Superconducting bulk magnets: Very high trapped fields and cracking

Abstract

Improved trapped fields are reported for bulk melt-textured YBa2Cu3O 7−δ (YBCO) material in the temperature range between 20 K 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 mini-magnets, 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 K and 30 K. 7460Ge, 7462Bf, 7480BjBulk type II superconductors can trap high magnetic fields that are generated by superconducting persistent currents circulating macroscopically within the superconductor. The main features of the resulting field distribution are a maximum trapped field B 0 in the center of the superconducting domain and a field gradient towards the sample edge which is determined by the critical current density j c of the supercurrents. Therefore, high trapped fields B 0 in bulk superconductors require a high critical current density and a large size of the current loops. Large single-domain bulk YBa 2 Cu 3 O 7−δ based (YBCO) samples can be produced by melt texture processing, especially by using SmBa 2 Cu 3 O 7−δ (Sm-123) as a seed crystal.[1] The pinning effect in melt textured samples can be improved by irradiation methods [2,3] and alternatively, by zinc-doping. [4,5] Cracking of the samples was found to limit the trapped field of bulk YBCO at temperatures below 77 K which can be explained by tensile stresses that occur during the magnetization process due to the stored flux density gradient and may exceed the tensile strength of the material.[6] The mechanical properties of bulk YBCO and its tensile strength can be improved considerably by the addition of Ag. [7,8] Another possibility to avoid cracking during magnetizing is to encapsulate the bulk YBCO disks in steel tubes. [9] A steel tube leads to stress compensation by generating a compressive stress on YBCO after cooling from 300 K to the measuring temperature which is due to the higher thermal expansion coefficient of steel compared to that of YBCO in the a, b-plane. Maximum trapped fields B 0 of 11.5 T at 17 K and of 14.4 T at 22 K were reported for single YBCO disks and mini-magnets consisting of two single YBCO disks, respectively. [9] In both cases, Ag was added to the YBCO disks which were placed into austenite Cr-Ni steel tubes. Previous attempts to combine the two beneficial effects of Ag and Zn additions failed due to the solubility of Zn in Ag in oxygen atmosphere. High trapped fields have also been reported for bulk Sm-123 with additions of Ag. At the surface of Sm-123 disks, trapped fields of 2.1 T at 77 K and of 8 T at 40 K have been observed. [10] In the present paper, t...

Influence of ZnO doping on the properties of single domain YBCO bulks fabricated by RE+011 TSIG process

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Comparative study of critical current density, pinning force and levitation force behavior in Yb211 doped-Sm123 bulk superconductors

“…It is well known that achieving high J c , irreversibility field B irr and magnetic field trapping ability in high temperature superconductors requires effective flux pinning. Therefore, several preparation techniques were developed to enhance pinning properties of HTS such as melt-textured growth (MTG) [7], liquid phase process [8], powder melting process (PMP) [9], melt powder melt growth (MPMG) [10], and top-seeded melt-growth (TSMG) [11,12] process.…”

Coaxial magnetic brakes using single-domain YBCO

“…However, in some applications, such as hypervelocity research, the fields produced by permanent magnets will not stop projectiles at the speeds and masses of interest in a reasonable distance. While the field produced by the best permanent magnets is approximately 0.5 T, magnets composed of melt-textured YBCO and related materials have been demonstrated to produce fields as high as 3.7 T at a temperature of 77 K [1], 11 T at 47 K [2], and 17 T at 29 K [3]. Because the braking force is proportional to the square of the field, monolithic YBCO magnets can therefore produce forces hundreds of times as high as a comparably sized permanent magnet.…”