2.1　LTS materials

The development of superconducting materials has experienced a long tortuous path from initial discovery to HTSC（Fig. 2.1.1）.LTSC material typically refers to the Nb-based alloy（most commonly Nb-Ti）and A15（Nb3Sn and Nb3 Al etc.）superconductors with lower critical temperature Tc .The most commonly conven tional superconductors used in applications are Nb-Ti（niobium-titanium）alloy and Nb3Sn（niobium-tin）compounds1.The well-established Nb-Ti and Nb3Sn 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 Nb3Sn 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 field Hc2 of 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 developed3.Consequently, the first practical su perconducting 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 106A/cm2at 4.2 K and 5 T4，5.

Fig.2.1.1　Critical temperatures of superconductors from LTSC to HTSC2.

In 1961，Kunzler et al.6reported that the compound Nb3Sn was capable of sustaining enormous current densities without resistance in very high magnetic fields. Since Nb3Sn has higher critical temperature Tc , higher critical current den sity Jc , and higher critical field Hc2 ，it is used for higher field applications.The maximum critical fields Hc2 are 14.5 T（Nb-Ti）and 30 T（Nb3Sn），respectively7，8.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 technology9，10，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 cer tain temperature and field conditions. It can be seen from Fig.2.1.2，that the critical current density12of Nb-Ti is lower than that of Nb3Sn.The performance of Nb3Sn 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 Nb3Sn（8 T at 4.2 K），i.e.Nb3Sn can generate a magnetic field higher than 12 T（4.2 K）.However, due to the stress and the strain, especially under trans-verse compression, the degradation in Jc restricts the use of Nb3Sn 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 Nb3Sn.

Fig.2.1.2　Critical current density of Nb-Ti, at 4.2 K and 1.9 K, compared with that of Nb3Sn at 4.2 K11.

The Nb3 Al superconductor has the outstanding features of a high critical field and an excellent strain tolerance in critical current performance. The test results demonstrate that the Nb3 Al conductor is suitable for applications in high field magnets.So far, the critical current density Jc of Nb3 Al superconducting wire has achieved 9×104A/cm2（20% higher than that of conventional wires）at 4.2 K and15 T13.This demonstrates that Nb3 Al superconducting wires can be used in the accelerator magnets.

In 2001，Nagamatsu et al. 14 discovered superconductivity in a magnesium di-boride（MgB2 ），which had a transition temperature of 39 K.This is the highest critical temperature so far achieved in a conventional superconductor.The mate-rial has attracted considerable interest because of its potential for low cost, higher stability and relatively simple deposition techniques, which enable MgB2 to be comparative with Nb-Ti in the future.The operating temperature of the MgB2 can be kept around 20 K by a cryocooler, but Nb-Ti requires 4.2 K and even lower.However, in comparison to standard Nb-Ti, MgB2 has some disadvantages in the current density, mechanics and uniformity.

The properties of major LTSC materials are shown in Table 2. 1.1.

Table 2. 1.1 characteristic parameters of LTSC materials15，16，17，18

HTSC wires of Bi-2212 or YBCO are interesting for magnetic fields above 16 T, since this field is barely achievable with Nb3Sn at 1. 9 K.The limits of the upper critical field Hc2 of Nb3Sn drive the development of HTSC materials with higher Hc2 values and greater critical current density（Jc ）for ultra-high field applications.