28 Ë 2 Superconducting materials
rare earth elements such as cerium, samarium, neodymium, and praseodymium,
superconductivity can also be found. In addition to higher critical temperature,Tc, the
iron-based superconductors also have very low anisotropy, very high critical magnetic
field,Hc2, and large critical current density,Jc. It is very suitable for high magnetic
field application. For the engineering application of the 20-K temperature region, the
iron-based superconductors may be the better choice than Nb-Ti and Nb 3 Sn. Recent
progress about iron-based superconductors can be found elsewhere [60].
The properties of main HTSC materials are shown in Tab. 2.3.
2.4 HTS bulk materials
HTS bulk materials have a series of unique features in terms of processing techniques,
characteristic properties, and areas of applications. HTS bulk materials are especially
attractive for practical applications because of their large current transport capacity
in the presence of strong magnetic fields and their ability to trap very high fields in a
compact space, particularly essential features for self-stable levitation without active
control systems. Although the critical current density of bulk HTSC is about two orders
of magnitude lower than that of thin films, the bulks offer a large effective critical
current density(Jc)since metal substrates are not required. Thus, HTS bulks are widely
used in flywheel energy storage system, transportation vehicles, current leads, fault
current limiters, wind power generators, ship motors, compact NMR/MRI, etc [62]. In
order to realize these applications, large size and excellent HTS bulks are required. It
is necessary to improve the bulk properties further. Specific details about HTS bulk
materials have been dissussed elsewhere [63]. This section only directly relates to the
basic knowledge of HTS bulks for magnetic levitation.
2.4.1Sintering HTS bulk materials
A single crystal is important for characterizing materials properties. However, single
crystals do not have large transport current because the effective flux-pinning pro-
perties are only provided by defects and impurities; thus, they have no value for
engineering applications.
In the early stage of HTS bulk material development, HTS bulks were synthe-
sized by a sintering technique, which is a common ceramic processing route and
generally easy to apply. The sintering techniques have a severe weak-link problem
associated with grain boundaries; as a result, the techniques are most often employed
in the search for and development of new compounds. The sintering bulk process
includes the precursor powder preparation, pressing and shaping, melt casting, and
recrystallization. HTS bulk materials can be achieved by a more conventional sinte-
ring approach. In June 1987, scientists in Hefei Research Institute of Cryogenics and