1
  MgB2 superconductor processed in high magnetic 
fields 
Yanwei MA Institute of Electrical Engineering,
Chinese Academy of Sciences, Beijing 100080, China
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Candidate HTS for large scale applications
Background

• Commercial production
• Bi2223 / silver tape - 1st Generation HTS
• Industrial laboratory
• YBCO 2nd Generation HTS coated conductor
• Pre-commercial
• MgB2

D. C. Larbalestier et al., Proceedings of the
IEEE, vol. 92, 2004, 1639
3
Benefits of SC Materials
Merits of superconductors
1. Low losses, 2. Small volume, 3. Light
weight, 4. High efficiency.
4
Structure of HTS superconductors
YBCO
Bi2223
MgB2
Kinds Bi2223 YBCO MgB2
Anisotropy parameters 50-100 5-7 2-3
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Magnetic aligning technique

• 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.
6
First report in Nature, in 1991

• Effect of magnetic field

gt kT
c-axis alignment is strongly preferred.
Vvolume of particle Hexternal magnetic field D
magnetic anisotropy (??-?//)

• For YBCO superconductors

?? 6.1 x 10-7 emu/g ??? 4.3 x 10-7 emu/g
At 1000?C, 10 T, orientation takes place for
grains with the size of 40 nm.
P. de Rango et al., Nature 349 (1991) 770
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Present situation

• 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.

• 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.

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Hot topic Superconductors synthesized in high
magnetic fields

• 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? Maeda, Awaji, et al. Bi-bulk,YBCO bulk
In 2000? Ma, et al., YBCO film
In 1997 Ma, et al. Bi-2212 tape
Improved the degree of texture and enhanced Jc by
MMP!
?
MgB2 processed in fields
Motivation
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Introduction-MgB2
The recent discovery of magnesium diboride
(MgB2) 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).
a3.086 Å c3.524 Å
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Experimental procedure
Tape
Characterization
MgB2 Powder
Heated in magnetic fields
Pressing
Bulk
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Temperature profile for processing of MgB2 in
magnetic fields
Temp.
600-800?C in vacuum
Magnetic field
0.5 h
1 h
Furnace cooling
Time
 
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Description of samples used in this work
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XRD patterns for tapes with and without filed
Broadening of the FWHM indicates inferior
MgB2 crystallinity and lattice distortion of the
core MgB2, closely related to the enhanced flux
pinning.
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Transport critical current density Jc (Tapes)
600 ? C
Ha is parallel to the tape surface
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Magnetc critical current density Jc measured by
magnetization (Tapes)
700 ? C
Ha is perpendicular to the tape surface
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Microstructure (SEM)-Tapes
14 T
0 T
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 MgB2 core obviously
occurred, resulting in the quite uniform
microstructure and the better connectivity
between the MgB2 grains.
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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.

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XRD patterns for different bulks
XRD results also reveal a larger FWHM
of the MgB2 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.
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Critical temperature Tc (Bulk)
The magnetic fields hardly affect Tc !
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Magnetc critical current density Jc measured by
magnetization (Bulk)
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Conclusions

1. The effect of high magnetic fields on the current
   carrying properties of both MgB2 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 MgB2 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 MgB2 samples
   in the absence of magnetic field.

A magnetic field is a more effective
method to enhance Jc for MgB2 superconductors.
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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

• G. Nishijima, S. Awaji, K. Watanabe IMR,
  Tohoku Univ.
• K. Togano
  NIMS, Tsukuba
• Aixia Xu, Xianping Zhang
  IEE, CAS, China

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Thank you for your attention!
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Barrier for applications

• Many groups have already demonstrated
  powder-in-tube (PIT) conductors for applications.
  The key problem of MgB2 superconductors for
  large-scale applications is the irreversibility
  field (17 T) is less than for Nb3Sn (27 T).

D. C. Larbalestier, et al., Nature 411 (2001) 368