2.2　LTS bulk materials

There are no practical applications of LTS bulk superconductors, mostly due to their thermal instability19. Local heat generated inside the LHS bulk materials easily leads to a giant magnetic flux jump, which quenches the superconduct-ing state.However, the specific heat of YBCO is different from these conven-tional BCS superconductors.Thus, HTS bulks are thermally stable even in large sample sizes due to their relatively large specific heat in the superconducting state.

The MgB2 in LTS materials is a special case, since it has a higher specific heat. Flux jump issues of MgB2 arise at very low temperatures, but tend to disap-pear at temperatures higher than 10 K20.Thus, MgB2 can be used to make large bulk material that produces more trapped flux.Thus, MgB2 bulk has a promising potential as a superconducting permanent magnet（SCPM）.The HTS REBaCuO bulk magnet can produce tesla-order SCPMs.However, it is difficult to fabricate a large single-domain REBaCuO bulk over 60 mm in diameter.In comparison to the REBaCuO bulk magnet, the MgB2 bulk magnet has several attractive properties, for instance, low-cost, the easy manufacturing of large samples, high mechanical strength and a homogeneous trapped field distribution.Furthermore, the problem of weak-links at grain boundaries can be ignored in the MgB2 polycrystalline bulk due to their long coherence lengthζ21.These characteristics enable us to produce better and larger polycrystalline MgB2 bulk magnets for use below their transi-tion temperature Tc =39 K.Bulk MgB2 PMs may have the potential to be used in power systems and magnetic levitated trains using liquid H2 or a cryocooler operation at 20 K.

Several groups have already reported the trapped fields in the MgB2 bulk by the field-cooled（FC）magnetization, and obtained a trapped field over 1. 5 T at low temperatures20，22，23，24，25.Naito et al.26have reported that the maximum of the trapped field at the top surface is 1.43 T at 13.4 K for MgB2 bulk 20 mm in diameter and is 1.50 T at 16.4 K for one that is 30 mm in diameter.

The achievable maximum flux density in a bulk material is the key factor in the development of high-power-density superconducting electrical machines. The engineering issues relevant to the development of rotors of synchronous machines using magnetized bulk MgB2 have been experimentally investigated by Marignetti et al.27，and they have verified the possibility of using bulk MgB2 magnets for the field excitation of synchronous machines.Both the experiments and the analysis indicate a maximum trapped field limit of 1.2 T at 15 K.