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    MgBMgB22 superconductor processed in  superconductor processed in 

high magnetic fields high magnetic fields 

Yanwei MAInstitute of Electrical Engineering, Chinese Academy of Sciences, Be

ijing 100080, China

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Candidate HTS for large scale applications

D. C. Larbalestier et al., Proceedings of the IEEE, vol. 92, 2004, 1639

• Commercial production:– Bi2223 / silver tape - 1st Generation HTS

• Industrial laboratory:– YBCO 2nd Generation HTS “coated conductor”

• Pre-commercial:– MgB2

Background

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Benefits of SC Materials

1. Low losses,

2. Small volume,

3. Light weight,

4. High efficiency.

Merits of superconductors:

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Kinds Bi2223 YBCO MgB2

Anisotropy parameters

50－ 100 5－ 7 2－ 3

Structure of HTS superconductorsStructure of HTS superconductors

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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 techniqueMagnetic aligning technique

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First report in First report in NatureNature, in 1991, in 1991

E 1

20

VH 2

V:volume of particleH:external magnetic fieldD: 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 1000C, 10 T, orientation takes place for grains with the size of 40 nm.

For YBCO superconductors:

IEEIEE 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 situationPresent 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.

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In 1991, Rango et al. at Grenoble reported the effect of magnetic field on the growth of YBCO bulk samples.

In 1999Liu, 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

MgB2 processed in fields ？

In 1997Ma, et al.

Bi-2212 tape

Hot topic: Superconductors synthesized in high magnetic fields

Motivation

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Introduction-MgB2

a=3.086 Å; c=3.524 Å

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).

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Experimental procedure

MgB2

Powder

Pressing BulkHeated in magnetic fields

Tape

Characterization

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Temperature profile for processing of MgB2 in magnetic fields

0.5 h 1 h Furnace cooling

600-800C in vacuum

Time

Tem

p.

 

Magnetic field

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Group Sample typeSintering

temperature/ time

Applied field during

sintering

Sample surface and field direction during sintering

Tc

Group IFe clad tape 600C/1 h 10 T Parallel 35.2

Fe clad tape 600C/1 h 0 T   35.5

Group IIFe clad tape 700C/1 h 14 T Perpendicular

Fe clad tape 700C/1 h 0 T  

Group III

Pellet 800C/2h 14 T Perpendicular 37 K

pellet 800C/1 h 8 T   36.9 K

pellet 800C/1 h 0 T   37.1 K

Description of samples used in this work

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10 T

(100

)

(101

)

(002

)

(110

)

(201

)

(102

)

*

FeFe

(200

)

(001

)

20 30 40 50 60 70 80

(100

)

(101

)

Fe

(002

)

(110

)

(102

)

* (200

)

(201

)

2 ( deg. )

Inte

nsi

ty (

arb

. un

it)

0 T

XRD patterns for tapes with and without filed

Broadening of the FWHM indicates inferior MgB2 crystallinity and lat

tice distortion of the core MgB2, closely related to

the enhanced flux pinning.

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10

100

1000

104

105

2 4 6 8 10 12 14 16

0 T

10 T

Jc (

A/c

m2 )

H (T)

4.2 K

Ha

Transport critical current density Jc (Tapes)

Ha is parallel to the tape surface

600 C

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1000

104

105

106

0 1 2 3 4 5

0 T

14 T

Jcm

(A

/cm

2 )

Tape

5 K

H (T)

Ha

Ha is perpendicular to the tape surface

Magnetc critical current density Jc measured by magnetization (Tapes)

700 C

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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 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

0

500

1000

1500

2000

14 T

(100

)

x

*

(101

)

(002

) (110

) *

(201

)

*

Cou

nt

(a.u

.)

0

500

1000 8 T

(100

)

(101

)

(002

)

(110

)

*

(201

)

*

Cou

nt

(a.u

.)x

0

500

1000

1500

2000

30 40 50 60 70 80

0 T

2 ( deg. )

Cou

nt

(a.u

.)

(100

)

(101

)

(002

) (110

)

(102

)

(201

)

**

x*

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)

-1

-0.8

-0.6

-0.4

-0.2

0

0 10 20 30 40 50

Bulk

0 T8 T14 T

M/M

max

Temperature (K)

10 OeZFC

-0.15

-0.1

-0.05

0

0.05

36 36.5 37 37.5 38

Bulk

0 T8 T14 T

M/M

max

Temperature (K)

10 OeZFC

The magnetic fields hardly affect Tc !

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100

1000

104

105

106

0 1 2 3 4 5

0 T 8 T

Jcm

(A

/cm

2 )

H (T)

20 K

5 K

30 K

Bulk

Magnetc critical current density Jc measured by magnetization (Bulk)

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ConclusionsConclusions

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

1. The effect of high magnetic fields on the current carrying properties of both MgB2 bulks and Fe-sheathed tapes was investigated fol

lowing 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.

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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:

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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