Performance Test of Bi-2212 HTS Current Leads Prepared by the Diffusion Process

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Nov 15, 2013 (3 years and 11 months ago)

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1

17
th

International Conference on Magnet Technology


Abstract
--
Bi
-
2212 HTS
bulk conductors
have

been prepared
by the
two components
diffusion process

for
current lead
application. The Bi
-
2212
HTS

layer is synthesized through
a
diffusion reaction between a Sr
-
Ca
-
Cu oxide substrate and a Bi
-
Cu oxide coating

with Ag addition. The
HTS
diffusion layer
ab
out 150
µ
m in thickness formed around the cylindrical tube
27/19mm in

outside/inside diameter

and 200mm in length
.

The transport

c
urrent

of the tub
ular

specimen exceeds 4,000A at
4.2K and

self
-
field
, which corresponds to a

transport current
density of 20,0
00A/cm
2
.

The Joule heating of the joint
at an
end
of the specimen is
estimated to be 200m
W from
the overall joint
resistance of 12n
Ω
.
Therefore, the total heat load, including heat
leakage conducted through
the
tubular specimen between 4K
and 40K, is expected to be less than 400mW at 4,000A. Present
Bi
-
2212 HTS conductors with large transport current as well as
low
joint
resistance an
d thermal conductivity, seem to be
promising as
a
current lead for superconducting magnets.



Index Terms
--
Bi
-
2212 oxide superconductor, diffusion process,
HTS current lead, transport current

I.

INTRODUCTION

IGH
-
Tc superconductors can be synthesized
through


the diffusion process in a
n appreciably

shorter reaction
time than that of
the HTS

prepared by the conventional
sintering process. In the Bi
-
Sr
-
Ca
-
Cu
-
O system, a thick and
homogeneous HTS

layer of Bi
2
Sr
2
CaCu
2
O
8+X

(Bi
-
2212) is
easily synthesized by the di
ffusion reaction between Bi
-
free
Sr
-
Ca
-
Cu oxide substrate and Bi
-
Cu oxide coating [1]. The
addition of Ag to the coating was found to enhance the
diffusion reaction [2]. In the present study, the transport
current performance
of Bi
-
2212 HTS tubular bulk sp
ecimen
will be reported. Referring to the transport performance, Joule
heating at the joints and heat leakage through the bulk
specimen, the Bi
-
2212
HTS
conductors synthesized by the
diffusion process seem to be promising for a current lead in
both
cryocoo
ler
-
cooled superconducting magnets [3] with
small allowable heat load
and
conventional cryogen
-
cooled
superconducting magnets with large transport current [4]
-
[6].


Manuscript
received

September 24, 2001.

Y.Yamada, O.Suzuki, M.Enomoto and K.
Tachikawa

are w
ith the school
of Engineering, Tokai University, Hiratsuka, Kanagawa 259
-
1292, Japan.

(telephone:+81
-
463
-
58
-
1211, e
-
mail:yyamaday@keyaki.cc.u
-
tokai.ac.jp)

H.Tamura, A.Iwamoto and T.Mito are with the National Institute for
Fusion Science, Toki, Gifu 509
-
529
2 Japan (telephone:+81
-
572
-
58
-
2123,

e
-
mail:mito@nifs.ac.jp)


Geneva, September 24
-
28,2001, TUPO3D2
-
09

II.

E
XPERIMENTAL

The preparation procedure of Bi
-
2212
HTS tubular
specimen
through the diffusion process is schematically
shown in Fig. 1. The substrate is c
omposed of Bi
-
free Sr
-
Ca
-
Cu oxide with
the
composition ratio of Sr:Ca:Cu=2:1:2
(referred to
as

0212

). The calcined 0212 oxide powder was
formed into cylindrical tubes 27/19mm in outside/inside
diameter
and 200mm in length
by cold isostatic pressing

(CIP)

of 200 MPa.
It was

then sintered at 1000
°
C in open air.
The coating is composed of Bi
-
Cu oxide with
the
composition
ratio of Bi:Cu=2:1(referred to
as
2001). The calcined 2001
oxide powder with 30wt%Ag
2
O addition was mixed
with
wax
to form
slurry
, and
was
coated around the tubular substrate.

The
diffusion reaction
was performed at 850
°
C for 20h in
open air to produce the Bi
-
2212
HTS

layer.

Ag added to the
coating
accumulates

on the surface of the specimen after the
reaction. Then, the Ag paste was coated ar
ound both ends of
the diffusion specimen, and was heat
-
treated at 800
°
C

in air

























Fig. 1. Preparation procedure of the Bi
-
2212 HTS tubular specimen by

the diffusion process.

Performance Test of Bi
-
2212 HTS Current
Leads
Prepared by the Diffusion Process

Y.
Yamada
, O.
Suzuki
,
M. Enomoto,
K.

Tachikawa
,
H. Tamura, A. Iwamoto
and T. M
ito

H

Bi
-
free Oxide

Bi:Sr:Ca:Cu

(0:2:1:2)

Cold Isostatic

Pressing

Sintering

Coating

T
u
bular

Substrate

Ag Coating

Bi
-
Cu Oxide

Bi:Sr:Ca:Cu

(2:0:0:1)

+

Ag
2
O

Heat treatment

HTS Current Lead

Coa
ting

Bi
-
2212

Ag contact

Substrate

Coating

Diffusion Reaction

re
Re
Re
ac
R
Rr
eR
eR
Re
act
ion

Bi
-
2212
Diffusion Layer

LllllLLlLL
ayer


2

to form the Ag contacts. Som
e of the specimens were
reinf
orced by glass
-
fiber
-
reinforced
-
plastic (GFRP) and
epoxy resin to
improve

the mechanical properties [7].

The structural properties of the prepared specimen were
studied by a scanning electron microscope

(SEM) and X
-
ray
diffractometry

(XRD). The Tc and Ic
of the specimens were
measured resistively by a dc four
-
probe method.
The Jc was
obtained by dividing Ic by the cross
-
sectional area of

the diffusion layer.
The
large
transport

currents
at 4.2K and

self
-
field were measured by the facilities of National In
stitute
for Fusion Science

(NIFS).

III.

R
ESULTS AND
D
ISCUSSIONS

A.

Structural Properties of Bi
-
2212 HTS Specimens

Fig. 2 shows Bi
-
2212 HTS current lead specimen of
27/19mm in outside/inside diameter and 200mm in length.
The Ag contacts of
about

100

µ
m

in thicknes
s are formed
around both ends of the tubular specimen. The low resistive
contacts result from the good bonding between the Ag paste
and the
precipitated

Ag on the HTS layer after the diffusion
reaction.


SEM micrograph taken on the cross
-
section in the
ci
rcumference side of the HTS specimen is shown in Fig. 3.














Fig. 2. Bi
-
2212 HTS current lead specimen 200mm in length with Ag

contacts at both ends.
















Fig. 3. SEM micrograph taken on the cross
-
section of the Bi
-
2212 HTS

current

lead specimen.

The Bi
-
2212 HTS diffusion layer of about 150
µ
m

is
synthesized around the tubular substrate. The diffusion layer
is composed of thin plate
-
like grains grown along the
diffusion direction, that is, the radial direction of the tube.
The charac
teristic structure of the diffusion layer results from
the preferred grain growth.

Fig. 4 indicates the XRD pattern taken on the outside
surface of the specimen after removing the Ag precipitation
by an H
2
O
2
+NH
4
OH etching solution. The XRD pattern
indicate
s strong (200) peak. Some CuO particles unsolved by
the etching solution remain on the surface. Therefore, the
diffusion layer is found to be composed of a
-
axis oriented
grains. The transport current
longitudinally

passes through the
tubular specimen along

the a
-
b planes of the grains.


















Fig. 4. X
-
ray diffraction pattern taken on the outside surface of the Bi
-
2212

HTS current lead specimen.


B.

Transport Performance of Bi
-
2212 HTS Current Lead

Fig. 5 shows the set
-
up of the Bi
-
2212 HTS curren
t lead
specimen connected to the Cu bus bar of 30kA and NbTi
superconducting wires. The Bi
-
2212 HTS specimen is
soldered to both Cu end caps using commercial 60wt.%Sn
-
Pb
solder. Eight voltage taps were attached on the Cu caps (V1,
V3, V6, V8), HTS specimen

(V4, V5) and in the Sn
-
Pb
solder pool (V2, V7). The voltage taps not shown in Fig. 5
are installed in inside or back of the specimen. SUS304
stainless steel boards serve as role of the shunt, and prevent
thermal stress that works in HTS bulk specimen.

The

transport performance for the Bi
-
2212 HTS specimen
at 4.2K and self
-
field is shown in Fig. 6. The transport current
was passed at a ramp rate of 50A/s. No voltage on the HTS
part(120mm between V4 and V5) was generated at a transport
current of 4,000A. The

HTS voltage appeared at near 4,200A,
and a voltage of 90
μ
V on HTS part was
generated

at 4,500A.
However, the transport current was stably run for 30s without
any

rise of the voltage in HTS part.

The voltages of both joints increased with increasing
current, and were 50
μ
V at positive joint and 80
μ
V at negative
10
20
30
40
50
60
70
(008)
(0010)
(200)
(0012)
CuO
(1115)
(139)
CuO
(400)
(228)
Intensity (arb. unit)
2
θ 
(deg.)
CuO
(2010)
(220)
(135)
CuO
CuO
CuO

Surface

Substrate

100
μ
m


3

j
oint after reaching 4,000A. Therefore, the Joule heats are as
small as 0.2W and 0.32W at positive and negative joint,
respectively. The joint voltage rose sharply at near 4,500A,
and especially, the negative joint voltage rose to 350
μ
V.

Although the sweep

rate was quickened with 150, 400 and
1,000A/s, there was hardly the change in the voltage of each
part.

The voltages at V1 to V8 as a function of the transport
current up to 4,500A at 4.2K and self
-
field are shown in

Fig. 7. The voltages at both joints w
ere mainly caused by the
resistance between V2 and V4 and between V5 and V7,
respectively. The resistance consists of Sn
-
Pb solder, Ag
























Fig. 5. Left : Set
-
up of the Bi
-
2212 HTS current lead specimen connected to
Cu bus bar of 30kA

and NbTi superconducting wires,

right : enlarged Bi
-
2212 HTS specimen prepared with Cu end caps,
SUS304 plates and voltage taps.

















Fig. 6. Transport current performance of Bi
-
2212 HTS current lead at 4.2K

and self
-
field.

contact and Bi
-
22
12 HTS part. The voltages of Cu end cap
(V1
-
V2 and V7
-
V8) are in order of several
μ
V.

Fig. 8 shows the temperature dependence of the resistivity
for Sn, Pb and commercial Sn
-
Pb solders, 50wt.%Sn
-
50wt.%Pb(50Sn
-
Pb), 60wt.%Sn
-
40wt.%Pb(60Sn
-
Pb) and
63wt.%Sn
-
37wt.%Pb (eutectic 63Sn
-
Pb). The resistivity of
the Sn
-
Pb solders is about 2 n
Ω·
m in the magnetic field above
0.08T of the critical field (Hc) for Pb metal at 4.2K [5].

The resistivity of the 60Sn
-
Pb solder is slightly lower than
that of other two solders
. The resistivity of the commercial
solders at 40K is about one order higher than that of them at
4.2K. The resistance of the 60Sn
-
Pb solder calculated from
the contact area and thickness in soldering is in order of n
Ω

at
4.2K. Thus, the voltage due to the

solder at 4,000A is less
than several tens
μ
V at most. The voltage of the Ag contact is
below
μ
V since the resistivity of Ag at 4.2K is two orders of
magnitude lower than that of the commercial solders.
Therefore, The high voltage of the joints above 4,00
0A seems
to include the resistance generated on Bi
-
2212 HTS bulk.



















Fig. 7. Voltages at V1 to V8 as a function of the transport current up to


4,500A at 4.2K and self
-
field.

















Fig. 8. Temperature dependence of resistivi
ty for Sn, Pb metals and


commercial Sn
-
Pb solders.

SUS

304

Bi
-
2212

HTS C.L


Nb
-
Ti

Cu

bus bar

V1

V4

V5

V8

Cu

Cu

SUS

304

VI.

1


V.

3


IV.

4


III.

7


II.

8


5

4

3

1

I.

6



6

7

8

(
+)

2

(
-
)

V1-2
V2-4
V4-5
V5-7
V7-8
V3-6
V1-8
Transport Current (A)
Voltage (
μ
V)
4.2K
0
1000
2000
3000
4000
5000
0
200
400
600
0
20
40
60
80
100
1
5
10
50
100
Resistivity(n
Ωm
)
Temperature(K

50Sn-Pb
60Sn-Pb
63Sn-Pb
Pb
Sn
200
300
300
0
200
400
600
800
Ti me (s)
Voltage (
μ
V)
V4-5
V1-4 (+)
V5-8 (-)
Transport Current (A)
Current
4.2K
200
400
600
800
1000
2000
3000
4000
5000
6000
0
0

4

C.

Estimation of Heat Loads for the HTS Current Lead

The heat loads Qt of current lead consist of heat leakage
Qc conducted through current lead and Joule heat Qj at
joint
between current lead and metal pa
rts such as Cu
electrode.
The h
eat Q conducted through a solid body of uniform cross
-
section is expressed in (1).



Q =

2
1
T
T
L
S

(T) dT


(1)

T1 ; Temperature at cold end of conductor (K)

T2 ; Temper
ature at warm end of conductor (K)

S ; Cross
-
sectional area of conductor (cm
2

)

L ; Length of conductor between
warm

and

cold end

(cm)

λ

;
Thermal conductivity (W/cm K)



Using (1), we calculate
d

the heat leakage of
1
60mW for

the Bi
-
2212 HTS tubular bulk 27/19mm in outside/inside
diameter and 200 mm in length between 4.2K and 40K. Here,
the thermal conductivity integral between 4.2K an
d 40K is
1.1W/cm [2], and which is considerably as small as 1/400 of
that of electrolytic copper used for conventional current leads

and less than 1/10 of that of Ag
-
Au and Ag
-
Mg alloy sheaths
used for HTS current leads [8]
.


The Joule heat
at positive
joint
is 188mW at transport

current of 4,000 A. Hence, total heat loads Qt is estimated to
be 348mW, which is less than 1/10 of that for conventional
gas
-
cooled current lead. Thus, the Bi
-
2212
tubular
bulk
conductor prepared by the diffusion process is att
ractive for

a current lead in cryocooler
-
cooled superconducting magnet
s

with small allowable heat leakage and high current
conventional cryogen
-
cooled superconducting magnets.

IV.

C
ONCLUSIONS

1.

Bi
-
2212 HTS
tubular bulk specimens,

200mm in length,


have been
prepared

by the diffusion process

for current


lead application.
The
HTS
diffusion layer, about 150µm in




thickness, is mostly consist

of thin plate
-
like
and
a
-
axis


textured grains.

















2.

The transport current
of 4,000A
for the
Bi
-
2212
HTS


specimens

runs
at 4.2K and self
-
field
. The voltage of


the joint is as low as 50
μ
V, and which results in the small


heat load at the joints.

3.

The
total
heat load for the Bi
-
2212 HTS specimen is
evaluated to be
less than
350mW at a transport

current of
4,000A between

4.2K and 40K.


4.

Bi
-
2212 HTS
tubular bulk conductors

prepared by the



diffusion process

may be

attractive for current leads in



superconducting

magnets.


V.

R
EFERENCES

[1]

K. Tachikawa, Y. Yamada, M. Satoh and Y.Hishinuma,

Structur
e

and
superconducting properties of oriented Bi
-
2212 oxide layer

synthesized
by a diffusion process,


Proc. 1994 Topical

International Cryogenic
Materials Conference, World Scientific
,

1995, pp.307
-
310.

[2]

Y.Yamada, F.Yamashita, K.Wada and K.Tachikawa,

Struc
ture and
superconducting properties of Bi
-
2212 cylinders prepared by diffusion
process

,
Advances in cryogenic Engineering(Materials)
vol.44(1998)

pp.547
-
554.

[3]

K. Watanabe, S. Awaji, T. Fukase, Y. Yamada, J. Sakuraba, F. Hata,
C. K. Chong, T
. Hasebe and M. Ishihara,

Liquid helium
-
free
superconducting magnets and their applications,


Cryogenics,Vol.34,
1994, pp. 639
-
642.

[4]

Y.

Yamada, M.

Takiguchi, K.

Tachikawa, A. Iwamoto and T.

Mito,

Bi
-
2212
cur
rent leads prepared by the diffusion process
,



IEEE Trans
.

Appl
.

Supercond
uc
t., Vol.10, 2000,
p
p
.
1481
-
1484.

[5]

Y.

Yamada, M.

Takiguchi, O.

Suzuki, K.

Tachikawa, A.

Iwamoto, H.

Tamura and T.

Mito,

Transport

performance of Bi
-
2212 current leads
prepared by a diffusion process
,


IEEE Trans.

Appl. Superc
onduct
.
,
Vol.11, 2001, p
p.
2555
-
2558.

[6]

T. Mito, K. Takahata, R. Heller, A. Iwamoto, R. Maekawa, H. Tamura,
Y.Yamada,
K.

Tachikawa, K.

Maehata, K.

Ishibashi, G.

Friesinger, M.

Tasca, A.

Nishimura, S.

Yamada,

S.

Imagawa, N.

Yanagi, H.

Chikaraishi, S.

Hamaguchi
, M.

Takeo, T.

Shintomi, T.

Satow and

O.

Motojima,

Development of high
-
temperature superconducting current
feeders for a large
-
scale superconducting experimental fusion system
,"

IEEE Trans. Appl.

Superconduct
.
, Vol.11, 2001, p
p.
2611
-
2614.

[7]

H. Tamura, A. Iw
amoto, T. Mito, K. Tachikawa and Y. Yamada,

Mechanical properties and reinforcement of Bi2212 cylindrical bulk
superconductor for current lead,


MT
-
17, 2001, TUP03D2
-
08.

[8]

M. Putti, M. R. Cimberle, C. Ferdeghini, G. Grasso, A. Manca and W.
Goldacker,

Stud
y of Bi(2223) tapes with low thermal conductivity,


IEEE Trans. Appl.

Superconduct
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-
3288.