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Perspective
Superconducting materials: Challenges and
opportunities for large-scale applications
Chao Yao1,2 and Yanwei Ma1,2,*

SUMMARY
Superconducting materials hold great potential to bring radical changes for elec-
tric power and high-field magnet technology, enabling high-efficiency electric
power generation, high-capacity loss-less electric power transmission, small light-
weight electrical equipment, high-speed maglev transportation, ultra-strong
magnetic field generation for high-resolution magnetic resonance imaging
(MRI) systems, nuclear magnetic resonance (NMR) systems, future advanced
high energy particle accelerators, nuclear fusion reactors, and so on. The perfor-
mance, economy, and operating parameters (temperatures and magnetic fields)
of these applications strongly depend on the electromagnetic and mechanical
properties, as well as the manufacturing and material cost of superconductors.
This perspective examines the basic properties relevant to practical applications
and key issues of wire fabrication for practical superconducting materials, and de-
scribes their challenges and current state in practical applications. Finally, future
perspectives for their opportunities and development in the applications of
superconducting power and magnetic technologies are considered.

INTRODUCTION
For many metals and compounds, when cooled to a sufficiently low temperature, their resistivity suddenly
drops to zero. This phenomenon, known as superconductivity, was first observed by Dutch physicist Heike
Kamerlingh Onnes. In 1908, Kamerlingh Onnes succeeded in liquefying helium at a temperature of 4.2 K,
and then in 1911, when he measured the low-temperature resistivity of metals, he found the superconduc-
tivity of mercury at 4.2 K (Kamerlingh Onnes, 1911). After discovering the zero resistance of the supercon-
ductor, in 1933, German physicists W. Meissner and R. Ochsenfeld found that if a superconductor was
cooled below the transition temperature Tc in the magnetic field, the magnetic field would be completely
ejected from the superconductor. This phenomenon is called the Meissner effect (Meissner and Ochsen-
feld, 1933), which is another essential characteristic of superconductivity. After that, researchers observed
superconductivity in many other substances, and some of them have higher superconducting transition
temperatures. At the same time, due to the exotic nature of superconductors, people have also carried
out extensive research of their practical applications. Zero resistance and high current density have a pro-
found impact on electrical power transmission and also enable much smaller and more powerful magnets
for motors, generators, energy storage, medical equipment, industrial separations, and scientific research,
while the magnetic field exclusion provides a mechanism for superconducting magnetic levitation, as
shown in Figure 1. Owning to the different operating temperature ranges and required magnetic fields,
and also the cooling approaches and material properties, currently the industrial applications of supercon-
ductors can be categorized into applications such as power cables, fault current limiters, transformers, and
induction heaters at 65-77 K with liquid nitrogen as coolant and field <1 T, applications such as motors, 1Key Laboratory of Applied
generators, maglev, energy storage devices, magnetic resonance imaging (MRI) systems and magnetic Superconductivity, Institute
separations at temperatures below 50 K and fields above 1 T, and high-field magnets (>10 T) for fusion, of Electrical Engineering,
Chinese Academy of
accelerator, high-field MRI, nuclear magnetic resonance (NMR), and scientific research at low-temperature Sciences, Beijing 100190,
regions (usually at 4.2 K using liquid helium as coolant). China
2Universityof Chinese
Since the discovery of superconductivity in mercury, lots of superconducting materials have been found. Ac- Academy of Sciences,
Beijing100049, China
cording to their constituents and structures, superconducting materials can be divided into several categories:
*Correspondence:
1) Metallic materials (Rogalla and Kes, 2012), which include pure metals (mercury, lead, niobium, etc.), alloys ywma@mail.iee.ac.cn
(such as Nb-Ti and Nb-Ge), and intermetallic compounds (such as Nb3Sn, Nb3Al, and MgB2); 2) ceramic com- https://doi.org/10.1016/j.isci.
pounds, including Chevrel compounds (e.g., PbMo6S8 and SnMo6S8) (Chevrel et al., 1971), copper-based oxides 2021.102541

iScience 24, 102541, June 25, 2021 ª 2021 The Author(s). 1

This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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Areas of superconducting applications

Science
Medical Accelerator NMR
Transportation
Industrial
Energy
SMES MRI Fusion High-field magnet

Si-crystal growth Magnetic separation Maglev Electric aircraft

Induction heaters SFCL Motors Generators

Cables Transformers

Figure 1. Main applications of superconducting power and magnetic technologies with their typical operating
magnetic fields
The areas of each application are marked by colored frames.

(Rogalla and Kes, 2012), ruthenium-based oxides (Maeno et al., 1994), and iron-based pnictides and chalcogen-
ides (Hosono et al., 2015); 3) organic materials (e.g., K3C60, Cs3C60, and Ba4C60) (Rosseinsky et al., 1993; Palstra
et al., 1995; Baenitz et al., 1995); 4) semiconductor, semi-metal, and insulators (e.g., SiC, diamond, and graphite)
(Ekimov et al., 2004; Muranaka et al., 2008; Cao et al., 2018). In the early research for superconductors, it was
found that the superconducting state is not only related to the temperature, but also to the external magnetic
field and the current in the superconductor. When the magnetic field applied to the superconductor is larger
than a certain critical value Hc, the superconducting state will be destroyed. When the current passing through
a superconductor is higher than a critical current Ic, the superconducting state will also be destroyed, even if the
external magnetic field is not applied. Therefore, the applicable range of superconducting materials is primarily
limited by these three parameters. So far, though thousands of superconductors have been discovered, the
ones with practical value are limited to Nb-Ti, Nb3Sn, copper-based oxide superconductors, MgB2, and iron-
based superconductors, as summarized in Table 1.

During the years from 1911 to 1932, low-temperature superconductors (LTS) such as lead, tin, niobium, and
other metals were found to be superconductors, and among them niobium has the highest Tc of 9.2 K. In
the following decades, many alloys and carbon- and nitrogen-based compounds with superconductivity
were discovered. Among these superconducting alloys and intermetallic compounds, Nb-Ti and Nb3Sn re-
ported in 1961 and 1954, respectively, are the most promising ones for practical applications, with a Tc of
9.5 K and 18.1 K, respectively. At 4.2 K, Nb-Ti and Nb3Sn have an upper critical field of 11 T and 25 T, respec-
tively. Both of them have current densities over 105 A/cm2, which are about 2 orders of magnitude higher
than that of copper conductors. Therefore, Nb-Ti and Nb3Sn enabled the construction of superconducting
magnets that can generate much higher magnetic fields than conventional resistive magnets.

In 1986, J. Bednorz and K. Muller discovered LaBaCuO superconductors with a Tc of 35 K, which opened the
gate of searching for high-temperature superconductors (HTS) (Bednorz and Muller, 1986), as shown in Figure 2.
In 1987, the Tc in this system was rapidly increased above the liquid nitrogen temperature (77 K) for the first time
because of the discovery of YBa2Cu3Ox (YBCO or REBCO, RE = Rare Earth) superconductors with Tcs up to 93 K

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Table 1. Basic material and critical current density relevant parameters for practical superconductors and their
wire fabrication technology and typical forms at present

Tc, max Hc2, 4.2 K Jc, 4.2k Coherence Anisotropy Wire Typical
Material (K) (T) (A/cm2) length εab (nm) gH technology wire forms
Nb-Ti 9.5 11.5 43105 (5 T) 4 Negligible \ round wire
Nb3Sn 18 25 ~106 3 Negligible bronze process round wire
internal Sn process
powder-in-tube
MgB2 39 18 ~106 6.5 2–2.7 powder-in-tube wire or tape
internal Mg diffusion
REBCO 92 >100 ~107 1.5 5–7 coated conductor flat tape
Bi-2223 108 >100 ~106 1.5 50–90 powder-in-tube flat tape
Bi-2212 90 powder-in-tube wire or tape
1111 IBS 55 >100 ~106 1.8–2.3 4–5 powder-in-tube wire or tape
6
122 IBS 38 >80 ~10 1.5–2.4 1.5–2 powder-in-tube wire or tape
11 IBS 16 >40 ~105 1.2 1.1–1.9 powder-in-tube wire or tape

(Zhao et al., 1987; Wu et al., 1987). Then bismuth-based cuprate superconductors (BSCCO) including Bi2Sr2Ca-
Cu2O8 (Bi-2212) and Bi2Sr2Ca2Cu3O10 (Bi-2223) with Tcs up to 110 K were discovered (Michel et al., 1987). As
presented in Figure 3, regarding the operating temperatures and magnetic fields, Bi-2223 and REBCO can carry
large supercurrents up to 30-50 K in field and at 77 K in self-field, so they are promising not only for high-field
magnets operated in low or moderate temperature regions but also for electro-technical applications with the
much cheaper liquid nitrogen as coolant. On the other hand, though Bi-2212 can be used only at low temper-
atures (<20 K), it has its own advantages for high-field applications. In 2001, the superconductivity at 39 K in
MgB2 was discovered by the Akimitsu group in Aoyama-Gakuin University (Nagamatsu et al., 2001). This tran-
sition temperature is the highest so far for the bulk binary intermetallic compounds. MgB2 is promising for ap-
plications at around 20 K that can be easily achieved by liquid hydrogen or cryocoolers, and is considered to
replace traditional LTS such as Nb-Ti and Nb3Sn used in liquid helium. Moreover, the abundant raw materials
and light weight of MgB2 also make it attractive for large-scale practical applications. In 2008, the discovery of
iron-based superconductors (Kamihara et al., 2008) by the Hosono group in the Tokyo Institute of Technology
marked the coming of the ‘iron age’ of high-Tc superconductivity after the ‘copper age’ marked by cuprate su-
perconductors. According to different chemical compositions and crystal structures, iron-based superconduc-
tors can be categorized into several types, such as ‘1111’ type (e.g., LaFeAsO1-xFx and SmFeAsO1-xFx), ‘122’ type
(e.g., Ba1-xKxFe2As2 and Sr1-xKxFe2As2), ‘111’ type (e.g., LiFeAs) and ‘11’ type (e.g., FeSe, FeSe1-xTex). Similar to

300 Room temperature

C-H-S (267 GPa)

Metallic superconductors
250 LaH10 (170 GPa)
Transition temperature (K)

Cuprate superconductors
Iron-based superconductors
MgB2
H3S (155 GPa)
200 Hydride superconductors

HgBa2Ca2Cu3O9- (31 GPa)

150
Tl2Ba2Ca2Cu3O10+ HgBa2Ca2Cu3O9-
Bi2Sr2Ca2Cu3O10+
100
LN2 (77 K) YBa2Cu3O7-
FeSe (film)
50 (La,Ba)2CuO4 MgB2 SmFeAs(O,F)
LH2 (20.3 K) Nb NbN Nb3Sn Nb3Ge (Ba,K)Fe2As2
Pb LaFeAs(O,F)
Hg
0
1900 1920 1940 1960 1980 2000 2020

Year

Figure 2. Evolution of high temperature superconductivity over time

The boiling temperatures of coolants such as liquid nitrogen (LN2) and liquid hydrogen (LH2) are marked by dashed lines.

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LHe LH2 Cryocoolers LN2

50

FeSeTe Ba-122 YBCO

40
Magnetic field (T)

MgB2 Bi-2212

30

20
Nb3Sn

Bi-2223
10

Nb-Ti
0
0 20 40 60 80 100
Temperature (K)

Figure 3. Comparative H-T phase diagram for representative cuprates, iron-based superconductors, MgB2 and
conventional superconductors
The solid and dashed lines show, respectively, upper critical field Hc2(T) and irreversibility field H*(T) parallel to the c-axis.
For anisotropic superconductors, the Jc(H) vanishes at H*(T), which can be much smaller than Hc2(T). The data are
collected from (Gurevich, 2014).(LHe: liquid helium; LN2: liquid nitrogen, LH2: liquid hydrogen).

cuprate superconductors, iron-based superconductors have layered crystal structure and small coherence
lengths, showing high upper critical fields and electromagnetic anisotropy. Though the Tcs of iron-based super-
conductors (up to 38 K for ‘122’ system and 56 K for ‘1111’ system) are not as high as that of cuprate supercon-
ductors, their anisotropy is remarkably lower, especially for ‘122’ and ‘11’ systems. The high upper critical fields
and low anisotropy make iron-based superconductors quite attractive for high-field applications that can work
at liquid helium temperature and also in a moderate temperature range around 20 K.

Very recently, room temperature superconductivity, which had always been a dream of researchers over the
past 100 years, was reported in a carbonaceous sulfur hydride with a critical temperature up to 287.7 K
(15 C) under an extremely high pressure of 267 GPa (Snider et al., 2020), as shown in Figure 2. However,
there is still doubt whether this result belongs to a novel kind of superconductivity different from the stan-
dard conventional and unconventional superconductivity, or was misinterpreted as superconductivity
(Hirsch and Marsiglio, 2021). From the viewpoint of practical applications, the operation at such high pres-
sures is much more difficult than that at low temperatures. Nevertheless, this result will encourage the
exploration for practical roomtemperature superconducting materials in the future.

MATERIAL CHALLENGES AND CURRENT STATE OF PRACTICAL APPLICATIONS

For high current and/or high magnetic field applications, superconductors must be made into composite
wires for cabling or coil winding. Except for large current-carrying capacity (indexed by critical current
density Jc, for which 105 A/cm2 at the operating temperature and magnetic field is widely accepted as
the threshold for practical applications), superconducting wires are required to have high mechanical
strength to withstand electromagnetic and thermal stress during operation, fine superconducting fila-
ments in metal matrix to protect against flux jumps, and thermal quenching and sufficient length of hun-
dred/thousand meters for large-scale use (Larbalestier et al., 2001a). As shown in Figure 4, nowadays LTS
such as Ni-Ti and Nb3Sn have been fully commercialized and widely applied to the electric power indus-
try. The first generation HTS (Bi-2223 and Bi-2212) are on the stage of medium-term commercialization,
with which many commercial demonstration projects have been completed. In the recent years, the sec-
ond generation HTS (REBCO) has undergone rapid development, and entered the stage of early
commercialization. MgB2 wires produced with powder-in-tube (PIT) method are also in the early stages
of commercialization, while the ones made with internal Mg diffusion method are still being researched
in labs. For iron-based superconductors, which are still under laboratory research and development,
rapid progress has been made for 122-type wires, and so far some attempts for experimental demonstra-
tions have been made.

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fully commercialized early stage of commercializa on

Nb-Ti Nb3Sn MgB2

medium-term commercializa on
Bi-2223 Bi-2212

early stage of commercializa on laboratory research

REBCO IBS

Figure 4. Illustration and conductor forms for practical superconducting materials

Typical images of the cross-sections of wires and tapes and the development status are presnted for practical
superconducting materials including niobium-based superconductors (Nb-Ti and Nb3Sn) (Lee, 2018), copper-based
oxides (Bi-2223 (Osabe et al., 2019), Bi-2212 (Jiang et al., 2019), REBCO (Zhang, 2019; Lee, 2018)), MgB2 (Li et al., 2013;
Braccini et al., 2007), and iron-based superconductors (IBS) (Yao et al., 2015).

1. Nb-Ti and Nb3Sn superconductors

Owing to its nice ductility, Nb-Ti alloy can be directly deformed into long wires from monofilament billets
with a copper matrix and multifilament billet made by assembling tens of thousands of monofilament wires.
Soon after its discovery, Nb-Ti superconducting wires were fully industrialized in 1968 and widely used in
practical applications. It should be noted that currently Nb-Ti alloy is still the cheapest practical supercon-
ducting material for applications in the liquid helium temperature region, because the raw materials and
manufacturing costs are much lower than other superconducting materials. On the other hand, due to
the brittleness of materials, Nb3Sn cannot be directly deformed into wires as done for Nb-Ti alloy. Several
types of processes can be used for wire fabrication, including the bronze process (started with Nb alloy rods
clad in ductile Cu-Sn bronze and then assembled inside another Cu-Sn tube to obtain multifilament struc-
ture), internal Sn process (started with placing a Sn core in the center of an Nb-filament embedded Cu ma-
trix), and PIT process (started with packing Sn-rich Nb-Sn powders such as Nb6Sn5 and NbSn2 in Nb tubes).
By heat treatment for cold-worked wires, the Nb3Sn superconducting phase can be obtained by reaction
between Sn and Nb. The key to improve the non-Cu Jc of Nb3Sn wires is the enhancement of their pinning
capacity. Since the grain boundary pinning in Nb3Sn is neither as efficient nor as dense as a-Ti pinning in
Nb-Ti alloy, grain refinement and artificial pinning centers are effective for flux pinning (Balachandran et al.,
2020). Due to the increased complexity of wire manufacture, the commercialization of Nb3Sn supercon-
ducting wires was realized after 1970, with a relatively higher price than that of Nb-Ti superconductors.

Since the 1960s, Nb-Ti and Nb3Sn superconductors have greatly promoted the development of supercon-
ducting magnets and thus stimulated the industry for superconducting materials and technologies. Nb-Ti
superconductors are usually used to manufacture superconducting magnets that generate magnetic fields

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up to 9 T at 4.2 K or 11 T at 1.8 K. At present, Nb-Ti superconducting wires are mainly used in the construc-
tion of MRI systems, superconducting magnets for laboratories, magnetic levitation trains, and so on, with a
consumption of about several thousands of tons each year. For Nb3Sn superconductors, their niche in the
market is high-field applications beyond the capability of Nb-Ti and in the range up to 23 T. The important
application areas of Nb3Sn superconductors include MRI systems, NMR devices, particle accelerators,
tokamak fusion devices, and high-field magnets for scientific research. Taking the fusion applications for
example, from 2008 to 2015 more than 500 tons of Nb3Sn wires were procured for the international ther-
monuclear experimental reactor (ITER) project, which led to a ten-fold boosted global Nb3Sn production
capability. Now the cutting-edge Nb3Sn wires are the internal-tin restacked-rod-process (RRP) wires with
the highest non-Cu Jc of 1.6–1.7 3 105 A/cm2 (4.2 K, 15 T), which is getting close to the requirement
(1.5 3 105 A/cm2 at 4.2 K, 16 T) of more than 5000 dipole magnets for the Future Circular Collider (Xu,
2017). For further improving the Jc performance, the addition of Hf and Zr to form ternary alloys Nb-Ta-
Hf and Nb-Ta-Zr and nano inclusions introduced with internal oxidation technique to reduce the grain
size have shown great potential (Xu et al., 2015, Balachandran et al., 2019). Industrial fabrication for Nb-
Ta-Hf alloy with average grain size less than 50 mm has been achieved by ATI metals (Balachandran et
al., 2020).

2. Copper-oxide superconductors
The copper-oxide superconducting materials have high Tc above the liquid nitrogen temperature (77 K)
and even liquefied natural gas (LNG) temperature (113 K). Due to the extremely rich abundance of nitrogen,
the cost of refrigeration with liquid nitrogen is much lower compared with liquid helium, making it possible
for large-scale industrial applications based on superconducting technology. Though the Tc of the three
cuprate superconducting compounds Bi-2223, Bi-2212, and REBCO are much higher than that of Nb-Ti
and Nb3Sn, they are much more difficult to be processed into wires and tapes due to their ceramic brittle-
ness. The plate-like crystals of cuprate compounds are formed during a high temperature heat treatment,
during which the oxygen content in these ceramic compounds should be controlled to obtain an optimal
superconducting phase. In addition, there is weak-linking effect between their grains with large grain
boundary angles, which is also not beneficial to the current carrying ability and can be reduced by grain
texture. For Bi-2223 superconductors, aligned grains with misalignment at the c-axis <15 is essential for
high inter-grain critical currents. Since Bi-2223 is a metastable phase and will decompose during melting,
the grain orientation can be obtained by mechanical deformation (rolling) and heat treatment process.
Therefore, based on the PIT process, high performance Bi-2223 strands are manufactured with an appear-
ance of tape. For Bi-2212 superconductors, PIT wires or tapes can be fabricated because well connected
and textured grains can be obtained from the liquid phase at a temperature lower than the melting point
of silver sheath. The PIT technique is not applicable to REBCO since it carries large critical current only in
highly biaxially textured tape samples with grain misalignment <5 , which is hard to achieve by the PIT tech-
nique. It only took a few years from the discovery to the commercialization of the first Bibased wires and
tapes because of the achievement of mechanical deformation induced grain texture. In contrast, REBCO
coated conductor tapes (a few meters long) were first manufactured almost 20 years after the discovery
of the compound and after about 10 years of intense R&D effort in the US, Europe, and Japan. The
1990s were a highly active period in the development of Bibased wires and tapes, while most of the break-
throughs in REBCO coated conductors were achieved during the 2000s (Uglietti, 2019).

2.1. BSCCO
For the fabrication of BSCCO wires and tapes, the PIT method introduced above for Nb3Sn is used for both
Bi-2223 and Bi-2212 compounds. Starting powders, such as oxides and carbonates, are mixed and calcined
to obtain precursors that are packed into a metal sheath, which is then mechanically deformed into wires
and tapes. Ag or Ag alloy is required as sheath material instead of Cu and other metals for BSCCO wires
and tapes, because they are chemically compatible (i.e., Ag hardly reacts with the precursor) and trans-
parent to oxygen (i.e., Ag can transmit the oxygen released from or absorbed into oxide superconductors
during heat treatment). Multifilamentary wires can be fabricated by further restacking monofilamentary
wires in an Ag sheath and then repeating the cold deformation process. To strengthen the coupling be-
tween BSCCO grains, it is necessary to make the randomly oriented grains aligned. Since BSCCO oxides
are crystallized with a plate-like appearance due to their high anisotropy, the grain orientation is relatively
easy. However, the approaches of grain orientation are different for Bi-2212 and Bi-2223. For Bi-2212 wires,
partial melting followed by gradual cooling is adopted, while Bi-2223 tapes are processed by rolling-
induced texture. In both cases, microstructures with uniaxial (c-axis) orientation are obtained. In 2005,

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Sumitomo (Kobayashi et al., 2005) developed a controlled over-pressure process which decreased the
porosity and improved grain connectivity; as a consequence, the production yield was increased and Ic ex-
ceeded 200 A at 77 K, self-field. While for Bi-2212 wires, the transport Jc did not change much until 2011,
when it was found that the porosity in the ceramic (due to the formation of gas bubbles during heat treat-
ment), rather than grain misalignment, was the main obstacle to transport supercurrents(Kametani et al.,
2011). With a high pressure heat treatment the transport Jc of Bi-2212 wires can be increased by two times
to 4 3 105 A/cm2 (4.2 K, 15 T) (Jiang et al., 2011). Moreover, homogeneous precursor powder with low im-
purity can help to reduce the porosity, thus further raising the transport Jc to 6.6 3 105 A/cm2 (4.2 K, 15 T)
(Jiang et al., 2019).

For Bi-2223 commercial tapes, companies including American Superconductor (AMSC), Sumitomo (Japan),
Bruker (Germany), and Innova Superconductor Technology (China) were able to produce kilometer-class
long tapes. Bi-2223 tapes have been used in many demonstration projects for power cables, motors, gen-
erators, transformers, and fault current limiters across the world (Sato et al., 2012). In 2012, the world’s first
HTS power substation, which was developed by the Institute of Electrical Engineering, Chinese Academy of
Sciences (IEECAS), was put into operation officially in the power grid in Baiyin city, Gansu province, China.
The substation, which integrates a superconducting magnetic energy storage device, a superconducting
fault current limiter, a superconducting transformer and an AC superconducting transmission cable, can
enhance the stability and reliability of the grid, improve the power quality and decrease the system losses
(Xiao et al., 2012). With laminated mechanical reinforcement technique by Sumitomo for the weak Ag
sheathed Bi-2223 tapes, the mechanical strength of the tapes is significantly enhanced, making them an
alternative for high-field applications. A 24.6 T cryogen-free magnet with a Bi-2223 insert and an Nb3Sn out-
sert has developed in Tohoku University, Sendai, Japan (Awaji et al., 2017). However, in the past years RE-
BCO is gaining more and more interest and research activities on Bi-2223 have been gradually reduced.
Sumitomo is now the only manufacturer producing kilometer class long wires with an Ic>200 A or wires
of several hundred meters in length with Ic>250 A (Nakashima et al., 2012).

In contrast to Bi-2223 tapes showing high anisotropy, Bi-2212 round wires are supposed to be promising in
high-field applications for their isotropic properties. As LTS wires, such round Bi-2212 wires can be easily
made into insert coils for high-field NMR applications, Rutherford cables for the magnets of accelerators, or
cable-in-conduit conductors for large magnets for fusion and detectors. At present, companies such as
Showa (Japan), Oxford Superconducting Technologies (OST, USA) and Alcatel/Nexans (France) are able
to produce kilometer class multifilamentary Bi-2212 long wires. In 2003, a Bi-2212 superconducting insert
magnet generated a magnetic field of 5 T in a 20 T background field by Showa, which was used for a 950
MHz NMR system. In 2014, using OST Bi-2212 wires, the National High Magnetic Field Laboratory in the
USA achieved a 34 T magnet that consisted of a small Bi-2212 insert coil in a 31 T background field, demon-
strating the potential of Bi-2212 superconductors for NMR system above 1 GHz (Larbalestier et al., 2014).
Berkeley National Laboratory and Brookhaven National Laboratory in the USA have studied the construc-
tion of dipole magnets for accelerators using Bi-2212 Rutherford cables. In China, the Institute of Plasma
Physics, Chinese Academy of Sciences (IPPCAS) considered the Bi-2212 CIC conductor for the Chinese
Fusion Experimental Tokamak Reactor (CFETR) (Zhang et al., 2013). In 2017, a sub-size, three-stage
rope-type cable containing 42 strands was manufactured by IPPCAS using Bi-2212 wires provided by the
Northwest Institute for Non-Ferrous Metal Research, China (Qin et al., 2017).

2.2. REBCO
Compared with BSCCO superconductors, REBCO has much lower anisotropy and a much higher in-field Jc
at 77 K. However, the uniaxial (c-axis) aligned grain structure achieved in BSCCO wires by rolling- or
melting-induced texture is unsatisfactory for REBCO to obtain a high inter-grain Jc and a biaxial grain
texture is necessary. In order to realize a biaxial texture, other than the PIT fabricating route, REBCO films
are deposited on biaxially textured buffer layers, which are coated on a long and flexible tape-like metal
substrate. In addition, for environmental protection and thermal stabilization, an Ag layer a few mm thick
and a thicker Cu layer are coated on the conductor. Since 2003, varieties of approaches for producing
coated conductors have been implemented at an industrial level (Senatore et al., 2014). The textured sub-
strate techniques include ion beam assisted deposition (IBAD), rolling assisted biaxially textured substrates
(RABiTS), and inclined substrate deposition. The deposition of epitaxial REBCO layer can be also achieved
via various routes, including chemical routes, such as metal organic deposition (MOD) and metal organic
chemical vapor deposition (MOCVD), or via physical routes such as pulsed laser deposition (PLD) and

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10 APC-introduced

Critical current density (MA/cm )

2
YBCO film (4.2 K)

1
Nb3Sn (4.2 K)

0.1

APC-introduced
0.01 Nb-Ti (4.2 K) YBCO film (77 K)

Biaxially textured
1E-3 Polycrystalline YBCO film (77 K)
YBCO tape (77 K)

0 2 4 6 8 10 12 14 16 18
Magnetic field (T)

Figure 5. Enhancement of the critical current density Jc(B) in REBCO tape conductors
There are two big boosts for the Jc of REBCO tape conductors: (1) Development of the biaxial texture onto metallic
substrates, and (2) introduction of the nanocomposite artificial pinning centers (APCs) in the coated conductors. With the
combination of biaxial texture structure and nanocomposite APCs, the Jc of coated conductors is much higher than that of
Nb-Ti and Nb3Sn wires. The data are collected from (Obradors and Puig, 2014) and (Puig, 2015).

reactive co-evaporation (RCE). Among them, MOD and RCE are ex situ processes incorporating two steps:
deposition of the pre-cursors and subsequent conversion of the precursors into REBCO. On the other
hand, deposition and formation occur simultaneously during in situ processes such as PLD and MOCVD.
The PLD process provides wide-ranging defect pinning structure, high-quality grain alignment, and feasi-
bility for thick-film deposition, but needs more expensive equipment and has a lower deposition rate. The
MOCVD and RCE techniques have a high deposition rate and large deposition area, but their products
have relatively poor inherent pinning microstructures and crystallinity. The MOD process has advantages
such as low system cost and the least solution waste, but its products have larger pinning particles than
that obtained in PLD growth, and the pinning at higher fields is less effective. The coated conductors man-
ufactured with different routes can be used for different applications according to their performance (Ob-
radors and Puig, 2014; MacManus-Driscoll and Wimbush, 2021).

So far, long-length (>1000 m) REBCO coated conductors with critical current over 300 A/cm-width at 77 K in self-
field have been achieved by several companies. In 2008, the world’s first kilometer class REBCO tape was devel-
oped by SuperPower in the USA with IBAD/MOCVD process (Ic = 227 A/cm at 77 K, in self-field) (Shiohara et al.,
2012). In 2016, SuNAM developed a 1000 m long 12 mm wide REBCO tape with Ic >800 A/cm via IBAD/RCE
process (Moon et al., 2016). In recent years, commercialization of REBCO-coated conductors has been achieved
by companies in the USA (ASMC, SuperPower, STI), Japan (SWCC, Fujikura, Sumitomo), Korea (SuNAM), Ger-
many (THEVA, d-NANO, Bruke), Russia (SuperOx), and China (Shanghai Superconductor, Samri, SCSC), and
rapid progress has been made for high-performance long-length REBCO coated conductors. The Ic of their
commercial coated conductors covers a wide range of values, from 100 A to over 250 A at 77 K, self-field (for
a 4 mm wide tape) (Senatore et al., 2014). At present, the research for improving the current-carrying ability
of coated conductors focuses on introducing artificial pinning centers (APCs) in the ceramic layer and increasing
the thickness of the ceramic layer. The combination of the biaxial structure of the REBCO tapes with APCs such
as non-superconducting nanoparticles or nanorods efficiently pins vortices at high temperatures, and thus
significantly enhances the in-field Jc performance, as presented in Figure 5. Moreover, it was observed that
the APCs can be used to adjust the Jc anisotropy, and are promising to further lower the anisotropy of REBCO
tapes (Senatore et al., 2014). It was also found that the APCs can suppress the degradation of Jc in REBCO tapes
with the increased thickness of superconducting layers. With such improved flux pinning, ceramic layer with
thickness of 5 mm, which is 5 times thicker than that in commercial coated conductors, was achieved by SuNAM
and KERI in Korea and at the University of Houston with a very high Ic reaching 1300–1500 A/cm (77 K, self-field),
respectively (Dürrschnabel et al., 2012; Kim et al., 2014; Majkic et al., 2018).

With the commercialization of REBCO-coated conductors, lots of demonstration projects based on them
for superconducting electric power devices such as power cables, motors, generators, transformers, and

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A B
NHMFL 45.5 T hybird magnet & 32 T superconducting IEE CAS 32.35 T superconducting magnets
magnet LTS outsert: 15 T, YBCO insert 17.35 T

Water cooled 31.1 T，YBCO14.4 T

HTS coil assembly of

the 32 T magnet

C SuNAN 26.7 T, all RE-123 magnet D RIKEN 27.6 T magnet E Tohoku Univ. 24.6 T
no insulation coils (radial current sharing) for 1.3 GHz (30.5 T) NMR project cryogen free magnet

Figure 6. State-of-the-art high-field solenoid magnets based on cuprate HTS materials

(A) a 45.5 T hybrid magnet with a 14.4 T REBCO insert in a 31.1 T resistive magnet (Hahn et al., 2019) and a 32 T magnet with a 15 T LTS oursert and a 17 T
REBCO insert (Berrospe-Juarez et al., 2018); (B) a 32.35 T magnet with a 15 T LTS oursert and a REBCO insert (Liu et al., 2020); (C) a 26.7 T all-REBCO magnet
(Yoon et al., 2016); (D) a 27.6 T magnet with a Bi-2223 coil and a small REBCO test coil in a 17 T LTS background magnet (Yanagisawa et al., 2016); (E) a 24.6 T
cryogen free magnet with a Bi-2223 insert and an Nb3Sn outsert (Awaji et al., 2017).

fault current limiters have been developed in Europe, USA, Japan, Korea, and China (Obradors and Puig,
2014; Shiohara et al., 2012). For example, in 2019 Korea Electric Power Corporation has fully funded and
completed the first commercial project of HTS power cables in the real grid, called the Shingal Project,
to connect two substations with a 23 kV HTS cable over a distance of 1 km (Lee et al., 2020). Besides the
power application projects there is continuous and remarkable progress of high-field applications using
coated conductors, as shown in Figure 6. At present, the world record of DC magnetic field is 45.5 T, which
was achieved by the National High Magnetic Field Laboratory (NHMFL, USA), with a 14.4 T REBCO insert in
a 31.1 T resistive magnet (Hahn et al., 2019). For all-superconducting magnets, a 27.6 magnet with a Bi-2223
coil and a small REBCO test coil in a 17 T LTS background magnet was developed in RIKEN, Japan (Yana-
gisawa et al., 2016), a 32 T magnet with a 15 T LTS oursert and a 17 T REBCO insert was developed in
NHMFL in 2017 (Berrospe-Juarez et al., 2018), and in 2019 a 32.35 T magnet with a 15 T LTS oursert and
a REBCO insert was developed in IEECAS (Liu et al., 2020). A 26.7 T all-REBCO magnet was also developed
by SuNAM (Yoon et al., 2016). These achievements in pursuing new records for the high magnetic field
clearly indicate the great potential of REBCO tapes in magnet applications.

3. MgB2 superconductors
The MgB2 superconductor discovered in early 2001 has a superconducting critical transition temperature
as high as 39 K, setting a record for the Tc of intermetallic superconducting materials. In contrast to cuprate
HTS, the supercurrents in MgB2 are not sensitive to the weak-linked grain boundaries Larbalestier et al.,
2001b, so MgB2 is quite promising for fabricating high-performance wires. However, due to the weak
flux pinning ability, the critical current density of MgB2 drops rapidly with the increase of applied magnetic
field, which limits the application of MgB2 in a high magnetic field region. Because of its relatively high Tc,

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low raw material cost, simple chemical composition and light weight, the MgB2 superconductor has also
attracted wide attention from the applied superconductivity community. It is generally believed that
MgB2 superconducting materials have obvious technical and cost advantages in the application of super-
conducting magnets in MRI systems at 1-2 T and 10-20 K regions.

There are two main fabrication routes for MgB2 wires. (1) PIT method: This process is relatively simple and
has been widely used in the preparation of Bi-2223 and Bi-2212 wires and tapes. MgB2 wires have been
made both by in situ and ex situ PIT method. The in situ process starts by packing the powders of unreacted
raw materials into metallic tubes, while the ex situ PIT method starts with reacted MgB2 precursor powder
packed into metallic tubes. Unlike Bi-2223 and Bi-2212 wires for which Ag or Ag alloy must be used as
sheath materials, and Nb3Sn wires for which Cu alloys can be used as sheath materials, for MgB2 wires since
elemental diffusion and chemical reaction will occur between Ag and Mg or Cu and Mg, they are usually
prepared by using Fe, stainless steel, and carbon steel sheaths, or composite sheaths composed of Cu al-
loys outer sheath and Nb, Ta, Fe inner barrier sheath. (2) Internal Mg diffusion process (IMD): This process
involved an Mg rod placed in the center of a metal tube, with B powder filled between the metal tube and
the Mg rod. After the assembly is cold deformed into wires, the Mg melts and diffuses into the surrounding
B powder to form the MgB2 superconducting phase during the final heat treatment. Compared with the
commercialized PIT process, the IMD process is easy to obtain high-density MgB2 phase, thus achieving
high transport Jc, so it has been the hotspot for the research of MgB2 wires (Ye and Kumakura, 2016).

For MgB2 superconductors, due to the lack of pinning centers inside the material, their current-carrying
ability was seriously suppressed by increasing field. The improvement of flux pinning is the key to enhance
the Jc performance of MgB2 wires in strong magnetic fields. Currently, carbon doping is the most effective
and widely used method to enhance Jc. The doped carbon into MgB2 lattice can substitute on the boron
site, causing a decrease in the mean free path of electrons, which results in an increase of upper critical
field. The most effective carbon dopents are recognized as nano-SiC and nano-C (or C-containing com-
pounds) (Dou et al., 2002; Ma et al., 2006). With the carbon doping, the transport Jc of MgB2 wires based
on PIT method can be significantly increased to 6 3 104 A/cm2 at 4.2 K and 10 T, while the Jc of wire samples
by IMD method can be enhanced as high as 1.5 3 105 A/cm2 (Li et al., 2013).

At present, 100-meter-class MgB2 wires based on IMD method have been achieved in several labs (Ye and
Kumakura, 2016; Wang et al., 2016), while companies such as Hyper Tech in the USA, Columbus in Italy,
Hitachi in Japan, Sam Dong in South Korea, and Western Superconducting Technologies in China have
possessed a production capacity of kilometer-level practical MgB2 wires (Jc = 1-23105 A/cm2 at 4.2 K
and 4 T) based on the PIT method. Among them, the MgB2 wires produced by Hyper Tech and Columbus
have been successfully employed in applications such as MRI, superconducting fault current limiter, and
superconducting cables. In 2006, the Columbus developed the world’s first MgB2-based open MRI system.
The system, which uses a GM refrigerator for cooling and can generate a magnetic field of 0.5–0.6 T at 20 K,
obtained the first scan of the human brain (Flukiger, 2014). So far, they have produced more than 20 sets of
the above-mentioned MgB2-based MRI system. It can be expected that the operated field strength will be
increased to 1-2 T by using MgB2 wires with higher Jc performance in the future to satisfy the various med-
ical diagnoses. Recently, the application of MgB2 superconductor in the motors of wind power generation
has received a significant boost. Hyper Tech has developed 8-20 MW wind power generators with super-
conducting stator and rotor based on low AC loss MgB2 wires. Taking a 10 MW superconducting generator
as an example, its weight is just about 50–60 tons, which is much lighter than the about 350 tons of a con-
ventional generator.

4. Iron-based superconductors
Studies on the grain boundary nature in iron-based superconductors (IBS) suggested that intergrain cur-
rents across mismatched grains in IBS are deteriorated to a lesser extent than in REBCO superconductors
(Katase et al., 2011). Therefore, the low-cost PIT method, which has been utilized in commercial Nb3Sn, Bi-
2223, and MgB2 wires, is promising for IBS wires manufacture. At present, silver is widely used as sheath
material for wires made of iron pnictides such as Sr1-xKxFeAs (Sr-122) and Ba1-xKxFeAs (Ba-122), since silver
is chemically stable and not easy to react with iron pnictides during heat treatment of wires. On the other
hand, in contrast to BSCCO wires, whose sheath material was limited to Ag or some Ag-rich alloys due to
the requirement for oxygen permeability, the IBS wires have more choices for sheath materials. For
instance, a composite sheath consisting of other cheap and stiff metals as the outer sheath and silver as

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4.2 K
Critical current density (A/cm )
2
5
10

NbTi IBS wire IBS tape

4
10
Iron-based superconductors
Nb3Sn
Ba-122 tape
Ba-122 wire
3
10
0 5 10 15 20 25 30 35
Magnetic field (T)

Figure 7. State-of-the-art critical current density Jc of 122-IBS based on PIT technique showing much weaker field
dependence up to 33 T compared with that of Nb-Ti and Nb3Sn LTS conductors
Data source for IBS short samples: Ba-122 tapes (Jc at 10-13 T, 24-27 T and 25-33 T were measured in a 14 T
superconducting magnet, a 28 T hybrid magnet and a 35 T water-cooled magnet, respectively) (Yao and Ma, 2019; Huang
et al., 2018); Ba-122 wires (Pyon et al., 2020). Data for LTS conductors were collected from nationalmaglab.org (Lee, 2018).

the inner sheath can be used for IBS wires. The composite sheath can reduce the ratio of silver cost, provide
sheath chemical stability, and enhance mechanical properties at the same time. Therefore, the low-cost,
high-strength, and high Jc performance IBS wire and tape conductors are very promising based on the
PIT method.

In 2008, the first iron-based superconducting wires were developed in IEECAS by in situ PIT method, which
starts by packing the powders of unreacted precursor materials into a metallic tube in a high purity Ar at-
mosphere. However, the defects in the material such as micro-cracks, low density, phase inhomogeneity,
and impurity phase restricted the transport current in wires. By using ex situ PIT method, in which reacted
and well-ground superconducting materials are packed into metallic tubes, the mass density and phase
homogeneity of the wire after the final heat treatment are significantly improved in iron pnictide wires
(Ma, 2012). In the past years, mechanical deformation processes such as flat rolling, isostatic pressing,
and uniaxial pressing have significantly improved the Jc of 122-type IBS in USA, Europe, China, Japan,
and Australia (Weiss et al., 2012; Malagoli et al., 2015; Zhang et al., 2014; Gao et al., 2017; Shabbir et al.,
2017). High transport Jc above the practical level of 105 A/cm2 (4.2 K, 10 T) has been achieved by hot or
cold uniaxial press combined with flat rolling process for PIT wires and tapes (Hosono et al., 2018; Yao
and Ma, 2019). A synergetic microstructural tailoring for mass density, grain alignment and micro-cracks
is the key to realize such high Jc performance. By using an optimized hot press process to achieve a higher
degree of grain texture, the transport Jc was further increase to 1.5 3 105 A/cm2 (Ic = 437 A) with a small Jc
anisotropy of 1.37 at 4.2 K and 10 T in Ba-122 tapes, which exhibits very weak field dependence up to 33 T,
as presented in Figure 7. The transport Jc measured at 4.2 K under high magnetic fields of 27 T is still on the
level of 5.5 3 104 A/cm2, showing a great application potential in moderate temperature range which can
be reached by liquid hydrogen or cryogenic cooling (Huang et al., 2018). On the other hand, a high Jc of 4 3
104 A/cm2 was measured at 4.2 K and 10 T for Cu/Ag sheathed Ba-122 round wire processed with a hot
isostatic pressing densification (HIP) (Pyon et al., 2020), as shown in Figure 7. A local grain alignment
perpendicular to the wire axis can be observed and ascribed to drawing or groove rolling process. Last
year, a practical level critical current density up to 1.1 3 105 A cm 2 at 4.2 K and 10 T was achieved in
Cu/Ag sheathed Ba-122 tapes by combing flat rolling to induce grain texture and a subsequent HIP densi-
fication, which is a scalable and cost-effective manufacturing route (Liu et al., 2021).

For 11-type iron chalcogenides wires based on the traditional PIT process, the most extensively used
sheath material is Fe, since it has been proved to be the most chemically compatible with the 11-IBS phase.
It also allows a PIT-based diffusion process, where the Fe near the inner surface of the sheath and the Se
and Te powder (for the in situ method) or FeTe1-xSex powder (for the ex situ method) inside the Fe tube
form the superconducting phase by chemical reaction during the heat treatment process. However, the

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difficulty in controlling the Fe content and the decomposition of the Fe (Se,Te) phase during the heat treat-
ment process restricts the transport Jc of 11-IBS wires (Hosono et al., 2018). On the other hand, thin film
technology exhibits its application potential for the fabrication of 11-IBS coated conductors. An almost
isotropic Jc of 1.7 3 105 A cm 2, which is lowered by less than one order of magnitude in high fields up
to 18 T, was achieved in Fe(Se,Te) film deposited on a RABiTS template (Sylva et al., 2019). Considering
the simple elemental constituent, the moderate Tc (16 K), the high upper critical fields, and the relative
ease of fabrication for 11-IBS thin films, it is very promising for high-field applications at a low-temperature
range (Pallecchi et al., 2020). Moreover, the newly discovered iron chalcogenides KxFe2Se2 (Guo et al.,
2010) (Tc> 30 K) and FeSe-based (Li,Fe) OHFeSe (Lu et al., 2015) (Tc  40 K) may broaden the application
range of iron chalcogenides.

For practical applications of IBS, fabricating wires and tapes with multifilaments in metal matrix to protect
against flux jumps and thermal quenching is an important step. Based on the techniques used in the single-
core IBS wires, Fe/Ag clad 7-filament 122-IBS wires and tapes were successfully fabricated with the PIT pro-
cess by IEECAS in 2013 (Yao et al., 2013). After that, Fe/Ag sheathed 114-filament 122-IBS wires and tapes
were also produced (Yao et al., 2015). Processed with hot press, a high transport Jc of 3.6 3 104 A/cm2 (4.2 K,
10 T) can be achieved in 7-filament Monel/Ag sheathed 122-IBS tapes, which exhibit an improved mechan-
ical strength and very weak field dependence for transport Jc (Yao and Ma, 2019). Though high Jc proper-
ties can be obtained in short ‘122’ IBS samples, practical applications need wire and tape conductors with
sufficient length. In 2014, the IEECAS group fabricated the first 11 m long 122-IBS tape by a scalable rolling
process. After carefully optimizing the long-length wire fabricating process to achieve a higher-level uni-
formity of deformation, the world’s first 100 meter-class IBS tapes was produced by the same group (Zhang
et al., 2017). This 115 m long 7-filament Sr-122 tape shows a uniform Jc distribution throughout the tape with
a minimum Jc of 1.2 3 104 A/cm2 (4.2 K, 10 T). Very recently, by improving the fabrication process, a 100-m 7-
filament Ba-122 tape with Jc above 5.0 3 104 A/cm2 (4.2 K, 10 T) was achieved, demonstrating great poten-
tial in large-scale manufacture and a promising future of IBS for practical applications.

In 2018, an IBS single pancake coil was firstly fabricated with Ba-122 superconducting tapes and tested un-
der a 24 T background field (Wang et al., 2019), then another single pancake coil and a double pancake coil
were developed and tested at a 30 T background field (Qian et al., 2021), showing very weak dependence of
critical currents on such high magnetic field. In 2020, by using 100-meter 7-filament Ba-122 tapes provided
by IEECAS, IBS racetrack coils were firstly fabricated at the Institute of High Energy Physics, Chinese Acad-
emy of Sciences (IHEPCAS). The racetrack coils were tested in a low-temperature superconducting com-
mon-coil dipole magnet which provided a maximum background field of 10 T at 4.2 K. One of the best
IBS racetrack coil quenched at 4.2 K and 10 T showed an operating current of 65 A, which is still as high
as 86.7% of the Ic of short samples at 10 T (Zhang et al., 2021). As presented in Figure 8, these results
demonstrate that the IBS conductor is a promising candidate for the application of high field magnets,
especially for future high-energy accelerators. At present, a conceptual design study of 12 T dipole mag-
nets is ongoing with the IBS technology to fulfill the requirements and need of a large-scale superconduct-
ing accelerator proposed by IHEPCAS (The CEPC Study Group, 2018).

FUTURE PERSPECTIVES OF MATERIALS AND THEIR APPLICATIONS

Nowadays, the inner force promoting the development of superconducting materials with higher performance
is mainly the great demand from high-field magnet applications such as MRI, NMR, and large scientific projects
including accelerators and fusion, since they greatly challenge the performance limits such as transport critical
density, upper critical field, mechanical strength, and magnetic and thermal stability. The generation of mag-
netic fields exceeding the limits of LTS conductors provides considerable opportunities for HTS materials,
and will speed up their commercialization. Compared to HTS materials which can be used in higher tempera-
tures and magnetic fields, the advantages of LTS materials are cost, technology maturity, and batch stability,
which make them still the priority choice if the required temperature and field strength are within their perfor-
mance limit. Moreover, while today’s target magnetic fields of new superconducting magnets are higher than
the working limit of LTS wires and HTS wires are indispensable, LTS magnets are still used to provide back-
ground fields for central HTS magnets to save the cost of the whole superconducting magnet.

For commercial applications with superconducting magnets, MRI and NMR systems are growing exponen-
tially and have fostered a corresponding exponential growth in the wire production capacity. In 2019, the
Iseult 11.7 T whole body MRI system has been constructed by CEA in France, and for the next goal will be a

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Figure 8. Iron-based superconducting coils

Pancake coils (Wang et al., 2019; Qian et al., 2021) and racetrack coils (Zhang et al., 2021) were made with iron-based superconductors and their in-field
critical currents, which show weak field dependence up to 30 T and 12 T, respectively.

>14 T whole body MRI system, for which the superconducting wires used should be Nb3Sn or HTS instead
of the current Nb-Ti (Vedrine, 2020). The development of biology and novel drugs is calling for >1-1.3 GHz
NMR systems, which need 25-30 T superconducting magnets using HTS wires is indispensable. On the
other hand, large scientific projects such as the Large Hadron Collider at CERN (built between 2002 and
2008) and ITER have also significantly benefited the superconducting material industry. Some large pro-
jects ahead are the Future Circular Collider (FCC) at CERN and the large tokamaks (such as EU-DEMO
in Europe, K-DEMO in Korea, and CFETR in China). While a Circular Electron Positron Collider (CEPC)
and its upgraded version Super Proton Proton Collider (SPPC) are under a consideration in China (The
CEPC Study Group, 2018). The accelerator magnets for FCC and SPPC are required to generate 16-20 T
fields, for which HTS conductors are preferred.

On the other hand, superconductivity applications of electric power technologies are still exploring their
niche in the market. Some application scenarios such as superconducting electric power cables and super-
conducting maglev trains for big cities, superconducting power station connected to renewable energy
network, and liquid hydrogen or LNG cooled electric power generation/transmission/storage system at
ports or power plants may achieve commercialization in the future. In Japan, the superconducting maglev
test track in Yamanashi, which has set a new speed record of 603 km/h of rail vehicles in 2015, is planned to
be expanded into a commercial line linking Tokyo and Nagoya in 2027. In the USA and Europe, NASA and
Airbus have started their own development project for electric aircraft. With the power distributed electri-
cally from turbine-driven superconducting generators to superconducting motors that drive electric fans
for propulsion, the E-aircraft have high fuel efficiency, less engine noise, and can contribute to the reduc-
tion of greenhouse gas emissions. Due to the limited capability of cooling system on aircraft, the HTS po-
wer and transmission technology will be used. In China, starting in 2020, two commercial demonstration
projects of REBCO power cables cooled with liquid nitrogen are under construction in the downtown of

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Shanghai and Shenzhen by the State Grid Corporation of China and the China Southern Power Grid,
respectively.

For the aforementioned large machines and big projects, the cost of superconductors becomes a crucial
issue. For cuprate superconductors that are stepping into commercialization, the product price is still the
main obstacle for their large-scale application. The current price is about $5/kA m for Nb3Sn, $60-80/kA m
for Bi-2212 and Bi-2223 and $100-200/kA m for REBCO conductors for use at 4.2 K and 10 T (Uglietti,
2019).Their price is still higher than the ideal $25/kA m for large-scale applications. Therefore, lowering
the price is the top priority for their research and development. For REBCO-coated conductors, one of
the reasons for their high price seems to be the low manufacturing yield of long-length tape products,
especially for the ones longer than 500 m (Uglietti, 2019). Another factor that raises the price is the quite
low ratio (just a few percent of the whole cross-section of the tapes, much lower than the 20-40% for BSCCO
PIT wires and tapes) of the superconducting layer. Therefore, increasing the thickness of the superconduct-
ing layer is another important issue for the low-cost produce for REBCO tapes.

For Bi-2212 and Bi-2223 wires and tapes, besides the price, their relatively weak mechanical strength due to the
presence of soft Ag or Ag alloys sheath is another critical issue that should be taken into consideration for ap-
plications under high electromagnetic stress. The weak mechanical strength of wires and tapes usually requires
a wind-and-react method for magnet constructions. However, the reaction by heat treatment can largely
degrade the mechanical strength of the structural materials of magnets, which have become one of the main
obstacles for ultra-high magnets with field strength close to 40 T. Laminated mechanical reinforcement tech-
nique has been successfully employed by Sumitomo for commercial Bi-2223 tapes, but they are still facing
the competition with the fast-growing REBCO tapes. On the other hand, Bi-2212 round wires can still find a mar-
ket for high-field applications because of their isotropic property and flexible architecture. Bi-2212 round wires
can be inserted in high strength alloy tubes for reinforcement, and there have been several studies on the struc-
tural material for the wind-and-react process for Bi-2212 wires (Tixador et al., 2015; Shen et al., 2015).

For MgB2 and IBS, as mentioned in the above sections, metallic composite sheaths can be used for wire fabri-
cation. Therefore, their mechanical strength would not be the main drawback for their practical applications. For
PIT MgB2 wires, the critical current density in fields is still needed to improve to make them to be better used at
around 20 K; for IMD MgB2 wires, the proportion of superconducting layer is needed to increase as for REBCO
tapes. For iron-based superconducting wires, it seems that grain texture is crucial for obtaining superior Jc, since
now the best Jc in textured tapes is about three times higher than that in wires. The composite sheaths for iron-
based superconducting wires contain an inner Ag barrier sheath, whose melting temperature (lower than that of
IBS phase) restricts the use of melting texture process for round wires as used for Bi-2212. Nevertheless, for the
tape conductors, it is possible to orient the tapes properly in the preferred direction to the magnetic field in
high-field applications, so as to make use of their best transport and mechanical properties. Moreover, the
relatively smaller critical bending radius of tapes would enable the react-and-wind process rather than the
wind-and-react process, thus expanding the choice of structural and insulating materials for the construction
of magnets (Uglietti, 2019). In addition to their low intrinsic anisotropy, IBS can still become an ideal candidate
for high-field applications after Jc being further increased in the future.

CONCLUSION
At present the great demand from high-field magnet applications provides a technology pull for HTS con-
ductors, and will speed up their commercialization. On the other hand, superconductivity applications for
electric power technologies are still exploring their niche in the market. Compared with HTS materials
which can be used in higher temperatures and magnetic fields, the advantages of LTS materials are
cost, technology maturity, and batch stability, which make them still the priority choice if the required tem-
perature and field strength are within their performance limit. The main obstacle for the large-scale appli-
cation of BSCCO and REBCO conductors is still the high price, so it is urgent to increase their performance/
cost ratio. The feasibility of metallic composite sheaths is advantageous for the high-mechanical strength
and low-cost fabrication for MgB2 and IBS, but the current carrying capability still needs to be enhanced
before gaining large-scale applications.

ACKNOWLEDGMENTS
This work is supported by the National Key R&D Program of China (Grant Nos. 2018YFA0704200 and
2017YFE0129500), the National Natural Science Foundation of China (Grant Nos. 51861135311,

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U1832213 and 51721005), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant
No.XDB25000000), and the Key Research Program of Frontier Sciences of Chinese Academy of Sciences
(Grant No.QYZDJ-SSW-JSC026).

AUTHOR CONTRIBUTIONS
Writing - original draft, C.Y.; writing - review & editing, C.Y., and Y.M.; supervision, Y.M.

DECLARATION OF INTERESTS
The authors declare no competing interests.

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