High Temperature Superconducting Magnetic Levitation

26 Ë 2 Superconducting materials

(e.g. 20 K). The Bi-2212 wires have better transport properties than Bi-2223 at 4.2 K. First-
generation HTS wire was commercially available since the late 1990s. The commercial
long-length wires have reached 1000 km in 2003. Moreover, several power system
devices [38] have been developed. The commercial manufacturing technology for HTS
BSCCO tapes is listed in Tab. 2.2. [39]. Table 2.2 shows that the HTS BSCCO tapes
made in China have achieved a higher technical level, long length of 100–1000 m,
critical current (Ic) of 90 A at 77 K and 0 T, and engineering critical current density
(Je)>9 kA/cm^2 at 77 K and 0 T. However, the high cost of Ag-sheathed Bi-2223 tapes,
their lower irreversible fieldHirr, and their high AC losses are limiting factors for
power applications.
Therefore, even though the BSCCO has a higher upper critical field than YBCO, its
properties limit its use in making high-field magnets. Since HTS BSCCO wires exhibit
a poor pinning performance, they cannot be used for power devices even with liquid
nitrogen cooling. It is for this reason that coated conductors of YBCO are preferred
to BSCCO. The corresponding YBCO wires are referred to as second-generation (2G)
HTS wires. 2G HTS wire is fabricated by depositing rare earth, barium-copper-oxide
(REBCO) HTS materials layered onto flexible thin metal substrate tapes with buffering
metal oxides. The main advantages of 2G HTS wire [40–42] are high critical current
density, large-scale production, lower manufacturing cost, better mechanical proper-
ties, and better performance under high magnetic fields. The 2G HTS REBCO wires are
very attractive for various low- and high-field applications.
YBCO (yttrium barium copper oxide, YBa 2 Cu 3 O 7 −x) contains the proportions of
the 3 different metals in the mole ratio of 1 to 2 to 3 for yttrium to barium to
copper, respectively. YBCO research focuses on optimizing properties in relation to
applications of HTS bulk (to see next section) and tape materials. The powder-in-
tube process (PIT) is successful for manufacturing LTS Nb 3 Sn, MgB 2 and HTS Bi-
family superconductors. However it cannot be used for YBCO conductors. Thin films of
YBCO have demonstrated better superconducting properties for use at liquid nitrogen
temperatures with higher critical current densityJc. The manufacturing process of
the 2G HTS REBCO tapes is mainly concentrated on substrates, buffer layers, the
superconducting film over layer, and their complex interactions. Preparation of YBCO

Tab. 2.2:Technology level of the HTS BSCCO wires [39].

Nations Developer
companies

Wires

lengths (m)

Critical currents

currentIc(A) at

77 K, 0 T

Engineering critical

densitiesJe(kA/cm^2 )

at 77 K, 0 T

U. S. A. AMSC 100 – 400 115 > 12
China Innova 100 – 1000 90 > 9
Japan Sumitomo 100 – 400 100 > 10
Germany EAS 100 – 1000 70 > 7
Germany Trithor 100 – 1000 60 > 7