All chemical YBa 2 Cu 3 O 7 superconducting multilayers: Critical role of CeO 2 cap layer flatness
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R. Hu¨hne and B. Holzapfel Leibniz Institute for Solid State and Materials Research Dresden (IFW Dresden), Institute for Metallic Materials, 01171 Dresden, Germany
Y. Morilla and J. Garcı´a-Lo´pez Centro Nacional de Aceleradores, E-41092 Sevilla, Spain
A. Pomar, F. Sandiumenge, T. Puig, and X. Obradorsb) Institut de Ciencia de Materials de Barcelona–Consejo Superior de Investigaciones Cientificas (ICMAB/CSIC) Campus de la UAB, Barcelona 08193, Spain (Received 30 June 2008; accepted 8 December 2008)
New advances toward microstructural improvement of epitaxial CeO2 films grown by chemical solution deposition and their use as buffer layers for YBa2Cu3O7 (YBCO) films are presented. We demonstrate that the degree of epitaxy and the fraction of (001) atomically flat surface area are controlled by the incorporation of tetravalent (Zr4+) or trivalent (Gd3+) cations into the ceria lattice. The degree of epitaxy has been investigated by means of Rutherford backscattering spectroscopy-channeling and reflection highenergy electron diffraction. In addition, we use a new methodology to quantify the fraction of (001) atomically flat area from atomic force microscopy images. Results are further correlated with the superconducting properties, microstructure, and texture of YBCO films grown by the trifluoroacetate route. A comparison with pulsed laser deposition and YBCO films grown on the same ceria layers is also presented. This growth procedure has allowed us to obtain all chemical multilayer films with controlled microstructure and critical current densities above 4 MA cm2 at 77 K.
I. INTRODUCTION
The production of affordable YBa2Cu3O7 (YBCO) coated conductors (CC) is an extremely challenging issue for electric power applications.1,2 Great effort is being devoted to finding simple multilayer architectures, which simultaneously satisfy an effective oxidation protection for the metallic substrate, an effective chemical barrier to cation diffusion, and a good lattice matching with a substrate and superconducting layer. Physical vapor deposition (PVD) techniques can certainly produce high-quality YBCO CC using numerous oxide buffer architectures where Jc values of several MA cm2 at 77 K are routinely achieved.3–5 However, the major goal now for CC technology to be competitive is to demonstrate the suitability of low-cost deposition techniques, such as chemical solution deposition (CSD) in which YBCO is grown by the trifluoroacetate route (TFA)6,7 Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/JMR.2009.0160 1446
J. Mater. Res., Vol. 24, No. 4, Apr 2009
and buffer layers by metalorganic decomposition (MOD).8–10 MOD- SrTiO3 and MOD- CeO2 have capitalized the attention of the researchers for its high potential as cap layers. Indeed, attractive Jc values [1 MA/cm2 (77 K)] have been obtained using MOD(Nb-doped) SrTiO3 as single buffer layer on Ni-rollingassisted biaxially textured substrates (RABiTs),11 MODCeO2 on MOD-La2Zr2O7 buffered Ni-RABITs,12 or MOD-CeO2 on
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