Physica C 313 Ž1999. 232–240
Refinement of secondary phase particles for high critical current
densities in žNd,Eu,Gd / –Ba–Cu–O superconductors
M. Muralidhar ) , M.R. Koblischka, M. Murakami
Superconducting Research Laboratory, ISTEC, 1-16-25, Shibaura, Minato-ku, Tokyo, 105, Japan
Received 26 August 1998; revised 8 December 1998; accepted 13 January 1999
Abstract
We have prepared ŽNd 0.33 Eu 0.33 Gd 0.33 .Ba 2 Cu 3 O y Ž‘NEG’. superconductors by means of melt-processing under a partial
oxygen pressure of 0.1%. In these samples, we achieved a large critical current density, Jc , of 6.8 = 10 4 Arcm2 at T s 77 K
and in an applied magnetic field, Ha s 2.5 T. Furthermore, the irreversibility field, Hirr , exceeds 7 T at 77 K for fields
applied parallel to the c-axis of the sample. As additional pinning sites, we introduced up to 40 mol% ŽNd,Eu,Gd. 2 BaCuO5
ŽNEG-211. particles plus Pt in analogy to YBa 2 Cu 3 O y ŽYBCO. melt-processed superconductors. Microstructural observations clarified that the Pt addition was effective in reducing the size of NEG-211 second-phase particles, which led to a
dramatic increase in Jc as compared to melt-processed YBa 2 Cu 3 O y and NdBa 2 Cu 3 O y ŽNdBCO.. A pronounced secondary
peak effect was observed in all magnetization curves obtained at 77 K, however, there is a considerable change in the shape
of the magnetization loops as a function of NEG-211 content, which demonstrates the pinning provided by the NEG-211
particles in addition to the dTc-pinning caused by fluctuation in the transition temperature, Tc , like in ordinary NdBa 2 Cu 3 O y
superconductors. q 1999 Elsevier Science B.V. All rights reserved.
PACS: 74.60 Ge; 74.60 Jg; 74.72 Jt; 74.80 Bj
Keywords: ŽNd,Eu,Gd.Ba 2 Cu 3 O y ; ŽNd,Eu,Gd. 2 BaCuO5 ; Melt processing; Pt addition; Microstructure; Peak effect; Flux pinning; Critical
current density
1. Introduction
The critical current density, Jc , is one of the most
important properties for practical applications of
high-temperature superconductors. Although Jc of
melt-processed YBa 2 Cu 3 O y ŽYBCO. already surpassed the lower limit of 10 4 Arcm2 at T s 77 K for
some applications, further improvement will be critically important to facilitate the bulk applications,
)
Corresponding author. Tel.: q81-196-35-9016; Fax: q81196-35-9017; E-mail: miryala1@istec.or.jp
e.g., the fabrication of quasi-permanent magnets
which can generate several tesla at 77 K.
Yoo et al. w1x found that NdBa 2 Cu 3 O 7 ŽNdBCO.
superconductors prepared by the melt process in a
reduced oxygen atmosphere exhibited high Jc values
in a high field region accompanied by a secondary
peak effect. The secondary peak effect is ascribed to
the so-called dTc-pinning w2–8x provided by RE-rich
ŽRE denoting rare earths. 123 clusters with a diameter of 10–50 nm, which are uniformly distributed in
the RE-123 matrix w9x. These clusters have been
observed by means of scanning tunneling mi-
0921-4534r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 4 5 3 4 Ž 9 9 . 0 0 0 1 7 - 9
M. Muralidhar et al.r Physica C 313 (1999) 232–240
croscopy ŽSEM. w10x and transmission electron microscopy ŽTEM. w11x; and the different features of
the dTc-pinning mechanism were discussed in detail
in Refs. w5–7x. Furthermore, it was experimentally
confirmed that the melt process under low oxygen
pressure Ž pO 2 ., i.e., the oxygen-controlled-meltgrowth ŽOCMG. process, is also effective in achieving high Tc and Jc values for other light rare earth
superconductors w12x. Thus, these light rare earth
LRE-Ba 2 Cu 3 O y ŽLRE: Nd, Sm, Eu, Gd. superconductors are promising for practical applications, since
in addition to an enhanced Tc of 96 K with a sharp
superconducting transition, they also exhibit Jc values larger than that of good quality YBa 2 Cu 3 O 7 in a
high magnetic field region particularly at high temperatures.
Recently, it was found that LRE-Ba 2 Cu 3 O y superconductors in which two LRE elements were
mixed together show similar features w13–15x. It was
also reported that LRE-Ba 2 Cu 3 O y composites with
three different LRE elements also exhibit high Tc
and Jc values w16–21x. However, the peak field and
Jc values were found to be dependent on the kind of
rare earth elements and on the processing conditions.
We have found that among these mixed LREBa 2 Cu 3 O y superconductors, the ternary compound
ŽNd,Eu,Gd.Ba 2 Cu 3 O y ŽNEG. with a ratio of Nd:
Eu:Gds 1:1:1 showed a record high Jc of 50 000
Arcm2 at an applied field of 2 T and the irreversibility field Ž Hirr . exceeding 7 T at 77 K for fields
parallel to the c-axis w19,20x. This improvement of
Jc was ascribed to a more uniform distribution of the
dTc-pinning sites w22x.
In the present study, we have succeeded in further
enhancing the critical current densities of
ŽNd,Eu,Gd.Ba 2 Cu 3 O y by adding NEG-211 particles
to the starting composition. The size of the NEG-211
particles could be refined by Pt addition as in the
case of melt-processed YBCO. We also discuss the
possible sources for enhanced flux pinning.
This paper is organized as follows: in Section 2,
we outline some details of the preparation of NEG
samples, and the experimental procedures are described. In Section 3.1, the microstructural analysis
by means of scanning tunneling microscopy ŽSEM.
and transmission electron microscopy ŽTEM. microcopy is presented. Section 3.2 presents the magnetic
characterization of the samples, and we discuss the
233
origin of the enhanced flux pinning in our samples.
Finally, in Section 4, some conclusions are drawn.
2. Experimental
Powders of Nd 2 O 3 , Eu 2 O 3 , Gd 2 O 3 , BaCO 3 and
CuO were weighed to have a nominal composition
of ŽNd 0.33 Eu 0.33 Gd 0.33 .Ba 2 Cu 3 O 7yx . These powders
were ground thoroughly and calcined at 8808C for 24
h with intermediate grinding, which was repeated
three times and then pressed into pellets. These
pellets were sintered at 10208C for 48 h and ground
thoroughly.
NEG bulk samples with a volume fraction of
20%, 30% and 40 mol% of NEG-211 second phase
were prepared using a mixture of sintered NEG and
commercial ŽNd,Eu,Gd.-211 powders. Here, it should
be born in mind that the formed second phase differs
depending on the kind of rare earth elements. For
NEG composites, we have found that the second
phase has the 211 structure. Since the addition of Pt
is known to be effective in refining the size of Y-211
in the YBCO system w22x, we added 0.5 mol% Pt in
all the samples.
Differential thermal analysis ŽDTA. measurements were performed on the samples pretreated at
9508C under low oxygen partial pressure Ž0.1% O 2 .
for 15 h to determine the peritectic decomposition
temperature. The heat treatment profiles in the melt
process for the NEG composites were scheduled on
the basis of the DTA results.
The precursor ŽNd,Eu,Gd.Ba 2 Cu 3 O y powders
were first pressed into pellets with a diameter of 20
mm and a thickness of 15 mm, and subsequently
subjected to CIP Žcold isostatic press. under a pressure of 2000 kgrcm2 . An MgO Ž100. seed was
placed on the center of the pellet, which was then
OCMG processed in 0.1% partial pressure of O 2
with a gas flow rate of about 300 mlrmin. The final
structure is a single domain, even though we did not
apply any temperature gradient. The samples could
be grown into a single domain, showing that the
growth of large grain bulk superconductors is possible with this system, that is, the mixture of three
different LRE elements did not inhibit the grain
growth of LRE-123 system, which is critically important for the future application of bulk samples.
234
M. Muralidhar et al.r Physica C 313 (1999) 232–240
The details of the heat treatment schedules can be
found elsewhere w19x. Rectangular samples with similar dimensions a = b = c f 1.5 = 1.5 = 0.5 mm3
were cut from as-grown crystals in order to enable a
direct comparison of the magnetic data. The oxygen
annealing was performed on these small rectangles
in flowing oxygen gas in the temperature range of
300–6008C w19x.
Microstructural features of the samples were observed with an optical polarization microscope, SEM,
and a TEM. The size and volume fraction of the
NEG-211 second phase trapped in the NEG matrix
were calculated from the SEM micrographs using an
image processing system. The average size of the
NEG-211 particles was determined assuming that the
particles are spherical in shape. EDX analyses were
also performed to analyze the chemical composition
of the matrix and the second phases.
Measurements of the superconducting transition
temperature, Tc , were performed with a commercial
SQUID magnetometer ŽQuantum Design, model
MPMS-7. in an applied magnetic field of 1 mT.
Magnetization loops ŽMHLs. are measured at T s 77
K in fields up to 7 T applied parallel to the c-axis.
The Jc values were calculated based on the extended
Bean critical state model Jc s Ž20D M .rw a 1 y
arŽ3b .4x, where D M is the magnetization hysteresis
during the increasing and decreasing external field
processes, a and b Ž a - b . are the dimensions of the
sample cross-section perpendicular to the applied
magnetic field.
3. Results and discussion
3.1. Microstructural analysis
Now, we will discuss the addition of second-phase
particles ŽNEG-211. in order to further increase the
flux pinning and hence, the critical current densities
in the NEG samples.
In Fig. 1a–c, we present SEM micrographs of
NEG samples with addition of 20, 30, and 40 mol%
of NEG-211 second phase. All the samples were
melt processed in 0.1% O 2 atmosphere. The size of
the 211 particles dispersed in the NEG matrix lies in
the range of 1–20 mm, with even larger particles
being present. This result is comparable to YBCO
Fig. 1. Scanning electron micrographs of 211 second phase in
OCMG processed samples fabricated without Pt addition: Ža.
ŽNd,Eu,Gd.Ba 2 Cu 3 O y q20 mol% of 211; Žb. ŽNd,Eu,Gd.Ba 2 Cu 3 O y q30 mol% of 211; and ŽNd,Eu,Gd.Ba 2 Cu 3 O y q40 mol%
of 211.
and NdBCO samples without the refinement by addition of Pt.
M. Muralidhar et al.r Physica C 313 (1999) 232–240
In contrast to Fig. 1, Fig. 2a–c shows SEM
micrographs of the NEG samples which contain
Fig. 2. Scanning electron micrographs of 211 second phase in
OCMG processed samples fabricated with Pt addition: Ža.
ŽNd,Eu,Gd.Ba 2 Cu 3 O y q20 mol% of 211; Žb. ŽNd,Eu,Gd.Ba 2 Cu 3 O y q30 mol% of 211; and ŽNd,Eu,Gd.Ba 2 Cu 3 O y q40 mol%
of 211.
235
about 20, 30 and 40 mol% of NEG-211 second phase
with 0.5 mol% Pt addition. All the samples were
melt-processed in 0.1% O 2 atmosphere. From the
figures, it is evident that the NEG-211 second-phase
particles are finely distributed in the NEG matrix and
their average size is smaller than 1 mm. To further
highlight these observations, we calculated the volume fraction of the second phase trapped in the NEG
matrix from SEM micrographs using an imageprocessing system. The results are presented in Fig.
3a–c. The average diameter of the NEG-211 particles for samples 20, 30 and 40 mol% is 0.786, 0.08
and 0.140 mm, respectively. It is evident that the Pt
addition is very effective in reducing the size of the
NEG-211 particles as in the case of the YBCO
system w23–25x.
Fig. 4a shows a TEM image of an NEG sample
with 40 mol% NEG-211 and addition of 0.5% Pt.
One can observe relatively large LRE-211 particles
about 1 mm in diameter and some tiny LRE-211
particles smaller than 0.1 mm. EDX analysis showed
that large LRE-211 inclusions contain Nd, Eu, and
Gd in an even ratio, which is identical to the nominal
composition of the precursor powder, i.e., NEG-211.
It is then interesting to note that the very small
LRE-211 particles mainly consist of Gd in the rare
earth site. Therefore, the extremely fine LRE-211
particles in our NEG samples are mostly Gd-211,
which must have been produced during the melt
processing. To allow for a comparison with former
results on melt-processed YBCO, Fig. 4b presents a
TEM image of a melt-processed YBCO sample with
an addition of 40 mol% 211 and 0.5 wt.% Pt. The
211 particle size is also here of the order of 1 mm.
The volume fraction of the 211 phase trapped in the
NEG matrix was calculated from SEM micrographs
w26x. This demonstrates that Pt addition is very effective in reducing the size of the NEG-211 particles
like the case of the YBCO system.
In the partially melted region, there are two different kinds of LRE-211 inclusions: one added as the
initial powder and the other produced by the peritectic decomposition of LRE-123. Since the decomposition temperature of Gd-123 is the lowest among the
four rare earth elements w12x, Gd-211 has the chance
to nucleate at relatively low temperatures, which
may lead to small second-phase particle size. As the
formation temperature of Gd-123 is lowest, the small
236
M. Muralidhar et al.r Physica C 313 (1999) 232–240
Gd-211 particles may not be consumed for the growth
of LRE-123 and so have the chance to survive.
Although further study is necessary to clarify why
Fig. 4. Ža. Transmission electron micrograph of melt-processed
ŽNd,Eu,Gd.Ba 2 Cu 3 O y with 40 mol% NEG-211 and 0.5 mol% Pt.
Note that the very fine LRE-211 inclusions are dispersed in the
matrix, which mainly comprises Gd in the rare earth site. Žb.
Transmission electron micrograph of melt-processed YBCO with
40 mol% Y-211 and 0.5 mol% Pt.
only the size of Gd-211 is extremely small, it definitely provides a method to enhance flux pinning in
LRE-Ba 2 Cu 3 O y with mixed LRE elements.
3.2. Magnetic characterization
Fig. 3. Size distribution of NEG-211 second phase in OCMGprocessed samples fabricated with Pt addition: Ža. ŽNd,Eu,Gd.Ba 2 Cu 3 O y q20 mol% NEG-211; Žb. ŽNd,Eu,Gd.Ba 2 Cu 3 O y q30
mol% NEG-211; and ŽNd,Eu,Gd.Ba 2 Cu 3 O y q40 mol% NEG-211.
Fig. 5 represents the temperature dependence of
the DC-susceptibility for the NEG samples with
different amounts of NEG-211 in zero-field-cooled
M. Muralidhar et al.r Physica C 313 (1999) 232–240
237
formed AC susceptibility measurements on the oxygenated samples. The samples also exhibit onset Tc
of 93.4 K with a sharp superconducting transition
Fig. 5. Temperature dependence of the normalized susceptibility
for OCMG-processed ŽNd,Eu,Gd.Ba 2 Cu 3 O y superconductors with
different volume fractions of the 211 phase.
ŽZFC. and field-cooled ŽFC. processes in the presence of a magnetic field of 1 mT. All the samples
show an onset Tc of about 93.2 K and a superconducting transition width of 1 K. We have also per-
Fig. 6. Magnetization hysteresis loops measured at T s 77 K, and
the external magnetic field Ha applied parallel to the c-axis of the
sample for ŽNd,Eu,Gd.Ba 2 Cu 3 O y superconductors with different
volume fractions of the LRE-211 phase.
238
M. Muralidhar et al.r Physica C 313 (1999) 232–240
Ž DT s 1 K. similar to DC magnetic susceptibility.
Such a sharp superconducting transition is almost
comparable to other LRE-Ba 2 Cu 3 O y melt processed
under low oxygen partial pressures.
Fig. 6 shows magnetization loops of the NEG
samples with different amounts of NEG-211 measured at T s 77 K. The external magnetic field is
always applied parallel to the c-axis of the samples.
All samples studied were found to exhibit a pronounced secondary peak effect. Note the position of
the secondary peak, which is considerably larger
than in most YBCO samples, and similar to that of
NdBCO w27x. This secondary peak effect has always
been observed in LRE-Ba 2 Cu 3 O y melt-processed
under reduced oxygen atmosphere, and is ascribed to
dTc-pinning provided by LRE-rich 123 clusters dispersed in the matrix w7,22x. The fact that our NEG
samples exhibit a similar peak effect suggests that
such LRE-rich 123 clusters are also dispersed in the
NEG compound as discussed in Ref. w22x. The combination of three LRE elements in a sample leads
evidently to a very uniform distribution of the LRErich clusters, which in turn enhances the flux pinning
due to a constant pinning wavelength w22,28x. Therefore, the formation of an homogeneous NEG-123
matrix is essential for the high Jc values exhibited
by the ternary compounds at elevated temperatures,
i.e., 77 K. Any disturbance of the matrix, e.g., by the
addition of another LRE element or by excess 211
additions, will lead to a degradation of the high peak
Jc .
The Jc Ž Ha . curves calculated from the MHLs of
Fig. 6 are displayed in Fig. 7. The maximum Jc
values for samples 20, 30 and 40 mol% are 46 000,
54 000 and 68 000 Arcm2 at the respective peak
field, h 0 , of 2.4, 2.2 and 2.5 T Ž Ha 5 c, at 77 K..
68 000 Arcm2 is the highest Jc value ever reported
in the literature at 2.5 T for fields parallel to the
c-axis in melt-processed RE–Ba–Cu–O bulk superconductors, which emphasizes that the ternary LRE
compounds are very promising superconductors for
bulk applications.
This is illustrated in Fig. 8, which gives a compilation of typical data of melt-processed YBCO Žwith
addition of 211 and Pt. w29x, NdBCO prepared by the
OCMG method Žwith Nd-422 particles. w27,30,31x,
the ternary compound ŽNd,Eu,Gd.Ba 2 Cu 3 O y ŽNEG.
with a ratio of Nd:Eu;Gds 1:1:1 w19x and our pre-
Fig. 7. Field dependence of the critical current density ŽT s 77 K,
Ha parallel to the c-axis. for ŽNd,Eu,Gd.Ba 2 Cu 3 O y superconductors with different volume fractions of the NEG-211 phase.
sent NEG sample with addition of 40 mol% NEG-211
and 0.5 mol% Pt. Characteristically, the meltprocessed YBCO sample does not exhibit the peak
effect, only the shape of the Jc Ž Ha . curve is some-
M. Muralidhar et al.r Physica C 313 (1999) 232–240
239
Fig. 8. Comparison of the field dependence of the critical current density ŽT s 77 K, Ha parallel to the c-axis. for melt-processed YBCO,
OCMG-processed NdBCO, ŽNd,Eu,Gd.Ba 2 Cu 3 O y , and ŽNd,Eu,Gd.Ba 2 Cu 3 O y with addition of 40 mol% NEG-211.
what deformed at intermediate fields. OCMGNdBCO presents a well-developed peak effect, which
is due to the distribution of Nd-rich phase in the
sample, providing the dTc-pinning. The Jc Ž Ha . curve
also reflects that the Nd-422 particles are still too
large Žsize f 3 mm. to act as effective pinning sites
w32x.
Let us now discuss the shape of the Jc Ž Ha . curves
of the NEG samples. The samples with 30 and 40
mol% of second phase showed a peak Jc value
similar to that of Jc in zero field. Note the considerable change of shape of the Jc Ž Ha . curves as a
function of LRE-211 content. The pure NEG samples Žas studied in Ref. w22x. and the present samples
with small amounts of LRE-211 additions have a
peak Jc Ž h 0 . being larger than that at zero field,
Jc Ž0.. In contrast, NEG q 30 mol% NEG-211 shows
similar values, and for NEG q 40 mol% NEG-211,
the zero-field Jc is larger than that at the secondary
peak. However, the overall Jc increases continuously with increasing NEG-211 content. Pinning provided by normal inclusions is mainly expected to be
active at low temperatures andror low fields. Therefore, the NEG-211 additions increase the overall Jc
Ž‘background’ Jc . as well as the Jc at intermediate
fields. Furthermore, as compared to samples with 20
mol% NEG-211 additions, the irreversibility field is
shifted slightly towards higher fields. Note that the
peak position, h 0 , is practically unaffected by the
NEG-211 content, as long as the NEG-211 content is
40 mol% or less. This indicates clearly that the main
mechanism for the peak formation is different from
the pinning provided by the normal inclusions. The
flux pinning in the present NEG samples with addition of LRE-211 is definitely caused by two different
main sources, thus the scaling of pinning forces as
discussed in Refs. w5–8,22x is apparently not working
in a wide temperature range. This topic will be
discussed in detail in a forthcoming publication w33x.
4. Conclusions
ŽNd,Eu,Gd. –Ba–Cu–O melt processed under
oxygen partial pressure of 0.1% exhibits a high Jc
value of 68 000 Arcm2 at 77 K and 2.5 T for Ha
applied parallel to the c-axis of the samples. These
values could be reproduced successfully in several
experiments. Further work will consider the control
of the initial particle size; this will be addressed in a
separate paper w34x. Microstructural observations
240
M. Muralidhar et al.r Physica C 313 (1999) 232–240
along with compositional analysis showed that the
addition of Pt is effective in refining the LRE-211
size. By means of TEM investigations, we have
found that extremely fine NEG-211 particles contain
only Gd on the rare earth site. These particles provide, therefore, effective flux pinning centers. The
samples exhibit a pronounced secondary peak effect
in the magnetization loops. The shape of the
Jc Ž Ha .-curves is found to change with increasing
NEG-211 content; but the position of the secondary
peak is practically unaffected by the NEG-211 content. This clearly demonstrates that the secondary
peak effect is due to a different pinning mechanism,
i.e., the dTc-pinning is also active in the NEG-system. The NEG-211 particles are mainly effective at
low fields, thus filling the gap between central peak
and secondary peak, which is very important for bulk
applications.
Acknowledgements
This work was partially supported by NEDO for
the R & D of Indust. Sci. and Tech. Frontier program.
MMD is thankful to the Iwate Techno Foundation,
Iwate, Japan for providing the financial assistance,
and MRK thanks the Japanese Science and Technology Agency ŽSTA. for providing the fellowship.
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