2.7 Trapped fluxes in HTS bulk Ë 45conditions. In addition, other environments, such as fabrication, handling, transport,
and storage, would also cause similar irreversible degradation. In particular, the
degradation is very serious when HTS REBCO bulks are exposed to humid atmosphere
environments. In order to prevent the performance degradation for REBCO bulk ma-
terials exposed to harmful environments, it is very necessary to investigate protection
methods for REBCO bulk materials.
Rod et al. [148] studied the degradation of textured YBCO under operation in
high humidity conditions and experimentally proved that the YBCO samples after
degradation contain a high amount of Y-211 phases in the degraded region due to the
reaction of YBCO with water. The results show that the initial formation of barium
hydroxide and the non-superconducting green phase, Y 2 BaCuO 5 , induces surface
degradation and degradation of mechanical properties.
In the research and development of the first man-loading HTS Maglev test vehicle
[149] in the world in the last century, thin copper box sealing technology was used.
YBCO bulks were placed in a copper box, then sealed by welding, vacuum pumped,
injected with dry nitrogen with a pressure slightly higher than atmosphere, and then
permanently sealed. Dry nitrogen improves heat conduction. The eight HTS Maglev
equipments are composed of 344 blocks of YBCO. The magnetic levitation forces
of each were measured every 6 months. The results showed that there was almost
no change in the magnetic levitation forces after more than 10 years. After opening
the copper boxes enclosing the HTSC, the YBCO bulks were almost unchanged. For
permanent applications, this may be a very effective method (see Sections 6.24 and
6.29). There is another method that uses the epoxy and cryogenic adhesive to seal the
YBCO bulks; however, long-term protection is not as effective as the above-mentioned
methods.
2.7 Trapped fluxes in HTS bulk
Rare-earth PMs with high energy density and high intrinsic coercive force are widely
used, bringing great technical and economic benefits. Superconductors can trap a
high magnetic flux and thereby can become a PM. The superconducting PMs have
much higher energy density than the rare-earth PMs, and their potential industrial
applications are very promising.
Trapped fields in LTS bulk Nb 3 Sn were investigated at the temperature of 4.2 K
(liquid helium) to 1.5 K in 1965. A Nb 3 Sn hollow cylinder, 18.6 mm long, 10.2 mm
outer diameter, and 2.7 mm inside diameter, was synthesized by combines niobium-
tin powder and sintering technique. The sintered bulk Nb 3 Sn hollow cylinder had
demonstrated that a magnetic field of 0.2, 0.4, and 2.7 T can be trapped at 4.2, 2.2,
and l.5 K, respectively [150].
In 1977, a hollow Nb 3 Sn superconducting cylinder of 2.45 cm in diameter was used
to trap a magnetic field of 2.24 T at 4.2 K. [151]. The cylinder of 7.62 cm was constructed