Observation of the three-dimensional distribution of flux pinning centers in Dy-doped YBa_(2)Cu_(3)O_(7-x) coated conductors

摘要

The spatial distribution of flux pinning centers (such as defects, secondary phases, etc.) has a critical effect on the superconducting properties of YBa_(2)Cu_(3)O_(7-x) (YBCO) coated conductors. To increase the flux pinning in coated conductors, impurities such as Dy can be intentionally added during processing to induce the formation of distributed insulating nanostructures. In this work, we show that the addition of Dy to a particular YBCO coated conductor gives rise to a high density of secondary nanoparticles of composition (Y_(s)Dy_(1-s))_(2)Cu_(2)O_(5) with s approx 0.6. Using high-angle annular dark-field tomographic methods, the three-dimensional distribution of these insulating nanoparticles is determined. The size of the nanoparticles and the uniformity of their distribution are found to depend on their interaction with the grain boundary network within the YBCO layer. A bimodal distribution in the size of the particles is seen, where areas away from grain boundaries have a favorable uniform distribution of small particles (approx25 nm) while unfavorable isolated large particles 100-140 nm are seen to intersect the grain boundaries. A grain boundary diffusion model is proposed to account for the agglomeration of the small nanoparticles to form the large ones to the detriment of flux pinning within these local regions of the superconductor.