2.7 Trapped fluxes in HTS bulk Ë 47Fig. 2.10:The effect of temperature on trapped-field distribution. The field was trapped between two
26.5-mm-diameter YBCO disks for 29, 46, and 78 K [110].
longer stable. When the flux jump is serious enough, the HTS bulk will be destroyed
due to the crack of formation from the large electromagnetic forces. Thus, for the
superconducting PMs, it is very important to maintain the thermal stability.
These are serious challenges for superconducting materials experts. It is necessa-
ry to prevent the generation of microcracks and micro-pores during the melt proces-
sing. There are some methods to improve the HTS bulk performance, for example,
addition of silver, sample reinforcement with metal rings, etc. Tomita and Murakami
have developed a post-fabrication treatment, which improves the mechanical proper-
ties as well as the thermal conductivity of a YBCO bulk magnet, thereby increasing its
field-trapping capacity. These methods use resin impregnation and wrap the materials
in carbon fiber and also include a small hole drilled into the center of the magnet to
allow the impregnation of BiPbSnCd alloy into the superconductor. These methods
greatly enhance the thermal stability and internal mechanical strength. As a result,
17.24 T could be trapped, without fracturing, in a bulk Y-Ba-Cu-O sample of 2.65 cm
diameter at 29 K (Fig. 2.10) [110].
There are three typical ways to magnetize a HTS bulk: field cooling (FC) magneti-
zation, zero-field cooling (ZFC) magnetization, and pulse field magnetization (PFM).
When the HTS bulk is magnetized in a magnetic field, the HTS bulk can trap flux.
After removing the field, a superconducting permanent magnet can be obtained. In
comparison, the ZFC magnetization is more difficult, because a very high external
magnetic field is generally needed. The FC magnetization is convenient. The applied