Calcium Doping YBCO to Increase Tc

       Superconductors.ORG herein reports the discovery of a second calcium-doped YBCO variant with critical transition temperature (Tc) higher than pristine YBCO. A resistive transition temperature near 99 K has been observed in Y₃CaBa₄Cu₈O₁₈₊. The structure type is 1223C with planar weight disparity between alternating (Y, Ca)Cu0₂ planes. (see graphic below left)

It's been known for years that adding calcium (Ca) to YBCO can increase its current-carrying capability across grain boundaries (GB) by as much as 35% [1]. And it seems to shield from "the effects of magnetic fields on transportation properties" [2]. Unfortunately adding Ca to YBCO also typically depresses the critical superconducting temperature.

       This problem was tackled in November 2006 when Superconductors.ORG announced the discovery of the first calcium-doped YBCO variant with critical transition temperature higher than pristine YBCO. That compound, (Y₂Ca)Ba₄Cu₇O₁₆₊ , produced a Tc near 96K (see plot above right).

       The newest YBCO variant, Y₃CaBa₄Cu₈O₁₈₊ , has an even higher transition temperature due to a larger unit cell with 16 layers. The Ca-doping level has also been reduced from 33% to 25% by the addition of another YCuO₂ layer.

       The "jitter" just to the right of Tc in the plot at page top is likely a result of minority phases. However, the two additional phases most likely to result both have Tc's near 96K. Y₃Ba₄Cu₇O₁₆ and (Y₂Ca)Ba₄Cu₇O₁₆ both have the 1223C/1212C structure (shown above). This insures that the volume fraction is high for superconducting phases above 96K.

       The value of this discovery lies in giving YBCO manufacturers a means of maximizing trans-GB current transport properties while simultaneously facilitating a Tc increase.

       Synthesis was by the solid state reaction method. Stoichiometric amounts of the below precursors were mixed and pressed at 70,000 psi and sintered for one hour at 775C; then for another 11 hours at 890C. Lastly, the pellet was annealed for 10 hours at 500C in flowing O2.

Y₂O₃ 99.99%   (Alfa Aesar)   3.39 moles (gr.)
BaCuO_x 99.9%   (Alfa Aesar)   8.67 moles
CaCO₃ 99.95%   (Alfa Aesar)   1.00 mole
CuO 99.995%   (Alfa Aesar)   3.18 moles


- E. Joe Eck
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[1] Yimei Zhu and Marvin Schofield, Brookhaven National Laboratory News, June 7, 2004
[2] K.X. Xu and Z.Y. feng, Shanghai University, October 7, 2004 ASC

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