Study Of Grain Boundary Transparency In (Yb1-Xcax)Ba2cu3o Bicrystal Thin Films Over A Wide Temperature, Field, And Field Orientation Range

The residual low-angle grain boundary (GB) network is still the most important current-limiting mechanism operating in biaxially textured rare-earth barium-copper-oxide (REBCO) coated conductors. While Ca doping is well established to improve supercurrent flow across low-angle GBs in weak fields at high temperatures, Ca doping also depresses T-c, making it so far impractical for high-temperature applications of REBCO coated conductors. On the other hand, high-field-magnet applications of REBCO require low temperatures. Here we systematically evaluate the effectiveness of Ca doping in improving the GB transparency, r(GB) = J(c)(GB)/J(c)(grain), of low-angle Yb1-xCaxBaCuO [001] tilt bicrystal films down to 10 K and with magnetic fields perpendicular and parallel to the film surfaces, while varying the Ca and oxygen doping level. Using low-temperature scanning laser microscopy and magneto-optical imaging, we found r(GB) to strongly depend on the angle between magnetic field and the GB plane and clearly identified regimes in which J(c)(GB) can exceed J(c)(grain) (r(GB) > 1) where the GB pinning is optimized by the field being parallel to the GB dislocations. However, even in this favorable situation, we found that r(GB) became much smaller at lower temperatures. Calculations of the GB Ca segregation profile predict that the high-J(c) channels between the GB dislocation cores are almost Ca free. It may be therefore that the positive effects of Ca doping seen by many authors near T-c are partly a consequence of the higher T-c of these Ca-free channels.