C:\My Documents\MS_&_OH\CONDUCTO\1999\iwcc_intergrowths.doc 12/05/9916:28
c
J. E. Everett, G. K. Perkins and A. D. Caplin
Centre for High Temperature Superconductivity, Blackett Lab., Imperial College, London SW7 2BZ, UK
Martino Leghissa, Marijn P. Oomen,
Siemens AG, P.O.Box 3220, 91050 Erlangen, Germany
Abstract —We report transport measurements on t he adventitious intergrowths that often occur in multi-filamentary
BSCCO tapes. Typically, they are capable of carrying an average
transverse current density about an order of magnitude less than the conventional Jc. Since their areal cross-section appears to be rather small, the Jc of the intergrowths themselves, and also of their grain boundaries with the filaments, must be very high.
I. INTRODUCTION
The AC losses of high temperature superconducting
BSCCO-2223/Ag multi-filament tapes are often much higher than expected, sometimes as high as if the superconductor were a monocore. The low resistivity of the matrix tends to reduce the effectiveness of the filamentary architecture, but in addition there is direct evidence of supercurrent flow between filaments from transport [1], magnetisation [2] and Hall probe [3] studies; occasional BSCCO phase intergrowths can often be seen in cross-sectional micrographs [4].
Given the rather low density of such intergrowths that are visible, and that they are not crystallographically-aligned to the filaments themselves, it is surprising that they are capable of carrying sufficient current to be troublesome. Put the other way about, the material of the intergrowths, and also the grain boundaries to the filaments that they bridge, must have rather large J c
’s, and deserve more detailed examination. We report here on a transport study focused on the distinguishing the behaviour of the intergrowths from the filaments themselves.
II. EXPERIMENTAL DESIGN
In order to analyse the data, consider two extreme cases:
A. No Bridges
If there are no bridges between the filaments, the edge filaments behave (at low currents anyway) as superconducting bus bars, and are equipotentials along their length.
In that case the transverse current flow is macroscopically uniform, and it flows through a composite medium of the metallic sheath and (perfectly conducting) superconductor.
The effective resistivity
ρ eff
of this composite medium should take account of the current flow being short-circuited through the superconducting filaments (although this may be inhibited by the long current transfer length). Because the latter are interleaved and of high aspect ratio, they reduce
ρ eff by a substantial factor from the matrix resistivity. Even though it is difficult to gauge
ρ eff
accurately, in the absence of bridges the transverse I-V characteristics should be ohmic.
B. Fully-bridged
On the other hand, if there are so many superconducting bridges in the conductor that directions longitudinal and transverse to the filaments become equivalent, the transverse current crowds close to the current contacts, and the transverse voltage decreases rapidly along the length. In the ohmic regime above T c
, this is precisely what happens, and the current and electric field distributions are calculable. Because of the non-linear I-V characteristic of the superconductor, analysis of the current distribution below T c
is much less tractable, but the current-crowding should be accentuated.
The contact arrangement (Fig. 1) is somewhat different from that used in [1], and is designed to allow for the very long lengths (~ mm) over which current transfers between superconductor and the Ag (or Ag alloy) matrix [5]. The slots cut at the ends ensure that the current is transferred fully from the metallic sheath to the edge filament(s).
When current is fed in at one of the I
1
contacts, and out from an I
2
contact, the voltage across the pair V
4
, gives the
“standard” current-voltage (I-V) characteristic. Above T c
, this configuration gives also the sheath resistivity. The “transverse” I-V characteristics are obtained with current fed in and out through the contact pair I
1
, and monitoring the voltage pairs V
1
, V
2
and V
3
; the experiment can be repeated using the current pair I
2
.
Figures 2 and 3 compare results on two 55 filament (untwisted) tapes. Sample A has a AgMg sheath, but the filaments are embedded in a Ag matrix; for sample B, the matrix
I
1
III. RESULTS & DISCUSSION
V
2 0 m m
1
V
2
V
3
I
2
This work was supported financially by the UK Engineering & Physical
Sciences Research Council and by European Commission BRITE EURAM
Contract No BRPR-CT95-0030
V
4
Fig. 1. Contact configuration for measuring “standard” (V
4
) and “transverse” (V
1
, V
2
,V
3
) I-V characteristics

C:\My Documents\MS_&_OH\CONDUCTO\1999\iwcc_intergrowths.doc 12/05/9916:28
1E-5
No-bridges
1E-5
No bridges
1E-6
Fully-bridged
1E-7
V1
V2
77 K
V3
1E-8
1 10
Current (A)
Fig. 2 Measured transverse current-voltage characteristics of sample A for current injected and extracted at the contacts I
1
; the “No bridges” and
“Fully-bridged” lines are explained in the text.
is AgPd. Both tapes have J c vicinity of 3x10 4 A/cm 2 .
’s at 77 K and in self-field in the
The lines “No bridges” indicate the ohmic response expected from the effective medium of resistive matrix and sheath, as described above. The “Fully-bridged” curve is derived from the conventional (longitudinal) I-V characteristic, scaled by the relevant geometrical factors. It represents the voltage that would be observed close to the current contacts if there are so many bridges that the tape behaves identically in longitudinal and transverse directions; in that case, because of current crowding, the voltage should drop off rapidly at the further sets of contacts.
The response observed in Sample A at the voltage contacts
(V
1
) closest to the current contacts is indeed ohmic, and of the anticipated "No bridges" order of magnitude; those at the contacts V
2
and V
3
are also close to ohmic. These characteris-
1E-6
77 K
1E-7
I1-V1
Fully bridged I1-V2
I1-V3
1E-8
1 10
Current (A)
Fig. 3 Measured transverse current-voltage characteristics of sample B for current injected and extracted at the contacts I
1
; the “No bridges” and
“Fully-bridged” lines are explained in the text.
tics shows conclusively that in this sample there are no significant superconducting segments in the transverse current path. However, our superconducting bus-bar model is clearly an oversimplification, in that the measured voltages decrease with distance away from the current contacts.
In contrast, the I-V characteristics of sample B are strongly non-ohmic, and at V
1
, are almost as steep as the “Fullybridged” curve. Consequently, at low currents, the transverse voltage is very much smaller than in Sample A; e.g., at 1 A, the voltages at V
1
in the two samples differ by a factor of
~10
2
. Hence we may infer the presence of significant superconducting transverse current paths in Sample B.
A comparison of the I-V characteristic at the contacts V
1
of sample B with the calculated “Fully-bridged” curve suggests that the average transverse critical current density is about a factor of 5 lower than the longitudinal J c
. A few intergrowths are indeed visible in micrographs of a sample from the same batch (Fig. 4), but it has to be remembered that the sampling depth is very shallow, so that it may convey a misleadingly low impression of their number. Even so, the areal crosssectional density of the intergrowths appears to be substantially smaller than the cross-section of the filaments themselves. Thus the J c
of these adventitious intergrowths -and equally important, also of their grain boundaries with the filaments - appears to be very high .
ACKNOWLEDGMENT
We thank B. Fischer, Vacuumschmelze GmbH for sample preparation.
REFERENCES
[1] S.P. Ashworth et al., IEE Trans.Appl.Supercond., vol. 7, pp. 1662-
1665, 1997
[2] J. Everett et al., Physica C, vol. 310, pp. 202-207, 1998.
[3] A.V. Volkozub et al., Physica C, vol. 310, pp. 159-162, 1998.
[4] J. Everett et al., IEE Trans.Appl.Supercond., in press.
[5] M. Polak et al., Supercond. Sci. Technol., vol. 10, pp. 769-777, 1997.
Fig. 4 Cross-sectional optical micrographs of sample A (top) and B (bottom); intergrowths (circled) are visible in sample B.