Critical Fields Critical Fields and and

Critical CurrentsCritical Currents in MgB in MgB

22

David Caplin and Judith Driscoll

Imperial College, London

Work supported by EPSRC

Commercial Uses of Magnesium Diboride Superconductor in the Electric Power Industries.  Cambridge 12th April 2002

ICSTM Centre for High Temperature Superconductivity

Generators &

Motors

JcE A /cm2

B / Tesla

106

105

104

103

2 640

FaultCurrent Limiters

Cables

Trans-formers

Current leads for magnets

BiSrCaCuOtapes @ 77K

YBa2Cu3O7

ribbons @ 77K

What’s needed for power applicationsWhat’s needed for power applications

Super-conducting

energy storage

ICSTM Centre for High Temperature Superconductivity

Key issues for power applicationsKey issues for power applications

• Overall current density JJcEcE of conductor, not just of superconductor

• Performance in field• Filamentary

architecture essential for AC applications

• Anisotropy of JcE

with respect to field direction

• Cost!– Conductor itself– Cooling

• Scaleability of fabrication

• Mechanical – Strength, bend

radius, …..

• Conductor shape– tape or wire

ICSTM Centre for High Temperature Superconductivity

ICSTM Centre for High Temperature Superconductivity

MgBMgB22

Mg

Mg

Mg

Mg

B

B

B

ICSTM Centre for High Temperature Superconductivity

Useful for high Useful for high current current applicationsapplications

Phase Diagram pure Phase Diagram pure MgBMgB22

Crystal H || ab

Hc2 /Teslalimit of

superconductivity

T /K

10

5

4020

Crystal H || c

ICSTM Centre for High Temperature Superconductivity

JJcc(B,T)(B,T)

Y. Bugoslavsky et al., Nature 410, 563 (2001)

0.1 1

103

104

105

10 K

35 K30 K 25 K

20 K

0H / T

Jc

A/cm2

Alfa-Aesar powder

Copper wireCopper wire

ICSTM Centre for High Temperature Superconductivity

How much current can be carried How much current can be carried by a Type II superconductor?by a Type II superconductor?

JJ

FL

E

1. Magnetic fields (self- and applied) generate vortices

2. Lorentz force FL between current J and vortices

3. Vortices drift4. Moving flux generates E-field

ICSTM Centre for High Temperature Superconductivity

JJcc is zero, is zero,

unless the vortices can be unless the vortices can be “pinned”“pinned”

• Microscopic defects provide pinning sites, e.g:– Precipitates– Dislocations

• JJc c drops as H increases, material useful only for H<Hirr

• Hirr is set by number and strength of these pinning defects (but Hirr < Hc2)

ICSTM Centre for High Temperature Superconductivity

Useful for high Useful for high current current applicationsapplications

Irreversibility field:Irreversibility field: “clean” MgB “clean” MgB22

Hirr /Tesla

T /K

20

10

4020crystal H||c

For H < Hirr,

Jc >~103 A/cm2

ICSTM Centre for High Temperature Superconductivity

IssuesIssues • Can Hc2 (superconductivity) be

increased?• Can Hirr (useful current densities) be

increased?• What is the impact of anisotropy?• Can all these parameters be optimised

simultaneouslysimultaneously, and by scaleable scaleable routesroutes?

ICSTM Centre for High Temperature Superconductivity

Hc2 can be raised by cutting the electron mean free

path, e.g. alloying

Cutting reduces dirty (clean

for << clean.

HHc2 c2 enhancementenhancement

Thin film

Hc2 /Tesla

T /K

20

10

4020Crystal H || c

ICSTM Centre for High Temperature Superconductivity

Irreversibility fieldIrreversibility field(At H = Hirr , Jc ~103 A/cm2)

• Commercial powder has Hirr ~<0.5 Hc2

• Thin films have Hirr ~0.8 Hc2 (??)

• Create pinning defects by controlled modification of MgB2

L. Cowey et al., MgB2 + additive

ICSTM Centre for High Temperature Superconductivity

• Vortices in MgB2 are always line-like (in contrast to the weakly-pinned pancake vortices of BiSrCaCuO).

• For conductors, unlikely to be worth texturing the material (in contrast to HTS). May be useful that at high fields, percolative current paths survive through grains of appropriate orientation.

AnisotropyAnisotropy• Crystal Hc2(ab) / Hc2(c)2.

cf. ~5 in YBaCuO, and >30in BiSrCaCuO.

20 25 30 35 400

1

2

3

4

T / K

J. Moore et al.

ICSTM Centre for High Temperature Superconductivity

MgBMgB22: : JJcc at 20Kat 20K

USEFUL

USEFULATTAINABLE??

ATTAINABLE??

ICSTM Centre for High Temperature Superconductivity

Generators &

Motors

JcE A /cm2

B / Tesla

106

105

104

103

2 640

FaultCurrent Limiters

Cables

Trans-formers

Current leads for magnets

BiSrCaCuOtapes @ 77K

YBa2Cu3O7

ribbons @ 77K

What’s needed for power applicationsWhat’s needed for power applications

Super-conducting

energy storage

MgB2 @ 20K

ICSTM Centre for High Temperature Superconductivity

Niche DC Application?Niche DC Application?

Open magnet MRI

MgB2 looks applicable

Higher field, better resolution

Lighter magnet

Not just imaging, but also minimally-intrusive surgery

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Simultaneous Simultaneous optimisation??optimisation??

Need to:

1) Add scattering centres so as to decrease and so also e.g. by alloying. Of itself, will not affect vortex pinning .

2) Add pinning defects of scale ~10nm, which will introduce also some electron scattering.

Any deleterious effects???Any deleterious effects???

ICSTM Centre for High Temperature Superconductivity

• MgB2 is a Type II superconductor, but when clean has a very low Hc2, and very weak vortex pinning.

• To be useful for high current applications, it has to be modified: – Alloy to increase Hc2

How short can How short can be made?? be made??– Defects to strengthen pinning.

Inside grains, not at boundaries!!Inside grains, not at boundaries!!

CONCLUSIONS 1CONCLUSIONS 1

ICSTM Centre for High Temperature Superconductivity

• So far, there has been little So far, there has been little systematic study of the basic systematic study of the basic science, chemical modificationscience, chemical modification

• Performance on lab scale Performance on lab scale what is needed for niche what is needed for niche applicationsapplications

• It’s still early days!!It’s still early days!!

CONCLUSIONS 2CONCLUSIONS 2