Critical Fields and Critical Currents in MgB 2

1. Commercial Uses of Magnesium Diboride Superconductor in the Electric Power Industries.  Cambridge 12th April 2002 Critical Fields and Critical Currents in MgB2 David Caplin and Judith Driscoll Imperial College, London Work supported by EPSRC

2. 106 Current leads for magnets 105 YBa2Cu3O7 ribbons @ 77K 104 BiSrCaCuO tapes @ 77K 103 0 2 4 6 What’s needed for power applications JcE A /cm2 Super- conducting energy storage Cables Trans- formers Fault Current Limiters Generators & Motors B / Tesla ICSTM Centre for High Temperature Superconductivity

3. Overall current density JcEof 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 Key issues for power applications ICSTM Centre for High Temperature Superconductivity

4. ICSTM Centre for High Temperature Superconductivity

5. Mg B Mg B Mg B Mg MgB2 ICSTM Centre for High Temperature Superconductivity

6. moHc2 /Teslalimit of superconductivity 10 Useful for high current applications 5 Crystal H || ab Crystal H || c T /K 20 40 Phase Diagram pure MgB2 ICSTM Centre for High Temperature Superconductivity

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

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

9. Jc is zero, unless the vortices can be “pinned” • Microscopic defects provide pinning sites, e.g: • Precipitates • Dislocations • Jc drops as H increases, material useful only for H<Hirr • Hirris set by number and strength of these pinning defects (but Hirr < Hc2) ICSTM Centre for High Temperature Superconductivity

10. 20 Useful for high current applications 10 crystal H||c 20 40 Irreversibility field: “clean” MgB2 moHirr /Tesla For H < Hirr, Jc >~103 A/cm2 T /K ICSTM Centre for High Temperature Superconductivity

11. Issues • Can Hc2(superconductivity) be increased? • Can Hirr (useful current densities)be increased? • What is the impact of anisotropy? • Can all these parameters be optimised simultaneously, and by scaleable routes? ICSTM Centre for High Temperature Superconductivity

12. moHc2 /Tesla 20 10 Thin film Crystal H || c T /K 20 40 Hc2 enhancement Hc2 can be raised by cutting the electron mean free path, e.g. alloying Cutting L reduces x: xdirty  (xclean L) for L<< xclean. ICSTM Centre for High Temperature Superconductivity

13. L. Cowey et al., MgB2 + additive Irreversibility 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 ICSTM Centre for High Temperature Superconductivity

14. J. Moore et al. Anisotropy • Crystal Hc2(ab) / Hc2(c)2.cf. ~5 in YBaCuO, and >30in BiSrCaCuO. • 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. ICSTM Centre for High Temperature Superconductivity

15. ATTAINABLE?? USEFUL MgB2: Jcat 20K ICSTM Centre for High Temperature Superconductivity

16. 106 Current leads for magnets 105 YBa2Cu3O7 ribbons @ 77K 104 MgB2 @ 20K BiSrCaCuO tapes @ 77K 103 0 2 4 6 What’s needed for power applications JcE A /cm2 Super- conducting energy storage Cables Trans- formers Fault Current Limiters Generators & Motors B / Tesla ICSTM Centre for High Temperature Superconductivity

17. 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

18. Simultaneous optimisation?? Need to: 1) Add scattering centres so as to decrease L and so also x, 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

19. 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 Hc2How short can L be made?? Defects to strengthen pinning.Inside grains, not at boundaries!! CONCLUSIONS 1 ICSTM Centre for High Temperature Superconductivity

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