Applied Superconductivity Center

Research Spotlight

Low Temperature, Nb- base supercomductors [LTS]:

The superconducting properties of two internal-Sn wires were measured after receiving widely varying Cu-Sn mixing heat treatments followed by a standard Nb₃Sn reaction heat treatment of 180 hours at 650°C. We can find no influence on the inductively measured superconducting properties due to the Cu-Sn mixing step, whether it lasted 200 hours or ~3 minutes. We conclude that the superconducting properties of fully reacted internal-Sn wires are independent of the level of Cu-Sn mixing. View Paper

Coated Conductors [CC]:

This nature insight article reviews the choice of superconducting materials for electric power applications with the emphasis on key mechanisms, which limit current-carrying capability of HTS conductors. Advantages and disadvantages of YBCO coated conductors, Bi-2223 multifilamentary composites, and the newly discovered MgB₂ are discussed and compared with those of low-Tc superconductors like NbTi and A-15 compounds. View Paper

Local Joule heating due to planar defects can cause thermal instabilities, which limit current-carrying capability of YBCO coated conductors. Based on exact solutions that describe distributions of electric field in superconductoirs, instability criteria are obtained for a planar defect in a film and for a grain boundary. Instabilities can be triggered by low-angle grain boundaries or small planar defects, which block only a small fraction of the sample cross-section. Hot spots near defects and overheating of grain boundaries can result in significant variations of critical currents along HTS coated conductors. View Paper

In this paper, current flow is examined at the level of the substrate grain, ~20-100 µm. Transport measurements of the YBCO were made within single substrate grains and across single substrate grain boundaries. Results show current percolation, intra-grain J_c values exceeding 5 MA/cm2, and a J_c dependence consistent with thin film bicrystal studies. View Paper

We obtained exact solutions for distributions of transport current and electric field near planar defects in superconductors of different geometries, taking into account the highly nonlinear E-J characteristics. These solutions reveal novel features of nonlinear percolative current flow, such as current domain walls of variable width and extended domains of strong electric field near defects. It is shown that even small defects can effectively block current flow, causing highly inhomogeneous distribution of E(x,y) and localized hotspots, which significantly limit the current-carrying capability of HTS polycrystals. View Paper

BSCCO:

The attainable J_c of Bi-2223 is determined by the electrical connectivity of each filament which is itself an uncertain compromise between minimizing the significant porosity produced by the Bi-2223 formation reaction and the extensive crack network that porosity reduction by intermediate rolling produces. Better control of microstructure through process will allow for substantial performance improvements in Bi-2223 composites. View Paper

Heat treatments of monocore and multifilament Ag-sheathed 2223 tape were carried out using overpressure (OP) processing in a static and a flow OP system between 125 and 180 bar. Mass density and critical current density were improved in overpressure processed Ag-sheathed Bi-2223 tapes. View Paper

This paper shows that the number of 2201 half-cell intergrowths in the 2212 structure is influenced by the cooling rate during melt processing, with faster rates leading to more 2201 intergrowths. The fraction of 2201 intergrowths is determined quantitatively by analyzing the shape of 2212 (00l) x-ray diffraction lines. A model is proposed to account for the growth of these 2201 intergrowths. View Paper

MgB₂:

Magneto-optical results indicate a uniform Bean critical state behavior in polycrystalline MgB₂ with no electromagnetic granularity characteristics. From the measured magnetic flux profiles the temperature dependence of the critical current density has been extracted, which is in qualitative agreement with the result of global magnetization measurements. View Paper

Using in-field resistivity measurements, the electronic anisotropy in c-axis textured MgB₂ thin films is estimated to be 1.9 ± 0.2. It is also observed that Hc2|| is strongly enhanced up to 39T by alloying, more than twice that is seen in bulk samples. The H - T phase diagram indicates that flux pinning disappears at H* ~ 0.8 Hc2^, indicating a weak but noticeable effect of thermal fluctuations. View Paper

Grain Boundaries:

We performed a combined theoretical and experimental analysis, which enabled us to prove the existence of mixed Abrikosov-Josephson vortices on low-angle grain boundaries in YBCO. Using a new exact solution for the nonlinear flux flow resistance of vortices on grain boundaries, we were able for the first time to extract the vortex core size and intrinsic depairing current density between grain boundary dislocations from transport measurements. The method gives a simple yet direct way of extracting intrinsic superconducting properties of grain boundaries at nanoscales. View Paper

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Created August 31,2005