Physica C 309 Ž1998. 39–42
Improvement of the superconducting properties in
žNd, Sm, Eu / –Ba–Cu–O using melt-quenched precursors
M. Muralidhar ) , M. Murakami
SuperconductiÕity Research Laboratory, ISTEC, 1-16-25, Shibaura, Minato-ku, Tokyo 105, Japan
Received 17 August 1998; accepted 9 October 1998
Abstract
We have studied the effect of precursor preparation methods on the superconducting properties of ŽNd, Sm, Eu. –Ba–Cu–O
melt-textured in 1% O 2 –Ar. X-ray diffraction analyses confirmed that melt-quenching was effective in suppressing the
formation of RE rich RE 1qx Ba 2yx Cu 3 O y precursor powders compared to a sintering method, and thus resulted in a sharp
superconducting transition with an onset Tc exceeding 94.5 K. A clear secondary peak effect was observed in the
magnetization curves, which shows that the field induced pinning is active like the other RE–Ba–Cu–O ŽRE; Nd, Sm, Eu,
Gd.. One can conclude that poor superconducting properties of ŽNd, Sm, Eu. –Ba–Cu–O obtained in the previous reports are
not intrinsic to the system but to an unoptimized precursor preparation method. q 1998 Published by Elsevier Science B.V.
All rights reserved.
Keywords: ŽNd, Sm, Eu. –Ba–Cu–O; Melt quenching; Melt process; Critical temperature; Peak effect; Critical current density
1. Introduction
RE–Ba–Cu–O ŽRE: Nd, Sm, Eu, and Gd.
bulk superconductors fabricated by the oxygen-controlled-melt-growth ŽOCMG. process w1,2x, which is
the melt process in a reduced oxygen atmosphere,
are highly attractive for bulk type applications since
they possess large critical current density Ž Jc . values
in high fields along with high irreversibility fields
exceeding 7T Ž H 5 c axis. even at 77 K w1–6x.
It was also experimentally confirmed that the
OCMG process is effective in achieving high Tc and
large Jc values for the intermixture RE–Ba–Cu–O
systems, in which the RE site was compounded with
)
Corresponding author. Tel.: q81-196-35-9016; Fax: q81196-35-9017; E-mail: miryala@istec.or.jp
three different rare earth elements w7–11x. X-ray
diffraction measurements showed that all the samples are single phase RE123 with a small amount of
RE211. Amongst these intermixture RE–Ba–Cu–O
systems, ŽNd,Eu,Gd. –Ba–Cu–O exhibits fairly good
pinning properties with the Jc value reaching 50 000
Arcm2 at 77 K and 2T Ž H 5 c axis.. Recently,we
have also succeeded in enhancing flux pinning properties of ŽNd, Eu, Gd. –Ba–Cu–O by dispersing fine
Gd211 particles about 0.1 mm in diameter, in which
a Jc value of 60,000 Arcm2 in 3T was achieved at
77 K for H 5 c axis w12x. Das et al. w13x studied flux
penetration and trapping behavior of ŽNd, Eu, Gd. –
Ba–Cu–O superconductor with magneto-optical
techniques and found that the critical state is well
established in the whole sample, showing that the
sample is a single grain without weak-links. They
0921-4534r98r$19.00 q 1998 Published by Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 4 5 3 4 Ž 9 8 . 0 0 5 9 2 - 9
40
M. Muralidhar, M. Murakamir Physica C 309 (1998) 39–42
also confirmed that the ŽNd, Eu, Gd. –Ba–Cu–O
sample is really good in quality with high pinning
performance.
In contrast to ŽNd, Eu, Gd. –Ba–Cu–O, however,
the ŽNd, Sm, Eu. –Ba–Cu–O sample always exhibited a rather broad superconducting transition and
poor Jc –B properties, despite the fact that the same
precursor preparation method was employed for both
systems w10x. It is hard to understand why this system exhibits a broad superconducting transition, since
both ŽNd, Sm. –Ba–Cu–O and ŽNd, Eu. –Ba–Cu–O
exhibit a sharp superconducting transition w14x. In
our previous work w15x, we have found that Gd
addition is effective in improving Tc values for ŽNd,
Sm, Eu. –Ba–Cu–O composite and such improvement was attributable to the fact that the formation
of RE rich RE 1q x Ba 2yx Cu 3 O y ŽRE123ss. phase
could be suppressed during the precursor preparation
when Gd was added to the system w15x.
For the preparation of precursor ŽNd, Sm, Eu.123
powders, we used a conventional sintering method in
that the mixed powders were heated to 8808C in
Ar–1% O 2 for three times with intermediate grinding. Our results show that such a sintering process is
not effective in suppressing the formation of RE rich
RE123ss phase in the ŽNd, Sm, Eu. –Ba–Cu–O system. Melt-quenching is known to be effective in
suppressing the formation of RE rich RE123ss in the
precursor powders, since the range of RE–Ba solid
solution is smaller at higher temperatures. Therefore,
we employed melt-quenching for the preparation of
precursor powders for the ŽNd, Sm, Eu. –Ba–Cu–O
and succeeded in improving the superconducting
properties.
2. Experimental
High purity commercial powders of Nd 2 O 3 ,
Sm 2 O 3 , Eu 2 O 3 , BaCO 3 and CuO were weighed to
have a nominal composition of ŽNd 0.33 Eu 0.33 Sm 0.33 .
Ba 2 Cu 3 O 7yx . These powders were ground thoroughly and calcined at 8808C for 24 h with intermediate grinding, which were repeated three times and
pressed into pellets. These pellets were sintered at
10208C for 48 h and ground thoroughly. To control
the formation of RE rich RE123ss phase, sintered
powders were melted in a platinum crucible at 14008C
for 30 min and splat-quenched by two copper plates.
These powders were first pressed into pellets 20
mm in diameter and 15 mm in thickness, which were
then subjected to CIP Žcold isostatic press. with a
pressure of 2000 kgrcm2 . A MgO Ž100. seed was
placed on the center of the pellet, which was then
OCMG processed in Ar–1% O 2 with a gas flow rate
of 300 mlrmin. The details of the heat treatment
schedules can be found elsewhere w10x.
For oxygen annealing the rectangular samples
with dimensions of about 1.5 = 1.5 = 0.5 mm3 were
cut from grown crystals and annealed in flowing O 2
gas in the temperature range of 300–6008C w10x.
X-ray diffraction studies were carried out with a
Rigaku RU-300 powder diffractometer equipped with
a rotating Cu anode X-ray tube and Cu K a radiation. Microstructural features of the samples were
observed with an optical microscope and a scanning
electron microscope ŽSEM..
Tc measurements were performed with a Quantum
Design MPMS-7 SQUID magnetometer with a magnetic field of 10 Oe. Magnetization loops are measured at 77 K using a 7 T superconducting magnet
with fields applied parallel to the c axis. The Jc
values were calculated based on the extended Bean
critical state model from the following equation:
Jc s Ž 20D M . r a 1 y ar Ž 3b . 4 ,
where Jc is in Arcm2 , D M is the magnetization
hysteresis during the increasing and decreasing field
processes in emurcm3, a and b Ž a - b . in centimeters, are cross sectional sample dimensions perpendicular to the applied magnetic field in centimeters.
3. Results and discussion
Fig. 1 shows the XRD patterns for ŽNd, Sm,
Eu.Ba 2 Cu 3 O y samples prepared from sintered and
melt-quenched powders, respectively. Both samples
mainly consist of the RE123 phase along with RE211.
The intensity and peak positions of the prominent
peak reflections are in good agreement with our
previous results w10x. Here it is important to note that
the amount of RE211 second phase is larger for the
sample prepared from sintered powders than that
prepared from melt-quenched powders. Since we
prepared the samples in RE123 stoichiometric ratio,
M. Muralidhar, M. Murakamir Physica C 309 (1998) 39–42
Fig. 1. XRD patterns for ŽNd, Sm, Eu.123 superconductors prepared from sintered and melt-quenched precursor powders,
OCMG-processed in 1% partial pressure of O 2 .
the larger amount of RE211 indicates that more
phase imbalance has taken place in the sample prepared from sintered powders. It can thus be concluded that melt-quenching is effective in avoiding
the formation of RE rich RE123ss phase.
Fig. 2 shows the temperature dependence of dc
magnetic susceptibility for ŽNd, Sm, Eu.Ba 2 Cu 3 O y
samples in zero-field-cooled ŽZFC. and field-cooled
ŽFC. processes in the presence of a magnetic field of
10 Oe. From the figure, it is clear that the sample
prepared from melt-quenched precursor powders exhibits superconducting transition sharper than that of
a sintered one, supporting the fact that the formation
of RE rich RE123ss was suppressed by using meltquenched precursors.
Fig. 3 displays Jc –B properties for ŽNd, Sm,
Eu.Ba 2 Cu 3 O y samples OCMG processed in 1% O 2
41
Fig. 3. Jc – B properties Ž77 K, H 5 c axis. for ŽNd–Sm–Eu.123
superconductors prepared from sintered and melt-quenched precursor powders, OCMG-processed in 1% partial pressure of O 2 .
measured at 77 K for fields parallel to the c axis. No
secondary peak effect is observed in the sample
made from sintered powders, while the sample prepared from melt-quenched powders exhibits the secondary peak effect, which indicates that the field-induced pinning centers are active like other RE–Ba–
Cu–O systems fabricated by the oxygen-controlledmelt-growth ŽOCMG. process w4x. The absence of
the peak effect in the sample prepared from sintered
powders can be attributed to the fact that RE rich
RE123ss clusters are already normal conducting in
zero field. These results clearly show that poor superconducting properties of ŽNd, Sm, Eu.123 reported in the previous report w10x are not intrinsic to
the material, but can be attributed to an improper
preparation method. Therefore, one can conclude
that if the sample preparation method is optimized,
all three rare earth elements RE–Ba–Cu–O systems
exhibit high Tc and Jc values accompanied by the
secondary peak effect.
4. Conclusions
Fig. 2. Temperature dependence of normalized susceptibility for
ŽNd, Sm, Eu.123 superconductors prepared from sintered and
melt-quenched precursor powders, OCMG-processed in 1% partial
pressure of O 2 .
We have successfully prepared high Tc ŽNd, Sm,
Eu. –Ba–Cu–O superconductors by using meltquenched precursor powders. Magnetization measurements showed that the sample exhibits onset Tc
of 94.5 K with a sharp superconducting transition. A
peak Jc value of 24 000 Arcm2 was recorded with
an applied field of 2T Ž H 5 c . at 77 K. Such drastic
42
M. Muralidhar, M. Murakamir Physica C 309 (1998) 39–42
improvement in Tc and Jc –B properties in ŽNd, Sm,
Eu.123 can be understood in terms of the suppression of RE rich RE123ss formation with meltquenched precursor powders.
Acknowledgements
This work was partially supported by NEDO for
the R & D of Industrial Science and Technology
Frontier program. MMD would like to thank the
Iwate Techno Foundation, Iwate, Japan for providing
the financial assistance.
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