Studies of cracking behavior in melt-processed YBCO bulk superconductors

An important phenomenon in bulk superconductors fabricated by top-seeded-melt growth (TSMG) is the formation of cracks due to the inherent brittleness of the YBa2Cu3O7-δ (Y-123) phase matrix. These form during the fabrication of the superconducting monolith and play an important role in the limitation of current flow. However, cracks may also form during cooling cycles of the sample to liquid nitrogen temperatures. In this investigation, macrocracks along the c-direction, in particular were analyzed microscopically before and after cooling. In addition we attempt to resolve the c-axis macrocrack formation pattern using the magnetoscan technique

Function elements of melt-textured YBCO for cryomagnetic applications

Function elements can be constructed by cutting, machining and bonding of batch processed melt-textured YBCO. Depending on the application structures with different complex geometry can be fabricated. They consist of several monoliths, e.g. rings, bars and plates, and may be passivated, if necessary. Melt-textured YBCO monoliths with different shape and a maximum size of 60 mm can be reproducible prepared in a batch process. Seeding by self-made SmBa2Cu3Ox leads to single domain material. Zero field cooled levitation forces at 77 K higher than 80 N were achieved. In single domain cylinders with a diameter of 30 mm and in quadratic shaped monoliths with an edge length of 35 mm maximum inductions of 1.3 and 1.2 T are frozen at 77 K. The trapped magnetic field of our function elements, e.g. plates and bars, are checked at 77 K. Results of our function elements in HTSC reluctance motors using single domain material are shown. Also, we will report about a fly wheel system, called "Dynastore" and a system to levitate persons.</p

Structure and properties of melt-textured YBa2Cu3O7-delta, high pressure-high temperature treated and oxygenated under evaluated oxygen pressure

Conference: International Cryogenic Materials Conference Location: Univ Wollongong, Wollongong, AUSTRALIA Date: FEB 10-13, 2004International audienceMT-YBCO samples oxygenated under controlled oxygen pressure exhibited at 77 K a critical current density j(c) = 85 kA cm(-2) in zero field and more than 10 kA cm(-2) up to 5 T field when the external magnetic field was perpendicular to the ab-plane of Y123, and a j(c) = 23 kA cm-2 in zero field and jc close to I kA cm-2 in 10 T field when the magnetic field was perpendicular to the c-axis of Y123. The microstructure of these samples contained an unusually high density of twins (about 30 twins mum(-1)) as well as a lot of stacking faults around Y211 inclusions. Using quasi-hydrostatic high pressure-high temperature (HP-HT) treatment we may vary the twin and dislocation densities in the material by changing the sample orientation in high pressure apparatus, while the oxygen content of Y123 phase as well as the lattice parameters remain unchanged. The microstructure of the material in the case where the highest pressure has been applied in the direction perpendicular to the ab-plane of Y123 is characterized by a very low twin density, perfect dislocations stepped along (110) directions and small faulted loops corresponding to CuO intercalating in the matrix. For this material j(c) = 10 and 8 kA cm(-2) in zero field were observed (when the external magnetic field was perpendicular to the ab-plane and perpendicular to the c-axis of Y123, respectively). High pressure-high temperature treatment causes an increase in the material density (up to near the theoretical one), microhardness and fracture toughness