Evidence for extensive grain boundary meander and overgrowth of substrate grain boundaries in high critical current dens
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Holesinger Los Alamos National Laboratory, Los Alamos, New Mexico 87545
R. Feenstra, A.A. Gapud, and E.D. Specht Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (Received 17 January 2005; accepted 19 April 2005)
It has been generally accepted that YBa2Cu3O7−x (YBCO) films deposited on deformation textured polycrystalline metal tapes result in YBCO grain boundary (GB) networks that essentially replicate the GBs of the underlying substrate. Here we report that for thicker YBCO films produced by a BaF2 ex situ process, this is not true. Using electron backscatter diffraction combined with ion milling, we have been able to map the evolution of the YBCO grain structure and compare it to the underlying template in several coated conductors. For thin (艋0.5 m) YBCO films deposited on rolling-assisted biaxially textured substrates (RABiTS), the YBCO GBs nearly directly overlap the substrate GBs. For 0.7–1.4 m YBCO films, the GBs were found to meander along the substrate GBs and along the sample normal, with displacements several times the film thickness. In very thick films (2.5–2.9 m), the YBCO grains can completely overgrow substrate grains and GBs, resulting in a substantial disconnection of the YBCO and substrate GB networks. Similar behavior is found for BaF2 ex situ YBCO films on ion-beam-assisted deposition-type templates. The ability of the YBCO to overgrow substrate grains and GBs is believed to be due to liquid-phase mediated laminar grain growth. Although the behavior of the YBCO GB networks changes with YBCO film thickness, the samples maintained high critical current density (Jc) values of >2 MA/cm2 for films up to 1.4 m thick, and up to 0.9 MA/cm2 for 2.5–2.9-m-thick films. I. INTRODUCTION
One approach to the development of practical high temperature superconducting (HTS) wires is to deposit epitaxial YBa2Cu3O7−x (YBCO) on oxide buffered flexible metal tapes.1–4 Cost-effective commercial production of these coated conductors (CCs) will require large critical currents (Ic) and fast YBCO growth processes. The latter requirement may be met by the ex situ conversion of YBCO precursors,5–9 metalorganic chemical vapor deposition,10,11 or pulsed laser deposition methods.12 A recent electromechanical study of CCs suggests that YBCO layers up to 3 m thick meet the strain requirements of most applications.13 The strong dependence of the critical current density (Jc) of YBCO with grain boundary (GB) misorientation angle14–17 also requires CC templates with excellent biaxial texture. The a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0262 2012
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J. Mater. Res., Vol. 20, No. 8, Aug 2005 Downloaded: 15 Mar 2015
long, flexible, metal-based templates used to support the epitaxial oxide growth of the buffer and superconducting films are typically based either on ion-beamassisted deposition (IBAD) of a biaxially oriented oxide layer on a randomly oriented polycrystalline metal strip,2,12,18,19 or a biaxially textured metal strip
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