Buffer layers based on SOE tapes

An alternative approach for a coated conductor architecture based on RABiTS tapes is to start from a NiO coated Ni-alloy tape prepared by self oxidation epitaxy (SOE). The NiO layer is stable to over 1000°C in subsequent processing and virtually eliminates the problem of oxygen permeation through the buffer layer system to the Ni-alloy substrate. A subsequent second buffer layer on NiO is necessary due to the lattice misfit of 8.3% with YBCO as well as to prevent the poisoning of the superconducting layer with Ni.

Different perovskite buffers were successfully grown on microalloyed Ni tapes using pulsed laser deposition. Among them, BaZrO₃ and SrZrO₃ buffers show an excellent epitaxial growth on NiO with an in-plane orientation similar to the underlying NiO. The subsequent deposition of YBa₂Cu₃O_7-x on top of these buffers requires a thin intermediate SrTiO₃ layer and results in epitaxial layers with a T_c of about 89 K and J_c values up to 1.6 MA/cm². Alternatively, epitaxial SrRuO₃ was grown on NiO using a thin intermediate BaZrO₃ layer leading to YBCO films of similar quality. This buffer layer architecture was successfully applied on SOE-Ni substrates having an elongated grain structure. Transport measurements on the YBCO film in magnetic fields revealed an increase of the critical current density J_c in direction of the grain elongation reaching values of up to 2.4 MA/cm² at 77 K in self-field.

Fig. 1	Fig. 2	Fig. 3

Fig. 1: Standard buffer architecture of coated conductors based on SOE-NiO/Ni tapes

Fig. 2: Texture of the NiO/Ni-Ag substrate: Pole figures of (a) Ni-Ag substrate; (b) SOE-NiO layer grown on the Ni-Ag substrate; (c) EBSD image of the NiO layer (2x2 mm²), grain boundaries with a misorientation angle between 4° and 10° are marked black, whereas grain boundaries above 10° are marked white.

Fig. 3: Resistive J_c measurements on a YBCO film prepared on a Ni-Ag/NiO/BaZrO₃/SrRuO₃ buffer architecture with an elongated grain structure showing higher values along to the grain elongation compared to the perpendicular direction

recent publications

Hühne, R., Eickemeyer, J., Selbmann, D. Schultz, L., Holzapfel, B.
Improved critical current densities of coated conductors by high aspect ratio grains
IEEE Trans. Appl. Supercond. 17 (2007) 3239-3242

Hühne, R., Selbmann, D., Eickemeyer, J., Holzapfel, B.
Preparation of advanced buffer layer architectures for YBa₂Cu₃O_x coated conductors based on surface oxidized Ni tapes
Physica C 460-462 (2007) 1413-1414

Eickemeyer, J., Selbmann, D., Hühne, R., Wendrock, H., Hänisch, J., Güth, A., Schultz, L., Holzapfel, B.
Elongated grains in textured substrate tapes and their effect on transport currents in superconductor layers
Appl. Phys. Lett. 90 (2007) 012510

Hühne, R., Selbmann, D., Eickemeyer, J., Hänisch, J., Holzapfel, B.
Preparation of buffer layer architectures for YBa₂Cu₃O_7-x coated conductors based on surface oxidized Ni tapes
Supercond. Sci. Technol. 19 (2006) 169-174

Hühne, R., Holzapfel, B., Kursumovic, A., Evetts, J.E., Cavallaro, A., Sandiumenge, F., Pomar, A., Puig, T., Obradors, X.
Preparation of MZrO₃ (M=Ba,Sr) buffer layers on surface oxidized Ni/NiO templates by PLD and MOD
IEEE Trans. Appl. Supercond. 15 (2005) 3024-3027