2.3 HTS materials Ë 27tapes is achieved by coated film conductor processing technology. REBCO-coated
conductors are one of the most promising candidates for widespread applications
because of their higherJcvalues in external magnetic fields. TheJcof 2G tapes strongly
depends on the direction of the applied field, and it requires a near-perfect texture
to achieve a highJc. The 2G tapes have a magnetic anisotropy. The upper critical
magnetic fieldHc2⊥is about 120 T for fields perpendicular andHc2‖about 250 T for
fields parallel to the CuO 2 planes [43]. Li et al. [44] have investigated the influence
of deposition parameters on the orientation, in-plane texture, surface morphology,
and roughness of films at a tape speed of 20–50 m/h. Li et al., in Shanghai Jiao Tong
University, have achieved 2G YBCO tapes with 1 cm width and 100 meters length, and
a critical currentIc>500 A. [45] 2G HTS YBCO wires [46, 47] are now available in long
lengths of high quality suitable for practice applications. A HTS magnet using 2G HTS
wire was developed by Song et al. [48] in SuperPower Inc.
The irreversible fieldHirrfor the BSCCO superconductors is much lower than for
YBCO. One restriction on YBCO is the critical temperatureTcof only 92 K, which could
be an issue for applications. TlBaCaCu0 (Tl-1223) or HgBaCaCu0 (Hg-123) offer better
in field performance with a higher critical temperatureTcand irreversible fieldHirr.
The TlBaCuO (thallium barium copper oxide) HTSC, critical temperatureTcof
90 K, was discovered by Sheng et al. [49] in 1988. Shortly later, this group succeeded
in preparing TlBaCaCuO superconductors withTc=120 K [50]. The thallium-based
superconductor family is large and includes Tl-1223, Tl-2223, Tl-1212, Tl-2212, and Tl-
2223 compounds. It holds the record for the highest critical temperatureTcof 125 K.
These compounds are very interesting for applications. Tl-2223 not only has a high
critical temperatureTc, but also has a high irreversibility field (atTcof 120 K). In
addition, both Tl-1212 and Tl-2212 have low microwave surface resistance. Therefore,
they are used widely in microwave passive devices.
Superconductivity in mercury barium copper oxide (HgBaCuO) was discovered by
Putilin et al. in 1993. [51]. Shortly thereafter, the mercury barium calcium copper oxide
(HgBaCaCuO) superconductor with aTcof 135 K (Tcof 150 K under high pressures,
around 30 GPa) was discovered by Schilling et al. [52] and Chu et al. [53] The HTS Hg-
1223 not only exhibits the highestTcand high irreversibility fieldsHirr, but also shows
remarkable critical current densities above 77 K. This is more important for power
system applications.
In addition to the above HTSCs, a new HTS LaFeAsO family, iron-based super-
conductors [54, 55], was discovered in 2006 in Tokyo Institute of Technology. This
new type of superconductor is based on conducting layers of iron and pnictide.
It is a chemical compound which contains iron layers and the copper elements.
In 2008, superconductivity of the analogous material LaFFeAsO was found at 43 K
[56, 57]. Soon after, the discovery, the superconducting transition temperatureTc
was raised up to 55 K by replacing La with other rare earth (RE) elements in the
Institute of Physics, China Academy of Sciences [58, 59]. The following researches from
other groups suggest that, by replacing the lanthanum in LaO 1 −xFxFeAs with other