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ItemTexture 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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ItemThermal 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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ItemStudies 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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ItemStrong, 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.
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ItemLocal texture and percolative paths for long-range conduction in high critical current density TlBa₂Ca₂Cu₃O₈₊ₓ deposits(Georgia Institute of Technology, 1994-01-03) Kroeger, D. M. ; Goyal, A. ; Specht, E. D. ; Wang, Z. L. (Zhong Lin) ; Tkaczyk, J. E. ; Sutliff, J. A. ; DeLuca, J. A.A possible microstructural origin of the high critical current densities which have been obtained in c-axis-aligned, polycrystalline TlBa₂Ca₂Cu₃O₈₊ₓdeposits has been identified. The results of x-ray diffraction determinations of basal plane texture of Tl-1223 deposits prepared by spray pyrolysis are observed to depend on the size of the x-ray beam. Furthermore, most grain boundaries were found from transmission electron microscopy to have small misorientation angles. It is concluded that although overall the basal plane orientations are nearly random, there is a high degree of local texture indicative of colonies of similarly oriented grains. The spread in a-axis orientation within a colony is ~10°–15°. Intercolony conduction, it is suggested, may be enhanced by a percolative network of small-angle grain boundaries at colony interfaces.
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ItemGrain-boundary chemistry and weak-link behavior of polycrystalline YBa[subscript 2]Cu[subscript 4]O[subscript 8](Georgia Institute of Technology, 1993-10) Wang, Z. L. (Zhong Lin) ; Brynestad, J. ; Kroeger, D. M. ; Sun, Y. R. ; Thompson, J. R. ; Williams, R. K.Magnetic-susceptibility and grain-boundary-chemistry data were obtained on dense YBa[subscript 2]Cu[subscript 4]O[subscript 8] (Y1:2:4) samples. Transmission-electron microscopy showed that the grain boundaries were free of second phases and the dislocation density was low. Cation content, oxygen composition, and hole density were determined by the combined techniques of nanoprobe energy-dispersive x-ray spectroscopy and electron-energy-loss spectroscopy. A total of 25 pairs of grains was analyzed and the results indicated that grain boundary and bulk compositions do not differ. The relative orientations of the crystallites were determined, and the results show that a wide variety of misorientations was sampled. Almost all of the grain boundaries were fully oxygenated and there are no hole deficiencies, but the magnetic susceptibility measurements showed that the material is granular weak linked. These results indicate that Y 1:2:4 is a material in which clean, stoichiometric boundaries still form weak links.
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ItemStructural and chemical disorder near the Y₂BaCuO₅/YBa₂Cu₃O₇-δ interface and its possible relation to the flux-pinning behavior in melt-textured YBa₂Cu₃O₇-δ(Georgia Institute of Technology, 1993-03-01) Wang, Z. L. (Zhong Lin) ; Goyal, A. ; Kroeger, D. M.Transmission-electron microscopy and energy-dispersive x-ray spectroscopy were used to study the microstructure of the YBa₂Cu₃O₇-δ (1:2:3) region adjacent to the Y₂BaCuO5/YBa₂Cu₃O₇-δ (2:1:1/1:2:3) interface. It is found that there exists a high local density of stacking faults in 1:2:3, near the 2:1:1/1:2:3 interfaces. The stacking faults lie parallel to the (001) basal plane and are inhomogeneously distributed around the 2:1:1 particles. They tend to be disk shaped with diameters ranging from a few to ∼30 nm. Calculations made using simple energy considerations suggest that these stacking faults may act as effective flux pinners for magnetic fields directed both parallel and perpendicular to the basal plane. They may account for the observed increase of Jc with volume fraction of 2:1:1 and also explain the angular dependence of transport Jc in melt-processed 1:2:3. An unusual tendency for the formation of facets on the incoherent, randomly oriented 2:1:1 particles parallel to the {001}-type planes in the 1:2:3 matrix was also observed. Microanalysis of the 1:2:3 region around the 2:1:1 particles, which contain few or no stacking faults, consistently shows an enrichment of Y and a corresponding depletion in Ba concentration. Such cation nonstoichiometry may result in the formation of numerous point defects, which could also result in pinning. The presence of ledges on some facets at the 2:1:1/1:2:3 interfaces and the observed compositional nonstoichiometry in the 1:2:3 phase in the vicinity of these interfaces suggest that 2:1:1 particles continue to change in size after entrapment in 1:2:3. The observed compositional variation is consistent with dissolution of trapped 2:1:1. Such diffusion effects and stresses due to the thermal and elastic mismatch between 2:1:1 and 1:2:3 provide mechanisms for generating the observed defects aroung the 2:1:1 particles in the 1:2:3 matrix.