j.1744-7402.2007.02138.x

Overview of MgB2 Superconductor Applications

Michael Tomsic,* Matthew Rindfleisch, Jinji Yue, Kevin McFadden, and JohnPhillips

Hyper Tech Research Inc., Columbus, Ohio 43212

Michael D. Sumption, Mohit Bhatia, Scot Bohnenstiehl, and Edward W. Collings

LASM, Department of Material Science and Engineering, The Ohio State University, Columbus, Ohio 43210

Since 2001, when magnesium diboride (MgB2) was first reported to have a transition temperature of 39 K, conductordevelopment has progressed to where MgB2 superconductor wire in kilometer-long piece-lengths has been demonstrated incoil form. Now that the wire is available commercially, work has started on demonstrating a MgB2 wire in superconductingdevices. This article discusses the progress on MgB2 conductor and coil development, and the potential for MgB2 supercon-ductors in a variety of commercial applications: magnetic resonance imaging, fault current limiters, transformers, motors,generators, adiabatic demagnetization refrigerators, magnetic separation, magnetic levitation, superconducting magnetic en-ergy storage, and high-energy physics applications.

Magnesium Diboride (MgB2) Wire Developmentat Hyper Tech Research Inc. (Hyper Tech)

Strand Design and Manufacturing Process

Hyper Tech currently offers a commercial MgB2

superconductor wire designated as an ‘‘1811 Nb/Cu/Monel’’ multifilament strand. This designation, in theorder as it is listed, corresponds to the number of mono-filaments in the strand, followed by the chemical barrier,monofilament sheath, and multifilament sheath materi-als. Figure 1 shows a typical wire cross section of thecommercial wire currently fabricated at Hyper Tech.

Hyper Tech uses a patented process for manufac-turing MgB2 superconductors; this process is called thecontinuous tube filling and forming (CTFF) process.During the CTFF process, powder is dispensed onto astrip of metal as it is formed into a tube. The CTFFprocess results in an overlap-closed tube filled with pow-der in continuous lengths; fabrication of precursor wirebillets continuously is the fundamental advantage of theCTFF process. Hyper Tech has used the ex situ tech-nique (filling the tube with already formed MgB2 pow-der), but primarily uses the in situ technique forfabricating MgB2 superconductor wire. The in situ tech-nique involves direct filling of a metallic tube with el-emental magnesium and boron powder and subsequentdrawing, followed by heat treatment, during whichthe elements react to form MgB2. The in situ processoffers the advantages of simplicity of fabrication, lower

Int. J. Appl. Ceram. Technol., 4 [3] 250–259 (2007)

Ceramic Product Development and Commercialization

*[email protected]

r 2007 The American Ceramic Society

reaction temperatures, and a more manageable way foradding dopants or other additives into the wire.1 Thelower reaction temperature is particularly important asit helps to minimize the possibility of powder–barrierreactions.

The strand fabricated by the CTFF process mustnot only contain the powder but must also be chemi-cally compatible in the heat-treatment range where thepowder is being reacted; the barrier material is typicallypure niobium but Ni, Fe, and Ti can alternatively beused. In general, the Nb barrier is enclosed in an outersheath, or sheaths, to aid the wire drawing and eventu-ally to provide electrical stabilization. For most of thewire made at Hyper Tech, the overlap-closed niobiumstrand is inserted into a seamless copper (or copper al-loy) tube and is drawn to a predetermined size, creatinga monofilament MgB2 strand. The monofilamentstrands are then restacked into another seamless tube;the diameter and length of the restack tube will deter-mine the final piece length of the wire. Billets are pres-ently being sized to produce continuous wire lengths of5 km for a typical 0.8 mm diameter wire. Experimentalwork will continue to take the 0.8 mm conductor piecelength up to 30 km, the typical continuously fabricatedpiece length for NbTi superconductor wires. Thesecond seamless tube, or multifilament outer sheath,is typically a nickel–copper alloy such as Monelt(Huntington Alloys, Huntington, WV) but can also be

copper-rich Cu–Ni alloys. Work is continuing to de-velop a wire, with the outer tube being all Cu. Thenumber of filaments in the restack can vary and usuallyCu stabilizer filaments are located in the center of themultifilament restacked wire. The ‘‘1811’’ designationfor the commercial wire therefore refers to 18 MgB2

monofilaments and one center copper filament.Hyper Tech has fabricated a variety of experimental

strands for various application-related projects, includ-ing the following:� Nb barrier, all-copper matrix. The Monel outer

restack sheath is replaced with pure 101 copper.The use of copper as the outer sheath improvesthe ductility and stability of the strand.

� Oxide-dispersion-strengthened (ODS) copper, tradename Glidcopt (SCM Metal Products, ResearchTriangle Park, NC). The Monel outer restacksheath is replaced with ODS Cu in order tocombine the benefits of lower resistivity withoutsacrificing the significant degree of strengthneeded for drawing.

� High filament count. MgB2 superconductor wirewith up to 61 total filaments has been fabricated.The strand design was a Nb/Cu/Monel.

� Very small diameter. Small-sized MgB2 supercon-ductor strands have been fabricated, such as a0.07 mm round monofilament and a 0.117 mmround seven filament Nb/Cu/Monel MgB2 con-ductor. The size of a MgB2 filament in the caseof the 0.117 mm seven filament-restack strandwas 17 mm. The size of a MgB2 filament in thestandard 1811 multifilament wire at 0.8 mm is76 mm.

� Rectangular. MgB2 superconductor wire hasbeen fabricated in a rectangular shape with a0.5 mm� 1.0 mm aspect ratio in various multi-filament strand designs.

Transport Current Properties

A wire in short sample form, in long length onspools, or wound on coils is heat treated to react theelemental magnesium (99%) and boron (99.9%, amor-phous) to form MgB2 in the strand. Heat treatments aresingle step and are performed under an argon atmo-sphere. The heat-treatment soak temperature is normal-ly 7001C, held for 20–40 min. The Ohio StateUniversity (OSU) has characterized the majority of theMgB2 conductors manufactured by Hyper Tech.2–4

CuNi alloySuperconductor

powder

Copper Niobium

Fig. 1. Cross section of a typical 1811 multifilament MgB2 wire.

www.ceramics.org/ACT MgB2 Superconductor Applications 251

Usually, two types of wire samples are used in transportcurrent density measurements: short samples and 1-mITER barrel coil samples. The Jc criterion for both sam-ple types is 1 mV/cm. Four-point Jc measurements aremade with background fields of up to 15 T appliedtransversely to the strand. The ITER barrel-type sampleshave a gauge length of 50 cm and are made with a wind-and-react protocol. ITER barrel measurements are takenat 4.2 K in liquid helium. Short samples are 3 cm inlength with a gauge length of 5 mm and are measured atelevated temperatures. Figure 2 shows the typical criticalcurrent and current density measurements versus appliedfield at various temperatures of the standard multifila-ment MgB2 conductor manufactured at Hyper Tech.Typical Jc values are 1� 105 A/cm2 at 5 T, 4.2 K and2� 105 A/cm2 at 1 T, 20 K. The Ic at 1 T and 20 K isaround 200 A. The average fill factor in the standardmultifilament strand is 15%. Hyper Tech is working toincrease fill factor to 30%, which would increase the 20 K,1 T Ic to 400 A. Thus far, experimental multifilamentstrands with a fill factor of 24% have been produced.

n-Value

Along with transport current density measure-ments, the corresponding exponential n values forHyper Tech’s MgB2 superconductor wire have been de-termined. The n values shown in the variable temper-ature graph (Fig. 3) are typical values for the ‘‘standard’’1811 multifilament wire described previously.

Nano-SiC Doping

Work is ongoing to improve the in-field perfor-mance of MgB2 wires.5,6 The University of Wollongongas well as Hyper Tech and OSU have shown that SiCnanoparticles can significantly improve the properties ofwire made from the binary Mg1B compound.7,8 Shortsample measurements by OSU have found enhance-ments in (m0Hirr) and the upper critical field (Bc2) with aSiC-doped wire made by Hyper Tech. Depending onthe heat-treatment schedule, SiC doping yielded Bc2s ashigh as 29.7 T and Birrs as high as 25.4 T. Measure-

H (T)0 2 4 6 8

I c (

A)

10

20

40

60

80

100

200 4.2K6K10K15K20K25K30K

J c(A

/cm

2 )

J e(A

/cm

2 )

20,000

40,000

60,000

80,000

100,000

200,000

2,000

4,000

6,000

8,000

40,000

20,000

10,000

Fig. 2. Typical Ic, Je, and Jc properties of the standard Hyper Tech multifilament MgB2 strand. Data on the graph are limited toJc4104 A/cm2.

B (T)2 4 5 63 7 8

n-va

lue

0

5

10

15

20

25

30

354.2K6K10K15K20K25K

Fig. 3. n value versus B for a multifilament MgB2 strand; shortsample, variable temperature measurements performed by the OhioState University.

252 International Journal of Applied Ceramic Technology—Tomsic, et al. Vol. 4, No. 3, 2007

ments showed that Hyper Tech wires with SiC-dopedstrands increase the current density in higher fields. Fig-ures 4 and 5 compare the current density of a MgB2

SiC-doped strand with an undoped one at 4 and 15 K.

Strain Tolerance

NIST has measured the irreversible strain limit eirr

of several Hyper Tech-fabricated multifilament strandsusing a stress-free cooling apparatus.9 NIST found thateirr increases with the number of filaments in the wire,and therefore reduction of filament size improves thestrain tolerance of the conductor. The strain tolerancelimits reported are 0.37% for a 7-filament conductor,0.40% for a 19-filament conductor, and 0.48% for

37-filament conductor. Figure 6 shows the relationshipbetween Jc and strain for a 37-filament wire.

Alternating Current (AC) Losses

The most near-term application to generate wirevolume to help drive the MgB2 wire price down is MRIapplications. As wire price is driven down by high-vol-ume requirements from the MRI industry, the potentialfor MgB2 to be used in electrical power applicationsincreases. Most of these applications operate on AC, andthe few applications that are DC powered involve someAC ripple, or ramping up and down. AC losses are anissue in power system design; the potential of MgB2 tobecome a good low AC loss superconductor operatingin the 15–30 K range will be an important advantage forsystem integrators. Fortunately, the MgB2 strand is suchthat the barrier and sheath material can be easily inter-changed for fabricating a conductor with a more resis-tive matrix than the standard multifilament wire toreduce external field-induced eddy current losses. Thedominant factor in external field losses is the filamentsize, which is reliably in the 50 mm range at present;Hyper Tech is working on reducing the filament sizefurther. Nevertheless, the filamentation and reductionof filament size is trivial as compared with YBCO-coat-ed conductors. Twisting, exceedingly difficult withYBCO conductors, is simple and straightforward forMgB2. With respect to transport current losses, MgB2

has the traditional advantage of a wire as compared witha tape—the losses are proportional to the width or ra-dius, and therefore can be quite low for MgB2.

Normal Zone Propagation

As MgB2 superconductor wire development ad-vances and more and more coils are fabricated, quenchprotection is becoming increasingly important in mag-net design. For the protection of superconducting de-vices, a high normal zone propagation velocity (vnz) ispreferred. The University of Twente studied the vnz ofMgB2 superconductors from several manufacturers, in-cluding Hyper Tech.10 For a standard Hyper Techmultifilament strand, Twente found a vnz of 15–50 cm/s at 5 K, 4.8 T. For comparison, the vnz ofNbTi and Nb3Sn ranges from 1 to 100 m/s; the vnz ofYBCO-coated conductors ranges from 0.2 to 1.0 cm/s.Twente found that vnz will decrease with lower n values,

H (T)0 2 4 6 8 10 12 14

1e+2

1e+3

1e+4

1e+5

1e+6

1e+7869 - Undoped 18+1 Nb/Cu/Monel (HT 700/20)1011 - 30nm SiC-10% (HT 675/40)

J ,

(A/c

m )

Fig. 4. Comparison between SiC-doped and undoped MgB2 wiresat 4 K, Jc versus field.

H (T)0 2 4 6 8 10 12

J C (

A/c

m2 )

1e+2

1e+3

1e+4

1e+5

1e+6626 -Undoped 18+1 Nb/Cu/Monel (HT 700/20) 1011 - 30nm SiC-10% (HT 625/3hr)

Fig. 5. Comparison between SiC-doped and undoped MgB2 wiresat 15 K, Jc versus field.


and will decrease by a factor of 2–5 when transitioningfrom 25 K (vs 5 K).

MgB2 Coil Development

Advancements in MgB2 wire manufacturing haveled Hyper Tech into superconductor coil and magnetdevelopment. The development of long-length multifil-amentary wire has enabled the design and fabrication ofa series of MgB2-wound solenoid and racetrack coils.The characterization of these coils allowed us to deter-mine the superconductivity properties of MgB2 wireover a long length. Establishing excellent properties overlength in early demonstration coils was followed by de-signing of more realistic coils using MgB2 supercon-ductor wire for specific applications. Long-lengthcharacterization in coil form is equally important forthe advancement of the MgB2 superconductor becausemost commercial applications will require many kilo-meters of wire.

Wind-and-React Solenoid Coils

Hyper Tech fabricated several solenoid coils woundwith long lengths of Nb/Cu/Monel-type multifilamentwire, using the wind-and-react approach. As coil fabri-cation techniques advanced, each successive coil waswound with increasing continuous lengths of wire. Themagnet bore size was either 3.8 or 7.6 cm, and the coilheight was 7.1 cm in the largest coil. Each coil was heat

treated in flowing Argon in a stainless-steel retort in afurnace and was then impregnated with epoxy. Themultifilament strands were insulated with an s-glassbraid.

One of the larger wind-and-react coils fabricated byHyper Tech was wound with 740 m of 0.83 mm strand,totaling 3463 turns around a 3.8 cm bore. This coil at-tained a field of 3.9 T at 4 K, 3.0 T at 15 K, 2.4 T at20 K, 1.8 T at 25 K, and 0.9 T at 30 K. Depicted in Figs7 and 8 is a photograph of the coil and a graph showing

Fig. 7. Photograph of a 740 m wind-and-react coil.

37 filaments 0.48%

50

75

100

125

150

175

0 0.1 0.2 0.3 0.4 0.5 0.6

Sample loadedSample unloaded

Cri

tical

Cur

rent

Je

(A/m

m2 )

Applied Strain (%)

T = 4 K; B = 6 T Initial n-value ~ 29

ε irr ~ 0.48 %

A BC D E F G H

G'A' B' C' D' E' F' H' I'

J'

K'

K

J

I

19 filaments 0.40%

7 filaments 0.37%

37 Filament Wire

Fig. 6. Critical current versus applied axial strain for a 37 filament MgB2 wire.9


the transport properties of the coil measured at varioustemperatures from 4.2 to 35 K.

A load line was generated for the 740 m solenoid coilto compare the test results of the coil with short samplemeasurements. Using the coil geometry and Ic measure-ments, a field distribution in the bore was calculated tofind the peak field in the coil at the innermost winding.11

At an Ic of 84.5 A at 4.2 K, the peak field in the coil wasestimated to be 3.96 T. The short sample was a 1 m ITERbarrel that was heat treated with the coil. The load line inFig. 9 shows that the coil performed better than the shortsample; the lower short sample value may be due to avariation in the properties in this particular strand.

React-and-Wind Solenoid Coils

Hyper Tech demonstrated a large solenoid coil fab-ricated with the react-and-wind approach; the bore size

in this coil was 53 cm. Over 820 m of an s-glass insu-lated Nb/Cu/Monel-type multifilament strand with atotal of 482 turns were wound on a copper coil former.A picture of the 53 cm coil is shown in Fig. 10. Theinsulated wire was heat treated on a stainless-steel spoolto react the wire to form MgB2. The reacted wire waswound on the coil bobbin from the stainless-steel spool.Epoxy was applied during coil winding to impregnatethe coil using a wet-wind-compatible epoxy. The coilgenerated an axial field of 0.12 T at 20 K, which was theexpected result.

Wind-and-React Racetrack Coils

Hyper Tech fabricated over 12 single-layer race-track coils for a cryogenic rotor in an LH2-cooled, su-perconducting generator demonstration under a NASAcontract. The racetrack coils were made using the wind-and-react approach. The wire was insulated with either asingle-layer s-glass braid or a ceramic sol–gel coating,and the wire length varied between 30 and 80 m. Thecoils were epoxy impregnated after heat treatment. Fig-ure 11 shows a picture of a racetrack coil. The best-per-forming coil reached 400 A at 4 K. This rotor coilgenerator project will lead to the fabrication of four ro-tor coils, with each coil being fabricated from one 600 mcontinuous length of a MgB2 superconductor wire. Thesuperconducting generator demonstration will be a

Temperature, T, K

0 40302010

Cri

tical

Cur

rent

, I ,

A

0

20

40

60

80

100

Bor

e Fi

eld,

B ,

T

1

2

3

4

0

5

10

15

20

25

J ,

kA/c

m

0

20

40

60

80

100

120

J ,

kA/c

m

Fig. 8. Ic, Je, and Jc as a function of temperature for 740 m wind-and-react solenoid coil.11

B, T0 2 4 6 8 10

I c, A

0

100

200

300

400

Nb/Cu/monel short sample at 4.2KSolenoid Coil Load Line

Fig. 9. Coil load line and short sample (1 m barrel) Jc versus Bcurve for the 740 m wind-and-react solenoid coil.11

Fig. 10. Fifty-three centimeters react and wind coil wound with820 m of an 1811 multifilament MgB2 wire.

Fig. 11. Typical MgB2 racetrack-type ‘‘rotor’’ coils.


2 MW generator operating at 20 K in liquid H2. Thefirst rotor coil has been fabricated, featuring a step pro-file, as shown in Fig. 12.

Potential Applications for MgB2 SuperconductingWire

MRI

Currently, the largest commercial market for su-perconducting wire is MRI systems. An MgB2 super-conducting wire has the potential to impact thisindustry.12 The degree of impact depends on the priceperformance ($/kA m) of the wire at a given tempera-ture and background field. The field of an MRI systemis typically described by the center bore field; however,the wire can potentially see up to twice that of the cen-ter. For example, a 1.5 T MRI system might experienceup to 3 T on the wire. The first MRI-type applicationswill most likely be for systems operating in the 10–27 Krange in background fields on the wire from 0.5 to3.0 T; these systems could be either an open or enclosedMRI system. By operating at temperatures higher than4 K (liquid He, operating temperature of present MRIdevices), there are potential savings in the constructionof a vacuum cryostat, especially with the open MRIsystems that typically require two separate cryostats. Be-cause MgB2 superconductors have a higher temperaturemargin compared with NbTi and Nb3Sn superconduc-tor wires, the MgB2 conductor can be constructed with

less copper (which is needed for stability), potentiallyincreasing the overall coil engineering current density.Also, for smaller bore magnets (sizes smaller than full-body MRI, o100 cm bore), less costly cryocoolers canpotentially be used. Therefore, the mostly likely systemsto be initially marketed using a MgB2 superconductingwire will be smaller bore extremity (arm and leg) MRIsystems, and open full-body MRI systems.

An increase in MgB2 wire usage beyond these initial10–27 K MRI system markets will depend on how lowthe price performance can be driven for a MgB2 wire op-erating at 4 K, especially at high fields. An MgB2 wireoperating at 4 K has the potential to exceed the perfor-mance of NbTi wire in high fields and become equal to orgreater than Nb3Sn wire performance at 4 K. For MgB2

to compete on a price performance basis with NbTi andNb3Sn at 4 K, increases in field performance and lowermanufacturing costs need to be demonstrated. As dis-cussed previously, dopants like nano-SiC are being addedto increase Bc2 and work is being conducted worldwide toincrease both pinning and connectivity in MgB2. Exper-imental work with MgB2 thin films13,14 has indicated thatan order of magnitude improvement in current density ata given magnetic field should be possible. With regard tothe $/m price of MgB2 wire, manufacturing processes aresimilar to that used for NbTi and Nb3Sn. A typical NbTisuperconductor wire at 0.8 mm sells for around $1/m,and Nb3Sn sells for $4–6/m. The selling price for MgB2

wire on a per-meter basis should be less than NbTi wirebased on manufacturing and material costs. Not only arethe basic materials Mg and B less costly than Nb and Ti,the density of MgB2 is one-third that of NbTi andNb3Sn. Thus, for the same kilogram of raw material,we obtain three times the wire length, meaning that the $/m selling price of MgB2 wire should become much lessthan NbTi wire. If the engineering current density of theMgB2 wire can be increased to exceed the NbTi or Nb3Snwire performance at 4 K in field, then the combination ofhigh superconductivity performance and low wire priceshould generate a much lower $/kA m than the presentNbTi or Nb3Sn wire at a given magnetic field. Therefore,for full-body MRI systems (1.5 and 3.0 T) and ultra-high-field (7 T plus) MRI and NMR, MgB2 wire could be-come the wire of choice based on basic wire economics.

Fault Current Limiters

Superconducting fault current limiters have beeninvestigated and developed over the past 15 years. Their

Fig. 12. The first full size MgB2 rotor coil fabricated for a 2 MWgenerator demonstration. The rotor coil is shown installed on a testplate.


main advantages are a negligible influence on an elec-trical network under normal operating conditions, prac-tically instantaneous limitation, and automatic responsewithout an external trigger. There are basically two typesof fault current limiters: resistive and inductive. MgB2 isknown to have a very sharp transition from a supercon-ducting to a normal state. This sharp transition makesMgB2 ideal for the resistive-type fault current limiter.Moreover, the normal zone heat propagation of MgB2 israpid compared with ceramic superconductors, whichminimizes hot spots, and the conductor can be designedwith a variety of highly resistive sheath materials. Withits potential low cost and ability to operate at moderatetemperatures (20–30 K), MgB2 is very attractive as acost-effective FCL element. Work has been reported15–17

on the current limiting properties of different kinds ofMgB2 wires at 50 Hz in the temperature range of 20–30 K. MgB2 wires showed good limiting propertiescharacterized by a fast transition to the normal stateduring the first half of the first cycle and a limiting effectduring the subsequent six cycles without damage to thewire samples. The temperature differences at the begin-ning of the transition in the first cycle smoothed outduring subsequent cycles; this was more evident for Cu-stabilized multifilament MgB2 wires. A stabilized twist-ed multifilament wire is also a low AC loss supercon-ductor, an important factor for fault current limitersthat operate on AC during the normal (no fault) con-dition. The other type of fault current limiters is induc-tive in nature, which involve superconductor coilsfabricated with considerably longer lengths of supercon-ductor. MgB2 is also suitable for potential low-cost coilsin the 20–30 K operating range. MgB2 fault currentlimiters will be in competition from inductive FCL coilsusing YBCO-coated conductors.

Transformers

Superconducting transformers will have reducedsize and weight and lower losses compared with trans-formers fabricated with conventional wire. In additionto these attributes, the superconducting transformermay significantly benefit the entire electrical system.Benefits come from reducing the short-circuit current inthe system and lower transformer impedance. The su-perconducting wire has current-limiting capability. Thiscan reduce the interrupting ratings of circuit breakersand in some cases, permit the use of mesh networks for atightly coupled power system. The lower transformer

impedance will improve voltage regulation and stabilityand increase real and reactive power availability to thepower system. There are many challenges in buildingand demonstrating a superconducting transformer, butwith regard to the wire the primary factor is producing asuperconductor wire with a price performance in the$1–3/kA m range at the targeted field and temperature.Several of the proposed transformer designs suggest thatthe field on the wire can be as low as 0.15–0.20 T, butsome designs could go up to 1.0 T. With a stabilized,multifilamentary, twisted, resistive matrix, low AC lossMgB2 wire at 20–30 K in fields of around 0.2–1.0 T, itappears that the targeted conductor cost can be met in afew years. The targeted power utility transformer forcommercialization has been identified at 30 MVA orlarger. If the other issues for superconducting trans-formers such as cryogenic insulation, high voltage ter-minations, and cryocooler refrigeration optimizationcan be addressed, there is the potential of commercial-izing a high-MVA transformer that is of a lower costthan a conventional transformer, based on low-costMgB2 superconductor wire. The competition to aMgB2 wire will be a YBCO-coated conductor operat-ing in the 65–77 K range in liquid N2.

Motors and Generators

Superconducting motors and generators have sev-eral potential advantages. They can be power-dense,lightweight, small volume, highly efficient, and reliable.MgB2 can offer advantages in several of the motor andgenerator systems being demonstrated. One design isthe superconducting homopolar motor being developedfor the Navy; the present design uses NbTi supercon-ductors operating at 4 K. As MgB2 current densitiesstart to exceed those of NbTi at 4 K, MgB2 offers thepotential of higher power densities, high temperaturemargins, and lighter weight coils for these supercon-ducting homopolar motors. Other superconductingmotor systems have been demonstrated using liquidneon and helium gas operating in the 20–30 K range;MgB2 superconducting coils have generated magneticfields in the 0.5–2.0 T field range at these temperatures.For all-electric aircraft motors, the fuel of choice is liq-uid H2; liquid H2 will be available for the turbine en-gine on an aircraft. Presently, for NASA, Hyper Tech isbuilding MgB2 superconducting rotor coils for a cryo-genic 2 MW generator demonstration that will becooled with liquid hydrogen at 20 K.


High Energy Physics (HEP) Applications

The long-term goal of MgB2 in HEP applications isthe production of a wire that will produce engineeringcurrent densities at least 1.2� 105 A/cm2 at 4 K in fields of12–16 T.18 While MgB2 has the potential to reach thisperformance, MgB2 should be able to satisfy several near-term accelerator-related requirements. Continually improv-ing the properties should enable MgB2 to be used in thewindings of undulator magnet installations and replace-ment wiggler magnets in accelerator applications. Somenear-term, lower field special applications include replace-ment light source bending magnets and solenoids for amuon collider. In synchrotron light sources, the benefit ofincreased brilliance of the photon beam at higher harmon-ics follows the replacement of permanent-magnet un-dulator magnets with superconducting ones for fieldso5 T. For such a modest field in a high-radiation appli-cation, MgB2 superconductor with its large thermal marginseems to be an ideal candidate superconducting material.

Adiabatic Demagnetization Refrigerators (ADR)

Future NASA instruments operating in space willfeature detectors operating well below 1 K. The coolingsystem of choice is ADR.19 These devices produce cool-ing by manipulating the entropy of a paramagnetic ma-terial, commonly referred to as a salt pill. This processinvolves ramping up and down the magnetic field seenby the pill while opening and closing a thermal switchbetween the pill and a heat sink. The heat is thenpumped up a chain of stages each at successively high-er temperatures, each stage requiring a superconductingmagnet. Targeted temperatures are 4, 10, and 15 K.While presently NbTi is being used at 4 K, and Nb3Snis being considered at 10 K, MgB2 is being consideredfor a 15 K operating temperature by NASA. As MgB2

performance exceeds that of NbTi at 4 K and Nb3Sn at10 K, MgB2 could be considered for coils operatinganywhere from 4 to 15 K or higher. This superconduct-ing application is unique because a very small diameterwire is required. The desired superconductor wire sizeneeds to be in the 0.075–0.200 mm range with an Ic inthe 3–30 A range, operating at the appropriate temper-ature (4, 10, 15 K) in 3–4 T magnetic fields.

Magnetic Separation

Superconducting magnetic separators using NbTioperating at 4 K in liquid He have been commercially

available since the early 1980s. The primary applicationof superconducting magnetic separators is removingiron from kaolin clay. These systems typically operatein the 2 T range. Experimental systems have been builtfor various wastewater treatment demonstration pro-jects. Higher field systems in the 2–5 T range have beensuggested for some magnetic separation systems. Be-cause these systems are typically large diameter boremagnets, that is, 1–2 m, they require considerable su-perconductor wire. Therefore, a low-cost superconduc-tor wire is needed. MgB2 superconductors offer thepotential of a low-cost wire with a high temperaturemargin that can enable large conduction-cooled mag-nets operating in the 10–25 K range.

Superconducting Magnetic Energy Storage (SMES)and Magnetic Levitated (MagLev) Trains

Both of these applications require the storage ofenergy in fairly large coils. The typical magnetic fieldsgenerated are in the 2–5 T range. The cost of thesuperconductor is important because a considerableamount of superconductor wire is required for theselarge coils. MgB2 superconductors have the potential tobecome a low-cost wire with a high temperature margin.If the desire is to operate at temperatures above 4 K withthese large coils, MgB2 superconductors will be a goodfit. There is also the potential for hybrid coils operatingin the 10–25 K range using MgB2 as the outer coil and aYBCO-coated conductor as the inner coil in the higherfield region.

Summary

Significant progress has been made since 2001 onthe development of long-length MgB2 superconductorwires. MgB2 superconductor wire performance is ex-pected to increase rapidly over the coming years throughimproved pinning and connectivity. The price perfor-mance at 4 K operation could exceed both NbTi andNb3Sn superconductor wire. At operating temperaturesfrom 4 to 30 K, the MgB2 wire price performance canpotentially enable several commercial applications. Theprimary competition to MgB2 in the 4–30 K tempera-ture range will be the ceramic superconductors,BSCCO, and YBCO tape; the main question is wheth-er BSCCO and YBCO will eventually be as cost effec-tive as MgB2 for superconducting devices operating inthis temperature range. Presently, the future is positive


for MgB2 superconductors, as there are several ongoingdemonstration projects that are directed toward com-mercial applications.

Acknowledgments

The authors thank Dr. Shi Xue Dou from the In-stitute of Superconducting and Electronic MaterialsDepartment of the University of Wollongong for hisassistance in the research and development of MgB2 su-perconductors presented in this paper, and Drs. NajibCheggour and Jack Ekin at NIST for providing axialstrain analysis.

References

1. M. D. Sumption, M. Susner, M. Bhatia, M. Rindfleisch, M. Tomsic, K.McFadden, and E. W. Collings. ‘‘High Critical Current Density Multifila-mentary MgB2 Strands,’’ Applied Superconductivity Conference, 5MB05,2006.

2. M. D. Sumption, M. Bhatia, M. Rindfleisch, M. Tomsic, and E. W. Col-lings, ‘‘Transport and Magnetic Jc of MgB2 Strands and Small Helical Coils,’’Appl. Phys. Lett., 86 102501-1–102501-3 (2005).

3. M. D. Sumption, M. Bhatia, X. Wu, M. Rindfleisch, M. Tomsic, and E. W.Collings, ‘‘Multifilamentary, In-Situ Route, Cu-Stabilized MgB2 Strands,’’Supercond. Sci. Technol., 18 [5] 730–734 (2005).

4. M. D. Sumption, M. Bhatia, M. Rindfleisch, M. Tomsic, and E. W. Col-lings, ‘‘Transport Properties of Multifilamentary, In Situ Route, Cu-Stabi-lized MgB2 Strands: One Meter Segments and the Jc(B, T) Dependence ofShort Samples,’’ Supercond. Sci. Technol., 19 155–160 (2006).

5. H. Kumakura, H. Kitaguchi, A. Matsumoto, and H. Hatakeyama, ‘‘UpperCritical Fields of Powder-in-Tube-Processed MgB2/Fe Tape Conductors,’’Appl. Phys. Lett., 84 3669 (2004).

6. R. Flukiger, H. L. Suo, N. Musolino, C. Beneduce, P. Toulemonde, andP. Lezza, ‘‘Superconducting Properties of MgB2 Tapes and Wires,’’ Physica C,385 286–305 (2003).

7. M. D. Sumption, M. Bhatia, M. Rindfleisch, M. Tomsic, S. Soltanian, S. X.Dou, and E. W. Collings, ‘‘Large Upper Critical Field and IrreversibilityField in MgB2 Wires With SiC Additions,’’ Appl. Phys. Lett., 86 092507-1–092507-3 (2005).

8. S. X. Dou, A. V. Pan, S. Zhou, M. Ionescu, H. K. Liu, and P. R. Munroe,‘‘Substitution-Induced Pinning in MgB2 Superconductor Doped With SiCNano-Particles,’’ Supercond. Sci. Technol., 15 1587 (2002).

9. N. Cheggour and J. W. Ekin, ‘‘Electromechanical properties of MgB2 su-perconductors,’’ (to be published).

10. H. van Weeren, M. Dhalle, A. den Ouden, and H. ten Kate. ‘‘Normal ZonePropagation in MgB2 Conductors,’’ Applied Superconductivity Conference,5MB01, 2006.

11. M. D. Sumption, S. Bohnenstiehl, F. Buta, M. Majoros, S. Kawabata, M.Tomsic, M. Rindfleisch, J. Phillips, J. Yue, S. Kawabata, and E. W. Collings.‘‘Wind and React and React and Wind MgB2 Solenoid, Racetrack and Pan-cake Coils,’’ Applied Superconductivity Conference, 2LC01, 2006.

12. Y. Iwasa, D. C. Larbalestier, M. Okada, R. Penco, M. D. Sumption, andX. Xi, ‘‘A Round Table Discussion on MgB2 Toward a Wide Market or a NicheProduction?—A Summary,’’ IEEE Trans. Appl. Supercond., 16 1457 (2006).

13. C. B. Eom, M. K. Lee, J. H. Choi, L. J. Belenky, X. Song, L. D. Cooley, M.T. Naus, S. Patnaik, J. Jiang, M. Riakl, A. Polyanskii, A. Gurevich, X. Y. Cai,S. D. Bu, S. E. Babcock, E. E. Hellstrom, D. C. Larbalestier, N. Rogado,K. A. Regan, M. A. Hayward, T. He, J. S. Slusky, K. Inumaru, M. K. Hass,and R. J. Cava. ‘‘High Critical Current Density and Enhanced IrreversibilityField in Superconducting MgB2 Thin Films,’’ Nature, 441 558 (2001).

14. W. N. Kang, E. M. Choi, H. J. Kim, H. J. Kim, and S. I. Lee, ‘‘Growth ofSuperconducting MgB2 Thin Films Via Postannealing Techniques,’’ PhysicaC, 385 24–30 (2003).

15. M. Majoros, L. Ye, A. M. Campbell, T. A. Coombs, A. V. Velichko, D. M.Astill, P. Sargent, M. Haslett, M. D. Sumption, E. W. Collings, M. Tomsic,and M. Husband, ‘‘Fault Current Limiting Properties of MgB2 Supercon-ducting Wires,’’ IEEE Trans. Appl. Supercond., 17 (2007).

16. Ye Lin, M. Majoros, A. M. Campbell, T. Coombs, S. Harrison, P. Sargent,M. Haslett, and M. Husband, ‘‘MgB2 Sample Tests for Possible Applicationsof Superconducting Fault Current Limiters,’’ IEEE Trans. Appl. Supercond.,17 (2007).

17. M. Majoros, L. Ye, A. M. Campbell, T. A. Coombs, M. D. Sumption, andE. W. Collings, ‘‘Modeling of Transport AC Losses in Resistive Fault CurrentLimiters Under no Fault Conditions,’’ IEEE Trans. Appl. Supercond., 17 (2007).

18. E. W. Collings, S. Kawabata, M. Tomsic, and M. D. Sumption, ‘‘MagnesiumDiboride Superconducting Strand for Accelerator and Light Source Applica-tions,’’ IEEE Trans. Appl. Supercond., 16 1445–1448 (2006).

19. J. Tuttle, S. Pourrahimi, P. Shirron, E. Canavan, M. DiPirro, and S. Riall, ‘‘A10 K Magnet for Space-Flight ADRs,’’ Cryogenics, 44 383–388 (2004).


j.1744-7402.2007.02138.x

Documents