1. IEE MgB 2 superconductor processed in high magnetic fields MgB 2 superconductor processed in high magnetic fields Yanwei MA Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100080, China

2. IEE Candidate HTS for large scale applications D. C. Larbalestier et al., Proceedings of the IEEE, vol. 92, 2004, 1639 Commercial production: –Bi2223 / silver tape - 1 st Generation HTS Industrial laboratory: –YBCO 2 nd Generation HTS “coated conductor” Pre-commercial: –MgB 2 Background

3. IEE Benefits of SC Materials 1. Low losses, 2. Small volume, 3. Light weight, 4. High efficiency. Merits of superconductors:

4. IEE YBCOBi2223 MgB 2 KindsBi2223YBCOMgB 2 Anisotropy parameters 50 － 1005－75－72－32－3 Structure of HTS superconductors

5. IEE The principal limitation to technological applications of high-Tc polycrystalline oxide superconductors is the low critical current density (Jc) found in these materials. This limitation is strongly correlated with the misorientation among the grains. Hence, to minimize the number of intergranular weak links, a high degree of crystallographic texture must be obtained. One possible route by which a strong crystallographic texture can be produced is to melt-process the HTS material under the effect of an elevated magnetic field. Magnetic aligning technique

6. IEE First report in Nature, in 1991 V:volume of particle H:external magnetic field D: magnetic anisotropy (   -   )   = 6.1 x 10 -7 emu/g   = 4.3 x 10 -7 emu/g > kT P. de Rango et al., Nature 349 (1991) 770 c-axis alignment is strongly preferred. Effect of magnetic field: At 1000  C, 10 T, orientation takes place for grains with the size of 40 nm. For YBCO superconductors:

7. IEE Bi-oxide superconductors (Bi2212 and Bi2223) and YBCO exhibit strong anisotropy in magnetic susceptibility. Many studies reported enhancements in texture for HTS superconducting bulks and tapes, by magnetic-melt processing (MMP), and showed that uniform high texture is achieved throughout the thickness. Present situation Bi-2223 tapes Yanwei Ma, et al., Physica C 282 (1997) 2619. Bi-2212 tapes H. B. Liu, et al., Physica C 303 (1998) 161. S. Awaji, Yanwei Ma, et al., Current Appl. Phys. 3 (2003) 391. Bi-based bulks H. Maeda, et al., Physica C 386 (2003) 115. W. Chen, et al., J. Crystal Growth 204 (1999) 69. YBCO bulks S. Awaji, et al., IEEE Appl. Supercond. 9 (1999) 2014. YBCO films Yanwei Ma, et al., Appl. Phys. Lett. 77 (2000) 3633. Yanwei Ma, et al., Phys. Rev. B 65 (2002) 174528.

8. IEE In 1991, Rango et al. at Grenoble reported the effect of magnetic field on the growth of YBCO bulk samples. In 1999 Liu, et al. Bi-2212 tape In 2000 年 Maeda, Awaji, et al. Bi-bulk,YBCO bulk Improved the degree of texture and enhanced Jc by MMP! In 2000 年 Ma, et al., YBCO film MgB 2 processed in fields ？ In 1997 Ma, et al. Bi-2212 tape Hot topic: Superconductors synthesized in high magnetic fields Motivation

9. IEE Introduction-MgB 2 a=3.086 Å; c=3.524 Å The recent discovery of magnesium diboride (MgB 2 ) with its superconducting transition temperature at 39 K has generated much interest in both fundamental research and applications. J. Akimitsu et al., Nature (London) 410, 63 (2001).

10. IEE Experimental procedure MgB 2 Powder PressingBulk Heated in magnetic fields Tape Characterization

11. IEE Temperature profile for processing of MgB 2 in magnetic fields 0.5 h1 hFurnace cooling 600-800  C in vacuum Time Temp. Magnetic field

12. IEE GroupSample type Sintering temperature/ time Applied field during sintering Sample surface and field direction during sintering Tc Group I Fe clad tape 600  C/1 h 10 TParallel35.2 Fe clad tape 600  C/1 h 0 T 35.5 Group II Fe clad tape 700  C/1 h 14 TPerpendicular  Fe clad tape 700  C/1 h 0 T  Group III Pellet 800  C/2h 14 TPerpendicular37 K pellet 800  C/1 h 8 T 36.9 K pellet 800  C/1 h 0 T 37.1 K Description of samples used in this work

13. IEE XRD patterns for tapes with and without filed Broadening of the FWHM indicates inferior MgB 2 crystallinity and lattice distortion of the core MgB 2, closely related to the enhanced flux pinning.

14. IEE Transport critical current density Jc (Tapes) H a is parallel to the tape surface 600  C

15. IEE H a is perpendicular to the tape surface Magnetc critical current density Jc measured by magnetization (Tapes) 700  C

16. IEE 14 T0 T Microstructure (SEM)-Tapes Clearly, well-developed grains can be seen in both samples. However, the core of the 0T sample is quite porous and loose. In contrast, with the application of strong magnetic field, densification of the MgB 2 core obviously occurred, resulting in the quite uniform microstructure and the better connectivity between the MgB 2 grains.

17. IEE Comparison between Group I and II tapes It is interesting to note the effect of a magnetic field seems different between Group I (Ha // the tape plane) and II (Ha  the tape surface) tapes. For Group I, although the enhanced Jc-B characteristic was observed in high field region due to induced poor crystallinity, the Jc improvement in low field area is small. The improved Jc by more than a factor of 2 for the field tapes of Group II was achieved. This indicates that the magnetic field works more effectively to enhance the Jc- B properties for Group II during processing.

18. IEE XRD patterns for different bulks XRD results also reveal a larger FWHM of the MgB 2 peaks for the pellet samples processed in the fields, implying that the pinning centers effective in a high-field region were possibly introduced by the applied magnetic field during processing.

19. IEE Critical temperature Tc (Bulk) The magnetic fields hardly affect Tc !

20. IEE Magnetc critical current density Jc measured by magnetization (Bulk)

21. IEE Conclusions A magnetic field is a more effective method to enhance Jc for MgB 2 superconductors. 1. The effect of high magnetic fields on the current carrying properties of both MgB 2 bulks and Fe-sheathed tapes was investigated following different thermal sequences. 2. It is found that application of a large magnetic field during processing not only results in the quite uniform microstructure and the better connectivity between the MgB 2 grains, but also induces the flux pinning centers effective in high-field region. 3. As a result, the Jc of these samples has showed much higher value than that of the MgB 2 samples in the absence of magnetic field.

22. IEE G. Nishijima, S. Awaji, K. Watanabe IMR, Tohoku Univ. K. Togano NIMS, Tsukuba Aixia Xu, Xianping Zhang IEE, CAS, China Acknowledgements Thanks to the Chinesisch-deutsches Zentrum fuer Wissenschaftsfoerderung. This work is supported by the NSFC, “Bairen” program of CAS, and also “973” national program. Collaborators:

23. IEE Thank you for your attention!

24. IEE Barrier for applications Many groups have already demonstrated powder-in-tube (PIT) conductors for applications. The key problem of MgB 2 superconductors for large-scale applications is the irreversibility field (17 T) is less than for Nb 3 Sn (27 T). D. C. Larbalestier, et al., Nature 411 (2001) 368