A. Polyanskii

High-Tc superconducting materials for electric power applications.

Large-scale superconducting electric devices for power industry depend critically on wires with high critical current densities at temperatures where cryogenic losses are tolerable. This restricts choice to two high-temperature cuprate superconductors, (Bi,Pb)2Sr2Ca2Cu3Ox and YBa2Cu3Ox, and possibly to MgB2, recently discovered to superconduct at 39 K. Crystal structure and material anisotropy place fundamental restrictions on their properties, especially in polycrystalline form. So far, power applications have followed a largely empirical, twin-track approach of conductor development and construction of prototype devices. The feasibility of superconducting power cables, magnetic energy-storage devices, transformers, fault current limiters and motors, largely using (Bi,Pb)2Sr2Ca2Cu3Ox conductor, is proven. Widespread applications now depend significantly on cost-effective resolution of fundamental materials and fabrication issues, which control the production of low-cost, high-performance conductors of these remarkable compounds.

Strongly linked current flow in polycrystalline forms of the superconductor MgB2.

The discovery of superconductivity at 39 K in magnesium diboride, MgB2, raises many issues, a critical one being whether this material resembles a high-temperature copper oxide superconductor or a low-temperature metallic superconductor in terms of its behaviour in strong magnetic fields. Although the copper oxides exhibit very high transition temperatures, their in-field performance is compromized by their large anisotropy, the result of which is to restrict high bulk current densities to a region much less than the full magnetic-field–temperature (H–T) space over which superconductivity is found. Moreover, the weak coupling across grain boundaries makes transport current densities in untextured polycrystalline samples low and strongly sensitive to magnetic field. Here we report that, despite the multiphase, untextured, microscale, subdivided nature of our MgB2 samples, supercurrents flow throughout the material without exhibiting strong sensitivity to weak magnetic fields. Our combined magnetization, magneto-optical, microscopy and X-ray investigations show that the supercurrent density is mostly determined by flux pinning, rather than by the grain boundary connectivity. Our results therefore suggest that this new superconductor class is not compromized by weak-link problems, a conclusion of significance for practical applications if higher temperature analogues of this compound can be discovered.

High critical current density and enhanced irreversibility field in superconducting MgB2 thin films

Larbalestier †§ N. Rogado*, K.A. Regan*, M.A. Hayward*, T. He*, J.S. Slusky*, K. Inumaru*, M.K. Haas* and R.J. Cava* † Department of Materials Science and Engineering, Univer-sity of Wisconsin, 1509 University Avenue, Madison, WI 53706 USA § Applied Superconductivity Center, University of Wisconsin, 1500 Engineering Drive, Madison, WI 53706 USA * Department of Chemistry and Princeton Materials Institute, Princeton University, Princeton, NJ 08544 USA

Thin Film Magnesium Boride Superconductor with Very High Critical Current Density and Enhanced Irreversibility Field

Larbalestier †§ N. Rogado*, K.A. Regan*, M.A. Hayward*, T. He*, J.S. Slusky*, K. Inumaru*, M.K. Haas* and R.J. Cava* † Department of Materials Science and Engineering, Univer-sity of Wisconsin, 1509 University Avenue, Madison, WI 53706 USA § Applied Superconductivity Center, University of Wisconsin, 1500 Engineering Drive, Madison, WI 53706 USA * Department of Chemistry and Princeton Materials Institute, Princeton University, Princeton, NJ 08544 USA

New Fe-based superconductors: properties relevant for applications

Less than two years after the discovery of high temperature superconductivity in oxypnictide LaFeAs(O, F) several families of superconductors based on Fe layers (1111, 122, 11, 111) are available. They share several characteristics with cuprate superconductors that compromise easy applications, such as the layered structure, the small coherence length and unconventional pairing. On the other hand, the Fe-based superconductors have metallic parent compounds and their electronic anisotropy is generally smaller and does not strongly depend on the level of doping, and the supposed order parameter symmetry is s-wave, thus in principle not so detrimental to current transmission across grain boundaries. From the application point of view, the main efforts are still devoted to investigate the superconducting properties, to distinguish intrinsic from extrinsic behaviors and to compare the different families in order to identify which one is the fittest for the quest for better and more practical superconductors. The 1111 family shows the highest Tc, huge but also the most anisotropic upper critical field and in-field, fan-shaped resistive transitions reminiscent of those of cuprates. On the other hand, the 122 family is much less anisotropic with sharper resistive transitions as in low temperature superconductors, but with about half the Tc of the 1111 compounds. An overview of the main superconducting properties relevant to applications will be presented. Upper critical field, electronic anisotropy parameter, and intragranular and intergranular critical current density will be discussed and compared, where possible, across the Fe-based superconductor families.

Electronic anisotropy, magnetic field-temperature phase diagram and their dependence on resistivity in c-axis oriented MgB2 thin films

An important predicted, but so far uncharacterized, property of the new superconductor MgB2 is electronic anisotropy arising from its layered crystal structure. Here we report on three c-axis oriented thin films, showing that the upper critical field anisotropy ratio Hc2?/Hc2? is 1.8 to 2.0, the ratio increasing with higher resistivity. Measurements of the magnetic field-temperature phase diagram show that flux pinning disappears at H*?0.8Hc2?(T) in untextured samples. Hc2?(0) is strongly enhanced by alloying to 39 T for the highest resistivity film, more than twice that seen in bulk samples.

High intergrain critical current density in fine-grain (Ba0.6K0.4)Fe2As2 wires and bulks

The K- and Co-doped BaFe(2)As(2) (Ba-122) superconducting compounds are potentially useful for applications because they have upper critical fields (H(c2)) of well over 50 T, H(c2) anisotropy γ < 2and thin-film critical current densities J(c) exceeding 1 MA cm(-2) (refs 1-4) at 4.2 K. However, thin-film bicrystals of Co-doped Ba-122 clearly exhibit weak link behaviour for [001] tilt misorientations of more than about 5°, suggesting that textured substrates would be needed for applications, as in the cuprates. Here we present a contrary and very much more positive result in which untextured polycrystalline (Ba(0.6)K(0.4))Fe(2)As(2) bulks and round wires with high grain boundary density have transport critical current densities well over 0.1 MA cm(-2) (self-field, 4.2 K), more than 10 times higher than that of any other round untextured ferropnictide wire and 4-5 times higher than the best textured flat wire. The enhanced grain connectivity is ascribed to their much improved phase purity and to the enhanced vortex stiffness of this low-anisotropy compound (γ~1-2) when compared with YBa(2)Cu(3)O(7-x) (γ~5).

Magnetic granularity, percolation and preferential current flow in a silver-sheathed Bi1.8Pb0.4Sr2Ca2Cu3O8+x tape

Abstract Magneto-optical imaging of the flux penetration into a Ag sheathed Bi1.8Pb0.4Sr2Ca2Cu3O8+x tape has been used to extract the current flow paths in a magnetic field applied perpendicular to the c-axis. Using the large aspect ratio of the plate-like grain structure and the slab geometry of the sample to simplify the current-flow geometry, we converted the magneto-optical signal into two-dimensional (2D) field and current distributions. We found that the current patterns were very non-uniform and sensitive to weak magnetic fields of ∼ 400–800 Oe, even at 10 K. Current streamlines show that the effective current-carrying cross-section of the tape strongly depends on the field. Magnetization currents flow preferentially near the silver sheath, while the tape center supports mainly percolative and granular current patterns consisting of an array of macroscopic current loops whose long dimensions are of the order of the tape thickness. By comparing contour maps of the local Jc values with the microstructural images, we found that the high-Jc regions correlate with colonies of well-aligned long grains which are preferentially located near the silver interface, while the less aligned structure of smaller grains in the central part of the tape is associated with the granular behavior and much lower current-carrying capability. The wide distribution of the local Jc(x, y) revealed by magneto-optical imaging indicates that the performance of BSCCO-2223 tapes can be significantly improved if a larger fraction of well aligned grains can be produced more uniformly throughout the tape cross-section.

Influence of nickel substrate grain structure on YBa2Cu3O7−x supercurrent connectivity in deformation-textured coated conductors

Coupled magneto-optical imaging and local misorientation angle mapping have been used to demonstrate the percolative nature of supercurrent flow in YBa2Cu3O7−x(YBCO) coated conductors grown on deformation-textured Ni substrates. Barriers to current flow occur at many YBCO grain boundaries (GBs) which have propagated through the buffer layers from the underlying Ni substrate, and all Ni GBs with misorientation angles >4° initiate percolative current flow. This type of current barrier is characteristic of the conductor form and has been found to exist in samples with Jc(0 T,77 K) values >2 MA/cm2. Sharpening of the local substrate texture or improving in low-angle GB properties should lead to higher Jc values.

Direct evidence for residual, preferentially-oriented cracks in rolled and pressed Ag-clad BSCCO-2223 tapes and their effect on the critical current density

We have studied the connectivity of rolled and pressed Ag-sheathed tapes through three deformation and heat treatment cycles using magneto-optical imaging. In pressed samples, a transport critical current density of (77 K, 0 T) was reached after two heat treatment steps, and increased to after a third heat treatment. Critical current density values in rolled samples also reached after two heat treatments, but decreased to after the subsequent deformation and third heat treatment. Magneto-optical imaging using fields applied perpendicular to the rolling plane revealed that flux penetrated the superconducting core mainly through defects oriented perpendicular to the direction of current flow in rolled samples, and parallel to the direction of current flow in pressed samples. In those samples which had received more than one heat treatment, the flux is believed to penetrate through cracks produced during the deformation steps which do not heal during the subsequent heat treatments due to lack of sufficient residual liquid phase.