22 Ë 2 Superconducting materials
Fig. 2.1:Critical temperatures of superconductors from LTSC to HTSC [2].
Hc2are 14.5 T (Nb-Ti) and 30 T (Nb 3 Sn), respectively [7, 8]. These superconducting
wires are widely used in magnetic resonance imaging (MRI) and nuclear magnetic
resonance (NMR), particle accelerators, magnetic separation, transformers, energy
storage devices, electric power cables, fault current limiters, electric motors or linear
motors, Maglev trains, etc. Since LTS devices which were cooled with liquid helium
turned out to be uneconomic, the magnet technology [9, 10], especially magnets for
particle accelerators and magnetic resonance imaging (MRI), are the only commercial
applications for those superconductors at present.
Magnet applications mainly depend on the critical current density at a certain
temperature and field conditions. It can be seen from Fig. 2.2 that the critical current
density [12] of Nb-Ti is lower than that of Nb 3 Sn. The performance of Nb 3 Sn has been
improved significantly over that of Nb-Ti. Although Nb-Ti is the most commonly used
superconducting material, its upper critical field is lower than Nb 3 Sn (8 T at 4.2 K), i.e.
Nb 3 Sn can generate a magnetic field higher than 12 T (4.2 K). However, due to stress
and strain, especially under transverse compression, the degradation inJcrestricts
the use of Nb 3 Sn in large-scale applications. In practical engineering, the selection of
superconducting materials is mainly based on the price. So far, the price of Nb-Ti wires
is cheaper than that of Nb 3 Sn.
The Nb 3 Al superconductor has outstanding features of high critical field and
excellent strain tolerance in critical current performance. The test results demons-
trate that the Nb 3 Al conductor is suitable for applications in high field magnets.
So far, the critical current densityJcof Nb 3 Al superconducting wire has achieved
9 × 104 A/cm^2. (20% higher than that of conventional wires) at 4.2 K and 15 T [13].