2.3 HTS materials Ë 25cheap and easily handled liquid nitrogen as a cryogen has increased the range of
practical applications of superconductivity.
The families of HTS materials include YBaCuO, BiSrCaCuO, TlBaCaCuO, HgBaCuO,
HgBaCaCuO, etc. Among them, only the bismuth strontium calcium copper oxide
(BSCCO) and the yttrium barium copper oxide (YBCO) are commercially available. In
general, HTSC can be fabricated in the shape of single crystal, bulk, thin film, and
wires or tapes. The forms used for electrical engineering applications include bulk
and wire materials. Wires and tapes have the potential to be used in magnet, power
device, and transportation as a more efficient and more compact replacement for
copper technology. Thin films have the potential for use in electronics. Bulk materials
have the potential to generate large trapped magnetic fluxes that are much greater
than those achievable in conventional PMs. HTS bulk is emphatically introduced in
the book of HTS Maglev. Melt-textured rare-earth Ba-Cu-O (REBCO, RE=Y, Nd, Sm, Eu,
Gd etc) bulk has high critical current density and high critical magnetic field, which
can produce strong and stable levitation. However, the critical current density of the
bulk is two orders of magnitude lower than that of thin films. It is necessary to improve
further the properties of HTS bulk.
In 1986, Bednorz and Müller [28] discovered superconductivity in a lanthanum-
based cuprate perovskite material (La 2 −xBaxCuO 4 ). Soon after, in January of 1987,
Chu, Wu, et al. [29] discovered ceramic yttrium barium copper oxide (YBa 2 Cu 3 O)
superconductors with a critical temperature above the boiling temperature of liquid
nitrogen. Meanwhile, Zhao, Chen, et al. [30] prepared YBa 2 Cu 3 O in February of 1987.
Thereafter, the research on HTS materials and their applications surged rapidly all
over the world.
BiSrCaCuO (BSCCO) is a new class of superconductors. Michel et al. [31] first
discovered superconductivity in Bi-2201 in 1987; shortly afterwards, the superconduc-
tivity of Bi-2212 [32] and Bi-2223 [33, 34] were discovered. The BSCCO system has three
phases, Bi-2201, Bi-2212, and Bi-2223, with transition temperatures of 20, 85, and 110 K,
respectively. The BSCCO family is analogous to the thallium family (TBCCO) and the
mercury family (HBCCO), which are all HTSCs.
BSCCO was the first HTSC material to be used to make practical superconducting
wires. Therefore, it is referred to as the first-generation (1G) HTS wires. Bi-2212 [35] wire
has many features, such as isotropic electromagnetic performance, filament structure,
compatibility with conventional cabling and braided strand insulation methods. The
upper critical fieldHc2in Bi-2212 polycrystalline samples at 4.2 K has been measured
as 200±25 T [36]. Bi-2212 is one of the most promising materials for superconducting
magnets which can generate a magnetic field above 25 T [37]. The irreversible fieldHirr
(id.H 3 ) is an important parameter for applications, since in practical applications, the
HTSCs are limited by the irreversible fieldHirr, above which the magnetic vortices melt
or decouple. To use Bi-2223 tapes in magnetic fields, the irreversible region lies below
2 T (at below 50 K); for Bi-2212 in the same field, it lies below 30 K. In order to produce
higher magnetic fields, the magnets have to be operated at very low temperatures