Magneto-optical imaging of flux patterns in multifilamentary
(BiPb)2Sr2Ca2Cu3Ox composite conductors
U. Welp, D. O. Gunter, G. W. Crabtree, J. S. Luo, and V. A. Maroni
Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439
W. L. Carter
American Superconductor Corporation, Westborough, Massachusetts 01581
V. K. Vlasko-Vlasov and V. I. Nikitenko
Institute for Solid State Physics, 142432 Chernogolovka, Moscow District, Russia
~Received 14 October 1994; accepted for publication 22 December 1994!
We present a study of the superconducting morphology of the transport current carrying cross
section of a 19-filament ~BiPb!2Ca2Cu3Ox ~Bi-2223! composite conductor using magneto-optical
imaging of magnetic flux patterns. In conjunction with electron microscopy on the same sample this
technique allows a unique correlation of superconducting and microstructural properties. Direct
evidence for enhanced superconducting properties in platelike regions along the silver/Bi-2223
interface and for weak properties near the core of the filaments is obtained. Misaligned grain
colonies are found to cause an interruption of the superconducting continuity in the
filaments. © 1995 American Institute of Physics.
A central goal of research on high-temperature superconducting ceramics is the establishment of a methodology for
the reliable fabrication of long length wire for use in magnet
systems and ac power transmission technology. At present,
the most promising systems are silver-sheathed
~BiPb!2Sr2Ca1Cu2Ox ~Bi-2212! and ~BiPb!2Sr2Ca2Cu3Ox
~Bi-2223! composites. Critical current densities J c at 77 K in
self-field in excess of 10 000 A/cm2 have been reported1 for
lengths exceeding a kilometer. Further improvement of the
performance of these conductors seems possible if the limiting factors in the present materials can be identified. In recent studies it has been shown that the conductor does not
respond uniformly across its cross section to an imposed
transport current. By slicing a Bi-2223 monofilament composite into strips parallel to the rolling direction Larbalestier
et al.2 showed that J c in the center of the filament is strongly
reduced as compared to J c near the edges which are in intimate contact with the silver sheath. In a systematic comparison of microstructure and critical current density Merchant
et al.3 observed that high performance conductors are characterized by a substantial degree of texturing of the Bi-2223
grains. This texture develops between superconductor and
silver sheath during the heat treatment phase of the thermomechanical processing cycle ~reaction induced texture!. A
recent transmission electron microscopy ~TEM! study4 on
cross sections of monofilamentary Bi-2223 composites confirms the presence of a highly textured almost phase-pure
Bi-2223 layer at the Ag/Bi-2223 interface, with second
phases more prominent toward the interior of the conductor.
Critical current densities of 100 000 A/cm2 in this thin interface layer were observed by Lelovic et al.5 by peeling off the
majority of a Bi-2223 monofilament. A recent magnetooptical study on a longitudinal cross section of a Bi-22231
and of a Bi-22126 conductor showed substantial variation of
J c along the rolling direction.
Here we report the first direct observation of the nonuniformity of the superconducting behavior in the transverse
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Appl. Phys. Lett. 66 (10), 6 March 1995
cross section ~i.e., the transport current carrying cross section! of a 19-filament Ag/Bi-2223 composite as function of
temperature and applied magnetic field using a highresolution magneto-optical technique. With increasing temperature and especially with increasing field a pronounced
superconducting granularity in the filament develops. We
present the first direct evidence for the occurrence of highpinning platelets near the Ag/Bi-2223 interface and a region
of weaker pinning in the center of the filaments. Also in
cross sections presented further on two interconnects between neighboring filaments are observed. Whereas one of
them does not carry a supercurrent at high temperatures and
fields the other is a strong link at all temperatures and fields
studied. The importance of such superconducting interconnects between filaments for the evaluation of ac losses is
discussed. A comparison with scanning electron microscopy
~SEM! images of the same cross section allows a close correlation of the observed magnetic flux patterns with the microstructure.
The magnetic patterns were recorded using a highresolution magneto-optical technique as described before.7 It
is based on the Faraday effect in a yttrium–iron–garnet film
placed on top of the sample. The enhanced optical sensitivity
allows the observation of flux patterns in fields up to 1000 G
at temperatures as high as 77 K with a spatial resolution of
about 3 mm. These temperature and field conditions are close
to the expected practical operating conditions of high-T c superconducting cables. The Bi-2223 composite conductor
studied here was prepared using the oxide-powder-in-tube
~OPIT! process which has been described in detail
elsewhere.8 The composite contains 19 filaments that lie in
an intermediate packing pattern9 of 3, 4, 5, 4 and 3 filaments
edge-to-edge. The critical current of this conductor is 35 A at
77 K in self-field. Details magnetic flux images were taken
near an edge pack and near the center pack of the composite.
These two areas experienced a large difference in total de-
0003-6951/95/66(10)/1270/3/$6.00
© 1995 American Institute of Physics
FIG. 1. SEM image @panel ~a!# and magnetic flux patterns at 30 K @panel
~b!# and 77 K @panel ~c!# of the filaments near the edge of an Ag/Bi-2223
19-filament composite. The total thickness of the composite is 175 mm. The
various labels are described in the text.
formation during the following process resulting in thinner
filaments near the center.
The magnetic flux patterns were recorded in a transverse
cross section, that is, in a plane perpendicular to the rolling
direction of the composite with magnetic fields applied parallel to the rolling direction. This geometry does not give
information on the variation of superconducting properties
along the transport current direction ~i.e., the rolling direction!. However, it yields directly the superconducting morphology of the cross section through which the transport current has to flow. Figure 1 shows the magnetic flux patterns at
30 and 77 K, respectively, in 855 G associated with the three
filaments near the edge of the tape together with the corresponding SEM image. The bright regions in the SEM image
@Fig. 1~a!# are the silver matrix which has an overall crosssection thickness of 175 mm. The cross section is characterized by a variety of microstructural nonuniformities, namely
second phase particles ~labeled ‘‘S’’!, irregularities in the
alignment of the grain colonies ~labeled ‘‘G’’!, platelike outgrowths into the silver matrix ~labeled ‘‘OG’’! and pullouts
generated in the polishing process ~labeled ‘‘P’’!. The second
phase
particles
contain
mostly
~Ca,Sr!2CuO3 ,
~Ca,Sr!14Cu24O41 , and CuO. In the flux images @Figs. 1~b!
and 1~c!# bright areas correspond to high levels of local field
and dark areas to low local field levels. Low local fields at
the sample surface are caused by effective shielding of the
applied field by the superconductor, i.e., these areas constitute zones of high local critical current. The nonuniformities
in the microstructure cause characteristic features in the flux
patterns.
Misoriented grain colonies give rise to sections of very
weak critical current that effectively decouple different parts
of the filament. This is particularly obvious near the middle
of the left filament ~G1, G4!. The misoriented Bi-2223 colonies show up as light gray bands in the SEM image. Since
these colonies contain well-aligned phase pure Bi-2223 they
exhibit good superconducting shielding causing dark bands
Appl. Phys. Lett., Vol. 66, No. 10, 6 March 1995
FIG. 2. SEM image @panel ~a!# and magnetic flux patterns at 30 K @panel ~b!
and 77 K ~panel ~c!# of the filaments near the center of an Ag/Bi-2223
19-filament composite. The total thickness of the composite is 175 mm. The
various labels are described in the text.
in the magnetic patterns. G1 is the dark region right above
the bright band across the middle of the left filament and G4
is the narrow dark line interrupting that bright band. Since
these colonies are misoriented by nearly 45° with respect to
the overall texture of the filament there exists a transitional
region above and below the colonies with strongly distorted
texture causing a depression of I c . These regions are the
bright bands in the flux image. Similar behavior occurs near
G2 and G3.
As discussed above the Ag/Bi-2223 interface has a positive influence on the structural and superconducting properties. The flux patterns allow for the first direct observation of
this phenomenon. The lower tip of the middle filament is
characterized by two ‘‘good’’ superconducting outer layers
~seen as well-aligned colonies in the SEM image! and a
‘‘weak’’ center. This weak center line extends throughout the
entire filament and is interrupted only by misoriented colonies. Similar behaviors are observed near the bottom tip and
the top left edge of the right filament. These images demonstrate that enhanced superconducting properties along the
Ag/Bi-2223 interface also exist in multifilament composites
which contain much thinner filaments ~as compared to
monofilament tapes!. The degree of development of this
sandwiched structure ~‘‘good’’ interfaces–‘‘weak’’ core! varies strongly among filaments, in particular the center section
of the middle filament in Fig. 2 appears almost uniform with
good current carrying capability. The origins for this variation have not been clarified yet; however, it is shown that the
OPIT process can yield filaments with uniform superconducting properties in their cross sections.
With increasing temperature critical currents in Bi-2223
composites decrease rapidly10 which causes a strong decrease in the observed magnetic flux patterns. Figure 1~c!
shows the magnetic patterns at 77 K. This image has been
amplified strongly ~as evidenced by the high level of the
background noise! in order to yield a legible figure. We note
that with increasing temperature the structure of the magnetic patterns stays essentially the same, that is, no new weak
links appear with increasing temperature. This implies that a
Welp et al.
1271
field of 855 G applied at low temperatures is strong enough
to break all the weak links and that the effect of increasing
temperature is merely a further weakening of these links. The
field dependence of these patterns will be discussed in more
detail elsewhere.11 Pullouts or second phase particles cause
disturbances of the flux patterns only when they cluster as
seen near the top of the right and left filaments in Fig. 1.
The SEM image and the flux patterns at 30 and 77 K of
the central area of the composite are shown in Fig. 2. The
magnetic patterns are characterized by a platelike structure
oriented along the Ag/Bi-2223 interface in a manner similar
to the behavior of the edgestack. This is clearly visible in the
two left and the far right filaments. The texture in this central
part of the tape appears to be better developed than near the
edges; the number of misaligned grain colonies which cause
an interruption of the supercurrent in the filament appears to
be reduced in the strongly deformed center stack. In addition
to these features this cross section is characterized by two
interconnects ~labeled ‘‘I’’! between the filaments. The flux
image at 30 K @Fig. 1~b!# shows that the large interconnect
~I1! carries a supercurrent whereas the small interconnect
~I2! is decoupled. Even though the flux patterns near the
large interconnect are somewhat diffused at 77 K, it appears
~by following the plates at the filament edges! that at this
elevated temperature the large interconnect is superconducting.
An important consideration in the design of superconducting power-transmission lines is the level of ac losses.
Depending on the geometry of a composite conductor and
the presence of longitudinal or transverse magnetic fields in
addition to the self-fields due to a transport current, rather
complex loss mechanisms develop12 in which the coupling
between the filaments is a crucial parameter. In the special
case of independent filaments it is expected that the ac losses
~at small field amplitudes! in a multifilamentary conductor
decrease like the inverse of the filament number ~for constant
superconducting cross section!.13 However, experimentally it
is found14 that the ac losses are essentially independent of the
number of filaments. This behavior can be caused by a sufficient number of superconducting interconnects which will
cause a strong coupling between filaments yielding an effective monofilamentary conductor.
In conclusion, using a high-resolution magneto-optical
technique we have obtained magnetic flux patterns of the
current carrying cross section of a multifilamentary Bi-2223
composite conductor at temperatures up to 77 K and fields of
855 G. In conjunction with the microstructure determined
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Appl. Phys. Lett., Vol. 66, No. 10, 6 March 1995
from electron microscopy this technique gives unique information on the superconducting morphology of the sample.
The images show that misaligned Bi-2223 grain colonies
cause an interruption of the supeconducting properties of the
filaments. Direct images of the enhanced superconducting
properties along the Ag/Bi-2223 interface and a depression
in the interior of the filaments have been obtained. Due to the
rapid decrease of the critical currents, the magnetic signal
decreases rapidly with increasing temperature. However, the
structure of the magnetic patterns, i.e., the superconducting
granularity, does not increase in any sufficient way. Also, we
imaged two interconnects between filaments one of which
was found to carry a supercurrent in high fields and discuss
its significance of ac losses.
This work was supported by the U.S. Department of Energy, BES—Materials Science ~U.W., G.W.C.! and the Office
of Energy Efficiency and Renewable Energy ~J.S.L., V.A.M.!
under Contract No. W-31-109-ENG-38 and the NSF-Office
of Science and Technology Centers under Contract No.
DMR91-20000 ~D.O.G.!.
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