Improved trapped fields are reported for bulk melt-textured YBa2Cu3O7−δ (YBCO) material in the temperature range between 20 and 50 K. Trapped fields up to 12.2 T were obtained at 22 K on the surface of single YBCO disks (with Ag and Zn additions). In YBCO minimagnets, maximum trapped fields of 16 T (at 24 K) and of 11,2 T (at 47 K) were achieved using (Zn+Ag) and Zn additions, respectively. In all cases, the YBCO disks were encapsulated in steel tubes in order to reinforce the material against the large tensile stress acting during the magnetizing process and to avoid cracking. We observed cracking not only during the magnetizing process, but also as a consequence of flux jumps due to thermomagnetic instabilities in the temperature range betweeen 20 and 30 K.

Superconducting bulk magnets: Very high trapped fields and cracking

New permanent magnets

The effects of Pr-doping on the critical current density Jc and the critical temperature Tc in YBa2Cu3O7−δ (YBCO) have been investigated. In spite of the monotonic decrease in Tc with increasing Pr-concentration (x), the increasing of Jc occurs as a field is applied and Jc displays a maximum at a certain x. Moreover, this maximum shifts to lower x by increasing fields. The characteristic behavior of Jc with Pr-doping can be explained in terms of the counterbalance of two different effects simultaneously caused by Pr-doping: the suppression of matrix superconductivity which leads to the decrease in depairing critical current density J0, and the formation of nanometric inhomogeneity which leads to the enhancement of pinning force.

The effects of Pr-doping on the critical current density in YBa2Cu3O7−δ

Effects of dilute impurity doping to the CuO chain of Y123 on the flux pinning properties were studied using single crystals and melt-solidified bulks. Cobalt-doped Y123 single crystals and iron-, cobalt- or gallium-doped Y123 melt-solidified bulks exhibited dramatically improved Jc-H characteristics accompanying huge secondary peaks in Jc-H curves. In field Jc of these samples are apparently higher than that of a low level zinc-doped bulk. These results strongly suggest that introduction of local disorders in the CuO chain by dilute impurity doping is a more promising way to improve Jc of RE123 materials than impurity doping to the CuO2 plane.

Enhanced flux pinning properties of YBa2Cu3Oy by dilute impurity doping for CuO chain

Large single-grain bulk RE–Ba–Cu–O (RE: rare earth elements) superconductors with high J c values can trapmagnetic ﬁelds much greater than conventional permanent magnets. At present, the maximum trapped ﬁelds are es-sentially limited by the mechanical property of RE–Ba–Cu–O, and therefore, its improvement is critically important. Inthis study, we measured mechanical properties of large-grain Y–Ba–Cu–O and Sm–Ba–Cu–O bulk superconductors bymonitoring the strain with the gauges directly mounted onto the sample surface. The stress was applied to the samplesby reducing the external ﬁeld from 10 to 0 T at 50 K. It is found that the present strain gauge method is useful for directcharacterization of mechanical properties of large grain materials. It is also conﬁrmed that Ag addition is e•ective inimproving the mechanical properties of large block Y–Ba–Cu–O and Sm–Ba–Cu–O. O 2000 Elsevier Science B.V. Allrights reserved. Keywords: Mechanical properties; Crack; Strain-gauge stress 1. IntroductionRecent progress in the bulk fabrication processenabled the production of large single-grain bulkRE–Ba–Cu–O superconductors with high J

Direct measurements of mechanical properties for large-grain bulk superconductors

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.

T Fahr

Papers

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