The discovery of superconductivity at 39 K in MgB 2 1 raises many issues. One of the central questions is whether this new superconductor resembles a hightemperature-cuprate superconductor or a lowtemperature metallic superconductor in terms of its current carrying characteristics in applied magnetic fields. In spite of the very high transition temperatures of the cuprate superconductors, their performance in magnetic fields has several drawbacks 2 . Their large anisotropy restricts high bulk current densities to much less than the full magnetic field-temperature (H-T) space over which superconductivity is found. Further, weak coupling across grain boundaries makes transport current densities in untextured polycrystalline forms low and strongly magnetic field sensitive 3,4 . These studies of MgB 2 address both issues. In spite of the multi-phase, untextured, nano-scale sub-divided nature of our samples, supercurrents flow throughout without the strong sensitivity to weak magnetic fields characteristic of Josephson-coupled grains 3 . Magnetization measurements over nearly all of the superconducting H-T plane show good temperature scaling of the flux pinning force, suggestive of a current density determine d by flux pinning. At least two length scales are suggested by the magnetization and magneto optical (MO) analysis but the cause of this seems to be phase inhomogeneity, porosity, and minority insulating phase such as MgO rather than by weakly coupled grain boundaries. Our results suggest that polycrystalline ceramics of this new class of superconductor will not be compromised by the weak link problems of the high temperature superconductors, a conclusion with enormous significance for applications if higher temperature analogs of this compound can be discovered.The principal samples were synthesized by direct reaction of bright Mg flakes (Aldrich Chemical) and sub-micron amorphous B powder (Callery Chemical). Starting materials were lightly mixed in half-gram batches, and pressed into pellets. These pellets were placed on Ta foil, which was in turn placed on Al 2 O 3 boats, and fired in a tube furnace under a mixed gas of 95% Ar 5% H 2 for 1 hour at 600 C, 1 hour at 800 C, and 1 hour at 900 C, and then lightly ground. The resulting powders were pressed into pellets and then hot pressed at 10 kbar at temperatures between 650 and 800 °C for periods between 1 and 5.5 hours. Disks ~4 mm in diameter and ~1 mm thick were cut from these pellets for property characterization. As noted later, this process cannot yet be considered optimum.Magnetization properties were examined in SQUID and vibrating sample magnetometers (VSM) in applied fields up to 14 T from 4.2 to above T c . Figure 1 shows onset T c values of 37-38 K for the above samples and for commercial MgB 2 powder (99.5%, ~2 µm diameter by examination, CERAC). Sample 1 and the commercial powder show smooth transitions with some temperature dependence of the zerofield cooled (ZFC) shielded moment, while sample 3 exhibits a step, indicative of non-uniformity in su...

The discovery of superconductivity at 39 K in magnesium diboride offers the possibility of a new class of low-cost, high-performance superconducting materials for magnets and electronic applications. This compound has twice the transition temperature of Nb3Sn and four times that of Nb-Ti alloy, and the vital prerequisite of strongly linked current flow has already been demonstrated. One possible drawback, however, is that the magnetic field at which superconductivity is destroyed is modest. Furthermore, the field which limits the range of practical applications-the irreversibility field H*(T)-is approximately 7 T at liquid helium temperature (4.2 K), significantly lower than about 10 T for Nb-Ti (ref. 6) and approximately 20 T for Nb3Sn (ref. 7). Here we show that MgB2 thin films that are alloyed with oxygen can exhibit a much steeper temperature dependence of H*(T) than is observed in bulk materials, yielding an H* value at 4.2 K greater than 14 T. In addition, very high critical current densities at 4.2 K are achieved: 1 MA cm-2 at 1 T and 105 A cm-2 at 10 T. These results demonstrate that MgB2 has potential for high-field superconducting applications.

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Chemical solution deposition (CSD) of YBa2Cu3O7−δ (YBCO) nanocomposites from colloidal precursor solutions containing double metal oxide preformed nanocrystals is a promising, costeffective and reproducible approach to produce superconducting films with high critical current density (Jc) and enhanced pinning. Here, the influence of the preformed nanocrystal composition on the microstructure and superconducting properties of the YBCO nanocomposite films is studied, with a focus on establishing a simple and scalable process to grow nanocomposites that can be transferred to grow nano-added coated conductors. Colloidal stable BaZrO3, BaHfO3, BaTiO3 and SrZrO3 nanocrystals (3-6 nm in diameter) were synthesized and added to an environment-friendly low-fluorine YBCO precursor solution. High-quality superconducting layers were grown on LaAlO3 single-crystal substrates from these four nanocomposite precursor solutions in a single deposition process, without the need of a seed layer, yielding Jc of 4-5 MA/cm² at 77 K in self-field. The different YBCO microstructures produced by the four types of nanocrystals and the resulting microstrain of the films are compared and related with the magnetic-field and angular dependence of Jc. We demonstrate the BaHfO3-containing nanocomposite as the best-performing with a homogeneous distribution of nanoparticles with 7 nm in average diameter and a high density of stacking faults, which leads to some of the best superconducting properties ever achieved via low-fluorine CSD. The Jc exhibits a much smoother decay in applied magnetic fields and a much more isotropic behaviour for non-parallel magnetic fields, and the pinning force is increased by a factor of 3.5 at 77 K and 1 T with respect to the pristine film.

The strain induced critical current degradation of overpressure processed straight Bi-2212/Ag wires has been studied at 77 K in self-field. For the first time superconducting properties, lattice distortions, composite wire stress and strain have been measured simultaneously in a high energy synchrotron beamline. A permanent Ic degradation of 5% occurs when the wire strain exceeds 0.60%. At a wire strain of about 0.65% a drastic n-value and Ic reduction occur, and the composite stress and the Bi-2212 lattice parameter reach a plateau, indicating Bi-2212 filament fracturing. The XRD measurements show that Bi-2212 exhibits linear elastic behaviour up to the irreversible strain limit.

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BSCCO tapes with dispersion hardened AgMg sheaths possess excellent mechanical properties but reach commonly only about 70 -80% of the critical currents of Ag sheathed reference tapes. Investigating the temperature dependence of I, and the (2223) phase content for two annealing steps on 19 filamentary Ag and AgMg sheathed tapes, characteristic differences with respect to the reaction kinetics and the resulting final phase content were found, A detailed characterization of the properties of the AgMg envelope, defining the boundary condition during the tape annealing process, gives first indications, that the oxygen exchange between filament and annealing atmosphere is modified with obviously significant influence on the phase formation kinetics and phase diagram shifts.