Speaker
Dr
Anatolii Polyanskii
(FSU, NHMFL, ASC)
Description
Low angle grain boundaries (GB) are still the most important current-limiting mechanism operating in rare earth barium copper oxide (REBCO) coated conductors. While Ca-doping is found to improve the transparency of low angle GBs in weak fields at high temperatures, the growing interest of building high field magnets using REBCO coated conductors requires evaluating the effectiveness of Ca-doping more broadly. In this study, we used Magneto-Optical-Imaging (MOI) to quantitatively study the GB transparency (rGB=JcGB/Jcgrain) in several (Yb0.7Ca0.3)Ba2Cu3O7 PLD thin films grown on SrTiO3 6 and 9 degree bi-crystals in the broad temperature range 5 to 77 K. JcGB and Jcgrain and their ratio rGB in various doping states obtained by varying the post-growth annealing conditions were independently determined by analyzing the MO images. We found that post-growth annealing affects JcGB and Jcgrain differently. In 1 bar oxygen annealing, JcGB stabilizes faster than Jcgrain. On the other hand, JcGB is less affected by annealing in reduced oxygen atmosphere. As a result, higher rGB is achieved when the sample is oxygen-deficient. Another major finding is that rGB of Ca-doped and pure GBs have different temperature dependence. Starting from T=0.6Tc, rGB of Ca-doped GB increases much faster with temperature than that of pure GBs. We found that on the other hand, the benefit of Ca-doping is very limited at the low temperature end. This is speculated to be a result of non-uniform Ca-segregation at the GBs. Overall, our results suggest that Ca-doping is not very valuable for low temperature application of REBCO.
*This work was performed at the National High Magnetic Field Laboratory, which is supported by the National Science Foundation Cooperative Agreement No. DMR-1157490.*
Author
Dr
Anatolii Polyanskii
(FSU, NHMFL, ASC)
Co-authors
Prof.
Aleksander Gurevich
(Department of Physics, Old Dominion University)
Prof.
David Larbalestier
(ASC, NHMFL, FSU)
Dr
Dmytro Abraimov
(FSU, NHMFL, ASC)
Dr
Fumitake Kametani
(ASC, NHMFL, FSU)
Dr
Pei Li
(Fermi National Accelerator Lab)
