2.3　HTS materials

High-temperature superconductors（HTSCs）constitute a large family of super-conducting ceramic materials. They offer the highest transition temperatures of all superconductors.The ability to use relatively cheap and easily handled liquid nitrogen as a cryogen has increased the range of practical applications of supercon-ductivity.

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 commer-cially 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 ap-plications 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 levi-tation.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 Müller28discovered superconductivity in a lanthanum-based cuprate perovskite material（La2−x Bax CuO4 ）.Soon after, in January of 1987，Chu and Wu et al.29discovered ceramic yttrium barium copper oxide（YBa2 Cu3 O）superconductors with a critical temperature above the boiling temperature of liquid nitrogen.Meanwhile, Zhao, Chen, et al.30prepared YBa2 Cu3 O in Febru-ary 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.31firstly discovered superconductivity in Bi-2201 in 1987，shortly afterwards the supercon-ductivity of Bi-221232and Bi-222333，34were discovered.The BSCCO system has three phases, Bi-2201，Bi-2212 and Bi-2223，with transition temperatures of 20 K，85 K and 110 K, respectively.The BSCCO family is analogous to the thallium fam-ily（TBCCO）and the mercury family（HBCCO），which are all high-temperature superconductors.

BSCCO was the first HTSC material to be used to make practical supercon-ducting wires. Therefore, it is referred to as the first-generation（1G）HTS wires.Bi-221235wire has many features, such as isotropic electromagnetic performance, filament structure, compatibility with conventional cabling and braided strand in sulation methods.The upper critical field Hc2 in Bi-2212 polycrystalline samples at 4.2 K has been measured as 200±25 T36.Bi-2212 is one of the most promising materials for superconducting magnets which can generate a magnetic field above 25 T37.The irreversible field Hirr （id.H3 ）is an important parameter for appli-cations, since in practical applications, the HTSCs are limited by the irreversible field Hirr ，above which the magnetic vortices melt or decouple.For use of Bi-2223 tapes work 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（e.g.20 K）.The Bi-2212 wires have better transport properties than Bi-2223 at 4.2K.First generation HTS wire was commercially available since the late 1990s.The com-mercial long length wires have reached 1000 km in 2003.Moreover, several power system devices38have been developed.The commercial manufacturing technology for HTS BSCCO tapes is listed in Table 2.3.139.Table 2.3.1 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 ）of＞9 kA.cm−2at 77 K and 0 T.However, the high cost of Ag sheathed Bi-2223 tapes, their lower irreversible field Hirr ，and their high ac losses are limiting factors for power applications.

Therefore, even though the BSCCO has a higher upper critical field than YBCO, but 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 ofYBCO 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 wire40，41，42are high critical current density, large scale production, lower manufacturing cost, better mechanical properties, and better performance under high magnetic fields.The 2G HTS REBCO wires are very attractive for various low field and high field applications.

Table 2. 3.1 Technology level of the HTS BSCCO wires39

YBCO（yttrium barium copper oxide, YBa2 Cu3 O7−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 Nb3Sn, MgB2 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 density Jc .The manufac turing 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 tapes is achieved by coated film conductor processing tech nology.REBCO coated conductors are one of the most promising candidates forwidespread applications because of their higher Jc values in external magnetic fields.The Jc of 2G tapes strongly depends on the direction of the applied field, and it requires a near-perfect texture to achieve a high Jc .The 2G tapes have a magnetic anisotropy.The upper critical magnetic field Hc2⊥ is about 120 T for fields perpendicular and Hc2 for fields parallel to the CuO2 planes is about 250 T43.Li et al.have investigated the influence of deposition parameters on the ori entation, in-plane texture, surface morphology and roughness of films at a tape speed of 20-50 m/h44.Li et al.in Shanghai Jiao Tong University have achieved 2G YBCO tapes with 1 cm width and 100 meters length, and a critical current Ic ＞500 A45.2G HTS YBCO wires46，47are now available in long lengths of high quality suitable for practice applications.A HTS magnet using 2G HTS wire was developed by Song et al.48in SuperPower Inc.

The irreversible field Hirr for the BSCCO superconductors is much lower than for YBCO. One restriction on YBCO is the critical temperature Tc of only 92 K, which could be an issue for applications.TlBaCaCu0（Tl-1223）or HgBaCaCu0（Hg-123）offer better in field performance with a higher critical temperature Tc and irreversible field Hirr .

The TlBaCuO（thallium barium copper oxide）HTSC, critical temperature Tc of 90 K, was discovered by Sheng et al.49in 1988. A short time later this group succeeded in preparing TlBaCaCuO superconductors with Tc =120 K50.The thallium based superconductor family is large and includes Tl-1223，Tl-2223，Tl-1212，Tl-2212，and Tl-2223 compounds.It holds the record for the highest criti-cal temperature Tc of 125 K.These compounds are very interesting for applica-tions.Not only Tl-2223 has a high critical temperature Tc ，but also Tl-1223（Tc of 120 K）has a high irreversibility field.In addition, both Tl-1212 and Tl-2212 have low microwave surface resistance.Therefore they are used widely in microwave passive devices.

Superconductivity in mercury barium copper oxide（HgBaCuO）was discovered by Putilin et al. in 199351.Shortly thereafter, the mercury barium calcium copper oxide（HgBaCaCuO）superconductor with a Tc of 135 K（Tc of 150 K under high pressures, around 30 GPa），was discovered by Schilling et al.52and Chu et al.53The HTS Hg-1223 not only exhibits the highest Tc , and high irreversibility fields Hirr ，but also shows remarkable critical current densities above 77K.This is more important for power system applications.

In addition to above HTSCs, a new HTS LaFeAsO family, iron-based supercon ductors54，55，was discovered in 2006 in the Tokyo Institute of Technology. This new type of superconductor is based on conducting layers of iron and pnictide.It is a chemical compound which contains iron layers and the copper elements.In 2008，superconductivity of the analogous material LaFFeAsO was found at 43 K56，57.Soon after the discovery, the superconducting transition temperature Tc was raised up to 55 K by replacing La with other rare earth（RE）elements in the Institute of Physics, China Academy of Sciences58，59.The following researches from other groups suggest that, by replacing the lanthanum in LaO1−x Fx FeAs with other rare earth elements such as cerium, samarium, neodymium and praseodymium, superconductivity can also be found.In addition to higher critical temperature, Tc ，the iron-based superconductors also have very low anisotropy, very high critical magnetic field, Hc2 ，and large critical current density, Jc .It is very suitable for high magnetic field application.For the engineering application of 20 K temper ature region, the iron-based superconductors may be the better choice than Nb Ti and Nb3Sn.Recent progress about iron-based superconductors can be found elsewhere60.

The properties of main HTSC materials are shown in Table 2. 3.2.