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School of Materials Science and Engineering

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Author: Kroeger, D. M. × Author: Wang, Z. L. (Zhong Lin) × End date: 1995 × Start date: 1995 × Type: Text ×

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Now showing 1 - 4 of 4
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    Texture and transport in spray pyrolyzed TlBa₂Ca₂Cu₃O₉ thick films
    (Georgia Institute of Technology, 1995-09) Tkaczyk, J. E. ; Sutliff, J. A. ; DeLuca, J. A. ; Bednarczyk, P. J. ; Briant, C. L. ; Wang, Z. L. (Zhong Lin) ; Goyal, A. ; Kroeger, D. M. ; Lowndes, D. H. ; Specht, E. D.
    The electron backscattering pattern technique has been applied to the microstructural investigation of Tl(1223) thick films formed by vapor-phase thallination of Ag-containing Ba–Ca–Cu–oxide precursors. For samples grown on polycrystalline YSZ, considerable biaxial alignment is found in localized, multigrain regions as wide a 100 µm or more. However, on scales above 1 mm the overall texture remains only uniaxial with the c-axes (i.e., [001]) aligned perpendicular to the plane of the substrate. On single-crystal KTaO₃ an epitaxial relationship is evident which persists to the surface of a 3 µm thick film. Modest variations in the processing protocol yield films containing grains oriented with the c-axis in the plane, resulting in the degradation of transport properties. The data suggest a growth mode in which sparse nucleation occurs at the substrate followed by rapid lateral crystallization.
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    Thermal stability and annealing of columnar defects in Bi₂Sr₂Ca₁Cu₂O₈/Ag superconductor
    (Georgia Institute of Technology, 1995-08-14) Thompson, J. R. ; Paul, David ; Wang, Z. L. (Zhong Lin) ; Kroeger, D. M. ; Christen, D. K.
    This work establishes the stability at elevated temperatures of columnar defects, artificially formed in the Bi-based cupric oxide superconductor Bi₂Sr₂Ca₁Cu₂O₈ for enhanced vortex pinning. Isochronal anneals, conducted in air, led to losses of critical current density in two stages. The defects were relatively stable up to ~ 550 °C, where second stage annealing began; above this, the pinning diminished rapidly. The recrystallization and loss of columnar defects were corroborated by transmission electron microscopy.
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    Studies on Superplastically Deformed 123/Ag Composites
    (Georgia Institute of Technology, 1995-05) Goyal, A. ; Wang, Z. L. (Zhong Lin) ; Sun, Y. R. ; Kroeger, D. M. ; Thompson, J. R. ; Chou, Y. T. ; Yun, Y. ; Harmer, A. P.
    Composites containing 25 vol% Ag were compressed at room temperature to over 110% at 850°C in air. Measurement of the strain rate sensitivity yielded a value of 0.5, characteristic of superplastic deformation. As deformed materials had sub-micron grain size and significant c-axis texture parallel to the pressing direction. TEM examination showed that the grains were highly defected and that the grain boundaries were clean. The T[subscript c] was however low with an onset of 50K and a width of ~10K. Annealing studies were carried out with an aim to "fully oxygenate" the material and anneal out a minimal number of defects to obtain higher transition temperatures, at the same time retaining a significant defect density for enhanced fluxpinning. Magnetization measurements were performed after most anneals in order to evaluate intragranular and intergranular properties. Results indicate the presence of unusually high J [subscript c]'s at low temperatures after the final anneal (T[subscript c] onset ~90°K). The observations may be explained by highly superior intragranular properties coupled with increased local current loop size.
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    Strong, Asymmetric Flux Pinning by Miscut-Growth-Initiated Columnar Defects in Epitaxial YBa[subscript 2]Cu[subscript 3]O[subscript 7—chi] Films
    (Georgia Institute of Technology, 1995-03-20) Lowndes, D. H. ; Christen, D. K. ; Klabunde, C. E. ; Wang, Z. L. (Zhong Lin) ; Kroeger, D. M. ; Budai, J. D. ; Zhu, Shen ; Norton, D. P.
    Strong flux pinning and a pronounced asymmetric angular dependence of J[subscript c]([eta, theta]) have been discovered in YBa[subscript 2]Cu[subscript 3]O[subscript 7—chi] films grown on miscut, mosaic LaAlO[subscript 3] substrate surfaces. A new pinning peak, which at low fields is nearly as strong as the “intrinsic pinning,” is observed for magnetic field orientations between the c axis and the CuO[subscript 2] plane. Cross-section transmission electron microscopy reveals columnar growth defects (2–3 nm diameter) aligned near the c axis, in concentrations ~10[superscript 10] cm[subscript —2]. The results demonstrate that deliberately modified substrate surfaces may introduce technologically useful flux pinning in YBa[subscript 2]Cu[subscript 3]O[subscript 7—chi] films.