30 Ë 2 Superconducting materials
Electronics Technology successfully prepared large bulk HTS samples using the sinte-
ring technique. The critical temperature of a sintered YBCO bulk sample with diameter
43 mm and thickness 2 mm was 86 K [64]. At the same time, the microwave surface
resistance of this large YBCO bulk was measured. The results were not published,
because the measured results were poor. Thereafter, the other group published results
are also poor, because the sintered bulks have no good surface of the conductive
properties and have severe weak link.
To compare with the conventional copper leads, HTS current leads can greatly re-
duce the heat leakage to the superconducting magnets operating at low temperatures.
This is because the thermal conductivity of the HTS is two orders of magnitude smaller
than that of copper. HTS bulks composed entirely of superconducting materials do
not have metallic sheaths of high thermal conductivity as in HTS wires. Thus, the
heat leakage of bulk HTS current leads is expected to be low. According to different
application needs, the HTS bulk-sintered materials can be processed into various
shapes (bars, tubes, plate, etc.). Both bulk-sintered BSCCO [65] and YBCO [66] can be
made for current leads. The sintering technique is an important technology to produce
BSCCO bars that can be used for current leads to supply power for superconducting
magnets. Ac-axis-oriented sintered BSCCO bulk can achieved aJcof 2.3× 104 A/cm^2
at 20 K, 1 kOe) [67]. The heat leakage of the commercial Bi-based HTS current leads is
a tenth that of copper current leads. In comparison to conventional copper leads, the
thermal load and consumption of liquid helium were greatly reduced. Sintering tech-
niques were also employed to fabricate tubes to shield external fields for the medical
diagnosis of the human brain or heart [68]. Nishikubo et al. [69] have developed a RF
shield using a HTS bulk. Rabbers et al. [70] have demonstrated the shielding of a DC
magnetic field at 4.2 K, in fields up to 2 T, with a thick LHT MgB 2 bulk cylinder, 70 mm
long and 18 mm bore.
A further improvement of HTS current leads is possible by replacing the Bi-based
conductor with a Y-based one, since the HTS YBCO bulk, especially the melt texture
growth (MTG) YBCO, can increase the current capacity and the reliability in magnetic
fields. Thus, the MTG bulk HTS YBCO can be used for current leads. Large single-
domain materials can shield higher fields. For instance, the shielding factor of a tube
with 5-mm single-domain YBCO exceeds the value for sintered YBCO by one order of
magnitude in the low-frequency range (1–200 Hz) [71]. Endoh et al. [66] has prepared
a 500-A class HTS current lead package using the YBCO rod whose size is휙 3 ×30 mm,
with the heat leakage under typical practical conditions, from 80 K hot end to 20 K
cold end, being 163 mW. Ohsemochi et al. [72] has successfully achieved a HTS bulk
current lead system with a rated current of 3000 A.
2.4.2Melt process HTS bulk materials
The energy density of magnetized HTS bulks at 77 K is greater than that with HTS
wires and tapes, since HTS bulks are capable of supporting a large persistent eddy