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Physics Procedia 58 (2014) 306 – 309

26th International Symposium on Superconductivity, ISS 2013

Shielding and trapped field properties of large MgB2 bulk
T. Ujiiea, T. Naitoa, H. Fujishiroa*, A. Figini Albisettib and G. Giunchic
aFaculity of Engineering, Iwate university, Morioka 020-8551, JapanEDISON S.p.A., R&D Division, Foro Buonaparte 31, 20121 Milano, Italy
cMaterials science Consultant, via Teodosio 8, 20131 Milano, ItalybAbstract
We have studied the magnetic shielding properties of MgB2 bulk fabricated by a reactive liquid Mg infiltration (Mg-RLI) method.
The magnetic shielding profiles indicated partially field-penetrated areas at the bulk periphery with weaker pinning force at 38 K.
We discuss about the superconductive inhomogeneityin the bulk by comparing the shielding profiles with trapped field ones by
the subsequent zero-field-cooled magnetization (ZFCM) and the pulsed-field magnetization (PFM). The present method isadvantageous to detect the inhomogeneity at the periphery of a large superconducting bulk.
©
The Authors.
Publishedby byElsevier
B.V.B.V.
This is an open access article under the CC BY-NC-ND license©2014
2014TheAuthors.
PublishedElsevier(http://creativecommons.org/licenses/by-nc-nd/3.0/).
Selection and peer-review under responsibility of the ISS 2013 Program Committee.
Peer-review under responsibility of the ISS 2013 Program Committee
Keywords: MgB2; bulk superconductor; shielding properties; pulsed field magnetization; magnetic field profiles; pinning force

1. Introduction
A MgB2 bulk is a metal superconductor with a critical temperature Tc=39 K, in which the problem ofintergranular weak links can be ignored even for polycrystalline samples because of their long coherence length [,compared with RE-Ba-Cu-O (RE: rare earth element) bulks [1]. Therefore, the polycrystalline MgB2 bulk can trap
the higher magnetic field and is expected as a material replacing conventional superconductors.
To evaluate the superconductive homogeneity of the large bulk without destruction, a magneto-optical imaging(MOI) method [2] and a magnetoscan method [3] are used to visualize the distribution of the magnetic flux
penetration and/or the critical current density Jc, both of which have the fundamental limitation for the measurement.

* Corresponding author. Tel.: +81-19-621-6363; fax: +81-19-621-6363
E-mail address:fujishiro@iwate-u.ac.jp

1875-3892 © 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/3.0/).
Peer-review under responsibility of the ISS 2013 Program Committee
doi:10.1016/j.phpro.2014.09.076

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T. Ujiie et al. / Physics Procedia 58 (2014) 306 – 309

On the other hand, Jc is directly measured magnetically for a small piece cut from the bulk. A new non-destructive
technique is desired to evaluate the superconductive homogeneity of a large MgB2 bulk. In this study, we measured
the magnetic shielding properties of a large MgB2 bulk just below Tc. The trapped field profiles were also measured
by the subsequent zero-field-cooled magnetization (ZFCM) and a pulsed-field magnetization (PFM), both of whichwere compared with the shielding profiles.
2. Experimental
The MgB2 bulk (54 mm in diameter and 15 mm thickness), fabricated by a reactive liquid Mg infiltration(Mg-RLI) method and mounted in a stainless steel (SUS) ring using Stycast 2850GT, was set on a cold head of a
Gifford-McMahon (GM) cycle helium refrigerator. To measure the shielding and ZFCM profiles, static magnetic
field of 28 mT was applied by solenoid-type copper coil after the bulk was cooled to Ts=36, 37, 38 and 39 K. For theshielding experiment, two-dimensional magnetic shielding profiles were mapped 2 mm above the bulk surface using
a Hall sensor under the magnetic field. For ZFCM, the trapped field profiles were measured after the appliedmagnetic field was decreased to zero. The PFM experiments were performed to the bulk cooled to 14 K. A magneticpulse Bex up to 1.97 T with a rise time of 0.013 s and a duration of 0.15 s was applied to the bulk by flowing thepulsed current from a condenser bank. The trapped field BT at the center of the bulk surface was monitored by Hallsensor using a digital oscilloscope. Two-dimensional trapped field profiles were mapped on the bulk with 1 mmabove the bulk surface. For each experiment, the time dependence of temperature T(t) was also measured at the sideof the SUS ring using Cernox thermometer.
3. Results and discussion
Figure 1 shows the magnetic shielding profiles on the bulk 2 mm above the bulk surface at various temperatures.
The position of the bulk is shown in each figure. The profile at 38 K indicated that magnetic flux penetrated
inhomogeneously from the bulk periphery, especially from the region A. However, the magnetic flux was shielded
uniformly at lower temperatures of 36 and 37 K due to the increase of Jc with decreasing temperature. For Ts= 39 K
as shown in Fig. 1(d), the magnetic flux penetrated inside of the bulk. Figure 2 shows the cross-sections of theshielding profiles shown in Fig. 1 along the line of Y=0 mm. The flux did not penetrate to the inside of the bulk
except for Ts=39 K. It should be noted that the inhomogeneous flux penetration can be observed around region A at38 K.
Figure 3 presents the trapped field profiles on the bulk surface for ZFCM. The bulk can trap the magnetic flux
25(a)

(b)

B[ mT ]2015region A

10A36 K
37 K
38 K
38 K
39 K

5A(c)

(d)

Fig. 1 The magnetic shielding field profiles onthe bulk surface at (a)Ts=36 K, (b) 37 K, (c) 38 Kand (d) 39 K.

0-40-30-20-100
10X [mm]

203040Fig. 2 Cross-section of the shielding profiles. Open symbols
were plotted along the line of Y=0 mm and solid symbols
were plotted along the diagonal line shown in Fig. 1(c).

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4Aregion A37 K
38 K38 K39 K

3(b)ABT[ mT ](a)210(c)-1-40

0.5-10010X [mm]20304026(a)PFM(b)

24
Temperature [K]Ts=14 K

0.3

TB [T]-20

Fig. 4 Cross-sections of the trapped magnetic
profiles for ZFCM. Open symbols were plottedalong the line of Y=0 mm and solid symbols wereplotted along the diagonal line shown in Fig. 3(b).

Fig. 3 Trapped field profiles for ZFCM at(a)Ts=37 K, (b) 38 K and (c) 39 K.

0.4

-30

0.2

221.97 T
1.79 T
1.54 T
1.36 T1.17 T0.93 T

2018
160.1

1400.8

11.2
B

1.4
1.6
[T]

ex1.8

202040

60

80

100

Time [sec.]

Fig. 5 (a) The applied pulsed field Bex dependence of the trappedfield BT. (b)Time dependence of temperature change for PFM.

only for the bulk periphery below 36 K, which indicated that the applied magnetic field of Bex=28 mT did notexceed the lower critical field Hc1, and the bulk shielded the magnetic flux. For Ts=37 K and 38 K as shown in Figs.
3(a) and 3(b), the magnetic flux penetrated in the bulk and was preferentially trapped around region A, at which Jc
was considered to be lower from the shielding measurement. For Ts=39 K as shown in Fig. 3(c), the magnetic flux
penetrated and trapped at the center of the bulk, in which the trapped field profile was conical and the maximum
trapped field was 1.2 mT. Figure 4 shows the cross-sections of the trapped field profiles for ZFCM shown in Fig. 3.
For Ts=38 K, it is noticed that the trapped field at region A was relatively higher compared with the cross-section
along the line of Y=0 mm.
Figure 5(a) shows the trapped field BT at the center of the bulk surface, as a function of the applied pulsed fieldBex for PFM. BT increases for Bex > 1.2 T, takes a maximum at Bex = 1.79 T and then decreases with increasing Bex.
The decrease of BT for Bex=1.97 T is due to the large temperature rise in the bulk, which was caused by the viscous
loss and pinning loss. The maximum BT was 0.41 T in this study, which was smaller than that on the MgB2 bulk
fabricated by a capsule method (BT=0.72 T at Ts=17 K) [4]. Figure 5(b) depicts the time dependence of thetemperature change at the side of the SUS ring for each applied field Bex. The temperature increases abruptly justafter the pulse application and slowly decreases with increasing time. The magnitude of the maximum temperaturerise increases with increasing Bex. The sharp temperature change results from the low specific heat and high thermal
conductivity of the MgB2 bulk, compared with those for the REBaCuO bulk at operating temperatures.
Figure 6 shows the trapped field profiles on the bulk 1 mm above the bulk surface for various applied pulsed
fields Bex. For lower Bex of 0.93 T as shown in Fig. 6(a), the magnetic flux was preferentially trapped around theregion A together with the bulk periphery. It is known that the magnetic flux is trapped at weaker pinning force

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T. Ujiie et al. / Physics Procedia 58 (2014) 306 – 309

0.6A

0.5

0.93 T
1.36 T
1.36 Tregion A

1.79 T
1.97 T(a)A(b)

BT [ T ]

0.4
0.3
0.2
0.1
0(c)

(d)

Fig. 6 Trapped field profiles on the bulk for PFMat (a) Bex=0.93 T, (b) 1.36 T, (c) 1.79 T and (d) 1.97

-0.1-40-30-20-10010X [mm]203040Fig. 7 Cross-sections of the magnetic profiles forPFM. Open symbols were plotted along the line of
Y=0 mm and solid symbols were plotted along thediagonal line shown in Fig. 6(b).

region for lower applied pulsed field [5]. Therefore, the critical current density Jc around region A seems to berelatively lower in the bulk, and it is consistent with the results of shielding and ZFCM experiments shown in Figs. 1and 3. For Bex=1.36 T as shown in Fig. 6(b), the trapped field around the region A was relatively lower because themagnetic flux can be preferentially trapped at strong pinning region for relatively higher Bex [5]. The line-shaped
disturbance at Y=-0.6 mm comes from the noise of the apparatus. For Bex=1.79 T and 1.97 T as shown in Figs. 6(c)and 6(d), the tapped field profiles was nearly the cone-shaped ones, but the profile was deformed due to theexistence of the region A. Figure 7 presents the cross-sections of the trapped field profiles for PFM shown in Fig. 6.
The trapped field profile changes from concave to convex with increasing Bex. For Bex=1.36 T, it is found that thetrapped field at region A was relatively lower compared with the cross-section along the line of Y=0 mm. The
distribution of the pinning force obtained by PFM was consistent with that obtained from the shielding measurement,
despite the time scale of the magnetic flux movement was fairy different. Thus, the shielding measurement isavailable for non-destructive evaluation technique of superconductive homogeneity at the periphery of the large bulk.
However, this technique is not necessarily effective to evaluate the superconductive homogeneity inside of the bulk.
In this case, the combination with the trapped field profiles by PFM and/or field cooled magnetization (FCM) isrequired.
4. Conclusion
We have studied the magnetic shielding profile and the trapped field profiles magnetized by ZFCM and PFM oflarge MgB2 bulk fabricated by Mg-RLI method. For the shielding measurement, the magnetic flux penetrated
inhomogeneously from the position with low Jc just below Tc. For the subsequent ZFCM, the magnetic flux wastrapped at this region. For PFM, at lower applied pulsed field, the magnetic flux was mainly trapped at sameposition, but at higher applied field, the magnetic flux was not trapped at this position. These results suggest that theshielding profiles can be used for non-destructive evaluation technique of inhomogeneity at periphery of the large
bulk.
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