Coated conductors (CCs) become highly resistive when quenching occurs. Thus, to manufacture reliable and safe wires for high-temperature superconducting applications, it is necessary to attach conducting (metal) layers with very low resistivity to stabilize and protect the wires from damage due to quenching. Presently, insulative oxides are used for buffer layers in commercially available CCs. Thick Ag and Cu layers are required to be deposited as the stabilizer layers on the superconducting REBa2Cu3O7 (REBCO) layer. However, high costs associated with Ag and the process itself are the major obstacles for achieving low-cost CCs. Use of a conductive buffer layer instead of the insulative oxides, combined with cube-textured pure Cu tape, can overcome the necessity of the expensive Ag stabilizing layer.
Several attempts using conductive buffers combined with {100}<001> textured pure Cu tape have been made toward developing new architectures. Although a number of oxides, metals, nitrides, and their combinations as the conductive buffer layers have been explored for developing low-cost CC architectures, thus far none have succeeded in fabricating a CC in which the substrate Cu tape functioned as the stabilizer layer as well as the template for biaxial crystal alignment of the REBCO.
Buffer layers must exhibit at least five properties simultaneously: (1) chemical inertness to both REBCO and Cu, (2) a low diffusion coefficient for metals in the layers, (3) a low diffusion coefficient of oxygen in the layers, (4) similar lattice constants for both REBCO and Cu, and (5) low electrical resistivity. It was not easy to find a material or a combination of a few materials fulfilling the above five requirements. Due to the ease of oxidation of Cu metal tape, a very low oxygen diffusion coefficient is required for the conductive buffer material to prevent generation of insulative CuO and/or Cu2O at the Cu/buffer interface to ensure good electrical coupling between REBCO and Cu tape.
We have reported on the advanced architecture for an electrical self-stabilized coated conductor composed of YBCO, Sr(Ti0.95Nb0.05)O3, Ni, and Cu tape laminated with stainless steel tape (SUS316).1, 2) Electrical resistivity of the Sr(Ti0.95Nb0.05)O3 thin film was quite low (2.5 × 10-3 Ω·cm at 77 K) before the deposition of the YBCO layer, and the Jc of the YBCO layer was 2.6 × 106 A/cm2 at 77 K under a magnetic self-field for the YBCO/Sr(Ti0.95Nb0.05)O3/Ni/Cu/SUS316 tape.1) We demonstrated that the {100}<001> textured pure Cu tape worked not only as the template for YBCO biaxial crystal alignment but also as the stabilizer layer for the YBCO/ Sr(Ti0.85Nb0.15)O3/Ni/Cu/SUS316 tape.3) We confirmed that some current flowed into the Cu tape from the YBCO layer through the conductive Sr(Ti0.85Nb0.15)O3 and Ni buffer layers in I > Ic regions. However, the resistivity of the Sr(Ti0.85Nb0.15)O3 layer was a few Ω·cm after YBCO deposition and oxygen annealing. The increase in resistivity of the Sr(Ti0.85Nb0.15)O3 layer suggested that oxygen diffused into the Sr(Ti0.85Nb0.15)O3 layer through the YBCO layer from the atmosphere during the YBCO deposition and oxygen annealing processes. The lower resistivity of the conductive buffer layer provides a shorter current transfer length. In this paper, we report a new conductive oxide buffer layer
A SUS316 tape was bonded to a {100}<001> textured Cu tape to strengthen the very soft annealed pure Cu tape, and a 0.5 - 2 μm thick Ni layer was electro-deposited on the surface of the laminated tape to provide Ni/Cu/SUS316 tape.4) The crystal orientation of the Ni layer in the Ni/Cu/SUS316 tape was 5.0 to 5.5° (full width at half maximum (FWHM) value in the X-ray (111) φ-scan measurement). La-doped SrTiO3 (La-STO) thin films were deposited in a 3% H2/97% Ar atmosphere by a pulsed laser deposition (PLD) method using sintered bulk (Sr1-XLaX)TiO3 as the targets. For deposition of La-STO, the chamber was maintained at 2 × 10-3 Pa pressure. A YBCO layer was then deposited by the PLD method in an oxygen atmosphere of 35 Pa at 740 - 760 °C, and finally annealed under oxygen flowing at 450 °C for 16 h.
We confirmed from X-ray diffraction measurements and scanning transmission electron microscope observations that the Ni, La-STO, and YBCO layers had excellent biaxial crystal orientations with the orientation relationship of (001)Cu ||(001)Ni || (001)La-STO || (001)YBCO and [100]Cu || [100]Ni || [100] La-STO || [100] YBCO. No NiO or Cu oxides were observed at the interfaces of YBCO/La-STO, La-STO/Ni, or Ni/Cu. Transport measurements for the YBCO/La-STO/Ni/Cu/SUS316 specimen were performed in liquid nitrogen in the absence of an external magnetic field. The Jc of the YBCO layer was 8.2 × 105 A/cm2, and the resistivity of the La-STO layer was 0.13 Ω·cm, which gave the resistance of 1.3 × 10-5 Ω between the YBCO and Ni layers per 1 cm2.
A part of this work was supported by JSPS KAKENHI Grant Number 21H01369 and JST ALCA Grant Number JPMJAL1109, Japan.
[References]
1) T. Doi et al., Materials Trans. 58(2017)1493.
2) A. Ichinose et al., Jpn. J. Appl. Phys. 56(2017)103101.
3) T. Doi et al., Appl. Phys. Espress 12(2019)023010.
4) N. Kashima et al., Jpn. J. Appl. Phys. 50(2011)063101.
Keywords: REBCO, coated conductor, textured Cu tape, conductive buffer layer