イットリウム系酸化物超電導体のテープ線材化

抄録

YBa₂Cu₃O_7-x oxide superconductor has intrinsically strong pinning properties at 77K, which suggests its potential for applications at liquid nitrogen temperatures. Formerly, YBa₂Cu₃O_7-x was considered difficult for use in forming conductors because its severe intergranular weak links could not be easily removed due to difficulty in crystalline alignment control. In recent years several possibilities of using flexible coated conductor have been indicated with the effective removal of intergranular weak links to produce “biaxially aligned structures.” Four kinds of methods have been proposed to produce “biaxially aligned structures, ” such as ion-beam-assisted deposition (IBAD), modified bias sputtering, inclined pulsed laser deposition, and rolling assisted biaxially textured substrates (RABiTS). This article reviews the current status of the development of YBa₂Cu₃O_7-x oxide superconducting tapes produced by vapor phase deposition. Biaxially aligned YBa₂Cu₃O_7-x coated conductors have the following advantages: (a) high J_c values: stable J_c values over 10⁵A/cm² (77K, 0T) were achieved, and J_c values over 10⁶A/cm² (77K, 0T) were obtained in short samples; (b) the possibility of large-current conductors: I_c (77K, 0T) values over 100A were obtained in 2-μm-thick YBa₂Cu₃O_7-x films; (c) strong pinning properties at 77K; (d) excellent flexibility and mechanical durability; and (e) the low a.c. losses caused by thin-strip geometry and the low conductivity of substrates. YBa₂Cu₃O_7-x tape conductors are unexplored technologies which require long-length, flexible tapes with uniform biaxial texturing. It has been a great challenge to remove the intergranular weak links and derive the intrinsic performance of oxide superconductors, which deserves further persistent research & development.