Showing papers in "Superconductor Science and Technology in 2002"

Hot-electron effect in superconductors and its applications for radiation sensors

Abstract: The paper reviews the main aspects of nonequilibrium hot-electron phenomena in superconductors and various theoretical models developed to describe the hot-electron effect. We discuss implementation of the hot-electron avalanche mechanism in superconducting radiation sensors and present the most successful practical devices, such as terahertz mixers and direct intensity detectors, for far-infrared radiation. Our presentation also includes the novel approach to hot-electron quantum detection implemented in superconducting x-ray to optical photon counters.

Substitution-induced pinning in MgB2 superconductor doped with SiC nano-particles

Abstract: By doping MgB2 superconductor with SiC nano-particles, we have successfully introduced pinning sites directly into the crystal lattice of MgB2 grains (intra-grain pinning). It became possible due to the combination of counter-balanced Si and C co-substitution for B, leading to a large number of intra-granular dislocations and the dispersed nano-size impurities induced by the substitution. The magnetic field dependence of the critical current density was significantly improved in a wide temperature range, whereas the transition temperature in the sample MgB2(SiC)x having x = 0.34, the highest doping level prepared, dropped only by 2.6 K.

Enhancement of critical current densities of powder-in-tube processed MgB2 tapes by using MgH2 as a precursor powder

Abstract: We investigated the effect of starting powder on the microstructure and superconducting properties of MgB2 tapes prepared by a powder-in-tube technique. Two kinds of powder mixtures, Mg + B and MgH2 + B, were used as precursors. When using the mixture of Mg + B, carbon-steel (CS) sheaths reacted with B, and FexB (x = 1 and 2) was formed. This brought about a decrease in the density of the MgB2 core and, hence, a low critical current density (Jc). On the other hand, we found that the mixture of MgH2 + B was effective in suppressing the formation of FexB and obtaining a high density of the core layer. The Jc values of the tapes prepared using the mixture of MgH2 + B were about twice as large as those prepared using the mixture of Mg + B and reached 1600 A cm−2 at 4.2 K and 10 T. This is higher than that of the CS-sheathed MgB2 tapes prepared with MgB2 powder.

On the Ginzburg-Landau analysis of the upper critical field Hc2 in MgB2

Abstract: We study the temperature dependence of the upper critical field Hc2(T) for superconducting magnesium diboride, MgB2 in the vicinity of Tc using a two-band Ginzburg-Landau (G-L) theory. The temperature dependence of Hc2(T) near Tc exhibits a positive curvature. In addition, the calculated temperature dependence and its higher-order derivatives are also shown to be in good agreement with the experimental data. In analogy with the multi-band character of the Eliashberg microscopic theory, the positive curvature of Hc2(T) is described reasonably by solving the two-band G-L theory.

Neutron irradiation of MgB2 bulk superconductors

Abstract: Sintered samples of MgB2 were irradiated in a fission reactor. Defects in the bulk microstructure are produced during this process mainly by the 10B(n,α)7Li reaction while collisions of fast neutrons with the lattice atoms induce much less damage. Self-shielding effects turn out to be very important and lead to a highly inhomogeneous defect distribution in the irradiated samples. The resulting disorder enhances the normal state resistivity and the upper critical field. The irreversibility line shifts to higher fields at low temperatures and the measured critical current densities increase following irradiation.

SQUID sensor array configurations for magnetoencephalography applications

Abstract: Electrophysiological activity in the human brain generates a small magnetic field from the spatial superposition of individual neuronal source currents. At a distance of about 15 mm from the scalp, the observed field is of the order of 10−13 to 10−12 T peak-to-peak. This measurement process is termed magnetoencephalography (MEG). In order to minimize instrumental noise, the MEG is usually detected using superconducting flux transformers, coupled to SQUID (superconducting quantum interference device) sensors. Since MEG signals are also measured in the presence of significant environmental magnetic noise, flux transformers must be designed to strongly attenuate environmental noise, maintain low instrumental noise and maximize signals from the brain. Furthermore, the flux transformers must adequately sample spatial field variations if the brain activity is to be imaged. The flux transformer optimization for maximum brain signal-to-noise ratio (SNR) requires analysis of the spatial and temporal properties of brain activity, the environmental noise and how these signals are coupled to the flux transformer. Flux transformers that maximize SNR can detect the smallest brain signals and have the best ability to spatially separate dipolar sources. An optimal flux transformer design is a synthetic higher-order gradiometer based on relatively short-baseline first-order radial gradiometer primary sensors.

Magnetic characterization of sintered MgB2 samples: effect of substitution or 'doping' with Li, Al and Si

Abstract: Powdered and sintered MgB2 samples, both pure and substituted or doped, have been prepared and characterized through magnetic measurements performed from T = 5 K up to a few degrees above the transition temperature of about 39 K. For all the samples, the irreversibility line (IL) appears much lower than the Hc2–T line and very far from that of both high-Tc (YBCO) and low-Tc (Nb3Sn) materials indicating the need to increase the pinning in this material to make it attractive for technological applications. Moreover, we have verified through different procedures that the sintered samples behave as well-connected bodies, showing no trace of granularity; therefore, the critical current density values Jc may be obtained by applying the 'critical state model' in a straightforward way. The hysteresis loop measurements allowed estimation of Jc both for powders and sintered samples and confirmed the strong field decrease of Jc, implicit in IL behaviour. We attempt to introduce defects in the MgB2 structure by different chemical treatments like substitution of lithium on the magnesium site and doping of the precursor boron powders with aluminium and silicon. Jc always increased in doped or substituted samples (up to a factor 3) and this fact is meaningful, in particular in the light of the small level of substitution or doping we performed. The best result in terms of Jc is achieved by silicon doping that, moreover, does not significantly decrease the transition temperature.

Chemistry and structure of Hg-based superconducting Cu mixed oxides

Abstract: The most relevant aspects of thermodynamics, preparation, crystal structure and superconducting properties under ambient and high pressure of HgBa2Can−1CunO2n+2+δ superconductors are reviewed in this contribution. The ideas which inspired authors to search for the new superconductors among Hg-based complex cuprates are discussed as an example illustrating the benefit of the concept of intergrowth structure formation for the design of new layered materials. Recent results on solid–gas equilibriums useful for reproducible syntheses of different homologues are summarized. The crystal structures are discussed with main attention focused on features such as concentration and location of extra oxygen atoms, static atomic displacements due to partial occupation of oxygen site in a Hg-layer, substitution in a Hg position and a presence of stacking faults. It is shown that these factors have to be taken into account for a correct structure description. The relationships between Tc, doping level, amount and formal charge of extra anions and external pressure are outlined to predict possible paths for enhancement of the superconducting properties in complex cuprates.

Microstructure and superconducting properties of hot isostatically pressed MgB2

Abstract: Bulk samples of MgB2 have been formed by hot isostatic pressing (HIPping) of commercial powder at 100 MPa and 950 °C. The resulting material is 100% dense with a sharp superconducting transition at 37.5 K. Microstructural studies have indicated the presence of small amounts of second phases within the material, namely MgO- and B-rich compositions, probably MgB4. Magnetization measurements performed at 20 K have revealed values of Jc = 1.3 × 106 A cm−2 at zero-field and 9.3 × 105 A cm−2 at 1 T. Magneto-optical (MO) studies have shown direct evidence for the superconducting homogeneity and strong intergranular current flow in the material.

Processing of high-performance Gd-Ba-Cu-O bulk superconductor with Ag addition

Abstract: We fabricated large-grain c-axis-oriented Gd–Ba–Cu–O bulk superconductors with very fine 211 particles and studied their field-trapping properties. Single domain Gd–Ba–Cu–O/Ag bulk samples with different molar ratios of Gd-123:Gd-211 = 10:x (x = 2 to 6) were melt-textured under controlled oxygen partial pressure of 1.0% by employing Gd-211 starting powders with a particle size of 1.0 μm. The maximum trapped magnetic field of 2.0 T was recorded at 77 K in the bulk 48 mm in diameter with the composition of x = 5. It is notable that the trapped field in the space of two Gd–Ba–Cu–O bulks reached an extremely high value of 3.3 T at 77 K. Furthermore, the field-trapping performance of the bulk sample was improved by applying the ultra-fine 211 powders of a size of 0.2 μm. The single-bulk Gd–Ba–Cu–O 50 mm in diameter exhibited a trapped field of 2.6 T at 77 K.

Transport currents of two-axially rolled and post-annealed MgB2/Fe wires at 4.2 K

Abstract: Single- and four-core MgB2 wires have been made by the powder-in-tube (PIT) method using a commercial MgB2 powder (Alfa Aesar) and two-axial rolling deformation in an Fe sheath. The deformed wires were subjected to annealing at temperatures from 950 ?C up to 1100 ?C for 30 minutes in argon. The interface layer thickness of 10?25.4 ?m caused by inter-diffusion and reaction between the MgB2 core and the Fe sheath has been observed in all annealed wires. Transport currents of as-deformed and post-annealed wires were measured at the temperature 4.2 K and external magnetic field B = 0?5.5 T. The presence of a ferromagnetic sheath in MgB2 composite wire influences the Ic(B) characteristics leading to Ic hysteresis and Ic anisotropy, which is more pronounced for those that have not been annealed (weakly linked grains) in comparison to well-connected MgB2 grains after recrystallization. A comparison of the overall current densities versus the external magnetic field of NbTi, Nb3Sn and Bi-2223 superconductors with four-core MgB2 wire is also presented.

High-Jc YBa2Cu3O7-x films on metal tapes by the metalorganic deposition method using trifluoroacetates

Abstract: High-Jc YBa2Cu3O7-x (YBCO) films on metal tapes have been successfully fabricated by the metalorganic deposition method using trifluoroacetates (TFA-MOD method) with optimal firing conditions In the firing process, the key factors by which to obtain high-Jc YBCO films on metal tapes are to prevent the films from deterioration by water vapour and to avoid BaCeO3 creation between YBCO and the CeO2 buffer layer With the optimal conditions, we have successfully formed a 024 µm thick YBCO film on a metal tape, having a Jc of 25 MA cm-2 (77 K, 0 T)

Vortex pinning by natural defects in thin films of YBa2Cu3O7- delta

Abstract: Although vortex pinning in laser-ablated YBa2Cu3O7−δ films on (100) SrTiO3 is dominated by threading dislocations (Dam B et al (1999) Nature 399 439), many other natural pinning sites are present. To identify the contribution from twin planes, surface corrugations and point defects, we manipulate the relative densities of all defects by post-annealing films with various as-grown dislocation densities, ndisl. While a universal magnetic field B dependence of the transport current density js(B, T) is observed (independently of ndisl, temperature T and the annealing treatment), the defect structure changes considerably. Correlating the microstructure to js(B, T), it becomes clear that surface roughness, twins and point defects are not important at low magnetic fields compared to linear defect pinning. Transmission electron microscopy indicates that threading dislocations are not part of grain boundaries nor are they related to the twin domain structure. We conclude that js(B, T) is essentially determined by pinning along threading dislocations, naturally induced during the growth process. Even in high magnetic fields, where the vortex density outnumbers ndisl, it appears that linear defects stabilize the vortex lattice by means of the vortex–vortex interaction.

Relaxation of transport current distribution in a YBaCuO strip studied by magneto-optical imaging

Abstract: The dynamics of magnetic flux distributions across a YBa2Cu3O7−δ strip carrying transport current is measured using magneto-optical imaging at 20 K. The current is applied in pulses of 40–5000 ms duration and of magnitude close to the critical one, 5.5 A. During the pulse some extra flux usually penetrates the strip, so the local field increases in magnitude. When the strip is initially penetrated by flux, the local field either increases or decreases depending on both the spatial coordinate and the current magnitude. Meanwhile, the current density always tends to redistribute more uniformly. Despite the relaxation, all distributions remain qualitatively similar to the Bean-model predictions.

Fabrication and superconducting properties of MgB2 composite wiresby the PIT method

Abstract: Dense MgB2/Cu wires with Ta as a buffer layer were successfully fabricated by the powder-in-tube (PIT) method. The microstructure was investigated by optical microscopy. Magnetization measurements were carried out by using a superconducting quantum interference device (SQUID) magnetometer at magnetic fields up to 7 T from 5 K to 35 K. The transition temperature of the MgB2 wire is around 38.4 K and the irreversibility field is 6.6 T at 5 K. The critical current density as high as 105 A cm−2 (5 K, self-field) and 104 A cm−2 (20 K, 1 T) has been obtained. The results suggest that the powder-in-tube (PIT) process is promising in preparing high-quality MgB2 wires.

Spin glass behaviour in ferromagnetic La2CoMnO6 perovskite manganite

Abstract: La2CoMnO6 perovskite manganite was synthesized by solid-state reaction. Rietveld refinement of powder x-ray diffraction data indicated that this compound crystallized with an orthorhombic structure containing distorted (Co/Mn)O6 octahedra (Pnma; a = 5.5 A; b = 7.2 A; c = 5.4 A. The dc magnetization and real part (x') of the ac susceptibility were measured at dc fields up to 1 T and an ac field of 0.1 Oe at frequencies of 21, 217, 600 and 2000 Hz over a wide range of temperatures from 300 K down to 4.2 K. A ferromagnetic transition was observed at Tc = 220 K. A spin glass ground state was also determined from the shift of a peak at 220 K in the real part of the ac susceptibility with frequency and slow spin relaxation for T < Tc as indicated by the time dependence of the remanent magnetization. The spin glass state arises from a ferromagnetic state and might be caused by competition between the ferromagnetic and antiferromagnetic interaction.

Processing and microstructure of single grain, uranium-doped Y-Ba-Cu-O superconductor

Abstract: Large grain Y–Ba–Cu–O (YBCO) superconductor doped with various amounts of depleted UO2 and containing excess Y2BaCuO5 (Y-211) and Y2O3 have been fabricated by top seeded melt growth (TSMG). The effect of depleted UO2 on the large grain microstructure has been studied systematically in samples with and without added Pt. It is found that UO2 refines the size of the second phase particles in the superconducting YBa2Cu3O7−δ (Y-123) matrix to dimensions of a few hundred nanometres with an approximately spherical morphology. Addition of Y2O3 to the uranium-doped precursor powder, rather than Y-211, yields a significantly finer distribution of second phase particles and an associated higher critical current density Jc at increased magnetic field.

Design and testing of the HTS bearing for a 10 kWh flywheel system

Abstract: Flywheels are of interest for a wide range of energy storage applications, from support of renewable resources to distributed power applications and uninterruptible power systems (UPS) (Day et al 2000 Proc. EESAT 2000 (Orlando, FL, Sept. 2000)). The use of high-temperature superconducting (HTS) bearings for such systems has significant advantages for applications requiring large amounts of energy to be stored with low parasitic losses and with minimal system maintenance. As flywheel systems increase in size, it becomes a significant challenge to provide adequate stiffness in these bearings without exceeding the strength limits of rotating magnet assemblies. The Boeing Company is designing and building a prototype flywheel of 10 kWh total stored energy and has focused much effort on the HTS bearing system. This paper will describe the general structure of the bearing and the steps taken to optimize its magnetic and structural performance and show recent test results.

Epitaxy-stabilized n-type superconducting cuprates

Abstract: We report the growth of n-type superconducting T'-(La,Ce)2CuO4 and infinite-layer (IL) (Sr,La)CuO2 thin films by means of molecular beam epitaxy (MBE). The bulk synthesis of T'-(La,Ce)2CuO4 and IL-(Sr,La)CuO2 requires complicated techniques: synthesis at low temperatures below 600 °C for the former and at high pressures above 3 GPa for the latter. This makes it difficult to grow bulk single crystals. We have found, however, that high-quality single-crystalline films of both compounds can be rather easily prepared by thin-film processes. Single-phase T'-(La,Ce)2CuO4 films can be obtained for a wide range of x (0.0 ≤ x ≤ ~0.4). The best Tcend is over 30 K, which is the highest in the T' family. For IL-(Sr,La)CuO2, by using KTaO3 substrates, high Tcend over 39 K and also metallic resistivity were achieved for the first time to our knowledge. We describe the key parameters in the growth and the properties of the resultant films.

The strain and temperature scaling law for the critical current density of a jelly-roll Nb3Al strand in high magnetic fields

Abstract: The engineering critical current density (JE) and the index of transition, N (where E = αJN), of a Nb3Al multifilamentary strand, mass-produced as a part of the Fusion programme, have been characterized as a function of field (B), temperature (T) and strain (e) in the ranges B ≤ 15 T, 4.2 K ≤ T ≤ 16 K and −1.79% ≤ e ≤ +0.67%. Complementary resistivity measurements were taken to determine the upper critical field (BC2(T, e)) and the critical temperature (TC(e)) directly. The upper critical field defined at 5%ρN, 50%ρN or 95%ρN, is described by the empirical relation BC2ρN(T, e) = BC2ρN(0, e)[1 −(T/TCρN(e))ν]. The upper critical field at zero Kelvin and the critical temperature are linearly related where BC2ρN (0, e) ≈ 3.6TCρN (e) − 29.9, although strictly BC2ρN (0, e) is a double-valued function of TCρN (e). JE was confirmed to be reversible at least in the range −0.23% < e < 0.67%. The JE data have been parameterized using the volume pinning force (FP) where FP = JE × B = A(e)BC2n (T, e)bp (1 − b)q and b = B/BC2(T, e). A(e) is taken to be a function of strain otherwise the maximum value of FP (found by varying the field) was a double-valued function of BC2 when the temperature was fixed and the strain varied. To achieve a very high accuracy for the parameterization required by magnet engineers (~1 A), the data were divided into three temperature–strain ranges, BC2(T, e) described by the empirical relation and the constants p, q, n and ν and the strain-dependent variables A(e), BC2(0, e) and TC(e) treated as free-parameters and determined in each range. A single scaling law that describes most of the JE data has also been found by constraining BC2(T, e) using the resistivity data at 5%ρN where ν = 1.25, n = 2.18, p = 0.39 and q = 2.16. When BC2(T, e) is constrained at 50%ρN or 95%ρN, the scaling law breaks down such that p and q are strong functions of temperature and q is also a strong function of strain. Good scaling provides support for identifying BC25%ρN (T, e) as the characteristic (or average) upper critical field of the bulk material. The JE data are also consistent with a scaling law that incorporates fundamental constants alone, of the Kramer-like form where the Ginzburg–Landau (GL) parameter κ is given by the relation γ is the Sommerfeld constant and t = T/TC(e). At an applied field equal to the upper critical field found from fitting the Kramer dependence (i.e. at BC2(T, e)), the critical current is non-zero and we suggest that the current flow is percolative. The functional form of FP implies that in high fields the grain boundary pinning does not limit JE, this is consistent with JE-microstructure correlations in other superconducting materials.

Magnetic relaxation and critical current density of the new superconductor MgB2

Abstract: Magnetic relaxation rate, critical current density and transport properties have been investigated on MgB2 bulks from 1.6 K to Tc at magnetic fields up to 8 T. A vortex phase diagram is depicted based on these measurements. A large separation between the bulk irreversibility field Hirr(T) and the upper critical field Hc2(T) has been found. It is thus proposed that there is a quantum vortex liquid due to strong quantum fluctuation of vortices at 0 K. It is also found that the magnetic relaxation rate is weakly dependent on temperature but strongly dependent on field indicating a trivial influence of thermal fluctuation on the vortex depinning process. Therefore, the phase line Hirr(T) is attributed to quantum vortex melting in the rather clean system at a finite temperature.

Fabrication and transport critical currents of multifilamentary MgB2/Fe wires and tapes

Abstract: Multifilamentary MgB2/Fe wires and tapes with high transport critical current densities have been fabricated using a straightforward powder-in-tube process. After annealing, we measured transport jc values up to 1.1 × 105 A cm−2 at 4.2 K and in a field of 2 T in a MgB2/Fe square wire with seven filaments fabricated by two-axial rolling, and up to 5 × 104 A cm−2 at 4.2 K in 1 T in a MgB2/Fe tape with seven filaments. For higher currents these multifilamentary wires and tapes quenched due to the insufficient thermal stability of the filaments. Both the processing routes and deformation methods were found to be important factors for fabricating multifilamentary MgB2 wires and tapes with high transport jc values.

Structure of the superconducting gap in MgB2 from point-contact spectroscopy

Abstract: We have studied the structure of the superconducting gap in MgB2 thin films by means of point-contact spectroscopy using a gold tip. The films were produced by depositing pure boron on a sapphire substrate, using e-beam evaporation, followed by reaction with magnesium vapour. The films have a Tc of 38.6 ± 0.3 K and resistivity of about 20 μΩ cm at 40 K. The point-contact spectra prove directly the existence of a multi-valued order parameter in MgB2, with two distinct values of the gap, Δ1 = 2.3 ± 0.3 meV and Δ2 = 6.2 ± 0.7 meV at 4.2 K. Analysis of the spectra in terms of the Blonder–Tinkham–Klapwijk model reveals that both gaps close simultaneously at the Tc of the film. Possible mechanisms that can explain the intrinsic coexistence of two values of the gap are discussed.

Influence of Ag, Cu and Fe sheaths on MgB2 superconducting tapes

Abstract: Copper, iron and silver MgB2 sheathed tapes have been manufactured under different conditions. It has been found that copper-sheathed tapes can show a higher critical current density than iron-sheathed tapes if heat-treated at temperatures below 850 °C. The influence of different overall mechanical deformation rates has been studied for tapes sheathed by all three types of metals. By increasing the deformation rate the critical current density was improved by about an order of magnitude in the case of the copper-sheathed tapes, while the critical current density of the iron-sheathed tapes remained constant.

Superconducting critical fields and anisotropy of a MgB2 single crystal

Abstract: Using a double axis vibrating sample magnetometer, we have made detailed magnetic measurements of the lower critical field Hc1 for fields parallel to the two crystallographic directions of MgB2 single crystals. Additionally, using a novel Hall probe magnetometer we have measured high precision magnetization loops, from which we directly determine the upper critical field Hc2 for both field orientations. Our results suggest that Hc1 is much larger than most previous estimates and that consequently the Ginzburg–Landau parameter κ is very low (less than 5). We find the anisotropy parameter γ ~ 2, independent of temperature over the measured range.

Fabrication of Gd–Ba–Cu–O films by the metal–organic deposition method using trifluoroacetates

Abstract: We have fabricated Gd–Ba–Cu–O (GdBCO) films on LaAlO3 single crystalline substrates by a metal–organic deposition method using trifluoroacetates (TFAs). The highest value of the critical temperature of superconductivity (Tc) of these TFA-derived GdBCO films was 92.9 K and the critical current density (Jc) reached 2.9 MA cm−2 at 77 K in the self-field. While this value of Jc is smaller than that of a previously reported Y–Ba–Cu–O (YBCO) film prepared by the same technique, the critical current density at large magnetic field surpassed that of the YBCO film. The Tc of the present GdBCO films depended only weakly on the crystallization temperature and the partial pressure of oxygen (pO2) for the range of parameters investigated in the present study. On the other hand, Jc was found to vary largely with the processing conditions. We discuss the reason for this strong processing parameter dependence of Jc by summarizing our results on a pO2 versus temperature diagram.

Bending and axial strain dependence of the critical current in superconducting BSCCO tapes

Abstract: A comparison is made between the critical current (Ic) versus bending strain and axial strain of superconducting multi-filamentary Bi2Sr2Ca2Cu3Ox (Bi-2223) AgMg sheathed tapes. For the bending strain measurement the tape is sandwiched between a curved base and cover plate. Six sets of bending plates introduce bending strains ranging from 0% to 1.0%. The measurements show a slight decrease in Ic after the first bending step after which the degradation becomes more pronounced. The Ic in a bent conductor is calculated assuming a linear axial strain profile inside the conductor. For this calculation the Ic degradation determined in an axial compression and elongation experiment is used. The model predicts an immediate decrease of Ic, caused by the compressive strain dependence. There is a good agreement (within 5%) between the measured data and the calculated values. Based on this good agreement it can be concluded that a possible shift in the neutral line or the formation of additional cracks due to bending has no significant influence on the Ic degradation. It is concluded that the influence of thermal contraction is crucial for a good calculation.

Joining Y123 bulk superconductors using Yb–Ba–Cu–O and Er–Ba–Cu–O solders

Abstract: We have succeeded in joining Y–Ba–Cu–O bulk superconductors using sintered Yb–Ba–Cu–O and Er–Ba–Cu–O bars as welding solders. The key to the fabrication of strongly coupled joints was the control of the welded surfaces such that both of the joined surfaces are parallel to the (110) plane. Another key was the use of highly dense sintered Yb–Ba–Cu–O and Er–Ba–Cu–O bars as welding solders. Microstructural observations have shown that welded regions were free from voids, residual liquid phase, and the segregation of second phase particles. The joint made with the Er–Ba–Cu–O solder was superior to that with Yb–Ba–Cu–O, presumably due to the inherently superior superconducting properties of Er–Ba–Cu–O.

Enhanced Transport Currents in Cu-Sheathed MgB2 Wires

Abstract: Copper-sheathed MgB2 wires, prepared by an in-situ process, were exposed to neutron radiation in order to introduce defects into the superconductor. The high level of disorder (4.6 × 10−2 dpa) leads to a decrease of the transition temperature by more than 4 K, but to an increase of the slope of the irreversibility line, thus resulting in higher irreversibility fields at low temperatures. The transport currents are significantly enhanced at 4.2 K for fields above 2 T.

Superconducting properties of Dy-Ba-Cu-O bulk superconductors

Abstract: We report on the superconducting properties of melt-textured Dy123 bulk superconductors containing 5 to 40 mol% Dy211. Dy?Ba?Cu?O bulk samples were fabricated with the cold-seeding method in air using a Nd123 seed crystal. The superconducting properties of the bulk samples were strongly affected by the Dy211 content and the oxygen-annealing temperature. In the Jc?B curve of the Dy123 sample with 5 mol% of Dy211, the secondary peak was clearly observed at the applied fields of 0.5?2.0 T. The peak Jc values were 32 000?37 000 A cm?2 at 77 K. The peak position shifted to lower fields with increasing oxygen-annealing temperature, presumably due to an increase in oxygen vacancies. We believe that the peak effect in Dy?Ba?Cu?O is ascribed to oxygen deficiency, since RE/Ba substitution is not present in the system. This idea was supported by the fact that the secondary peak decreased with increasing Dy211 contents and disappeared in the Dy123 sample with 40 mol% of Dy211.