Trapped field and critical current density of MgB2 bulk fabricated by a capsule method
Sasaki T., Naito T., Fujishiro H.
Faculty of Engineering, Iwate University, Morioka 020-8551, Japan
We have studied the trapped field BT and the critical current density Jc of thelarge MgB2 bulks fabricated by a capsule method. The maximum BT values of thebulks with 20 mm, 30 mm, and 38 mm in diameter, respectively, were 1.43 T at
13.5 K, 1.5 T at 16.5 K, and 1.77 T at 15.5 K. The increase in the BT value withdiameter at 20 K was smaller than that expected by the Bean’s critical statemodel. The Jc values were 1.2-1.6×105 A/cm2 for the 20 mm diameter bulk and0.7-1.3×105 A/cm2 for the 30 mm diameter bulk at 20 K in the self field, which
can explain the suppression in the BT for the larger bulks.
INTRODUCTION
Since the discovery of superconductivity in MgB2 with the critical temperature Tc=39 K, it has been
researched for the practical applications, such as wires and thin films. Several groups reported that MgB2
bulk trapped the tesla-order magnetic field [1, 2]. The BT value of about 2.3 T at 6 K was obtained for theMgB2 bulk with 28 mm in diameter and 11 mm in thickness which was sintered under high pressure of 2GPa [1]. And also, Yamamoto et al. reported the 1.2 T MgB2 magnet at 17 K with 20 mm in diameter and
5 mm in thickness [2]. Such a tesla-class superconducting bulk magnet has been mainly studied using asingle grain of RE-Ba-Cu-O (RE: rare earth elements) bulk by field cooling [3]. However the RE-Ba-Cu-O
bulk needs the crystalline orientation, because the intergranular weak links become lower the Jc. On theother hand, for the MgB2 bulk, we can ignore the problem of intergranular weak links even inpolycrystalline samples because of their long coherence length [4]. Therefore, the stronger MgB2 bulk
magnet can be produced by the enlargement of the bulk diameter.
In this paper, we report the trapped field, BT, and the critical current density, Jc, of large MgB2 bulkswith 20-38 mm in diameter fabricated by the capsule method.
EXPERIMENTAL
MgB2 bulks were prepared by the capsule method [5].
Figure 1 shows the schematic image of the capsule. Raw
powder of Mg (99% in purity, ≦180 μm in grain size)
and B (99% in purity, 300 mesh in grain size) wereweighted with 1.1:2.0 in molar ratio and ground. The
mixture was pressed into pellets under an uniaxialpressure. The precursor pellet was set in the hole of thestainless steel flange, covered by the stainless steel plates,
and finally closed by the other flange with the copper
gasket using the bolts and nuts. The stainless steel plates
were inserted to prevent the reaction of Mg with Cu andthe expansion of the pellet during the sintering. The
precursor pellet was prepared in air and the capsule wassealed in Ar-atomosphere using a glove box. The closed
capsule was sintered at 800 ℃ for 6 h in a box furnaceand cooled down to room temperature by furnace cooling. Figure 1 Schematic illustration of the closed capsule
The MgB2 bulks with 20 mm diameter bulk, 30 mm
diameter bulk, and 38 mm diameter bulk, respectively, were named M-20, M-30, and M-38. Sample
specifications are listed in Table 1.

Table 1 Specification of the MgB2 bulks.

Sample name

DiameterThickness

Sintering temp. (℃)

Sintering time (h)Tc (K)ΔTc (K)Max of BT

M-20

20.2

5800

6

38.1-38.4

1.1-1.3

1.43 T at 13.5 K

M-30

30.4

9800

6

38.3-38.4

1.2-1.3

1.5 T at 16.5 K

M-38

38.4

9800

6

1.77 T at 15.5 K

The MgB2 bulk was magnetized by field cooling (FC) in a magnetic field of 5 T, and then the appliedfield was decreased to 0 T at a rate of -0.22 T/min. Trapped field was measured by a cryogenic Hall sensormounted on center of the bulk surface, and the temperature of the bulk was measured by a Cernox
thermometer which was also mounted on the bulk surface. We evaluated Tc and Jc by measuring themagnetization using several small pieces cut from the bulk after the FC magnetization measuments. Themagnetization was measured using a commercial SQUID magnetometer. Jc was estimated from thehysteresis loop using the extended Bean model.

22RESULTS

T

B (T)

1.5

BT (T)

1.5

1T=20K

0.5

800 ℃,6 h020253035d (mm)4030351M-200.5FCM
B =5T

M-30

exM-38010152025T (K)

Figure 2 Temperature dependence of trapped field
BT(T). Inset shows the diameter dependence of thetrapped field BT at 20 K.

2MgB (112)2MgB (201)22MgB (200)MgB (110)
2←MgO(111)
MgB (102)
MgB 2(111)22MgB (002)

MgB (101)

MgB (100)
2Mg(111)

M-20

M-30

20

22MgCu (222)

MgCu (111)intensity(a.u.)

Figure 2 shows the temperature dependence of thetrapped field, BT(T), of the MgB2 bulks. Themaximum of the BT(T) values of M-20, M-30, andM-38 bulks, respectively, are 1.43 T at 13.5 K, 1.5
T at 16.5 K, and 1.77 T at 15.5 K. Inset of Fig. 2shows the diameter dependence of BT at 20 K,
which are estimated by interpolating BT(T) curve,for the MgB2 bulks. The BT (20 K) value increases
with the increase of diameter, however the increasein BT (20 K) is smaller than that expected by theBean’s model [6].
Figure 3 shows the powder X-ray diffraction
patterns of the M-20 and the M-30 bulks. Theobserved peaks of both bulks represent mainly theMgB2 phase, but a small amount of impurities suchas Mg, MgO and MgCu2 phases were observed.
The MgCu2 results from the reaction of liquid Mg
and Cu gasket. Since the distance between theprecursor pellet and Cu gasket for the 30 mm
diameter bulk is closer than that for the 20 mm
diameter bulk, the MgCu2 phase was detected onlyin the M-30. We consider that the amount of theMgCu2 phase contained at edge in M-30 is larger
than that at center in M-30.
Figure 4 shows the temperature dependence ofthe normalized magnetization, M(T), in a magnetic
field of 4 G after the zero-field cooling. Six pieces
were cut from each MgB2 bulk, whose positions areschematically shown in the figure. The criticaltemperature, Tc, defined at the midpoint of thetransition are 38.1-38.4 K for the M-20 bulk and38.3-38.4 K for the M-30 bulk. The widths of thetransition, ΔTc, are 1.1-1.3 K for the M-20 bulk
and 1.2-1.3 K for the M-30 bulk. The impurity
phases seem to deteriorate slightly the Tc.

304050602θθ (deg.)

7080

Figure 3 Powder X-ray diffraction patterns

90

-0.2
Normalized M

0ABCDEF-0.4ex(a) M-20

bulk-0.6
DEF-0.8

AB
C-1
2530

AB
CDEF-0.2

B =4G

Normalized M

0-0.4

ex

DEF-1
2545(b) M-30

bulk

-0.6
-0.8

3540
Temperature (K)B =4GA
B
C

30

3540
Temperature (K)453102101100DEF0c

00.05

0.1 0.15
B (T)

0.2

bulk

T=20KAB
C1105104103102c10ABCDEF

10.9
0.80.7J (A/cm2)

546(b) M-30

23B (T)

4522J (×10 A/cm )10105105(a) M-20

101100ABCDEF

100.05bulk
DEF010.90.80.7

c

6J (×10 A/cm )10c

J (A/cm2)

Figure 4 Temperature dependence of normalized magnetization for the M-20 (a) and M-30 (b) bulks. The positionsof the samples(A to F) cut from the bulk were shown in the inset.

0.1 0.15
B (T)0.2T=20KA
BC

2

345

B (T)

Figure 5 Magnetic field dependence of the critical current density at 20 K for the M-20 (a) and M-30 (b) bulks. Inset
shows the extended figure of Jc-B from 0T to 0.2T. The positions of the samples(A to F) cut from the bulk wereshown in the inset.

Figure 5 shows the magnetic field dependence of the Jc at 20 K. The measured samples are the sameas those for the M(T) measurements. The Jc values are 1.2-1.6×105 A/cm2 for the M-20 bulk and 0.7-1.3
×105 A/cm2 for the M-30 bulk. The Jc values of the M-30 bulk is lower than that of the M-20 bulk. Thismight mean that the MgCu2 phase does not act as pinning center and decreases the stoichiometric MgB2
phase in the M-30 bulk. This result is consistent with the suppression in the BT value of the bulk with 30mm in diameter.
CONCLUSION
We measured the trapped magnetic field and the critical current density of the MgB2 bulks with 20-38 mm
in diameter and 5-9 mm in thickness fabricated by the capsule method. The important results are describedbelow.
(1) The maximum BT(T) values of MgB2 bulks with 20, 30, and 38 mm in diameter, respectively, were1.43 T at 13.5 K, 1.5 T at 16.5 K, and 1.77 T at 15.5 K.
(2) The BT(T) values of MgB2 bulks come to 1.2-1.35 times when their diameter comes to 1.5-1.9 times,
which is contrary to the Bean’s model.
(3) The Jc values are 1.2-1.6×105 A/cm2 for the 20 mm diameter bulk and 0.7-1.3×105 A/cm2 for the 30mm diameter bulk at 20 K in the self field, which can explain the suppression in the BT of the larger bulks.
From the results of the powder X-ray diffraction patterns, the existence of MgCu2 phase only in a 30 mm
diameter bulk is confirmed.

ACKNOWLEDGMENTS
The authors would like to acknowledge Mr. Y. Takeda at the Advanced Prototype and Processing Center,
Iwate University for his help at the production of the capsule. This work was in part supported by Japan
Science and Technology Agency under a Research for Promoting Technological Seeds 2009 (02-051) anda Adaptable and Seamless Technology Transfer Program through target-driven R&D for a Exploratory
Research of FS stage (AS232Z02579B).
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