Supercond. Sci. Technol. 12 (1999) 376–381. Printed in the UK
PII: S0953-2048(99)02047-3
Microstructure, texture and critical
current of Ag-sheathed 2223
multifilament tapes
Y L Liu†, W G Wang‡, H F Poulsen† and P Vase‡
† Materials Research Department, Risø National Laboratory, PO Box 49, DK-4000 Roskilde,
Denmark
‡ Nordic Superconductor Technologies A/S, Priorparken 878, DK-2605 Brøndby, Denmark
Received 22 February 1999
Abstract. An Ag-sheathed 2223 multifilament tape was produced by the powder-in-tube
method. The various parts of the tape were heat treated at different temperatures under
reduced oxygen partial pressure. The microstructure and the texture were characterized by
synchrotron x-ray diffraction and SEM and correlated with Jc . In the low temperature range
(<826 ◦ C), the 2223 fraction and the c-axis alignment of 2223 grains increased with
increasing temperature. A significant increase of Jc (from 1 to 41 kA cm−2 ) was observed in
this range, indicating that the phase purity and the texture were the major controlling factors.
In the medium temperature range (826–830 ◦ C), the 2223 fraction and the grain alignment
tend to saturate, and Jc remains nearly constant at a level of 40 kA cm−2 . In the high
temperature range (830–836 ◦ C), the 2223 fraction and the grain alignment remained
unchanged but Jc decreased with increasing temperature. The drop in Jc was related to the
presence of an amorphous phase and a small amount of 2201 phase, indicating that the grain
connectivity has become the major current-limiting factor. The variations in the filament
shape, density and alignment within the multifilament tape were characterized. The influence
of the inhomogeneous structure on Jc is discussed.
1. Introduction
Since the discovery of high-temperature superconductivity in
1986–1987, steady progress has been attained in the research
and development of superconducting tapes. The critical
current density Jc of short (Bi, Pb)2 Sr2 Ca2 Cu3 (2223) tapes
has reached 70.5 kA cm−2 [1]. Multifilament tapes have been
industrially produced up to a length of 1250 m and with Jc
of 23.3 kA cm−2 [2]. However, the critical current density
obtained at present is still two orders magnitude lower than
that of 2223 thin film [3]. This implies that a substantial
increase in the critical current density of the tape would be
possible if the microstructure and the texture of the tapes
could be improved.
A great deal of work on optimizing the powder-intube (PIT) process with respect to Jc has been reported
[4, 5]. The optimization has mainly been carried out on
single-filament tapes with the focus on the relationship
between Jc and the processing parameters, such as the
deformation, the heat treatment temperature and the oxygen
partial pressure. The correlation between the processing
parameters and the resulting microstructure and texture is
only partially understood. The purpose of this work is
therefore to investigate the dependence of the microstructure
and texture on the heat treatment process of a multifilament
tape and to correlate the results with Jc . The variable is
the operation temperature, whereas the deformation route
0953-2048/99/060376+06$19.50
© 1999 IOP Publishing Ltd
and the controlling atmosphere are fixed. Synchrotron
x-ray diffraction and SEM are applied to characterize the
microstructure and the texture at the end of the heat treatment.
Synchrotron x-ray diffraction is an accurate technique for
quantifying the average bulk phase concentration and the
grain alignment of the Bi- and Pb-rich phases. SEM is, on the
other hand, a technique adequate for characterizing structures
such as the structural density and the grain alignment as well
as phase composition and distribution on a local scale.
2. Experimental details
An Ag-sheathed multifilament tape (with 19 filaments) was
produced by the PIT method. The composition of the
precursor powder was Bi 1.84, Pb 0.34, Sr 1.91, Ca 1.96
and Cu 3.06. The Ag:oxide ratio was 4–4.2:1 and the
final dimension of the tape was 2.8 × 0.16 mm2 . The
technical details of the PIT processing can be found in
previous publications [6]. To investigate the effect of the heat
treatment temperature, 20 short tapes (∼5 cm in length) were
cut from the same green tape and sintered in a temperature
gradient furnace [7] under reduced oxygen partial pressure.
The studied temperature range was 810–840 ◦ C and the
duration of the heat treatment was 150 h. After the heat
treatment the critical current Ic was measured at 77 K in
self-field by a standard four-point method with a criterion of
Properties of Ag-sheathed 2223 tapes
1 µV cm−1 . The texture and microstructure examinations
were carried out on selected short tapes.
The texture and phase concentrations were determined
using a dedicated x-ray diffraction set-up at beamline BW5
at the HASYLAB synchrotron in Hamburg [8]. In order
to penetrate the Ag sheeting hard x-rays with energies of
100 keV were used. The set-up involved a gradient Si/Ge
monochromator crystal and a two-dimensional (2D) detector
for data acquisition (an XIOS-II CCD camera). Texture
determinations for the 2212 and 2223 phases were based
on the azimuthal distribution of the (115) reflections, which
could be determined directly from the 2D images. The
distributions were well described by Lorentzian squared
functions with a resulting accuracy in the width (FWHM)
of the order of 1◦ . Phase concentrations were based on the
integrated intensities of selected peaks and structure values
from the literature. Only relative concentrations are given
here, owing to difficulties in determining the phase content
of the possible amorphous phases. Moreover, the signal-tonoise ratio in the experiment set a lower detection limit on
the concentrations—of the order of 2% for the Bi phases.
The microstructure was examined on longitudinal and
transverse sections of the tape using a JEOL JSM840
scanning electron microscope (SEM). For the phase
identification EDS was applied using a Noran energydispersive x-ray analysis system. In order to assess the bulk
density the tape thickness was measured using a micrometer.
To describe the local density the area fraction of voids in the
SEM micrographs was determined using an image processing
program IMAGE-PRO.
Figure 1. (a) Variations of Jc () and tape thickness ( ) with the
heat treatment temperature. (b) Variations of the 2223 fraction ()
and the alignment of 2223 grains around the tape normal (FWHM)
( ) with the heat treatment temperature.
3. Results
In figure 1(a), the measured Jc value is plotted as a function
of the heat treatment temperature. The relative concentration
(C2223 /(C2223 + C2212 )) and the texture, i.e. the alignment
of the 2223 grains around the tape normal (FWHM) as
determined by synchrotron x-ray diffraction, are included
in figure 1(b). Three temperature ranges can be defined.
In the first range (819–826 ◦ C), the 2223 fraction increases
and the texture of 2223 phase sharpens with increasing
temperature, and Jc also increases significantly from 1 to
41 kA cm−2 . In the second range (826–830 ◦ C), the 2223
fraction and the grain alignment tend to saturate at about
98% and 16◦ , respectively, and Jc remains nearly constant at
a level of 40 kA cm−2 . In the third range (830–836 ◦ C),
the 2223 fraction and the grain alignment are unchanged
as compared with the second range but Jc decreases with
increasing temperature from 38 to 25 kA cm−2 . In addition
to 2223 and 2212, the 3221 phase has been found in the tapes
in the low temperature range. The intensity of its peak is 30,
18, 12 for operation temperatures of 819, 823 and 826 ◦ C,
respectively. The raw data are used here owing to the lack of
a structure factor. The 2201 phase has not been found at any
temperature. In the tape heat treated at the high temperature
(836 ◦ C), a contribution of an amorphous phase is recorded.
The tape thickness as a function of the operation
temperature is given in figure 1(a). Despite the scattering of
the data the general trend is clear and shows that the thickness
decreases with increasing temperature.
Three specimens heat treated at low (820 ◦ C), medium
(827 ◦ C) and high (834 ◦ C) temperatures were selected for
SEM examination. In this paper they will be referred to as
LT, MT and HT, respectively. In the following results are
presented with regard to filament shape, structural density,
grain alignment, phase composition and the local variations
of these parameters.
3.1. Inhomogeneous filament shape
A detailed investigation was performed on the MT tape. In the
longitudinal direction, the interface of the filaments is rather
smooth without sausaging. A micrograph of the transverse
section of the tape is shown in figure 2. The filaments are well
aligned to the surface of the tape. The shape of the individual
filaments is not uniform. The filaments in the centre region
are thinner (∼5–10 µm) and wider, whereas those near the
edges are thicker (∼20 µm) and narrower. These variations
in the filament shape are representative of the other tapes as
well.
3.2. Density and grain alignment
The microstructure of the centre and edge filaments of the
MT and LT tapes is shown in figure 3. It can be seen that the
density (compactness) and the texture (grain alignment) are
related to each other. A structure with well-aligned grains
is usually also reasonably dense, whereas a structure with
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Y L Liu et al
Table 1. Phases present in tapes.
LT (820 ◦ C)
MT (827 ◦ C)
HT (834 ◦ C)
2223 conversion
2221
∼20%
Large platelets
2201
−
>90%
Thin platelets,
thickness 1 µm
−
Pb-rich phase
CuO
CaCuO2
(Ca, Sr)2 CuO3
SrO
++
++, up to 10 µm
++
++, 2–3 µm
+, 1 µm × 8 µm
+, 1–2 µm
+, up to 6–8 µm
+
+, 2–3 µm
+, 1 µm × 8 µm
>90%
Thin platelets,
thickness 1 µm
Thin platelets,
thickness <1 µm,
<1%
+, 2–3 µm
+, up to 6–8 µm
+
+, 2–3 µm
+, 1 µm × 8 µm
−, absent; + and ++, increasing concentration.
Figure 2. SEM micrograph showing the transverse section of the MT tape.
Table 2. A summary of Jc and microstructure.
Jc (A cm−2 )
Filament
Area fraction
of void (%)
Alignment
2223 fraction (%)
LT (820 C)
6100
Centre
Edge
6.1
9.5
Poor
Worse than centre
∼20
∼20
MT (827 ◦ C)
41 200
Centre
Edge
1.6
5.2
Good
Less good than centre
>90
>90
HT (834 ◦ C)
30 400
Centre
Edge
1.8
6.0
Good
Less good than centre
>90a
>90a
Tape ID
◦
a
A small amount of 2201.
poor grain alignment often contains voids which are present
between the grains. The density results (presented as the
area fraction of voids) and qualitative estimates of the 2223
grain alignment are given in table 2. The density and grain
alignment are seen to improve with increasing operation
temperature. The MT and HT tapes have a fairly dense
structure and the 2223 grains are well aligned especially in
the filaments in the centre regions. The dimensions of the
2223 grains are 0.5–1 µm in the c direction and ∼20 µm in
the a–b plane (figure 3(a)). The LT tape, on the other hand, is
porous and the alignment is poor even in the centre filaments
(figure 3(c)). As a matter of fact, the LT tape is not an ideal
example to illustrate the temperature effect on 2223 density
and alignment owing to its low 2223 fraction (see below).
Nevertheless, this temperature effect has also been seen in
parallel research [9], which shows that the 2223 formed at
low temperatures is less dense and less well aligned than
that formed at high temperatures. As well as the temperature
effect, we also note that the density and alignment vary locally
within each tape, i.e. the porosity level and the misalignment
increase from the filaments in the centre to those at the edges
(see figures 3(b) and 3(d)).
378
It is worth noting that the variations in density and grain
alignment with operation temperature observed by SEM are
in agreement with the thickness data of the bulk (figure 1(a))
and texture data determined by the synchrotron technique
(figure 1(b)), respectively.
3.3. Phase composition
SEM backscattered images are shown in figure 4. The
secondary-phase particles identified are 2201, Pb-rich phase,
CuO, CaCuO2 , (Ca, Sr)2 CuO3 and SrO. These phases are
typical for tapes heat treated in an atmosphere of reduced
oxygen [10]. The estimated amount and the size of these
phases are given in table 1. In both the MT and the HT
tapes, the 2223 conversion is very close to complete in all
the filaments despite the variations in the shape, density
and grain alignment from the centre to edge. However,
the backscattered image (figures 4(a) and 4(b)) indicates
the presence of very thin (1 µm) light grey layers in
2223 phase, which may be 2212 resulting from 2223 and
2212 layer-on-layer growth [11, 12]. The type and size of
secondary-phase particles found in the two tapes are quite
Properties of Ag-sheathed 2223 tapes
Figure 4. SEM backscattered image showing the microstructure
of (a) the MT tape, (b) the HT tape and (c) the LT tape.
Figure 3. SEM secondary electron images showing the
microstructure of: (a) The centre filament of the MT tape. The
black elements are either secondary-phase particles such as
Ca2 CuO3 or voids. (b) the edge filament of the MT tape. (c) the
centre filament of the LT tape. The alignment is worse and there
are more voids than for the MT tape. (d) The edge filament of the
LT tape.
similar (see table 1). The only difference which can be
detected in the backscattered mode is that the HT tape
contains a small amount of a white phase (figure 4(b)), too
thin to be identified by EDS. Based on the experience and
literature, we identify this phase as 2201. In the LT tape (see
figure 4(c)), a considerable amount of large 2212 platelets are
present. The 2223 conversion is far from complete. The type
of secondary-phase particles is similar to that in MT and HT,
while the amount of particles is much larger (see table 1).
It is interesting to note that, despite the low conversion in
the LT tape, in general a 2223 layer with a thickness of 0.5–
1 µm has developed at the Ag–oxide interface. A faster
transformation process at the interface has also been reported
by other authors [10].
It is also worth noting that the 2223 fraction estimated
on SEM micrographs (table 1) agrees qualitatively with the
results of synchrotron measurements (figure 1(b)). Results
of both techniques show that the concentration of the Pb-rich
phase decreases with increasing operation temperature.
However, it is difficult to identify by EDS analysis whether
379
Y L Liu et al
the Pb-rich phase is 3221 or Ca2 PbO4 . A very low
concentration (<1%) of 2201 can be seen by SEM but not by
synchrotron diffraction. On the other hand, the amorphous
phase can be detected by the synchrotron technique but not by
SEM. For other phases such as CuO, CaCuO2 , (Ca, Sr)2 CuO3
and SrO, synchrotron x-ray diffraction is not as sensitive as
SEM.
In summary, Jc and the results of microstructural
observations of the three tapes are listed in table 2. The low
Jc of the LT tape is attributed to its low 2223 conversion, low
density and poor alignment. The microstructures of MT and
HT tapes are rather similar with respect to density, alignment
and 2223 phase purity while Jc of HT is lower than that of
MT by 26%. This considerable drop in Jc is likely to be
related to the presence of the amorphous phase and the small
amount of 2201. It is reported that the 2201 phase tends to be
distributed at the 2223 grain boundaries and causes the grain
connectivity to deteriorate [13].
4. Discussion
4.1. 2223 phase conversion
The present work demonstrates a clear dependence of
the 2223 phase fraction in the final tape on the heat
treatment temperature (figure 1(b)). The temperature range
826–836 ◦ C may correspond to the thermodynamically stable
range of 2223 [14]. This temperature range also allows
formation of liquid phases which enhance the reaction
kinetics [15, 16]. In the present work the liquid phase cannot
be observed directly. However, an in situ study on similar
tapes by synchrotron x-ray diffraction [12] has shown that
there is a liquid above 820 ◦ C. The present findings also
indicate that if the heat treatment temperature is too high
(e.g. >830 ◦ C), an excess amount of liquid is left over at the
end of heat treatment prior to cooling. This liquid (usually Bi
rich) precipitates forming amorphous phases and/or the 2201
phase during cooling [14, 17].
Below 826 ◦ C the maximum 2223 conversion determined by the thermodynamics is reduced. The kinetics of
the 2223 transformation is significantly slowed down owing
to the lack of liquid phases. A large amount of alkaline earth
cuprate phases has not been used to convert to 2223; instead
under the heat treatment conditions these phases grow in size
(figure 4(c)).
4.2. Density and grain alignment
The observation that the variation of the 2223 texture with
heat treatment temperature follows the same pattern as the
variation of 2223 fraction (figure 1(b)) does suggest that the
texture formation and the phase transformation are correlated
processes during the heat treatment. This may be understood
on the basis of the texture transmission model proposed by
a number of authors [11, 12]. According to this model, the
2223 texture is mainly determined by the 2212 initial texture
as well as the formation and growth process of 2223. High
temperatures (for example, 826–836 ◦ C in the present work)
facilitate (1) a well-developed 2212 initial texture right before
the 2223 transformation takes place, (2) formation of liquids
which allow the new 2223 to form onto the free surfaces of
380
existing 2212 platelets or earlier-grown 2223 crystallites and
(3) a substantial grain growth of the 2223. As a result the 2223
formed in this temperature range is dense and well aligned
as in MT and HT (see figure 3(a)). In the low temperature
range, on the contrary, the initial 2212 texture is less well
developed, the formation of 2223 takes place in a different
environment owing to the lack of liquids and the growth is
limited. The resulting structure is therefore porous and with
a low degree of alignment.
4.3. Current limiting factors
Figure 1 demonstrates that Jc is more sensitive to the heat
treatment temperature than the 2223 fraction and the texture.
Müller et al [18] have reported similar phenomena but their
work did not include texture observations. In the low
temperature range (<826 ◦ C) the phase purity and texture
are the main controlling factors for Jc . However, in the
high temperature range the Jc value varies independently of
the 2223 fraction and texture. This indicates that structural
inhomogeneities which influence the current flow on different
scales have become the major current-limiting factors.
Potential current-limiting microstructures may be secondary
phases, amorphous phase and defects at boundaries between
regions, grains as well as colonies [19]. The present work has
shown the correlation between the presence of an amorphous
phase and 2201 and a drop in Jc . It has also been noted
that the 2223 generally contains 2212 intergrowth on a very
small scale (as observed in the MT tape). The effect of
intergrowths on Jc has been discussed in the literature but
not yet well understood [18, 19]. Further investigation on
grain and colony scale by TEM is required.
The variations in the density and texture between the
filaments will certainly influence the current capacity of the
individual filaments. The study of the current distribution
[20] has shown that Jc of the centre part of a similar
multifilament tape can be as high as 70 kA cm−2 whereas Jc
of the edge part is 6 kA cm−2 . In the literature a difference
in the current capacity between the centre and the edge of
a multifilament tape by a factor of 10 has been reported
[21].
5. Conclusions
The dependence of the phase composition, density, texture
and Jc of a 2223 multifilament tape on the heat treatment
temperature has been studied. The microstructure and texture
have been characterized on both a global and a local scale
using synchrotron x-ray diffraction and SEM. This work leads
to the following conclusions.
The 2223 phase transformation and the texture formation
are correlated processes during the heat treatment. In
tapes with the same deformation route the 2223 fraction
as well as its structural characteristics such as density,
grain size and alignment are dependent on the operation
temperature. This is understood as the temperature effects
on the thermodynamics, kinetics of transformation and the
growth behaviour of 2223.
In the high temperature range the average 2223 fraction
and texture have reached the optimum level and are no longer
Properties of Ag-sheathed 2223 tapes
major controlling factors of Jc . Further optimization should
be focused on reducing structural inhomogeneities on various
scales and improving current flow. Macroscopically it is
important to produce filaments with uniform shape, density
and alignment. Microscopically direct efforts to characterize
and control the impurities and defects within 2223 grains and
at boundaries are needed.
Acknowledgments
We thank J-C Grivel, Z Han and B Kindl for fruitful
discussions and Th Tschentscher, L G Andersen and
A-M Heie Kjær for assistance in experimental work. Wang
and Vase acknowledge the valuable contributions of the
members of production and development group in Nordic
Superconductor Technologies. The support for this work
was provided by Dansync, the Danish Energy Agency, the
companies ELSAM and ELKRAFT.
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