The project conducted by NEDO for developing a high-temperature superconducting flywheel energy storage system is introduced; the two test results of fundamental studies are described. One is the measurement of levitation force and rotation loss of superconducting magnetic bearings composed of oxide superconducting bulks and permanent magnet composite. Two types of superconducting magnetic bearings, axial and radial types, were fabricated and tested. The other test was the fabrication and testing of two functional models. A small-sized superconducting flywheel model of the 0.5kWh class was fabricated and tested. A medium-sized rotating functional model of the 10kWh class was fabricated as well.
Superconducting magnets made of high-Tc superconductors are promising for industrial applications. It is well known that REBa2Cu3O7-x superconductors prepared by melt processes have a high critical current density, Jc, at 77K and high magnetic fields. The materials are very prospective for high magnetic field application as a superconducting permanent/bulk magnet with liquid-nitrogen refrigeration. LREBaCuO bulks, compared with REBaCuO bulks, exhibit a larger Jc in high magnetic fields and a much improved irreversibility field, Hirr, at 77K. In this study, we discuss the possibility and trapped field properties of a superconducting bulk magnet, as well as the melt processing for bulk superconductors and their characteristic superconducting and mechanical properties. One of the applications is a superconducting bulk magnet for future magnetically levitated (Maglev) trains.
Irradiation of superconducting material with particles, such as electrons, protons, neutrons, ions etc., is known as a valuable tool to introduce pinning centers in a controlled way. Due to its extremely large penetration range, neutron irradiation is thought to be most suitable for the production of randomly distributed defects in bulk materials. Fast neutrons produce spherical cascade defects with a diameter of approximately 5-6nm. The trapped field of a Y-Ba-Cu-O bulk sample was found to be increased up to 3.7T at 77K by fast neutron irradiation, which suggests the practical importance of this technique. On the other hand, by using thermal neutron irradiation, we can create fission tracks in the sample doped with uranium, which causes a dramatic enhancement of Jc.