Oxidation induced formation of a-b planar defects in melt-textured YBa2Cu3O7-y containing Y2BaCuO5 inclusions

In order to understand the formation mechanism of the planar defects of a Ba-Cu-O platelet and a CuO stacking fault of melt-textured YBa2Cu3O7-y(123), two different types of 123 samples containing Y2BaCuO5 (211) inclusions (a tetragonal and orthorhombic samples) were prepared by melt-quenching heat treatment and/or subsequent annealing in oxygen atmosphere, and the microstructures were examined. In the tetragonal samples which are high-temperature non-superconducting phases, no remarkable defect formation was observed either around the trapped 211 inclusions or in the 123 matrix. After annealing of the tetragonal samples in oxygen atmosphere for the transformation of the tetragonal phase to the superconducting orthorhombic phase, meanwhile, various types of defects were observed such as dislocations, stacking faults, twins and the BaCuO platelets. Particularly, the planar defects of the CuO stacking faults and the BaCuO platelets were found to nucleate mostly at the 123/211 interface and then to grow toward the interior of the 123 matrix. In the sample with shorter oxygen annealing time, the formation of the planar defects was limited in the vicinity of the trapped 211 while in the sample with longer annealing time, they extended to the whole 123 matrix. It is concluded that the formation of the planar defects of the melt-processed Y-Ba-Cu-O is attributed to the low temperature annealing in oxygen atmosphere for the tetragonal-to-orthorhombic phase transformation.