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Z. L. Wang, A. Goyal, D. M. Kroeger, D. H. Lowndes, and E. D. Specht Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (Received 11 October 1994, accepted 15 May 1995)

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 /mm 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 KTaC>3 an epitaxial relationship is evident which persists to the surface of a 3 /JLTTI 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.

I. INTRODUCTION Large-scale applications of high temperature superconductors (HTS) await the fabrication of long lengths of a suitable conductor. Critical current density of the order 104—105 A/cm 2 at the service temperatures and fields appears necessary for many applications. However, long polycrystalline conductors contain grain boundaries which severely degrade the transport properties, especially in a magnetic field. The benefit of grain alignment was documented by experiments on bicrystals which showed good critical current density across small-angle grain boundaries,1 most recently in Tl(1223) thin-film bicrystals.2 Other researchers observed improved properties across special (so-called low X) grain boundaries.3 In Bi2Sr2Ca2Cu3Oz powderin-tube tapes where c-axis grain alignment is obtained through mechanical processing, critical current densities over 104 A/cm 2 at 77 K, zero field, have been obtained over lengths of the order of 100 m.4 This material serves as a paradigm for the brick-wall model which proposed an explanation of high critical current density in these uniaxially aligned materials.5 Significant enhancements above this current density level have been demonstrated in polycrystalline films of HTS materials where alignment of both the c-axes and a-axes has been achieved by the special preparation of a textured substrate.6 In these cases the critical current densities approach those of single crystal films. However, it appears difficult to prepare such substrates with lengths appropriate for large-scale applications. J. Mater. Res., Vol. 10, No. 9, Sep 1995 http://journals.cambridge.org

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Critical current densities above 105 A/cm 2 at 77 K, zero field, were obtained in spray pyrolyzed Tl(1223) films formed on polycrystalline YSZ substrates.7-8 The observation of high critical current density in these thick films offered some optimism for large-scale applications using this chemical system. Although clearly