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Trapped magnetic field between double stacked MgB2 bulks magnetized by pulsed field

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2014 J. Phys.: Conf. Ser. 507 032016(http://iopscience.iop.org/1742-6596/507/3/032016)
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11th European Conference on Applied Superconductivity (EUCAS2013)
Journal of Physics: Conference Series 507 (2014) 032016

IOP Publishingdoi:10.1088/1742-6596/507/3/032016

Trapped magnetic field between double stacked MgB2 bulks
magnetized by pulsed field
H Fujishiro1, T Ujiie1, T Naito1, A Figini Albisetti2 and G Giunchi3
1Iwate University, 4-3-5 Ueda Morioka, JapanEDISON S.p.A., Foro Buonaparte 31, 20121 Milano, Italy
3Materials Science Consultant, via Teodosio 8, 20131 Milano, Italy
2E-mail: fujishiro@iwate-u.ac.jp

Abstract. Pulsed field magnetization (PFM) was performed at Ts=14 K for the double
stacked MgB2 bulks (bulk pair) 55 mm diameter fabricated by a reactive liquid Mg infiltration(Mg-RLI) method, compared with that for the single bulk. The trapped field of Bz=0.80 T wasachieved between two bulks and Bz at the bulk surface was enhanced from 0.42 T to 0.50 T by
stacking of the bulks. The trapped field characteristics of the bulk pair can be qualitatively
explained by the model analyses.
1. Introduction
MgB2 bulk magnet has attractive natures such as low cost, light-weight, and weak-link-freehomogeneous current flow, which are clear contrast with REBaCuO (RE=rare earth element)
superconducting bulk magnets. To magnetize the superconducting bulks, a pulsed field magnetization(PFM) is an inexpensive and mobile experimental setup with no need of a superconducting magnet, asa substitute for field-cooled magnetization (FCM). We have investigated the PFM procedure for theREBaCuO bulks to enhance the trapped field [1-3]. Considering the obtained experimental results, we
proposed a new PFM technique, in which multi-pulses are applied under different temperatures, andrealized over 5 T on a GdBaCuO bulk, which is a record-high value by PFM to date [3]. For MgB2bulks, the results of the trapped field by FCM have been mainly reported. The higher trapped field wasreported to be 2.25 T at 15 K on the single MgB2 bulk [4] and 3.14 T at 17.4 K in the double stacked
MgB2 bulks [5]. We have performed the PFM procedure for the single MgB2 bulk fabricated by areactive liquid Mg infiltration (Mg-RLI) [6] and by a capsule method [7] with 20-50 mm in diameterat 16-30 K, where the trapped field of 0.47 T at 23 K and 0.71 T at 16 K were realized, respectively.
To analyze the flux dynamics during PFM, we performed a numerical simulation of PFM for theMgB2 bulk [8], which reproduced qualitatively the experimental results. The enhancement of the bulk
thickness can achieve the higher trapped field because of the elimination of the demagnetization factor.
In this study, we investigated the trapped field characteristics by PFM for large MgB2 of single
bulk and double stacked bulks (bulk pair) fabricated by Mg-RLI method. The magnetic flux intrusionand flux trap during PFM are discussed using a numerical simulation.
2. Experimental Procedure
The thick MgB2 bulk disk of 55 mm in diameter, which was fabricated by the Mg-RLI technique [9],was sliced and two bulk plates (A and B) were prepared. The thickness of bulk-A and bulk-B was 15.3

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11th European Conference on Applied Superconductivity (EUCAS2013)
Journal of Physics: Conference Series 507 (2014) 032016

IOP Publishingdoi:10.1088/1742-6596/507/3/032016

and 10.3 mm, respectively. The superconducting transition temperature Tc and the trapped field Bz ofthe bulk-B magnetized by FCM at 20 K were, respectively, confirmed to be 38 K and 2.0 T. Each bulk
was mounted in stainless steel (SUS) ring of 8 mm thickness filled with a stycast 2850GT resin. Figure
1 shows the experimental setups for PFM around the bulk. For the bulk pair as shown in figure 1(a),the bulk-A was set on a soft iron yoke cylinder and tightly anchored onto the cold stage of a Gifford–
McMahon cycle helium refrigerator. The bulk-B was also mounted on the bulk-A with the insertion ofthe brass plate 2 mm in thickness. The initial temperature TsA of the bulk-A was set to 14 K. A
magnetizing solenoid copper coil (94 mm i.d., 153 mm o.d., and 50 mm height), which was dipped inliquid nitrogen, was placed outside the vacuum chamber. A magnetic pulse Bex(t) with a rise time of0.012 s and a duration time of 0.15 s was applied to the bulk by flowing the pulsed current. The time
evolutions of the local fields BLC(t) and the subsequent trapped fields Bz between two bulks and at thecenter of the surface were, respectively, monitored by Hall sensors of HS-a and HS-b (F W Bell, BHT
921 and BHA 921). Two-dimensional trapped field profiles of Bz(1 mm) were mapped at a distance of
z=1 mm above the bulk surface. During PFM, the time dependence of temperature T(t) of each bulk
was measured at each SUS ring using CernoxTM thermometers of TM-a and TM-b. The similar
experiments were performed for the single bulk-A as shown in figure 1(b).
1double-stacked bulks
bulk-A surface (HS-a)bulk-B surface (HS-b)

0.8

HS-a

Bz (T)

single bulk

bulk-A surface (HS-a')

0.6
0.4

HS-b
HS-aHS-a'bulk-B

bulk-A

HS-bbulk-AHS-a'0.2
00

Figure 1. Experimental PFM setups for (a) thebulk pair and (b) the single bulk-A.

0.5

1

1.5Bex (T)2

2.5

Figure 2. Trapped field Bz for the bulk pair(HS-a and HS-b) and the single bulk (HS-a’),as a function of the applied pulsed field Bex.

3. Results and discussion
3.1. Experimental results
Figure 2 shows the trapped field Bz for the bulk pair and the single bulk-A, as a function of the applied
pulsed field Bex. For the bulk pair, Bz at the surface (HS-b) increases, takes a maximum of 0.50 T at
Bex=1.79 T and then decreases with increasing Bex. Bz in the bulk pair (HS-a) also increases withincreasing Bex and takes a maximum of 0.80 T at Bex=1.98 T, which is the highest Bz value by PFM todate. The Bz vs Bex curve in the bulk pair shifts the higher Bex side, compared with that for the surface
because the demagnetization factor between the bulks was eliminated and the shielding behavior of thestack is more effective. For the single bulk-A, the maximum Bz was 0.42 T. The maximum of total
trapped flux Φ for the bulk pair and single bulk-A was, respectively, 0.88 mWb for Bex=1.55 T and
0.69 mWb for Bex=1.36 T, which were obtained at Bex lower than that, at which Bz took a maximum.
Figure 3 presents the trapped field profiles on the MgB2 bulks for Bex=1.55 and 1.98 T. The
profiles for the bulk pair are shown in figures 3(a) and 3(b). For Bex=1.55 T, the trapped field profile isconcave because of the lower applied field than the optimum one, where the right lower positionindicated by the arrow is difficult to trap the magnetic flux, which may reflect the inhomogeneity ofcritical current density Jc in the bulk-B. On the other hand, for Bex=1.98 T as shown in figure 3(b), the211th European Conference on Applied Superconductivity (EUCAS2013)
Journal of Physics: Conference Series 507 (2014) 032016

IOP Publishingdoi:10.1088/1742-6596/507/3/032016

magnetic flux was intruded into the central region of the bulk and the trapped field profiles are nearly
the conical one. The profiles on the bulk pair may be influenced by the Jc distribution in the bulk-A.
The profiles for the single bulk-A are shown in figures 3(c) and 3(d). For Bex=1.55 T as shown infigure 3(c), the magnetic flux was intruded into the central region of the bulk with a small inflection
around the arrows. The conical Bz profile can be obtained Bex=1.79 T and, for Bex=1.98 T, the conicalprofile maintains with the decrease in the maximum value, as shown in figure 3(d). The trapped field
profiles of the bulk pair are not necessarily the same as that for each bulk. The complicated flux
intrusion and trap can be considered for the stacking, even though Jc distribution seem to be identical.
Figure 4 shows the time evolution of the applied field Bex(t) and local fields BLC(t) at the center ofthe bulk surface for Bex=1.55 and 1.98 T. For each Bex, BLC(t) at the bulk surface (HS-b) first starts toincrease and takes a maximum, and then decreases to a final value due to the flux flow. BLC(t) in thebulk pair (HS-a) shows the similar time dependence with a time delay because of the strong shielding
current in the bulk center. For lower Bex=1.55 T, the final Bz at HS-a is smaller than that at HS-b. On
the other hand, for higher Bex=1.98 T, the magnitude relation of the final Bz was reversed.
Figure 5 shows the time dependences of the temperatures T(t) of the bulk pair at TM-a and TM-bfor Bex=1.98 T. The temperatures increase abruptly just after the pulse application, and then quickly
decrease with increasing time, which are clear contrast with those for the REBaCuO bulk [1]. The
sharp temperature changes result from the low specific heat and high thermal conductivity of theMgB2 bulk. It should be noted that the early temperature profiles are quite different; T(t) at TM-b
shows the spike-like behavior, but T(t) at TM-a changes moderately. The difference may come fromthat of the thermal environment for each bulk. Furthermore, the initial temperature of the bulk-B at
TM-b was 17 K, which was 3 K higher than that of the lower bulk-A at TM-a. The result suggests thatthe thermal contact of the bulk-B is not sufficient to the bulk-A and/or the thermal radiation inducesfrom the surface of the bulk-B. The temperature gradient may exist along the thickness direction in theconduction-cooled bulk, even in the single bulk. In the inset, the applied field dependence of themaximum temperature Tmax was also shown. Tmax monotonically increases with increasing Bex.
2(b) Bex=1.98T(a) Bex=1.55T

B

ex

1.5

BB (T)

ex

HS-bHS-abulk-B

1bulk-A

0.5

HS-a

HS-b

HS-b

HS-a0Figure 3. Trapped field profiles on the bulk
pair ((a) and (b)) and the single bulk-A ((c)
and (d)) for Bex=1.55 and 1.98 T.

00.05
0.1
Time (s)0.15 00.05
0.1
Time (s)0.15

Figure 4. Time evolution of the applied fieldBex(t) and local fields BLC(t) at the centre of thebulk surface for Bex=1.55 and 1.98 T.

3.2. Results of numerical simulation and discussion
In order to understand the experimental results of the PFM for the bulk pair, we performed thenumerical simulation. Physical phenomena during PFM were described using electromagnetic andthermal fields. The power-n model (n=50) was supposed to describe the nonlinear E-J characteristic inthe bulk. The temperature and magnetic field dependence of the critical current density Jc(T, B) wasdefined. The details of the simulation are described elsewhere [2, 8].

3

11th European Conference on Applied Superconductivity (EUCAS2013)
Journal of Physics: Conference Series 507 (2014) 032016

3032

Bex=1.98T

240.8
TM-aTM-b

16

22

1

1.5

B (T)

22.5

ex

20
18

bulk-ATM-aTM-b

16
14020

4060
Time (s)bulk-A surface (a)bulk-B surface (b)

(b)
(a)

0.6
bulk-Bbulk-A

0.4

TM-bTM-a(b) TsA=14K, TsB=17K

bulk-A surface (a)bulk-B surface (b)20Bz (T)T

24T (K)(a) TsA=TsB=14Kmax

26bulk-Bsimulation

128(K)

28

IOP Publishingdoi:10.1088/1742-6596/507/3/032016

0.2

80

100

0Figure 5. Time dependence of the temperaturesof the bulk pair at TM-a and TM-b after
applying the magnetic pulse of Bex=1.98 T.

1

1.5
Bex (T)2

1

1.5
Bex (T)

2

Figure 6. Results of the simulation of the trappedfield Bz of the bulk pair at (a) TsA=TsB= 14 K and(b) TsA=14 K, TsB=17 K.

Figure 6(a) shows the results of the simulation of the trapped field Bz of the bulk pair at
TsA=TsB=14 K, as a function of Bex. Bz in the bulk pair (HS-a) and on the bulk surface (HS-b) show thesimilar Bex dependences; both of Bz take a maximum at Bex=1.8 T and then decrease with increasing
Bex. Bz at the position (b) is very slightly higher than that at the position (a) for Bex lower than 1.7 T.
These results reproduce the experimental ones qualitatively (see Fig. 2). Figure 6(b) shows the similar
results of the simulation of the trapped field Bz at TsA=14 K and TsB=17 K, which are the similar
condition to the experimental results as shown in figure 5. Both absolute Bz values at the positions of(a) and (b) decreased, compared with those in figure 6(a) and the Bz vs Bex profile at the position (b)shifts lower Bex side. The Jc value in the bulk-B decreases and the magnetic flux is easy to penetrate inthe bulk-B. As a result, the magnetic flux is easy to penetrate also into the bulk-A. The temperaturerise increases and the trapped field of both bulks decreases. The relation shown in figure 6(b) is morequalitatively similar than that in figure 6(a) to that obtained by the experimental results in figure 2.
In summary, pulsed field magnetization was performed at Ts=14 K for the double stacked MgB2
bulks fabricated by a reactive liquid Mg infiltration (Mg-RLI) method. The trapped field of Bz=0.80 T
was achieved between two bulks and Bz at the bulk surface was enhanced from 0.42 T to 0.50 T by
stacking of the bulks. The trapped field characteristics such as the Bz vs Bex relations at the bulk
surface and in the bulk pair can be qualitatively explained by the model analyses.
References[1] Fujishiro H, Oka T, Yokoyama K, Kaneyama M and Noto K 2004 IEEE Trans. Appl.
Supercond. 14 1054
[2] Fujishiro H and Naito T 2010 Supercond. Sci. Technol. 23 105021[3] Fujishiro H, Tateiwa T, Fujiwara A, Oka T and Hayashi H 2006 Physica C 445–448 334[4] Yamamoto A et al. 2011 Abstracts of The 15th Japan-US Workshop on Advanced
Superconductors, arXiv:1208.1854[5] Durrell J H, Dancer C E J, Dennis A, Shi Y, Xu Z, Campbell A M, Hari Babu N, Todd R I,Grovenor C R M and Cardwell D A 2012 Supercond. Sci. Technol. 25 112002[6] Fujishiro H, Tamura T, Arayashiki T, Oyama M, Sasaki T, Naito T, Giunchi G and Albisetti A
F 2012 Jpn. J. Appl. Phys. 51 103005[7] Fujishiro H, Naito T, Sasaki T and Arayashiki T 2013 Proc. ICEC24-ICMC2012 571[8] Fujishiro H, Naito T, Oyama M, Arayashiki T, Tamura T, Sasaki T and Giunchi G, 2013 IEEE
Trans. Appl. Supercond. 23 6800804[9] Perini E, Giunchi G, Saglietti L, Albisetti A, Matrone A, and Cavaliere V 2011 IEEE Trans.
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