Formation of c-macrocracks during oxygenation of TSMG YBa2Cu3O7/Y2BaCuO5 single-grain superconductors

The oxygenation process of the top seeded melt-grown (TSMG) single-grain YB2Cu3O7/Y2BaCuO5 (123/211) bulk superconductors was studied by microstructure observations. It is shown that the c-cracks in these bulks are formed in the oxygenated orthorhombic layer along the a/b-macrocracks. The lattice contraction of the 123 phase in a/b-plane induces tensile stresses in the oxygenated layer, which cannot be relaxed by twin boundary motion. The critical spacing of the c-macrocracks gimel(cr) = 14 mum, at oxygenation temperature 380 degreesC, estimated using Thoules's model, is in a good agreement with the c-macrocrack spacing measures on standard TSMG 123/211 single-grain samples. (C) 2003 Elsevier B.V. All rights. reserved.