The critical current density of yttrium barium copper oxide coated conductors

The critical current density Jc of YBa 2Cu3O7-x (YBCO) coated conductors is determined by the connectivity of the polycrystalline grain network and by vortex pinning. The aim of this work is to explore these two key scientific issues. Current obstruction effects of the grain boundary network were first studied by measuring variable width tracks in variously-textured ex situ coated conductors. We found that the global texture exercises a significant effect on Jc especially in low fields. We then grew low angle [001]-tilt bicrystal YBCO films with controlled doping so as to study the influence that variable size and charge segregants would have on the inter-grain Jc. The beneficial effects of Ca doping was already proven, but here we were able to show that the optimum doping of low angle grain boundaries was obtained with the smaller substitutions of 15% Ca for Y, rather than the 30% previously employed. We then studied Ca-doping in the small rare-earth (RE) ion Yb variant of the RE-123 structure, YbBa2Cu3O7-x, and the influence of substitutions for Y of the large RE ion Nd in YBa2Cu3O7-x. We found that strain- and charge-driven segregation to the grain boundary was consistent with the segregation model of Gurevich. Very interestingly, the 6° Nd-doped YBCO grain boundaries exhibited no degradation of intergrain Jc compared to the intragrain Jc without significant Tc reduction. Then, the vortex pinning was studied by sequential ion milling of YBCO films with various vortex pinning microstructures. YBCO films without strong pins exhibited two-dimensional collective pinning behavior and thermal fluctuation depinning effect, and thus Jc fell off fast with increasing thickness, magnetic field and temperature. In contrast, a YBCO film with dense, insulating, nanoscale pins exhibited strong three-dimensional pinning behavior and a high and uniform Jc through thickness. Analysis of the through-thickness properties of ex situ high Jc coated conductors showed them to be well into the three-dimensional strong pinning limit but to show some materials degradation effects in thicker films, due to artifacts of the ex situ conversion process. Collectively, this study clearly shows the route to high critical current density YBCO films.