T H. Kojo , S.I. Yoo , M. Murakami

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Physica C 289 Ž1997. 85–88
Melt processing of high-Tc Nd–Ba–Cu–O superconductors in air
H. Kojo a , S.I. Yoo
a
a,b
, M. Murakami
a,)
SuperconductiÕity Research Laboratory, International SuperconductiÕity Technology Center, 1-16-25, Shibaura, Minato-ku, Tokyo 105,
Japan
b
Railway Technical Research Institute, 2-3-38 Hikari-cho, Kokubunji-shi, Tokyo 185, Japan
Received 16 May 1997; accepted 30 June 1997
Abstract
We have succeeded in fabricating Nd–Ba–Cu–O bulk superconductors, which exhibit high Tc and a sharp superconducting transition, by the melt process in air without additional high temperature annealing in a reduced oxygen atmosphere. The
key to this achievement is the use of Ba-rich Nd 4y2 z Ba 2q2 z Cu 2yzO10y d as a precursor. Fabricated bulk samples exhibit
onset Tc values of 94–95 K and the secondary peak effect in the M–H loop like the case of Nd–Ba–Cu–O superconductors
produced by melt processing in a reduced oxygen atmosphere. The peak position and the irreversibility field for the best
sample were about 1.2 T and 5 T at 77.3 K, respectively, and the estimated Jc at the peak field was about 2.3 = 10 4
A cmy2 . q 1997 Elsevier Science B.V.
Keywords: Superconductor; Melt process; Nd–Ba–Cu–O; Critical current density; Solid solution; Phase diagram
1. Introduction
Unlike YBa 2 Cu 3 O y , which forms only a stoichiometric compound, large rare earth elements ŽRE: La,
Nd, Sm, Eu and Gd. are known to form a
RE 1q x Ba 2yx Cu 3 O 7q d solid solution ŽRE123ss. because the ionic radius of RE 3q is close to that of
Ba2q. The superconducting transition temperature
ŽTc . is depressed when RE 3q substitutes on the Ba2q
site, since the carrier concentration is decreased.
Owing to the presence of the RE–Ba substitution,
REBa 2 Cu 3 O 7y d ŽRE123. samples melt-processed in
air exhibit low Tc with a broad transition. Such
substitution can largely be suppressed by employing
low oxygen partial pressure pO 2 during the solidifi)
Corresponding author. Tel.: q81 3 34549284; Fax: q81 3
34549287; E-mail: [email protected]
cation of the RE123 phase, which is the oxygen-controlled-melt-growth ŽOCMG. process w1–3x.
OCMG-processed RE–Ba–Cu–O ŽRE: Nd, Sm,
Eu, Gd. superconductors exhibit high Tc and a sharp
transition. Furthermore, they are very promising for
practical applications since the critical current density Jc and the irreversibility field were higher than
those of melt-processed Y–Ba–Cu–O. The OCMG
process, however, has the disadvantage that pO 2
must be controlled during the process, which adds to
the cost and will require a complicated system to
seed the crystal for the fabrication of large grain
samples. To avoid these problems melt processing in
air has greatly been favoured.
Recently, Obradors et al. w4x and Hu et al. w5x have
reported that Nd–Ba–Cu–O superconductors meltprocessed in air exhibit high Tc of 95–96 K. It
should be noticed, however, that in both cases addi-
0921-4534r97r$17.00 q 1997 Elsevier Science B.V. All rights reserved.
PII S 0 9 2 1 - 4 5 3 4 Ž 9 7 . 0 1 5 7 2 - 4
H. Kojo et al.r Physica C 289 (1997) 85–88
86
tional Ar annealing at high temperatures above 9008C
is indispensable to the achievement of high Tc . We
suppose that during such treatment RE-rich phase
with depressed Tc will decompose into Nd 4 Ba 2Cu 2 O 10 ŽNd422. plus the liquid phase, and then the
phase with enhanced Tc solidifies from Nd422 and
the liquid, leading to the recovery of high Tc .
In the present paper, we show that Nd–Ba–Cu–O
superconductors exhibiting Tc of 94–95 K and a
sharp transition can be fabricated by the melt process
in air without additional high temperature annealing
in low pO 2 .
2. Experimental
Four batches having different nominal compositions were prepared using precursors of NdBa 2Cu 3 O 7y d , Nd 0.9 Ba 2.1Cu 3 O 7y d , Nd 4 Ba 2 Cu 2 O 10 and
Nd 3.6 Ba 2.4 Cu 1.8 O 10y d , which were independently
prepared by the solid state reaction. Nominal compositions for the samples are shown in Table 1. Chemical compositions of the Nd422 and Nd 3.6 Ba 2.4Cu 1.8 O 10y d , which were analyzed by inductively
coupled plasma ŽICP. spectrometry, are also displayed in the table. The results of powder X-ray
diffraction ŽXRD. analyses for the precursor materials suggested that all the precursors were phase-pure
at least on the X-ray level except Nd 0.9 Ba 2.1Cu 3 O 7y d
which contained a small amount of BaCuO 2 phase.
Although the solubility limit of Ba in Nd422ss was
reported to be z s 0.1 y 0.15 w6,7x, an XRD pattern
for Nd 3.6 Ba 2.4 Cu 1.8 O10y d indicates that a single
phase is formed even when z s 0.2, suggesting that
the solubility limit is higher than the reported values.
Well-mixed precursor powders were uni-axially
pressed into pellets of 20 mm diameter, and subjected to cold-isostatic pressing under a pressure of
200 kgrcm3. For the melt growth in air, the samples
were placed on yttria-stabilized zirconia plates and
ramped to 11208C in 3 h, held for 30 min, cooled to
10808C at a rate of 108Crmin, slowly cooled to
10108C at a rate of 18Crh, and then furnace cooled.
Melt-grown samples were composed of many textured Nd–Ba–Cu–O grains of 5–10 mm diameter.
Small samples with a rectangular cross section of
1.5 = 2.0 mm2 and 0.5–0.8 mm thickness were cut
from textured domains, and then annealed in flowing
pure O 2 gas at a rate of 300 mlrmin with the
following temperature schedule: heated to 5008C in 3
h, held for 1 h, cooled to 3008C in 25 h, held for 200
h and finally furnace-cooled.
DC magnetization measurements were performed
with a Quantum design MPMS SQUID magnetometer. A zero-field-cooled warming procedure was used
for the Tc measurement with the applied field of 10
Oe. The magnetization hysteresis loops were also
obtained with the SQUID magnetometer for fields
parallel to the c-axis. The Jc was estimated using
the extended Bean model w8x.
3. Results and discussion
Fig. 1 shows the temperature dependence of magnetization for the samples. The sample D, synthesized using stoichiometric Nd422 powder as the
precursor, exhibits onset Tc of 90 K with a broad
transition width of 10 K. In contrast, the samples A
to C, synthesized using Ba-rich Nd422ss as the
precursor, exhibit relatively high onset Tc values of
94–95 K with the transition width of 4–7 K. It is
Table 1
Nominal compositions for batches A to D and chemical compositions of Nd 3.6 Ba 2.4 Cu 1.8 O10y d and Nd 4 Ba 2 Cu 2 O10 determined by ICP
analyses
Batch
Nominal composition
Nd:Ba:Cu ratio in Nd422 phase
A
B
C
D
Nd 0.9 Ba 2.1Cu 3 O 7y d q 10 mol% Nd 3.6 Ba 2.4 Cu 1.8 O10y d
Nd 0.9 Ba 2.1Cu 3 O 7y d q 20 mol% Nd 3.6 Ba 2.4 Cu 1.8 O10y d
NdBa 2 Cu 3 O 7y d q 20 mol% Nd 3.6 Ba 2.4 Cu 1.8 O10y d
NdBa 2 Cu 3 O 7y d q 20 mol% Nd 4 Ba 2 Cu 2 O10
Nd:Ba:Cus 3.66:2.39:1.75
Nd:Ba:Cus 3.66:2.39:1.75
Nd:Ba:Cus 3.66:2.39:1.75
Nd:Ba:Cus 4.00:2.04:1.96
H. Kojo et al.r Physica C 289 (1997) 85–88
Fig. 1. Temperature dependence of magnetic susceptibility for the
batches A to D. The susceptibility was normalized with the value
at 10 K.
clear that the Tc and transition were greatly improved
when the Ba-rich Nd422ss was used, although the
transition is still broad compared to those of
OCMG-processed Nd–Ba–Cu–O samples w2,3x. It
should also be noted that even batch D shows muchimproved superconducting properties compared with
the previous result w2,3x. It is probable that the
formation of Nd-rich Nd123ss could be suppressed
in the present experiment, since we used separately
sintered Nd123 and Nd422 powders.
The formation of high-Tc phase in air can be
explained in terms of phase compatibility. The composition x of Nd 1q x Ba 2yx Cu 3 O 7q d , which is solidified through the peritectic reaction of Nd422ss and
the liquid phase, is determined by the liquid phase
composition which is compatible with the
Nd 4y 2 z Ba 2q2 z Cu 2yzO 10y d at just above the peritectic temperature. As shown in Fig. 2, which is the
schematic subsolidus NdO1.5 –BaO–CuO ternary
phase diagram in air, the tie line of high-Tc Nd123
phase is connected to the Nd422ss, in which Ba
content lies at the solubility limit. Therefore, the
composition of the liquid phase which is compatible
with high-Tc Nd123 phase should also be compatible
with Ba-rich Nd422ss, although the exact number of
z could not be determined in the present experiment.
It is also probable that the presence of Ba-rich
Nd422ss in the liquid will maintain Ba-rich liquid
component, which also favours the formation of
high-Tc Nd123 with suppressed Nd–Ba substitution.
87
We confirmed that Tc of 94–95 K could also be
obtained using Nd 3.8 Ba 2.2 Cu 1.9 O 10y d instead of
Nd 3.6 Ba 2.4 Cu 1.8 O 10y d , but the transition was
broader. This suggests that the excess Ba in the solid
solution of z s 0.1 is not sufficient to achieve the
preferential formation of high-Tc phase. It might also
be possible to simply prepare the Ba-rich liquid
phase, however, for the melt-textured process, the
reaction of the liquid phase with the substrate or the
crucible will easily shift the liquid composition toward Ba-poor direction. It is thus concluded that the
present success in growing high-Tc Nd123 phase in
air is ascribed to the use of Ba-rich solid phase, that
is, Ba-rich Nd422ss.
Fig. 3 shows Jc –B curves of the samples A to D
measured at 77.3 K for fields parallel to the c-axis.
For comparison, the Jc –B curve of a Nd–Ba–Cu–O
bulk with 20 mol% excess Nd422, which was
OCMG-processed in 1% O 2 , is also displayed. While
the absolute values of Jc for the samples A to C are
still lower than that of OCMG-processed sample,
they are much improved compared to the batch D.
The peak effect, which is commonly observed in
OCMG-processed RE123 superconductors is also
observed in batches A to C, suggesting that the
field-induced pinning center, which was first proposed by Murakami et al. w9x and later verified as the
Fig. 2. Schematic subsolidus phase diagram of the NdO1.5 –BaO–
CuO system in air. The range of the solid solution depends on the
temperature and pO 2 and is approximate in this phase diagram.
The tie lines are also schematic. Here it should be noted that
Nd123ss has a solid solution towards the Nd-rich direction, while
Nd422ss towards the Ba-rich direction, and high-Tc Nd123 with
suppressed Nd–Ba substitution is compatible with Nd422ss in
which the Ba content lies on the solubility limit.
88
H. Kojo et al.r Physica C 289 (1997) 85–88
Acknowledgements
The authors would like to thank Toshima Co. Ltd
for the preparation of sintered powders and DOWA
Mining for compositional analysis with ICP. This
work was partially supported by NEDO for the
R & D Industrial Science and Technology Frontier
Program.
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Fig. 3. Magnetic field dependence of the critical current density
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4. Conclusions
The utilization of Ba-rich Nd 4y 2 z-Ba 2q 2 zCu 2y zO10y d solid solution Ži.e. large z . as the precursor is the key to growing high-Tc phase of
Nd 1q x Ba 2yx Cu 3 O 7q d solid solution with small x
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be much simplified. Further improvement in both Tc
and Jc is expected by the optimization of the processing conditions.
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