Superconductor Science and Technology

Related content

PAPER

Optimization of vortex pinning at grain boundaries
on ex-situ MgB2 bulks synthesized by spark
plasma sintering- Ti-doping effects on magnetic properties ofdense MgB2 bulk superconductors
Tomoyuki Naito, Takafumi Yoshida and
Hiroyuki Fujishiro

To cite this article: Tomoyuki Naito et al 2017 Supercond. Sci. Technol. 30 095007- Potential ability of 3 T-class trapped fieldon MgB2 bulk surface synthesized by
infiltration-capsule method
Tomoyuki Naito, Arata Ogino and Hiroyuki
Fujishiro

View the article online for updates and enhancements.

- Trapped magnetic field and vortex pinning
properties of MgB2 superconducting bulk
fabricated by a capsule method
Tomoyuki Naito, Tomohisa Sasaki and
Hiroyuki Fujishiro

This content was downloaded by tomon from IP address 130.34.226.189 on 17/10/2017 at 12:30

Superconductor Science and Technology
Supercond. Sci. Technol. 30 (2017) 095007 (8pp)

https://doi.org/10.1088/1361-6668/aa6d14

Optimization of vortex pinning at grainboundaries on ex-situ MgB2 bulks
synthesized by spark plasma sintering
Tomoyuki Naito, Yuri Endo and Hiroyuki Fujishiro
Faculty of Science and Engineering, Iwate University, Morioka 020-8551, Japan
E-mail: tnaito@iwate-u.ac.jp
Received 25 February 2017, revised 30 March 2017
Accepted for publication 13 April 2017
Published 9 August 2017
Abstract

Grain boundaries are well known to be the predominant pinning centers in MgB2
superconductors. To study the effects of grain boundaries on the trapped ﬁeld properties of
MgB2 bulk, we prepared MgB2 bulks by a spark plasma sintering method using a ball-milled
starting powder. The trapped ﬁeld was maximized for the bulk made from the ball-milled powder
with crystallite size, τ, of 27 nm; the highest trapped ﬁeld, BT , of 2.3 T achieved at 19.3 K was
1.2 times larger than that of the bulk made from the non ball-milled powder (τ = 50 nm). The
degradation of the trapped ﬁeld for the bulk from ﬁner powder (τ = 6 nm) originated mainly
from the lowered Tc . The critical current density, Jc , and the pinning force density, Fp , were also
maximized for the bulk from τ = 27 nm. The competition between the increase of the numericaldensity of grain boundaries and the degradation of superconductivity determined the vortex
pinning properties for the MgB2 bulks with mechanically reﬁned grains. The scaling analysis forthe pinning force density suggested that the change in the dimension of the dominant pinningsource from 2D (surface) to 0D (point) was induced by grain reﬁning. Although the nanometric
impurity particles such as MgB4, MgO and Mg-B-O were created in the bulk during both ballmilling and spark plasma sintering processes, we considered the point-contact between thereﬁned grains was the predominant point pinning source.
Keywords: trapped ﬁeld, spark plasma sintering, grain reﬁnement, critical current density, pining
force density, MgB2 bulk
(Some ﬁgures may appear in colour only in the online journal)[17, 18], a long coherence length [19] and small electromagnetic anisotropy [20]. Although the highest trapped ﬁeldof 5.4 T at 12 K [9] among MgB2 bulks is about one third of
17.6 T, the homogeneity and stability of MgB2 bulk magnets[5, 10, 12] surpasses those of RE-Ba-Cu-O bulk magnets,
which is preferable for a magnet used for a compact NMR(nuclear magnetic resonance) system [21]. A target magnetic
ﬁeld of 9.4 T, which corresponds to the NMR frequency of
400 MHz, requires the enhanced BT to be more than double
the current level.
Vortices are well known to be pinned predominantly at
the grain boundaries in MgB2, thus a low-temperaturesynthesis [22, 23] suppressing the grain growth and/or ballmilled [24] or commercially available nanometer-size [25, 26]

1. Introduction
A vortex pinning effect realizes a type II superconducting
bulk quasi-permanent magnet [1]. A quite compact and strong
magnetic ﬁeld can be used in open space as a conventional
permanent magnet. The record-high trapped ﬁeld of 17.6 T at
26 K was realized at the center of a couple of single-grain GdBa-Cu-O disk-bulks (24.15 mm in diameter and 15 mm inthickness) reinforced by shrinkage ﬁtting of a stainless steel
ring [2]. In addition to the promising RE-Ba-Cu-O (RE: rareearth element) bulk magnets, a polycrystalline MgB2 bulk
magnet also offers a strong, homogeneous and highly stable
magnetic ﬁeld [3–16], because of the speciﬁc features of
MgB2 such as no weak-linked grains for supercurrent ﬂow
0953-2048/17/095007+08$33.00

1© 2017 IOP Publishing Ltd Printed in the UK

Supercond. Sci. Technol. 30 (2017) 095007

T Naito et al

starting powders are often employed to increase the numericaldensity of grain boundaries. For the MgB2 tapes sheathedwith Fe, which were fabricated by a powder-in-tube method
using ball-milled MgB2 powder via an ex-situ route, the Jc
increased with decreasing grain size up to a certain grain size,
however further ﬁner grains reduced Jc [24]. The Jc increase
was attributed to the increased numerical density of the grainboundaries. On the other hand, the Jc decrease for ﬁner grains
originated from the fact that the impurities diffusing at thegrain surfaces degraded the connectivity between the grains.
The similar Jc and BT characteristics optimized for the grain
size were also found in several reports [11, 27, 28]. In contrast, the monotonic increase of Jc with decreasing grain size
was observed for MgB2 being free from the mechanicalmilling process [26, 29]. Thus, the relationship between the Jc
and the grain size is still controversial.
The densiﬁcation by drawing and a high-pressure
synthesis also enhances the grain boundary pinning due toboth the increase of numerical density of grain boundaries andthe cross-section for supercurrent ﬂow. A spark plasma sintering (SPS) method with a uniaxial pressing is often
employed to obtain a dense MgB2 bulk [30, 31]. The most
attractive advantage of this method is quite a short synthesis
time, typically ∼15 min for MgB2, which is expected tostrongly suppress the grain growth. Actually, almost the same
crystallite size was observed between the raw starting and the
SPS-processed MgB2 powders [32]. After the discovery ofsuperconductivity in MgB2 [33], the BT of 1 T at 27.5 K waspromptly reported for a SPS-processed MgB2 bulk with 30
mm diameter [3]. Although ﬂux jumps prevented the bulk
from trapping higher ﬁelds at lower temperatures, this pioneering work demonstrated the potential of MgB2 bulk
magnets. Recently, 2.05 T was trapped by a SPS-processedbulk (30 mm in diameter and 9.7 mm in thickness) at 20 K
[15], which was comparable with the BT of the dense MgB2
bulks prepared by the conventional high-pressing methods
such as hot isostatic pressing (HIP) and hot pressing (HP). We
have developed the strong in-situ MgB2 bulk magnets trapping 3–5 T class by densiﬁcation using the HIP [8] andinﬁltration [16] methods and the doping of the impurities suchas Ti [12] and SiC [34]. To further realize the higher trapped
ﬁeld for the MgB2 bulk, in this paper, we studied the effects
of grain reﬁning and the SPS process on the trapped ﬁeld
properties of ex-situ SPS-processed MgB2 bulks using various
ball-milled powders.

a constant duration of 12 h in the ball-milling process, in which
the stainless steel (SUS304) balls (10 mm in diameter) andcontainer were used. Each starting powder was poured into a
set of graphite die and punches lined with graphite paper, andpelletized preliminarily under a uniaxial stress of 10 MPa. Thiswas sintered in the SPS apparatus in a vacuum of 20–100 Pa
for 5–15 min at 950 °C–1050 °C under the uniaxial pressure of
50 MPa; the temperature was monitored on the surface of thegraphite die using a radiation thermometer. The ﬁlling factors,
f, of the bulks, which were the ratio of the measured totheoretical mass densities, were typically ∼90%. The constituent phase and structure of the MgB2 samples were evaluated by x-ray diffraction using Cu-Kα radiation (with awavelength of 0.154 nm) operated with an acceleration voltageof 30 kV and electron current of 30 mA.
An MgB2 bulk was magnetized by ﬁeld cooling (FC) in
the following procedure. It was cooled down to a target
temperature in a magnetic ﬁeld of 5 T parallel to the thickness
direction using a 10 T cryogen-free superconducting magnet(JMTD-10T100, Japan Superconductor Technology, Inc.)
using a G-M type cryo-cooler. After the applied magnetic
ﬁeld was ramped down to 0 T at a rate of 0.022-0.22 T min−1,
the trapped ﬁeld was measured by a cryogenic Hall sensor(HP-VVP, AREPOC r.s.o.) mounted on the center of the bulk
surface using GE7031 varnish. The temperature of the bulk
was monitored by a Cernox thermometer which was adhered
beside the Hall sensor on the bulk surface. After the FC
magnetization experiments, the superconducting properties
such as the critical temperature, Tc , and the critical current
density, Jc , were evaluated using a small piece cut from thebulk. Magnetization, M, was measured as a function oftemperature and magnetic ﬁeld using a commercial SQUID
magnetometer (MPMS-XL, Quantum Design Inc.). Jc wasestimated from the magnetic hysteresis using the extended
Bean model [35, 36], Jc = 20DM a (1 - a 3b ), where DM
is the width of the hysteresis, and a and b (a < b ) are thecross-sectional dimensions of the sample perpendicular to theapplied magnetic ﬁeld. Microstructure and chemical composition were analyzed by a electron probe micro analyzer with
a ﬁeld emission electron gun (FE-EPMA; JXA-8100/8200,
JEOL Ltd.) and a scanning electron microscopy (SEM; JSM7001F, JEOL Ltd.) coupled with electron backscatter diffraction (EBSD; NordlysNano, Oxford Instruments plc).

3. Results and discussion
2. Experimental procedure

3.1. Characterization of the SPS-processed MgB2 bulks

Figure 1(a) shows the powder X-ray diffraction patterns of the
as-produced and ball-milled (BM) powders. The as-produced(non-BM) powder was conﬁrmed to be a single phase of
MgB2. On the other hand, the MgB4 impurity phase appeared
for the ball-milled powders in addition to the main phase of
MgB2. The diffraction peaks broadened noticeably owing to
the ball-milling. The crystallite size, τ, of the powder was

MgB2 bulks were fabricated by an ex-situ SPS method(LABOX-110, SINTER LAND Inc., Ltd.) using a commercially available MgB2 powder (99% in purity, Furuuchi Chemical Co.). As-manufactured MgB2 powder was pulverized bya planetary ball-milling device (PM100, Retsch GmbH). We
prepared three kinds of starting powders with different crystallite size, τ, by controlling the rotation speed (0−600 rpm) for2

Supercond. Sci. Technol. 30 (2017) 095007

T Naito et al

Figure 1. (a) Powder x-ray diffraction patterns of the as-produced

and ball-milled powders. (b) The raw intensity of MgB2(101) peak(left axis) and the intensity ratio of the MgB4(121) to MgB2(101)peaks (right axis) as a function of the crystallite size, τ.
Figure 2. (a) X-ray diffraction patterns of three SPS-processed MgB2

bulks. (b) The peak intensity ratios of MgO(200)/MgB2(101) and of
MgB4(121)/MgB2(101) as a function of the crystallite size, τ.

estimated by the following Scherrer’s relation [37],t=Kl,b cos J

(1 )where K is the shape factor, generally ∼0.9, λ the wavelengthof the Cu-Kα radiation, β the full width at half maximum(FWHM) of the diffraction peak, and ϑ the Bragg angle. Theτ value of 50 nm estimated for the non-BM powder was
reduced to 27 nm and 6 nm by ball-milling with rotation
speeds of 250 rpm and 600 rpm, respectively. Figure 1(b)shows the raw intensity of the MgB2(101) peak (left axis) andthe intensity ratio of the MgB4(121) to MgB2(101) peaks(right axis) as a function of τ. The monotonic decrease of the
MgB2(101) peak intensity suggests the degradation of thecrystallinity by the mechanical shock. The increase of therelative amount of MgB4 indicates that a decomposition
reaction, 2MgB2  Mg + MgB4, was promoted by the ball
milling. No detectable XRD peak relating to the residual Mg
indicates the existence of amorphized Mg and/or MgO below
the detection limit of XRD.
Figure 2(a) shows the x-ray diffraction patterns of the
MgB2 bulks made from the starting powders with different τ(50 nm, 27 nm, 6 nm). They were named Bulk#50, Bulk#27,
and Bulk#6, respectively. Both MgB4 and MgO impurity
phases appeared in all the MgB2 bulks. That there was no

MgB4 in the as-produced (non-BM) MgB2 powder suggeststhat MgB4 in the bulks was created via the decomposition
reaction (2MgB2  Mg + MgB4) during the SPS process.
This is consistent with the decomposition temperature of
MgB2, which was reported to be 897 °C–1071 °C [31, 38].
On the other hand, MgO was created from dissolved Mg
vapor which reacted with the oxygen absorbed on the surfaceof grain and/or the residual oxygen in the SPS apparatus. As
shown in ﬁgure 2(b), the peak intensity ratio of MgO(200)/
MgB2(101) slightly increased with decreasing τ, and thatof MgB4(121)/MgB2(101) is almost independent of theτ value. These results mean that almost the same amount of
MgB4 and MgO impurities should be contained in threebulks, despite each starting powder having different amountsof them.
Figure 3 shows the temperature dependence of the normalized magnetization at m0 H = 0.4 mT after zero-ﬁeld
cooling for the MgB2 bulks. All the bulks represented a sharp
superconducting transition. The critical temperatures, Tc ,deﬁned at the mid-point were 37.9 K, 37.4 K, and 35.0 K
for Bulk#50, Bulk#27, and Bulk#6, respectively. The
3

Supercond. Sci. Technol. 30 (2017) 095007

T Naito et al

obviously maximized at τ = 27 nm (for Bulk#27) at T = 20
K, 25 K, and 30 K.
Figure 5(a) shows the magnetic ﬁeld dependence of thecritical current density, Jc (m0 H ), at 10 K and 20 K for the
MgB2 bulks. The Jc (m0 H ) decreased monotonically withincreasing magnetic ﬁeld for all the MgB2 bulks, which iscommon behavior for MgB2. The Jc (m0 H ) for Bulk#27 wasevidently enhanced by the grain reﬁning. In particular, theirreversibility ﬁeld, m0 Hirr , was shifted from 4.2 T for
Bulk#50 to about 5 T for Bulk#27 at 20 K, where m0 Hirr
was deﬁned by a magnetic ﬁeld at Jc = 10 A cm−2. On theother hand, a small enhancement of Jc (m0 H ) was observed for
Bulk#6 with respect to the Jc (m0 H ) for Bulk#50. The Jc at
m0 H = 0 T and 3 T are plotted as a function of the crystallite
size, τ, for various temperatures in ﬁgures 5(b) and (c),respectively. Jc versus τ shows a peak at τ = 27 nm (forBulk#27) for both 0 T and 3 T, similarly to the BT versus τ asshown in the inset of ﬁgure 4.
To understand the obtained BT and Jc characteristics, weconsidered the pinning force density, Fp . Figure 6(a) shows
the magnetic ﬁeld dependence of the pinning force density,
Fp (m0 H ), at 20 K for the MgB2 bulks. The grain reﬁning
evidently enhanced the Fp (m0 H ). Bulk#27 took the maximum, Fpmax , of 1.6 GN m−3 at 1 T, which was 1.7 times
larger than the Fpmax of 0.92 GN m−3 at 0.8 T for Bulk#50.
Somewhat enhanced Fpmax of 1.1 GN m−3 at 1 T was
observed for Bulk#6. The dominant pinning mechanism inthe present bulks was estimated by two scaling functions[39, 40] as follows. As shown in ﬁgure 6(b), the conventional
scaling function [39], fp (=Fp /Fpmax )µh ip (1 - h i )q , reproduced the experimental fp with p = 0.74 and q = 2.7 for
Bulk#50, p = 0.96 and q = 2.8 for Bulk#27 and p = 0.94
and q = 3.4 for Bulk#6. Here h i is the normalized ﬁeld, h i =m0 H /m0 Hirr and the exponents p and q are characteristics forthe dominant pinning mechanism. Theoretical prediction of
p = 0.5 and q = 2 for the normal surface pinning and p = 1
and q = 2 for the normal core point pinning suggests thechange in the dominant pinning mechanism from the normal
surfaces to the normal cores by grain reﬁning, in spite ofsomewhat deviated p and q values. The large q (>2) values
were often found for various materials such as YBa2Cu3O 7 - d[41], Nb3Sn [42], and other MgB2 [29]. Since the theoreticalq = 2 is for a homogeneous isotropic superconductor, Wördenweber took the Gaussian distribution of a speciﬁc criticalﬁeld in channels for the vortex motion as the inhomogeneity
into account for the Fp (m0 H ) calculation, and then obtained
the simulated q values larger than 2 [43]. On the other hand,
Eisterer demonstrated that the anisotropy changed substantially the ﬁeld dependence of Fp and the theoretical Kramer plot [40], which possibly changes the conventional
theoretical parameters of p and q. Therefore, the inhomogeneity of the present MgB2 bulks, as found in ﬁgure 7, andthe anisotropic nature of MgB2 probably offered the large q
value of ∼3. To solve the problems in the conventional
scaling, Eisterer proposed a new Fp -scaling procedure [40] bytaking the anisotropy parameter γ and the percolationthreshold psh into account, where psh is deﬁned as the minimum fraction of the superconducting region for a continuous

Figure 3. Temperature dependence of the normalized magnetization
in a magnetic ﬁeld of 0.4 mT after zero-ﬁeld cooling for three SPSprocessed MgB2 bulks.

Figure 4. Temperature dependence of the trapped ﬁeld by the FCM
under a magnetic ﬁeld of 5 T for three SPS-processed MgB2 bulks.
Inset shows the trapped ﬁeld at T = 20, 25 and 30 K as a function of
the crystallite size, τ.

suppression of Tc probably originated from the degradedcrystallinity of the starting MgB2 powder.
3.2. Magnetic properties and microstructure of the
SPS-processed MgB2 bulks

Figure 4 shows the temperature dependence of the trappedmagnetic ﬁeld, BT (T ), for the MgB2 bulks. The BT increased
with decreasing temperature for all the bulks due to the Jc
increase by cooling. The BT of 1.9 T at 20 K for Bulk#50
was comparable with the BT of 2.05 T obtained for the larger
SPS-processed MgB2 bulk 30 mm in diameter [15]. The
BT (T ) curve of Bulk#27 was pushed up entirely with respect
to that of Bulk#50, and the highest BT was 2.3 T at 19.3 K.
The BT of Bulk#6 was smaller than that of Bulk#50 from 36
K down to 28 K because of low Tc . However, it started toincrease steeply around 30 K, exceeded the BT of Bulk#50,
and then approached the BT of Bulk#27. This indicates thatthe ‘positive’ effect of the increase of pinning centers exceeds
the ‘negative’ effect of the lowering Tc by the ball-milling
below 30 K. As shown in the inset of ﬁgure 4, the BT was
4

Supercond. Sci. Technol. 30 (2017) 095007

T Naito et al

Figure 5. (a) Magnetic-ﬁeld dependence of the critical current density, Jc (m 0 H ), at T = 10 and 20 K for three SPS-processed MgB2 bulks.
(b) and (c), respectively, show the Jc at m0 H = 0 and 3 T for the indicated temperatures as a function of the crystallite size, τ.

(p = 2, q = 1). The two scaling results suggest strongly the
predominant point pinning for the bulks made from the BM
powder.
Let us consider the point pinning source introduced bygrain reﬁning and SPS processes from the microstructural
analysis. Figure 7 shows the backscattered electron (compositional contrast) images for three MgB2 bulks. A dark grayspot in the light gray matrix increased and dispersed ﬁnely inthe order of Bulk#50, Bulk#27, Bulk#6. The typical chemical compositions at the indicated positions were examined
quantitatively by FE-EPMA, which were summarized intable 1. The chemical compositions of the light gray and of
dark gray regions, respectively, were MgBy (y = 2.4–2.7) and
MgBy (y = 2.8–4.2) with a small amount of oxygen. Thissuggests that the ﬁne MgBy (y > 2) and MgO particles dispersed in the MgB2 matrix (light gray region) and that the
MgB4 dominates in the dark gray regions. Bright regions
contained ﬁve to ten times as much oxygen compared to thesupercurrent path. In this scaling, a reduced peak-ﬁeld, h n,peak ,at the f pmax in the fp versus h n plot, as shown in ﬁgure 6(c),determines directly the dominant pinning center. Here, theh n (=m0 H /m0 Hn ) is the normalized ﬁeld and m0 Hn is deﬁned
as the magnetic ﬁeld at which Fp drops to half of Fpmax atmagnetic ﬁelds above the peak magnetic ﬁeld. The h n,peakvalues of 0.35, 0.43 and 0.43 obtained for Bulk#50,
Bulk#27 and Bulk#6, respectively, strongly support that thesurface pinning for the non-BM bulk changes to the point
pinning centers for the grain-reﬁned bulks, by referring to
the theoretically predicted h n,peak values with g = 4 and
psh = 0.25 for the typical pinning sources with the combination of p and q values in the following; (1) h n,peak = 0.34 fornormal surface pinning (p = 1/2, q = 2), (2) h n,peak = 0.47 fornormal point pinning (p = 1, q = 2), (3) h n,peak = 0.56 forsuperconducting volume pinning (p = 1, q = 1), (4) h n,peak =0.62 for superconducting surface pinning (p = 3/2, q = 1)
and (5) h n,peak = 0.65 for superconducting point pinning5Supercond. Sci. Technol. 30 (2017) 095007

T Naito et al

Figure 6. (a) Magnetic-ﬁeld dependence of the pinning force density, Fp (m 0 H ), at T = 20 K for three SPS-processed MgB2 bulks; (b) and (c),respectively, show the reduced pinning force densities, fp , versus the reduced ﬁeld h irr and h n at 20 K.

Figure 7. Backscattered electron (compositional contrast) images for three SPS-processed MgB2 bulks.

demonstrates the randomly oriented MgB2 grains. The grain
size apparently reduced, and concurrently an unanalyzed‘black’ area increased, in the order of Bulk#50, Bulk#27,
Bulk#6. The unanalyzed area probably corresponds to thevoids and grains ﬁner than the resolution limit of 50 nm for
the EBSD device and/or an amorphous phase; both result in
difﬁculties in Kikuchi pattern indexing. Considering both the
XRD patterns indexed mainly by MgB2 and the degradationof Tc , the ball-milling does not offer amorphous MgB2 butinstead offers the nanometric deformed MgB2 crystal for both
Bulk#27 and Bulk#6. The EBSD results conﬁrm thatBulk#6 has the largest numerical density of the grainboundary among the present bulks, however the trapped ﬁeld
is maximized for Bulk#27. This originated from the competition between two effects given by ball-milling, i.e. theTable 1. Typical chemical composition ratio of three SPS-processed
MgB2 bulks. LG, DG and BR mean light gray, dark gray and brightregions, respectively (see text and ﬁgure 7).

Bulk#50

Mg
B
O

Bulk#27

Bulk#6

LG1

DG1

BR1

LG1

DG1

LG3

DG3

BR3

1
2.50
0.06

1
4.23
0.07

1
2.14
0.54

1
2.41
0.10

1
2.82
0.11

1
2.69
0.22

1
3.52
0.07

1
3.41
0.44

amount of both gray regions. These are probably Mg-B-O
compounds as pointed out by previous papers [44–47].
Figure 8 shows the electron backscattered diffraction(EBSD) images for three bulks. The mosaic pattern
6

Supercond. Sci. Technol. 30 (2017) 095007

T Naito et al

Figure 8. Electron backscattered diffraction (EBSD) images for three SPS-processed MgB2 bulks.

increase of the pinning centers and the deterioration of thesuperconductivity of the MgB2 matrix due to the deformationof the MgB2 crystals, which degrades the elemental pinning
force. Since the contact cross-section between the grains
becomes small with decreasing grain size, the nature of thegrain boundaries should be changed from a planar to a pointcontact, that is, the 2D (correlated) to 0D (point-like) pinning
centers by grain reﬁning. This explains the characteristics ofpinning force density shown in ﬁgure 6. As found in ﬁgure 7,
the micrometric impurity-rich regions are obviously reducedby the ball-milling and SPS processes. They are too large tobe the effective pinning centers, considering the vortex-core
size, ~2x , where x ~ 10 nm was reported for MgB2 [48]. The
XRD results shown in ﬁgure 2 suggest that the amount of
impurities, approximately the product of their number andvolume (size), are comparable for three bulks. That there isalmost the same FWHM of MgB4(121) peaks and the fact thatMgO was created from the dissolved Mg vapor and residual
oxygen suggest the comparable size of both impurities forthree bulks. The Mg-B-O might be a nanometer-size Mg(B,O)2 precipitate [45]. Actually, the ﬁne MgBy (y > 2), MgO
and Mg-B-O particles must act as ‘point’ pinning centers[44–47]. Consequently, they seem not to play the principalrole in the ‘dimensional change’ of the pinning mechanism.
The similar grain size dependent BT properties were alreadyreported [11], in which the authors pointed out that theenhancement of BT was due to the increase of the grainboundary density and electron scattering. Contrary to thepresent results, both Jc and Fp increased monotonically with areduction in grain size for MgB2 made from the raw starting
powder without mechanical milling [26, 29]. This suggeststhat the use of such ﬁne starting powder, which probably
contains clean grain boundaries, further improved the trapped
ﬁeld properties of the ex-situ MgB2 bulks.

larger than the BT of 1.9 T for a bulk made from theτ = 50 nm powder. The vortex pinning characteristics
including the critical current density, Jc , and the pinning
force density, Fp , were also maximized for the τ = 27
nm bulk. The optimal grain size was determinedapparently by the balance of the increase of the pinning
centers with the degradation of superconductivity.
(ii) The dominant pinning source changed from the surface(two-dimensional) to the point (zero-dimensional) pinning by the grain reﬁning from the scaling analysis forthe pinning force density. We proposed that the grainboundaries possibly acted as the point-like pinning
centers when a contact cross-section of MgB2 grains
became smaller than a certain threshold value.
(iii) Both ball-milling and SPS processes created the MgB4,
MgO and Mg-B-O impurities as the nanometricparticles. These also acted as the point pinning centers,
but did not play the principal role in the dimensionalchange of the dominant pinning source.

Acknowledgments
We would like to thank Drs K Nonaka and T Murakami of
Iwate University for their help in the microstructure analysis
using FE-EPMA and SEM-EBSD apparatuses. This work waspartly supported by JSPS KAKENHI Grant Numbers
JP15K04718, JP15K04646.

References[1] Gotoh S, Murakami M, Fujimoto H and Koshizuka N 1992
J. Appl. Phys. 72 2404–10[2] Durrell J H et al 2014 Supercond. Sci. Technol. 27 082001[3] Murakami M and Yoo S I 2003 (in Japanese) http://moniko.
s26.xrea.com/shinbunhappyou/MgB2.htm[4] Viznichenko R V, Kordyuk A A, Fuchs G, Nenkov K,
Müller K H, Prikhna T A and Gawalek W 2003 Appl. Phys.
Lett. 83 4360–2[5] Naito T, Sasaki T and Fujishiro H 2012 IEEE Trans. Appl.
Supercond. 22 4401703[6] Naito T, Sasaki T and Fujishiro H 2012 Supercond. Sci.
Technol. 25 095012[7] Durrell J H, Dancer C E J, Dennis A, Shi Y, Xu Z,
Campbell A M, Babu N H, Todd R I, Grovenor C R M and
Cardwell D A 2012 Supercond. Sci. Technol. 25 112002

4. Summary
We have studied the trapped ﬁeld properties of MgB2 bulks
made from the as-produced and ball-milled starting MgB2
powders with different crystallite size, τ, 50, 27 and 6 nm,
using the spark plasma sintering (SPS) method. The obtained
important results are listed below.
(i) A bulk made from the τ = 27 nm powder offered thehighest BT of 2.3 T at 19.3 K, which was 1.2 times
7Supercond. Sci. Technol. 30 (2017) 095007

T Naito et al[27] Dancer C E J, Prabhakaran D, Crossley A, Todd R I and
Grovenor C R M 2010 Supercond. Sci. Technol. 23 065015[28] Vignolo M et al 2008 IEEE Trans. Appl. Supercond. 18 1175–8[29] Martínez E, Mikheenko P, Martínez-López M, Millán A,
Bevan A and Abell J S 2007 Phys. Rev. B 75 134515[30] Lee S Y, Yoo S I, Kim Y W, Hwang N M and Kim D Y 2003
J. Am. Ceram. Soc. 86 1800–2[31] Schmidt J, Schnelle W, Grin Y and Kniep R 2003 Solid State
Sci. 5 535–9[32] Aldica G, Batalu D, Popa S, Ivan I, Nita P, Sakka Y, Vasylkiv O,
Miu L, Pasuk I and Badica P 2012 Physica C 477 43–50[33] Nagamatsu J, Nakagawa N, Muranaka T, Zenitani Y andAkimitsu J 2001 Nature 410 63–4
[34] Naito T et al private communication[35] Bean C P 1962 Phys. Rev. Lett. 8 250–3[36] Gyorgy E M, van Dover R B, Jackson K A,
Schneemeyer L F and Waszczak J V 1989 Appl. Phys. Lett.
55 283–5[37] Cullity B 1956 Elements of X-ray Diffraction (Reading, MA:
Addison-Wesley)[38] Guo Y, Zhang W, Yang D and Yao R L 2012 J. Am. Ceram.
Soc. 95 754–9[39] Dew-Hughes D 1974 Phil. Mag. 30 293–305[40] Eisterer M 2008 Phys. Rev. B 77 144524[41] Li J, Boer F D, Roeland L, Menken M, Kadowaki K,
Menovsky A and Franse J 1990 Physica C 169 81–6[42] Cheggour N and Hampshire D P 2002 Cryogenics 42 299–309[43] Wördenweber R 1992 Phys. Rev. B 46 3076–83[44] Zhu Y, Wu L, Volkov V, Li Q, Gu G, Moodenbaugh A,
Malac M, Suenaga M and Tranquada J 2001 Physica C 356
239–53[45] Liao X Z, Serquis A, Zhu Y T, Huang J Y, Civale L,
Peterson D E, Mueller F M and Xu H F 2003 J. Appl. Phys.
93 6208–15[46] Prikhna T et al 2015 J. Supercond. Nov. Magn. 28 525–30[47] Naito T, Yoshida T, Mochizuki H, Fujishiro H, Basu R and
Szpunar J A 2016 IEEE Trans. Appl. Supercond. 26
6800805[48] Buzea C and Yamashita T 2001 Supercond. Sci. Technol.
14 R115[8] Sasaki T, Naito T and Fujishiro H 2013 Phys. Procedia 45
93–6[9] Fuchs G, Häßler W, Nenkov K, Scheiter J, Perner O,
Handstein A, Kanai T, Schultz L and Holzapfel B 2013
Supercond. Sci. Technol. 26 122002[10] Yamamoto A, Ishihara A, Tomita M and Kishio K 2014 Appl.
Phys. Lett. 105 032601[11] Sugino S, Yamamoto A, Shimoyama J and Kishio K 2015
Supercond. Sci. Technol. 28 055016[12] Naito T, Yoshida T and Fujishiro H 2015 Supercond. Sci.
Technol. 28 095009[13] Mochizuki H, Fujishiro H, Naito T, Albisetti A F and
Giunchi G 2015 Supercond. Sci. Technol. 28 105004[14] Bhagurkar A G, Yamamoto A, Anguilano L, Dennis A R,
Durrell J H, Babu N H and Cardwell D A 2016 Supercond.
Sci. Technol. 29 035008[15] Berger K, Koblischka M R, Douine B, Noudem J, Bernstein P,
Hauet T and Lévêque J 2016 IEEE Trans. Appl. Supercond.
26 6801005[16] Naito T, Ogino A and Fujishiro H 2016 Supercond. Sci.
Technol. 29 115003[17] Kambara M, Babu N H, Sadki E S, Cooper J R, Minami H,
Cardwell D A, Campbell A M and Inoue I H 2001
Supercond. Sci. Technol. 14 L5–7
[18] Larbalestier D C et al 2001 Nature 410 186–9[19] Finnemore D K, Ostenson J E, Bud’ko S L, Lapertot G and
Canﬁeld P C 2001 Phys. Rev. Lett. 86 2420–2[20] Bud’ko S L and Canﬁeld P C 2002 Phys. Rev. B 65 212501[21] Nakamura T, Tamada D, Yanagi Y, Itoh Y, Nemoto T,
Utumi H and Kose K 2015 J. Magn. Reson. 259 68–75[22] Kumakura H, Kitaguchi H, Matsumoto A and Hatakeyama H
2004 Appl. Phys. Lett. 84 3669–71[23] Yamamoto A, Shimoyama J, Ueda S, Katsura Y, Horii S andKishio K 2005 Supercond. Sci. Technol. 18 116[24] Flükiger R, Suo H, Musolino N, Beneduce C, Toulemonde P and
Lezza P 2003 Physica C 385 286–305[25] Chen S K, Yates K A, Blamire M G and MacManus-Driscoll J L
2005 Supercond. Sci. Technol. 18 1473
[26] Katsura Y, Yamamoto A, Ueda S, Iwayama I, Horii S,
Shimoyama J and Kishio K 2006 J. Phys.: Conf. Ser. 43 119

8