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

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.

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

The development of bulk high-temperature superconductors (HTSs) and their applications has today come to a point where the mechanical response to high magnetic fields may be more important than their critical-current density and large-grain property. Reviewed in this article are the recent studies of the magneto-elastic effects which are caused by flux pinning in the superconductors. This includes the work on the giant irreversible magnetostriction and internal stress, which often cause fatal cracking of the HTS bulks as they become magnetized. The cracking is a problem that today accompanies the quest for the highest trapped field values, and the latest development in this area is also presented. While the first part is an overview of experimental efforts, the second summarizes the work done to model the pinning-induced stress and strain under various magnetic and geometrical conditions.

https://iopscience.iop.org/article/10.1088/0953-2048/13/10/201/pdf

Flux-pinning-induced stress and magnetostriction in bulk superconductors

Y-Ba-Cu-O superconductors were prepared by the top-seeded melt growth process with multiseeding technique. By using several seeds at the same time, large samples could be fabricated in a short time with simple heat treatment. However, the samples fabricated by the normal multiseeding technique show a rapid decrease of trapped magnetic field value across the grain boundaries because of the residual liquid layer. To remove the residual liquid layer, various types of multiseeding were newly suggested. The optimum result was obtained when the seeds were arranged without spacing. The individual grains were combined as a single domain and did not show any deterioration of magnetic properties at the boundary. The formation mechanism of a well-combined single domain by the multiseeding technique is discussed.

Top-seeded melt growth of Y-Ba-Cu-O superconductor with multiseeding

Large, single-grain Y–Ba–Cu–O (YBCO) was fabricated via the infiltration of Ba–Cu–O liquid into a precursor body composed of solid, porous Y2BaCuO5 (Y-211) and observed to trap a magnetic field of 0.15 T at 77 K. In this process a NdBCO seed crystal was used to promote heterogeneous nucleation, which allows the fabrication of single-grain YBCO containing a uniform and very fine distribution of Y-211 inclusions in the YBa2Cu3O7−δ(Y-123) matrix without the addition of Pt. These superior microstructural features and significant field trapping ability compared with samples processed by conventional top-seeded melt growth suggest this technique could be a practical alternative for processing large, single-grain superconductors for engineering applications.

Fabrication of Large Single-grain Y–Ba–Cu–O Through Infiltration and Seeded Growth Processing

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.

https://iopscience.iop.org/article/10.1088/0953-2048/12/2/003/pdf

D Schläfer

Papers

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