THE PUZZLE OF TWO DIFFERENT SUB-MICROMETER TUNGSTEN-RICH DEPOSITS IN BULK YBCO: ONE ACTS AS PINNING CENTERS AND THE OTHER DOES NOT

Ravi-Persad Sawh, Roy Weinstein, Drew Parks, and Alberto Gandini

Beam Particle Dynamics Laboratory, Department of Physics, and Texas Center for Superconductivity, University of Houston
Houston, Texas, 77204, USA

ABSTRACT: Two types of large grain YBCO samples doped with tungsten oxide, one with platinum and the other without, were produced using a slow cooling process.  Observations of the trapped magnetic flux density showed that the flux density of the W-doped, Pt-free samples did not change with W doping levels of up to 2.1 mol%.  In contrast, the (W + Pt)-doped samples resulted in a monotonic improvement in trapped magnetic flux density as a function of W doping.  Microstructure studies indicate that both types of samples contain profuse sub-micrometer deposits of a W-rich compound.  The Pt-free samples contain (W_0.4Y_0.6)BaO₃ deposits while the (W + Pt)-doped samples contain deposits of a (W_0.5Pt_0.5)YBa₂O₆ compound.  Both types of deposits are of essentially the same size and have comparable number density.  The results are strikingly similar to an earlier experiment in which uranium doped, Pt-free, large grain YBCO also did not show any improvement in trapped magnetic flux density.  The U-doped, Pt-free samples contain profuse sub-micrometer deposits of a (U_0.4Y_0.6)BaO₃ compound, which have been shown to be ferromagnetic.  The inability of both the (W_0.4Y_0.6)BaO₃ and (U_0.4Y_0.6)BaO₃ sub-micrometer deposits to act as pinning centers in self-field, suggest that this behavior is systematic.

KEYWORDS:  Tungsten doping.  Profuse sub-micrometer deposits.  Ineffective pinning centers.  Ferromagnetic non-pinning centers.  Melt-textured YBCO high temperature superconductor.

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IEEE/CSC & ESAS EUROPEAN SUPERCONDUCTIVITY NEWS FORUM (ESNF), No. 10, October 2009On June 29, 2009, this manuscript was submitted for possible publication in “Advances of Cryogenic Engineering (Materials)”