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Vortex pinning properties and microstructure of
MgB2 heavily doped with titanium group elements- Optimization of vortex pinning at grainboundaries on ex-situ MgB2 bulks
synthesized by spark plasma sintering
Tomoyuki Naito, Yuri Endo and Hiroyuki
Fujishiro

To cite this article: Yuhei Takahashi et al 2017 Supercond. Sci. Technol. 30 125006- Ti-doping effects on magnetic properties ofdense MgB2 bulk superconductors
Tomoyuki Naito, Takafumi Yoshida and
Hiroyuki Fujishiro

View the article online for updates and enhancements.

- Potential ability of 3 T-class trapped fieldon MgB2 bulk surface synthesized by
infiltration-capsule method
Tomoyuki Naito, Arata Ogino and Hiroyuki
FujishiroThis content was downloaded by tomon from IP address 130.34.226.195 on 09/11/2017 at 00:27

Superconductor Science and Technology
Supercond. Sci. Technol. 30 (2017) 125006 (7pp)

https://doi.org/10.1088/1361-6668/aa91ed

Vortex pinning properties andmicrostructure of MgB2 heavily doped withtitanium group elements
Yuhei Takahashi, Tomoyuki Naito

and Hiroyuki Fujishiro

Faculty of Science and Engineering, Iwate University, Morioka 020-8551, Japan
E-mail: tnaito@iwate-u.ac.jp
Received 9 August 2017, revised 27 September 2017
Accepted for publication 9 October 2017
Published 1 November 2017
Abstract

The doping of titanium group elements (TGE; Ti, Zr, Hf) to MgB2 is known to improve vortex
pinning properties. However, the optimal doping level of each TGE is still a controversial issue.
In this study, to improve the critical current density, Jc , in high magnetic-ﬁeld by the TGEdoping, we have studied the vortex pinning properties of (Mg1 -x TGEx)B2 samples with x
ranging in the interval 0 < x < 0.5. The Jc of 1.2kAcm−2 at 20K in 3 T for the pristine sample
was maximized to 2.9kAcm−2 for the Ti20%-doped sample, 4.9kAcm−2 for the Zr30%doped one, and 5.2 kA cm−2 for the Hf30%-doped one under identical temperature and magnetic
ﬁeld. The irreversibility ﬁeld, m0 Hirr , of 3.8 T for the pristine sample was also shifted to a higher
ﬁeld of 4.2T, 4.3T and 4.6T by the Ti20%-, Zr30%- and Hf30%-doping, respectively.
Microstractural observations suggested that the grain size, shape and distribution of the MgB2
and impurity phases depended strongly on the species of TGE. We discuss the mechanism of theenhanced vortex pinning properties of the TGE-doped MgB2.
Keywords: MgB2, critical current density, titanium group elements, doping effects(Some ﬁgures may appear in colour only in the online journal)
Jc -properties of MgB2 under high magnetic ﬁelds [8, 15–19],
in which the optimal TGE-doping level of 1%–20% depended
on the species of TGE. The pioneering work, which examined
the Ti-doping effects, showed that the three orders magnitudelarger Jc was achieved for the Ti10%-doped MgB2 sample
compared to that of the non-doped one [15]. Furthermore, theZr-doping also offered the enhanced Jc characteristics [17].
The optimal Ti- and Zr-doping level of 10% was found in theliteratures [15, 17, 19]. Goto et al claimed that the Zr- and Hfdoping were more effective to enhance the Jc -properties than
the Ti-doping and obtained the maximized Jc at the doping
level of 1%–2% [18]. However, the doping level dependenceof the Jc , Jc (x), suggested that the higher Jc might be obtained
for the higher x than 10%. We also found that both Jc and thetrapped ﬁeld, BT , of Ti-doped MgB2 bulks were enhanced up
to the Ti-doping level of 20% [8, 20]. In the Ti-doped MgB2,
a non-superconductive TiB2 thin layer was created at theboundary of MgB2 grains, and then suppressed the grain
growth of MgB2 without the disturbance of supercurrent

1. Introduction
The MgB2 superconductor with the critical temperature, Tc , of
39 K[1] can be operated at liquid hydrogen temperature. No
weak coupling between the MgB2 grains [2, 3] enables anuntextured sample with high critical current density, Jc , over
105 A cm−2 at 20 K in the self-ﬁeld [4]. However, the Jc valueof MgB2 is known to strongly deteriorate in magnetic ﬁelds[4], which must be improved for practical superconducting
wire and bulk applications such as a magnet [5–8], motor [9]and generator [10]. Chemical doping or substitution is one of
the solutions to improve the vortex pinning properties of
MgB2. Substituting carbon (C), derived from various carbon
sources including SiC[11], B4C[12], the fullerene (C60)[13], carbohydrates [14] and so on, for the B-site enhances theupper critical ﬁeld, m0 Hc2 , by shortening the coherence
length, resulting in the highly enhanced Jc in higher magneticﬁelds. In addition to the carbon, the doping of titanium groupelements (TGE; Ti, Zr, Hf) also signiﬁcantly improved the0953-2048/17/125006+07$33.00

1© 2017 IOP Publishing Ltd Printed in the UK

Supercond. Sci. Technol. 30 (2017) 125006

Y Takahashi et al

ﬂowing [16]. On the other hand, the microscopic analysis for
the Zr- and Hf-doped MgB2 to clarify the pinning mechanism
has not been carried out yet. Consequently, the optimal
doping level of each TGE and the pinning mechanism of theZr- and Hf-doped MgB2 are still controversial issues to date.
In this paper, we studied the vortex pinning properties of the
MgB2 samples heavily doped with TGE (Ti, Zr, Hf) up to
50% and discussed the pinning mechanism for the heavily
doped region.

2. Experimental details
2.1. Fabrication of MgB2 samples(Mg1 -x TGEx)B2 sample was fabricated by a conventional insitu powder-in-closed-tube method. Raw powders with 99%
in purity of Mg (180 μm in grain size, Kojundo Chemical
Laboratory Co., Ltd), amorphous B (46 μm in grain size,
Furuuchi Chemical Co.), Ti (45 μm in grain size, Furuuchi
Chemical Co.), Zr (61 μm in grain size, Furuuchi ChemicalCo.) and Hf (50 μm in grain size, Furuuchi Chemical Co.)
were weighed with Mg:TGE:B=(1.1−x):x:2.0 in molar ratio
and mixed by agate motor and pestle for 1h. Well-mixed
powder was poured into a closed-end stainless steel (SUS)
tube with 6 and 5mm in outer and inner diameters, respectively, pressed into the tube using a SUS rod, and the otherend of SUS tube was closed by pressing. Subsequently, themain body was pressed into a plate, and ﬁnally both ends
were bent at 180. The precursor in the closed tube was sintered at 800°C for 1h in 0.1MPa ﬂowing Ar in a tube
furnace. The relative density was estimated to be about 50%of the ideal density for all the samples.
Figure 1. (a) Powder x-ray diffraction patterns for the pristine and

TGE30%-doped MgB2 samples. (b) The peak area of MgB2(101)peak as a function of the TGE-doping content, x. The lines are
guides to the eye.

2.2. Measurements

The constituent phase and structure of (Mg1 -x TGEx)B2
sample were evaluated by the x-ray diffraction using the CuKα radiation (the wave length of 0.154 nm) operated with theacceleration voltage of 30kV and electron current of 30mA.
Temperature and magnetic-ﬁeld dependences of magnetization, M(T) and M (m0 H ), were measured by a commercial
SQUID magnetometer (MPMS-XL, Quantum Design Inc.).
Critical current density, Jc , was estimated from the magnetic
hysteresis loop, M (m0 H ), using the extended Bean’s criticalstate model [21, 22]. Microstractural observation and analysisof chemical composition were performed by a scanningelectron microscopy with a ﬁeld emission electron gun (FESEM; JSM-7001F, JEOL Ltd.), coupled with energy dispersive x-ray spectroscopy and electron backscatter diffraction (EBSD; NordlysNano, Oxford Instruments plc).

Mg. For the Ti30%-doped sample, the main phase was MgB2
with a small amount of residual Ti and created TiB2. For the
Zr30%- and Hf30%-doped samples, the MgB2 phase was
conﬁrmed, but quite a large amount of impurities appeared.
The Zr30%-doped sample contained both Zr and ZrB2
impurities. Unreacted Hf metal appeared as the impurity for
the Hf30%-doped sample. In addition, no peak shift of the
MgB2 phase by the TGE-doping was observed, suggestingthat the Mg-site substitution by the TGE did not occur.
Figure 1(b) shows the peak area of MgB2(101) versus the
TGE-doping content, x, for the (Mg1 -x TGEx)B2 samples. The
peak area of the Ti-doped samples was almost independent ofx for x  0.1, and decreased monotonically with increasing x.
On the other hand, the steep decrease in the peak area with x
was observed for both Zr- and Hf-doped samples, and was
almost the same among three TGE-doped samples abovex=0.3. The reduction rate was obviously larger than that
expected from the volume fraction of MgB2, assuming thesame amounts and crystallinity of the MgB2 phase for all thesamples. In other word, a part of the MgB2 phase was

3. Results and discussion
Figure 1(a) shows the typical XRD patterns for the pristine
and TGE30%-doped MgB2 samples. The main phase of thepristine sample was MgB2 with a small amount of residual2Supercond. Sci. Technol. 30 (2017) 125006

Y Takahashi et al

Figure 2. Critical temperature deﬁned at the midpoint, Tcmid , as afunction of the TGE-doping content, x. Inset shows the temperaturedependence of the normalized magnetization, m(T), at
m0 H =0.4 mT after zero-ﬁeld cooling for the Hf-doped MgB2
samples for various x values.

distorted by the TGE-doping, which might offer the weaksuperconductivity regions acting as pinning centers [23].
Inset of ﬁgure 2 shows the temperature dependence of thenormalized magnetization, m (T )(=M (T ) M (10 K)), in amagnetic-ﬁeld of m0 H =0.4mT after zero-ﬁeld cooling for
the Hf-doped MgB2 samples with x=0, 0.1, 0.3 and 0.5. The
Ti- and Zr-doped samples also showed the sharp superconducting transition around 37–38K (not shown here). The
transition temperature, Tcmid , deﬁned at the midpoint of thetransition, was plotted as a function of x, in the main panel ofﬁgure 2. Although the Tcmid was almost insensitive to the
TGE-doping level, some TGE-doped samples showedsomewhat lower Tcmid than the pristine sample. The
Tc -reduction possibly originated from the introduction of lattice distortion and/or the Mg-site substitution by the TGEs.
The former scenario of the introduction of distortion in the
MgB2 crystal is preferable, because the deformed crystal
generally degrades the superconductivity [24]. The latter
scenario of the site substitution can be discarded here, because
no XRD peak shift of the MgB2 phase by the TGE-doping
was observed, as described in the preceding paragraph. Furthermore, the similar x-independent Tcmid (x) behavior was also
reported previously for the TGE-doped MgB2 bulk preparedby a HIP method [18, 20]; the authors also claimed no solid
solution between Mg and Ti.
Figure 3(a) shows the magnetic-ﬁeld dependence of thecritical current density, Jc (m0 H ), at 20K for the pristine and
TGE30%-doped samples. For the pristine sample, the Jc ofabout 0.9 ´ 10 5 A cm−2 in the self-ﬁeld decreased steeply
with increasing magnetic-ﬁeld and reached to 100Acm−2 at
m0 H =3.8T, which was deﬁned as the irreversibility ﬁeld,

Figure 3. (a) Magnetic-ﬁeld dependence of the critical current

density, Jc (m0 H ), at 20K for the pristine and TGE30%-doped
samples. Inset shows the magnetic-ﬁeld dependent Jc normalized by
the self-ﬁeld Jc , Jc0 . (b) The Jc s in m0 H =3T at 20K as a functionof the TGE-doping content, x, for the TGE-doped MgB2 samples.
The lines are guides to the eye.

m0 Hirr . The Jc (m0 H ) was enhanced clearly by the TGE-doping
and the irreversibility ﬁeld, m0 Hirr of 3.8T for the pristine
sample was also shifted to higher ﬁeld of 4.2T, 4.3T and
4.6T by Ti30%-, Zr30%- and Hf30%-doping, respectively.
Inset of ﬁgure 3(a) shows the magnetic-ﬁeld dependent normalized Jc , Jc Jc0 , where Jc0 is the self-ﬁeld Jc . The Jc Jc0 of
the TGE-doped samples started to deviate evidently from thatof the non-doped one above around 2T and showed higher
Jc Jc0 and m0 Hirr . This indicates that the TGE-doping introduced the pinning centers acting effectively at high ﬁelds,
especially for the Hf-doping. Figure 3(b) shows the Jc s at
20K in m0 H = 3 T as a function of TGE-doping content, x.
The Jc of 1.2 ´ 10 3 Acm−2 for the pristine sample was3Supercond. Sci. Technol. 30 (2017) 125006

Y Takahashi et alevidently maximized to 2.9 ´ 10 3 Acm−2 for the Ti20%doped sample, 4.9 ´ 10 3 Acm−2 for the Zr30%-doped one,and 5.2 ´ 10 3 Acm−2 for the Hf30%-doped one. The optimal doping level of 30% for both Zr and Hf is quite higher
than the reported values below 10% [17, 18]. The Jc -peakvalues of both Zr- and Hf-doped samples were nearly comparable, but the x dependence of Jc for the Hf-doped samples
was moderate. Furthermore, the Jc for the Zr50%- andHf50%-doped sample was still higher than the Jc of thepristine sample. The Jc -peak comes from the competition
among various factors of the number density of pinning
centers, the connectivity [25], the MgB2 volume fraction, thecrystallinity of MgB2 and so on, because the literatures
revealed that the TGE-doping offered the ﬁne and well-connected MgB2 grains [15, 16, 20, 26]. We suggest that the Hfdoping more effectively suppresses the grain growth and aids
the sintering than the Ti- and Zr-doping.
Figure 4 shows the backscattered electron (BSE; compositional contrast) images for the Ti30%-, Zr30%- andHf30%-doped MgB2 samples. Bright and dark gray regions
existed in the light gray matrix. The quantitative EDS analysis
revealed that the bright, dark gray and light gray regions wereunreacted TGE, MgB4 and MgB2, respectively. The peripheryof some bright region containing a streak, which was indicated by black arrows, was determined to be (TGE)B2 for the
Ti30%- and Zr30%-doped samples. HfB2 region was not
conﬁrmed around the Hf precipitates for the Hf30%-doped
sample, which is consistent with no HfB2 peak in the XRD
pattern shown in ﬁgure 1(a). The size of Zr-based particles
was rather smaller than that of Ti- and Hf-based ones,
regardless of the grain size of as-manufactured powders.
Figure 5 shows the EBSD and the secondary electron(SE) images for the pristine and TGE30%-doped MgB2
samples. The incident electron beam inclined by 20 from thesample surface was focused on the matrix. The EBSD analysis was applied for the MgB2, (TGE)B2 and TGE crystals,
which were represented by red, blue and green colors,
respectively. The voids, the residual Mg (not analyzed here)
and the ﬁne crystal smaller than the resolution limit of present
EBSD device of 50nm possibly existed in the unanalyzed
black area. The volume fraction of examined compounds andelements, which were estimated from the EBSD results, aresummarized in table 1. Only the red MgB2 and unanalyzed
black areas were observed in the pristine sample shown inﬁgure 5(a-1). The residual Mg, which was conﬁrmed by the
XRD, should exist in the unanalyzed black area, as reported
previously [27]. In addition to both MgB2 and unanalyzed
areas, some impurities found in the XRD patterns weredetected for the Zr30%- and Hf30%-doped samples. Both Zr
and ZrB2 particles were clearly observed for the Zr30%doped sample (ﬁgure 5(c-1)). For the Hf30%-doped sample, a
few nanometric Hf particles was observed but HfB2 did notexist in the EBSD image (ﬁgure 5(d-1)). On the other hand,for the Ti30%-doped sample, both Ti and TiB2 were notfound in the EBSD image (ﬁgure 5(b-1)), although the existence of both phases was conﬁrmed by the XRD patterns(ﬁgure 1(a)) and the BSE image accompanied with the EDS
analysis (ﬁgure 4(a)). Since the incident electron beam was

Figure 4. Backscattered electron (compositional contrast) images forthe (a) Ti30%-, (b) Zr30%- and (c) Hf30%-doped MgB2 samples.
Yellow and black arrows indicate a dark gray area and a streak in abright area, respectively (see text).

4Supercond. Sci. Technol. 30 (2017) 125006

Y Takahashi et al

Figure 5. EBSD and SE images for the (a) pristine and (b) Ti30%-, (c) Zr30%- and (d) Hf30%-doped MgB2 samples. For the EBSD images,
red is MgB2, blue is (TGE)B2, green is TGE and black is unanalyzed areas (see text). An IPF plot for the Hf30%-doped sample is shown.

found in the EBSD images (ﬁgures 5(a-1, b-1, c-1, d-1)), thesize of observable MgB2 grains decreased and the unanalyzed
area increased by the TGE-doping. Somewhat large MgB2
grains are conﬁrmed to be assembly of nanometric MgB2
grains having different crystal orientation by an inverse poleﬁgure plot (IPF), as shown typically for the Hf30%-doped
sample. The SE images (ﬁgures 5(a-2, b-2, c-2, d-2))
demonstrate that the volume fraction of voids was rather
smaller than that of unanalyzed area, thus the unanalyzed
black area originates from not only voids and residual Mg but
also ﬁne grains smaller than 50nm in diameter and/or distorted grains in the matrix. The grain reﬁning and the introduction of distortion are usually conﬁrmed by the broadeningof a XRD peak. The full width at half maximum of the
MgB2(101) peak increased from 0.26 for the pristine sample
to 0.29, 0.38 and 0.35 for the Ti30%-, Zr30%- and Hf30%doped samples, respectively. Therefore, the reﬁned and/or
distorted MgB2 grains probably exist in the unanalyzed area,
and then predominantly enhanced Jc -properties. The distorted
grains should offer the weak-superconductivity regions acting
as the ﬁeld-induced pinning centers, which is consistent with
the improved high-ﬁeld Jc , as shown in ﬁgure 3(a). The
volume fraction of MgB2 and unanalyzed area for the Hf30%-

Table 1. Volume fraction of compounds and elements in the pristine

and TGE30%-doped MgB2 bulks.
Composition of analyzed compounds and elements (%)
SampleMgB2(TGE)B2

TGE

Unanalyzed

Pristine
Ti30%doped
Zr30%doped
Hf30%doped

44
25

–
4

–
4

56
67

24

14

5

57

15

4

4

77

focused on the matrix avoiding the visible impurities, thelarge Ti precipitates with TiB2, as found in ﬁgure 4(a), were
not naturally detected by the EBSD. The EBSD image for the
Ti30%-doped sample suggests strongly the absence of any
small nanometric Ti particles in the matrix. TiB2 possibly
existed as a lamellar between the MgB2 grains [16], the sizeof which was naturally smaller than the present EBSD resolution limit of about 50nm. We note that the ZrB2 thin layers[17] also probably exist in addition to the ZrB2 grains. As
5Supercond. Sci. Technol. 30 (2017) 125006

Y Takahashi et al

doping are rather smaller and larger than those for the Ti30%and Zr30%-doping. This suggests the larger amount of ﬁne
MgB2 grains in the Hf30%-doped sample, whch is one of thepossibilities to the dramatically improved Jc -characteristics inthe Hf-doped MgB2 for the wide range of x. Although the
sub-micron size of impurity particles such as Zr, ZrB2 and Hf
is rather larger than 2x ~ 10 nm [28] (ξ is the coherence
length), they can act as the pinning centers and contributed
slightly the enhancement of Jc -properties.
Zhaoet al pointed out that the lamellar TiB2 region
created at the surface of MgB2 grain suppressed the grain
growth of MgB2 and realized the nanometric MgB2 grains[16]. The Hf-doping also offered the ﬁne MgB2 grains
regardless of the absence of HfB2. The result resembles to our
results reported previously [20], in which the Ti-doped MgB2
bulk without TiB2 layer offered the nanometric MgB2 grains.
It is noteworthy that the low sintering temperature of 700°C
for the previous Ti-doped MgB2 might not allow the creationof TiB2 phase. Our present and previous results suggest thatthe existence of (TGE)B2 phase does not necessarily obtainthe ﬁne MgB2 grains. However, the mechanism of grain
reﬁning without (TGE)B2 remains unsolved problem.

supported by JSPS KAKENHI Grant Numbers JP15K04718,
JP15K04646.

ORCID iDs
Tomoyuki Naito

https://orcid.org/0000-0001-7594-3466

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4. Summary
We have studied the doping effects of titanium group elements (TGE=Ti, Zr, Hf) on the vortex pinning properties
for the (Mg1 -x TGEx)B2 prepared by a powder-in-closed-tube
method. The critical current density, Jc , of 1.2kAcm−2 at
20K in 3 T for the pristine sample was evidently maximized
to 2.9kAcm−2 for the Ti20%-doped sample, 4.9kAcm−2
for the Zr30%-doped one, and 5.2kAcm−2 for the Hf30%doped one. The irreversibility ﬁeld, m0 Hirr , of 3.8T for thepristine sample was also shifted to higher ﬁeld of 4.2T, 4.3T
and 4.6T by Ti20%-, Zr30%- and Hf30%-doping, respectively. The optimal doping levels were x=0.2 for the Tidoping and 0.3 for the Zr- and Hf-doping. In addition, the Hfdoping enhanced the Jc value for a wider Hf-content compared to the Zr-doping. These results suggest that the Hfdoping is the most effective to improve the vortex pinning
properties of MgB2 among TGEs. Microstractural analysis
revealed that the TGE-doping offered the ﬁne MgB2 grains.
In particular, the Hf-doping promoted the creation of nanometric MgB2, which resulted in the highly improved Jc and
m0 Hirr . Finally, the Jc of the Zr50%- and Hf50%-doped samles
was still higher than that of the pristine one, which suggested
that the reﬁning MgB2 grains were preferable to enhance the
Jc-properties, compared to the volume fraction of MgB2.

Acknowledgments
The authors would like to thank Drs TMurakami andKNonaka and Mr KSasaki of Iwate University for theirassistance in microstractural analysis using FE-SEM-EDS/
EBSD and FE-EPMA apparatuses. This work was partly
6Supercond. Sci. Technol. 30 (2017) 125006

Y Takahashi et al[27] Ogino A, Naito T and Fujishiro H 2017 IEEE Trans. Appl.
Supercond. 27 6800905[28] Buzea C and Yamashita T 2001 Supercond. Sci. Technol.
14 R1157