IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 27, NO. 4, JUNE 2017

6800905

Optimization of Infiltration and Reaction Process for
the Production of Strong MgB2 Bulk Magnets
Arata Ogino, Tomoyuki Naito, and Hiroyuki Fujishiro

Abstract—An infiltration and reaction (IR) process can realize adense MgB2 bulk under an ambient pressure, which is preferable
for the industrial mass production in cost. However, the trappedfield of the IR-processed bulk is commonly rather smaller than thatof the high-pressure synthesized bulk, which is caused by the cracks
and the thread-like channels of residual Mg preventing the circularsupercurrent flow. To obtain the high-quality IR-processed MgB2
bulk, we investigated the optimization of heat-treatment condition
for the IR-process (sintering temperature, Tht = 700–900 ◦ C andholding time, th = 1–9 h). A volume fraction of the MgB2 phaseincreased monotonically with increasing Tht and/or th , and thegrain-size increased with increasing Tht . As a result, the highest
critical current density Jc and irreversibility field µ0 Hirr wereachieved for the sample sintered at 700 ◦ C for 9 h. The relation between the superconducting properties and the microscopic structure was also discussed.
Index Terms—MgB2 , infiltration and reaction process, criticalcurrent density, trapped magnetic field.

I. INTRODUCTION
gB2 superconducting bulk magnet realizes highly homogeneous and stable Tesla-class magnetic-field in acompact space [1]–[3]. A cryogenic apparatus equipped with
the MgB2 bulk can be operated in liquid hydrogen due to the superconducting transition temperature, Tc , of 39 K [4]. A simple
synthesis process, that is, the sintering of the stoichiometric mixture of Mg and B powders (in-situ) or MgB2 powder (ex-situ),
is also preferable for the industrial mass production in terms of
cost and time. MgB2 bulk is usually fabricated under a highpressure to achieve a high filling factor, f , over 90% of the ideal
density [4]–[6]. The dense MgB2 bulk can trap the strong magnetic field over BT = 2 T at 20 K, which is about 1.3 times larger
than that of the bulk with f ∼50% via the in-situ-route underambient pressure [7]. Contrary to the high-pressure synthesis,
an infiltration (diffusion) and reaction (IR) method also realizes
the dense bulk (f >90%) without the external physical pressure[8], [9], in which the liquid Mg infiltrates into and reacts withsolid B preform to create MgB2 above the melting point of Mg,
650 ◦ C. However, the IR-processed MgB2 bulks generally trap

M

Manuscript received September 7, 2016; accepted December 2, 2016. Date of
publication December 21, 2016; date of current version February 3, 2017. This
work was supported in part by the JSPS KAKENHI under Grant JP15K04718
and Grant JP15K04646. (Corresponding author: Tomoyuki Naito.)
The authors are with the Faculty of Science and Engineering, Iwate University, Morioka 020-8551, Japan (e-mail: cloud3254@gmail.com; tnaito@
iwate-u.ac.jp; fujishiro@iwate-u.ac.jp).
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.2016.2643500

a magnetic field lower than that of high-pressure synthesizedbulks [10]–[12], because the inhomogeneous structures, such as
cracks [8] and channels of residual unreacted Mg [12], preventthe circular supercurrent flow, which are caused by the fact thata precise mechanism of infiltration and reaction process has notbeen investigated in details. Recently, we succeeded the MgB2
bulk (30 mm in diameter and 6.6 mm in thickness) by IR methodwith BT of 2.4 T at 15.9 K (2.1 T at 20.0 K) by suppressing
the expansion of the B preform under the liquid Mg-infiltration
process using the specially designed home-made capsule [13].
Although the superconducting characteristics are comparablewith those of the high-pressure synthesized bulks, this bulk stillcontained small cracks. And a small amount of Mg2 B25 impurity phase also existed, which is an intermediate product in
the IR process but does not prevent supercurrent flowing [14].
However, the creation of Mg2 B25 reduces the superconducting
volume fraction and remains the unreacted Mg.
In this paper, we have studied the IR process by observing theformation region of the MgB2 phase of the samples synthesizedfor various heat-treatment conditions. We have also discussed
the optimal heat-treatment temperature and holding time byusing the X-ray diffraction pattern, microstructure, and superconducting properties including Tc , critical current density, Jc
and BT for the MgB2 phase.
II. EXPERIMENTAL DETAILS
MgB2 sample was fabricated by an infiltration and reaction
process in a closed stainless steel (SUS) tube with 10 mm and
8 mm in outer and inner diameters, respectively, as shown in
Fig. 1(a). Raw powders of Mg (99% in purity, ≤180 μm in grainsize, Kojundo Chemical Laboratory Co., Ltd) and crystalline
B (99% in purity, ≤45 μm in grain size, Kojundo Chemical
Laboratory Co., Ltd) were weighed with Mg : B = 1.1 : 2.0 in
molar ratio, in which the excess Mg is expected to compensate
the lack of liquid Mg source. The crystalline B powder was
firstly poured into the closed-end SUS tube and then pressedusing a SUS rod under a uniaxial pressure of 5 MPa in air. The
Mg powder was also put and pressed vertically on the B layer
in the same way. The height of both B and Mg layers was about
15 mm. The tube was fully sealed using a tube fitting in Ar
atmosphere in a glove box. It was fired at Tht = 700–900 ◦ C,
which is higher than the melting point of Mg (650 ◦ C), forvarious holding times, th = 0-9 h in the box furnace. A verticalcross-section was cut from the heat-treated objects using a wire
electric discharge machine in flowing water.

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6800905

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 27, NO. 4, JUNE 2017

Fig. 1. (a) Schematic image of the precursor powders filled in the closed
stainless steel tube. (b)–(d) Pictures of the vertical cross section of the specimens
cut from the objects which were heat-treated at (b-1) T ht = 700 ◦ C for th =0–30 min, (b-2) T ht = 700 ◦ C for th = 1-9 h, (c) T ht = 800 ◦ C for th = 1–9 h,(d) T ht = 900 ◦ C for th = 1–9 h.
Fig. 2. X-ray diffraction pattern of the 700◦ C-1h sample, which was measured
at x = 8 mm of the center of X-ray beam.

The constituent phase and crystal structure of the samples
were examined by the X-ray diffraction using the Cu-Kα radiation (the wave length of 0.154 nm) operated at the acceleration
voltage of 30 kV and electron current of 30 mA. Magnetization, M , was measured by a commercial SQUID magnetometer(MPMS-XL, Quantum Design Inc.) as functions of temperatureand magnetic field. Critical current density, Jc , was estimatedfrom the magnetic hysteresis loop, M (μ0 H), using the extended
Bean’s critical state model [15], [16]. The temperature dependence of the trapped field, BT (T ), after the field-cooled magnetization was measured by a cryogenic Hall probe (HP-VVP,
Arepoc s.r.o) set on the center of the cylindrical bulk surface.
Microstructural observation and analysis of chemical composition were performed by a scanning electron microscopy with afield emission electron gun (FE-SEM; JSM-7001F, JEOL Ltd.)
coupled with energy dispersive X-ray spectroscopy (EDS) andelectron backscatter diffraction (EBSD; NordlysNano, Oxford
Instruments plc).
III. RESULTS AND DISCUSSION
Figures 1(b)–(d) show the vertical cross section of the IRprocessed samples, in which the outer frame is the SUS tube.
An unreacted B precursor probably remained in an empty spacefound for the 700◦ C-0-4 h samples, which was removed by theflowing water during the cutting process and a hard reacted material was remained. It should be noticed that the Mg infiltration
tends to be promoted from the upper periphery of the Boronlayer at Tht = 700 ◦ C for shorter th , proceeds also at the central
region, and then reaches the bottom of the Boron layer at th =9 h, as shown in Fig. 1(b). On the other hand, at Tht = 800 and
900 ◦ C, the Mg infiltration was completely terminated withinth = 1 h as shown in Figs. 1(c) and 1(d). The IR-process can beclearly observed using this technique.
The XRD pattern was taken at various positions for each sample, as schematically shown in the inset of Fig. 2. The horizontal
distance, x, which was measured from the bottom of sample,
represents the central position irradiated by the X-ray beam.
Figure 2 shows a typical XRD pattern at x = 8 mm for the700◦ C-1h sample. The reaction product was confirmed mainly
to be MgB2 with impurities of Mg2 B25 and residual unreacted
Mg. The similar XRD patterns were obtained for all the samples
and at various positions.
Figures 3(a), 3(b) and 3(c), respectively, show the peak intensity of MgB2 (101) plane, IM gB 2 (101) , and the peak intensity
ratios of IM g 2 B 2 5 (324) /IM gB 2 (101) and of IM g(101) /IM gB 2 (101) ,as functions of the heat-treatment temperature, Tht , and holding
time, th . In Fig. 3(a), the IM gB 2 (101) of about 1,000 cps for the700◦ C-1h sample increased to about 2,400 and 3,600 cps forthe 800◦ C-1h and 900◦ C-1h samples, respectively. The higher
IM gB 2 (101) was also obtained for the longer th at each Tht .
These results suggest that the formation of the MgB2 phase waspromoted with increasing Tht and/or th . In Figs. 3(b) and 3(c),the monotonic decrease of both IM g 2 B 2 5 (324) /IM gB 2 (101) andIM g(101) /IM gB 2 (101) can be observed with increasing Tht andelongating th , except for the IM g(101) /IM gB 2 (101) for 700◦ C1h and 800◦ C-1h samples. These results indicated that thereaction between Mg2 B25 and Mg to form the MgB2 phase
was promoted by the heat treatment at higher Tht and for
longer th .
Figure 4 shows the temperature dependence of the normalized magnetization, m(T )(=M (T )/M (10 K)), for the MgB2
samples fabricated at 700–900 ◦ C for th = 1 and 9 h. All
the samples, which included the th = 4 h ones (not shown),
showed the sharp superconducting transition around 38 K. Inset shows the critical temperature, Tcm id , at the mid-point (m= 0.5) of the transition as functions of the heat-treatment temperature, Tht and the holding time th . As shown in the inset,
Tcm id tends to be higher at higher Tht and for longer th , which isqualitatively consistent with the IM gB 2 (101) behavior shown in
Fig. 3(a).
Figure 5 shows the magnetic-field dependence of the criticalcurrent density, Jc (μ0 H), at T = 20 K for the MgB2 samples
fabricated at 700–900 ◦ C for th = 1 and 9 h. The irreversibility
field, μ0 Hirr , which was defined as a magnetic-field at Jc =100 A cm−2 , of 4.1–4.3 T for the 700 ◦ C-samples was evidently

OGINO et al.: OPTIMIZATION OF INFILTRATION AND REACTION PROCESS FOR THE PRODUCTION OF STRONG MgB2 BULK MAGNETS

6800905

Fig. 5. (a) Magnetic-field dependence of the critical current density,
Jc (μ 0 H ), at 20 K for the MgB2 samples fabricated at 700–900 ◦ C for th= 1 and 9 h. (b) and (c), respectively, shows the Jc at 20 K under μ 0 H = 3 T
and the self-field (μ 0 H = 0 T) as functions of the heat-treatment temperature,
T ht and the holding time, th .

Fig. 3. (a) Peak intensity of MgB2 (101) plane and peak intensity ratios of(b) Mg2 B2 5 (324)/MgB2 (101) and of (c) Mg(101)/MgB2 (101) as functions of
the heat-treatment temperature, T ht , and duration time, th .

Fig. 4. The temperature dependence of the normalized magnetization, m(T ),at μ 0 H = 0.4 mT after zero-field cooling for the MgB2 samples fabricated at
T ht = 700, 800 and 900 ◦ C for th = 1 h and 9 h. Inset shows the T cm id vs. T ht
plots for the th = 1 h and 9 h samples.

higher than about 3.8 T for the 800 ◦ C- and 900 ◦ C-samples.
The higher Jc under a magnetic field was also realized for the
700 ◦ C-samples; the Jc at 3 T and 20 K was typically shownin Fig. 5(b). On the other hand, the self-field Jc at 20 K was
almost independent of the heat-treatment condition, as shownin Fig. 5(c).
Figure 6 shows the secondary electron images (SEI) and the
EDS (chemical mapping) images (EDSI) of Mg and B for the700◦ C-1h and 900◦ C-9h samples. Both SEI’s shown in Figs. 6(a)and 6(b) demonstrated the existence of two distinct light grayand dark gray (LG and DG) regions. A bright area in the EDSI
shown in Figs. 6(c)–6(f) represents the existence of the target
element. Boron was clearly observed as the absence in the Mgmapping. Although the surface morphology of the 700◦ C-1h
sample (Fig. 6(a)) does not reflect the chemical compositions(Figs. 6(c) and 6(e)), the large angular-shaped B particles withsmall amount of Mg are clearly found, and both Mg and residual
B coexisted in the matrix. The quantitative EDS analysis suggested that the large B-enriched particle and the matrix were,respectively, estimated roughly to the Mg2 B25 phase with theunreacted B and the MgB2 phase with a small amount of B. On
the other hand, the DG area in the SEI (Fig. 6(b)) corresponds
evidently to the B-enriched particle in the EDSI’s (Figs. 6(d) and6(f)) for the 900◦ C-9h sample. These were determined roughly
to be the Mg2 B25 phase by the quantitative EDS analysis. The
B-enriched particle was rounded and the amount substantiallydecreased with increasing Tht , indicating that the reaction ofliquid Mg and solid B at the periphery of the B particle was6800905

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 27, NO. 4, JUNE 2017

Fig. 6. (a) and (b) the secondary electron images and the chemical mapping
images of (c) and (d) Mg and (e) and (f) B for the 700◦ C-1h and 900◦ C-9h
samples.

promoted by the higher Tht and longer th . The grain growth of
the MgB2 was also facilitated for the 900◦ C-9h sample, because
the connected MgB2 grains can be found in the SEI (Fig. 6(b)).
Figure 7 shows the electron backscattered diffraction images(EBSDI) and EDS images (EDSI) for the 700◦ C-1h and 700900◦ C-9h samples. The EBSD analysis was applied for MgB2
and Mg phases, which are represented by red and blue colors,
respectively. In the EDSI for Mg and B, the bright region showsthe existence of the target element. Quite a small amount of
nanometer-size MgB2 grains and quite a large amount of unreacted Mg coexisted with an unanalyzed black-color area inthe EBSDI for the 700◦ C-1h sample (Fig. 7(a-1)). The EDSI’sof Mg and B (Figs. 7(a-2) and 7(a-3)) suggest the unreacted B
predominantly existed in the black area of EBSDI. As foundin the EBSDI of 700◦ C-9h sample (Fig. 7(b-1)), the creationand grain-growth of MgB2 were promoted by elongating theholding time, th . However the unreacted Mg still substantiallyremained. Therefore, the longer th than 9 h must be required
for Tht = 700 ◦ C. The MgB2 grains grew to micrometer-sizefor the 800◦ C-9h sample (Fig. 7(c-1)). The amounts of MgB2
also increased greatly, but at the same time, that of residual
Mg decreased for the 800◦ C-9h sample(Fig. 7(c-1)). The further larger MgB2 grain with a few micrometer-size was found
for the EBSDI of the 900◦ C-9h sample (Fig. 7(d-1)). In addition, the unreacted Mg still remained, but the unreacted B
fully disappeared. These results demonstrate that the existenceof fine nanometric MgB2 grains is preferable to obtain the highJc (μ0 H) and μ0 Hirr , in spite of the quite small volume fractionof MgB2 .
Figure 8 shows the temperature dependence of the trappedfield, BT (T ), of the IR-processed cylindrical bulk (8 mm inFig. 7. (a-1) Electron backscattered diffraction (EBSD) image and EDS(chemical mapping) images of (a-2) Mg and (a-3) B for the 700◦ C-1h sample. (b-), (c-) and (d-) series, respectively, are the same images for the 700 ◦ C,
800 ◦ C and 900 ◦ C samples for th = 9 h. In the EBSD images, red is MgB2
and blue is Mg metal. In the EDS images, red is Mg metal and green is B metal.

Fig. 8. Temperature dependence of the trapped field, B T (T ), of the IRprocessed cylindrical bulk (8 mm in diameter and 15 mm in thickness) synthesized at T ht = 700 ◦ C for th = 9 h.

diameter and 15 mm in thickness) synthesized at Tht = 700 ◦ C
for th = 9 h. The highest BT of 1.2 T was achieved at 13.7 K.
The numerical simulation [17] using the Jc (μ0 H) of this bulk
suggested that this BT will become approximately twice byenlarging the diameter to 30 mm. The BT value on the IRprocessed MgB2 bulk become enhanced with increasing both

OGINO et al.: OPTIMIZATION OF INFILTRATION AND REACTION PROCESS FOR THE PRODUCTION OF STRONG MgB2 BULK MAGNETS

Jc and the size by the improvement of IR process including theheat-treatment condition and the refinement of B particles.
IV. SUMMARY
We have studied the superconducting properties of the MgB2
sample fabricated by the infiltration and reaction (IR) process
under various heat-treatment conditions (the sintering temperature, Tht = 700–900 ◦ C and the holding time, th = 0–9 h). The
observation of IR process suggested that the infiltration processstrongly depended on the heat-treatment conditions. An empty
region without any hard materials was observed for the samples synthesized at Tht = 700 ◦ C for th = 0–4 h. The MgB2
phase was completely created in the preform under other heattreatment conditions (Tht = 700 ◦ C for th = 9 h, Tht = 800 and
900 ◦ C for th ≥1 h). The volume fraction of MgB2 increased
with increasing Tht and th , and simultaneously the nanometersize MgB2 grain for Tht = 700 ◦ C grew to the micrometer-size
for Tht = 900 ◦ C. On the other hand, the critical current density,
Jc and the irreversibility field, μ0 Hirr were degraded for higher
Tht . As a result, the MgB2 sample fabricated at 700 ◦ C for 9 h
demonstrated the best performance, Jc = 5 kA cm−2 under 3 T
and μ0 Hirr = 4.3 T at 20 K among the samples studied here.
These results suggest that the lower sintering temperature ispreferable to obtain the higher vortex pinning properties. Considering the balance of the grain-size with the volume fractionof MgB2 , the optimal th seems to be over 9 h for Tht = 700 ◦ C.
ACKNOWLEDGMENT
The authors would like to thank Dr. T. Murakami and Mr.
K. Sasaki of Iwate University (IU) for their assistance in microstructual analysis using SEM-EBSD and SEM-EDS apparatuses, and also Mr. Y. Takeda of IU for his help in fabricating
the MgB2 samples.
REFERENCES[1] T. Naito, T. Sasaki, and H. Fujishiro, “A proposal of new fabricating
technique of large MgB2 bulk by a capsule method,” IEEE Trans. Appl.
Supercond., vol. 22, no. 3, Jun. 2012, Art. no. 4401703.

6800905[2] G. Fuchs et al., “High trapped fields in bulk MgB2 prepared by hotpressing of ball-milled precursor powder,” Supercond. Sci. Technol.,vol. 26, 2013, Art. no. 122002.
[3] A. Yamamoto, A. Ishihara, M. Tomita, and K. Kishio, “Permanent magnet
with MgB2 bulk superconductor,” Appl. Phys. Lett., vol. 105, no. 3, 2014,
Art. no. 032601.
[4] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, and J. Akimitsu,“Superconductivity at 39 K in magnesium diboride,” Nature, vol. 410,
no. 6824, pp. 63–64, 2001.
[5] Y. Takano et al., “Superconducting properties of MgB2 bulk materials
prepared by high-pressure sintering,” Appl. Phys. Lett., vol. 78, no. 19,
pp. 2914–2916, 2001.
[6] D. Goto, T. Machi, Y. Zhao, N. Koshizuka, M. Murakami, and S. Arai,“Improvement of critical current density in MgB2 by Ti, Zr and Hf doping,” Physica C, vol. 392–396, pp. 272–275, 2003.
[7] T. Sasaki, T. Naito, and H. Fujishiro, “Trapped magnetic field of dense
MgB2 bulks fabricated under high pressure,” Phys. Procedia, vol. 45,
pp. 93–96, 2013.
[8] G. Giunchi, “High density MgB2 obtained by reactive liquid Mg infiltration,” Int. J. Mod. Phys. B, vol. 17, no. 04n06, pp. 453–460, 2003.
[9] S. Ueda et al., “High critical current properties of MgB2 bulks preparedby a diffusion method,” Appl. Phys. Lett., vol. 86, no. 22, 2005, Art. no.
222502.
[10] E. Perini, G. Giunchi, L. Saglietti, A. Albisetti, A. Matrone, andV. Cavaliere, “Magnetic field trapping in MgB2 bulks and inserts,” IEEE
Trans. Appl. Supercond., vol. 21, no. 3, pp. 2690–2693, 2011.
[11] H. Mochizuki, H. Fujishiro, T. Naito, A. F. Albisetti, and G. Giunchi,“Trapped field properties of a concentric-circled MgB2 bulk composite
magnetized by pulsed field and field cooling,” Supercond. Sci. Technol.,vol. 28, no. 10, 2015, Art. no. 105004.
[12] A. G. Bhagurkar et al., “A trapped magnetic field of 3 T in homogeneous,bulk MgB2 superconductors fabricated by a modified precursor infiltrationand growth process,” Supercond. Sci. Technol., vol. 29, no. 3, 2016, Art.
no. 035008.
[13] T. Naito, A. Ogino, and H. Fujishiro, “Potential ability of 3 T-class trappedfield on MgB2 bulk surface synthesized by infiltration-capsule method,”
Supercond. Sci. Technol., vol. 29, no. 11, 2016, Art. no. 115003.
[14] G. Giunchi, G. Ripamonti, T. Cavallin, and E. Bassani, “The reactive liquid Mg infiltration process to produce large superconducting bulk MgB2
manufacts,” Cryogenics, vol. 46, no. 23, pp. 237–242, 2006.
[15] C. P. Bean, “Magnetization of hard superconductors,” Phys. Rev. Lett.,
vol. 8, no. 6, pp. 250–253, 1962.
[16] E. M. Gyorgy, R. B. van Dover, K. A. Jackson, L. F. Schneemeyer, and J. V.
Waszczak, “Anisotropic critical currents in Ba2 YCu3 O7 analyzed using
an extended bean model,” Appl. Phys. Lett., vol. 55, no. 3, pp. 283–285,
1989.
[17] H. Fujishiro, T. Naito, and T. Yoshida, “Numerical simulation of thetrapped field in MgB2 bulk disks magnetized by field cooling,” Supercond.
Sci. Technol., vol. 27, no. 6, 2014, Art. no. 065019.