24 Ë 2 Superconducting materials
2.2 LTS bulk materials
There are no practical applications of LTS bulk superconductors, mostly due to
their thermal instability [19]. Local heat generated inside the LHS bulk materials
easily leads to a giant magnetic flux jump, which quenches the superconducting
state. However, the specific heat of YBCO is different from these conventional 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 MgB 2 in LTS materials is a special case, since it has a higher specific heat.
Flux jump issues of MgB 2 arise at very low temperatures, but tend to disappear
at temperatures higher than 10 K [20]. Thus, MgB 2 can be used to make a large
bulk material that produces more trapped flux. Thus, MgB 2 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 RE-
BaCuO bulk magnet, the MgB 2 bulk magnet has several attractive properties, for
instance, low-cost, easy manufacturing of large samples, high mechanical strength,
and homogeneous trapped field distribution. Furthermore, the problem of weak
links at grain boundaries can be ignored in the MgB 2 polycrystalline bulk due
to their long coherence length휁[21]. These characteristics enable us to produce
better and larger polycrystalline MgB 2 bulk magnets for use below their transition
temperatureTc=39 K. Bulk MgB 2 PMs may have the potential to be used in power
systems and magnetic levitated trains using liquid H 2 or a cryocooler operation
at 20 K.
Several groups have already reported the trapped fields in the MgB 2 bulk by
the field-cooled (FC) magnetization and obtained a trapped field over 1.5 T at low
temperatures [20–25]. Naito et al. [26] have reported that the maximum of the trapped
field at the top surface is 1.43 T at 13.4 K for MgB 2 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 MgB 2 have been experimentally investigated by Marignetti
et al., [27] and they have verified the possibility of using bulk MgB 2 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.
2.3 HTS materials
HTSCs constitute a large family of superconducting ceramic materials. They offer the
highest transition temperatures of all superconductors. The ability to use relatively