Kenneth H. Sandhage

Critical issues in the OPIT processing of high-Jc BSCCO superconductors

The oxide-powder-in-tube (OPIT) method has produced superconducting wires or tapes that exhibit high critical current densities in high magnetic fields when the superconducting oxide is the two-layer compound Bi2Sr2Ca1Cu2Oy or the three-layer compound Bi2−xPbxSr2Ca2Cu3Oy. For good properties to be obtained, however, numerous critical processing issues must be addressed.

Thermostability and decomposition of the (Bi,Pb)2Sr2Ca2Cu3O10 phase in silver‐clad tapes

The stability of the Bi2−xPbxSr2Ca2Cu3O10 (Pb‐2223) phase contained in silver‐sheathed oxide‐powder‐in‐tube specimens has been investigated by x‐ray diffraction, transmission electron microscopy, and energy dispersive x‐ray analysis. Silver tubes loaded with Pb‐2223 precursor powders were processed into tapes using established metallurgical techniques. The tapes were heat‐treated in a specially designed equilibration apparatus at selected temperatures (800–845u2009°C) for a range of times (10–5500 min) and quenched in liquid gallium held at ∼40u2009°C. The results showed that the Pb‐2223 phase is stable in a limited temperature interval between 810 and 830u2009°C in 7.5% oxygen. At 800u2009°C, this phase decomposes to Bi2Sr2CaCu2O8 (2212), Ca2PbO4, and CuO; while at temperatures ≥840u2009°C it partially melts with precipitation of Bi2Sr2CuO6 (2201) and Ca2CuO3. The effects of the silver cladding on the Pb‐2223 phase stability and microstructure are also discussed.

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.

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.