Solution Synthesis of Epitaxial Rare-Earth Oxide Thin Films on Roll-Textured Nickel
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ABSTRACT Using solution chemistry, epitaxial films of rare-earth oxides of the general formula RE 2 0 3 (where RE = Sm to Lu) were prepared on cubic-textured nickel tapes. Solutions of metal methoxyethoxides or metal acetate/methoxyethoxides in 2-methoxyethanol were used to coat rolltextured nickel tapes using either spin-coating or dip-coating. Coated tapes were subsequently heated in a reducing atmosphere at temperatures between 950 'C and 1160 'C for varying lengths of time. Film quality was determined using X-ray diffraction and electron and surface probe microscopy. Films were found to be oriented both in and out-of-plane of the substrate, free of pinholes and gross defects, and of sufficient quality for use as substrates for high T, superconductors. INTRODUCTION A "coated conductor" structure consisting of a polycrystalline copper oxide-based superconductor deposited on a mechanically strong metal tape is predicted to be the next generation of superconducting wire operating at liquid nitrogen temperature.' High critical currents can only
be achieved in copper oxide-based superconducting films if there is both good out-of-plane and inplane grain alignment. For out-of-plane alignment, c-axis texture, the copper oxide planes must lie parallel to the substrate. This c-axis texture is often achieved even on amorphous or glassy substrates due to the layered structure of the cuprate superconductors, but in-plane alignment is much more difficult to achieve. Two distinctly different approaches to inexpensive substrates are being pursued in order to achieve this alignment in practical coated conductors. One approach is a technique known as Ion Beam Assisted Deposition (IBAD), which involves the deposition of a textured buffer layer on a randomly oriented, oxidation resistant metal tape using multiple ion beams.2 The second approach to epitaxial substrates is a technique known as Rolling Assisted Biaxially Textured Substrates (RABiTS). The RABiTS process involves the deposition of epitaxial buffer layer(s) on a nickel tape which has been mechanically deformed by rolling and heat treated to obtain a highly textured metastable microstructure.3 Laser ablation was used to produce the first successful RABiTS buffer layer. An epitaxial layer of palladium on roll textured nickel was deposited by laser ablation followed by a layer of cerium oxide and then a layer of yttrium stablized zirconium (YSZ).4 Later, the use of the palladium layer was discontinued from the RABiTS architectures.5 In addition to the use of pulsed laser ablation, e-beam evaporation and sputtering have been used to deposit ceramic layers on the RABiTS to achieve high critical currents.6 The best results to date for a RABiTS process were obtained with the use of a cerium oxide layer (>500 A) in contact 203 Mat. Res. Soc. Symp. Proc. Vol. 619 ©2000 Materials Research Society
with the nickel, a yttrium stabilized zirconium (YSZ) oxide layer (I gim) on top of the cerium oxide layer, and a cap layer of cerium oxide (>500 A) on top of the YSZ. Recently, high c
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