It is shown that the a/b-and c-macrocracks starts to form on the surface of YBCO bulk superconductors at the initial stage of oxygenation. Shortening of the Y123 c-and (a + b)/2-lattice parameters with increasing oxygen concentration induces tensile stresses on oxygenated layer and causes its cracking.
PACS : 74.72.BkTop seeded melt-grown (TSMG) bulk single-grain YBa 2 Cu 3 O 7 /Y 2 BaCuO 5 (Y123/211) superconductors are highly attractive for practical applications (as well as bulks where Y is replaced by rare earth (RE) element). For good performance in applications the material has to be mechanically resistant in order to withstand large thermal stresses during cooling/heating cycles and during magnetization [1]- [4]. Unfortunately YBa 2 Cu 3 O 7 compound is inherently brittle therefore it is sensitive to cracking [5]. In this contribution we summarize our latest results on the cracking associated with oxygenation of TSMG samples .The study was performed on the TSMG single-grain bulks, which were annealed at 900 • C in oxygen for 1 hour and cooled down in argon with the cooling rate 600 • C/hour, to have starting tetragonal samples with low and defined oxygen content. For further treatment, bars of 2 x 2 mm 2 cross-section and with bar walls parallel to the a/c-planes were cut and oxygenated at different temperatures in a tubular furnace. Heating to and cooling from the oxygenation temperature was carried out in Ar atmosphere. Samples were oxygenated in a flow of oxygen. The microstructure was studied by optical microscopy.2 Results and discusion 1. a/b-macrocracks. The oxygenation process was very inhomogeneous at lower temperatures (Fig. 1a, b). Cracks, which were formed during oxygenation, provided a path for oxygen flow to the sample and faster transformation to the orthorhombic state along the a/b-macrocracks and consequently enhanced crack propagation. At higher temperatures the oxygenated layer becomes more regular (Fig. 1c).