Large-grain high-temperature superconductors of the form RE-Ba-Cu-O (where RE is a rare-earth element) can trap magnetic fields of several tesla at low temperatures, and so can be used for permanent magnet applications. The magnitude of the trapped field is proportional to the critical current density and the volume of the superconductor. Various potential engineering applications for such magnets have emerged, and some have already been commercialized. However, the range of applications is limited by poor mechanical stability and low thermal conductivity of the bulk superconductors; RE-Ba-Cu-O magnets have been found to fracture during high-field activation, owing to magnetic pressure. Here we present a post-fabrication treatment that improves the mechanical properties as well as thermal conductivity of a bulk Y-Ba-Cu-O magnet, thereby increasing its field-trapping capacity. First, resin impregnation and wrapping the materials in carbon fibre improves the mechanical properties. Second, a small hole drilled into the centre of the magnet allows impregnation of Bi-Pb-Sn-Cd alloy into the superconductor and inclusion of an aluminium wire support, which results in a significant enhancement of thermal stability and internal mechanical strength. As a result, 17.24 T could be trapped, without fracturing, in a bulk Y-Ba-Cu-O sample of 2.65 cm diameter at 29 K.

High-temperature superconductor bulk magnets that can trap magnetic fields of over 17 tesla at 29 K

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

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

https://cyberleninka.org/article/n/818074.pdf

Recent advances in iron-based superconductors toward applications

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

High trapped fields were found in zinc-doped, bulk melt-textured YBa2Cu3O7−x (YBCO) material showing a pronounced peak effect in the field dependence of the critical current density Trapped fields up to 11 T were found at 77 K at the surface of a YBCO disk (diameter 26 mm, height 12 mm) Very high trapped fields up to 1435 T were achieved at 225 K for a YBCO disk pair (diameter 26 mm, height 24 mm) by the addition of silver and using a bandage made of stainless steel The pinning forces and trapped fields obtained in bulk YBCO material are compared with results reported for melt-processed NdBa2Cu3O7−x and SmBa2Cu3O7−x

Trapped magnetic fields larger than 14 T in bulk YBa2Cu3O7−x

After the levitation force relaxation was studied for different field-cooling height and working-levitation height, the high-temperature superconductor (HTS) bulk was horizontally moved in the lateral direction above the permanent magnet guideway. Both levitation and guidance force were collected by the measurement system at the same time. It was found that the decay of levitation force is dependent on both the maximum lateral displacement and the movement cycle times, while the guidance force hysteresis curve does not change after the first cycle. This work provided scientific analysis for the HTS maglev system design.

Influence of the lateral movement on the levitation and guidance force in the high-temperature superconductor maglev system

It is planned to build a 50 MJ pulsed field laboratory at the Forschungszentrum Rossendorf (near Dresden) to obtain magnetic fields up to 100 T with 10 ms pulse duration. This facility would provide the appealing possibility to have access to Zeeman energies in the energy range of the infrared free electron lasers (5–250 μm) that is now under construction at the radiation source ELBE in Rossendorf. For gaining experience in the construction and operation of pulsed magnets, a smaller pulsed field laboratory was established at the Institute for Solid State Physics and Materials Research Dresden (IFW Dresden) in 1999. The laboratory includes pulse magnets with peak fields up to 50 T in a 24 mm bore and a pulse length of about 11 ms, and a 40 T long pulse magnet with a 24 mm bore and a pulse length of about 130 ms. The coils are energized by a 1 MJ, 10 kV capacitor bank with thyristor switches. The bank is subdivided into four identical and independent units. A particular advantage is the possibility to reverse the polarity of the magnetic field pulse by means of a novel circuit with industrial circuit breakers. The design of the power supply is explained in detail. High-precision measurements of magnetization and magnetotransport in the temperature range of 1.5–300 K have been performed.

P. Verges

Papers

Superconductively levitated transport system - the SupraTrans project

Superconducting bulk magnets: Very high trapped fields and cracking

Trapped magnetic fields larger than 14 T in bulk YBa2Cu3O7−x

Influence of the lateral movement on the levitation and guidance force in the high-temperature superconductor maglev system

The Dresden high magnetic field laboratory - overview and first results