Christopher A. Craven

Properties of high-Tc wires made by the metallic precursor process

The metallic precursor process, used in the production of high-temperature superconducting ceramics, exploits the relatively high formability of the metallic state to fabricate multifilament wires with excellent transport and mechanical properties. Multifilament wires, containing 9,583 filaments, exceed by tenfold the filament counts achieved by any other process. Oxide critical current densities of 17.7 kA/cm2 at 77 K in self-field surpass the best electrical performance reported for any other process used to fabricate multifilament composite wires. The mechanical properties of these wires approach the behavior of ideal composites, resulting in the flexibility, durability, and strain tolerance required for large-scale use in power generation, distribution, and end-use applications such as motors and current limiters.

Epitaxial growth of YbBa 2 Cu 3 O 7−δ films on (100)-oriented MgO and SrTiO 3 substrates by oxidation of a liquid alloy precursor

Textured superconducting films of YbBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} were grown on single crystals of MgO (100) and SrTiO{sub 3} (100) by oxidation of a liquid alloy precursor. The substrates were coated by dipping them in molten YbBa{sub 2}Cu{sub 3} (m.p. {similar to}870 {degree}C). After removal from the melt, the liquid layers on the substrates were oxidized in pure oxygen to form the tetragonal oxide phase, i.e., YbBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}, then annealed at 500 {degree}C to obtain the superconducting orthorhombic phase of the same compound. The microstructure of the films obtained in this way was found to be related to the nature of the substrate as well as to processing variables that included oxidation temperature and oxidation time. Films grown on MgO (100) showed {ital c}-axis texture as well as a random growth structure. Films prepared on SrTiO{sub 3} (100) showed either a {ital c}-axis texture or a mixture of {ital c}-axis and {ital a}-axis texture. The superconducting properties of the as-prepared films and the effects of key process parameters on film quality and microstructure are presented and discussed.

Bi-axial texture in Ca0.1Y0.9Ba2Cu4O8 composite wires made by metallic precursors

Abstract High filament count, silver-sheathed composite wires of Ca 0.1 Y 0.9 Ba 2 Cu 4 O 8 (Y–124) were prepared by a metallic precursor route. The ductility of the metallic precursor enabled one to manufacture tapes containing up to 962 407 filaments, with filament dimensions as fine as 0.25 μm thick and 1 μm wide. By using a thermal-mechanical treatment to texture the Y–124 grains, transport critical current densities in the oxide filaments of 69 500 A/cm 2 at 4.2 K in self-field were obtained. Moreover, in an applied field of 0.1 T, the samples retained 39% of their self-field critical current density. A TEM investigation revealed significant bi-axial crystallographic texture: in areas viewed, c -axis alignment of adjacent grains was within 10° and oriented perpendicular to the tape face; a -axis alignment of adjacent grains was within 15° and oriented parallel to the longitudinal direction of the filaments. Furthermore, c -axis texture alone did not adequately predict the performance of these Y−124 composite conductors. Instead, performance scaled with the degree of bi-axial texture. These wires exhibited among the best reported J c for a polycrystalline, sintered wire of YBCO in an applied magnetic field.

Advances in the processing and properties of YBa2Cu4O8

High-filament-count, silver-sheathed composite wires of YBa2Cu4O8 were prepared by a metallic precursor route. The ductility of the metallic precursor enabled the manufacture of tapes containing up to 962,407 filaments, with filament dimensions as fine as 0.25 μm thick and 1 μm wide. The combination of thermal-mechanical treatment and fine filament dimensions resulted in significant biaxial crystallographic texture. Transport critical current densities in the oxide filaments of 69.5 kA/cm2 at 4.2 Kin self-field with reduced weak-link behavior in an applied field were obtained. Critical current retention during bending was shown to increase as filament count increased, with the highest filament-count material showing close to one percent critical bending strain. These wires exhibited some of the best properties for a polycrystalline, sintered wire of YBCO in an applied magnetic field.

Growth of c-axis-oriented films of YbBa2Cu3O7−δ on single and polycrystalline MgO substrates by oxidation of a liquid alloy precursor

Abstract Textured superconducting films of YbBa 2 Cu 3 O 7–δ supported on both single and polycrystalline MgO substrates were prepared by oxidation of a liquid precursor alloy. The substrates were coated by dipping them into the molten alloy (YbBa 2 Cu 3 , melting point ≈870°C). After withdrawal from the melt, the adhering metal was oxidized to the corresponding oxide phase, i.e., Y YbBa 2 Cu 3 O 7−δ , which exhibited a superconducting transition at ≈80 K following annealing in pure O 2 at 500°C. With MgO(1 0 0) substrates, evidence was seen for the epitaxial growth of YbBa 2 Cu 3 O 7−δ crystals having their c -axis parallel to the [1 0 0] direction of the substrate. For polycrystalline MgO, X-ray diffraction and microstructural examination showed that the high- T c crystallites in the films were also oriented with their c -axis perpendicular to the substrate surface, but the directions of the a and b axes were disordered rather than epitaxial. The superconducting properties of as-prepared films are discussed together with the effects of key processing parameters on composition and microstructure.