Development of Rare Earth Niobate Buffer Layer for YBCO Coated Conductor Using Chemical Solution Deposition Approach
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Development of Rare Earth Niobate Buffer Layer for YBCO Coated Conductor Using Chemical Solution Deposition Approach M.S. Bhuiyan, M. Paranthaman, S. Sathyamurthy, and D.B. Beach Chemical Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6376, U.S.A. ABSTRACT We have grown epitaxial rare earth niobate (RE3NbO7; RE – La, Ce, Nd, Sm, Eu, Gd, Ho, Y, and Yb) buffer layers on biaxially textured Ni-W substrates using chemical solution deposition approach. Precursor solutions of 0.4 M total cation concentration were spin coated on short samples of Ni-3 at.%W (Ni-W) substrates and heat-treated at 1050 to 1100oC in a gas mixture of Ar-4%H2 for 15 minutes. Effect of solution chemistry and processing atmosphere on texture and microstructure were studied. Detailed X-Ray studies indicated that all the rare earth niobates were grown epitaxially. There was no significant change observed in texture for different precursor chemistry, however, dramatic effect on surface morphology observed. SEM and AFM investigations of RE3NbO7 films reveal noticeable difference in surface morphology and roughness for various processing atmospheres. Processing under a 20% wet Ar-4% H2 gas mixture was found to be the optimum condition for growing Gd3NbO7 films with high quality microstructure. INTRODUCTION YBa2Cu3O7-δ (YBCO) based coated conductors have spurred widespread interest with nearterm applications in prototype high-efficiency power transmission, power storage, transformers, high field magnets, and motor systems. Implementation of these high current and/or high magnetic field applications requires a cost-effective route to a high critical current density (Jc) superconducting wire structure, such as c-axis oriented YBCO thin films on inexpensive base metal or alloy tape substrates. However, material processing issues such as stoichiometry, out of plane and grain-to-grain orientation is critical to obtain YBCO film with Jc (77 K) > 1 MA/cm2 [1–3]. Deposition of YBCO on oxide-buffered, rolling-assisted biaxially (in-plane) textured (RABiT) Ni substrates is one promising approach to these issues [4-8]. Normally, high- Jc YBCO cannot be directly deposited onto RABiT (100) Ni tapes due to Ni oxidation and Ni poisoning issues [9]. As a result, a buffer layer of an intermediate material is generally required to preserve the biaxial texture of the cube- oriented Ni for subsequent deposition of YBCO and to prevent oxidation and diffusion of nickel during the YBCO growth. Many potential buffer layer materials have been identified, including CeO2, YSZ, Y2O3, Gd2O3, La2 Zr2O7, SrTiO3 and Gd3NbO7 for YBCO-coated conductors [10–16]. Considerations for candidate buffer layers include lattice matching and chemical compatibility with Ni and YBCO, development of biaxial orientation, obtaining low oxygen diffusivity, and demonstrating commercial viability (low cost and high process speed). RE3NbO7 has a pyrochlore structure with lattice parameter ranging from 10.7204 Å to 10.3894 Å for La to Yb niobates. These niobates appears to displ
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