Critical Fields and Critical Currents in MgB 2 David Caplin and Judith Driscoll Imperial College,...
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Transcript of Critical Fields and Critical Currents in MgB 2 David Caplin and Judith Driscoll Imperial College,...
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
ICSTM Centre for High Temperature Superconductivity
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