Zhong Lin Wang

Texture and transport in spray pyrolyzed TlBa 2 Ca 2 Cu 3 O 9 thick films

©1995 Materials Research Society. The original publication is available at: http://www.mrs.org/

Defects Near the Y2BaCuO5/YBa2Cu3O7−x Interface and their Effect on Flux-Pinning in Melt-Processed and Quench-Melt-Growth Processed YBa2Cu3O7−x

A detailed examination of the Y 2 BaCuO 5 (211)/ YBa 2 Cu 3 O 7−x (123) interface in several melt-processed 123 samples prepared using different methods was undertaken using analytical electron microscopy. It is found that there exists a significant increase in the a-b planar stacking fault density in 123, near the 211/123 interface. When viewed along [001], these faults appear as disks with diameter from a few to 30 nm and are bounded by dislocation loops. Most stacking faults are confined to the (001) basal plane. The size and density of defects around the 211 particles suggest that these defects could act as effective flux-pinning sites and may explain the observations of increased J c with increasing volume fraction of 211 and a maximum in J c when the applied field parallel to the c-axis.

Interface and grain boundary structures in YBa2Cu3O7-x and YBa2Cu4O8 materials

Y2BaCuO5YBa2Cu3O7-x (Y211/Y123) interfaces in melt-processed YBa2Cu3O7-x were studied by high-resolution transmission electron microscopy and energy dispersive X-ray spectroscopy. Yttrium enrichment and barium depletion were observed locally at the Y211/Y123 interfaces where Y123 (001) facets were present. This effect may be interpreted as the result of lattice substitution of Ba by Y near these interfaces. Cation nonstoichiometry was found near Y211/Y123 interfaces where liquid phases (Cu-Ba-O) were present. This chemical disorder introduces numerous point defects in the Y123, and these defects may act as additional pinning sites alongwith stacking faults. A comparison of grain boundary (GB) chemical composition in polycrystalline YBa2Cu3O7-x and YBa2Cu4O8(Y124), studied using nanoprobe parallel-detection electron energy-loss spectroscopy (EELS), is presented. The studies of Y124 show that stoichiometric grain boundaries can also form weak links between superconducting grains. It is suggested that weak-link behavior is determined largely by misorientation at grain boundaries.

Local texture and percolative paths for long-range conduction in high critical current density TlBa[sub 2]Ca[sub 2]Cu[sub 3]O[sub 8+][sub [ital x]] deposits

A possible microstructural origin of the high critical current densities which have been obtained in c‐axis‐aligned, polycrystalline TlBa2Ca2Cu3O8+x deposits 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.