Defect formation, distribution and size reduction of Y2BaCuO5 in melt-processed YBCO superconductors

High critical current density (J(c)) in high temperature superrranducting melt-processed Y-Ba-Cu-O (YBCO), which consists of a superconducting YBa2Cu3O7-y (Y123) phase matrix containing discrete Y2BaCuO5 (Y211) inclusions, can be achieved by control of the grain microstructure during processing. Y123 grain growth rate, Y211 particle size, morphology, distribution and density and the chemistry of the peritectically molten liquid are key parameters in the processing of this material and effectively determine its microstructure. In addition, YBCO processed with excess Y211 generates defects at the Y123/Y211 interface which form effective flux pinning sites and hence enhance J(c). The effect of addition of Y211 on the sample microstructure, its distribution and refinement by chemical doping are discussed in detail in this review.