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Physics Procedia 45 (2013) 93 – 96ISS2012

Trapped magnetic field of dense MgB2 bulks fabricatedunder high pressure
T. Sasaki, T. Naito*, H. Fujishiro
Department of Materials Science and Engineering, Faculty of Engineering, Iwate University, Morioka 020-8551, JapanAbstract
We have measured the trapped magnetic field BT, critical current density Jc and connectivity K between crystal grains for the MgB2
bulks fabricated by a HIP method and a capsule method. The maximum BT value is 2.51 T at 12.7 K for the bulk fabricated by HIP
method. From the SEM images of the bulks prepared by HIP at 98 and 980 MPa, the higher pressure enhances the grain growth.
These superconducting characteristic values for the bulks by the HIP method were higher than those for the bulks by the capsulemethod.

© 2013 The Authors. Published by Elsevier B.V.
© 2013 The Authors. Published by Elsevier B. V. Selection and/or peer-review under responsibility of ISS Program Committee.
Selection and/or peer-review under responsibilty of ISS Program Committee.
Keywords: MgB2; trapped magnetic field; hot isostatic pressing; density; grain boundary

1. Introduction
MgB2 has been researched for the practical applications such as wires and thin films, in general. A tesla-classsuperconducting bulk magnet has been mainly studied using a single grain of RE-Ba-Cu-O (RE: rare earth elements)
bulk [1]. However, the c-axis oriented single grain of RE-Ba-Cu-O bulk must be used, because the intergranular weaklinks lower the critical current density Jc. On the other hand, for the MgB2 bulk, we can ignore the problem ofintergranular weak links even in polycrystalline samples because of their long coherence length [2]. Although several
groups reported that MgB2 bulk trapped the tesla-order magnetic field [3-6]. We reported the trapped magnetic field
BT of 1.77 T at 15.5 K in a in-situ MgB2 bulk with 38 mm in diameter and 9 mm in thickness fabricated by capsulemethod, in which a precursor pellet was sealed with two commercial stainless steel flanges and copper gasket [3].
Tomita et al. suggested the BT of 2.25 T at 15 K for the in-situ MgB2 bulk with 30 mm in diameter and 10 mm inthickness [4]. Viznichenko et al. reported that the BT of about 2.3 T at 6 K was obtained for the MgB2 bulk with 28mm in diameter and 11 mm in thickness which was sintered under the pressure as high as 2 GPa [5]. Recently, Durrel
et al. reported the average Jc of 8.0 108 A/m2 in the self field at 20 K in the ex-situ MgB2 bulk with 25 mm indiameter and 5.38 mm in thickness fabricated by uniaxial hot pressing [6].
The in-situ MgB2 bulk gives us the good intergranular connection and high Jc. However, the density of in-situ
MgB2 bulk is about 50% value compared with the theoretical density (2.62 g/cm3), because Mg reacts with B inprecursor pellet. Therefore it is possible to improve the Jc using high-density bulk. In this study, we fabricated highdense MgB2 bulks using a HIP (Hot Isostatic Pressing) method and compared the trapped magnetic field properties
with those fabricated under ambient pressure. We discussed the correlation between the trapped magnetic field,density, and micro structure.

*Corresponding author. Tel./fax: +81-19-621-6362.
E-mail address: tnaito@iwate-u.ac.jp

1875-3892 © 2013 The Authors. Published by Elsevier B.V.
Selection and/or peer-review under responsibilty of ISS Program Committee.
doi:10.1016/j.phpro.2013.04.060

94T. Sasaki et al. / Physics Procedia 45 (2013) 93 – 96

Table 1. Specification of the MgB2 bulks.
SampleS-HIP#23
HIP#26
CAP#20
CAP#30

Diameter (mm)
23.1
26.1
20.2
30.4

Thickness (mm)
24.0
6.5
5.0
9.0

Heat treatment
3h900
900
3h6h800
8006h

Pressure980 MPa
98 MPaambientambient

Filling fraction
93.8%95.0%
59.9%
52.7%

2. Experimental
2.1. Sample preparation
Raw powder of Mg (99% in purity, 180 m in grain size) and B (99% in purity, 300 mesh in grain size) wereweighted with 1.05~1.1:2.0 in molar ratio and ground. High dense MgB2 bulks were prepared by the HIP method. The
mixture was pressed into pellets by CIP (Cold Isostatic Pressing) method. The precursor pellet was sealed in thestainless steel (SS) container by EBW (Electron Beam Welding) in vacuum. The sealed SS containers were set into theHIP machine and subsequently sintered at 900
for 3 h under a pressure of 98 or 980 MPa. Ambient pressure bulks
were prepared by the capsule method [3]. The precursor pellet was set in the hole of the SS flange, covered by the SSplates, and finally closed by the other SS flange with the copper gasket using the bolts and nuts. The SS plates wereinserted to prevent the reaction of Mg with Cu and the expansion of the pellet during the sintering. The precursor
pellet was prepared in air and the capsule was sealed in Ar-atomosphere using a glove box. The closed capsule wassintered at 800for 6 h in a box furnace and cooled down to room temperature by furnace cooling. The MgB2 bulk
prepared by the HIP at 980 MPa with 26.2 mm in diameter and 24.0 mm in thickness was named S-HIP#23. TheMgB2 bulk prepared by the HIP at 98 MPa with 26.1 mm in diameter and 6.5 mm in thickness was named HIP#26.
The MgB2 bulks fabricated by capsule method with 20.2 mm in diameter and 30.4 mm in diameter, respectively, werenamed CAP#20 and CAP#30. Sample specifications are listed in Table 1.
2.2. Measurements
The MgB2 bulk was magnetized by field cooling (FC) in a magnetic field of 5 T, and then the applied field wasdecreased to 0 T at a rate of -0.22 T/min. Trapped field was measured by a cryogenic Hall sensor mounted on thecenter of the bulk surface, and the temperature of the bulk measured by a Cernox thermometer which was also
mounted on the bulk surface. We measured the magnetization curve by using a commercial SQUID magnetometerusing several small pieces cut from the bulk after the FC magnetization measurements and evaluated Jc using theextended Bean model. Electrical resistivity was measured by a standard four probe technique. The micro structure ofthe MgB2 bulks was observed by SEM (Scanning Electron Microscope).
3. Results and discussion
Fig. 1 shows the temperature dependence of the trapped field, BT(T), of the MgB2 bulks. The maximum BT(T) of SHIP#23, HIP#26, CAP#20, and CAP#30, respectively, are 2.42 T at 13.4 K, 2.51 T at 12.7 K, 1.43 T at 13.5 K, and1.52 T at 16.6 K. The BT(T) values of the HIP bulks were higher than that of the bulks fabricated by the capsulemethod due to the high density of HIP bulks. Surprisingly, the BT(T) of the HIP#26 (98 MPa) bulk was higher thanthat of the S-HIP#23 (980 MPa) bulk. The BT values of both HIP#26 and S-HIP#23 exceed that of the bulk fabricatedunder pressure of 2 GPa [5]. The high pressure synthesis over 980 MPa seems to decrease the BT. Durrel et al.
reported the BT of 3.14 T at 17.5 K using the stack of two disk-shaped MgB2 bulks. Although there is no data of single
disk [6], our bulks also expected to give us the BT over 3 T in the similar configuration.
Fig. 2 shows the magnetic field dependence of the Jc at 20 K for four MgB2 bulks. The Jc values in the self field at
20 K of S-HIP#23, HIP#26, CAP#20, and CAP#30, respectively, are 3.0 105, 4.8 105, 1.4 105, and 1.1 105
A/cm2. The Jc values of the HIP bulks were 2-3 times higher than that of the bulks fabricated by the capsule method,
because the filling fraction of the HIP bulks was twice as high as that of the bulks fabricated by the capsule method.
This result of Jc qualitatively correlated with that of BT.

95T. Sasaki et al. / Physics Procedia 45 (2013) 93 – 96
610S-HIP#23
HIP#26
CAP#20
CAP#302

S-HIP#23
HIP#26
CAP#20
CAP#30510

4102J (A/cm )310c

Trapped Field (T)31FCM
B =5 T@20 K

110ex02

1000102030Temperature (K)4010012B (T)345Fig. 2. Temperature dependence of critical currentdensity Jc at 20 K.

Fig. 1. Temperature dependence of trapped field BT.

Resistivity (m cm)

0.06
S-HIP#23
HIP#26
CAP#30

0.05
0.04
0.03
0.02
0.01
0050

100 150 200Temperature (K)250300Fig. 3. Temperature dependence of electrical resistivity(a)(b)

Fig. 4. SEM images of the bulks by the (a) HIP method (S-HIP#23) and by the (b) capsule method (CAP#30).
Fig. 3 shows the temperature dependence of resistivity
expression [7],Kcrystalexp

100. The connectivity K was evaluated from the following

(1)

96

T. Sasaki et al. / Physics Procedia 45 (2013) 93 – 96(a)S-HIP#23(b)HIP#26

Fig. 5. High magnification SEM images for the (a) S-HIP bulk (S-HIP#23) and the (b) HIP bulk (HIP#26).
where crystal 6.32
cm [8], and exp = (300 K)- (40 K). The connectivity values of S-HIP#23, HIP#26, andCAP#30 are 70.2%, 81.0%, and 15.3%, respectively, which demonstrate that the good grain connection for the HIPbulks enhances the superconducting circular current flowing in the bulks and enhances the BT and Jc.
Fig. 4 shows SEM images of the HIP bulk (S-HIP#23) and the bulk by the capsule method (CAP#30). Many large
voids can be observed in the micro structure of the bulk by the capsule method. However, no large void was found in
the HIP bulk. The increase of the volume fraction of superconductivity was confirmed for the HIP bulk by the SEM
observation and was correlated with the BT and Jc values.
Fig. 5 shows the high magnification SEM images of the S-HIP#23 and HIP#26 bulks. The grain size of S-HIP#23
is about 0.2~2.0 m and that of HIP#26 is about 0.2~1.5 m. This suggests that the grain growth is enhanced by thehigher pressure sintering. Therefore, the number of the grain boundaries which seems to act as the pinning center inthe MgB2 decreases in the S-HIP#23 and the BT and Jc values are lower than those for HIP#26.
4. Conclusion
We measured the trapped magnetic field BT, critical current density Jc, and connectivity K, for the MgB2 bulks
fabricated by both HIP and capsule methods. The important obtained results are described in the followings.
(1) The highest BT of 2.51 T at 12.7 K was achieved for the HIP#26 (98 MPa). The BT and Jc of the HIP bulks werehigher than that of the bulks fabricated by the capsule method, and these results were qualitatively correlated with theresult of K.
(2) The SEM images of MgB2 bulks prepared by the HIP at 98 and 980 MPa suggest that the higher pressure enhances
the grain growth. From the results, the difference of the BT between S-HIP#23 and HIP#26 is explained.
Acknowledgements
The authors would like to acknowledge Dr. H. Teshima of Nippon Steel & Sumitomo Metal Corporation for hishelp at the production of the HIP bulks, and Mr. Y. Takeda of Advanced Prototype and Processing Center, Iwate
University for his help at the production of the capsule and the cutting bulks, and Mr. R. Koseki of Nakamura lab. inIwate University for his help at the cutting bulks. This work was in part supported by Japan Science and Technology
Agency under a Adaptable and Seamless Technology Transfer Program through target-driven R&D for a Exploratory
Research of FS stage (AS232Z02579B).
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