Enhancement of critical current density by a “MgB2-MgB4” reversible reaction in self-sintered ex-situ MgB2bulks

Article abstract of DOI:10.1016/j.jallcom.2016.09.312

Self-sintered ex-situ MgB₂polycrystalline bulks have experienced a two-step sintering process, initially at 900?°C for 0–20?min and then at 650?°C for 1?h. MgB₂was decomposed to MgB₄and Mg at 900?°C and composed again at 650?°C from MgB₄and Mg. The reversible reaction promotes the material migration, and thus eliminates pores and enhances the connectivity between the grains. The critical current density (J_c) is significantly improved due to both improved grain connectivity and the additive pinning centers such as MgB₄and new-born boundaries. This two-step sintering process can be a promising method to fabricate high-performance ex-situ MgB₂bulks and wires.

Full text of DOI:10.1016/j.jallcom.2016.09.312

Journal of Alloys and Compounds 694 (2017) 24e29
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Enhancement of critical current density by a “MgB2-MgB4” reversible
reaction in self-sintered ex-situ MgB2 bulks
,
Junming Peng a, Qi Cai a, Fang Cheng a, Zongqing Ma a *, Chong Li a, Ying Xin b,
,
**
Yongchang Liu a
a State Key Lab of Hydraulic Engineering Simulation and Safety, School of Materials Science & Engineering, Tianjin University, Tianjin 300072, People's
Republic of China
b Tianjin University, Futong Group Research Center of Applied Superconductivity, Tianjin University, Tianjin 300072, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Self-sintered ex-situ MgB2 polycrystalline bulks have experienced a two-step sintering process, initially at
900 ꢀC for 0e20 min and then at 650 ꢀC for 1 h. MgB2 was decomposed to MgB4 and Mg at 900 ꢀC and
composed again at 650 ꢀC from MgB4 and Mg. The reversible reaction promotes the material migration,
and thus eliminates pores and enhances the connectivity between the grains. The critical current density
(Jc) is significantly improved due to both improved grain connectivity and the additive pinning centers
such as MgB4 and new-born boundaries. This two-step sintering process can be a promising method to
fabricate high-performance ex-situ MgB2 bulks and wires.
Received 5 August 2016
Received in revised form
10 September 2016
Accepted 28 September 2016
Available online 29 September 2016
Keywords:
© 2016 Elsevier B.V. All rights reserved.
MgB2 superconductoro
Reversible reactiono
MgB4o
1. Introduction
properties in polycrystalline MgB2 samples are considerably sup-
pressed due to poor connectivity, reducing the effective current
MgB2 is the intermetallic compound with a relatively high
superconducting transition temperature of 39 K [1] and can be the
most promising candidates for application in engineering without
liquid helium [2], which also possesses the advantages such as
‘weak-link free’ grain coupling and low material cost. A great deal of
work has been done to improve the superconducting properties,
such as critical current density (Jc), the upper critical field (Hc2) and
the irreversibility field (Hirr), of MgB2. The introduction of addi-
tional flux pinning centers and connectivity enhancement of the
grains are both necessary to increase the Hirr and the Jc to the levels
required for commercial applications. As Jc is affected by the upper
critical field, improvement in its Hc2 is essential for practical ap-
plications of MgB2. Hc2 can be improved by the introduction of
disorder through various processes such as chemical doping [3e7],
irradiation [8] and various thermochemical processing techniques
[9e11]. However, the relatively weak flux pinning in pure MgB2 is
the main factor of limitation in Jc especially under high fields [12],
and recent studies have revealed that the current-transport
carrying cross-sectional area [13].
Two common methods are usually used for fabricating MgB2
bulks and wires. The first is by using an in-situ reaction that starts
with the heat treatment of mixed Mg and B powders. It is easy to
add impurities to improve flux pinning in the in-situ method
[14e16]. Since the chemical reaction between Mg and B forming
MgB2 results in about 30% volume reduction and porosity is known
to be unavoidable during the in-situ process [17]. This inevitably
results in a reduction of the engineering Jc. The second approach for
fabricating MgB2 bulks and wires is the ex-situ method. The ex-situ
method that employs prereacted MgB2 powder is favorable in
terms of bulk density compared to the in-situ method [18]. A higher
bulk density of 75% (the close-packing of rigid spheres) can be
expected for the ex-situ method. However, grain connectivity in ex-
situ MgB2 bulks and wires was found to be worse than that of the
in-situ fabricated MgB2 samples [19]. It is attributed to the weak
intergranular coupling and presence of impurities between the
MgB2 grains even after sintering at a high temperature of approx-
imately 1100 ꢀC [20]. Therefore, improving the intergranular
coupling for ex-situ MgB2 makes great sense in terms of fabrication
of high-performance MgB2 superconducting bulks and wires.
Previously, Tanaka et al. reported that the intergranular coupling
strength of ex-situ MgB2 could be significantly improved under
* Corresponding author.
** Corresponding author.
E-mail addresses: mzq0320@163.com (Z. Ma), licmtju@163.com (Y. Liu).
http://dx.doi.org/10.1016/j.jallcom.2016.09.312
0925-8388/© 2016 Elsevier B.V. All rights reserved.

J. Peng et al. / Journal of Alloys and Compounds 694 (2017) 24e29
25
ambient pressure by heating at high temperature (900 ꢀC) for a long
period (100 h) [21]. As a consequence, the connectivity was far
enhanced compared to that observed in conventional ex-situ MgB2
samples. Another result showed that, the connectivity was further
increased using high-purity laboratory-made MgB2 with less sur-
face oxidation and carbon-doped MgB2 as starting powders [22].
Moreover, finer initial MgB2 particle prepared by ball milling could
account for the enhancement of intergranular coupling with both
qualitative and quantitative [23]. Those results [21e23] also
showed that only with long period sintering can the self-sintering
among MgB2 occur. All of their experiments were conducted in a
sealed environment at a constant temperature, and the decompo-
sition and composition of MgB2 are not easy to achieve. They
thought that the considerable evaporation of Mg and generation of
MgB4 when MgB2 sample heated above 850 ꢀC in an open system is
the disadvantage of promotion of solid-state self-sintering.
In this paper, the ex-situ MgB2 bulks were prepared by a
reversible reaction through a two-step sintering process with the
aim of improving the poor grains connectivity. It was found that the
nature of self-sintering is material migration. Reversible reaction
among MgB2 bulks could promote the material migration. More-
over, we draw conclusions about the influence of two-step sinter-
ing process to the formation of new particles and the new heating
method to the superconducting properties. This two-step sintering
Fig. 1. XRD patterns of the MgB2 bulks of different preservation time at 900 ꢀC.
synthesized by one-step sintering contain MgB2 as the dominant
phase, and as the preserving time increases, the peak intensity of
MgB4 increases. The weight fraction of MgB4 in these samples after
one-step sintering was estimated based on the XRD data, and the
corresponding value are 2.54%, 4.40% and 6.16% for sample
‘900 ꢀC,0 min’, sample ‘900 ꢀC, 10min’ and sample ‘900 ꢀC, 20min’,
respectively. One can see that the amount of formed MgB4 increases
obviously with the preserving time prolonging. The main reason is
the decomposition of MgB2 with the product of MgB4 at 900 ꢀC
process can be
a promising method to manufacture high-
performance ex-situ MgB2 bulks and wires.
2. Experimental procedure
Commercial magnesium diboride powders (99% purity,
30e100 mm in size) used in present work were from Alfa Aesar
Chemicals Co. Ltd. The pure powder was pressed into cylindrical
pellets with 5 mm in diameter and 1.5 mm in thickness under a
pressure of 10 MPa. The pressed pellets were sintered at 900 ꢀC
varying from 0 to 20 min and then at 650 ꢀC for 1 h under flowing
high-purity Ar gas. All the heating programs were conducted in
differential thermal scanners apparatus (Mettler Toledo, TGA/DSC 1
HT/1611). To reduce the Mg loss during the reversible reaction, pure
Mg (3% of the weight of MgB2) was laid up around the sample
before heating. In this study, bulk samples are denoted by sintering
temperature and sintering time. For example, a bulk sample only
with 1 h preservation at 650 ꢀC is called ‘650 ꢀC, 1 h’; sample with
0 min preservation at 900 ꢀC and then with 1 h preservation at
650 ꢀC is called ‘900 ꢀC, 0 min - 650 ꢀC, 1 h’; sample with 10 min
preservation at 900 ꢀC and then with 1 h preservation at 650 ꢀC is
called ‘900 ꢀC, 10 min - 650 ꢀC, 1 h’; sample with 20 min preser-
vation at 900 ꢀC and then with 1 h preservation at 650 ꢀC is called
‘900 ꢀC, 20 min - 650 ꢀC, 1 h’.
(MgB2(s)
/
MgB4(s)þMg(g)) [24,25] has occurred and the
decomposition goes in depth as time prolonged. The intensity of
the Mg is too weak to recognize in Fig. 1 because of the Mg evap-
oration. Fig. 2 also shows that all samples synthesized by two-step
sintering contain MgB2 as the dominant phase. But compared with
one-step sintered samples, the difference is that the intensity of
MgB4 decreases (sample ‘900 ꢀC, 20 min - 650 ꢀC, 1 h’) and even
vanishes (sample ‘900 ꢀC,0 min - 650 ꢀC, 1 h’; sample ‘900 ꢀC,
10 min - 650 ꢀC, 1 h’). The decreasing amount of the MgB4 is the
evidence of composition between MgB4 and Mg (main from the
The phase composition and microstructure of the prepared
samples were examined by the powder X-ray diffraction (XRD, D/
MAX-2500) with Cu Ka radiation, scanning electron microscopy
(SEM, Hitachi Model No.S 4800) and transmission electron micro-
scopy (TEM, JEM-2100), respectively. Then the temperature
dependence of resistivity in the sintered samples was measured by
a DC four-probe method in the temperature range from 5 to 300 K.
The magnetization of the sintered samples was measured using a
physical properties measurement system (PPMS) at 20 K. Jc was
calculated from the width of magnetization hysteresis loops (DM),
based on the extended Bean model Jc ¼ 20 M/a (1 ꢁ a/3 b), where
D
D
M ¼ M þ ꢁ M ꢁ is the hysteresis loop width and a and b (a<b) are
the dimensions of a rectangular sample.
3. Results and discussion
Fig. 2. XRD patterns of the MgB2 bulks of different preservation time at 900 ꢀC and
The XRD results presented in Fig. 1 show that all samples
then with 1 h preservation at 650 ꢀC.

26
J. Peng et al. / Journal of Alloys and Compounds 694 (2017) 24e29
addition of Mg laid out of bulks) with the product of MgB2 during
the 1 h preservation period at 650 ꢀC.
decompose to Mg and MgB4, meanwhile Mg vapor can migrate
easily to the areas with the low vapor pressure (for example necks
and small pores). At the second step (650 ꢀC for 1 h) of this stage,
composition between Mg and MgB4 occurs and new boundary
forms. Moreover, Fig. 3 c shows better connectivity and the smaller
pore than Fig. 3 b because of extra 10-min preservation at 900 ꢀC
and more MgB2 particles being taken part in the reversible reaction.
Given this, it's not difficult to explain Fig. 3 d (sample ‘900 ꢀC,
20 min - 650 ꢀC, 1 h’) which is depicted by stage III hold the best
connectivity which with 20 min preservation at 900 ꢀC. At this
stage III, more MgB2 is decomposed and reformed, the original
surface become closer and closer, pores are eliminated progres-
sively and the new boundary formed at last. However, some
unreacted MgB4 and new-formed MgO are wrapped around the
new-born boundary which can damage the connectivity. Isolated
and closed pores and large grains can be seen in Fig. 3 d.
Fig. 3 presents typical microstructural features of MgB2 poly-
crystalline bulks prepared by ex-situ with different heating pro-
grams. Here, gray, black, and white contrasts in the secondary
electron images correspond to MgB2 grains, pores, and impurity
phases such as MgO, respectively. Fig. 3 a, sample ‘650 ꢀC, 1 h’,
clearly shows microstructure with many pores between the parti-
cles. Obviously, it is owing to deficient self-sintering of ex-situ
MgB2. One can also see that the intergrain coupling of sample
‘650 ꢀC, 1 h’ (Fig. 3 a) between MgB2 grains/particles is poor in
contrast with that of sample ‘900 ꢀC,0 min - 650 ꢀC, 1 h’, ‘900 ꢀC,
10 min - 650 ꢀC, 1 h’, and ‘900 ꢀC, 20 min - 650 ꢀC, 1 h’ (Fig. 3 b, c, d),
in which more strongly linked MgB2 grains are observed. We can
also learn that as the preservation time prolonged at 900 ꢀC, the
intergrain coupling enhanced and the boundaries become vaguer
(Fig. 3 b, c, d). All of these suggested that solid-state self-sintering
occurred during the heat treatment.
Fig. 4 shows schematic diagrams of self-sintering of three grains
[21], which can describe the results obtained from the different
sintering programs properly in present work. Stage I (correspond-
ing to sample ‘650 ꢀC,1 h’) is the early stage of sintering and contact
angle of grains becomes shallower (formation of necks) at this
stage. The obvious characteristic of this stage is the reserve of many
open pores. We can confirm this phenomenon from Fig. 3 a. At this
stage, MgB2 grains can hardly been decomposed because of low
temperature, but the surface of grains has changed to form necks
which are result from slow migration of MgB2 at 650 ꢀC. In the next
stage (stage II, corresponding to sample ‘900 ꢀC,0 min ꢁ650 ꢀC, 1 h’
and sample ‘900 ꢀC, 10 min - 650 ꢀC, 1 h’), pores are gradually
eliminated by the formation of new grain boundaries (The detailed
information of new boundary will be shown in Fig. 5 and discussed
later). At this stage, the surface contact area increased and the bulk
became denser. Fig. 3 b (sample ‘900 ꢀC,0 min ꢁ650 ꢀC, 1 h’) and
Fig. 3 c (sample ‘900 ꢀC, 10 min - 650 ꢀC, 1 h’) show this situation
appropriately. At the first step of this stage, MgB2 grains begin to
The HRTEM images of the sample ‘900 ꢀC, 20 min - 650 ꢀC, 1 h’
are shown in Fig. 5. The new-born boundary was found between
MgB2 grains (as marked by a white line in Fig. 5b), and MgB4 and
MgO were recognized within the new-born boundary in Fig. 5 a.
The corresponding lattice was confirmed by FFT (Fast Fourier
Transformation). The decomposition of MgB2 is easier to occur
close to gaps and pores than in the particle center because gaps and
pores can provide a highway for new formed Mg diffusing from
original matrix, then some Mg may react with O2 which exists in
pores and gaps forming MgO. From Fig. 5 a, the scale of MgB4
crystalline grains was measured varying 5e10 nm and those grains
can act as flux pinning centers. When heating at 650 ꢀC, pores and
gaps are gradually eliminated and substituted by the new-born
boundary. This also agrees with the stage IV in Fig. 4. The new-
born boundary was built up by the unreacted MgB4, impurity
MgO, and new formed MgB2. The width of this boundary was
measured varying 10e15 nm.
According to the Rowell connectivity analysis, the active cross-
sectional area fraction (AF) represents the connectivity factor be-
tween adjacent grains [26,27]. Here the AF is estimated as:
Fig. 3. SEM images of the sintered MgB2 bulks of (a) 1 h preservation at 650 ꢀC, (b) preservation 0 min at 900 ꢀC and then with 1 h preservation at 650 ꢀC, (c) preservation 10 min at
900 ꢀC and then with 1 h preservation at 650 ꢀC, (d) preservation 20 min at 900 ꢀC and then with 1 h preservation at 650 ꢀC.

J. Peng et al. / Journal of Alloys and Compounds 694 (2017) 24e29
27
Fig. 4. Schematic diagrams of self-sintering of three grains which can describe the different results obtained from the sintering programs properly. Stage I: early stages of sintering;
a contact angle of grains becomes shallower (formation of necks). Stage II: the middle stage of sintering; further evolution of necks and pores diffuse through and accumulate at
grain boundaries (GBs). Stage III: the final stage of self-sintering; some pores are eliminated and isolated and closed pores appear [21]. The contact GB area between grains
progressively increases and densification occurs during sintering.
Fig. 5. HRTEM images for self-sintered MgB2 sample ‘900 ꢀC, 20 min - 650 ꢀC, 1 h’.
AF ¼ Drideal=ðr300K
Drideal ridealð300KÞ
ꢁ
r40K
Þ
(1)
(2)
¼
ꢁ ridealð40KÞ
Where rideal is the resistivity of a reference crystal and rT is the
measured resistivity. According to previous studies [27], here the
Drideal is 7.3 mU*cm. The results are listed in Table 1. The AF values
suggest that the connectivity of these samples increased as the
duration time prolonged at 900 ꢀC. However, the value of AF for all
the ex-situ samples are lower than that of in-situ samples, implying
worse grain connectivity in ex-situ samples. This result is consistent
with previous studies [19e21], where intergranular coupling of ex-
situ MgB2 samples was reported to be generally lower than in-situ
ones due to the impurity phase at grain boundaries and the low
bulk density. Actually, how to improve the grain connectivity of ex-
situ MgB2 sample is always a great challenge and also one of the
research hotspots in terms of MgB2 application. To further analyze
the intergrain connectivity, the residual resistivity ratio (RRR) was
also estimated (see Table 1). The value of RRR increases from
sample ‘900 ꢀC, 0 min - 650 ꢀC, 1 h’ to ‘900 ꢀC, 20 min - 650 ꢀC, 1 h’.
These results also indicate that samples with longer preservation at
900 ꢀC possess much better grain connectivity than the typical
MgB2 grains. Fig. 6 shows the resistivity versus temperature plots of
sintered MgB2 bulks with different preservation time at 900 ꢀC and
Fig. 6. The temperature dependence of resistivity of sintered MgB2 bulks with
different preservation time at 900 ꢀC and then with 1 h preservation at 650 ꢀC.
then with 1 h preservation at 650 ꢀC. The resistivity of samples
sintered at high temperature is lower than sintered at low
Table 1
Tc and electrical resistivity properties of sintered MgB2 bulks with different preservation time at 900 ꢀC and then with 1 h preservation at 650 ꢀC.
Sample
Tc(K)
r300K
(mU*cm)
r40K
(
mU*cm)
Dr
(mU*cm)
AF
RRR
650 ꢀC, 1 h
37.4
38.7
39.2
38.7
10116.4
2329.6
1598.0
740.0
6650.2
1571.6
946.9
3466.2
758.0
651.1
377.6
0.0021
0.0096
0.0112
0.0193
1.92
1.48
1.68
2.04
900 ꢀC, 0 mine650 ꢀC, 1 h
900 ꢀC, 10 mine650 ꢀC, 1 h
900 ꢀC, 20 mine650 ꢀC, 1 h
362.6

28
J. Peng et al. / Journal of Alloys and Compounds 694 (2017) 24e29
temperature and as the duration time at 900 ꢀC prolonged, the
resistivity decreased significantly.
Fig. 7 shows the magnetic field dependence of Jc for all samples.
It is particularly noteworthy that self-sintering can improve Jc
across all the measured magnetic field. Jc of three one-step sintered
samples without heating at 650 ꢀC are all lower than that of two-
step sintered samples with heating at 650 ꢀC not only at the self-
field but also at high fields. This phenomenon is explained by
that one-hour preservation at 650 ꢀC can reduce MgB4 and enhance
the connectivity, as discussed above. It was also found that Jc under
self-field increases as the duration time prolonged at 900 ꢀC. The
improvement of Jc under self-field is attributed to the increase of
coupling between grains, as discussed above and shown in Figs. 3
and 4. This is believed to contribute significantly to an effective
path for the transport current in the superconducting states [20].
We can also note that Jc of sample ‘ 650 ꢀC, 1 h’ is a little higher than
the sample ‘900 ꢀC, 0 min - 650 ꢀC, 1 h’ at the self-field. However,
this situation reversed at high fields because of the grown effective
pinning centers such as new-born boundaries and MgB4 [28] in
sample ‘900 ꢀC, 0 min - 650 ꢀC, 1 h’. The value of Jc is still lower than
the best values reported in previous studies [21,22], and there are
two main reasons for this result. Firstly, due to our limited exper-
imental condition, the inevitable impurity such as MgO scattered in
the grains boundary degrade the value of Jc significantly. Then
another more important reason is the intrinsic low connectivity of
ex-situ sintered MgB2 bulks despite some improvement by pro-
longing the duration time at 900 ꢀC as discussed above. It can be
boldly predicted that longer duration at 900 ꢀC could result in more
material migration and the improvement of connectivity.
Fig. 8. Normalized pinning force Fp/Fpmax as a function of reduced magnetic field h ¼ H/
Hirr at 20 K for the ex-situ sintered MgB2 bulks with different preservation time at 900
oCand then with 1 h preservation at 650 ꢀC.
reaction may also act as effective pinning centers. Moreover, we
also can note that peaks of sample ‘900 ꢀC, 10 min - 650 ꢀC, 1 h’ and
sample ‘900 ꢀC, 20 min - 650 ꢀC, 1 h’ shifted to higher fields and the
sample ‘900 ꢀC, 20 min - 650 ꢀC, 1 h’ to the highest fields. At this
stage, more new-born boundaries and MgB4 impurity can act as
effective pinning centers. This agrees with the curves trend in Fig. 6
that at high fields the Jc was increased as the duration time pro-
longed at 900 ꢀC.
Fig. 8 shows the normalized flux pinning force f ¼ Fp/Fpmax as a
function of the reduced field h ¼ H/Hirr at 20 K for these ex-situ
samples. Here, the flux pinning force Fp is calculated by the equa-
tion Fp ¼ m0HJc (m0 ¼ 4
p
*10ꢁ7), Fpmax is the maximum value of Fp(H),
and Hirr is defined as the field at Jc ¼ 100 A cmꢁ2 [21]. In present
study, the peak of Fp/Fpmax in the sample ‘650 ꢀC, 1 h’ and sample ‘
900 ꢀC, 0 min - 650 ꢀC, 1 h’ is located at about h ¼ 0.2, which in-
dicates that the grain boundary pinning is the predominant pinning
mechanism in these samples [29]. And compared to the sample
‘650 ꢀC, 1 h’, the peak of sample ‘900 ꢀC, 0 min - 650 ꢀC, 1 h’ moved
to high fields. Besides, the original grain boundary and impurity,
the new-born boundaries and the MgB4 formed by the reversible
4. Conclusions
To improve the grain connectivity of ex-situ MgB2 poly-
crystalline bulks, we attempted to take advantage of the decom-
position of MgB2 at relatively high temperature (900 ꢀC) and then of
the composition between MgB4 and Mg at relatively low temper-
ature (650 ꢀC) with the product of MgB2. By prolonging the pres-
ervation time (from 0 to 20 min) at high temperature, the
decomposition was promoted and thus the amount of MgB4 was
increased. By heating at relatively low temperature (650 ꢀC) for 1 h,
MgB2 has formed again and the amount of MgB4 was decreased.
This reverse reaction can promote material migration and thus
enhance the connectivity. It's the critical factor for the enhance-
ment of connectivity. The value of Jc was not only improved at the
self-field but also at high fields. The reasons for these results are the
connectivity between MgB2 grains was enhanced, and also the
new-born grain boundary and the impurity (MgB4) served as
effective pinning centers at high field. The employment of this
reversible reaction is a promising method to fabricate high-
performance ex-situ MgB2 bulks and wires.
Acknowledgement
The authors are grateful to China National Funds for Distin-
guished Young Scientists (Grant No. 51325401), the National Nat-
ural Science Foundation of China (Grant No. 51302186 and
51574178) and the Natural Science Foundation of Tianjin (Grant
No.14JCQNJC03300). This work is also supported by the National
Magnetic Confinement Fusion Energy Research Project (grant
No.2014GB125006).
Fig. 7. Measured Jc-H characteristics of the sintered MgB2 bulks with different pres-
ervation time at 900 ꢀC and then with 1 h preservation at 650 ꢀC.

J. Peng et al. / Journal of Alloys and Compounds 694 (2017) 24e29
on superconductor properties of MgB2, Acta. Mater. 70 (2014) 298e306.
29
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