IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 22, NO. 3, JUNE 2012

4401703

A Proposal of New Fabricating Technique of Large
MgB Bulk by a Capsule Method
Tomoyuki Naito, Tomohisa Sasaki, and Hiroyuki Fujishiro

Abstract—We proposed a new fabricating technique of a large
MgB superconducting bulk (20–30 mm in diameter) by a capsulemethod and measured the trapped field distribution for the MgBbulks. The capsule consists of a couple of commercial stainless steel
flanges, a stainless steel plate and a copper gasket. The MgB bulks
showed a superconducting transition at 38.3–38.6 K with a transition width of 0.6–1.1 K. The trapped magnetic field of the centerof the bulk surface reached 1.65 T at 14 K for the bulk 30 mm indiameter. These results indicate that the capsule method is usefulto produce the tesla-order large MgB bulk magnet.
Index Terms—Capsule method, MgB , superconducting bulk
magnet, trapped magnetic field.
Fig. 1. Schematic image of the capsule.

I. INTRODUCTIONABULK superconductor which traps the magnetic field canbe used as a strong permanent magnet. A single grainof RE-Ba-Cu-O (RE:rare earth elements) superconducting bulk
fabricated by a top seeded melt-growth method [1]–[3] is one ofthe promising materials for a bulk magnet because of their highsuperconducting transition temperature and high critical current
density below 77 K. Actually, a Y-Ba-Cu-O bulk reinforced byan epoxy resin trapped the magnetic field of 17 T at 29 K by fieldcooling [4], which is one or two orders of magnitude larger thanthat of a permanent magnet. In view of the Bean’s critical statemodel [5] the trapped magnetic field is proportional to the diameter and critical current density, , therefore a larger bulk witha large
is desirable. To obtain the single grain RE-Ba-Cu-O
bulk with diameter over 100 mm, we have to use a special technique [6]–[8].
Because of the developments of the fabrication techniques,
the polycrystalline MgB samples showed a highoverA/cm [9], [10], which is as high as that of the single grain
RE-Ba-Cu-O bulks [11]. Since MgB has a long coherence
length, the problem of the weak link can be ignored even foruse of polycrystals [12].
Several groups have already developed the MgB bulk
magnet [13]–[15]; the MgB bulk (28 mm in diameter andManuscript received September 13, 2011; accepted November 08, 2011. Dateof publication November 22, 2011; date of current version May 24, 2012. Thiswork was supported in part by the Japan Science and Technology Agency under
Research for Promoting Technological Seeds 2008 (02-051).
T. Naito and H. Fujishiro are with the Faculty of Engineering, Iwate University, Morioka 020-8551, Japan, and the Soft-Path Engineering Research Center(SPERC), Faculty of Engineering, Iwate University, Morioka 020-8551, Japan(e-mail: tnaito@iwate-u.ac.jp).
T. Sasaki is with the Faculty of Engineering, Iwate University, Morioka 0208551, Japan.
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TASC.2011.2176896

11 mm in thickness) fabricated under high pressure, trapped amagnetic field of about 2.3 T at 6 K [13]. The 1.2 T MgB bulk
magnet (20 mm in diameter) at 17 K was also reported [14].
Therefore, the MgB bulk is a promising material to produce
a large superconducting bulk magnet.We have developed acapsule method for fabricating a large MgB bulk. In this paper,
the superconducting properties and the trapped magnetic fieldof MgB bulks fabricated by a capsule method, are evaluated,
and the possibility of the capsule method are discussed.
II. EXPERIMENTAL
A. Preparation of MgB Bulks
MgB bulks were prepared by the capsule method. Fig. 1
shows the schematic image of the capsule. The capsule consistsof a couple of commercial stainless steel flanges (70 mm in diameter, 10 mm in thickness; ICF flange), a stainless steel plate
and a copper gasket. Raw powders of Mg and B were weighted
with 1.1:2.0 in molar ratio and ground. The mixture was pressed
into pellets 20–30 mm in diameter and 5–9 mm in thickness
under an uniaxial pressure. The precursor pellet was prepared inair, and the preparation of the capsule was performed in Ar-atmosphere using a glove box. The precursor pellet was set in
the hole of the stainless steel flange, covered by the stainlesssteel plate, and finally closed by the other flange with the coppergasket. The closed capsule was sintered at 600–800 C for 6–48
h in a box furnace and cooled down to room temperature by furnace cooling. Sample specifications are listed in Table I. Fig. 2shows the photograph of the #20 bulk 20 mm in diameter. The
circle at the center is the MgB bulk. Superconducting transition temperature, , at the mid-point of the transition and the, were obtained by the magnetization meatransition width,surements. Powder X-ray diffraction pattern (not shown here)
was taken at the room temperature and the almost single-phase1051-8223/$26.00 © 2011 IEEE4401703

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 22, NO. 3, JUNE 2012

TABLE ISPECIFICATIONS OF THE MgB BULKS

Fig. 2. Photograph of the #20 MgB bulk (center circle) with the stainless steel
flange.

Fig. 3. Temperature dependence of the trapped magnetic field of the MgBbulks.

structure was confirmed with small amount impurities such as
MgO and MgCu .
B. Measurements
To measure the trapped magnetic field, the MgB bulk wasset on a cold head of a Gifford-McMahon cycle helium refrigerator by inserting a thin indium plate to obtain good thermal
contact, and cooled down under a magnetic field of 5 T using
a 10 T cryogen-free superconducting magnet. The temperatureof the bulk was measured by a Cernox thermometer mounted
on its top surface. Trapped magnetic field was measured bya cryogenic Hall sensor (BHT-921, F.W. Bell Inc.) adheredon the center of the bulk surface. We also measured the profile of the trapped magnetic field at 4 mm up from the bulk
surface by scanning the axial-type Hall sensor. The superconducting properties were evaluated by measuring the resistivity
and the magnetization using a small piece cut from the bulk
after the measurement of the trapped magnetic field. Resistivity
was measured by a standard four probe technique. Magnetization was measured under the magnetic field of 0.4 mT after
zero-field cooling using a commercial superconducting interference device (SQUID) magnetometer (MPMS-XL, Quantum
Design Inc.).
III. RESULTS AND DISCUSSION
A. Trapped Magnetic Field
Fig. 3 shows the temperature dependence of the trapped magnetic field,, of the MgB bulks. Thelines in theprevious reports [13], [14] are also shown. The
value of #20
bulk is 1.3 T at 17 K, which is slightly larger than the reported
one of 1.2 T at the same temperature for the MgB bulk with 20mm in diameter [14]. The quality of our bulk seems to be comparable to that of the reported one. Thelines of #30-1 and-2 bulks coincide with each other and increase with decreasing

Fig. 4. Profile of the trapped magnetic field of the #30-2 bulk at 30 K.

temperature. These results suggest that the critical current density, , increases with decreasing temperature. The highest
value is 1.65 T at 14 K for #30-2 bulk. This value is somewhat
smaller than the reported one which was achieved for the bulk
with 28 mm in diameter [13]. This probably come from the factthat the MgB bulk in Ref. [13] was fabricated under high pressure, i.e., their bulk is denser than ours. Thecurves of#30-1 and -2 bulks suggest that the superconducting characteristics do not depend on the sintering condition. The magnitude of
at 17 K for #30-1 and -2 bulks is approximately 1.5 T, whichis estimated by the interpolation of thecurve. Thisvalue is about 1.2 times larger than that of #20 bulk (20 mmin diameter), the value of which is less than 1.5 expected fromthe ratio of the diameter. Because Bean’s critical state model
gives the fact that the trapped magnetic field is proportional tothe diameter of a disk-shaped bulk, if theis the same. Thus,
this might originate from the difference inproperties
between 20 mm and 30 mm bulks. Fig. 4 shows thedistribution at 30 K for #30-2 bulk. The symmetric conical-shaped

NAITO et al.: NEW FABRICATING TECHNIQUE OF LARGE MgB BULK BY A CAPSULE METHOD

4401703

stainless flanges, a stainless steel plate and a copper gasket andevaluated their superconducting properties and ability to trap
the magnetic field. 1.65 T bulk magnet was realized at 14 K
using a bulk with 30 mm in diameter, of which magnetic field
was somewhat less than the MgB bulk magnet (28 mm in diameter) synthesized under high pressure. The superconducting
transition temperature and width of the transition were about
38.3–38.6 K and 0.6–1.1 K, respectively. These results indicate
the MgB bulks fabricated by the capsule method have good superconductivity and high ability to trap the magnetic field. Our
capsule method is found to be useful to produce larger MgBbulk magnets than 50 mm in diameter.
ACKNOWLEDGMENT
We thank Dr. S. Kobayashi for his help at the magnetization
measurements using SQUID magnetometer.
REFERENCES

Fig. 5. Temperature dependence of (a) the resistivity and (b) the normalized
magnetization of the small pieces cut from the center of the #30-1 and 2 bulks.

profile indicates that superconducting characteristics of theMgB fabricated by the capsule method is homogeneous. To
study deeply the mechanism of the trapping magnetic field in
the MgB bulks, the evaluation ofdistribution using smallpieces cut from the bulk is now in progress.
B. Superconducting Properties
Fig. 5(a) and (b), respectively, show the temperature dependence of the resistivity and the normalized magnetization of thesmall pieces cut from the center of the #30-1 and -2 bulks. Resistivity decreases monotonically with decreasing temperature andshows the sharp superconducting transition. Inset of Fig. 5(a)is the enlarged figure around transition region. The superconducting transition temperatures, ’s, defined at the midpointof the transition, are approximately 38.5 K for #30-1 bulk and38.6 K for #30-2 one. The transition widths,’s, are about
0.6 K for #30-1 bulk and 1.0 K for #30-2 one. In Fig. 5(b), thereduced magnetization also demonstrates the sharp superconducting transition, and the ’s at the midpoint and’s areapproximately 38.3 K and 0.8 K for #30-1 bulk and 38.5 K and
1.1 K for #30-2 one. These results indicate that almost the stoichiometric MgB bulks are fabricated by the capsule method.
IV. SUMMARY
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