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.

The thermal diffusivity thermal barrier coatings (zirconia stabilized with 7-8 wt% yttria) deposited by air plasma spraying has been measured by the laser flash method. Free-standing samples (0.5-0.6-mm thick) polished on both sides wen coated with graphite. Deduced values of thermal conductivity (e.g., 1.5 W m(-1) K-1 at 200 degrees C in 1 atm. of nitrogen) are in the range measured in other studies for this type of thermal barrier coating. The % increase in thermal conductivity in I arm. of gas compared to its value in vacuum was found to be about 10% in nitrogen at 500 degrees C, close to a calculated value of 7% using a model in the literature. A simple extension of the model to higher temperatures and gas pressures predicts a significant rise, so that by 10 atm. of gas pressure the % increase would be 30% (with little sensitivity to temperature). (C) 1997 Elsevier Science S.A.

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.

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.

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.

Spray pyrolyzed T1(1223) films deposited on polycrystalline YSZ substrates are characterized before and after heavy ion irradiation. A factor of 2 decrease in zero field critical current is observed. However, significantly improved critical current is found at fields above 1 T, where intragranular effects dominate. The irreversibility line at 5 T is shifted by ∼20 K to higher temperatures. Scaling of the data before and after irradiation and at different temperatures is modeled by an expression which interpolates between single vortex pinning and collective creep.

Long lengths of flexible silver-cell Bi-2223 high-T/sub c/ (critical temperature) superconductors have been fabricated by the powder-in-tube technique. By improving process conditions DC transport measurement at liquid helium (4.2 K) and hydrogen (20 K) temperatures yield J/sub c/'s (critical current densities) greater than 10/sup 5/ A/cm/sup 2/ at zero field and exceed 3*10/sup 4/ A/cm/sup 2/ at liquid nitrogen (77 K) temperature in short tape samples. Detailed microstructural analysis and J/sub c/ measurements with applied fields up to 20 T are reported. Coils have been fabricated from 1-m lengths of conductor with J/sub c/'s at 77 K approaching 10/sup 4/ A/cm/sup 2/. Measurements on small pancake coils made from 10-m tapes are also reported.<>

Superconducting joints between Ag-clad, Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub z/ tapes have been obtained with approximately 1/2 the current capacity of the tapes themselves. The Ag sheath is removed from one side of each tape without significantly disturbing the superconducting core. The exposed superconducting core of the two tapes is brought into contact and pressed so as to again seal the superconductor in a Ag sheath. A reaction anneal is performed to join the two cores together and repair damage associated with the removal of Ag and the pressing. Transport measurements using multiple voltage taps have been found useful in characterizing the critical current variation across the joint.< >

Progress is reported in the understanding and development of the silver‐addition process for the preparation of ‘‘1223’’‐TCBCO thick films. Described are improvements in film homogeneity achieved via modification of the spray deposition procedure, increases in Jc achieved via post synthesis heat treatment in oxygen, and the results of studies of the film growth mechanism which indicate that presence of silver, barium oxide, and thallium oxide are all necessary for the development of a liquid phase responsible for the accelerated film formation obtained by this technique. Also reported are studies of the preparation of films on metallic substrates which include Jc vs Field data at 4.2 K and 77 K for ‘‘1223’’ film on silver foil exhibiting Tc(0)=105 K and Jc(zf‐77 K)=10,000 A/cm2.

A process is described for the preparation of superconducting films of “1223” TlxCa2Ba2Cu3Oy (0.6510 000 A/cm2 was measured at 60 K-2 T with the magnetic field applied parallel to the crystallographic c-axis of the film.

Large critical currents with a relatively weak magnetic field dependence are obtained in thick films of TlBa2Ca2Cu3Oz. Transport measurements indicate Jc ≳105 A/cm2 at 77 K, zero field, and Jc≳104 at 60 K in a 2 T field applied along the c-axis. The observed behavior is attributed to a large degree of uniaxial alignment of platelike grains, and to superior intragranular flux pinning. These results are consistent with recent theories concerning the nature of vortices in highly anisotropic (layered) superconductors and ‘‘brick wall’’models of intergranular current transport.

Large critical currents with a relatively weak magnetic field dependence are obtained in thick films of TlBa2Ca2Cu3Oz. Transport measurements indicate Jc > 105 A/cm2 at 77K, zero field, and Jc>104 at 60K in a 2T field applied along the c-axis. Reduced weak link behavior at low fields is attributed enhanced intergranular coupling which accompanies the uniaxial aligned, plate-like microstructure. The improved performance at high fields is attributed to strong intragranular flux pinning. Scaling of the high field critical current at different temperatures is observed and the data can be fit to an expression which interpolates between single vortex pinning and collective creep regime.