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 KTaO 3 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.

be 0.2-1 pm thick (in the c-direction) and 3-10 pm in the plane [2]. Jc measured resistively with voltage contact spacing in the range of 1 to 4 mm (therefore spanning hundreds of grains) is typically 0.5 1 x lo5 A/cm2 in self-field at 77 K and has good field dependence [l]. At 40 K in fields up to 9T, Jc remains well above 104 A/cm2 [l]. X-ray phi scans were used to study the local and macroscopic in-plane texture [2-41. A large x-ray beam (4 x 8 mm) which illuminated most of the specimen surface indicated random

A possible microstructural origin of the high critical current densities which have been obtained in c-axis-aligned, polycrystalline TlBa&a,Cu,Os +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 -lo”-15”. Jntercolony conduction, it is suggested, may be enhanced by a percolative network of small-angle grain boundaries at colony interfaces.

Dissipation and critical current densities in high-Tc superconductors can depend on both intragranular flux creep and intergranular weak links. Separating these effects in polycrystals can be a formidable task. We present current-voltage data which can in some cases identify weak-link , and verify it by using ion irradiation to greatly diminish the contribution from flux creep. We conclude that the occurrence of significant weak links in series with so-called strongly-coupled current paths may be more common than previously deduced.

Magneto-optical imaging was used to visualize the inhomogeneous penetration of magnetic flux into polycrystalline TlBa 2 Ca 2 Cu 3 O x films with high critical current densities, to reconstruct the local two-dimensional supercurrent flow patterns and to correlate inhomogeneities in this flow with the local crystallographic misorientation. The films have almost perfect c -axis alignment and considerable local a - and b -axis texture because the grains tend to form colonies with only slightly misaligned a and b axes. Current flows freely over these low-angle grain boundaries but is strongly reduced at intermittent colony boundaries of high misorientation. The local (<10-micrometer scale) critical current density J c varies widely, being up to 10 times as great as the transport J c (scale of ∼1 millimeter), which itself varies by a factor of about 5 in different sections of the film. The combined experiments show that the magnitude of the transport J c is largely determined by a few high-angle boundaries.

In several Tl- and Hg-based high-Tc superconducting materials, vortex-pinning defects were formed by irradiation with 0.8 GeV protons. The protons cause heavy constituent nuclei (Tl, Hg, Bi, …) to fission, which generates randomly oriented tracks. These columnar defects lead to significant enhancements in the current-conducting properties. We investigated bulk materials and thin films, irradiated with or without Pb or Au “amplifier foil” overlayers, which increase the number of column-creating fission fragments.

Tape applications require high values of critical current density in thick films of TlBa2Ca2Cu3Oy (Tl-1223). Previous work on polycrystalline YSZ substrates has shown average values of Jc(77 K, 0 T) = 60 kA cm−2 for films 3–5 μm thick. The effect of varying the substrate temperature (853–868°C) during thallination in a two-zone flow-through furnace is studied here for thicker films (7–8 μm thick). Film density and the zero-resistance transition temperature increase with increasing processing temperature, as does the amount and size of the plate-like features on the film surface. X-ray diffraction shows that alignment improves with increasing temperature, but is still below values obtained for 3 μm thick films. Phases other than Tl-1223 were found at all processing temperatures. Values of Jc for the 7–8 μm thick films are generally lower than for the thinner 3–5 μm thick ones, but the current per sample width can be higher (the highest value for the thinner films was 5 A mm−1, whereas at substrate temperatures > 860°C, 7% of the segments of the 7–8gmm thick films had values < 10 A mm−1.

Optimum thermomechanical processing conditions have been developed to achieve a high current density in powder-in-tube (PIT) thallium-based (Tl,Pb)-1223 and (Tl,Bi)-1223 tapes. Critical currents in excess of 25 A corresponding to a current density of 20 000 A/cm2 have been achieved in these tapes at 77 K. Heat treatment and intermediate deformation sequences, heating rates, and starting precursor phase assemblages have been examined to optimize the current density. A combination of processing conditions that result in incremental densification, less microcracking and liquid formation in the early and final stages of thermomechanical treatment is found to result in the highest current densities.

X-ray diffraction rocking curves are used to measure the c axis alignment of TlBa2Ca2Cu3Ox films grown on polycrystalline substrates with thickness varying from 3 to 10.5 μm. Films thicker than 3 μm are found to contain two layers: a well aligned (3.5° FWHM) bottom layer, and a poorly aligned (greater than 12° FWHM) top layer. Azimuthal scans show that the component with good long-range out-of-plane alignment retains its characteristic colony microstructure of local in-plane alignment as film thickness increases. The length dependence of the critical current density may be accounted for by assuming that all the supercurrent is carried by the well-aligned component.

Uniaxially textured TlBaSrCa/sub 2/Cu/sub 3/O/sub 9/ bulk ceramics have been fabricated by slip casting in magnetic fields up to 9 Tesla followed by sintering and reduction annealing. The measured transport properties are compared with those of spray pyrolyzed thick films where the degree of <001> texture is controlled through processing. Transport anisotropy is correlated to the degree of texture but not to the magnitude of the critical current density. In fact, although the critical current density of the aligned ceramic has the lowest value and is the most sensitive to small magnetic fields, it is also the most anisotropic with respect to the field direction. These data indicate that <001> texture is necessary but not sufficient for high critical current density in the Tl(1223) system.<>

It is imperative for tape applications to produce thicker films of TlBa2Ca2Cu3Oy with high critical current density on polycrystalline substrates (a useful figure of merit is the critical current per unit width, with a reasonable goal being 10 A mm−1). We have studied the critical current density at 77 K, 0 T (Jc) as a function of film thickness in the range 2 to 10 μm. A key parameter for obtaining a high Jc was found to be the film density; films with relative density > 82% showed much higher values of Jc. Based on average values, Jc was found to peak at 60 kA cm−2 for samples 2.5 to 5 μm thick (corresponding to 3 A mm−1 for 5 μm thick samples), and to decrease for thicker samples, yielding 3 to 3.5 A mm−1 from 5 to 10 μm. There are individual segments with values as high as 8.4 A mm−1.

X-ray microdiffraction has been used to map the orientational distribution of grains in a 4 × 5mm2 polycrystalline film of Tl1−yBa2Cu2Cu3Ox (Tl-1223) grown on yttira-stabilized zirconia (YSZ). The film consists of “colonies”, each containing ∼ 104 grains with their a-axes aligned; the c-axes of all grains are aligned normal to the film, but there is no overall a-axis alignment. While the grain boundaries within a colony are small angle, colonies are separated by large-angle tilt boundaries. The typical colony size is 0.4 mm. Measured critical currents are compared with simulations based on observed distributions of grain alignment; we find that variations in grain alignment account for the observed variations in the critical current density. Simulations over longer distances indicate that a colony structure can substantially increase the critical current, especially in magnetic fields.