Effect of Carbon-Doping in Bulk Superconducting MgB2 Samples

Bulk superconducting samples of MgB2 were prepared by solid state reaction of stoichiometric quantities of Mg turnings and B in a sealed Ta cylinder at 890 C for 2 hours. The as-synthesized MgB2 samples had a Tc of 39 K, as defined as the onset of diamagnetism. The crystal symmetry was found to be hexagonal with lattice parameters, a=3.0856 A, and c=3.5199 A, similar to the literature values. To study the effect of carbon doping in MgB2, various C-containing samples of x varying from 0 to 1.00 in MgB2-xCx were prepared. Magnetic characterizations indicate that the Tc onset is same for pure and C-doped samples for x = 0.05, and 0.10. However, the shielding signal decreased monotonically with C content, apparently due to the presence of carbon on the grain boundaries that isolates grains and prevents flow of supercurrents on the perimeter.Comment: 10 pages, 3 figure

Superconducting magnesium diboride films with Tc \approx 24K grown by pulsed laser deposition with in-situ anneal

Thin superconducting films of magnesium diboride (MgB2) with Tc \approx 24K were prepared on various oxide substrates by pulsed laser deposition (PLD) followed by an in-situ anneal. A systematic study of the influence of various in-situ annealing parameters shows an optimum temperature of about 600C in a background of 0.7 atm. of Ar/4%H2 for layers consisting of a mixture of magnesium and boron. Contrary to ex-situ approaches (e.g. reacting boron films with magnesium vapor at 900C), these films are processed below the decomposition temperature of MgB2. This may prove enabling in the formation of multilayers, junctions, and epitaxial films in future work. Issues related to the improvement of these films and to the possible in-situ growth of MgB2 at elevated temperature are discussed.Comment: 5 pages, 4 figure

Ni-Cr textured substrates with reduced ferromagnetism for coated conductor applications

A series of biaxially textured Ni(1-x)Cr(x) materials, with compositions x = 0, 7, 9, 11, and 13 at % Cr, have been studied for use as substrate materials in coated conductor applications with high temperature superconductors. The magnetic properties were investigated, including the hysteretic loss in a Ni-7 at % Cr sample that was controllably deformed; for comparison, the loss was also measured in a similarly deformed pure Ni substrate. Complementary X-ray diffraction studies show that thermo-mechanical processing produces nearly complete {100} cube texturing, as desired for applications.Comment: PDF only; 19 pp., incl 10 figure

Vortex pinning in high-Tc materials via randomly oriented columnar defects, created by GeV proton-induced fission fragments

Extensive work has shown that irradiation with 0.8 GeV protons can produce randomly oriented columnar defects (CD's) in a large number of HTS materials, specifically those cuprates containing Hg, Tl, Pb, Bi, and similar heavy elements. Absorbing the incident proton causes the nucleus of these species to fission, and the recoiling fission fragments create amorphous tracks, i.e., CD's. The superconductive transition temperature Tc decreases linearly with proton fluence and we analyze how the rate depends on the family of superconductors. In a study of Tl-2212 materials, adding defects decreases the equilibrium magnetization Meq(H) significantly in magnitude and changes its field dependence; this result is modeled in terms of vortex pinning. Analysis of the irreversible magnetization and its time dependence shows marked increases in the persistent current density and effective pinning energy, and leads to an estimate for the elementary attempt time for vortex hopping, tau ~ 4x10^(-9) s.Comment: Submitted to Physica C; presentation at ISS-2001. PDF file only, 13 pp. tota

Final Report on CRADA ORNL05-0703

The work of this CRADA has been focused on the development of Rolling-Assisted Biaxially Textured Substrate (RABiTS)-based high-temperature superconducting (HTS) coated conductor technology that is in the pre-commercial development stage. Metal-Oxide Technologies, Inc. (MetOx) is a Houston-based small business that is developing and manufacturing second-generation (2G) HTS wire using an all-Metallo-Organic Chemical Vapor Deposition (MOCVD) process, including the buffer layers and HTS coating. Advances toward commercialization were enabled by coordinated interactions that facilitated the synthesis, characterization, and iterative optimization of prototype 2G wire segments

Anisotropy in the flux-line lattice symmetry of a low-kappa Type-II superconductor

Correlation between the symmetry of the two-dimensional flux-line lattice (FLL) and the real crystal lattice (CL) has been studied in a superconducting niobium sphere by means of small-angle neutron diffraction. A double-perfect silicon-crystal diffractometer enabled precise determination of the three inter-fluxoid distances corresponding to th e FFL basic cell. A systematic study of the anisotropic behavior was made as a function of temperature and magnetic-field amplitude for fields parallel to a few high-symmetry CL axes in the (1 anti 1 0) plane. In addition, at T = 4.30 K progressive deformation of the FLL was studied as the sample was rotated in the (1 anti 1 0) and (100) planes. The FLL was found to be hexagonal only for fields parallel to the three-fold CL axis. Two-fold symmetry prevailed for other CL directions in these planes except near the four-fold axis, where either of two distorted triangular lattices existed, preserving the reflection symmetry in composite, but not individually. This behavior will be discussed in terms of current models for fluxoid-CL interactions; in particular, in terms of the extension of the Takanaka theory done by Delrieu, Roger, and Kahn

Fabriacation and properties of high-J{sub c}, biaxially aligned YBa{sub 2}Cu{sub 3}O{sub 7-delta} thick films on metallic tape ...

We report the synthesis and properties of high-J{sub c}, biaxially aligned YBCO films deposited on thermo-mechanically textured Ni tapes. Sharply cube-textured Ni tapes, 125 {mu}m thick, were produced by mechanical rolling followed by recrystallization anneal. Short segments were coated with epitaxial oxide buffer layers, followed by fully aligned YBCO films to thicknesses of 1 to 3 {mu}m. In-plane textures of 7-10{degree} FWHM are achieved, with c-perpendicular alignment to 1{degree} FWHM. Typical zero-field J{sub c} values are in the range 5-9x10{sup 5} A/cm{sup 2} at 77 K, with strong behavior in magnetic fields comparable to that of epitaxial films on single crystal oxides. Assessment of properties necessary for a tape conductor technology are discussed

Transport properties of Tl1223 deposits for possible conductor applications

We have investigated the microstructure and electrical transport properties of high-temperature superconducting (HTS) TlBa{sub 2}Ca{sub 2}CU{sub 3}0{sub 8{plus}x} (Tl1223) deposits on polycrystalline substrates of YSZ and silver. Samples were formed by deposition of a n-free or TI-poor precursor film, followed by thermal reaction with Tl{sub 2}0 vapor. The resulting deposits have c-axis perpendicular texture with local in-plane alignment, which leads to a colony-type microstructure and improved electrical transport properties. Transport properties on different samples, on the same samples at different widths, and on samples with artificially-induced strong flux pinning defects lead to the conclusion that a relatively small fraction of the material comprises strongly-coupled current paths. Results on 3-20 {mu}m thick deposits on silver indicate that these materials may be useful as a new class of HTS conductors