IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 26, NO. 3, APRIL 2016

6800805

Vortex Pinning Properties of Dense Ti-Doped MgB2
Bulks Sintered at Different Temperatures
Tomoyuki Naito, Takafumi Yoshida, Hidehiko Mochizuki, Hiroyuki Fujishiro, Ritwik Basu, and Jerzy A. Szpunar

Abstract—We have studied the trapped field by field-cooledmagnetization and microstructure of the Ti-doped MgB2 bulks
fabricated at 700 ◦ C by an in situ hot isostatic pressing method,which were compared with those at 900 ◦ C. The maximum trappedfield BT of 3.0 T at 19 K was realized at the Ti-doping level of
10%–20%. In spite of the lower filling factor of 700 ◦ C Ti-doped
bulks, i.e., f ∼ 80%, the obtained BT = 3.0 T was 1.3 times
larger than that of the nondoped bulk (f ∼ 90%) and was comparable with those for the 900 ◦ C Ti-doped bulks (f ∼ 90%) withthe same Ti contents. The quantitative analysis of the chemicalcomposition revealed that fine particles of higher borides, i.e.,
MgBy (y > 2), existed in the MgB2 matrix. The low sintering
temperature of 700 ◦ C and the Ti doping strongly suppressed thegrain growth of the MgB2 matrix. The increase in both the grainboundaries of MgB2 and the nonsuperconducting fine particles
enhanced the trapped field of the present 700 ◦ C Ti-doped MgB2
bulks.
Index Terms—Critical current density, electron backscattereddiffraction (EBSD), field-cooled magnetization (FCM), MgB2 ,superconducting bulk magnet, Ti doping, trapped magnetic field.

I. I NTRODUCTIONSUPERCONDUCTING bulk magnets with a Tesla-ordertrapped magnetic field, BT , are very promising for developments of a novel superconducting apparatus [1], in which
the BT is used in an open space as a conventional permanent
magnet. In addition to single grain RE-Ba-Cu-O (RE is rareearth element) bulk magnets, MgB2 bulk magnets have been
extensively studied, especially for recent years [2]–[8]. This isbecause the specific features of MgB2 including weak-link-freegrain boundaries [9], [10], RE free, light weight and high yieldstress [11] possibly overcome the shortcomings of RE-Ba-Cu-O
bulks.
Major problems of MgB2 are a low filling factor, f , of about
50–75% of the theoretical density and a strongly suppressed
critical current density, Jc , in magnetic fields. Highly dense
MgB2 can be achieved by a sintering under high pressure [12],[13]. We have reported that the trapped field by field-cooled
Manuscript received September 8, 2015; accepted February 9, 2016. Dateof publication February 23, 2016; date of current version April 25, 2016. This
work was supported in part by Hitachi Metals·Materials Science Foundationand in part by JSPS KAKENHI Grants 23560002, 15K04718, and 15K04646.
T. Naito, T. Yoshida, H. Mochizuki, and H. Fujishiro are with the Facultyof Engineering, Iwate University, Morioka 020-8551, Japan (e-mail: tnaito@
iwate-u.ac.jp).
R. Basu is with the Department of Mechanical Engineering, The Northcap
University, Gurgaon-122 017, India.
J. A. Szpunar is with the Department of Mechanical Engineering, Universityof Saskatchewan, Saskatoon, SK S7N 5A9, Canada.
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.2016.2531426

magnetization (FCM) was enhanced from 1.5 T for the porous
MgB2 bulk (f ∼ 50%) sintered at Tsin = 900 ◦ C under anambient pressure [4] to 2.5 T for the dense bulk (f ∼ 90%)
fabricated at the same Tsin by a hot isostatic pressing (HIP)
method [6]. The increase in both the cross-section of the supercurrent path and the number density of grain boundaries acting
as the pinning centers enhanced the BT . Fuchs et al. realized
BT of 5.4 T at 12 K for the uniaxially hot pressed MgB2 bulk
using the nanometric ball-milled MgB2 powder [14], demonstrating that the simultaneous grain refining and densification
are quite effective to enhance the BT . The introduction of
pinning centers by chemical doping is an another route toimprove the vortex pinning properties. Mg1−x Tix B2 sample
with x = 0.1 synthesized at the ambient pressure demonstrated
the enhanced Jc of 106 A cm−2 at 20 K in the self-field, which
was about three orders of magnitude larger than that of thex = 0 sample [15]. This originated from the fact that the TiB2
phase with lamellar structure at the boundaries of MgB2 grains
suppressed concurrently the grain growth of MgB2 and thecreation of MgB4 impurity phase at the grain boundaries [16].
We reported the trapped field properties of the highly dense Tidoped MgB2 bulks sintered in the HIP furnace at Tsin = 900 ◦ C[8]. BT of 3.6 T was obtained at 13 K on the surface for theTi5–20%-doped bulks, which was 1.3 times larger than thatof the pristine bulk. The higher BT of 4.6 T was realized at
14 K in the pair of Ti5% and Ti10%-doped bulks due to theelimination of the demagnetization effect. The element maps
obtained by an electron probe micro-analyzer suggested thatthe nanometric particles of unreacted B, Mg-B and TiB2 were
expected to act as the strong pinning centers in high magnetic
fields. As found in several reports, the oxygen also plays animportant role on the Jc -increase [17], [18]. The nanometric
MgO and B- and O-enriched inclusions acted as the pinning
centers [17], [18], and Ti-doping assisted the formation ofsuch nanostructures and offered the further increase in Jc [18].
The volume fraction and the size of the nanometric particles
might depend on the fabricating temperature. In this paper,
we have fabricated Ti-doped MgB2 bulks at Tsin = 700 ◦ C by
HIP method and investigated the trapped field properties andmicrostructure which were compared to those of the Ti-doped
bulks fabricated at Tsin = 900 ◦ C.
II. E XPERIMENTAL P ROCEDURE
A. MgB2 Bulk Preparation
Ti-doped MgB2 bulks were fabricated by an in situ HIP
method, which was detailed elsewhere [8]. The nominal

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6800805

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 26, NO. 3, APRIL 2016

TABLE IS PECIFICATIONS OF THE M EASURED MgB 2 B ULKS

composition of the precursors was Mg : Ti : B = (1 − x) : x : 2(x = 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.5) in molar ratio. The precursor pellet sealed in the evacuated stainless steel container was
sintered at Tsin = 700 ◦ C for 3 h under an argon-gas pressure of
98 MPa and cooled down to the room temperature by furnace
cooling. The MgB2 disk-bulks with 38 mm in diameter and
7 mm in thickness were machined. The critical temperatures,
Tc ’s, were about 38 K for all the bulks, which were determinedat the mid-point of the transition in the magnetization curve,
M (T ). The filling factor of the bulk, f , which was the ratioof the measured mass density to the theoretical one of the
MgB2 -Ti composite, was 87% for the pristine bulk, decreased
monotonically with increasing Ti-content, and finally degradedto 68% for the Ti50% bulk, which is contrast to the larger fover 90% for the bulks sintered at 900 ◦ C. We also estimatedthe volume fraction, Vf , of the MgB2 phase in the bulks. The
specification of the measured MgB2 bulks are shown in Table I.

Fig. 1. Temperature dependence of the trapped field, BT (T ), of the pristineand four Ti-doped MgB2 bulks sintered at Tsin = 700 ◦ C. The BT (T ) curvesof the MgB2 bulks (x = 0 and 0.1) fabricated at Tsin = 900 ◦ C are also shown,which are referred to in Ref. [8]. Inset shows the trapped field at 20 K as afunction of Ti contents for MgB2 bulks with Tsin = 700 ◦ C and 900 ◦ C.

B. Measurements
The experimental setup of the FCM is as follows. The MgB2
bulk was set on the cold-stage of the G-M type cryocooler byinserting a thin indium sheet. A Pt-Co thermometer and 50 W
heater were employed to control the temperature of the coldstage and a Cernox thermometer to monitor the temperatureof the bulk surface. The MgB2 bulk was magnetized by FCM
around 15 K under an external magnetic-field of μ0 H = 5 T
parallel to the thickness direction. After the μ0 H was decreased
to 0 T at a rate of 0.022 T min−1 , the trapped field, BT , wasmeasured at the center of the bulk surface using a cryogenic
Hall sensor (HP-VVP, AREPOC s.r.o.). During FCM, the temperature rise on the bulk surface was as small as 0.1 K for
all the bulks. The temperature and magnetic-field dependencesof magnetization, M (T ), and M (μ0 H), were measured bya SQUID magnetometer (MPMS-XL, Quantum Design, Inc.)
using a small piece cut from the bulk. The critical current
density, Jc , was estimated from the M (μ0 H) loop using theextended Bean model [19], [20]. The microstructure and chemical composition of bulks were analyzed by an electron probe
micro-analyzer with a field emission gun (FE-EPMA) and ascanning electron microscopy (SEM) coupled with electronbackscatter diffraction (EBSD).
III. R ESULTS AND D ISCUSSION
Fig. 1 shows the temperature dependence of the trappedfield, BT (T ), of the Mg1−x Tix B2 bulks (x = 0, 0.1, 0.2,
0.3, 0.5) sintered at Tsin = 700 ◦ C. The BT (T ) curves of theMg1−x Tix B2 bulks with Tsin = 900 ◦ C for x = 0 and 0.1 arealso depicted for comparison, which are referred to in Ref. [8].

Hereafter, the bulks fabricated at Tsin = 700 ◦ C and 900 ◦ C are
denoted using the prefixes “700d-” and “900d-”, respectively.
The BT of 2.7 T was obtained for the 700d-pristine bulk at
the lowest temperature of 14.8 K, was decreased monotonicallywith elevating temperature, and finally reached zero at Tc =38 K. The BT of 2.7 T for the pristine bulk was enhanced to
3.5 T for the Ti10–20%-doping. However, the BT decreases for
the Ti30% and BT for Ti50%-doping is lower than that of 700dpristine bulk. Inset of Fig. 1 shows the Ti-content dependenceof BT at 20 K for both 700d- and 900d-bulks. The BT seems
to be insensitive to the sintering temperature Tsin . The BT is
maximized at the wide optimal Ti-contents of 10–20%, whichis inconsitent with the optimal doping level of 10% in previous
reports [15], [21].
Fig. 2(a) shows the magnetic field dependence of the critical current density, Jc (μ0 H), at 20 K for the 700d-MgB2
bulks. The Jc (μ0 H)’s of the 900d-bulks with x = 0 and 0.1
are also plotted [8]. The moderate decay of Jc (μ0 H) was
observed above 3 T for the 700d-pristine bulk in contrast tothat of the 900d-pristine bulk. Especially, the irreversibility
field, μ0 Hirr , of the 700d-pristine bulk evidently exceeds 5 T,
which is defined as the magnetic field for Jc = 10 A cm−2 .
The Jc (μ0 H) curves of the Ti-doped bulks seem not to depend
strongly on neither the sintering temperature nor Ti-contents,
except for the Ti50%-doping. The Jc versus Ti-contents at
μ0 H = 0 and 3 T, respectively, are plotted for both the 700dand 900d-bulks in Fig. 2(b) and (c). The Jc at 0 T for the
700d-bulks initially increases with Ti-doping, takes a peakat Ti5%-doping, and decreases with increasing Ti-contents.
Ti50%-doping for the 700d-bulk obviously reduces the Jc .

NAITO et al.: PINNING PROPERTIES OF Ti-DOPED MgB2 BULKS SINTERED AT DIFFERENT TEMPERATURES

6800805

Fig. 2. (a) Magnetic field dependence of the critical current density, Jc (μ0 H),
at 20 K of the pristine and four Ti-doped MgB2 bulks sintered at Tsin =700 ◦ C. The Jc (μ0 H) curves of the MgB2 bulks with Tsin = 900 ◦ C for
x = 0 and 0.1 are also shown, which are referred to in Ref. [8]. Ti contents
dependence of Jc at (b) μ0 H = 0 and (c) 3 T for the Ti-doped MgB2 bulkswith Tsin = 700 ◦ C and 900 ◦ C.

The similar behavior was also found for the 900d-bulks, inwhich the Ti10%-doping maximizes Jc . Under 3 T, the Jc isenhanced from 6.4 × 103 A cm−2 for the 700d-pristine bulk
to 1.2 − 1.4 × 104 A cm−2 for the 700d-Ti5–30% bulks. The
somewhat small Jc ’s of the 700d-Ti10–30% bulks compared tothat of 900d-Ti5–20% bulk originates from the reduced volume
fraction of MgB2 for the 700d-bulks. Consequently, the Tidoping effectively enhanced the Jc (μ0 H) in high fields.
Fig. 3(a) shows the powder X-ray diffraction (XRD) patterns
for the 700d-bulks (x = 0, 0.3) and 900d-bulks (x = 0, 0.2)[8]. The MgB2 phase is dominant for all the bulks. A residual
unreacted Mg for both pristine bulks and Ti, Ti2 O3 , TiB2
for the both Ti-doped bulks appear as the impurity phases.
Fig. 3(b) and (c) show the ratios of the peak intensity of the
impurities to the MgB2 (101) plane for the 700d- and 900dbulks, respectively. For the 700d-bulks, the intensity ratio of
the Ti(100) abruptly increases from 0 for both the Ti0% andTi1% to 0.14 for Ti5%, increases moderately with increasing
Ti-contents, and finally reached 0.32 for Ti50%. On the other
hand, the amount of the Mg phase decreases monotonicallywith increasing Ti-contents. The tendency of the increase of Ti
for the 900d-bulks is resemble to that for the 700d-bulks. The
amount of TiB2 phase increases monotonically with Ti-contents
for the 900d-bulks. In contrast to the 900d-Ti-doped bulks, noclear impurity peaks of the TiB2 phase appeared for the 700dTi-doped bulks, indicating that the different types of pinning
centers might be created.

Fig. 3. (a) Powder X-ray diffraction patterns taken at the room temperature
for the Mg1−x Tix B2 bulks sintered at 700 ◦ C (x = 0, 0.3) and 900 ◦ C(x = 0, 0.2). Peak intensity ratios of impurity peaks to the main peak of the
MgB2 (101) for the bulks with (b) Tsin = 700 ◦ C and (c) 900 ◦ C.

Fig. 4. Backscattered electron (compositional contrast) images of the (a) pristine and (b) Ti30%-doped MgB2 bulks sintered at 700 ◦ C. The chemicalcomposition was quantitatively examined at the indicated points (A-G).

Fig. 4 shows the backscattered electron (compositional contrast) images for the 700d-bulks (x = 0, 0.3). For the 700dpristine bulk, many dark spots are found in the dominant brightgray area and some darker (black) parts represent the voids. The
quantitative chemical composition was analyzed at the indicatedpoints (A-G), which was summarized in Table II. The chemicalcomposition at points A and D are the MgBy phases with
y = 10.5 and 6.4, respectively, suggesting that the dark parts
are strongly B-enriched compounds, that is, higher borides. The
creation of higher borides results in the residual unreacted Mg,
as found in the XRD pattern [Fig. 3(a)]. The compositions of6800805

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 26, NO. 3, APRIL 2016

TABLE IIC HEMICAL C OMPOSITION OF THE MgB 2 B ULKS S INTERED AT 700 ◦ C

MgBy with y = 2.1 and 3.3, respectively, was obtained forpoints B and C, therefore the dominant bright gray area isconfirmed to be the MgB2 phase with a small amount of higher
borides. For the 700d-Ti30% bulk, the dominant gray area
(point F) and the dark region (point E) have the compositionof MgBy with y = 3.1 and 9.2, respectively. Therefore, thechemical composition of main and impurity phases for the
Ti-doped bulk is almost the same as the nondoped bulk. Quite
large bright regions appear in the Ti30% bulk are Ti, because of
the composition of Mg : B : Ti = 0 : 0 : 1 at point G. The similar
B-enriched MgBy (y > 2) phases and large Ti precipitates werealso observed for the Ti-doped 900d-bulks [8]. The B-rich areain the MgB2 matrix is probably nanometric size smaller thanthe present resolution of FE-EPMA. Therefore, the present
nanometric B-rich particles are expected to trap effectively thevortices. Several reports suggested that an oxide in the starting
Mg powder due to the high affinity of Mg to O often forms
the Mg-B-O inclusions with nanometer-size as the pinning
centers [17], [18]. Furthermore, Prikhna et al. pointed out that
the Ti-doping reduced the oxygen content in the MgB2 matrix
by absorbing oxygen and concurrently promoted the creationof the nanometric Mg-B-O inclusions, which resulted in thefurther Jc -increase. [18]. In addition, an accumulated B layeron the Ti particles, which was found in 900d-Ti-doped bulks[8], is not confirmed for the 700d-Ti30% bulks, which suggests
no TiB2 phases in the bulk. To clarify precisely the pinningmechanism of the Ti-doped bulk without TiB2 , the role ofoxygen should be examined.
Fig. 5 shows the electron backscattered diffraction (EBSD)
images for the pristine and Ti10% bulks for Tsin = 700 ◦ C
and 900 ◦ C. The mosaic structure with various colors suggeststhat the bulk consists of the randomly oriented grains. The
grain size of MgB2 for the 700d-bulks is smaller than thatfor the 900d-bulks, which comes from the suppression of thegrain growth due to the lower sintering temperature. The Tidoping evidently further reduced the grain size of MgB2 forboth 700d- and 900d-bulks. An unanalyzed area shown by blackcolor indicates amorphous and/or the quite fine grain smaller
than the resolution limit of 50 nm for the EBSD apparatus.
This results in difficulties in indexing EBSD patterns in theseregions. This area increases in the overall region for the 700dbulks, especially for the Ti10% bulk. The wide unanalyzed
areas were found around the Ti particles for the 900d-Ti10%
bulk indicated by the thick arrows, in which the TiB2 phase wasincluded [8]. Both smaller MgB2 grains and the increase of theamorphous phase bring about the fact that the 700d-Ti-doped
bulks with the relatively low f ∼ 80% show the high BT values
comparable to those of the highly dense 900d-Ti-doped bulks(f ≥ 90%). Therefore, the 700d-Ti-doped bulks with f ≥ 90%

Fig. 5. Electron backscattered diffraction images for the (a) pristine and(b) Ti10%-doped MgB2 bulks sintered at Tsin = 700 ◦ C and for the (c) pristineand (d) Ti10%-doped MgB2 bulks at Tsin = 900 ◦ C. The thick arrows in(b) and (d) indicate Ti particles.

should offer at least 10% higher BT with respect to the presentbulks. The origin of the grain refining by Ti-doping in the 700dbulk without the TiB2 phase is believed to differ from the TiB2
mechanism [16].
IV. S UMMARY
We have studied the trapped field properties and the microstructure for the Ti-doped MgB2 bulks sintered at Tsin =700 ◦ C by the in situ HIP method compared to those sintered atTsin = 900 ◦ C. The trapped fields, BT ’s, for the Tsin = 700 ◦ Cbulks with relatively low filling factor of f = 70–90% werecomparable with those for the Tsin = 900 ◦ C bulks with higherf ≥ 90%. The critical current densities, Jc (μ0 H)’s, were also
almost independent of both the sintering temperature and Ticontents (except for Ti50%-doping), which is consistent withthe results of BT properties. The quantitative analysis of thechemical composition suggested that the nonsuperconducting
nanometric particles of higher borides, MgBy (y > 2), existed
in the Tsin = 700 ◦ C bulk. The smaller MgB2 grains and/or
larger amount of the amorphous MgB2 phase were obtained
for the Tsin = 700 ◦ C bulks compared to the Tsin = 900 ◦ Cbulks by both the lower sintering temperature and Ti-doping.
The increase of these pinning centers resulted in the sintering
temperature independent BT . To clarify the mechanism of thegrain refining by Ti-doping for the Tsin = 700 ◦ C bulks without
TiB2 phase, further study is now in progress.
ACKNOWLEDGMENT
We would like to acknowledge Dr. H. Teshima of Nippon
Steel & Sumitomo Metal Corporation for his assistance inthe preparation of the MgB2 bulks using the HIP furnace and
Mr. K. Nonaka of Iwate University for his help in the microstructure analysis using FE-EPMA.

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