Suppression of grain boundary weak link by Ca doping in YBa2Cu3O7 coated conductor

YBa2Cu3O7 coated conductors are a strategic material for superconducting applications such as high field magnets, fusion, and motors. Grain boundaries reduce the critical current density (J(c)) even at a tilt angle as low as 5 degrees, but the successful development of the highly oriented substrates seemed to overcome the weak link problem at grain boundaries. However, it reappears when we visit the homogeneity of the coated conductors. To suppress the weak link in the coated conductors, the Ca doping was investigated. The Ca-doped YBa2Cu3O7 films were fabricated on the moderately oriented substrates. While the grain boundaries in the moderately oriented substrates significantly degraded the J(c) without Ca doping, the Ca doping improved the J(c) especially at low temperature. This indicates that the tilt angle dependence of J(c) was varied by the Ca doping. While the J(c) for the moderately oriented substrate was 20 times smaller than that for the highly oriented substrate, the Ca doping restored 1/2 of the J(c) for the highly oriented substrate at 40 K and 9 T. The vortex structure changed from Abrikosov Josephson vortices to the Abrikosov vortices with increasing the Ca content. The combination of Ca doping and moderate substrate texture is another design of coated conductors. The Ca doping can patch the local degradation of the substrate texture to mass produce the practical coated conductors with improved homogeneity.