Browsing by Author "Bringmann, B."

Since several years, technical applications of bulk HTS YBCO superconductors are of growing interest. However, shapes of HTS tiles needed for complex applications require the joining of two or more single-domain monoliths. The welding technique leads to the formation of a low-angle grain boundary and then requires a high connectivity between two adjacent single-domain materials. This study was devoted to the characterization of the microstructure and the superconducting properties of natural and artificial low-angle grain boundaries. The natural grain boundary formed during the multi-seeded melt growth (MSMG) process exhibits a non-uniform microstructure during the growth. An energy dispersive spectroscopy (EDS) analysis carried out across the grain boundaries always reveals a depletion of copper and an accumulation of yttrium. Mechanical joining of two single-domain monoliths leads to the formation of an artificial grain boundary. This kind of joining was performed either without or with a welding agent, i.e., YbBa2Cu3O7-x (Yb-123), The first method leads to a good connectivity between welded Y-123 platelets. In contrast, the control of the mechanical welding process with a welding agent is more difficult. At 940 degrees C, Yb-123 decomposes into Yb2BaCuO5, BaCuO2 and CuO. This decomposition deteriorates the superconducting properties between two adjacent domains. (C) 2000 Elsevier Science B.V. All rights reserved.

High quality bulk YBaCuO superconductors with maximum trapped fields up to 1.3T and transport critical current densities up to 1.3A/cm(2) in self field at 77K are prepared by the Top Seeded Melt Growth. On the basis of this process two methods for the processing of large tiles and tiles in complex shapes are presented. Superconducting rings with a radial orientation of the crystallographic c-axis are processed by a multi-seeding method. These rings are used in the magnetic bearing of a cryotank designed for the storage of liquid hydrogen. The joining of single domains with an ErBaCuO compound leads to the formation of a superconducting joint between single domains. Large tiles prepared by this process are to be used in the circumferential magnetic bearing of a flywheel.

Large-volume HTS tiles with low-angle grain boundaries are obtained by cutting melt textured YBa2Cu3O7-x (YBCO) samples parallel to their c-direction and rejoining them with ErBa2Cu3Ox, applying a pressure of 0.5 MPa. This joining technique enables the preparation of large monoliths as well as of monoliths with complex shape which are required for technical application. Samples prepared by this technique exhibit high intergranular critical current densities up to 10(9) A/m(2) at 55 K and good mechanical connectivity.

CeO2-doped YBaCuO monoliths synthesized with a top-seeded melt growth process in a conventional box furnace exhibited values of trapped magnetic field of up to 1.33 T at 77 K. To our knowledge, this is the highest value of trapped field reported for a melt-textured YBaCuO monolith. A suitable temperature profile and the use of high-density Y2BaCuO5 substrates led to reproducible single-domain crack-free samples investigated by optical and scanning electron microscopy and crapped field measurements. The zero-field-cooled levitation forces at 77 K of standard samples amounted to 70-83 N. A transport critical current density of up to 1.3 x 10(5) A/cm(2) in self field at 77 K was obtained.

For several years, technical applications of bulk HTS YBCO superconductors are of growing interest. However, for applications which require a complex geometry of the monoliths, the Multi-Seeding Melt Growth (MSMG) and the mechanical joining of two single-domain tiles becomes necessary. However, these two techniques lead to the formation of a low-angle grain boundary (GB). The influence of the seed distance (d(S)) on the properties of the GB has been studied in order to obtain a precipitate free grain-boundary. On the top surface of one monolith, two distances between two seeds were chosen, i.e. 4mm and 11mm. Regarding d(S)similar to 11mm, some rich-copper phases and large pores can be observed throughout the Ga In contrast, for d(S)similar to 4mm, neither residual copper-rich phases nor pores are observed within the GB. Magneto-optical measurements performed on the sample with d(S)similar to 4mm, revealed that the critical current density through the GB is influenced by the growth directions. Regarding the mechanical joining, a high connectivity between two adjacent domains can be obtained by using the PLD technique for coating of the monoliths.

Seeded melt growth of YBCO high-temperature superconductors is one of the most promising preparation techniques to obtain high-quality HTS tiles for application, e.g. in magnetic bearings. Semi-finished HTSL products of complex shapes have to be developed by different seeding and multi-seeding techniques. To obtain large hollow cylinders designed for application in the magnetic bearing of a cryotank a modified multi-seeded melt growth (MSMG) process was employed. This cryotank will be mounted for testing in a vehicle of a major German car manufacturer. The MSMG process introduces grain boundaries into the HTS tiles. For transport current investigations of [0 01]-tilt grain boundaries in melt textured YBCO a series of MSMG bicrystals have been prepared. They exhibit a dependence of the critical current density on misorientation angle which is much weaker than the one observed in thin-film bicrystals. The bulk samples have dimensions larger than the magnetic penetration depth along the grain boundary. Thus, flux pinning has to be taken into account. Different contributions to the longitudinal pinning force have to be considered: vortices at grain boundaries can be pinned by magnetic interaction with Abrikosov vortices in the banks, by defects in the grain boundary itself or by defects which are located next to the grain boundary. (C) 2002 Elsevier Science B.V. All rights reserved.