Using quantitative magneto-optics and an inversion scheme of Biot-Savart’s law the local current carrying capability of various types of grain boundaries (GB’s) and interfaces in YBaCuO thin films and bulk material was investigated. In all GB’s and interfaces a spatial variation of the local critical current density jc was observed which, however, has different reasons, such as microstructural inhomogeneities, magnetic field dependence of jc and size effects in extended GB’s and networks of low angle grain boundaries (LAGB’s).

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 [001]-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.

The influence of the seed distance (dS) on the properties of grain boundaries (GBs) has been studied in order to obtain a precipitate-free GB. On the top of one monolith, two distances between two seeds were chosen, i.e. 4 and 11 mm. For dS≃11 mm, some copper-rich phases and large pores can be observed throughout the GB. In contrast, for dS≃4 mm, neither residual copper-rich phases nor pores are observed within the GB. The growth direction for the two samples seems to be parallel or nearly parallel to the GB. It reveals that the absence of misorientation between the two domains cannot prevent the formation of a residual copper phase layer throughout the GB; only a shorter seed distance will do this. However, element-mapping measurements revealed the occurrence of macrosegregation of the Y-211 particles on top of the monolith and centred on the GB. This tends to vanish towards the bottom of the sample and only a thin layer remains. Magneto-optical measurements performed on the sample with dS≃4 mm, revealed that the critical current density through the GB differs according to the growth directions.

High quality bulk YBaCuO superconductors with maximum trapped fields up to 1.3 T and transport critical current densities up to 1.3 A/cm/sup 2/ in self field at 77 K 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.

The local current density distribution of grain boundaries in differently processed melt-textured YBa2Cu3O7-δ (YBCO) is investigated by a magneto-optical technique and inversion of Biot-Savart's law. By correlating the local current density with the microstructure revealed by a scanning electron microscope, we are able to distinguish the depression of the critical current density jc at clean grain boundaries (GBs) from that occurring at normal phases and cracks at and in the vicinity of GBs. The GBs are formed by different multi-seeding melt-growth processes using a non-stoichiometric precursor which is optimized with respect to the bulk critical current. In order to obtain GBs with extended regions free of normal phases and with large critical current densities the amount of liquid phase and the density of Y2BaCuO5 particles has to be controlled. The properties of GBs can be tailored by both controlling the amount of liquid phase and the seed distance. The related changes in the growth and microstructure are analysed and we show how to grow extended GBs almost free of normal phases with optimized critical current densities in multiple-seeded melt-growth YBCO.

For several years, the 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/sub 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. 4 mm and 11 mm. Regarding d/sub s//spl sim/11 mm, some rich-copper phases and large pores can be observed throughout the GB. In contrast, for d/sub s//spl sim/4 mm, neither residual copper-rich phases nor pores are observed within the GB. Magneto-optical measurements performed on the sample with d/sub s//spl sim/4 mm, 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.

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 109A/m2 at 55 K and good mechanical connectivity.

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.

The correlation between the local current distribution and the microstructure of different types of grain boundaries in YBaCuO monoliths is investigated by magneto-optical technique and inversion of Biot-Savart's law. The grain boundaries (GB's) are formed by a multi-seeding melt-growth process and bonding of monoliths by annealing two monoliths at an external pressure with and without chemical additions.

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 trapped 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 × 105 A/cm2 in self field at 77 K was obtained.

Y2BaCuO5 (Y-211) particles distributed in the textured matrix play an important role for the superconducting properties. The Top-Seeded Melt Growth (TSMG) process always leads to a macrosegregation of Y-211. For TSMG and Multi-Seeded Melt Growth (MSMG) monoliths, the characterization of the distribution of the Y-211 particles was performed over slices in the (001) plane and (001) direction, respectively. As soon as the growth commences, a zone with a low content of less than 10 mol% of Y-211 appears. The low content of Y-211 can mainly be explained by the pushing/trapping model. This model relates the radii of the smallest trapped Y-211 particles to the growth rate. Furthermore, when the undercooling ΔT reaches 25°C, the growth front stops to push the particles and starts to trap them.

Sm1Ba2Cu3O7-delta (Sm123) seed crystals have been fabricated by a Top-Seeded-Solution-Growth (TSSG) method using BaZrO3 crucibles. Crystals up to a size of 100 mm(2) in the a-b plane have been grown. In this presentation, the crystal growth will be described and the growth parameters will be discussed. Furthermore, the superconducting and structural properties of the Sm123 seed crystals will be presented. These seed crystals have been successfully employed for the melt texturing of Y1Ba2Cu3O7-delta (Y123) monoliths of diameters up to 50 mm. The superconducting and structural properties of the melt-textured Y123 determined by hall probe measurements and optical microscopy will be presented.