Current Distribution in High-Tc Composite Tapes

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Nov 15, 2013 (3 years and 4 months ago)

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Current Distribution in High
-
Tc Composite Tapes


Yinshun Wang, Liye Xiao, Liangzhen Lin, Shaotao Dai and Ming Qiu



Abstract

The current distributions within HTS tapes are
important for the study of heat dissipation and thermal
stability of the composite
conductors. In this presentation,
c
urrent distributions
between superconducting ceramic core
and silver sheath

are

measured explicitly under
different
DC
magnetic fields
with different

orientation at
77K
.
Three new
parameters
I
cc
,
I
00
and B
cc

are defined t
o describe the I
-
V
characterization of HTS tapes when the current is over the
critical current, where I
cc

is almost equal to I
00

when
external

field is higher than B
cc
.



Index Terms

-

Bi2223/Ag tapes, Current distribution, critical
current, parameter


I
. INTRODUCTION


iigh Temperature Superconductor (HTS) have
electrrenewed the interest in application study of supercon
-

ductivity in electric power apparatuses[1]
-
[4]. The
replacement of copper by superconductors results in
considerable improvements in the

power
equipment

with
respect to their efficiency, size, performances and
environmental

impact. It

s likely that the first HTS power
transmission system will enter into the commercial market
over the next few years. Study of current distribution within
the

cross section of high Tc superconducting composite tape
or wire is fundamentally important in the development of
practical applications, such as cable, fault current limiter
(FCL), superconducting magnetic storage system (SMES)
etc. When the current flowi
ng in the composite tape
surpasses its critical current, the sharing current between the
superconducting core and the silver sheath must be
considered.

In this paper, we present the I
-
V characterizatics of
BSCCO/Ag
-
2223 tapes which are subject to DC field
s with
different strength and orientation at 77K. The I
-
V
characterizatics is then used to study the current distribution
in the superconducting core and silver sheath. At same time,
we defined three interesting parameters which is related to


Manuscrip
t received September 2
5
, 2001.

Yinshun
W
ANG is with Applied Superconductivity
Lab.
, Institute of
Electrical Engineering, Chinese Academy of Sciences, Beijing 100080,
P. R.
China
and
with
Dept. of Physics, Hebei University,
Baoding
071002,HebeiProvince,
P.R.
China
(e
-
mail:

yswang@mail.iee.ac.cn
).


Liyie Xiao is with Applied Superconductivity Lab., Institute of
Electrical Engineering, Chinese Acadeny of Sciences, Beijing100080,
P.R.China (e
-
mail:xiao@mai.iee.ac.cn)

Liangzhen LIN is with Applied Superconductivi
ty Lab., Institute of
Electrical Engineering, Chinese Academy of Sciences, Beijing100080,
P.R.China (e
-
mail:xiao@mai.iee.ac.cn).

Shaotao DAI is with Applied Superconductivity Lab., Institute of
Electrical Engineering, Chinese Academy of Sciences, Beijing10
0080,
P.R.China (e
-
mail:stdai@mai.iee.ac.cn)

Ming QIU is with Applied Superconductivity Lab., Institute of
Electrical Engineering, Chinese Academy of Sciences, Beijing100080,
P.R.China (e
-
mail:qiuming@mai.iee.ac.cn)

critical current to describe the I
-
V cha
racterization when the
current is over the critical current and
external

field is higher
than a certain value.


II. EXPERIMENTAL


The test samples are two multifilamentary Bi2223/Ag
tapes, which have some differences in their geometries,
electromagnetic an
d mechanical properties. The
specifications of the samples are listed in Table I.


Table I

SPECIFICATIONS OF SAMPLE TAPES


Sample A B


Superconductive Bi2223/Ag tape Bi2223/Ag tape

Material

Length(cm) 10 1
0

Dimension(mm
2
) 5.1

0.22 4.1

0.21

I
c
(A) 20 102

(1

V/cm at 0T, 77K)

DC background field Applied Applied

(0
~
250mT)



The DC characterization was measured with four
-
probe
technique. E
--
I curve of sample A was measu
red under
different background field and different field orientation, and
that of sample B was measured under perpendicular field
only. The angle between field orientation and tape surface is
shown as Fig. 1.










Fig. 1. Schematic of ang
le between field and tape surface



The background field is produced by a racetrack
-
shape
magnet, the homogeneity of the field space where the
samples were tested is smaller than 1%. The magnet is
cooled by liquid
nitrogen

together with sample in a cryosta
t.
Sample frame can rotate from

90
0

to 90
0

continuously, so
measurement can be done at different fields and directions.
The samples are 4.5cm long between voltage taps. Critical
current I
c
(B)

was measured by criterion of 1

V/cm. During
the measurement, th
e current flowing in the tapes was
raised
till the

electric field of the tapes was up to 200

V/cm,


B


Tape


H


n


obtaining complete current
-
distribution picture in the
composites, while the current was set under 2Ic to avoid
temperature rise above 77K.


III. RESULTS AND

DISCUSSION


A.

Current distribution results


The current flowing in the silver sheath can be simply
calculated by[5]:




B
R
V
I
Ag
Ag


(1)

where V is the voltage drop along the sample, and


B
R
Ag
is
the resistance o
f silver sheath which is related to magnetic
field B[6]
. The cross section of silver sheath was calculated
through the measurement of the resistance of the BSCCO
tape at room temperature, thus we could get the resistance of
silver sheath at 77 K by neglect
ing the thermal contraction of
the sample.





































Fig.2 refers to the critical current reduction of sample A
due to the field orientation change at fixed external field
B=40mT.

The critical current I
c
(

) is quite sensitive t
o field
orientation when


is smaller than 35
0
; when


is larger than
35
0
, I
c
(

) is almost a platform.
Fig.3 presents the critical
current reduction in sample B due to different perpendicular
external field. The I
c
(100mT) is still about 50%I
c
(0).









In order to show clearly, logarithm scale coordinate were
chosen in Fig.5. According to Fig.4 and Fig.5, we can get
that the current flowing in silver sheath is less than 1% I
c
,
more than 99% of the current flows in superconducting core.
And it

s also note
d that the current distribution between the
core and sheath is greatly affected by the nonlinear nature of
the superconducting materials, and the repartitions strongly
depend on the field orientation.






Fig.4 shows the current distribution of sample A
in the
superconducting core and the silver sheath at fixed external
field B=40mT, and Fig. 5 presents the same picture of
sample B. Obviously, the current entering the silver sheath
increases with the increasing of the field angle and magnetic
field. The c
urrent flowing in silver sheath is less than 1%I
c

when tape carries current I
c
.





















A.

Overload characterization results


Fig. 6 is a typical V
-
I curve. When the tape is totally
0
50
100
150
200
0
20
40
60
80
100
120
sample B
Critical current I
c
(A)
Perpendicular background filed B(mT)

Fig.3. Critical current
reduction due to vertical
background field B(mT) for sample B

0
4
8
12
16
20
24
28
32
0
7
14
21
28
SC at



Ag at


SC at


Ag at


SC at


Ag at


Current in SC and Ag Sheath (A)
Total Current I
T
(A)

10
1
10
2
10
-2
10
-1
10
0
10
1
10
2
SC at B=0mT
Ag at B=0mT
SC at B=20mT
Ag at B=20mT
SC at B=40mT
Ag at B=40mT
SC at B=80mT
Ag at B=80mT
Current Distribution (A)
Total Current in tapes I
T
(A)






Fig. 5. Current repartition between superconducting core and silver
sheath for different applied perpendicular fields for sample B.


Fig.2. Critical current reduction due to different angle between
filed B and tape surface at B=40mT for sample A



Fig.4. Current repartition between superconducting core and silver
sheath for different angle


at external field B=40mT for sample A

0
20
40
60
80
100
0
5
10
15
20
25
Sample A
Critical Current I
c
(

)(A)
Angle

(Degree)

normal conductor, it behaves as resistance, then it can be
fit
ted as follows:


b
aI
E





(2)

a
b
I

00



(3)

c
cc
I
I
2
3






(4)


Where
we defined two parameters: I
00

and I
cc
.
According to the experimental resul
ts, an interesting relation
Eq.(5) was found while the external magnetic field is over
certain value for different tape:


cc
I
I

00

(5)








































Fig.7 shows I
00

and I
cc

under field B, an
d Fig. 8 refers
to the I
00

and I
cc

with different angle


at B=40mT for
sample A. When external magnetic fields are over B
cc
=10mT
and B
cc
=30mT for perpendicular and
parallel

respectively
(Here define B
cc

as a criterion value over which I
00

and I
cc

are appr
oximately equal, the parameters I
00

and I
cc

are almost
the same, so are this case at B=40mT for different angle.
Both I
00

and I
cc

almost equal at this fixed magnetic field and
are independent of angle

. The surprising fact is that
parameter B
cc

is also a
function of angle

.









































Fig. 9 presents the I
00

and I
cc

under perpendicular
external magnetic field for sample B, while magnetic field is
over B
cc
=30mT, the two parameters are also almost the
same.

Two multifilament
ary Bi2223/Ag tapes, which were
fabricated by different companies with the PIT technique,
have the same relation though they have different ratio of
Ag/SC and different processing method. Therefore, this
characteristic is possibly intrinsic property of Bi
-
2223
0
20
40
60
80
100
120
0
20
40
60
80
100
120
140
160
180
I
00
I
cc
Parameter I
cc
and I
00
(A)
Magnetic Field B(mT)

Fig. 9. I
00
and I
cc

under perpendicular magnetic field for sample B

0
50
100
150
200
250
0
5
10
15
20
25
30
35
I
cc
at

=0
0
I
00
at

=0
0
I
cc
at

=90
0
I
00
at

=90
0
Parameters I
cc
and I
00
(A)
External magnetic field B(mT)

Fig. 7 I
00

and I
cc

under different external field sample A


Fig.6. Schematic of a typical E
-
I curve for fitted parameters.


0
10
20
30
40
50
60
70
80
90
0
5
10
15
20
25
I
cc
I
00
Parameters I
cc
and I
00
(A)
Angle

(Degree)

Fig. 8. I
00
and I
cc

with angle


between field and tape surf
ace in sample A


0
20
40
60
80
100
120
0
30
60
90
120
150
180
210
I
00
Test E--I Curve
Linear-Fitted Part on The Curve
E=aI-b, I
00
=b/a
E(

V/cm)
Current I(A)


ceramic material and is almost independent of sheath. We
supposed that:


c
cc
kI
I
I


00

(6)


where
k

is a constant which might depend on the intrinsic
properties of superconducting materials. As I
00

is obtained
by fitting the E
-
I

curve when current is much more than I
c

and magnetic field is above B
cc
, it can be used to describe
the characteristics of tape operating in overload. Therefore,
I
00

is intrinsic parameter of superconducting tape, just like
that of n value.

In this experi
ment, we investigated current distribution
of the Bi2223/Ag multifialmentary tapes at 77K under low
magnetic field region (
<
250mT). How about the situation
when the magnetic field is above 250 mT, or the temperature
is below 77 K? Whether do other HTS mate
rials such as
Yi
-
based bulk and low Tc superconductors have the same
behavior? All of these questions are needed to study in detail
in near future.


IV. CONCLUSION


Current distributions
within superconducting ceramic
core and silver sheath

have been inves
tigated explicitly
under different DC magnetic fields
and field

orientation at
77K.
The current flowing in silver sheath is less than 1%I
c

while the tape carries critical current I
c
. Three new
parameters I
cc

, I
00

and B
cc

which might be
intrinsic

for HTS,
are obtained in overload conditions. I
00

is approximately
equal to 3/2I
c

when the DC background field is above B
cc
,



























which may be another characteristic of high temperature
superconducting material. However, we will further study

this problem and suggest a theory to explain this behavior in
the future.


V. ACKNOWLEDGMENTS


The authors would like to thank Dr. Huaming Wen,
Prof. Yubao Lin and Huidong Li for their help in
experiments.


REFERENCES


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