IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 25, NO. 3, JUNE 2015

6801204

Development of 4 T Class MgB2 Bulk
Magnets Doped by Ti
Takafumi Yoshida, Tomoyuki Naito, and Hiroyuki Fujishiro

Abstract—We have studied the trapped field properties of Tidoped MgB2 bulks that were fabricated by a hot isostatic pressing
method. The trapped field values of Ti5% doped bulk with 38 mm
in diameter and 7 mm in thickness were about 3.6 T at 13.2 K and
2.8 T at 20 K, which were 1.3 times larger than those of the pristinebulk at each temperature. The critical current density was alsoenhanced by the Ti-doping from 1.7 × 102 A/cm2 for the pristinebulk to 3.0 × 103 A/cm2 for the Ti5% doped bulk at 20 K under
4 T. The improvement of the flux pinning properties originatedfrom the TiB2 thin layer at the periphery of the Ti precipitates,
which was acted as a strong pinning center.
Index Terms—Critical current density, MgB2 superconductors, microstructure, superconducting bulk magnet, trapped field
properties.

I. I NTRODUCTIONM

gB2 has the highest superconducting transition temperature Tc = 39 K among intermetallic compounds. Since
its coherence length is long in comparison with RE-Ba-Cu-O(RE: rare earth elements) system [1], we can discard the weaklinks at grain boundaries, and can realize a superconducting
bulk magnet using a polycrystal. Several groups have reported
that a MgB2 bulk trapped the tesla-order magnetic field [2]–[6].
We reported the trapped magnetic field, BT , of 1.77 T at 15.5 K
for an in-situ MgB2 bulk with 38 mm in diameter and 9 mm inthickness fabricated under an ambient pressure by the capsulemethod [2]. This bulk had a low filling factor of about 50%,
which was intrinsically caused by the in-situ reacting process.
Therefore, the high-pressure sintering was expected to enhance
the filling density and the critical current density, Jc , i.e., BT .
We have succeeded a 3 T class MgB2 bulk magnet using thehighly dense MgB2 bulk fabricated by a hot isostatic pressing(HIP) method under a pressure of 98 MPa [3]. The similar
result was reported by Viznichenko et al., who achieved the BT
value of about 2.3 T at 6 K for the MgB2 bulk with 28 mm
in diameter and 11 mm in thickness which was sintered undera much higher pressure of 2 GPa [4]. Recently, Durrel et al.
reported the BT value over 3 T at 17.5 K in stacked MgB2 bulks(diameter 25 mm and thickness 5.4 mm) which were fabricatedby uniaxial hot pressing method under a pressure of 25 MPa [5],Manuscript received August 9, 2014; accepted December 9, 2014. Date of
publication December 18, 2014; date of current version February 6, 2015. This
work was supported in part by JST A-STEP under Grant AS242Z02673L, byHitachi Metals—Materials Science Foundation, and by JSPS KAKENHI underGrant 23560002.
The authors are with the Faculty of Engineering, Iwate University, Morioka
020-8551, Japan (e-mail: 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.2014.2381558

and Fuchs et al. obtained quite high BT of 5.4 T at 12 K for the
hot pressed MgB2 bulk (diameter 20 mm and thickness 8 mm)[6]. Therefore, the densification is an essential way to develop
the strong MgB2 bulk magnet.
Chemical doping is usually performed to improve the flux
pinning properties; C and Ti are known to be effective dopantsfor MgB2 [7]–[9]. Dou et al. demonstrated that Jc and theirreversibility field, Birr , of MgB2 wire prepared by a powderin-tube method were significantly enhanced by SiC-doping, inwhich Jc was 2.4 × 105 A/cm2 at 20 K under 2 T for the 10wt%
SiC doped sample [7]. On the other hand, Zhao et al. reported
that Ti-doping also brought about high Jc for the Ti10%-doped
MgB2 sample, in which Jc was 1.3 × 106 A/cm2 in the selffield and 9.4 × 104 A/cm2 under 2 T at 20 K [8]. Doped Ti did
not occupy the atomic site in the MgB2 crystal, but formed athin TiB2 layer between the grain boundaries. Besides, MgB2
grains were greatly refined by Ti-doping, resulting in a strongly
coupled nanoparticle structure [9]. They concluded that theenhancement in Jc originated from both the existence of TiB2
layer as the novel flux pinning center and the densification.
In this paper, for the purpose of the improvement of thetrapped field properties of MgB2 , we fabricated Ti-doped MgB2
bulks by HIP method and studied the effect of Ti-doping on
their flux pinning properties and microstructure.
II. E XPERIMENTAL D ETAILS
A. Sample Preparation
MgB2 bulks were fabricated by the HIP method [3]. Raw
powders of Mg (99.5% in purity,  180 µm in grain size,
Kojundo Chemical Laboratory Co., Ltd), amorphous B (99% in
purity, 300 mesh in grain size, Furuuchi Chemical Corp.) andTi (99% in purity,  45 µm in grain size, Kojundo Chemical
Laboratory Co., Ltd) were weighted with Mg : B : Ti = (1 − x) :2 : x (x = 0, 0.05, 0.1, and 0.2) in molar ratio and ground. The
mixture was pressed into pellet 40 mm in diameter and 20 mm
in thickness under an uniaxial pressure of about 12 MPa, andthe pellet was further densified by a cold isostatic pressing
method under a pressure of 196 MPa. The precursor pellet wassealed in the stainless steel container by electron beam welding
in vacuum, and subsequently was sintered at temperature of
900 ◦ C for 3 h under a pressure of 98 MPa in the HIP furnace.
Typical size of the shaped bulk is 38 mm in diameter and 7 mmin thickness. The filling factor, F, which was the ratio of
the measured mass density to the theoretical mass density of the
MgB2 -Ti composite, was larger than 90%. This means that thefilling factor of the MgB2 part in the Ti-doped bulk was above1051-8223 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 25, NO. 3, JUNE 2015

TABLE IS PECIFICATION OF THE P RISTINE AND Ti-D OPED MgB2 B ULKS

90%. We also estimated the volume fraction, Vf , of the MgB2
phase in the Ti-doped bulks as follows, 87% for Ti5%-doped,
84% for Ti10%-doped, and 72% for Ti20%-doped bulks, which
are also found in Table I. Specifications of the bulks are listed
in Table I.
Fig. 1. Temperature dependence of trapped field, BT (T ), of the pristine andTi-doped MgB2 bulks with various Ti-doping.

B. Measurements
The MgB2 bulk was magnetized by field cooling (FC) in amagnetic field of 5 T parallel to the thickness direction using
a 10-T cryogen-free superconducting magnet (JMTD-10T100,
Japan Superconductor Technology (JASTEC) Inc.), and thenthe applied field was decreased to 0 T at a rate of 0.022–
0.222 T/min. The trapped field value, BT , at the center of thebulk surface was measured by a cryogenic Hall sensor (BHT921, F.W. BELL Inc.). After the FC experiment, we evaluatedthe critical temperature, Tc , and the critical current density, Jc ,by measuring the magnetization of a small piece cut from thebulk using a commercial SQUID magnetometer (MPMS-XL,
Quantum Design Inc.). Jc was estimated from the hysteresis loop using the extended Bean model. Crystal structure
and phase evaluation were investigated by the powder X-ray
diffraction (XRD) with a Cu Kα radiation using the X-ray diffractometer (Multi Flex, Rigaku Corp.). Microstructure andcomposition analysis of the Ti-doped MgB2 bulks were investigated by electron probe micro analyser (EPMA) (JXA-8100,
JEOL Ltd.).
III. R ESULT AND D ISCUSSION
Fig. 1 presents the temperature dependence of the trappedfield, BT (T ), of the pristine and Ti-doped MgB2 bulks. The
maximum BT values of the pristine bulk, Ti5%-, Ti10%-, andTi20%-doped bulks, respectively, are 2.5 T at 16.0 K, 3.6 T at
13.2 K, 3.5 T at 14.4 K, and 3.4 T at 15.3 K. For comparison,
the BT values at 16 K are estimated by interpolation for all
the bulks, as described in Table I. Those of Ti-doped bulks areabout 1.3 times larger than that of the pristine bulk. Although
we succeed in the improvement of the trapped magnetic field by
Ti doping, the BT (T ) curves of Ti-doped bulks hardly dependon the Ti-doping levels, which cannot be explained by the factthat the volume fraction decreases with increasing Ti-contents.
Fig. 2 shows the temperature dependence of the normalized magnetization, M(T ), in a magnetic field of 4 Oe after
zero-field cooling. The critical temperature, Tc , defined at the
midpoint of the transition is 38.1–38.4 K for all the bulks regardless of the Ti-doping levels. This indicates that Ti does notsubstitute for the Mg-site, because, in general, the site substitution suppresses the Tc value.

Fig. 2. Temperature dependence of normalized magnetization for the pristine
and Ti-doped MgB2 bulks, in 4 Oe after the ZFC process.

Fig. 3(a) shows the magnetic field dependence of the criticalcurrent density, Jc (B), at T = 10–30 K for the pristine and Tidoped MgB2 bulks. The Jc (B) values of the Ti-doped bulks aremuch higher than that of the pristine bulk for all the measured
temperatures. Fig. 3(b) and (c) shows the Ti-doping amount
dependence of Jc at T = 10 K and 20 K for various magnetic
fields. The Jc value increases clearly by Ti-doping for eachapplied field, takes a slight maximum at the 10% doping and decreases at the 20% doping except for at 20 K under 3 T and 4 T
at which the maxima are observed at the 5% doping. However,the doping dependence of Jc higher than 5% is very small,which is a clear contrast to the results reported previously [8]and can explain the Ti-content independent BT curves in Fig. 1.
Difference in Jc between the pristine and Ti-doped bulks tends
to increase conspicuously with the increase in the magnetic
field. The Jc values of the pristine and Ti10%-doped bulks arelisted in Table II, in which the Jc values of pristine bulk are
8.3 × 103 A/cm2 at T = 10 K and 1.7 × 102 A/cm2 at T = 20 K
under 4 T. On the other hand, the Jc values of Ti10%-doped
bulks are 3.5 × 104 A/cm2 at T = 10 K and 2.7 × 103 A/cm2 at
T = 20 K under 4 T. It is noteworthy that although the absolutevalue of Jc of the Ti10%-doped bulk at 10 K under 0 T iscomparable to the reported value [8], that of the Ti10%-doped
bulk at 20 K under 4 T is about two orders of magnitude larger
than the reported value [8]. This indicates that the Ti-contained
impurities act as the effective pinning center at higher magnetic
fields in comparison with [8].
Fig. 4 shows the X-ray diffraction patterns of the pristine andTi-doped bulks at room temperature. For the pristine bulk, only

YOSHIDA et al.: DEVELOPMENT OF 4 T CLASS MgB2 BULK MAGNETS DOPED BY Ti

6801204

Fig. 4.

X-ray diffraction patterns of the pristine and Ti-doped MgB2 bulks.

Fig. 5.

Composition images of the Ti-doped MgB2 bulks.

Fig. 3. (a) Magnetic field dependence of the critical current density Jc at 10–
30 K for the pristine and Ti-doped MgB2 bulks. Ti-doping amount dependenceof the critical current density Jc at (b) T = 10 K and (c) 20 K in various magnetic
fields.
TABLE IIJc VALUES OF THE P RISTINE AND Ti10% D OPED MgB2 B ULKS

the peaks of MgB2 are observed. For the Ti-doped bulks, inaddition to the dominant MgB2 phase, the peaks of Ti and TiB2
are found as the impurity phases. The peak intensity of TiB2
and Ti increases with increasing Ti-doping level.
Fig. 5 shows the composition images on the surface of the Tidoped MgB2 bulks, which was obtained by the backscatteredelectrons using EPMA. The dominant gray region is the MgB2
matrix phase and the black parts are voids and cracks. The white
portion of the images represents Ti-metal which is identified inFig. 6 and increases with increasing Ti-doping. Fig. 6 shows
the characteristic X-ray images of Mg, B, and Ti for the Ti5%
doped bulk; white color represents the corresponding element.
From Figs. 5(a) and 6(c), the white regions in the composition
image are Ti and their size is typically several tens micrometers.
In addition, it should be noted that B accumulates at the
periphery of Ti particles as found in Fig. 6(b). This thin layer
containing both Ti and B may be the TiB2 phase observed in the

XRD patterns. We consider that the TiB2 layers act as the strong
flux pinning center and enhance both Jc and BT values, because
the size of Ti particles is too large to pin the flux effectively. The
size of TiB2 is quite different from that observed in the literature[9], which possibly gives higher Jc in higher magnetic fields.
On the other hand, the trapped field is generally degraded withdecreasing the volume fraction Vf of MgB2 phase in the bulk.
Therefore, the trapped field independent of the Ti-contents asshown in Fig. 1 may originate from the competition between
the increase of the number of the vortex pinning centers and thedecrease of the volume fraction of MgB2 phase.
IV. C ONCLUSION
We have investigated the trapped field, BT , the critical current
density, Jc , microstructure and composition of the Ti-doped
MgB2 bulks which were fabricated by the HIP method. The
TiB2 thin layer found at the periphery of the Ti precipitate acted
as the effective vortex pinning center at higher magnetic fields,
and, as a result, highly improved both BT and Jc .
The important obtained results are described in the following.
1) The highest BT of 3.6 T was realized at 13.2 K for the
Ti5% doped bulk, which was 1.3 times larger than that

6801204

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 25, NO. 3, JUNE 20154) TiB2 was found as the thin layer at the periphery of theTi particles, which acted as the strong pinning center andenhanced both BT and Jc .

ACKNOWLEDGMENT
The authors would like to acknowledge Dr. H. Teshima of
Nippon Steel & Sumitomo Metal Corporation for his help at
the production of the HIP bulks.
R EFERENCES

Fig. 6. Characteristic X-ray images of (a) Mg, (b) B, and (c) Ti for the Ti5%
doped MgB2 bulk.

of the pristine bulk. However, further doping did notenhance BT (T ); the BT (T ) curves of both Ti10% andTi20% doped bulks were almost the same as that of Ti5%
bulk. Extra Ti was precipitated as Ti particles.
2) The critical temperature, Tc , was about 38.1–38.4 K for
all the bulks regardless of the Ti-doping levels, whichindicated that doped Ti did not substitute for the Mg-site.
3) The Jc values of the Ti-doped bulk were about one orderof magnitude higher than that of the pristine bulk in themagnetic fields. The Jc value increased clearly by Tidoping for each applied fields, took a slight maximum
at the 10% doping and decreased at the 20% doping.

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