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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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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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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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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.