Influence of variable Ca-doping on the critical current density of low-angle grain boundaries in YBa₂Cu₃O_7-d

The rapid decrease in the critical current density Jc with grain boundary (GB) misorientation angle θ strongly limits the current-carrying capability of yttrium-barium-copper-oxide (YBCO), and the residual low-angle GB distribution is the most important current-limiting mechanism in biaxially textured YBCO coated conductors. To deepen the understanding of the Ca doping in low-angle GBs in YBCO, transport characteristics of low-angle GBs in YBa2Cu3O7-d bicrystals with different Ca contents x (Y1-xCaxBa2Cu3O7-d) were examined to verify the influence of the carrier density at the GB due to the Ca doping level. Y1-xCaxBa2Cu3O7-d (x = 0, 0.10, 0.15, and 0.30) epitaxial films were deposited by pulsed laser deposition on single crystal SrTiO3 (STO) substrates and 7° and 9° symmetric [001]-tilt bicrystal STO substrates. We found that 15% Ca doping was the most effective at increasing the Jc behavior across the 7° GB in intermediate fields, completely eliminating the GB dissipation signature in the V-I characteristics. For the 9° GB, 10% Ca doping yielded the highest depairing current Jd across the GB, calculated from the flux flow resistivity, even though a significant GB dissipation signature remained in the V-I curves.