From meandering to straight grain boundaries: Improving the structures of artificially induced grain boundaries in super
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From meandering to straight grain boundaries: Improving the structures of artificially induced grain boundaries in superconducting YBa2 Cu3 Oy bicrystals Xiao-Feng Zhanga) , Volk R. Todt, and Dean J. Miller Materials Science Division and Science and Technology Center for Superconductivity, Argonne National Laboratory, Argonne, Illinois 60439 (Received 10 March 1997; accepted 2 July 1997)
This paper presents several key aspects of our approach to preparing artificially induced [001] tilt grain boundaries (GB’s) with uniform, well-defined structures in YBa2 Cu3 Oy (YBCO) superconductors. GB structures formed in thin film and bulk bicrystals, respectively, will be compared. In YBCO thin film bicrystals, meandering rather than planar GB’s are formed. Using a low film deposition rate has been demonstrated to reduce the magnitude of meander significantly, but complete elimination of the meander has not yet been accomplished. Thus, we have developed a dual-seeded-melt-texture process to produce uniform, planar GB’s with controllable misorientation angles in YBCO bulk bicrystals. Transmission electron microscopy (TEM) studies reveal a remarkably planar and simple configuration on different length scales. Such a simple structure allows for an insightful interpretation of transport behavior across individual GB’s.
I. INTRODUCTION
The fabrication of high transition temperature (Tc ) superconducting GB’s with controlled microstructures is an important step in developing an improved understanding of GB transport properties. Various techniques for fabricating such GB’s have been developed. One of the more common methods is to deposit epitaxial superconducting thin films on suitable bicrystal substrates. As an example, the geometry of a [001] tilt bicrystal is shown schematically in Fig. 1. Epitaxial growth of the thin film on each half of such bicrystals yields an artificially induced GB. Chaudhari et al. demonstrated that the critical current density (Jc ) across artificial GB’s formed in YBCO thin film bicrystals is always significantly less than that of either adjacent grain.1 Dimos et al. and Ivanov et al. further demonstrated an inverse relationship between the Jc across the GB and the misorientation angle (u) of the YBCO film bicrystals.2,3 It has generally been assumed that the GB in the film will follow the straight GB’s in underlying substrate bicrystals, and in these previous studies, a planar GB configuration with well-defined orientations was assumed and used as a reference for orientation when electric current and/or magnetic fields were applied for characterization of transport properties. However, recent TEM studies revealed a much more complicated GB configuration a)
Current address: Materials Science and Technology Division, Mail Stop K765, Los Alamos, New Mexico 87545. J. Mater. Res., Vol. 12, No. 11, Nov 1997
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formed in YBCO thin film bicrystals. The GB planes are meandering rather than planar.4–10 The devi
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