Development of (Re)BaCuO Coated Conductors by Liquid Phase Epitaxy

Abstract

Since the discovery of high-temperature superconductors, there has been a worldwide effort towards the development of processes for fabricating coated conductors for power applications. Most of these processes are based on vapour phase deposition techniques that have relatively low growth rates. A high-rate processing route was proposed based on the observation of high growth rate of (RE)Ba2Cu3O7- δ superconducting compounds (RE = rareearth element) from a flux supersaturated with one or more RE elements by liquid phase epitaxy (LPE). LPE has been successfully used to grow YBCO thick films with both c- and a,b-orientations on (110) NdGaO3 substrates and pure c-oriented films on YBCO seeded (100) MgO and (100) SrTiO3 under carefully controlled growth temperature and undercooling. The film growth mode (c- or a,b-oriented) is determined by the growth rate, which is directly related to the level of RE supersaturation that could be controlled by the undercooling used along with the amount of total RE solubility in the solution. The LPE grown films were highly epitaxial and biaxially aligned with good in-plane and out-of-plane textures. YBCO thick films grown on NdGaO3 by LPE showed high Tc of ~92 K and zero-field Jc at 77 K of 2.5×105 A/cm2. The initial growth of YBCO was found to be a multi-nucleation process. However, above a critical film thickness, dislocations started to form as a lattice-misfit stress relieving mechanism that led to step formation and spiral growth around dislocation cores. The growth kinetics from an unstirred solution was found to obey a $t$ law, whereas the growth rate from a stirred solution (substrate rotation) was found to have two growth stages with initialtransient and steady-state regimes. The transient regime extended to ~180 s with $t growth kinetics. In the steady-state regime, diffusion across an established diffusion boundary layer led to a linear increase of film thickness with time. Detailed study of the film growth confirmed the presence of interface kinetics that limited the growth on a competitive basis with volume diffusion in the solution. Furthermore, film growth on seeded substrates was found to be dependent on the stability of the seed layers where the mechanism of the dissolution of seed films was investigated. The major obstacle towards continuous deposition of coated conductor in long lengths by LPE is the lack of non-vacuum techniques capable of producing long lengths of suitably buffered substrate or a closely-matched substrate where YBCO can be deposited directly. However, despite the lack of suitably buffered and/or seeded substrates in long lengths, continuous metallic tape processing had been tested at a preliminary level showing scalability of the process.