P. Schätzle

Zn doping of YBa2Cu3O7 in melt textured materials: peak effect and high trapped fields

Abstract The previously introduced modified melt crystallization process (MMCP) has been applied to prepare single grain YBCO bulk material with Zn partially substituting for Cu. Hole doping was controlled by an appropriate oxidizing treatment of the as-grown bulk. A field induced pinning was indicated by a well pronounced peak of the critical current density jc in the jc vs. H relationship for the maximal oxidized (overdoped) material containing Zn, whereas pure overdoped YBCO shows no peak effect. The peak effect for Zn-doped YBCO appearing for T=77 K at a relatively high field of about 3 T can be attributed to pair breaking by locally induced magnetic moments due to in plane Zn for Cu substitution. Besides high quality of the bulk YBCO, the peak effect is the reason for the trapped field as large as 1.12 T at 77 K in the cylinder of only 25 mm in diameter.

Multi-seeded melt crystallization of YBCO bulk material for cryogenic applications

YBCO bulk materials were crystallized by a multi-seeded melt growth process. Up to four biaxial oriented Sm-123 seeds were placed on the top of rectangular shaped YBCO bars (up to ). The oriented grown single grains are joined by partially coherent grain boundaries with an improved current transport capability and high levitation forces with overall values of 55-65 N. Segregation of nonsuperconducting rest melt and microcracks limit the current transport across the grain boundary.

Modified melt texturing process for YBCO based on the polythermic section YO1.5“Ba0.4Cu0.6O” in the YBaCuO phase diagram at 0.21 bar oxygen pressure

Abstract Y 2 O 3 admixed to 123 significantly influences the mechanism of melt texturing of YBCO. Based on phase diagram considerations, it is shown that the process window for stable growth conditions can be considerably increased. Furthermore, a lower process temperature ( T a =960°C to 985°C) becomes possible. Samples have been grown by both directional solidification and zone melting techniques. The superconducting properties are determined by local and integral levitation force measurements.

THE INFLUENCE OF INITIAL COMPOSITION AND OXYGEN PARTIAL PRESSURE ON THE PROPERTIES OF MELT-TEXTURED NDBACUO

Abstract The influence of several admixtures on the microstructure formation and the superconducting properties is explained on the base of phase-equilibrium considerations. Conditions are derived which permit the stabilization of Ba rich Nd-123 phase to occur which is a precondition for preparing textured NdBaCuO materials with Tc > 90 K and a sufficient high jc. A melt-textured bulk material with Tc = 91 K and jc(77 K, 0 T, B ∥c) = 21.8 kA/cm2 was produced at p(O2) ≈ 100 Pa using an initial mixture consisting of NdBa2Cu3O7−δ and Nd2BaO4.

Melt processing of (Nd, Y)BaCuO and (Sm, Y)BaCuO composites

Solid solutions of and high-temperature superconducting materials were synthesized. The (Nd, Y)BaCuO composite samples melt textured in air atmosphere show high critical current densities of about . A double exchange of both and by (or by , respectively) was observed in these materials. The results indicate vacancies on barium sites to exist additionally as defects in the substituted 123 phase of the melt textured materials. The microstructure reveals (Nd, Y)-211 or (Sm, Y)-211 inclusions with different Nd,Y and Sm,Y contents, respectively.

YBCO/Ag bulk material by melt crystalization for cryomagnetic applications

Based on thermoanalytic investigations a melt crystallization process was developed with reduced process temperatures and process times to produce YBCO/Ag monoliths (d=26 mm) achieving the homogenous distribution of small sized Ag inclusions. The mechanical properties are improved, but the superconducting properties at 77 K, B/sub 0/ (480 mT) and F/sub N/ (40 N) are slightly reduced compared with YBCO monoliths without an addition of Ag (750 mT and 65 N). The maximum trapped flux was increased to the value B/sub 0/ 9.4 T at 26.5 K, instead of 8.4 T in the Ag-free bulk monolith.

High trapped fields in melt-textured YBCO samples

Abstract Bulk melt textured YBCO samples in the shape of disks were developed for use as trapped field magnets. The trapped field in single grains with diameter of 24 mm reached maximum values up to 0.56 T at 77 K and a value of 5.5 T at 30 K. Degradation of the trapped field was observed in samples after the activation process at 20 K in a high applied magnetic field. This is interpreted in the framework of a model of magnetic stress induced damage.

Melt processing and high performance of binary (Nd, Sm)-Ba-Cu-O superconductors

Binary (Nd, Sm)-Ba-Cu-O superconductors with Tc of 96 K and a high irreversibility field up to 8 T have been fabricated in air via a newly developed technique. The initial composition was controlled by using binary (Nd, Sm)2BaO4 as an addition. In comparison with both NdBaCuO and YBaCuO melt-textured superconductors, the present composition showed a remarkable enhancement of critical current densities in an intermediate field regime (1-3 T). Microstructural observations and compositional analysis showed that the distributions of Nd and Sm in the (Nd, Sm)-123 matrix were homogeneous while the atomic ratio between these two rare earth elements was inconsistent with that in the nominal composition. These new characteristics were thought to cohere with the different solubilities between Nd and Sm in the liquid phase. Finally, trapped magnetic field mapping showed a single-domain shape of the trapped field, which indicated the growth of a single grain in the samples.

Trapped fields beyond 14 tesla in bulk YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//

Bulk YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// material with single-domain grains up to 35 mm in diameter was prepared by a modified melt-texturing process. The pinning properties were improved by chemical doping of Zn on Cu plane sites resulting in a well pronounced peak effect in the field dependence of the critical current density. Trapped fields of more than 1.1 T at 77 K have been obtained in a zinc-doped YBCO disk of only 27 mm in diameter. Higher trapped fields are observed at lower temperatures due to the increasing critical current density, however, at temperatures between 50 and 20 K, the trapped field is limited by the mechanical properties. Using a bandage made of steel, very high trapped fields were achieved in the 1 mm gap between two YBCO disks. Trapped fields up to 14.4 T were found at 22.5 K in such minimagnets containing silver precipitates, whereas the trapped fields of zinc-doped minimagnets reached values of 11.2 T at 47 K.

High trapped fields in melt-textured YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//

Bulk melt textured YBCO material with single-domain grains up to 35 mm in diameter was prepared by a modified melt-texture process. The maximum trapped field B/sub o/ measured in the 1 mm gap between two single-grain disks was found to increase from 1.2 T at 75 K up to 9.6 T at 46 K, which is the highest trapped field achieved in nonirradiated samples. A strong exponential decrease of the trapped field with increasing temperature found in most of the cases can be explained by flux creep within the framework of collective pinning assuming weak pinning. A weaker B/sub o/(T) dependence and higher B/sub o/ values at temperatures between 55 and 75 K were observed in newly processed YBCO material. A pronounced peak effect found in the field dependence of the critical current density suggests strong pinning in this material. For applications of YBCO samples in superconducting magnetic bearings, an effective method for magnetizing the superconductor is required. Results of magnetizing YBCO samples by applying pulsed magnetic fields are presented.