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Enhancement of the critical current density and <ux pinning of MgB2 Enhancement of the critical current density and <ux pinning of MgB2
superconductor by nanoparticle SiC doping superconductor by nanoparticle SiC doping
S X. Dou
University of Wollongong
, shi@uow.edu.au
Saeid Soltanian
University of Wollongong
, saeid@uow.edu.au
J. Horvat
University of Wollongong
, jhorvat@uow.edu.au
Xiaolin Wang
University of Wollongong
, xiaolin@uow.edu.au
S. H. Zhou
University of Wollongong
, sihai@uow.edu.au
See next page for additional authors
Follow this and additional works at: https://ro.uow.edu.au/engpapers
Part of the Engineering Commons
https://ro.uow.edu.au/engpapers/93
Recommended Citation Recommended Citation
Dou, S X.; Soltanian, Saeid; Horvat, J.; Wang, Xiaolin; Zhou, S. H.; Ionescu, M.; Liu, Hua-Kun; Munroe, P.; and
Tomsic, M.: Enhancement of the critical current density and <ux pinning of MgB2 superconductor by
nanoparticle SiC doping 2002.
https://ro.uow.edu.au/engpapers/93
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Enhancement of the critical current density and flux pinning
of MgB
2
superconductor by nanoparticle SiC doping
S. X. Dou,
a)
S. Soltanian, J. Horvat, X. L. Wang, S. H. Zhou, M. Ionescu, and H. K. Liu
Institute for Superconducting and Electronic Materials, University of Wollongong, Northfields Avenue,
Wollongong, NSW 2522, Australia
P. Munroe
Electron Microscopy Unit, University of New South Wales, Sydney, NSW 2001, Australia
M. Tomsic
Hyper Tech Research Incorporated, 110 E. Canal Street, Troy, Ohio 45373
共Received 11 July 2002; accepted 5 September 2002兲
Doping of MgB
2
by nano-SiC and its potential for the improvement of flux pinning were studied for
MgB
2⫺ x
(SiC)
x/2
with x⫽0, 0.2, and 0.3 and for 10 wt % nano-SiC-doped MgB
2
samples.
Cosubstitution of B by Si and C counterbalanced the effects of single-element doping, decreasing T
c
by only 1.5 K, introducing intragrain pinning centers effective at high fields and temperatures, and
significantly enhancing J
c
and H
irr
. Compared to the undoped sample, J
c
for the 10 wt % doped
sample increased by a factor of 32 at 5 K and 8 T, 42 at 20 K and 5 T, and 14 at 30 K and 2 T. At
20 K and 2 T, the J
c
for the doped sample was 2.4⫻ 10
5
A/cm
2
, which is comparable to J
c
values
for the best Ag/Bi-2223 tapes. At 20 K and 4 T, J
c
was twice as high as for the best MgB
2
thin films
and an order of magnitude higher than for the best Fe/MgB
2
tapes. The magnetic J
c
is consistent
with the transport J
c
which remains at 20 000 A/cm
2
even at 10 T and 5 K for the doped sample,
an order of magnitude higher than the undoped one. Because of such high performance, it is
anticipated that the future MgB
2
conductors will be made using a formula of MgB
x
Si
y
C
z
instead of
pure MgB
2
.©2002 American Institute of Physics. 关DOI: 10.1063/1.1517398兴
The critical current density (J
c
) in MgB
2
has been a
central topic of research since superconductivity in this com-
pound was discovered.
1
High J
c
values of 10
5
to 10
6
A/cm
2
have been reported for MgB
2
by several groups. However, J
c
drops rapidly with increasing magnetic field due to its poor
flux pinning. To take advantage of its high T
c
共39 K兲, im-
provements in the irreversibility field (H
irr
) and J
c
(H) were
achieved by oxygen alloying of MgB
2
thin films
2
and by
proton irradiation of MgB
2
powder.
3
However, for practical
applications, the pinning centers should be introduced by a
simple process, such as chemical doping. Most of element
substitution studies were aimed at raising T
c
and were thus
limited to a low doping level. Improvement of flux pinning
was also attempted, using doping by chemical compounds.
Here, the results are largely limited to the addition of the
compounds, rather than substitution of Mg or B by the com-
pounds. Additives alone appear to be ineffective for the im-
provement of pinning at high temperatures.
4–6
Recently, we
found that chemical doping of nano-SiC into MgB
2
can sig-
nificantly enhance J
c
in high fields with only slight reduc-
tions in T
c
up to a doping level of 40% of B.
7
This finding
clearly demonstrated that cosubstitution of SiC for B in
MgB
2
induced intragrain defects and a high density of
nanoinclusions as effective pinning centers. However, the
processing conditions used were far from optimized and the
sample density was only 50% of the theoretical value. There-
fore, it was not possible to assess the full potential of nano-
SiC doping for the improvement of J
c
. In this work, we
show that nanometer size SiC-doped MgB
2
gives the highest
J
c
values in high magnetic fields at 20 K reported for any
form of MgB
2
, including thin films.
MgB
2
pellet samples were prepared by an in situ reac-
tion method, which has been described in detail elsewhere.
7
Powders of magnesium 共99% pure兲 and amorphous boron
共99% pure兲 were well mixed with SiC nanoparticle powder
共size 10 to 100 nm兲 with the atomic ratio of
MgB
2⫺ x
(SiC)
x/2
, where x⫽0, 0.2, and 0.3, for samples des-
ignated as 1 to 3, respectively. A sample with 10 wt % of SiC
addition to MgB
2
was also made as sample 4. Pellets 10 mm
in diameter and 2 mm in thickness were made under uniaxial
pressure, sealed in Fe tubes and then heated at temperatures
of 700–900 °C for 1 h in flowing high-purity Ar. This was
followed by furnace cooling to room temperature. The same
powders used in samples 1 and 4 were made into wires using
the powder-in-tube method.
10
These are designated as
samples 5 and 6, respectively.
The magnetization of samples was measured over a tem-
perature range of 5 to 30 K using a physical property mea-
surement system 共PPMS兲共Quantum Design兲 in a time-
varying magnetic field with sweep rate 50 Oe/s and
amplitude up to 8.5 T. All the samples were cut to the same
size of 0.56⫻2.17⫻3.73 mm
3
from as-sintered pellets. A
magnetic J
c
was derived from the height of the magnetiza-
tion loop ⌬ M using a Bean model: J
c
⫽ 20⌬M/
关
a(1
⫺ a/3b)
兴
. Irreversibility field (H
irr
) was obtained from mea-
suring the field-cooled and zero-field-cooled magnetic mo-
ments as a function of temperature for several values of the
a兲
Author to whom correspondence should be addressed; electronic mail:
shiគdou@uow.edu.au
APPLIED PHYSICS LETTERS VOLUME 81, NUMBER 18 28 OCTOBER 2002
34190003-6951/2002/81(18)/3419/3/$19.00 © 2002 American Institute of Physics
Downloaded 19 Jun 2006 to 130.130.37.6. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
field. The critical temperature (T
c
) was obtained as the onset
of the diamagnetic transition in the magnetic ac susceptibil-
ity measurements. The transport J
c
was measured with the
four-probe method using a pulsed current source.
Figure 1 shows x-ray diffraction 共XRD兲 patterns for the
SiC doped and nondoped samples. The XRD pattern for the
nondoped sample 共sample 1兲 reveals about 5% MgO, beside
MgB
2
as the main phase. Samples 2 and 3 consist of MgB
2
as the main phase, with Mg
2
Si as the major impurity phase
共crosses in Fig. 1兲. The estimated fraction of Mg
2
Si was
10%. The energy dispersive spectroscopy 共EDS兲 analysis re-
sults showed that the Mg:Si ratio was identical over the en-
tire sample area, indicating a homogeneous phase distribu-
tion.
Figure 2 shows magnetic J
c
(H) curves for the SiC-
doped MgB
2
samples at 5 K, 20 K, and 30 K, for different
doping levels. It is noted that all the J
c
(H) curves for doped
samples show a crossover with the nondoped samples at
higher fields. Although SiC doping caused a slight reduction
of J
c
in low fields, it is much larger than for the nondoped
samples in high fields for all the measured temperatures.
Compared to the nondoped sample, J
c
for the 10 wt % doped
sample increased by a factor of 32 at5Kand8T,23at15K
and 6 T, 42 at 20 K and 5 T, and 14 at 30 K and 2 T. This is
the best J
c
(H) performance ever reported for MgB
2
in any
form. It is noted that the J
c
(H) curves for the nondoped
sample showed a rapid drop in high fields and a plateau near
H
irr
. Earlier, we ascribed this phenomenon to the grains de-
coupling at higher fields, as a consequence of impurities at
the grain boundaries.
8
In contrast, none of the SiC-doped
samples show this phenomenon, as either the substitutions or
the induced nanoinclusions are incorporated into the grains.
Figure 3 shows a comparison of magnetic J
c
(H)fora10
wt % SiC-doped sample at 20 K with data reported in litera-
ture. J
c
for this sample exhibits a better field performance
and higher values of J
c
in high field than any other element
doped samples
4–6
or nondoped wires.
9
Our SiC-doped MgB
2
is even better than the thin-film MgB
2
共Fig. 3兲, which had
exhibited the strongest reported flux pinning and the highest
J
c
in high fields to date. At 20 K, the best J
c
for the 10 wt %
SiC-doped sample was 10
5
A/cm
2
at 3 T, which exceeds the
J
c
values of state-of-the-art Ag/Bi-2223 tapes. At 20 K and 4
T, J
c
was 36 000 A/cm
2
, which is twice as high as for the
best MgB
2
thin films
2
and an order of magnitude higher than
for the state-of-the-art Fe/MgB
2
tapes.
9
The temperature dependence of H
irr
for nano-SiC-doped
MgB
2
, as well as for the pellets and tapes prepared previ-
ously 共round symbols兲, is shown in the inset to Fig. 3. Ap-
parently, H
irr
for x⫽0 overlaps with H
irr
for the previous
samples, even though the latter had significantly smaller val-
ues of J
c
. Doping with SiC significantly improved H
irr
. For
example, H
irr
for the SiC-doped samples reached 7.3 T at 20
K, compared to 5.7 T for the nondoped one. This is consis-
tent with improvement of the field dependence of J
c
with the
doping. Because H
irr
for the nondoped control sample (x
⫽ 0) is the same as for the previously prepared samples, the
improvement of J
c
(H) definitely occurred because of the
improvement of flux pinning by the doping and not because
of improved sintering of MgB
2
.
Figure 4 shows the transport J
c
(H) values for the un-
doped and 10 wt % SiC-doped MgB
2
wires 共samples 5 and
6兲 at 5 K, 10 K, and 20 K. It is evident that the transport J
c
results for both the undoped and doped wires are in excellent
agreement with the magnetic J
c
. It is also clear that the
enhancement in transport J
c
due to SiC doping is consistent
with the magnetic J
c
. The transport I
c
for the 10 wt % SiC-
doped MgB
2
/Fe reached 660 A at 5 K and 4.5 T (J
c
FIG. 1. XRD patterns for the nondoped and SiC-doped samples.
FIG. 2. The magnetic J
c
dependence at 5, 20, and 30 K for samples 1, 2, 3,
and 4, shown by solid, dashed, and dotted lines, and crosses, respectively.
FIG. 3. A comparison of magnetic J
c
(H) at 20 K for the 10 wt % SiC-doped
sample 共sample 4兲 and for samples that were: Ti doped 共see Ref. 4兲,Y
2
O
3
共see Ref. 5兲 doped, thin film with strong pinning 共see Ref. 2兲 and Fe/MgB
2
tape 共see Ref. 9兲. Inset: temperature dependence of the irreversibility field
for SiC-doped MgB
2
with different SiC content 共triangles and squares兲 and
for previously prepared nondoped MgB
2
共round symbols兲.
3420 Appl. Phys. Lett., Vol. 81, No. 18, 28 October 2002 Dou
et al.
Downloaded 19 Jun 2006 to 130.130.37.6. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
⫽133 000 A/cm
2
) and 540 A at 20 K and 2 T (J
c
⫽ 108 000 A/cm
2
). The transport J
c
for the 10 wt % SiC-
doped MgB
2
wire is more than an order of magnitude higher
than for the best Fe-sheathed MgB
2
wire reported to date at
5 K and 10 T and 20 K and 5 T,
5,9
respectively.
The inset in Fig. 4 shows T
c
for the nondoped and 10
wt % doped samples. The T
c
for the undoped sample is 37.6
K. For the doped samples, T
c
decreased with increasing dop-
ing level. The transition width was typically 0.5 K. It is
striking to note that T
c
has only dropped by 1.5 K for the 10
wt % SiC-doped sample. In contrast, T
c
was depressed by
almost 7 K for 10 wt % C substitution for B in MgB
2
.
10
These results suggest that the higher tolerance of T
c
to SiC
doping in MgB
2
is attributable to the cosubstitution of B by
C and Si. This is because the atomic radii of C 共0.077 nm兲
and Si 共0.11 nm兲 atoms are close to that of B 共0.097 nm兲.
Codoping with SiC counterbalanced the negative effect on
T
c
of the single element doping.
Regarding the mechanism behind the enhancement of J
c
at higher fields, it is necessary to recognize the special fea-
tures of SiC doping. First, in contrast to previous work on
doping for improving J
c
, SiC doping has no densification
effect, as evidenced by the fact that the density of doped
samples is 1.2 g/cm
3
, independent of the doping level. In
addition, SiC doping takes place in the form of substitution
and/or addition,
8
while in the previously reported work,
4–6
the doping was in the form of additives, not incorporated into
crystalline lattice of MgB
2
.
The transmission electron microscopy images showed a
high density of dislocations and a large number of ⬃10 nm
inclusions inside the grains 共Fig. 5兲. Their concentration in-
creased with the doping level. EDS analysis of the grains
revealed the presence of uniformly distributed Mg, B, C, Si,
and O 共inset to Fig. 5兲. This, and the results of XRD, sug-
gests that the inclusion nanoparticles were made of Mg
2
Si,
or unreacted SiC. All the intragrain defects and the inclu-
sions act as effective pinning centers. Our results suggest that
a combination of substitution-induced defects and highly dis-
persed additives are responsible for the enhanced flux pin-
ning. When SiC reacts with liquid Mg and amorphous B at
the sintering temperatures, the nanoparticles of SiC will act
as nucleation sites to form MgB
2
and other nonsupercon-
ducting phases which can be included within the grains as
inclusions. Thus, the reaction-induced products are highly
dispersed in the bulk matrix.
Given the ease of production of SiC-doped MgB
2
, our
results significantly strengthen the position of MgB
2
as a
competitor to more expensive conventional and high-
temperature superconductors. It is evident that future MgB
2
conductors will be made using a formula of MgB
x
Si
y
C
z
in-
stead of pure MgB
2
. In summary, we have demonstrated that
both the transport and magnetic J
c
, the irreversibility field
and the flux pinning of MgB
2
are all significantly enhanced
through nano-SiC doping, significantly improving the poten-
tial of MgB
2
for many applications.
The authors thank Dr. T. Silver, Dr. M. J. Qin, Dr. A.
Pan, Dr. E. W. Collings, Dr. M. Sumption, and R. Neale for
their helpful discussions. This work was supported by the
Australian Research Council, Hyper Tech Research Inc.,
Ohio, and Alphatech International Ltd, NZ.
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FIG. 4. Transport J
c
for SiC doped 共sample 5兲 and undoped MgB
2
共sample
6兲 wires 共the lines only serve for guiding the eyes兲. Inset shows T
c
for these
samples.
FIG. 5. TEM image showing the intragrain dislocations and nanoparticle
inclusions within MgB
2
grains. Inset: EDS element analysis of MgB
2
grains.
3421Appl. Phys. Lett., Vol. 81, No. 18, 28 October 2002 Dou
et al.
Downloaded 19 Jun 2006 to 130.130.37.6. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
