Comparison of growth texture in round Bi2212 and flat Bi2223 wires and its relation to high critical current density development

Abstract

Why Bi2Sr2CaCu2Ox (Bi2212) allows high critical current density Jc in round wires rather than only in the anisotropic tape form demanded by all other high temperature superconductors is important for future magnet applications. Here we compare the local texture of state-of-the-art Bi2212 and Bi2223 ((Bi,Pb)2Sr2Ca2Cu3O10), finding that round wire Bi2212 generates a dominant a-axis growth texture that also enforces a local biaxial texture (FWHM <15°) while simultaneously allowing the c-axes of its polycrystals to rotate azimuthally along and about the filament axis so as to generate macroscopically isotropic behavior. By contrast Bi2223 shows only a uniaxial (FWHM <15°) c-axis texture perpendicular to the tape plane without any in-plane texture. Consistent with these observations, a marked, field-increasing, field-decreasing Jc(H) hysteresis characteristic of weak-linked systems appears in Bi2223 but is absent in Bi2212 round wire. Growth-induced texture on cooling from the melt step of the Bi2212 Jc optimization process appears to be the key step in generating this highly desirable microstructure.

Full text

Comparison of growth texture in round
Bi2212 and flat Bi2223 wires and its
relation to high critical current density
development
F. Kametani, J. Jiang, M. Matras, D. Abraimov, E. E. Hellstrom & D. C. Larbalestier
Applied Superconductivity Center, National High Magnetic Field Laboratory, Florida State University, Tallahassee FL 32310 USA.
Why Bi
2
Sr
2
CaCu
2
O
x
(Bi2212) allows high critical current density J
c
in round wires rather than only in the
anisotropic tape form demanded by all other high temperature superconductors is important for future
magnet applications. Here we compare the local texture of state-of-the-art Bi2212 and Bi2223
((Bi,Pb)
2
Sr
2
Ca
2
Cu
3
O
10
), finding that round wire Bi2212 generates a dominant
a
-axis growth texture that
also enforces a local biaxial texture (FWHM ,156) while simultaneously allowing the
c
-axes of its
polycrystals to rotate azimuthally along and about the filament axis so as to generate macroscopically
isotropic behavior. By contrast Bi2223 shows only a uniaxial (FWHM ,156)
c
-axis texture perpendicular to
the tape plane without any in-plane texture. Consistent with these observations, a marked, field-increasing,
field-decreasing
J
c
(H)
hysteresis characteristic of weak-linked systems appears in Bi2223 but is absent in
Bi2212 round wire. Growth-induced texture on cooling from the melt step of the Bi2212
J
c
optimization
process appears to be the key step in generating this highly desirable microstructure.
Superconducting magnets require high critical current density J
c
in long-length wire forms, which are
inevitably polycrystalline
1
. One of the most significant barriers to applying high temperature supercon-
ducting (HTS) materials in random polycrystalline forms has been the current-obstructing effects of
randomly oriented grain boundaries (GBs)
1,2
. Thus the two HTS conductors available in the market today, (Bi,
Pb)
2
Sr
2
Ca
2
Cu
3
O
x
(Bi2223) and YBa
2
Cu
3
O
7-d
(YBCO) (or more generally REBCO, where RE denotes rare earth)
are fabricated in complex ways determined by the need to minimize grain misalignments so as to reduce or even
eliminate high angle GBs (HAGBs). High J
c
in Bi2223, which allows operation at 77 K due to its high critical
temperature T
c
of ,110 K
3
, requires a complex deformation- and reaction-induced texturing process which aims
to preferentially align the c-axes of grains normal to the tape plane
4
. Present Bi2223 tapes have a uniaxial texture
of ,15ufull-width at half-maximum (FWHM), allowing a J
c
of order 500 A mm
22
at 77 K and self-field
5–7
.
REBCO coated conductors
8–11
have taken much attention away from Bi2223 because their growth on quasi-single
crystal templates allows a much better, biaxial texture (2–5uFWHM) which generates much higher J
c
(77 K, sf) of
,3310
4
Amm
22
12
. However, a major impediment to magnet applications of these HTS conductors is their 20–
40:1 shape anisotropy, their large electromagnetic anisotropy and their delivery in fixed sizes determined by their
complex manufacturing process, rather than by the desires of their users. On the other hand, Bi
2
Sr
2
CaCu
2
O
x
(Bi2212) is the only HTS cuprate in which high J
c
can be developed in a round wire form
13,14
which also allows it to
be supplied in multiple multifilament architectures
15
and in the twisted state that benefits low hysteretic losses,
isotropic properties and high magnetic field quality. Another huge advantage of round wires is that they can also
be flexibly cabled into conductors of arbitrary current capacity, as is well demonstrated by many uses of Nb-Ti and
Nb
3
Sn wires
16
. The opportunity to develop similar capabilities in a round wire cuprate conductor with high
current density capability to fields well above 50 T, more than twice that possible with any Nb-based conductor,
suggests the value of a detailed understanding of how Bi2212 differs from its anisotropic HTS sibling Bi2223.
Although its T
c
of 90–95 K restricts applications at 77 K, its high irreversibility field H
irr
below 10 K
17
and its
round wire form are exactly the desirable characteristics required for high field magnets. Especially because J
c
of
Bi2212 has now reached 3200 and 2500 A mm
22
at 12 and 20 T, 4.2 K in fully dense, round wire form with an
irreversibility field H
Irr
(4 K) .100 T
18–20
, it has become a very serious candidate for high field NMR
21,22
and
particle accelerators such as future upgrades of the Large Hadron Collider (LHC)
23
.
OPEN
SUBJECT AREAS:
SUPERCONDUCTING
PROPERTIES AND
MATERIALS
SCANNING ELECTRON
MICROSCOPY
Received
7 October 2014
Accepted
13 January 2015
Published
10 February 2015
Correspondence and
requests for materials
should be addressed to
F.K. (kametani@asc.
magnet.fsu.edu)
SCIENTIFIC REPORTS | 5 : 8285 | DOI: 10.1038/srep08285 1

The Bi2212 round wires have long been assumed to be macro-
scopically untextured. Indeed, since high J
c
could only be
developed in very short lengths, it never received much attention
and many assumed that the low J
c
values of long length wires were
due to its roundness which was presumed to deny the mac-
roscopic texture believed to be essential to high J
c
. In fact, the
key breakthrough to high J
c
in Bi2212 round wires was to under-
stand that the principal critical current-limiting mechanism
(CLM) was blockage of supercurrent flow by filament-diameter
bubbles formed during the melt step of heat treatment
18–20,24–26
,
and not by HAGBs generated by any lack of macroscopic texture,
as seemed totally plausible considering the need to make Bi2223
and REBCO in tape forms so as to generate strong crystal-
lographic texture. Considering that supercurrent flow occurs pref-
erentially in the CuO
2
planes of all HTS cuprate materials, Bi2212
included, we felt that it was time to open the issue of Bi2212
texture in round wires again. Actually we know that planar,
[001] tilt thin film bicrystal GBs of YBCO, Bi2212 and Bi2223
all possess very similar exponential decay of intergranular J
c
as a
function of misorientation angle
2
. Whether such experiments are
valid for much more general GBs is less clear. For example, sub-
sequent work has shown that non-planar GBs in REBCO have
much higher J
c
than their planar GB counterparts
27,28
but the
effect of the non-planarity is only to about double the critical
angle for current blocking from about 3uto 6–7urather than
make a transformational change in GB behavior. Nevertheless,
this relaxation of critical angle is what enables ex situ coated
conductor processes to generate high J
c
29
. Thus the question of
whether a similar process was at work in filaments of Bi2212
presented itself to us as an urgent question now that we have
been able to establish a reliable overpressure process (OP) for
making very high J
c
Bi2212 round wires
18
. We wanted to under-
stand the true grain structure of these OP round wires so as to get
a more general understanding of the role of naturally occurring
GBs formed by the controlled solidification from the melt origin-
ally developed by Heine et al. in 1989
30
.OPdoesdensifythe
Bi2212 filaments, making their physical connectivity high, but
OP does not change the filament shape
18–20
and it seems hard
to imagine that their grain structure is enhanced by the
densification.
The motivation for this paper is therefore to address a long-
standing conundrum: why is high J
c
possible in round wire Bi2212
but not in round wires of its close sibling Bi2223. To perform the
study, we used Electron Backscatter Diffraction Orientation
Imaging Microscopy (EBSD-OIM) to reveal the texture of state-
of-the-art high J
c
Bi2223 tape and overpressure processed round
wire Bi2212. We found that while the Bi2223 tape shows the
expected uniaxial texture of about 15uaround [001], the Bi2212
round wire is actually not macroscopically untextured. In fact it
possesses a strong a-axis texture parallel to the filament axis that
imposes thus too a marked biaxial texture with similar 15uin-
plane spread (the bdirection [010] is thus normal to the wire
axis). Electromagnetic isotropy is enforced by a gradual rotation
of the c-axis of grains around the azimuth about the wire axis such
that a conductor containing hundreds of filaments is locally aniso-
tropic, while isotropic as a whole. This unique grain structure,
especially the in-plane biaxial texture, correlates well to the much
higher J
c
of Bi2212 compared to Bi2223, as well as to the lack of
weak-link signature in the J
c
(H) characteristics of Bi2212 and its
presence in Bi2223 that we here present as a striking difference
between the two conductors. The study also opens up the intri-
guing possibility of making isotropic, multifilamentary round
wires of other HTS materials if similar grain and GB structures
could be realized in them too. This would be especially powerful
for compounds with the REBCO structure because of their inher-
ently much lower electronic anisotropy and higher irreversibility
field, properties that would allow operation in high fields, even at
liquid nitrogen temperatures.
Results
In order to clarify the often complex and sometimes confusing ways
of referring to texture in these wires, we need a few words about the
principal ways that we present our experimental data in this paper.
The external perspective of the experimenter naturally defines two
principal axes, the axis along the wire or that normal to the wire (or
the tape). In crystallographic parlance, the wire axis view is normally
called the rolling direction (RD), while the perpendicular or side view
made at 90uto RD is called the normal direction (ND). In the case of
the tape there are clearly two preferred NDs, one parallel to the broad
face of the tape (as used here) and one perpendicular. For a round
wire there is no preferred ND but, since we found that the c-axis
rotates around the wire axis, it is possible to choose a local ND such
that low index planes dominate in the ND view, as they do in either
preferred-direction ND view of a Bi2223 tape. This is what we do
here. Figure 1a further relates the macroscopic, wire-level view to the
crystallographic view by showing the Bi2212 and Bi2223 crystal
structures drawn with VESTA software
31
. Both are perovskites in
which oxide layers of Bi, Sr, Ca and Cu stack alternately in an inher-
ently anisotropic structure. We note that this produces an inherent
electronic anisotropy in both Bi2212 and Bi2223 that can be hidden if
there is a random grain orientation distribution. Accordingly, we use
both in-plane and out-of-plane projections on inverse pole figures
(IPF) to define the individual grain misorientations, as shown in
figure 1b. For in-plane rotations or misorientations, the a-orb-axes
of the crystal ([100] or [010]) change direction while keeping the
direction of the c-axis [001] constant, while only out-of-plane rota-
tions are needed to change the c-axis orientation, as is illustrated in
figure 1c. As such, we can visualize both in-plane and out-of-plane
components of the grain misorientations. It should be noted that the
a and b lattice parameters are so close that [100] cannot be distin-
guished from [010] by EBSD. However, Bi2212 is known to form
plate-like grains with a high a/b aspect ratio, presumably because of
the lattice modulation along the b-axis
32,33
. Thus, in this paper, we
presume that Bi2212 grains are always longer along the a-axes than
the b-axes. In this interpretation the dominant texture of Bi2212 is
then [100] along the wire axis (RD), while [010] lies normal to the
wire axis.
We first wished to compare the J
c
behavior of Bi2212 and Bi2223,
as shown in Figure 2. Two striking features are clear: one is that the
J
c
(H) at 4.2 K of round wire Bi2212 is about 3 times higher than that
of Bi2223 and second that Bi2223 flat tape showed a significant
dependence of the magnitude of J
c
(H) according to whether it was
measured in increasing or decreasing field. As usual, J
c
(H) was always
higher in decreasing field (figure 2 inset). Strikingly, since such hys-
teresis is normally attributed to the presence of weak links in the
polycrystalline network
34
, the Bi2212 round wire showed no J
c
(H)
hysteresis (figure 2b). Although both conductors are fully (.95%)
dense because OP densification was used in both
5–7,18
, it is clear that
weak links are present in Bi2223, but also that the signature is appar-
ently absent in Bi2212.
Figure 3a and c compare longitudinal SEM cross sections of the
Bi2223 flat filament viewed parallel to the broad filament face (ND, as
illustrated) with that of a Bi2212 filament also viewed along ND. Grey
regions of Bi2223 or Bi2212 are dominant in both cases with occa-
sional darker alkaline earth cuprate (AEC) or lighter, low-T
c
phase
(Bi2212 and Bi2201 for Bi2223 and Bi2212, respectively) regions in
the cross-sections. The Bi2223 filament is remarkably phase-pure,
whereas the Bi2212 filament is less phase-pure, having larger AEC,
Cu-free (CF) or Bi2201 2
nd
phase regions, as well as a small residual
void fraction. The grain textures and grain-to-grain misorientations
of the same regions are visualized as inverse pole figures (IPF) and
GB maps in Figures 4–5. In figure 3b and d, the Bi2223 and Bi2212
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SCIENTIFIC REPORTS | 5 : 8285 | DOI: 10.1038/srep08285 2

grains are colored based on their grain orientation when viewed
parallel to ND. Due to the strong uniaxial c-axis texture of the
Bi2223 tape, the plate-like Bi2223 grains appear as thin laths with
ab-planes parallel to ND. Their grains are typically ,0.3–1 mm thick
and 5–20 mm in diameter. Because of their marked uniaxial texture,
almost all Bi2223 grains appear green, blue or their mixture, indi-
cating a rather continuous in-plane misorientation distribution
between [100] and [110] about a common [001] axis. The dominant
GBs are straight and generally parallel to the ab-plane of either of the
adjacent grains and to the tape surface. Indeed, GBs parallel to the c-
axis are very rare. The uniaxial [001] texture that is the object of the
deformation- and growth-induced texture processing route is quite
clear.
As mentioned above, in Bi2212 there is no preferred axis of ND,
except that it is normal to the wire axis. Here we chose a length of
filament where there was a dominant set of Bi-2212 grains with ab-
planes lying parallel to ND, for which the ND-IPF map of the Bi2212
filament (figure 3d) shows a dominant green color corresponding to
a dominant [010] orientation. As for Bi2223, the Bi2212 grains are
also plate-like in shape and of similar thickness to Bi2223, but they
are ,50–300 mm in length, almost 10 times larger than the Bi2223
grains. Note however that the lateral growth of the grains is strongly
constrained by the nominal 15 mm diameter filaments. Even though
some grain growth from filament to filament occurs, few grains are
wider than 30 mm. As with Bi2223, the dominant GBs are those lying
parallel to the ab-planes. As for Bi2223 and also for Bi2212, the grains
have a strong tendency to stack on top of each other in a colony
structure in which all grains share a common c-axis. We made a
600 mm long ND-IPF cross section montage of this Bi2212 filament
(supplemental figure S1) and confirmed that the GBs//c-axis are very
rare.
A way to compare the Bi2212 and Bi2223 texture relevant to their
grain boundary connectivity is by coloring GBs according to their
local grain-to-grain misorientation. In this case we choose a misor-
ientation of 20u(figure 4a and b). Figure 4b makes it immediately
clear that the majority of Bi2212 GBs have local misorientations
#20u(green), while GBs with .20umisorientation (dark brown)
are significantly more evident in Bi2223 (figure 4a). A further com-
parative view of the texture can be obtained from binning the indi-
vidual grain-to-grain misorientations, as shown in Figure 4c and d,
which plot the misorientations of all grains examined in the EBSD
scans. In the Bi2223 flat filament, the grain-to-grain misorientation
angles, although peaking at ,12u, are broadly distributed from ,5u
to 45u(figure 4c). On the other hand, the distribution of Bi2212
grain-to-grain misorientation angles shows a sharp peak around 8u
(figure 4d). We note that figure 4c and d contain angles .45u, which
occurs when the out-of-plane texture component dominates the
misorientation, as in some Bi2212 colony GBs. In Bi2223, due to
the uniaxial texture, the dominant component of misorientations
is in-plane, so the angular range is confined to 45ubecause of the
inability of the EBSD to distinguish between [100] and [010] due the
very small difference of the aand blattice parameters in either
compound.
The most significant way of comparing the difference between the
Bi2212 and Bi2223 grain textures may be however by their ND and
RD inverse pole figures. As already shown in the ND-IPF grain map
of figure 3, the distribution of Bi2223 grain orientations is ab-plane
dominant in both the ND and RD projections. Comparing figure 5a
and b, the uniaxial texture viewed along the RD which perceives
rotations normal to the RD is slightly better than along the ND.
The out-of-plane grain misorientations are ,10ualong the filament,
whereas this distribution creeps up to ,15uacross the filament.
However, both the ND- and RD-IPF share the same random in-plane
distributions, as judged by the uniform distribution of orientations
between [100] and [110] in figure 5a and b (recall that [100] and
[010] cannot be distinguished by EBSD). This is a natural character-
istic of a uniaxial texture in which only the c-axis of grains is aligned
normal to the tape surface.
By contrast a rather different picture emerges from the inverse
pole figures of Bi2212 in figure 5c and d. The two very distinctive
features are a cluster of points within about 15uof [100] in two
orthogonal directions in the RD view but a rather continuous distri-
bution of grain orientation points in the ND view within 15uof the
axis connecting [100] and [001]. What this latter distribution sig-
nifies is that there is a continuous rotation of the [001] axis of Bi2212
grains along the wire axis. Note that no positional information is
present in the inverse pole figures, thus the grain-to-grain misorien-
tation angle of each GB cannot be measured by figure 5. However
figure 4b, d and 7c (shown later) where the majority of GBs have local
misorientation ,20u(,8uin average) and high angle GBs such as
,90uGBs are very minor, suggest that the continuous distribution of
points in figure 5c is caused by gradual rotation of grains with GBs of
Figure 1
|
(a) Crystal structures of Bi2212 and Bi2223 and definition of
unit axes. (b) Location of grain misorientations on the inverse pole figure
(IPF). (c) Schematic illustration of in-plane and out-of-plane rotations of
the BSCCO grains. The broad flat surface of a BSCCO grain is always
parallel to its ab-plane. Note that the axes of out-of-plane rotations can
take any directions containing its ab-plane.
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SCIENTIFIC REPORTS | 5 : 8285 | DOI: 10.1038/srep08285 3

Figure 2
|
Comparison of the superconducting critical current density
J
c
(H, 4.2 K) of the Bi2223 flat tape and the Bi2212 round wire. The field H was
applied normal to the wire axis for Bi2212, and to the tape face for Bi2223, respectively. The triangles of Bi2212 and Bi2223 indicate J
c
measured in
increasing field, whereas the circles of Bi2212 and Bi2223 represent J
c
in decreasing field. Note that J
c
of Bi2212 round wire is approximately 3 times larger
than that of the Bi2223 flat tape and is without any field hysteresis, which is quite evident in Bi2223 and is shown more clearly in the inset.
5 μm 5 μm
//ND
5 μm
5 μm
//ND
[110]
[001] [010] [110]
[001]
[100]
[010]
(a) Bi2223 (b) Bi2223
(c) Bi2212
(d) Bi2212
Figure 3
|
Comparison of the grain structure of Bi2223 and Bi2212 filaments as viewed along the ND parallel to the tape plane in Bi2223 and along an
ab
-plane-oriented section of a Bi2212 filament. A longitudinal cross section of a representative area of Bi2223 is shown in (a) by a backscattered electron
SEM image, and in (b) by a corresponding ND-IPF map, whereas those of Bi2212 are shown in (c) and (d). In the ND-IPF maps, Ag and second
phase regions are blacked out and low T
c
phase regions (Bi2212 in Bi2223 and Bi2201 in Bi2212) are colored gray (the outer black regions are the Ag
sheath). Colors in (b) and (d) correspond to the grain orientations defined by the crystallographic directions parallel to the ND projection axis which is
perpendicular to the wire direction.
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SCIENTIFIC REPORTS | 5 : 8285 | DOI: 10.1038/srep08285 4

small grain-to-grain misorientations, rather than by having many
high angle GBs. It is also clear from extended scans (like the
600 mm long one mentioned earlier) that there is a continuous rota-
tion of [001] along the filament axis as new Bi2212 grains grow on
solidifying from the melt (supplemental figure S1). Grains cluster
within ,15uof [010] in the ab-plane. Figure 5d shows that the grain
orientations along the wire axis (RD) are also preferentially [100] with
only a ,15uvariation of misorientation in both in- and out-of-plane
directions and this makes clear that the Bi2212 grains also possess a
biaxial texture, the a-axis ([100]) lying parallel toRD which is also the
filament axis. As noted above, the [001] c-axis can rotate easily about
the filament axis, allowing a continuous range of orientations per-
pendicular to the wire direction. Another feature of the Bi2212 grain
structure is plastic bending and/or twisting of individual grains. The
ND-IPF of figure 5c shows that many grains have orientations that
continuously vary between the [100]/[110] and [001] directions due
5 μm
5 μm
0 102030405060708090100
0
5
10
15
20
25
30
Angle (deg.)
Fraction of GB length (%)
0 102030405060708090100
0
5
10
15
20
25
30
Angle (deg.)
Fraction of GB length (%)
(a) Bi2223
(b) Bi2212
(c) Bi2223 (d) Bi2212
Figure 4
|
(a) (b) GB maps of the areas of figure 3a (Bi2223) and 3c (Bi2212), respectively. GBs with misorientation angle of #20uare traced in green,
while misorientations .20uare colored in dark brown. Based on these GB maps, the fractional GB length is plotted as a function of misorientation
angle for (c) Bi2223 and (d) Bi2212. The shaded area represents the fraction of GBs having misorientations of #20uthat appear green in (a) or (b). Note
the large difference in the most frequent misorientation (,14uin Bi2223 versus ,8uin Bi2212) between the two conductors.
//ND //RD
//ND //RD
(a) Bi2223
(c) Bi2212
(b) Bi2223
(d) Bi2212
[001]
[110]
[010]
[001]
[110]
[010]
[001]
[110]
[100]
[001]
[110]
[100]
15° 10° 15° 10°
15°
15°
15°
15°
Figure 5
|
ND and RD Inverse Pole Figures (IPF) of the grain orientations in Bi2223 and Bi2212 are shown in (a)(b) and (c)(d), respectively. The IPFs of
Bi2223 are derived from the image of figure 3b, whereas those of Bi2212 are a combination of figure 3d and 7b. They are stereographic projections
of the grain orientations parallel to the ND projection axis (which is normal to the filament direction and parallel to the tape plane for Bi2223) in (a) and
(c), and in (b) and (d) the RD which lies parallel to the filament direction. The black dotted lines in (a) and (b) mark the dominant misalignments of 15u
and 10uaway from the ab-plane that defines the dominant [001] texture of Bi2223, while the very different 15u‘‘corner-pocket’’ texture of Bi2212 that
defines a significant biaxial, in-plane alignment around [100] is shown in (d).
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SCIENTIFIC REPORTS | 5 : 8285 | DOI: 10.1038/srep08285 5

to this grain plasticity that must occur during grain formation and
growth at high temperature (also shown in supplemental figure S2).
Discussion
The key microstructural finding of this paper is that there is a very
marked texture in melt-processed Bi2212 round wire and that the
texture is, rather surprisingly, biaxial. From a superconducting prop-
erty point of view, the J
c
(H,4 K) of the Bi2212 conductor is about 3
times higher than that of the Bi2223 tape conductor, in spite of the
significantly higher T
c
of Bi2223 (110 versus ,80 K). As has been
reported before
35
, there is a markedly hysteretic J
c
(H) characteristic
for this state-of-the-art Bi2223 conductor, but surprisingly we find
almost no weak link signature in the Bi2212 conductor.
A key benchmark for applications is that overall conductor current
density J
E
must exceed 300–500 A/mm
2
in a domain of H and T of
practical interest and it is clear that both Bi2223 and Bi2212 can do
this at 4 K to fields of well over 20 T, given that superconductor
filling fractions are of order 40% for Bi2223 and 25–30% for
Bi2212. But, from a future development point of view, it would be
highly desirable to understand what fraction of the superconductor
cross-section actually carries current. Since both conductors have
been heat treated under an external overpressure, they are both well
over 95% dense, thus ruling out voids as current-limiting obstacles in
either conductor and most plausibly implicating GBs as the dominant
current-limiting mechanism. However, it is notoriously hard to evalu-
ate the extent to which the J
c
developed by flux pinning within grains
is throttled by blocking grain boundaries, non-superconducting
second phases, cracks or other obstacles. The striking differences in
texture shown by this study suggest a first focus on the distribution of
misorientations within the grain boundary network.
There are two principal and different models that have been pro-
posed to describe supercurrent flow in uniaxially textured Bi2223 flat
tapes, the Brick-wall
36
and Railway-switch model
37
. In the Brick-wall
model, the J
c
of the current path is limited by c-axis current flow
across basal-plane faced, c-axis twist GBs, resulting in a Josephson-
like weak link behavior that can however sustain finite J
c
in high fields
because of the very large surface area of the basal-plane–faced GB
junctions
36
. On the other hand, the Railway-switch model postulates
that the many small-angle, c-axis tilt GBs found in Bi2223 where a-
or b-axes of grains terminate at the broad ab-plane face of an adjacent
grain provide a strongly coupled, vortex-pinning dominated current
path
37
. Neither mechanism is believed to allow supercurrent flow
over 100% of the cross-section and there is great uncertainty on what
the active cross-section might be, because measurements, either by
transport or by magnetization, measure the product of J
c
and A
(where A is the actual section carrying supercurrent, rather than
the total measured cross-section) without either J
c
or A being
known independently. Bulaevskii et al. have laid out procedures
for separating the different current paths
38
, but a significant prob-
lem is that current transfer across the matrix Ag can confuse the
shapes of the V–I curves that are an important part of the evalu-
ation procedure. Earlier we did succeed in measuring V–I curves
on an extracted, bare Bi2223 filament with about half the J
c
of the
present conductor and did conclude that they gave evidence of c-
axis transport
39
. More generally, however, Bulaevskii et al. con-
cluded that vortex pinning dominates the V–I curves and J
c
(H)
characteristics rather than the Josephson-like Brick-wall beha-
vior
38
. In any case it seems most likely that both mechanisms
operate and that a complex series-parallel current path determines
the actual superconducting characteristics. The weak-link hyster-
esis in Bi2223 observed here supports such an interpretation (note
that this hysteresis was much larger in earlier conductors of lower
overall J
c
35
). The very intriguing possibility raised by the quasi-
biaxial texture of the round Bi2212 wires which show almost no
hysteresis is that their higher J
c
can indeed be explained by, not
just correlated to, their better texture.
Within the framework of the exponential decay of J
c
with misor-
ientation angle h
2
in planar bicrystals of all cuprates, it is quite natural
to believe that the much better, quasi-biaxial texture of Bi2212 com-
pared to Bi2223 shown in Figure 5 would correlate well to the lack of
weak-link behavior in Bi2212 and its three times higher J
c
(figure 2).
Figure 6 shows that that magnetization J
c
(H) behavior of both Bi2212
and Bi2223 is rather well linearized by the Kramer function
(Dm
1/2
(m
0
H)
1/4
, where m and H are the magnetic moment and field,
respectively) used to describe vortex depinning by flux line lattice
shear
40
, which again is suggestive of a dominant, strongly coupled
current path. All of these data are consistent with a larger fractional
cross-section current path in Bi2212 and fewer weak-linked grain
boundaries in the biaxially aligned Bi2212 filament.
Under this interpretation, the reason why the two textures are so
different becomes of prime importance. One key difference between
the two compounds is in their processing. In the case of Bi2223, the
powder is a mixture of Bi2212 and ‘‘stuff’’, mostly Ca and Cu oxides.
Formation of Bi2223 occurs largely by solid state diffusion (though a
small amount of liquid is present) after a complex, rolling-induced
alignment of the Bi2212 grains in the precursor, which then convert
in situ to Bi2223 during heat treatments. Bi2223 is often described as
possessing a deformation and reaction-induced texture. In Bi2223
0 5 10 15 20
0.0
0.2
0.4
0.6
0.8
40 K
35 K 30 K
25 K
20 K
17.5 K
15 K
μ
0
H (T)
(∆m)1/2(μ0H)1/4
(a) Bi2223, H ⊥ Tape face
0 5 10 15 20
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
30 K
25 K 20 K 17.5 K
15 K
12.5 K
10 K
(∆m)1/2(μ0H)1/4
μ
0
H (T)
(b) Bi2212, H ⊥ Wire
Figure 6
|
Kramer function plots of the Bi2212 round wire and Bi2223
flat tape, derived from magnetization measurements over the
temperature range 15–40 K for Bi2223 in (a) and 10–30 K for Bi2212 in
(b). A considerable linear regime exists for both conductors, which is
generally taken to be consistent with a vortex pinning dominated J
c
behavior. Dm5m(field down) – m(field up), where m denotes the
magnetic moment. Note that the magnetization J
c
is proportional to Dm.
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SCIENTIFIC REPORTS | 5 : 8285 | DOI: 10.1038/srep08285 6

tapes, the filament width is ,150–250 mm
7
, far larger than the dia-
meter of Bi2223 grains (5–20 mm, as is seen in figure 3b and 4a), thus
allowing freedom for the a-orb-axis of grains to grow in any dir-
ection within the ab-plane without degrading the uniaxial rolling
texture. By contrast, the Bi2212 texture develops on slow cooling
after melting of the starting Bi2212 powder. Large Bi2212 grains
grow up to several 100 mm long along the ab-plane, a size ,10 times
larger than the filament diameter which is typically 15–20 mm.
Perhaps crucially, Bi2212 grains tend to grow faster along the a- than
the b-axis, and much faster than along the c-axis, thus forming very
thin grains with a high in-plane a/b aspect ratio .5, rather than the
round disks of Bi2223
33
. But growth from the melt confers another
benefit, because the surrounding Ag restricts the longest grains to
those with the fastest growth a-axes aligned along the filament axis.
This preferred growth within the narrow filament cavities results in
both in-and out-of-plane grain orientation along the wire axis, as is
seen in figure 5d. The almost round cross section of filament cavities
also allows the grains to grow with their c-axis oriented in any dir-
ection normal to the wire axis (ND). As the Bi2212 grains grow, the
multiple regions that have the local biaxial texture can form in par-
allel. They can join with each other having different azimuthal orien-
tations, as is shown by the reddish regions in figure 7b. However, the
ND-IPF of figure 5c indicate that the plastic bending and twisting of
Bi2212 grains that appear on the ND-IPF as continuously changing
orientation plots can presumably compensate large misorientations
over long lengths by local, small misorientations. In addition, our
earlier TEM studies suggested that the Bi2212 ab-planes can easily
bend in distances of ,10–20 nm so as to minimize the out-of-plane
misorientations.
It should be noted that Bi2212 round wires are not as perfectly
biaxial as the REBCO coated conductors in which the out-of-plane
and in-plane misorientation is minimized to less than 5u
12
. Indeed, as
is clearly seen in figure 4, the general misorientation of the GBs in
Bi2212 is typically 10–15uwhich is significantly higher than the
critical angle (2–3u) found in experiments on thin film bi-crystals
with planar GBs parallel to the film normal
2
. But, in spite of this
higher misorientation, the weak link signature of Bi2223 is absent
in Bi2212 (and in biaxially textured coated conductors too),
implying that Bi2212 GBs are less obstructive. Three additional
factors may play an important role in enhancing current flow
across Bi2212 round wire GBs. One is the very large, basal-
plane-faced GB area per grain. As an earlier study of YBCO coated
conductors pointed out, the J
c
reduction across GBs becomes less
pronounced as the GB area increases due to the larger current-
carrying cross section
27
. Actually we estimate the area of basal-
plane-faced GBs in the Bi2212 to be ,500–5000 mm
2
per grain
due to the large ab-plane grain boundary area. This is very favor-
able compared to the transverse cross-section of ,200 mm
2
for the
whole filament. The second factor is that the GBs in these Bi2212
round wires are almost parallel to the direction of macroscopic
current flow. Because of the rotation of the c-axis about the wire
axis, there will always be regions where any c-axis current across
these basal-plane GBs will be parallel to the magnetic field in a
Lorentz force-free configuration, which minimizes depinning of
weakly pinned vortex segment at GBs
27,41
. The third factor is the
possibility to significantly overdope Bi2212 well beyond that pos-
sible in Bi2223 or YBCO, which is certainly beneficial for increas-
ing the superfluid density at the GB and for strengthening the
vortex line tension, both of which enhance the GB current den-
sity
42
. We thus conclude that multiple factors enhance the cap-
ability for developing higher J
c
in Bi2212 round wires that operate
much less effectively in Bi2223 tapes. A vital one is almost cer-
tainly the favoring of a-axis growth along the filament axis from
which the biaxial texture evolves.
//ND
5 μm
5 μm
5 μm
(a)
(b)
(c)
[110]
[001] [010] [110]
[001]
[100]
[010]
Figure 7
|
SEM image, ND-IPF and GB maps of another portion of a 600 mm long longitudinal cross section, a portion of which was also shown in
Figure 3c and d, showing that comparatively rare, but highly misaligned regions do exist in the Bi2212 filament. As shown in (b), most grains
are well textured and in this case green, which denotes that [010] is parallel to the ND. There is also a highly misoriented red-orange region indicating that
c-axis [001] oriented grains are present. (c) Although the majority of GBs are colored green (,20u), the colony boundaries have much larger
misorientation, signifying that they contain a large component of out-of-plane misorientation.
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SCIENTIFIC REPORTS | 5 : 8285 | DOI: 10.1038/srep08285 7

Conclusion
In summary, we have presented the grain textures of state-of-the-art
Bi2223 tapes and Bi2212 round wires. In contrast to the uniaxially
textured Bi2223 tape in which the in-plane GB misorientations are
essentially random with an out-of-plane misorientation or c-axis
texture of ,15u, the Bi2212 round wires exhibit a quasi-biaxial tex-
ture as a result of the growth of long, high aspect ratio grains within
the narrow filament cavities. Along the wire axis, the in-plane GB
misorientation in Bi2212 is constrained within ,15u, and to ,15u
for the out-of-plane misorientation too. In addition, the large grain
size allows large-area, ab-plane basal-plane-faced GBs favorable for
intergrain current transport. Our study demonstrated that a unique
biaxially aligned microstructure is present in high J
c
Bi2212 round
wire which suggests the possibility of making round wires from other
HTS materials if a similar melt-driven path can be found for them
too.
Methods
The Bi2212 wire of 0.8 mm diameter was fabricated by Oxford Superconducting
Technology using the Powder-in-Tube (PIT) technique. A typical conductor (though
many architectures are possible
15
) uses an 85 filament first stage drawn into hexa-
gonal form, 18 of them being then stacked inside a Ag alloy tube to make a final size
conductor containing ,1500 (85 318) filaments, each about 15 mm in diameter. For
our EBSD study, we used a specially designed 27 37 variant of this architecture
described in our previous paper
24
. The Bi2212 wires were heat treated by the OP
technique
18
. The Bi2223 tape was fabricated by Sumitomo Electric Inc. also using a
PIT process. In contrast to the Bi2212 wire fabrication, rolling of the Bi2223 wire was
performed both before the first heat treatment and between the first and second heat
treatment in order to optimize the deformation- and reaction-induced uniaxial tex-
ture
6
. The final tape was ,4.3 mm wide by 0.23 mm thick, containing ,140 flat
filaments, each about 230 mm wide and 10 mm in thickness. I
c
values were measured
at 4.2 K applying magnetic field up to 15 T by the four probe method. The I
c
values
are converted to J
c
using the stated filling factor of 40% for the Bi2223 tape and that
measured on cross-sections of the Bi2212 wires densified by OP just before melting
when the filaments are still round. The J
c
data presented here are actually for a Bi2212
round wire of 37 318 architecture which was heat-treated by the same OP technique.
This wire had a J
c
(H) characteristic within 10% of that exhibited by earlier mea-
surements of the 27 37 architecture wire.
Longitudinal cross sections were prepared by mechanical grinding and polishing
on SiC papers and diamond lapping film, a final gentle polish with 50 nm colloidal
alumina, followed by ion-milling at 2.0 kV in a Gatan PECS ion mill. Electron
Backscatter Diffraction Orientation Imaging Microscopy (EBSD-OIM) to visualize
the filament grain structure was performed in a Carl Zeiss 1540EsB scanning electron
microscope with an EDAX Hikari camera, and TSL OIM Collection software.
1. Larbalestier, D., Gurevich, A., Feldmann, D. M. & Polyanskii, A. High-T
c
superconducting materials for electric power applications. Nature 414, 368–377
(2001).
2. Hilgenkamp, H. & Mannhart, J. Grain boundaries in high-T
c
superconductors.
Rev. Mod. Phys. 74, 485–549 (2002).
3. Sato, K. et al. High-J
c
silver-sheathed Bi-based superconducting wires, IEEE
Trans. Magn. 27, 1231 (1991).
4. Rupich, M. W. & Hellstrom, E. E. One Hundred Years of Superconductivity
671–689 (CRC Press/Taylor & Francis, 2012).
5. Yuan, Y. et al. Significantly enhanced critical current density in Ag-sheathed
(Bi,Pb)
2
Sr
2
Ca
2
Cu
3
O
x
composite conductors prepared by overpressure processing
in final heat treatment. Appl. Phys. Lett. 84, 2127–2129 (2004).
6. Kobayashi, S. et al. Controlled over pressure processing of Bi2223 long length
wires, IEEE Trans. Appl. Supercond. 15, 2534–2537 (2005).
7. Nakashima, T. et al. Overview of the recent performance of DI-BSCCO wire.
Cryogenics 52, 713–718 (2012).
8. Selvamanickam, V. et al. Recent progress in second-generation HTS conductor
scale-up at SuperPower. IEEE Trans. Appl. Supercond. 17, 3231–3234 (2007).
9. Rupich, M. W. et al. The development of second generation HTS wire at American
Superconductor. IEEE Trans. Appl. Supercond. 17, 3379–3382 (2007).
10. Shiohara, Y., Yoshizumi, M., Izumi, T. & Yamada, Y. Present status and future
prospect of coated conductor development and its application in Japan.
Supercond. Sci. Technol. 21, 034002 (2008).
11. Usoskin, A. et al. Processing of long-length YBCO coated conductors based on
stainless steel tapes. IEEE Trans. Appl. Supercond. 17, 3235–3238 (2007).
12. Malozemoff, A. P. & Yamada, Y. One Hundred Years of Superconductivity
689–702 (CRC Press/Taylor & Francis, 2012).
13. Marken, K. R. et al. BSCCO–2212 conductor development at Oxford
Superconducting Technology. IEEE Trans. Appl. Supercond. 13, 3335–3338
(2003).
14. Miao, H. et al. Development of round multifilament Bi-2212/Ag wires for high
field magnet applications. IEEE Trans. Appl. Supercond. 15, 2554–2557 (2005).
15. Miao, H. et al. Bi-2212 round wire development for high field applications,
1MOr2A-01. Applied Superconductivity Conference, August 10–15, Charlotte NC
(2014).
16. Scanlan, R. M., Malozemoff, A. P., Larbalestier, D. C. Superconducting materials
for large scale applications. Proceedings of the IEEE 10, 1639 (2004).
17. Chen, B. et al. Two-dimensional vortices in superconductors. Nature Physics 3,
239–242 (2007).
18. Larbalestier, D. C. et al. Isotropic round-wire multifilament cuprate
superconductor for generation of magnetic field above 30 T. Nature Materials 13,
375–381 (2014).
19. Jiang, J. et al. Doubled critical current density in Bi-2212 round wires by reduction
of the residual bubble density. Supercond. Sci. Technol. 24, 082001 (2011).
20. Jiang, J. et al. Reduction of gas bubbles and improved critical current density in Bi-
2212 round wire by swaging. IEEE. Trans. Appl. Supercond. 23, 6400206 (2013).
21. Markiewicz, W. D. et al. Perspective on a superconducting 30 T/1.3 GHz NMR
spectrometer magnet. IEEE Trans. Appl. Supercond. 16, 1523–1526 (2006).
22. Maeda, H. & Yanagisawa, Y. Recent developments in high-temperature
superconducting magnet technology. IEEE Trans. Appl. Supercond. 24, 4602412
(2014).
23. Godeke, A. et al. Development of wind-and-react Bi-2212 accelerator magnet
technology. IEEE Trans. Appl. Supercond. 18, 516–519 (2008).
24. Kametani, F. et al. Bubble formation within filaments of melt-processed Bi2212
wires and its strongly negative effect on the critical curren t density. Supercond. Sci.
Technol. 24, 075009 (2011).
25. Malagoli, A. et al. Evidence for long range movement of Bi-2212 within the
filament bundle on melting and its significant effect on J
c
.Supercond. Sci. Technol.
24, 075016 (2011).
26. Scheuerlein, C. et al. Void and phase evolution during the processing of Bi-2212
superconducting wires monitored by combined fast synchrotron micro-
tomography and x-ray diffraction. Supercond. Sci. Technol. 24, 115004 (2011).
27. Feldmann, D. M. et al. Mechanisms of enhanced supercurrent across meandered
grain boundaries in high-temperature superconductors. J. Appl. Phys. 102, 083912
(2007).
28. van der Laan, D. C. et al. The effect of strain on grains and grain boundaries in
YBa
2
Cu
2
O
7-d
coated conductors. Supercond. Sci. Technol. 23, 014004 (2010).
29. Feldmann, D. M. et al. Evidence for extensive grain boundary meander and
overgrowth of substrate grain boundaries in high critical current density ex situ
YBa
2
Cu
3
O
7-x
coated conductors. J. Mater. Res. 20, 2012 (2005).
30. Heine, K., Tenbrink, J. & Tho¨ner, M. High-field critical current densities in
Bi
2
Sr
2
CaCu
2
O
81x
/Ag wires. Appl. Phys. Lett. 55, 2442–2443 (1989).
31. Momma, K. & Izumi, F. VESTA 3 for three-dimensional visualization of crystal,
volumetric and morphology data. J. Appl. Crystallogr. 44, 1272–1276 (2011).
32. Zhu, Y. Q. et al. Structural origin of misorientation-independent superconducting
behavior at [001] twist boundaries in Bi
2
Sr
2
CaCu
2
O
81d
.Phys. Rev. B. 57, 8601
(1998).
33. Uemoto, H., Mizutani, M. Kishida, S. & Yamashita, T. Growth mechanism of Bi-
based superconducting whiskers. Physica C 392–396, 512–515 (2003).
34. D’yachenko, A. I. Hysteresis of transport critical current of high T
c
superconductors in strong magnetic fields. Theory, Physica C. 213, 167–178
(1993).
35. Marti, F., Grasso, G., Huang, Y. & Flu
¨kiger, R. High critical current densities in
long lengths of mono- and multifilamentary Ag-sheathed Bi(2223) tapes. IEEE
Trans. Appl. Supercond. 7, 2215–2218 (1997).
36. Bulaevskii, L. N., Clem, J. R., Glazman, L. I. & Malozemoff, A. P. Model for the
low-temperature transport of Bi-based high-temperature superconducting tapes.
Phys. Rev. B 45, 2545 (1992).
37. Hensel, B., Grasso, G. & Flu
¨kiger, R. Limits to the critical transport current in
superconducting (Bi,Pb)
2
Sr
2
Ca
2
Cu
3
O
10
silver-sheathed tapes: The railway-
switch model. Phys. Rev. B 51, 15456 (1995).
38. Bulaevskii, L. N., Daemen, L. L., Maley, M. P. & Coulter, J. Y. Limits to the critical
current in high-Tc superconducting tapes. Phys. Rev. B 48, 13798 (1993).
39. Cai, X. Y. et al. Current-limiting mechanisms in individual filaments extracted
from superconducting tapes. Nature 392, 906–909 (1998).
40. Kramer, E. J. Scaling laws for flux pinning in hard superconductors. J. Appl. Phys.
44, 1360 (1973).
41. Durrell, J. H. et al. Critical current of YBa
2
Cu
3
O
7-d
low-angle grain boundaries.
Phys. Rev. Lett. 90, 247006 (2003).
42. Shen, T. et al. Development of high critical current density in multifilamentary
round-wire Bi
2
Sr
2
CaCu
2
O
81d
by strong overdoping. Appl. Phys. Lett. 95, 152516
(2009).
Acknowledgments
This work was supported by the US Department of Energy (DOE) Office of High Energy
Physics under grant number DE-SC0010421, by the National High Magnetic Field
Laboratory (which is supported by the National Science Foundation under NSF/
DMR-1157490), and by the State of Florida. We acknowledge the help of V. S. Griffin in
Vibrating Sample Magnetometer measurements and N. C. Craig for transport critical
current measurements. We are also grateful for discussions with U. P. Trociewitz at
NHMFL, M. Rikel at NEXANS and C. Scheuerlein at CERN.
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SCIENTIFIC REPORTS | 5 : 8285 | DOI: 10.1038/srep08285 8

Author contributions
F.K. performed the metallography and electron backscatter diffraction, and prepared the
manuscript. J.J. and M.M. reacted the samples. J. J. and D.A. performed the transport critical
current measurements. E.E.H. and D.C.L. directed the research and contributed to
manuscript preparation. All authors discussed the results and implications and commented
on the manuscript.
Additional information
Supplementary information accompanies this paper at http://www.nature.com/
scientificreports
Competing financial interests: The authors declare no competing financial interests.
How to cite this article: Kametani, F. et al. Comparison of growth texture in round Bi2212
and flat Bi2223 wires and its relation to high critical current density development. Sci. Rep.
5, 8285; DOI:10.1038/srep08285 (2015).
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SCIENTIFIC REPORTS | 5 : 8285 | DOI: 10.1038/srep08285 9