The ability of large-grain (RE)Ba2Cu3O7−δ ((RE)BCO; RE = rare earth) bulk superconductors to trap magnetic fields is determined by their critical current. With high trapped fields, however, bulk samples are subject to a relatively large Lorentz force, and their performance is limited primarily by their tensile strength. Consequently, sample reinforcement is the key to performance improvement in these technologically important materials. In this work, we report a trapped field of 17.6 T, the largest reported to date, in a stack of two silver-doped GdBCO superconducting bulk samples, each 25 mm in diameter, fabricated by top-seeded melt growth and reinforced with shrink-fit stainless steel. This sample preparation technique has the advantage of being relatively straightforward and inexpensive to implement, and offers the prospect of easy access to portable, high magnetic fields without any requirement for a sustaining current source.

https://iopscience.iop.org/article/10.1088/0953-2048/27/8/082001/pdf

A trapped field of 17.6 T in melt-processed, bulk Gd-Ba-Cu-O reinforced with shrink-fit steel

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TOPICAL REVIEW: Superconducting bearings

This paper describes the present status of high temperature superconductors (HTS) and of bulk superconducting magnet devices, their use in bearings, in flywheel energy storage systems (FESS) and linear transport magnetic levitation (Maglev) systems. We report and review the concepts of multi-seeded REBCO bulk superconductor fabrication. The multi-grain bulks increase the averaged trapped magnetic flux density up to 40% compared to single-grain assembly in large-scale applications. HTS magnetic bearings with permanent magnet (PM) excitation were studied and scaled up to maximum forces of 10 kN axially and 4.5 kN radially. We examine the technology of the high-gradient magnetic bearing concept and verify it experimentally. A large HTS bearing is tested for stabilizing a 600 kg rotor of a 5 kWh/250 kW flywheel system. The flywheel rotor tests show the requirement for additional damping. Our compact flywheel system is compared with similar HTS–FESS projects. A small-scale compact YBCO bearing with in situ Stirling cryocooler is constructed and investigated for mobile applications. Next we show a successfully developed modular linear Maglev system for magnetic train operation. Each module levitates 0.25t at 10 mm distance during one-day operation without refilling LN2. More than 30 vacuum cryostats containing multi-seeded YBCO blocks are fabricated and are tested now in Germany, China and Brazil.

http://iopscience.iop.org/article/10.1088/0953-2048/25/1/014007/pdf

Superconductor bearings, flywheels and transportation

SupraTrans is an innovative transportation concept based on the principle of superconductive magnetic levitation. The aim of the project is to create a fully working prototype, which proves its ability for passenger transport by explicit consideration of the compatibility between systems for propulsion, safety, positioning, power supply, transport logistics and the levitation system itself. The SupraTrans technology uses the flux pinning in high temperature superconductors (HTS) to stabilize the lateral and vertical position of the vehicle on the magnetic track. This self-stabilizing system is the main advantage of the superconductive levitation in comparison to all other levitation systems, which need electronic control and power to keep a constant distance between the train and the track.

Superconductively levitated transport system - the SupraTrans project

We fabricated a single-domain Gd?Ba?Cu?O bulk superconductor 65?mm in diameter and studied the microstructure, superconducting and field-trapping properties. Melt-processing was performed under a controlled oxygen partial pressure of 1.0% using a precursor containing Gd123 and Gd211 powders in a molar ratio of 2:1, with 0.5?wt% of Pt and 20?wt% of Ag2O added. The distribution of Ag and Gd211 particles was almost homogeneous. The addition of Ag was very effective in reducing the amount of cracking in the sample. The maximum trapped magnetic field recorded was 3.05?T at 77?K. We also measured the trapped field between two Gd?Ba?Cu?O bulk samples in order to minimize the demagnetizing effect and found that the trapped field reached 4.3?T at 77?K.

https://iopscience.iop.org/article/10.1088/0953-2048/18/2/026/pdf

Melt-processed Gd–Ba–Cu–O superconductor with trapped field of 3 T at 77 K

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

Magnetic properties of melt-processed YBCO doped with different content of Li, Ni and Pd impurities are reported. The increase of critical currents of doped materials has been observed in intermediate magnetic fields. The normalized pinning force is scaled into a single curve for all measured isotherms (60–77 K) for the samples with higher impurities level. The analysis of magnetic relaxation measurements at a field of 1 T and in the temperature interval between 10 and 70 K reveals a logarithmic dependence of the effective activation energy vs. current density. The observed time dependence of the magnetization in doped materials can be described in the framework of collective pinning theory.

Pinning behavior and magnetic relaxation in melt-processed YBCO doped with Li, Ni and Pd

This paper reports on the trapped fields of 14.3 and 11.4 T measured in the gap between two YBCO cylinders and on top of a single YBCO cylinder, respectively. The magnetic flux has been trapped at 17 and 22 K, respectively. The composite material consists of a YBCO-matrix with Ag inclusions and was reinforced by a bandage of steel. A modified melt crystallisation process was applied to grow the bulk superconductors which takes into account the features in the phase diagram due to the additional silver component. On the other hand, the trapped fields at 77 K benefit from the newly proposed chemical doping by Zn on Cu plane sites which results in a well pronounced peak effect near 3 T. This peak in relatively high fields is thought to result from pair breaking by induced magnetic moments due to in plane substitution of Cu by Zn. Thus, more than 1.1 T has efficiently been trapped in a cylindrical sample of only 25 mm in diameter.

YBCO — monoliths with trapped fields more than 14 T and peak effect

Trapped fields up to 16 T were achieved at 24 K in superconducting mini-magnets consisting of two bulk YBCO disks. 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 of the material. Additionally, Ag and Zn additions were used for improving the mechanical and the pinning properties of the superconductor, respectively.

High trapped fields in bulk YBCO encapsulated in steel tubes

The present paper reports on the trapped fields of 9.0 and 11.4 T (on top of a single YBCO cylinder) which have been trapped at 42.5 and 17 K, respectively. The composite material consists of a YBCO-matrix with Ag inclusions and was reinforced by a bandage from steel. A modified melt crystallisation process was applied to grow the superconducting bulk. On the other hand, the trapped fields at 77 K benefit from the newly proposed chemical doping by Zn on Cu plane sites which results in a well pronounced peak effect between 1 and 3 T. Thus, more than 1.1 T has efficiently been trapped in a cylindrical sample of only 25 mm in diameter.

S. Gruss

Papers

Superconducting bulk magnets: Very high trapped fields and cracking

Pinning behavior and magnetic relaxation in melt-processed YBCO doped with Li, Ni and Pd

YBCO — monoliths with trapped fields more than 14 T and peak effect

High trapped fields in bulk YBCO encapsulated in steel tubes

Large YBCO — Monoliths with Peak Effect and High Trapped Fields