Oxygenation Induced Cracking in YBCO Bulk Superconductors

Abstract

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).

Reduced twinning efficiency and tri-dimensional crack structure in melt-textured NdBa2Cu3O7−δbulk samples fragmentation process

“…The observed macroscopic cracks are then simply the largest members of these fissure families, as shown in the figure where the two micro-fissure families appear close to a crack: it is easy to verify that all fissures are only directed either parallel or perpendicular to the crack. This well-defined pattern shown by the a-b plane cracks observed in the bulk of oxygenated orthorhombic TSG Nd123 samples, is completely different from the irregular structure characterizing the similarly oriented cracks reported in analogous Y123 samples [16,17,27]. These are apparently due to a high precipitate density, but the only analogous structures found in Nd123 samples are the irregular cracks that can be observed in the peripheral growth bands of samples obtained by high initial Nd210 content precursor powders [25], which are clearly due to a very high local density of 422 particles (figure 2(a)) as well.…”

“…Pellet oxygenation in high-pressure furnaces, which allows one to thwart c axis tensile stresses by compression, has been proposed in earlier works as a possible a-b cracks preventing mechanism [15]. In Y123, some cracks are also observed along the c direction: they present a more irregular shape than a-b ones, and are normally associated with precipitates, pores or other crystal defects [16,17].…”