Critical Fields and Critical Currents in MgB 2 David Caplin and Judith Driscoll Imperial College, London Work supported by EPSRC Commercial Uses of Magnesium Diboride Superconductor in the Electric Power Industries. Cambridge 12 th April 2002

ICSTM Centre for High Temperature Superconductivity Generators & Motors J cE A /cm 2 B / Tesla Fault Current Limiters Cables Trans- formers Current leads for magnets BiSrCaCuO 77K YBa 2 Cu 3 O 7 77K Whats needed for power applications Super- conducting energy storage

ICSTM Centre for High Temperature Superconductivity Key issues for power applications J cEOverall current density J cE of conductor, not just of superconductor Performance in field Filamentary architecture essential for AC applications Anisotropy of J cE 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

MgB 2 Mg B B B

ICSTM Centre for High Temperature Superconductivity Useful for high current applications Phase Diagram pure MgB 2 Crystal H || ab H c2 /Tesla limit of superconductivity T /K Crystal H || c

ICSTM Centre for High Temperature Superconductivity J c (B,T) Y. Bugoslavsky et al., Nature 410, 563 (2001) Alfa-Aesar powder Copper wire

ICSTM Centre for High Temperature Superconductivity How much current can be carried by a Type II superconductor? JJ FLFL E 1.Magnetic fields (self- and applied) generate vortices 2.Lorentz force F L between current J and vortices 3.Vortices drift 4.Moving flux generates E-field

ICSTM Centre for High Temperature Superconductivity J c is zero, unless the vortices can be pinned Microscopic defects provide pinning sites, e.g: –Precipitates –Dislocations J cJ c drops as H increases, material useful only for H<H irr H irr is set by number and strength of these pinning defects (but H irr < H c2 )

ICSTM Centre for High Temperature Superconductivity Useful for high current applications Irreversibility field: clean MgB 2 H irr /Tesla T /K crystal H||c For H < H irr, J c >~10 3 A/cm 2

ICSTM Centre for High Temperature Superconductivity Issues Can H c2 (superconductivity) be increased? Can H irr (useful current densities) be increased? What is the impact of anisotropy? simultaneouslyscaleable routesCan all these parameters be optimised simultaneously, and by scaleable routes?

ICSTM Centre for High Temperature Superconductivity H c2 can be raised by cutting the electron mean free path, e.g. alloying Cutting reduces dirty ( clean for << clean. H c2 enhancement Thin film H c2 /Tesla T /K Crystal H || c

ICSTM Centre for High Temperature Superconductivity Irreversibility field (At H = H irr, J c ~10 3 A/cm 2 ) Commercial powder has H irr ~<0.5 H c2 Thin films have H irr ~0.8 H c2 (??) Create pinning defects by controlled modification of MgB 2 L. Cowey et al., MgB 2 + additive

ICSTM Centre for High Temperature Superconductivity Vortices in MgB 2 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. Anisotropy Crystal H c2 (ab) / H c2 (c) 2. cf. ~5 in YBaCuO, and >30 in BiSrCaCuO. J. Moore et al.

ICSTM Centre for High Temperature Superconductivity MgB 2 : J c at 20K USEFUL ATTAINABLE??

ICSTM Centre for High Temperature Superconductivity Generators & Motors J cE A /cm 2 B / Tesla Fault Current Limiters Cables Trans- formers Current leads for magnets BiSrCaCuO 77K YBa 2 Cu 3 O 7 77K Whats needed for power applications Super- conducting energy storage MgB 20K

ICSTM Centre for High Temperature Superconductivity Niche DC Application? Open magnet MRI MgB 2 looks applicable Higher field, better resolution Lighter magnet Not just imaging, but also minimally-intrusive surgery

ICSTM Centre for High Temperature Superconductivity Simultaneous 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???

ICSTM Centre for High Temperature Superconductivity MgB 2 is a Type II superconductor, but when clean has a very low H c2, and very weak vortex pinning. To be useful for high current applications, it has to be modified: How short can be made?? –Alloy to increase H c2 How short can be made?? Inside grains, not at boundaries!! –Defects to strengthen pinning. Inside grains, not at boundaries!! CONCLUSIONS 1

ICSTM Centre for High Temperature Superconductivity So far, there has been little systematic study of the basic science, chemical modificationSo far, there has been little systematic study of the basic science, chemical modification Performance on lab scale what is needed for niche applicationsPerformance on lab scale what is needed for niche applications Its still early days!!Its still early days!! CONCLUSIONS 2