Advances in MgB 2 Strand Development by Compositional Optimization and Doping

1. Advances in MgB2 Strand Development by Compositional Optimization and Doping E.W. Collings with M.D. Sumption M.A. Susner Laboratories for Applied Superconductivity and Magnetism The Ohio State University, Columbus, OH, USA Prepared for EUCAS, Brussels, Belgium, Sept.16-20, 2007

2. Development of increased Bc2 and Jc in practical bulks and wires ----outline of the presentation • Upper critical fields in films – a wire development goal • Structure and the role of dopants • Critical current density in films – a wire development goal with focus on Intrinsic improvements to Jc(B) Increasing Hirr/Bc2/ Jc with dopants Extrinsic improvements to Jc(B) Reducing porosity Increasing connectivity

3. Intrinsic Properties of the MgB2 Crystal (Grain) High Upper Critical Fields in Films ---- the initial impetus to high critical field strand development

4. -- after A. Gurevich, cond mat 0701281v1 -- after Ferdeghini, Ferrando et al.

5. Structure, upper critical field, charge-carrier scattering, and the role of dopants The presence of Mg in the compound MgB2 stabilizes the B sub-lattice in the form a honeycomb-like stack of hexagonal networks – a “new allotrope of B” • The B honeycomb which dominates the electronic structure • derives from: • -bonding orbitals within the B planes and • -bonding orbitals both within and out of the plane. • The in-plane  orbitals give rise to a 2D -band • The in-plane and out-of-plane  orbitals form the 3D -band.

6. The Upper Critical Field Hc2 in films, and its enhancement through impurity scattering, provided the initial impetus for MgB2 development for applications In dirty MgB2 films the fact that D << D (i.e  scattering much stronger than  scattering) leads to : and Thus decreasesinDproduce increases in Hc2nearTc And further decreases inD produce increases in Hc20 These relationships in principle allow Hc2 to be manipulated by selectively introducing dopants into the MgB2 lattice – for example:

7. Some dopants and their expected effects on the Hc2s • carbon when substituting for B provides strong  scattering and enhances Hc2nearTc • metals substituting into the Mg sublattice increase the  scattering and enhance Hc20 • macroscopic particles that contribute to lattice distortion increase both  and  scattering ----- after Gurevich and Larbalestier some practical examples follow

8. Zr substituting into the Mg sublattice can increase the  scattering and enhance Hc20 ZrB2 particles that contribute to lattice distortion increase both  and  scattering A – magnetization loop closure B – from Jc(B) at 100A/cm2 C – resistive transition C B A ZrB2 increases moHirr over the entire temp. range with more pronounced effect at low temperatures

9. Birr Bc2 Bulk MgB2 doped with SiC, C, and TM-diborides Resistive critical field measurement

10. High Critical Current Densities in Films ---- a goal and incentive for high Jc development in bulk MgB2 and strands

11. The Critical Current Densities of Films

12. The first stage – just “standard” binary PIT strands

13. Improved strands with dopants Selected thin film data: Kan 03 Kom 02 Eom 01 Zha 05 in Kit 05 OSU strand data: SiC+excess Mg excess Mg 19 filament binary

14. Further improvements with dopants

15. On the trail of higher Jc s – a catalog of dopants Nitrides, borides, and silicides Carbon and carbon inorganics Metal oxides Metallic elements Organic Compounds

17. What do we expect to achieve by doping? (1) Increase in the crystal’s Hirr and Bc2 (2) Introduction of flux pinning centers (elementary strength, fp) (3) Localized lattice strain by leftover dopant Both (1) and (2) either separately or together serve to increase the bulk pinning force, Fp(N/m3), since: Fp = fp x (force summation function) Hc2m(T).f(h) (h  H/Hc2) where f(h) is often seen in the formhp(1-h)q

18. 7-fil. Strand glidcop sheathed 19-fil strand Monel sheathed Data fitted with arbitrary p and q values Lack of scaling indicates a mixture of pinning sites

19. Some recognized pinning functions -- after D. Dew-Hughes, Phil. Mag. 30 (1974) 293

20. SiC-doped MgB2 • surface (GB) pinning at low temperatures and low fields • point pinning at high temperatures and low fields • shift to “volume pinning” at high fields

21. Finally, in pursuing the mixed-pin approach one needs to be alert to the possibility of compositional inhomogeneity in doped samples PPMS-measured heat capacity Low temperature heat capacity data

22. Extrinsic Properties of the MgB2 Crystal (Grain) Extrinsic improvements to Jc(B) by: Reducing porosity Increasing connectivity

23. The porosity of in-situ MgB2 SEM/back-scattered-electron micrograph of an Mg1.15B2 monocore strand after 40min/700oC (left) and its high resolution SEM/secondary-electron images. 44-130 nm

24. Porosity of in-situ MgB2 Assume a piece of Mg buried in mass of B powder compacted to various B-powder-packing densities The published densities of the component crystals are: Mg =1.77 B = 2.34 MgB2 = 2.67 The following Table is for a B-powder packing-density of 65% The accompanying plot is for B-powder packing-densities varying from 50% to 100%

25. B Powder Packing Density of in-situ MgB2 Overall final packing density of the in-situ-reacted body = 1721/(13.74+14.24) = 61.5% 65% dense 100% dense MgB2 hole Mg

26. General Porosity actual pores intergranular film partially disconnecting some of the grains

28. Normal-state resistivity measurement as a gauge of general connectivity Since general porosity is a geometrical phenomenon, normal-state resistivity can be used as a measure of effective cross-sectional area for transport (a/A  1/F) – in both normal and superconducting states Our connectivity analysis uses a modified Rowell approach based on the Bloch-Gruneisen function which recognizes the large variation in Debye- exhibited by heavily doped specimens

29. The Bloch-Gruneissen (B-G)-Based Connectivity Analysis The Bloch-Gruneissen (B-G) extension of Rowell’s method requires detailed temperature-dependence measurement but at the same time provides some extra information. It consists of replacing the previously assumed universal i(T) with a -dependent one in the form of the B-G function: The measure resistivity is fitted to to: the constant k having first been determined by fitting a set of single crystal data (for which F= 1) --- finally, the percent connectivity , 100/F, is determined for all of the samples

30. Measuredm(T) = 0 + i(T) and i(T) for wires of binary MgB2 and heavily doped (with TiB2, SiC) MgB2 designated MB700, MBTi800, and MBSiC700

31. B-G-fits to in-house and published  (T)data [1] J.J. Tu et al., Phy. Rev. Lett. 87 277001 (2001) [2] Private Communication [3] P. C. Canfield, D. K. Finnemore, S. L. Bud'ko, J. E. Ostenson, C. Lapertot, C. E. Cunningham, C. Ptrovic, Phys. Rev. Lett. , vol. 86 p. 2423 (2001) [4] Yu. Eltsev, S. Lee, K. Nakao, et al., et al., Phys. Rev. B 65 140501 (2002)

32. B-G-fitted Results * Assumed for the single crystal ** Measured – after Eltsev ** * Obtained by fitting the B-G function toEltsev’s resistivity data

33. Normal-state resistivity measurement associated with Bloch-Grüneisen analysis of the data is a very powerful tool, yielding: • The effective c/s area for transport, in principle enabling • a true intragrain Jc to be determined • (2) Provides the intragrain residual resistivity, ro, which as a measure of • intragrain electron scattering should correlate with Bc2 • Yields a resistive debye temperature, R, which may be compared • with its calorimetric counterpart, D • (4) D , combined with a measured Tc, and with the aid of the • McMillan formula , • provides a value for the electron-phonon coupling constant,  .

34. Debye Temperature and Electron –Phonon Coupling Constant using PPMS Data Heat Capacity and Debye Temperature

35. Conclusion A potential performance box is defined by the exceptionally high Bc2 exhibited by films, and the usefully high Bc2 exhibited by doped wires This property awaits full exploitation -- Intrinsically, in terms of flux pinning -- Extrinsically, in terms of porosity connectivity

36. The End ----- thank you

37. SPARE SLIDE COLLECTION

38. * Measured – after Eltsev ** Obtained by fitting the B-G function toEltsev’s resistivity data

39. i(T) (1/F)i(T)

40. m(T) = 0 + i(T) and i(T) for wires of binary MgB2 and heavily doped (with TiB2, SiC) MgB2

41. previous undoped binary result (1/F)i(T) for wires of binary MgB2 and heavily doped (with TiB2, SiC) MgB2

42. --- from hydrocarbons C substituting into the B sites --- from SiC based on Kumakura et al