Dietmar Selbmann

Highly cube textured Ni–W-RABiTS tapes for YBCO coated conductors

Abstract The investigation of Ni–W alloys in the concentration range of solid solubility of tungsten in nickel proved the potential of this metallurgical system for rolling assisted biaxially textured substrate (RABiTS) tapes of high performance. The cube texture of Ni+5 at.%W was characterized by a maximum of the deviation frequency at about 4°. For this material the beneficial effect of the initial grain size on the final texture quality was shown. The YBCO (YBa 2 Cu 3 O 7− δ ) type coatings on CeO 2 buffer layers carried absolute currents up to I =130 A at 77 K. A critical current density of j c =1.2 MA/cm 2 was achieved at 77 K and zero field on tapes up to 20 cm long.

Elongated grains in textured substrate tapes and their effect on transport currents in superconductor layers

The development of cube textured flexible metallic substrate tapes is of basic interest for the economic production of YBa2Cu3Ox (YBCO) coated superconductors. Nickel tapes microalloyed with silver were prepared with the specific feature of an elongated grain structure. Cube textured NiO films were grown by surface oxidation epitaxy on these tapes retaining the elongated grains. Further, buffer layers and YBCO were deposited thereon. The YBCO layer transports an up to four times higher critical current density (Jc=2.35MA∕cm2) in the direction of grain elongation compared to the transversal one (Jc=0.55MA∕cm2).

Continuous YBCO deposition onto moved tapes in liquid single source MOCVD systems

Abstract In a small scale MOCVD system YBCO films with j c(77 K ) > 1 MA were deposited onto substrates, which were continuously moved through the deposition chamber. Two routes for the buffer layer deposition onto textured Ni tapes were chosen: a) CeO 2 , LaMnO 3 and LaCrO 3 films were deposited in hydrogen atmosphere by MOCVD. On LaAlO 3 and R-Al 2 O 3 crystals epitaxial films were obtained b) textured NiO layers were grown by oxidation of textured Ni tapes. Based on the small scale system an enlarged MOCVD reactor was constructed.

Reel-to-reel MOCVD for YBCO coated conductor

A reel-to reel MOCVD system has been developed for a fast and economic process to get high quality YBCO coated conductor tapes. Several oxides buffer layer sequences (based on YSZ, CeO/sub 2/, Y/sub 2/O/sub 3/...) were then grown epitaxially on two kinds of tape. Effective stacking for MOCVD reported here are YSZ/Y/sub 2/O/sub 3//YBCO and YSZ/CeO/sub 2//YBCO, respectively, on oxidized Ni RABiTS microalloys (0,1% W) and on SS/YSZ (IBAD) tapes. First, EBSD and AFM techniques have been used to characterize the substrates. Superconducting YBCO films were fully characterized by SEM, XRD and AC susceptibility measurements Epitaxial growth was depicted with a strong in-plane and out-of plane texture: FWHM of YBCO /spl phi/-scans were 8.5/spl deg/ on Ni tape and 9/spl deg/ on IBAD Hastelloy tapes. High performances were also measured by AC susceptibility, with J/sub c/ values of 0.6 MA/cm/sup 2/ at 77 K (0 T) on Ni tape and close to 1 MA/cm/sup 2/ on IBAD substrates, with a sharp transition (/spl Delta/T/sub c/ < 0.4 K) and a T/sub c/ onset at 90 K. Deposition of 2 meter buffered YSZ and CeO/sub 2//YBCO films have also been performed showing good in-plane alignment (/spl phi/-scan YSZ: FWHM = 11/spl deg/) with a tape velocity set at 4 m/h.

Effect of silver on cube texture formation in nickel substrate tapes

The effect of silver in pure nickel on the quality of the primary recrystallization cube texture was investigated after intense cold rolling and annealing. Texture sharpness and thermal stability in the nickel tapes could be improved by micro-alloying with silver in the range of 100–500 at. ppm Ag. The sharpest cube texture was observed for the Ni + 100 at. ppm Ag material after annealing at 550 °C (FWHM(111) = 4.4°). Further, the nickel oxide layer grown thereon had a sharp cube texture rotated by 45° relative to the metallic substrate. Grain boundaries showed a typical zigzag image and the grain form was fractal-like.

YBCO films on buffered Ni RABiT substrates by pulsed injection MOCVD

Abstract Biaxially textured buffer layers and YBa 2 Cu 3 O 7 (YBCO) films were grown on NiO/Ni based substrates by pulsed injection MOCVD. High quality biaxially textured NiO layers have been prepared following an optimised recrystallisation–oxidation process. The yttria stabilized zirconia (YSZ) and its combination with CeO 2 and/or Y 2 O 3 were investigated in the role of buffer layers for YBCO. Thin biaxially textured buffer layers were deposited in a temperature range from 600 to 800 °C. The best superconducting properties correspond to the Ni/NiO/YSZ/Y 2 O 3 /YBCO architecture: YBCO layers were grown epitaxially and exhibited a critical temperature T c ∼90 K ( ΔT c ⩽1 K) and critical current densities J c ⩾5×10 5 A/cm 2 at 77 K.

YBaCuO deposition by MOCVD on metallic substrates: a comparative study on buffer layers

YBaCuO films of about 1 /spl mu/m thickness have been deposited by metal organic chemical vapor deposition (MOCVD) on biaxially textured Ni-based substrates. Different buffer layers (MgO, YSZ, CeO/sub 2/), and also NiO epitaxially grown on Ni, have been tested with subsequent YBCO deposition in the same reactor. When using NiO as first buffer layer, Ni-based substrates were oxidized in a rapid thermal processing system or in a conventional furnace previous to deposition of the second buffer layer. In the present work, we study different conductor architectures from the point of view of T/sub c/, J/sub c/ (77K), epitaxial growth, and chemical stability of the structure.

Improved Critical Current Densities of Coated Conductors by High Aspect Ratio Grains

The critical current density Jc is limited by the network of small angle grain boundaries in coated conductors based on biaxially textured metal substrates up to a texture dependent cross-over field. Below this cross-over field Jc can be improved by optimizing the grain boundary network. One possibility to enhance Jc at low magnetic fields is to use substrates with high aspect ratio grains. Therefore, cube textured Ni tapes micro-alloyed with silver were prepared having such an elongated grain structure with an aspect ratio of about 4. Highly textured NiO layers have been grown on these substrates using surface oxidation epitaxy (SOE). YBCO films were deposited afterwards on these substrates by pulsed laser deposition using a BaZrO3/SrRuO3 buffer layer architecture. Transport measurements revealed a significant enhanced Jc value along the rolling direction of the tape compared to the transverse direction. Similar results were obtained in numerical simulations based on measured EBSD texture maps.