Some physical properties and Vickers hardness measurements of Fe diffusion-doped Bi<Subscript>1.8</Subscript>Pb<Subscript>0.35</Subscript>Sr<Subscript>1.9</Subscript>Ca<Subscript>2.1</Subscript>Cu<Subscript>3</Subscript>O<Subscript>y</Subscript> superconductors

In this study we have investigated the influence of iron diffusion and diffusion-annealing time on the mechanical and the superconducting properties of bulk Bi_1.8Pb_0.35Sr_1.9Ca_2.1Cu₃O_y superconductors by performing X-ray diffraction (XRD), scanning electron microscopy (SEM), Vickers hardness, dc resistivity (ρ-T) and critical current density (J_c) measurements. The samples are prepared by the conventional solid-state reaction method. Doping of Bi-2223 was carried out by means of iron diffusion during sintering from an evaporated iron film on pellets. Then, the Fe layered superconducting samples were annealed at 830 °C for 10, 30 and 60 h. The mechanical properties of the compounds have been investigated by measuring the Vickers hardness (H_v). The mechanical properties of the samples were found to be load dependent. The load independent Vickers hardness (H₀), Young’s modulus (E), yield strength (Y), and fracture toughness (K_IC) values of the samples are calculated. These all measurements showed that the values of the Vickers hardness, critical current density, and critical transition temperature and lattice parameter c increased with increasing Fe doping and diffusion-annealing time.     

Electrical Resistivity and Magnetoresistance Studies of?(Bi,Pb)-2223 Phase Substituted by Ru

Series of Bi_1.8Pb_0.4Sr₂Ca_2.1Cu_3−x Ru _x O_10+δ , (Bi,Pb)-2223, superconducting samples (with x=0.0, 0.025, 0.05, 0.075, 0.1125, 0.15, 0.3, and 0.4) were prepared by a solid-state reaction technique. The structural and the superconducting properties of our prepared samples were investigated using X-ray powder diffraction (XRD), scanning electron microscope (SEM), electron dispersive X-ray (EDX), electrical resistivity, and magnetoresistance measurements. The XRD showed the enhancement of (Bi,Pb)-2223 phase formation untiluntil x=0.05, beyond which a strong phase transition from (Bi,Pb)-2223 phase to (Bi,Pb)-2212 phase was observed. In addition, the superconducting transition temperature and the hole-carrier concentrations, determined from electrical resistivity measurements, increased as the Ru-contents increased untiluntil x=0.05. A systemic study of the magnetoresistance measurements for our prepared samples was carried out under applied weak magnetic fields started from 0.0 up to 4.4 kG, and their results were explained according to thermally activated flux creep and Ambegaokar and Halperin models. The calculated critical current density and upper critical magnetic field increased untiluntil x=0.05, and then decreased with further increased of Ru-contents. This indicated that the low-contents of Ru, 0.0<x≤0.05, can generate locally weak superconducting region that enhance flux pinning strength and hence improve the physical properties of (Bi,Pb)-2223 superconducting phase.     

Effect of Ce substitution on structural and superconducting properties of Bi-2212 system

In this study, Bi₂Sr₂Ca_1?xCe_xCu₂O_y, where x = 0, 0.01, 0.05, 0.1 and 0.25, superconducting samples were prepared by solid state method and subsequently used as feed in a laser induced directional solidification (LFZ) process. The physical properties of the samples were investigated by powder X-ray diffraction, scanning electron microscopy, dc-electrical resistance, magnetization, and magnetic-hysteresis loops measurements. It has been found that no significant difference has been observed in the critical transition temperatures of samples except for the sample with the highest Ce additions, which shows the lowest T _C compared with the other doped samples. Magnetic hysteresis measurements have shown that the hysteresis loop is greater than the doped samples. In addition, critical current density values obtained from the hysteresis loops measurements by using Bean’s critical state model show a decrease with Ce-addition. All the results indicate that Cerium substitution for Ca produces the deterioration on the superconducting properties, compared with the undoped sample.     

Study and Application of Controlled Vortex Dynamics in?Patterned YBCO Films

Quantitative imaging of the local magnetic field and of current density distribution in superconductors (with microscopic resolution over macroscopic length scales) is achieved by means of the Magneto-Optical Imaging technique with an indicator film. We exploit this technique to study the vortex arrangement and the corresponding supercurrent distribution in high temperature superconducting YBa₂Cu₃O_7?x films. Several patterned superconducting films were studied, either non-simply connected structures, which imply macroscopic flux quantization, and superconductors whose local properties were tailored by means of confined heavy-ion irradiation. Moreover, by means of electrical transport measurements coupled with the real-time imaging of the magnetic pattern, it is directly shown how the local current distribution in patterned superconductors is affected by the electrical transport both in the Meissner and in the vortex regimes. The relevance of a controlled and localized dissipation induced by the confined vortex motion in tailored superconducting films is demonstrated for direct applications of this phenomenology to superconducting devices, such as magnetic field and photon detectors.     

Pore Structure and Transport Properties in Bulk YBa2Cu3O7?δ Doped with Sb2O3

In this work we have studied the pore structure and electrical transport properties of superconducting YBa₂Cu₃O_7?y polycrystalline samples doped by the addition of different Sb₂O₃ concentrations, i.e. resulting in (YBa₂Cu₃O_7?y )_1?x (Sb₂O₃) _x . The samples were prepared through the solid-state reaction method. Rietveld analyses of X-ray diffraction data were used to investigate how the lattice parameters are modified by doping. Specific superficial area measurements identified the principal characteristics of the pore structure of the samples and how these properties change with doping. The superconducting properties were studied by using zero field cooling magnetization and transport critical current measurements. The critical temperature of the samples does not depend on the doping level, but their transport critical current density strongly decreases as the Sb₂O₃ concentration is increased. Our experimental results suggest that for the samples studied here there is not a direct correlation between the modification by doping of both, the pore structure and the transport critical current density.     

Screening properties and critical current of superconductor-MgO composites

A series of composites of the high temperature superconducting (HTSC) phase (Bi,Pb)₂Sr₂Ca₂Cu₃O_x (2223) with MgO were prepared. AC susceptibility measurements showed a percolation effect in the screening properties and critical current around 50 vol.% superconducting material, while maximum screening is reached at 70 vol.% of the HTSC phase. The shielding efficiency of superconducting material added to the composite is highest in the composition range between 40 and 70 volume percent HTSC phase. this behaviour can be understood in terms of random distribution of superconducting and non-superconducting phases and the occurrence of intergrain coupling effects.     

Reactively evaporated films of copper molybdenum sulfide

Films of superconducting Chevrel-phase copper molybdenum sulfide Cu_xMo₆S₈ were deposited on sapphire substrates by reactive evaporation using H₂S as the reacting gas. Two superconducting temperatures (10.0 K and 5.0 K) of the films were found, corresponding to two different phases with different copper concentrations. All films were superconducting above 4.2 K and contained Chevrel-phase compound as well as free molybdenum. The critical current was measured as a function of applied field. One sample was found to deviate from the scaling law found for co-evaporated or sputtered samples, which possibly indicates a different pinning mechanism or inhomogeneity of the sample.     

Influence of Boron-Containing Dopants on Superconducting Properties of (Bi, Pb)-2223 HTS

The influence of addition of lead borate Pb(BO₂)₂ and boron oxide B₂O₃ on the phase evolution and superconducting properties of (Bi, Pb)-2223 HTSs synthesized by the solid-state reaction method in alumina crucibles has been studied. X-ray diffraction, resistivity, critical current density, and AC susceptibility measurements were performed on the prepared compounds. Obtained results have shown that boron-containing dopants lead to the drastic enhancement of the (Bi, Pb)-2223 phase formation. Boron-doped samples reveal a significant increase in both the zero resistivity temperature and transport critical current density compared to the undoped specimen. On the other hand, a high content of boron-containing dopants causes the appearance of a very low-T _c 2201 phase and leads to a deterioration of coupling between superconducting grain boundaries. Obtained results could enable us to develop a cheap and energy efficient fabrication technology for nearly single (Bi, Pb)-2223 phase superconducting materials via heat treatment of boron-incorporated precursors in an alumina crucibles.     

Structure and superconducting properties of pure and variously doped bulk MgB2 obtained by uniaxial and isostatic hot pressing

The preparation of dense MgB₂ bodies, undoped and doped with different atomic species (Na, Ag, Y), was performed via reactive sintering by uniaxial and isostatic hot pressing, starting from the pure elements, and compared with undoped samples obtained by commercial MgB₂ powder. The superconducting characteristics of the obtained materials, namely critical temperature (T_c) and current (J_c), were obtained through ac susceptibility measurements and compared to their structural features, like phase purity and secondary phases formation and distribution in the MgB₂ matrix. Both the adopted hot pressing techniques gave rise to undoped MgB₂ pieces exhibiting phase purity in the range 85–95% and relative density above 80%; although in most cases the doped samples underwent higher extents of phase decomposition and lesser densification, they all exhibited higher critical temperature and current compared to the corresponding undoped material, indicating a net influence of the doping on the superconducting behaviour of MgB₂, particularly Ag and Y. An opportune quality factor was adopted, to obtain a more reliable comparison between the different MgB₂ samples and evaluation of the samples goodness, in relation to their superconducting characteristics. It was put in evidence that low amounts of doping can improve the superconducting behaviour of MgB₂ and that this influence can be addressed in terms of pinning centres, as there was no experimental evidence of an actual atomic substitutions in the MgB₂ crystal.     

Jc enhancement and flux pinning of Se substituted YBCO compound

Y_2/3 Se_1/3Ba₂Cu₃O_x compound was fabricated by using solid state fabrication technique. Optimum heat treatments conditions for Y_0.77Se_0.33Ba₂Cu₃O_x were investigated. It was determined that the XRD results of these samples were similar to Y-123 phase with some impurities. Magnetization dependence of applied magnetic fields was measured in the range of 0–9 T at 10–50 K. The symmetric and asymmetric M–H loops were obtained for the samples. Magnetization loops obtained from measurements were successfully described by the extended Valkov–Khrustalev model. The temperature and applied magnetic field dependencies of magnetization of sample were estimated and critical current density of samples was calculated by Bean model and pinning force of samples was calculated by using Lorentz force. It is found from critical current density values that Se additions were acted as a pinning center which increased critical current density.     

Superconductive YBa2Cu3O7−x fibres from an extrudable dispersion of a particulate precursor

A particulate precursor of YBa₂Cu₃O_7–x was dispersed in a concentrated acetic/formic acid solution of yttrium, barium and copper acetates to form an extensible, viscous mixture. This dispersion was stable, unlike one prepared from superconductive YBa₂Cu₃O_7–x itself. Fibres extruded from the viscous dispersion were fired at 935 °C in the presence of oxygen.Although the fired fibres were not fully densified, they had a sharp Meissner transition temperature at 90 K and an apparent critical current density (J _c) of 900 A cm^–2 (1800 A cm^–2 corrected for porosity) at 77 K. The superconducting fibre was sensitive to magnetic field, which is evidence of weak coupling between the superconducting grains.     

Current distribution measurement in YBCO thin film for a superconducting fault current limiter

Current distribution in the superconducting film for a resistive fault current limiter is important, because it influences AC loss and a uniformity of S/N transition. The lateral current distribution of the film was reconstructed from the magnetic field distribution which is measured by multiple Hall probes. The following results were obtained. (1) Non-uniform current distribution in the superconducting film was observed when the current was less than 1.3 times of critical current (I_c). (2) The current in a superconducting film was uniform when the current was much higher than I_c. The current can be considered uniform when the film works as a fault current limiter, because the S/N transition starts about twice of I_c. (3) The validity of the measurement was verified by the comparison with the electric circuit simulation.     

Time-Dependent Diffusion Coefficient of Fe in MgB<Subscript>2</Subscript> Superconductors

The iron (Fe) diffusion in superconducting MgB₂ bulk samples has been studied for sintering time durations of 15 min, 30 min, 1 h, 2 h, and 4 h at 900^°C. Fe coating bulk polycrstalline superconducting MgB₂ samples for Fe coating were prepared by pelletizing and used in the diffusion experiments with initial sintering at 800^°C for 1 h. A thin layer of Fe was coated on MgB₂ pellets by evaporation in vacuum. Effects of Fe diffusion on the structural, electrical, and superconducting properties of MgB₂ have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), infrared spectroscopy (IR), energy-dispersive X-ray spectroscopy (EDS), and resistivity measurements. Fe diffused samples have slightly increased critical transition temperatures and have larger lattice parameter c values, in comparison with bare samples. Fe diffusion coefficients were calculated from depth profiles of c parameter and room temperature resistivity values. Depth profiles were obtained by successive removal of thin layers from Fe diffused surfaces of the samples. Our results have shown that the Fe diffusion coefficient decreases with increasing sintering time and resistivity measurements can be utilized for determination of diffusion coefficient.     

Effect of Nb addition on magnetic,structural and superconducting properties of (Bi,Pb)-2223 superconductors

In this work, the effects of Nb₂O₅ addition with different ratios on the structural and magnetic properties of Bi_1.7?xPb_0.3Nb_xSr₂Ca₂Cu₃O_y (x = 0.00–0.20) superconducting samples were investigated. (Bi, Pb)-2223 superconducting samples were prepared by conventional solid-state reaction method. The phase formation, phase fraction and lattice parameters were determined from X-ray powder diffraction (XRD) measurements, the microstructure, surface morphology analyses of the samples were carried out using scanning electron microscope (SEM). Additionally, ac susceptibility measurements were done in order to determine the critical current density (J_c) and hole concentration (p) of the samples. AC susceptibility measurements were done at various ac fields (ranging from 20 to 160 A/m) to understand the effect of Nb addition on magnetic properties of Bi_1.7?xPb_0.3Nb_xSr₂Ca₂Cu₃O_y superconductor. Critical onset (T _c ^on ) and loss peak temperatures (T_p) were estimated from the ac susceptibility curves. It was observed from ac susceptibility measurements that the critical onset temperatures decreased from about 108–98 K with increasing Nb addition (x = 0.00–0.20). The imaginary part of susceptibility was used to calculate the intergranular critical current density (J_c) by means of the Bean’s model. X-ray diffraction analysis revealed that the samples consisted of a mixture of Bi-2223 and Bi-2212 phases as the major constituents and non-superconducting phase Ca₂PbO₄ as the minor. It was also shown from XRD measurements that volume fraction of high-T_c phase decreases with increasing Nb addition up to x = 0.20. The sample with Nb addition of x = 0.20 showed the highest volume fraction of Bi-2223 phase (86 %). When Nb addition was increased, the surface morphology and grain connectivity are found to degrade, the grain sizes decrease and porosity of the samples were observed to increase from SEM images except the sample with x = 0.20 Nb addition.     

Improvement of High T c Phase Formation in BPSCCO Superconductor by Adding Vanadium and Substituting Titanium

We have produced the (BiPb)₂V _x Sr₂Ca₃Cu_4?y Ti _y O_12+?? with x=0.1 and y=0.050,0.10,0.2 and 0.3 compounds by melt-quenching method. Structural and superconducting properties of the produced samples were investigated by Scanning Electron Microscopy, X-ray diffraction patterns, electrical resistance measurements and dc-magnetic hysteresis loop measurements. The pure high-T _c phase (2223) is nearly found with Ti substitution for x=0.05 and 0.10. The onset critical temperature (T _c._onset) of the samples increases up to 111 K with doping up to x=0.20. In addition, considerable large values of the critical current densities (J _c), calculated from the hysteresis loop measurements by using Bean??s critical state model are obtained for the samples in the same doping range. Our data have indicated that J _c decreases with increasing temperature and Ti concentration.     

Improving the Characteristics of Powder-In-Tube MgB2 Superconductor Fabricated Using the Extrusion Technique

High-density cylindrical MgB₂ superconductors were manufactured using an ex situ powder in tube extrusion technique. The superconducting properties of the steel-sheathed MgB₂ samples were examined with respect to their magnetisation responses, critical temperature and current density. The critical current density of the superconductor was determined by means of magnetisation measurements using Bean’s critical state model. J _c reached a maximum of 4×10⁵ A/cm² at 5 K.     

Effect of Eu2O3 Nanoparticles Addition on Structural and Superconducting Properties of BSCCO

To study the effects of Eu₂O₃ nanoparticles addition to BSCCO superconducting system, four bulk polycrystalline samples with general formula of Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O _y +xEu₂O₃ (where x=0.0, 0.3, 0.5, 1.0 wt%) were prepared by chemical sol–gel method. X-ray diffraction, SEM, and TEM were used for structural characterization of the samples. DC electrical resistivity, critical current, and AC magnetic susceptibility were measured. XRD analysis showed that both (Bi,Pb)-2223 and Bi-2212 phases coexist in the samples having orthorhombic crystal structure. DC electrical resistivity, J _c , and AC magnetic susceptibility measurements reveal that adding Eu nanoparticles to BSCCO improves superconducting properties of this system and enhances its critical current density. The enhancement of the J _c may be caused by improvement of the grain connectivity with Eu nanoparticle additions.     

Fracture behaviour and its relation to critical current of silver-sheathed Ba2YCu3O7-x superconducting composite wires and tapes

Silver-sheathed Ba₂YCu₃O_7-x superconducting composite wires and tapes were prepared by rolling, drawing, swaging and pressing methods. The fracture behaviour and its influence on critical current at O T at 77 K of the Ba₂YCu₃O_7-x were investigated. The oxide was found to show multiple fracture under applied tensile stress, and the critical current density and tensile strength of the oxide in the rolled, swaged and pressed samples were higher than those in the drawn samples. When the working amount was high, the current density and the strength of the oxide were found to become high. Within the present conditions, there was a correlation between critical current density and cracking stress: the higher the cracking stress, the higher the critical current density became. The cracking stress of the present oxide was determined to be 50MPa at most, being far lower than that of the Nb₃Sn compound (800 to 2000 MPa). The critical current density of the rolled, swaged and pressed samples was reduced rapidly when exerted stress on the oxide exceeded the cracking stress, while the reduction in the drawn samples occurred gradually. A strong dependence of the critical current, as a function of applied stress and cracking stress of the oxide, on the measured portion due to scatter in the size of defects contained in the oxide, was found.     

“Peak effect” in the magnetic field dependence of the transport critical-current density in high-T c superconducting ceramics

The maximum observed in the magnetic field dependence of the transport critical-current density of high-T _c superconducting ceramics was analyzed. Transport critical current and magnetization measurements performed on Bi(Pb)-Sr-Ca-Cu-O bulk sintered samples allowed us to conclude that such a maximum may result from the influence of demagnetization effects at low applied field values and an increase of the intergrannular pinning at higher fields.     

Synthesis and superconducting properties of CaC6

Among the superconducting graphite intercalation compounds, CaC₆ exhibits the highest critical temperature T_c=11.5 K. Bulk samples of CaC₆ are obtained by immersing highly oriented pyrographite pieces in a well-chosen liquid Li–Ca alloy for 10 days at 350 °C. The crystal structure of CaC₆ belongs to the space group. In order to study the superconducting properties of CaC₆, magnetisation was measured as a function of temperature and direction of magnetic field applied parallel or perpendicular to the c-axis. Meissner effect was evidenced, as well as a type II superconducting behaviour and a small anisotropy. In agreement with calculations, experimental results obtained from various techniques suggest that a classical electron-phonon mechanism is responsible for the superconductivity of CaC₆. Application of high pressure increases the T_c up to 15.1 K at 8 GPa.