32 Ë 2 Superconducting materials
2.4.3Developments of HTS bulk materials
The rare-earth Ba-Cu-O (REBCO, RE=Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm and Yb etc.)
HTS bulks have high critical current densityJcand high trapped magnetic flux. The
maximum trapped magnetic field is approximately an order of magnitude higher than
that for the presently used conventional PMs. Early studies of HTS bulk focused on
YBCO. The critical current of HTS YBCO bulk materials had increased one thousand
times in a few years from 1987 to 1991 [82]. Since then, HTS bulk materials and their
applications have been considerable [79]. With a modified SIG process, the value of
Jcis up to 2.3× 105 A/cm^2 under zero field and 10^4 A/cm^2 up to 7 T at 77 K in YBCO
[83]. Muralidhar et al. [84, 85] in the Railway Technical Research Institute (RTRI)
reported achievement in the (Nd, Eu, Gd) BaCuO (NEG-123) system of superiorJcvalues
(9.25× 105 A/cm^2 at 65 K, 6.40× 105 A/cm^2 at 77 K, and even 1× 105 A/cm^2 at 90 K). A
trapped fieldBTnear 10 T may be possible at 77 K if proper chemical pinning centers
are created [86].
A breakthrough in the inexpensive growth of large amounts of high-quality YBCO
samples was made by the TSMG method [87]. In this TSMG technique, RE123 (Sm123
or Nd123) crystals with a higher melting temperature than Y123 were put on top of the
precursor pellet. The processing conditions for the fabrication of NdBCO and SmBCO
are considerably more complex than those for YBCO. A large number of single domain
REBCO samples of various shapes were prepared with this method [88–93].
The melt process for the RE (Nd, Sm, or Eu) BCO systems under controlled oxygen
partial pressure (pO 2 ) is known as the oxygen-controlled melt growth (OCMG) process
[94]. The OCMG-processed REBCO superconductors exhibit a largerJcin high magnetic
fields and a considerably improved irreversible fieldHirrat 77 K, implying that stronger
flux pinning can be realized in a commercially feasible way. The GdBCO sample
exhibited the highest zero-field critical current density of 8.8× 104 A/cm^2 at 77 K for
H⊥c, while the highest secondary peakJcof 6.5× 104 A/cm^2 was achieved in NdBCO at
1 T and 77 K forH‖c[95].
A critical current densityJcvalue above 10^5 A/cm^2 for melt processed YBCO
superconductors has been obtained especially under high magnetic fields. For an
external field parallel to botha-bplanes and the direction of the current flow, critical
current densities of 1.6× 104 A/cm^2 in self field and 1.1× 104 A/cm^2 under 1 T have been
obtained [96].
In the following years, many efforts have been made to increase the textured
domain size and trapped flux. A single bulk GdBCO sample 50 mm in diameter
exhibited a trapped field of 2.6 T at 77 K, with the Hall sensor in contact with the
surface [97].
A single bulk GdBCO sample 65 mm in diameter exhibited a trapped field of 3.05 T
at 77 K. One study measured the trapped field between two GdBCO bulks in order
to minimize the demagnetizing effect and found that it reached 4.3 T at 77 K [98].
Enlargement of the single-grain bulk REBCO materials is effective in improving their