Jia-Su Wang and Su-Yu Wang
2 Superconducting materials
Superconductors are divided into conventional and unconventional superconductors
and can also be divided into high- and low-temperature superconductors (HTSC
and LTSC, respectively). The HTSCs can be cooled with liquid nitrogen (77 K) in-
stead of liquid helium (4.2 K). This is commercially important because liquid ni-
trogen is much cheaper, which makes HTSCs particularly attractive for practical
applications.
The superconducting material can be produced in the forms of single crystals,
films, wires, and tapes. The most common LTSCs in applications are the Nb-Ti
(niobium-titanium) and the Nb 3 Sn (niobium-tin) wires and tapes. Superconducting
films are mainly used for electronic devices. In principle, for the HTSC materials, there
are bulk and wire materials for the electrical engineering applications. The HTSC bulks
are mainly used for the HTS Maglev train, Maglev bearings, Maglev flywheel energy
storage devices, and so on.
2.1 LTS materials
The development of superconducting materials has experienced a long tortuous path
from initial discovery to HTSC (Fig. 2.1). A LTSC material typically refers to the Nb-
based alloy (most commonly Nb-Ti) and A15 (Nb 3 Sn and Nb 3 Al etc.) superconductors
with lower critical temperatureTc. The most commonly conventional superconductors
used in applications are Nb-Ti (niobium-titanium) alloy and Nb 3 Sn (niobium-tin)
compounds [1]. The well-established Nb-Ti and Nb 3 Sn LTSCs have occupied a large
market share of commercial market of superconducting wires.
Long and flexible wires are required to build a high-field magnet. Although Nb 3 Sn
was found before Nb-Ti, it is a brittle material and hard to fabricate into wires. Despite
a lower transition temperature of 10 K and a lower upper critical fieldHc2of 14.5 T, the
Nb-Ti wire offers the significant advantage of flexibility and formability. Therefore,
magnets using Nb-Ti wires were the first commercially available superconducting
one in the 1960s. In 1962, the first commercial Nb-Ti alloy superconducting wire
was developed [3]. Consequently, the first practical superconducting magnet was
manufactured using the Nb-Ti wire. It opened a new epoch for the application of
superconducting magnets. Up to now, the Nb-Ti has achieved a critical current density
in excess of 10^6 A/cm^2 at 4.2 K and 5 T [4, 5].
In 1961, Kunzler et al. [6] reported that the compound Nb 3 Sn was capable of sustai-
ning enormous current densities without resistance in very high magnetic fields. Since
Nb 3 Sn has higher critical temperatureTc, higher critical current densityJc, and higher
critical fieldHc2, it is used for higher field applications. The maximum critical fields