Thermal and phase characterization of Bi-2223 superconductors

Article abstract of DOI:10.1016/S0921-4526(02)00859-1

Advances in processing and fabrication of high critical current density (J_c) long length conductors, silver sheathed superconducting magnets and coils from silver sheathed Bi-2223 tapes, pancake coil magnets from 1 to 10m length and mono-multif

Full text of DOI:10.1016/S0921-4526(02)00859-1

Physica B 321 (2002) 257–264
Thermal and phase characterization of Bi-2223
superconductors
M.N. Khana,*, M. Khizara, B.N. Mukashevb
a GIK Institute of Engineering Science and Technology, Topi, NWFP, 23460 Swabi, Pakistan
b Institute of Physics and Technology, Ministry of Sciences, Academy of Sciences, Republic of Kazakhstan
Abstract
Advances in processing and fabrication of high critical current density ðJcÞ long length conductors, silver sheathed
superconducting magnets and coils from silver sheathed Bi-2223 tapes, pancake coil magnets from 1 to 10 m length and
mono-multifilament Bi-2223/2212 high critical temperature superconductors by powder in tube technique continue to
bring these materials closer to commercial applications. In our laboratory Bi-2223 conductors doped with Sm, Nb, Ag
and Gd were prepared by the heat treatment of rapidly quenched glass precursors. Activation energies and frequency
factors, employing different models were evaluated. It was observed that both peritectic transition and reaction rates
were dependent on ambient atmosphere. The resistivity measurements revealed that the critical temperature decreases
with increasing Gd, Sm, Nb, and Ag concentration. The critical current density Jc measured at 77 K from I2V data
shows an increase with silver addition. The critical current densities of silver sheathed tapes using the powder in tube
were found to be in the range 7 ꢀ 108 A/m2. X-ray diffraction results showed that the volume fraction of the high-Tc
(2223) phase decreases and that of low-Tc (2212) phase increases with increase of these rare-earth ion contents. These
results are explained on the basis of possible variation of hole concentration with trivalent rare-earth ion substitution
and also by considering the magnetic nature of the substituted ion in the composition.
r 2002 Elsevier Science B.V. All rights reserved.
Keywords: Phase characterization; Critical current density; Silver sheated Bi-2223 tapes; Peritectic transition
1. Introduction
long lengths of wire, i.e. high Jc at various
magnetic fields, is required. Critical current
densities (Jc’s) of short length of Bi2Sr2Ca1-
Cu2O8+x (Bi-2212) tapes processed on silver
substrates by partial-melt slow cooling (PMSC)
One of the most important applications of high-
Tc superconductors is in the form of super-
conducting wires. Wires may be used for power
transmission, electrical magnets for MRI, scientific
equipment, particle accelerators and mineral se-
parators. In these cases, conventional designs
indicate that meter-lengths of wires are required
for viable applications. Thus, good performance of
have
reached
sufficiently
high
values
(B5.9 ꢀ 105 A/cm at 4.2 K and zero applied field)
[1] for practical applications. Recently, Bi-2212/
Ag/Bi-2212 double-sided uncovered (DSU) tap
sections 1.5 m in length were prepared by dip-
coating and PMSC with long-length transport
critical current densities (Jc’s) reduced to 25% of
short-length values [2]. A disadvantage of the DSU
*Corresponding author.
E-mail address: mnkhan@giki.edu.pk (M.N. Khan).
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 0 8 5 9 - 1

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M.N. Khan et al. / Physica B 321 (2002) 257–264
tape configuration is that the oxide layers break
significantly more under bending strain than other
tape configurations such as powder in tube.
Alternatively, powder-in-tube (PIT) methodology
has been used to process tape sections up to 300 m
in length for the (Bi,Pb)2Sr2Ca2Cu3O10+x/Ag
system. Fabrication of high Jc long-length con-
ductors, their various micro structural and proces-
sing problems and some alternative approaches
are described in the literature [3,4].
Discovery of cuprate HTS with transition
temperatures over 130 K has created opportunities
for many microwave and antenna applications and
holds out promise of future digital application [4].
A major recent development towards the commer-
cialization of high-Tc superconductors is the use of
these materials for viable superconducting motors.
US Department of Energy and American Super-
conductor Corporation have already demon-
strated a 149 kW motor [5] operating at 1800 rpm
with the coils at a temperature of 27 K. For power
cables, only rolled multifilamentary power-in-tube
(Bi,Pb)2Sr2Ca2Cu3Ox(Bi-2223) tape currently ap-
pears practicable. The main limitation with Bi-
2223 is its anisotropy and difficulty in reproducing
high critical current densities ðJcÞ obtained on
some very short, pressed samples. This is due to
the granular nature of Bi-2223 and short coherence
lengths in HTS.
We have made a wide range of Jc studies [6,7]
for Ba2Cu3O7 and Bi2Sr2Ca2Cu3O10 (BSCCO) in
both wire and bulk forms. Although high Jc’s have
been obtained for Bi2Sr2Ca2Cu3O10 by a PIT
method, there is no clear evidence for effective
pinning centers in the BSCCO. On the other hand,
it has been pointed out [8] that Y2BaCuO5 can
provide pinning centers and/or can be introduced
by specific processing method. Moreover, studies
have revealed that Yba2Cu3O7 has intrinsically
superior characteristics for pinning centers than
that of Bi- and Ti-based superconductors [9].
In this paper, we have summarized the progress
made in improving the manufacturability of these
brittle materials and report out results of various
ion substitutions for Ca in Bi1.7Pb0.3Sr2Ca2ꢁxRx
Cu3Oy (where R=Nb, Eu, Yb and Ag) on
superconducting, structural, electrical and kinetic
parameters.
2. Experimental
A solid-state reaction scheme was chosen for the
powder synthesis. PbO, Bi2O3, SrCo3 and CuO
were prepared by decomposition of metal nitrate
solution in the cation ratio Bi1.72Pb0.3Sr2-
Ca2ꢁxNbxCu3Oy. The powder was calcined at
8201C for 10 h, uniaxially pressed into pellets,
and sintered at 8401C for different times in order
to alter the phase assemblage. It was found that
nitrate solution route provided highly reactive,
homogenous and carbonate-free powders from
which the 2223 phase can be formed effectively. It
should be, however, pointed out that at this stage
the powders contain a small fraction of the 2223
and the 2212 is the major phase. This incompletely
reacted precursor will facilitate the liquid phase
formation in the subsequent processes. The
average particle size was 2–3 mm. The pulverized
gray powder was melted in a covered alumina
crucible in an electric furnace at 12501C for 0.5 h.
The crucible was removed from the furnace and
melt was cast quickly on to a steel plate (30 mm
thick by 14 cm diameter) and sheets of glass
B1 mm thick. From the same crucible the melts
were pumped up into fused silica tubes (E2 mm
diameter and 30 cm long) resulting in the forma-
tion of the crystal-free glassy-rod specimens. The
inner glass rods were removed mechanically from
the outer silica tube for the subsequent experiment.
For the heat treatment the glassy rods were cut
into 20-mm long pieces. The samples were placed
on an alumina plate and re-heated in air at a
heating rate of 21C/min to a given temperature
(840–8551C) at which they were held for various
times so as to be crystallized and then cooled to
room temperature in the furnace.
Fabrication of long-length PIT Bi2Sr2Ca-
Cu2O8+x/Ag tapes is described elsewhere [10,11].
Differential thermal analysis (DTA) was carried
out in various atmospheres at different heating
rates using a Perkins Elmer DSC-7. The kinetics of
crystallization of the samples were investigated
from non-isothermal DTA using a Perkin Elmer
DTA-7 at a heating rate of 5–201C/min. Glass
samples were annealed in air for various lengths of
time at temperatures selected from the DTA
results. Crystalline phases formed in the annealed

M.N. Khan et al. / Physica B 321 (2002) 257–264
259
specimens were identified from the powder XRD
patterns, recorded at room temperature. The
electrical resistance was measured from 77 to
160 K by the standard four-probe configuration
with 1.0 mV/cm used as the criteria for Jc
measurements.
Metallic behavior and sharp transitions are
observed at 77 K for 2212 and 99 K for 2223.
Fig. 3 shows the DTA curve of a typical Ag-
sheathed 2212 tape [13]. The endotherm at 8841C
was due to incongruent melting of the 2212 phase.
During cooling, an exotherm was observed be-
tween 8501C and 8651C. This peak, as will be
discussed later, was due to solidification of 2212
phase.
3. Results and discussions
The delay in solidification relative to melting
occurred because nucleation required under cool-
ing to overcome the nucleation energy barrier to
form critical nuclei. Fig. 4 shows X-ray diffraction
patterns of the undoped (with x ¼ 0) samples
annealed at 8501C for 25 h (A) and 240 h (B) in the
air in the 2y range up to 701. The intensity and
peak positions of (0 0 2), (0 0 1 0), (1 1 5), (1 0 9),
(0 0 1 2), (1 1 9), (2 0 0) and (0 0 1 4) reflections are
in good agreement with the values reported in the
literature [6,12,13] for the high Tc 2223 phase. It is
evident from Fig. 4 that the intensity of reflections
3.1. Improvements in the properties of PIT BSCCO
tapes
Advances in the processing and fabrication of
mono and multifilament Bi-2223/2212 high critical
temperature superconductors by the PIT technique
continue to bring these materials closer to
commercial application. Consistently high critical
current densities ðJcÞ >104 A/cm2 have been
achieved along the entire length of a 1260 m long,
35 filament Bi-2223 tape [12].
Fig. 1 shows XRD pattern of bismuth family of
superconductors. Orthorhombic lattice cell para-
meters were calculated and they are presented in
Table 1. The values are in good agreement with the
available literature. Resistivity of bulk samples
were measured using Van der Pauwn four-probe
method and the results are shown in Fig. 2. Closed
cycle He cryostat was used for resistivity measure-
ments down to 15 K for 2212 and 2223 phases.
Table 1
Lattice parameters of orthorhombic phases of bismuth family
of superconductors
(
a (A)
(
b (A)
(
c (A)
System
Tc:zero (K)
2201
2212
2223
5.375
5.230
5.218
5.310
5.412
5.632
24.525
29.180
30.815
—
77
100
Fig. 1. XRD Patterns of Bi-family of superconductors (a) 2223 phase (b) 2212 phase and (c) 2201 phase.

260
M.N. Khan et al. / Physica B 321 (2002) 257–264
corresponding to high-Tc phases increased and
those corresponding to low Tc phases decreased
with increasing sintering time. The ‘c’ value of
(
high-Tc phase is 38 A and that of low-Tc phase is
31 A1. The peaks at 2y ¼ 4:41 and 4.61 correspond
to the (0 0 2) reflections of the high- and low-Tc
phases, respectively.
In the sample sintered for 25 h, distinct peaks at
2y ¼ 4:41 and 4:61 appeared indicating that the
sample consisted of both low- and high-Tc phases.
With increasing sintering time one can notice a
gradual increase in the intensity of the peak at
2y ¼ 4:41 corresponding to (0 0 2) reflection of the
high-Tc phase with a decrease in the intensity of
peak at 2y ¼ 4:61: For the sample sintered for
240 h the low-Tc phase peak at 2y ¼ 4:61 is almost
absent and peaks corresponding to the high-Tc
phase are relatively sharper.
Fig. 2. Resistivity plots for 2212 and 2223 phases.
These results indicate that on heating the glass
Bi2Sr2CaoCu1O8 (2201) phase crystallizes out first
followed by the formation of 80 K Bi2Sr2Ca1-
Cu2O8 (2212) phase at the higher temperature. The
110 K Tc phase is formed at still higher tempera-
ture just below the melting point, probably by
reaction between the low-Tc 2201 and 2212 phases.
Fig. 5 shows XRD patterns for Bi1.7Sr2-
Ca2ꢁxAgxCu3Oy ðx ¼ 0Þ annealed at 8501C for
240 h in air.
We obtained the values of lattice constants
(
(
(
a ¼ 5:60 A, b ¼ 5:20 A and c ¼ 37:75 A. The lattice
constants for the sample annealed for 25 h are
Fig. 3. DTA curve for Ag-sheathed 2212 tape [13].
Fig. 4. X-ray diffraction patterns for Bi1.65Pb0.32AgxSr1.73Ca1.33Cu3.85Oy superconductor with A ¼ 25 h, B ¼ 240 h. The peaks denoted
by (H) and (L) correspond to the high-Tc (2223) and low-Tc (2212) peaks, respectively.

M.N. Khan et al. / Physica B 321 (2002) 257–264
261
Fig. 5. XRD for Bi1.7Pb0.3Sr2Ca2AgxCu2O ðx ¼ 0Þ annealed.
Fig. 7. Annealing time dependence of the current density Jc of
Bi1.7Pb0.3Sr2Ca2ꢁxAgxCu3Oy ðx ¼ 0Þ:
Fig. 6. Temperature dependence of resistance Bi1.68Pb0.32
Ag0.6Sr1.73Ca1.85Cu2.85Oy.
Fig. 8. Temperature dependence of resistance Bi1.7Pb0.3Sr2
Ca2ꢁxAgxCu3Oy.
(
(
(
a ¼ 5:39 A, b ¼ 5:51 A and c ¼ 30:82 A. This con-
firmed that the high-Tc (2223) and low-Tc (2212)
phases differ mainly in the length of c-axis. Fig. 6
shows the temperature dependence of resistance of
Bi1.68Pb0.32Ag0.6Sr1.73Ca1.85Cu2.85Oy. It is evident
that the material exhibits a semiconducting beha-
vior. In fact, the doping of silver in our samples
decreases the Tcð0Þ which varied from 109 to 7 K
with ðx ¼ 0:020:6Þ: Fig. 7 shows the annealing
time dependence of critical current density ðJcÞ at
77 K under zero magnetic fields. From Fig. 7 one
can notice a clear increase in Jc with sintering time.
Fig. 8 shows the temperature dependence of
resistance for Bi1.7Pb0.32Sr2Ca2ꢁxAgxCu3Oy. The
DTA curve (Fig. 9) of the polycrystalline material
showed that the glass transition temperature was
about 4961C and the crystallization temperature
was 5201C. The liquid temperature of the glass is
about 8601C.
(Bi1.6Pb0.4)Sr1.7Ca2.3Cu3Oy glass showing the glass
A
typical DTA scan of

262
M.N. Khan et al. / Physica B 321 (2002) 257–264
was very slow. We note that the endothermic peak
appears at 779.81C in the bulk sample. It is of
particular interest to clarify the origin of this
endothermic peak because the Bi2Sr2Ca2Cu3Ox
phase precipitates and grows above this endother-
mic temperature. It is clear that the reaction
among the Bi2Sr3ꢁxCaxOy ðx ¼ 1Þ CuO and
Bi2Sr2CuOx phases in the interior of the sample
is closely related at around 779.81C. On the basis
of these results, the crystallization mechanism may
be speculated as follows. The 2212 phase first
precipitates out followed by the formation of 2212
phase at higher temperatures. The 110 K Tc phase
is formed at 861.51C just below the melting point,
probably by reaction between the low-Tc 2201 and
2212 phases and the residual calcium and copper
oxides.
Fig. 9. DTA of Bi1.68Pb0.32Sr1.75Ca1.85Cu2.85Oy glasses melted
at 12501C.
The variable heating rate DSC method was used
to evaluate the kinetics of crystallization. The
values of kinetic parameters are determined by
using the kinetic model of Bansal and Doremus
[14] which is expressed by the relation:
In½TP2=aꢂ ¼ In½E=Rꢂ ꢁ In v þ E=RTP;
ð1Þ
where TP is the peak maximum temperature, a the
heating rate, E the activation energy, R the gas
Fig. 10. A typical scan of (Bi1.6Pb0.4)Sr1.7Ca2.3Cu3Oy glass in
argon atmosphere at a heating rate of 101C/min.
transition ðTgÞ and crystallization temperature
ðTxÞ is depicted in Fig. 10.
The Tg and Tx temperatures for this system are
3801C and 4631C, respectively. Several other
exothermic and endothermic peaks are also
observed at temperatures above Tx:
The thermogravimetry ðTgÞ curve in air for the
bulk sample is also shown in Fig. 10. The weight
gain begins around 7001C and the maximum gain
occurs at about 8131C. These results indicate that
the diffusion of oxygen into the interior of the
sample and at temperatures below around 6001C
Fig. 11. DTA curves in N2 atmosphere for niobium-doped
samples recorded at heating rates.

M.N. Khan et al. / Physica B 321 (2002) 257–264
263
from X-ray diffractions, decreases with increasing
dopant concentration. The c lattice parameter is
also found to decrease with increase in dopant
concentration. Thus the volume fraction of the
2212 phase depends on the c lattice parameter.
Our results have revealed that the Ag-sheathed
tapes should be fabricated from the BPSCCO
pellets having high concentration (>50%) of 2223
phase and sintered for short duration at lower
temperature better Jc values. The critical current
densities of silver-sheathed tapes were found to be
in the range of 7 ꢀ 108 Amꢁ2. These results are
explained on the basis of possible variation of hole
concentration with trivalent rare-earth ion sub-
stitution and also by considering the magnetic
nature of the substituted ion in the composition.
Fig. 12. A plot of LnðTP2=aÞ vs ð1=TPÞ for the boron-doped
sample.
Acknowledgements
The support funded by GIK Institute of
Engineering Sciences and Technology is gratefully
acknowledged. The authors would like to thank
Ministry of Sciences of the Republic of Kazakh-
stan and Ministry or Science and Technology of
the Islamic Republic of Pakistan for providing
financial support for this project.
constant and v the frequency factor. The kinetic
parameters (E and v) are related to the reaction
rate constant ðKÞ by an Arrhenius equation:
K ¼ v exp ½ꢁE=RTꢂ:
ð2Þ
Eq. (1) is an extension of Johnson–Mehl–Avremi
isothermal kinetic model for use in non-isothermal
methods. In Eq. (1) it was assumed that the rate of
reaction is maximum compensated DSC. DTA
runs recorded at different heating rates between
51C/min and 201C/min in an N atmosphere are
shown in Fig. 11. A plot of In½TP2=aꢂ vs. ½1000=TPꢂ
for crystallization of glass was linear as shown in
Fig. 12, verifying applicability of the kinetic
model. From linear-square fitting, values of the
kinetic parameters were calculated.
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