A basic study on high-temperature superconductor cables available in liquid nitrogen

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A lot of high-temperature superconductors have layer structure and their critical current density is depressed under magnetic fields applied parallel to crystallographic c-axis. This anisotropy is a crucial factor for designing superconducting cables operating in magnetic fields, and limits the range of the practical use of high-temperature superconductors. In this study, superconducting characters of Yba_2Cu_3O_<7δ>(YBCO) have been investigated from the practical viewpoints of electric power cables of YBCO. YBCO films used in this study have been prepared by sputtering. Optimum deposition conditions for producing YBCO films on MgO (100) have been made clear. The growth of YBCO phase is dominated by deposition temperature, one of deposition conditions, and c-axis oriented YBCO films indicative of high critical temperature T_c and high critical current density J_c grow around 900℃. 45°-rotated YBCO grains appear in films with increasing deposition temperature above 900℃ and J_c of films is appreciably reduced. Substrate material and surface treatments of substrates give effects the growth of YBCO grains. On SrTiO_3 (100) substrates and MgO(100) substrates surface-treated by acid etching, well-regularized spiral growth of YBCO is seen. These films show high-J_c values in excess of 3×10^6 A/cm^2.In conclusions, c-axis oriented films with highly bi-axially aligned structure provides high-J_c character. Precise control of deposition temperature, lattice-matching substrates with YBCO and surface treatments of substrates are required in preparing high-J_c YBCO films.