Bi-based High-temperature Superconductors Research Articles

High-pressure studies on pristine and Pb-substituted Bi-based high-temperature superconductor

High-pressure studies on pristine and Pb-substituted Bi-based high-temperature superconductor

15 MeV proton irradiation effects on Bi-based high temperature superconductors

AbstractNowadays, superconducting magnetic coils are used in some tokamaks such as EAST, KSTAR, JT-60, and T-15 to generate strong magnetic fields and also in ITER magnetic fields of about 13 tesla will be produced with the help of superconductors. The tokamak superconductors are exposed to the variety of radiations (neutron, ions beam, and gamma) from plasma nuclear reactions which will affect some of the superconductor properties. Therefore, study of the irradiation effects on the superconductor structure and properties are very crucial from technological and scientific point of view. One of the superconductor irradiation effects to be investigated under different conditions of energy and dosage is the potential resistance of the material used in tokamak reactor magnetic coils against activation by radiation. In this work, pellets of high TcBi-based superconductors have been prepared and after measurement of parameters, a sample of pellet has been irradiated with 15 MeV protons using Karaj cyclotron facility. The sample's parameters have been measured again after irradiation treatment. X-ray diffraction patterns and SEM images of the sample before and after irradiation treatment have been studied.

Texturing of Micaceous Superconductor Powder and Fabrication of Wire to Preserve the Texture

The Bi-based high-temperature superconductors are micaceous: grains cleave preferentially in the ab plane when fractured, so that powders consist of thin platelets. Superconducting current transport is similarly planar, and supercurrent flow is largely limited to the ab plane. We report studies of two methods to align the particles of such powder in a thin layer so that most particles are aligned with parallel ab planes. We are developing a `jelly-roll' method for wire fabrication that could preserve this desirable texture.

Tuning the Interlayer Spacing of High-TcBi-Based Superconductors by Intercalation: Measuring the Penetration Depth and the Two-Dimensional Superfluid Density

Substantial control of the interlayer spacing in Bi-based high temperature superconductors has been achieved through the intercalation of guest molecules between the superconducting layers. Measurements using implanted muons reveal that the penetration depth increases with increasing layer separation while T_{c} does not vary appreciably, demonstrating that the bulk superfluid density is not the determining factor controlling T_{c}. Our results strongly suggest that for Bi-based high temperature superconductors the superfluid density appearing in the Uemura scaling relation rho_{s} proportional, variantT_{c} should be interpreted as the two-dimensional density within the superconducting layers, which we find to be constant for each class of system investigated.

Optimization of Cross-sectional Shapes of the Bi-2223/Ag Wires before Flat Rolling

Rolling process plays an important role in the manufacture of Bi-based high temperature superconductor tapes, and the plastic flow regularities of the superconducting wires during deformation will directly affect the ultimate quality of the tapes. In order to investigate the effect of cross-sectional shapes before flat rolling on the performance and homogeneity of the tapes, some numerical models of Bi-2223/Ag wires with different cross-sectional shapes including circular, square, elliptical and racetrack cross-sections are constructed during the rolling process. By comparing the relative density, logarithmic strain ratio and length-width ratio on the filaments, it is revealed that Bi-2223/Ag wire with special-shaped cross-section can achieve better conductivity than the round wire, in particular, the racetrack cross-sectional wire has the second best performance among four wires. Based on material processability and experimental condition, tri-pass racetrack drawing technique is employed to optimize the process and obtain racetrack cross-sectional wire. The rolling process of Bi-2223/Ag wire with racetrack cross-section causes more intensive deformation of filaments in the center of the tape and achieves the filaments with larger length-width ratio. Also, the deformation distribution of filaments verifies the numerical results. Consequently, the racetrack drawing technique can be utilized for a reference during the mechanical processing and to increase the current transmission capacities of Bi-2223/Ag tapes.

Study of current–voltage characteristics of Bi-based high-temperature superconductors with fractal cluster structure

We study the effect of the structure on critical currents and current–voltage characteristics (CVC’s) of foamed bismuth-based polycrystalline high-temperature superconductors (HTSC). The fractal cluster structure of superconducting foams has been observed and the fractal dimension of boundaries between superconducting and normal clusters has been determined. Based on the magnetic and transport properties of the foamed polycrystalline superconductors, we have shown that the initial parts of CVC’s of the superconducting foams are described well by the model that accounts the magnetic flux trapping in fractal clusters of a normal phase.

Microstructure control of Bi-based high-temperature superconductors heat-treated in high magnetic fields

Microstructure control of Bi-based high-temperature superconductors heat-treated in high magnetic fields

Superconductivity‐induced phononic effects in high‐temperature superconductors: Raman study

Raman scattering spectra of (123) and Bi-based high-temperature superconductors (HTSC) with different doping levels have been investigated. It is demonstrated that phonons in HTSC can provide unique information on energy, symmetry, temperature- and doping-dependencies of the superconducting gap and pseudogap. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of doping on properties of Bi‐based superconductors

The influence of alkaline metals on the properties of Bi-based high temperature superconductors has been studied. The samples Bi3.2Pb0.8Sr4Ca5Cu7AxOy (A = Li or Cs, x = 0, 0.2, 0.4, 0.6, 0.8) were prepared and characterized by XRD analysis, resistivity and susceptibility measurements. The results showed that all the doped samples had Tc in the range 104–110.5 K. The transition width ΔT of the Bi-2223 phase in the lithium doped samples increased with higher concentration of dopant. In the cesium doped samples the transition width decreased with increasing Cs level up to x = 0.6 then slightly increased. Values of Jc of lithium containing samples decreased with increasing amount of doped lithium. Samples doped with lower amount of cesium (x = 0.2, 0.4) had notably higher critical current densities than the undoped standard. Nevertheless, Jc's of samples with higher amount of cesium were worse than that of the standard. Lithium had a positive influence on the formation of Bi-2223 phase in all concentration except x = 0.6. In samples doped with cesium the volume of Bi-2223 phase was higher than in the undoped sample as well, the values were not as high as in lithium doped samples. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Quantitative analysis of twist boundaries and stacking faults in Bi-based superconductors by parallel recording of dark-field images with a coherent electron source

Using a 300-keV transmission electron microscope equipped with a field-emission gun, we study grain boundaries by forming a coherent electron probe 100--200 \ensuremath{\mu}m above the specimen and observe in the diffraction mode the angular distribution of the transmitted electrons within the convergent beam disks. We studied Bi-based high-temperature superconductors containing stacking faults and twist grain boundaries with accompanying displacements perpendicular to the boundary plane. We observed strong intensity oscillation from the planar faults even when they were viewed edge on, in agreement with model calculations. At twist boundaries we observed rigid body translation normal to the boundaries as large as $0.03\ifmmode\pm\else\textpm\fi{}0.01\mathrm{nm},$ and determined the displacement vector associated with the addition or subtraction of one bilayer of $({\mathrm{CuO}}_{2}+\mathrm{Ca})$ to be $0.320\ifmmode\pm\else\textpm\fi{}0.002\mathrm{nm}$ for the intrinsic and extrinsic layers in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8},$ as well as for the intrinsic layer in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Ca}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{10}.$ We also briefly address the possibility of determining the profile of the inner potential across grain boundaries using this coherent electron probe.

AC losses of HTS tapes and bundles with de-coupling barriers

Plastronic Inc. is working on a project for the Ballistic Missile Defence Organization to improve the strength and AC loss characteristics of Bi-based high temperature superconductors (HTS) for AC applications like motors, generators, and transmission cables. High strength, high amperage conductor is made by encapsulating bundles of HTS tapes with a nonsilver sheath. With this technique, Cu-alloy spacers can be placed between Bi-2223/Ag HTS tapes to de-couple the tapes under AC currents. AC losses were measured on conductor bundles with and without Cu-alloy spacers at AC currents up to 70 A and frequencies from 50 to 400 Hz. At frequencies up to about 100 Hz, it was found that the bundle with resistive barriers had AC loss equal to the sum of individual tapes as expected from the Norris ellipse model. At higher frequencies, the Cu-alloy barrier was not as effective in reducing the AC loss. For single tapes, Zr-oxide was used to coat the HTS filaments before stacking to form a multifilament tape. AC loss measurements of 6-filament tapes with and without the oxide coating showed little effect of the coating when the loss was normalized to the square of the critical current as the theory suggests.

Fluctuation conductivity and microscopic granularity in Bi-based high-temperature superconductors

Systematic conductivity measurements near ${T}_{c}$ in granular (Bi,Pb)-Sr-Ca-Cu-O superconductors are presented, and focus is given on the interplay between thermal fluctuations and disorder at micro- and mesoscopic levels. Experiments show that the resistive transition is a two-step process. In the normal phase, Gaussian and critical fluctuation conductivity regimes were identified. Both are affected by local disorder. Particularly, the critical regime is characterized by a power law with exponent ${\ensuremath{\lambda}}_{\mathrm{cr}}\ensuremath{\sim}3,$ which we interpret as resulting from microscopic granularity. Below ${T}_{c}$ and in the regime describing the approach to the zero-resistance state, fluctuation conductivity diverges as expected in a paracoherent-coherent transition of a mesoscopic granular superconductor. The results show that, instead of trivially rounding the transition, disorder at micro- and mesoscopic levels preserves a true critical phenomenology in granular superconductors.

A cluster description of structural modulation in Bi2Sr2CaCu2O8+δ

Abstract We describe the deviation from the average structure of Bi-based high-temperature superconductors by introducing one single type of cluster that includes the displacement field. These clusters are easily seen in medium-resolution electron microscopy images, and thus their spatial arrangement provides a simple way of characterizing the material, enabling us to relate superconducting properties to the salient features of the crystal structure.

Influence of actual structure on the luminescence of high-temperature superconductors

Though the luminescence of high-temperature superconductors (HTSC) came to be studied just after the discovery of high-temperature superconductivity, the problem of identification of the intrinsic emission of these compounds has not yet been resolved. This is due, first of all, to the fact that the luminescent properties of HTSC are influenced both by intense light irradiation and the environment. Previously such consideration has been given to interpretation of the luminescence band with a maximum at 2.8 eV typical of copper-oxide HTSCs. The goal of the present study is to elucidate the nature of the luminescence band with a maximum at 2.4 eV. The authors have investigated the luminescence spectra of single crystals and polycrystals (ceramics) of HTSC samples of YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} (YBaCuO), ceramic samples of YBa{sub 2}Cu{sub 4}O{sub 8}, and ceramic and monocrystalline Bi-based HTSC samples. Moreover, the influence of mechanical and chemical treatment of their surfaces, and intense light irradiation on the luminescent properties of the actual structure of the samples was studied.

Preparation and properties of Bi(Pb, Sb)-Sr-Ca-Cu-O superconductors

The influence of Pb and Sb dopants on the properties of Bi-based high-temperature superconductors was systematically investigated. The samples with nominal composition BiaPbbSbcSr4Ca5Cu7O (a+b+c=3.6-4.4; b=0.2-0.8; c=0.1-0.6) were prepared and characterized by DTA, XRD analysis, resistivity and susceptibility measurements. Our results showed that the addition of a small amount of Sb for a constant or decreasing concentration of Pb has little influence on Tc but suppresses T0. The important characteristic of this system is the presence of a new phase for higher concentrations of Sb with strong reflections near 2 Theta =35 degrees (Cc K alpha ) in the XRD pattern.

Influence of initial precursors on Bi-based high-temperature superconductors real structure and related properties

Bi-based HTSC-phases 2212 and 2223 were synthesized from different sets of initial precursors. By means of XRD semiquantitative analysis, XRD line broadening, ac-susceptibility measurements, investigation of samples magnetization M vs. applied magnetic field H and HTSC phase degradation in water it was found that the choice of precursors has strong effect on formation processes and real structure peculiarities of Bi-based superconductors.

Nonequilibrium effects in the thermal conductivity of Bi-based high temperature superconductors

Thermal conductivity of two Bi-based high temperature superconducting ceramics, 2212 and 2223, with a markedly different microstructure have been examined. A pulse technique has been used. Depending on the heating pulse duration, the more weak-links-containing 2223 sample shows either equilibrium or nonequilibrium behavior. The temperature dependence of the thermal conductivity between 110 K and 180 K is found to obey the Arrhenius law. The experimental observations are interpreted as a thermally activated heat flux motion through the polycrystalline grain boundaries with an energy barrier of nearly 10 meV. It is likely due to a phase slip mechanism. An extra relaxation process due to the pore content/structure is thus emphasized in such regimes.

Synthesis of Bi-based high temperature superconductors using melt technologies

To obtain high J c Bi-based superconducting materials melt-texture techniques were applied. Amorphous precursors were prepared by rapid quenching (cooling rate about 10 3 °C/s) of melted powders. Chemical composition and preparing routes of the starting mixtures were varied. The precursor with starting composition Pb 0.2Bi 1.8Ca 1Sr 2Cu 2O 8 was chosen among others for further investigations because of a maximal content of amorphous phase in it. The sample was crystallized by zone melting and gradient treatment. The dependence of texture extent on sample feeding was stated. The transition from isometric to directional crystallization at the cooling rate about 10°C/min was found. The samples demonstrated J c values about 6·10 4 A/cm 2 (4.2 K, H = 0 T). Some amorphous impurities on grain boundaries were found by TEM in these samples. One can assume that high J c values were reached due to a good grain alignment but not to a grain boundary purification.

Pulsed laser deposition - a versatile technique only for high-temperature superconductor thin-film deposition?

One of the most stringent prerequisites for the successful deposition of high-temperature superconductor (HTS) thin films of high quality is the stoichiometry control. This prerequisite will be met easily using the pulsed laser deposition technique (PLD) by the nature of the process. This paper highlights the principles of the preparation of Y-based and Bi-based HTS thin films by PLD, giving some emphasis to the study of the in-plane epitaxy of c-axis oriented films by X-ray techniques and Raman scattering. In particular, new results will be reviewed concerning the fabrication and properties of Pr-substituted Y-Ba-Cu-O thin films and Pr-Ba-Cu-O/Y-Ba-Cu-O superlattices with different periodicity. Focussing on the more methodological aspects of these efforts, some possibilities will be explored to prepare new complex materials of technological interest with the pulsed laser deposition technique.

The influence of Pb on the stability of the Bi-type modulated structure

The mechanism of the Bi-type modulated structure was studied in Bi-based high-temperature superconductors with nominal compositions of Bi 2Sr 2CaCu 2O y , Bi 1.85Pb 0.15Sr 2CaCu 2O y and Bi 1.8Pb 0.35Sr 2Ca 2Cu 3O y . It was found that the three samples annealed in oxygen flow showed the presence of Bi-type modulation in electron diffraction patterns; after the samples were annealed in nitrogen atmosphere the Bi-type modulation still existed for the sample Bi 2Sr 2CaCu 2O y but disappeared completely for the Pb- doped samples Bi 1.85Pb 0.15Sr 2CaCu 2O y and Bi 1.8Pb 0.35Sr 2Ca 2Cu 3O y . This suggests that doping Pb in the Bi-Sr-Ca-Cu-O system strongly changes the stability of the Bi-type modulation. It is confirmed that the driving force of Bi-type modulation is not the consequence of extra oxygen for the charge balance, but the adjustment of the BiO layer to the lattice mismatch.