toughnesses of both the monolithic BPSCCO and the (MgO)w/BPSCCO HTS composite are investigated. Detailed mechanical properties are reported for the first time for the (MgO)w/BPSCCO HTS composite. Mechanisms of strengthening and toughening in the MgO-whisker-reinforced HTS composite are also discussed.

Solid-state processing and phase development of bulk (MgO)w/BPSCCO high-temperature superconducting composite

The inherently weak mechanical properties associated with monolithic high-temperature superconductors (HTS) can be improved by introducing properly selected strong ceramic whiskers into the HTS materials. In this research, processing and superconducting properties of monolithic Pb-doped Bi-2223 (BPSCCO) and MgO whisker-reinforced BPSCCO HTS composite materials have been systematically studied. A solid-state processing method is successfully developed to fabricate the (MgO)w/BPSCCO composite. The HTS composite contains a dense and highly pure BPSCCO matrix phase with a preferred grain orientation, which is reinforced by MgO whiskers randomly oriented in the plane perpendicular to the hot-pressing direction. The HTS composite material is shown to exhibit excellent superconducting properties. For example, a transport Jc measured at 77 K in a zero field has been obtained to exceed 5000 A/cm2 in a (MgO)w/BPSCCO composite with 10% MgO whiskers by volume. Relationships among solid-state processing variables, HTS phase development, and superconducting properties of the monolithic BPSCCO and the HTS composite are established in the paper.

Electrochemical behaviour of copper, lead, and bismuth solid compounds and their ternary mixtures at solid state at a carbon paste electrode with electrolytic binder. Application to the study of the high transition temperature superconductor Bi 0.7 · Pb 0.3 · Sr · Ca · Cu 1.8 · O x

The electrochemical behaviour of solid species of different oxidation states of copper, lead, and bismuth and their ternary mixtures is studied by scan voltammetry using a carbon paste electrode with electrolytic binder and electroactive compound incorporated (CPE-EB-ECI). The electrolytic binder was a 10N KOH solution, the scan potential range was between +0.50 and −1.50 V ( sce), and the scan rate was 0.5 mVs −1. The obtained electrochemical voltammograms were employed to elucidate the corresponding ion electrochemical transformations. In this way, the obtained conclusions were utilized to characterize the different peaks of the electrochemical profile of the high transition temperature superconductor Bi 0.7 · Pb 0.3 · Sr · Ca · Cu 1.3 · O x.

Neutron scattering from the flux-line lattice in Bi 2Sr 2CaCu 2O 8 + y

Neutron small-angle diffraction has been used to investigate the flux-line lattice structure within single crystals of the high-temperature superconductor Bi 2.15Sr 1.95CaCu 2O 8 + x . The diffracted intensity goes rapidly to zero as the magnetic field or the temperature is increased. Melting at low fields as a function of temperature coincides with the appearence of finite resistance within the superconducting state. At low temperatures the diffracted intensity disappears in fields greater than ∼ 70 mT, probably due to the decomposition of the flux-line lattice into randomly pinned 2d “pancake” vortices.

Conceptual design of a 20-kA current lead using forced-flow cooling and Ag-alloy-sheathed Bi-2223 high-temperature superconductors

High-temperature superconductors (HTSs) consisting of Bi-2223 HTS tapes sheathed with Ag alloys are proposed for a 20-kA current lead for the planned stellarator WENDELSTEIN 7-X. Forced-flow He cooling is used, and 4-K He cooling of the whole lead, as well as 60-K He cooling of the copper part of the lead, is discussed. Power consumption and behavior in the event of loss of He flow are given. >

Performance characterizations of Bi-2223 composite powder-in-tube conductor elements

Abstract A 30 MW superconducting magnetic energy storage (SMES) system for electric utility application is currently under design by Babcock & Wilcox. High-temperature-superconductor (HTS) current leads have been designed to reduce the refrigeration load of the SMES system's 16 kA capacity electrical connection between the 300 K power conditioning system and the 4 K low-temperature-superconductor energy storage magnet. Each lead employs an array of 18 parallel HTS conductor elements. Each conductor element consists of a sintered stack of Bi-2223 HTS powder-in-tube tapes, each having an Au-alloyed Ag sheath. Laboratory performance characterization of preproduction conductor elements included critical current with applied field at 77 and 60 K, thermal and electrical cycling effects, critical current vs axial strain, and end-connection electrical resistivity. Details of the conductor element application and construction are described. The methodology and results of the performance characterization are presented. The testing demonstrates mechanical robustness and electrical stability required for long-term practical application in the SMES system.

Low temperature scanning tunneling microscopy and spectroscopy of single crystals of the high temperature superconductor Bi 2Sr 2CaCu 2O 8, cleaved “in-situ” and at room temperature

A low-temperature scanning tunneling microscope is used for simultaneous topography and superconducting energy gap measurements of Bi 2Sr 2CaCu 2O 8+ d (Bi2212) single crystals. The samples were cleaved either in situ at 4.2 K or at room temperature with the surface parallel to the a– b planes. The interpretation of the conductance data suggests a superconducting gap value along the c-direction 2 Δ/ k B T c–9 for the room temperature cleaved crystals and 2 Δ/ k B T c–6.7 for the low-temperature cleaved samples.

Microwave absorption in the high temperature superconductor Bi 2Sr 2Ca 1−xGd xCu 2O 8

Microwave absorption in the high temperature superconductor Bi 2Sr 2Ca 1−xGd xCu 2O 8

Temperature dependence of the upper critical field in high-temperature superconductors: Localization effects

It is shown that the anomalous temperature dependence of the orbital part of the upper critical field H c2 observed for epitaxially grown films of high-temperature superconductors Bi  Sr  Cu  O (in wide temperature interval) can be satisfactorily explained by the influence of localization effecs in two-dimensional(quasi-two-dimensional) case.

Anderson-like insulator - metal transition in rare earth doped Bi 2Sr 2(Ca z RE 1 − z)Cu 2O 8 + y

The transition from the insulating compounds Bi 2Sr 2RE 1Cu 2O 8 + y (RE=rare earth) to the High-Temperature superconductor Bi 2Sr 2Ca 1Cu 2O 8 + y was studied. The substitution of Ca 2+ for RE 3+ has two effects: It introduces hole-like carriers and creates local disorder. The disorder causes localization of the electronic states in the tails of the conduction band and thus an Anderson-like transition. The carrier density scales with the rare earth valence and concentration justifying a simple chemical picture. For low Ca-content the samples are insulating and charge is transported by hopping processes. By increasing the Ca-concentration the hole density increases which leads to an insulator-metal transition at a critical concentration z c. The RE-ions show Curie-Weiβ like magnetic susceptibility and are coupled to each other through the delocalized carriers.

Giant magnetostriction of cuprates in superconducting state

Abstract Unusually large magnetostriction was found in a single crystal of high temperature superconductor Bi 2 Sr 2 CaCu 2 O 8 . The sample length change was measured along the ab -plane under magnetic field applied parallel to the c-axis up to 6 T. The relative sample length change which exceeded the order of 10 −4 at 4.8 K was negative during the field increasing process, and became positive by sweeping the field back to zero, giving rise to a large hysteresis. For a (La 1−x Sr x ) 2 CuO 4 ( x = 0.05) sample, the magnetostriction was smaller by two orders, but the shape of the magnetostriction curve exhibited many similarities with that of Bi 2 Sr 2 CaCu 2 O 8 . The behavior of these magnetostriction curves can be well-explained by the compressive and expansive forces internally exerted within the crystal due to the flux pinning.