IEEE TRANSACTIONS ON MAGNETICS, VOL. 30, NO. 4, JULY 1994

1645

Low Thermal Conductive Bi-2223 Tapes
Sheathed with Ag-Au Alloys
Hiroyuki Fujishiro, Manabu Ikebe, Koshichi Noto and Michiaki Matsukawa
Department of Materials Science and Technology,
Faculty of Engineering, Iwate University,
Morioka 020, Japan
Takaaki Sasaoka, Katsumi Nomura, Jun-ichi Sat0 and Shoji Kuma
Advanced Research Center, Hitachi Cable, Ltd.,
Tsuchiura 300, JapanA b s r a c t - W i t h t h e view of a p p l y i n g t o power cur-r e n t leads for superconducting magnet systems andf o r o t h e r cryogenic power handling s y s t e m s , lowthermal conductive Bi-2223 superconductingt a p e s s h e a t h e d with Ag-Au alloy were fabricateda n d t h e i r t h e r m a l con d u c t I v l t ies we r e measured
f r o m 1 2 to 260K. T h e c r i t i c a l current density( o v e r n f l - J ) was about 1700A/cmz a t 7 7 K , OT a n dremained nearly constant irrespective ofAu
c o n c e n t r a t i o n u p t o 11 a t . % . T h e t a p e s h e a t h e d
w i t h A g + l l a t . % A u alloy, of which t h e supercond u c t o r c r o s s - s e c t i o n r a t i o f,, w a s 0 . 6 5 , h a d at h e r m a l c o n d u c t i v i t y v a l u e a b o u t O.ZW/cmK a t
77K. This value is :as low as t h a t of Cu-Zn. I t
w a s f o u n d t h a t thle t h e r m a l c o n d u c t i v i t y o f
the t a p e was close to the calculated oiie basedo n f s c and the independently measured thermal
c o n d u c t i v i t i e s , o f t h e A g - A u a l l o y a n d t h e Bi22 2 3 s u p e ir o n d u c t o r .Th e s u p ercon d u c t ing t a pes
sheathed with the alloy were c o n f i r m e d to bes u i t a b l e f o r t h e a p p l i c a t i o n a s power c u r r e n tleads.

I. INTRODUCTION
Since the discovery of high-T, oxide superconductors, agreat deal of research has been performed for their application
to the cryogenic equipment such as high field superconducting magnets (SCM) and SCM energy storage (SMES) systems. For the power current lead use, both bulk oxide superconductors without metal stabilizer and oxide superconducting
tapes sheathed with pure silver (Ag) have been proposed inplace of conventional copper leads [1]-[3]. Due to the small
thermal conductivity of bulk oxide superconductors, heat intrusion through the leads can be kept low. Unfortunately, therelatively low critical current density (J) and fragility are disadvantages of the bulk superconductors which must be overcome. Oxide superconducting tapes sheathed with pure Ag
have several advantages; the high-J characteristics due to theC

Manuscript received September 20, 1993.

increase of the crystal orier ation in the core superconductor,
the flexibility in use and an increase in thermal stability, etc.
[4],[5]. The high thermal conductivity of the pure Ag sheath,
however, causes a serious problem of the heat intrusion into
the cryogenic apparatus. For power current leads, it is necessary to develop a new sheathing material to replace pure Ag
which has a thermal conductivity as low as possible andwhich does not cause degradation of the superconducting
characteristics such as Tcand J,.
A popular method of reducing the thermal conductivityof metals is alloying. As Ag is generally congenial to oxide
superconductors, we fabricated the Ag-Au and Ag-Cu alloys
as sheathing materials and investigated the dependence of thethermal conductivity and the electrical resistivity on the Au
and Cu contents in thealloys [6]. It was found that the thermal and electrical conductivities of the Ag-Au alloy tapes,
which were subjected to necessary heat treatment in oxidizing
atmosphere to realize the superconducting tape, drastically decreased with increasing Au content up to llat.%. By contrast,
the thermal and electrical conductivities of Ag-Cu alloy tapes
hardly decreased with an increase in Cu content because of theoxidation of Cu during the heat treatment. The superconduct.
ing characteristics of Bi2S5CaCu20x (Bi-2212) ,thick films
on these alloy substrates prepared by the doctor blade method[7] and Bi2Sr2Ca2Cu,0x (Bi-2223) superconducting tapes
sheathed with the same alloys [8] were previously investigated. The Ag-Cu alloys reacted with the Bi-basedtsuperconductors and caused degradation of the superconducting
characterislics. On the other hand, Ag-Au alloys did not reactwith the Bi-based superconductors and are very promisingmaterials for the metal stabilizer as a possible replacement forpure Ag.
In the following, we investigate the possibility of applying Bi-2223 superconducting tapes sheathed with Ag-Au
alloys to the power current'leads. Firstly, we report the tlierma 1 conductivities of the Bi-2223 superconducting tapes
sheathed with Ag-Au alloys from 12 to 260K. The thermal
conductivity is also calculated by use of the thermal conductivities of both the Ag-Au alloy and the Bi-2223 superconductor, and the superconductor cross-section ratio f,,which refers to the ratio of the core superconductor crosssection to the total cross section of the tape. The calculated
thermal conductivity is compared with the measured one andthe estimation of the thermal conductivity is discussed for thecurrent lead design. Secondly, the relation between the0018-9464/94$04.00 0 1994 TEEE

1646

overall-J, at 77K, OT and the Au content in the Ag-Au alloy
sheath is investigated. Thirdly, the heat intrusion through alOOcm current lead of lOOOA with a stacking structure madeby assembling Ag-Au sheathed tapes is theoretically determined using the characteristics of the tape. Lastly, the electrical resistivities of the Ag-Au alloy tapes are measured undermagnetic fields up to 12T in order to estimate the effect ofthe magnetoresistance.The influence of magnetic field on tlie
thermal conductivity of the core superconductor is also discussed.

magnetic field up t o 12 Tesla. The frcwas decided fromobservations of the tape cross section using an optical microscope.
111. RESULTS and DISCUSSION

A. Thermal conductivity and electrical resistivity of Ag-Au
alloysFig. 1shows the temperature dependence of the thermal
conductivity of the heat treated Ag-Au alloy tapes (Klll0S
I1 EXPERIMENTAL
with various concentrations of Au up to 30at.%. Kplloy drastically decreasedwith increasing Au content at low temperaA. Sample preparationtures. The thermal conductivity of the tape with llat.%Au
Ag-Au alloys containing Au from 0 to 30 at.% were pre- was by about three orders smaller than that of the pure Ag
pared using an RF induction furnace in a pure AI atmosphere. tape at 20K. In the Au concentration range from 11 to
Ag and Au of 4N (99.99%) grade were used as raw materials. 30at.%, the further decrease of the thermal conductivity be
For the preparation of the superconducting tapes, the alloy came gradual. This may correspond to the saturation tendency
was made into the tape-shape. Bi,.,Pbo.4S~,oC"2.2Cu3,00~associated with the Au content getting nearer to 50at.%.
Thus, the thermal conductivity of the Ag-Au alloy tape with
(Bi-2223) oxide superconducting powder was fabricated as Au content up to about 1lat.% is c o n f m e d to decrease drasfollows; Bi203, PbO, SrC03, CaC03 and CuO raw powder tically with increase in Au content. The thermal conductivity
data shown in Fig.1 are fairly consistent with those by Crispof 4N grade were mixed and calcined at 800°C for 20 hours.
After pulverizing the calcined material, the resulting powder et a1 [lo].
Fig. 2 shows the temperature dependence of the electrical
was wrapped with alloy tape and then cold-rolled into a tape.
Finally, the tape was heat treated at the maximum temfiera- resistivity of the heat treated Ag-Au alloy tapes. The
ture of M O T in air which was necessary for Bi-2223 super- electrical resistivity drastically increased with an increase in
the Au content at low temperatures. For example, theconducting tape. The finished tape was about 300pm i n
thickness and about 8mm in width. Sintered Bi-2223 super- electrical resistivity of the Agtllat.%Au alloy tape was byconducting polycrystal was also fabricated under the same about three orders larger than that of the pure Ag tape at 20K.
heat treatment conditions using the calcined powder in order A s t h e electronic thermal conductivity and the electricalt o e v a l u a t e t h e t h e r m a l conductivity of the core resistivity are related to one another by the Wiedemann-Franzsuperconductor. Ag-Au alloy tapes without the super- law, the Au content dependences in Figs.1 and 2 a r e
conducting core were also fabricated with about 50pm i n reasonable.
thickness and about 2mm in width to measure the thermal
conductivity and electrical resistivity. The alloy tapes wereheat treated under tlie same conditions as the superconducting
1 o2
tapes.

B. Measurements
The thermal conductivities of the sheathed Bi-2223 superconducting tapes, the alloy tapes and the sintered Bi-2223
polycrystal were measured using a steady state heat flow
method from 12 to 260K with an automated measuring system of our own design [9]. The system made use of a Giffordcycle helium refrigerator as a cryostat. One
McMahon (G-M)
end of the sample was soldered to the cold head of therefrigerator and a small resistance heater was attached to theother end of the sample by GE7031 varnish. AutO.O7at.%Fe- chrome1 thermocouples of 73pm in diameter were useddifferentially to measure the differences in temperature. The
electrical resistivity was measured by a conventional fourprobe method in the same refrigerator from 12 to 300K, andin liquid helium at 4.2K. In order to investigate the influenceof the heat treatment on the sheathing alloy, the electricalresistivity of as-rolled alloy tapes was also measured. To
study the effect ofthe magnetoresistance, the electrical resis
tivities of the alloy tapes were measured at 4.2K under aP
O.Oltiat.%Au

10100
Temperature (K)!

Fig. 1. Temperature dependence of the thermal conductivity ofheat treated Ag-Au alloy tapes with the various
concentrations of Au.

1647

B Thermal conductivity of the Bi-2223 superconducting tapes
lo2

T
15at.%Au

30at.XAu

1l a t . % A ul.Oat.

1o

- ~

10

100
Temperature (K)Fig. 2. Temperature dependence of the electrical resistivity ofheat treated Ag-Au alloy tapes with the various concentrations of Au.

To investigate the influence of heat treatment i n air on
the physical properties of the Ag-Au alloys, the electricalresistivity of the as-rolled alloy tapes was also measured. Fig.
3 shows the temperature dependence of the electrical resistivities of both the as-rolled tapes (AR) and the heat treatedtapes (HT) with Au content from 0.22 to 11 at.%. It was
found that the heat treatment results in a slight decrease of theresistivity values at low temperatures and also a slightincrease of the temperature dependence of the resistivity. The
decrease of the resistivity at low temperatures may come fromthe decrease of the dislocation density and the increase of thetemperature dependence may come from the softening of thealloy caused by the heat treatment. However, both effects arerather small and, according to the Wiedeman-Franz law, also
the thermal conductivity of the alloys should not be muchinfluenced by the heat treatment in air.
I

-5

I

1

m

11.111

1

The thermal conductivities of the Bi-2223 superc o n d u c t i n g t a p e s w i t h v a r i o u s A u contents andsuperconductor cross-section ratios f,, were investigated. The
superconducting transition temperatures of these tapes wereabout 104K and independent of the Au content and f,& Figs.
4 and S show the temperature dependence of the thermal
conductivity of the Bi-2223 superconducting tapes (K&AUT)
sheathed with Ag+ 1lat.%Au alloy (fsc=0.6S) (hereinafterreferred to as No.1 sample), and sheathed with AgtlSat.%Aualloy (fsc=0.33) (hereinafter as No.2 sample) with closed
circles, respectively. In these figures, the thermal ConduG
tivities of the Ag-Au alloy tapes with the same Au content(
K in Fig.1)~~ and~ the ~Bi-2223~
sintered polycrystal (KPOlS
are also shown with open circles and closed triangles, respeG
tively. K
was 24 mW/cmK at 200K and decreased m o mPolytonically with decreasing temperature.K
showed aPolysignificant upturn just below Tc(=lOSK) and took a local
maximum around 70K, and then decreased steeply withfurther lowering temperatures. The value of K
i s aboutPol!
two orders smaller than that of Ag-Au alloy tape with 11 andlSat.% Au content. It is found that the magnitude Of KAnAuT-- lies between those Of Kalloy and K

--

Poly'

I

loo

5

Y

*
E 1 0 -1
-

10

c.

100
Temperature (K)

c1
v)
v)

K

1 o-2'
1

I

I

10

100

I

.

Temperature (K)Fig. 4. Temperature dependence of the thermal conductivity
Kof Bi-2223 superconducting tape (No.1 sample)
AgAuT
sheathed with Ag+llat.%Au alloy for ratio fs, 0 . 6 5(closed circles). T h e thermal conductivity of
Ag+llat.%Au alloy tape K(open circles) and that ofalloy

Fig. 3. Temperature dependence of the electrical resistivities ofboth the as-rolled Ag-Au tapes(AR) and the heat treatedtapes @IT)in which Au contains from 0.22 to llat.%.

Bi-2223 polycrystal K

POLY(closed triangles) and thecalculated thermal conductivity K (thick line) were also
Gilshown.

1648i[' 10- tI-

No.2 sample

K(Agtl)at.%Au)

3

i. n-

100
Temperature (K)

I

I

2

1

-1
Temperature (K)Fig. 5. Temperature dependence of the thermal conductivity of
Bi-2223 superconducting tape (No.2 sample) sheathedwith Ag+lSat.%Au alloy for core ratio fsc 0.33 (closed
circles). The thermal conductivity of AgtlSat.%Au alloy
tape (open circles), that of Bi-2223 polycrystal (closed
triangles) and the calculated thermal conductivity (thickline) are also shown.

Fig. 6. Temperature dependence of thermal conductivities of boththe Bi-2223 superconducting tapes sheathed with pureAg (fSc= 0.23) and the high purity copper of 4N grade.

As the superconducting tape consists of the Bi-2223
superconducting core and the Ag-Au alloy sheath, the heatshould flow in parallel through both the core and the sheathalong the tape. In this case, the thermal conductivity Ka, ofthe superconducting tape can be calculated using the values ofKK
andfsZ byconductivity of No.1 and 2samples is about 0.2W/cmK at
77K which is comparable to that of the Cu-Zn alloy.

poly'

alloy

For comparison, the, thermal conductivities of the pureAg(4N) tape, No.1 and 2 samples, a Cu-Znalloy and Bi2223 polycrystal are also shown.

C. Characteristics of overall-Jc
The overuZZ-Jccharacteristicsof the Bi-2223 tape sheathedwith Ag-Au alloy for various Au contents were measured [8].
Overall-J, of the tape is defined by

K was also shown in Figs. 4 and 5 with thick solid lines.
IC
caloverall-Jc- The calculated K~ is very close to the experimentally ob-S
tained thermal conductivity K~ This ~confirms~
that~ the .
where IC is the critical current and S is the total cross section
thermal conductivity of a superconducting tape with arbitrary
superconductorcross-section ratio f,, and with some Au con- of the tape. Fig. 7 shows the relation between the Au content
in the Ag-Au alloy sheath and the overall-J, of the tape, incentration in the Ag-Au alloy sheath can be practically estL
mated by use of Eq.(l). It is useful in power current lead which f,, is kept to 0.65, at 77K and OT. The overdl-J,,
design to be able to estimate the heat flow through the lead remained almost constant at about 1700 Ncmz, irrespective
rather than to measure it. For comparison, the thermalof Au contents in the Ag-Au alloy sheath up to 11 at.%. As
conductivities of Bi-2223 tapes (f,, =0.23) sheathed with a the J, of Bi-2223 sintered polycrystal was about 500NcnQ at
pure Ag sheath (Kand the high purity copper (4N) (Kc)
77K and OT [ l l ] , it was found that the overall-J, of the BiAg
used in conventional power current leads were also measured. 2223 superconductingtape was about three times larger thanof the Bi-2223 sintered sample. Fig. 8 shows theFig. 6 shows the temperature dependence Of K
and K ~ ~ that,
AgT
magnetic field dependence of the overall-J, of the Bi-2223
together with the thermal conductivities of samples No.1 and
2 , the heat treated pure Ag(4N) tape, a Cu-Zn alloy andtape (fSc=0.65) sheathed with Ag-Au alloy with various Au
K
K
is
only
slightly
smallerthanthethermalconduccontents at 77K. The magnetic fields were applied parallel topoly' AgTthe tape surface. The overall-Jc vs, magnetic field curves oftivity of the pure Ag tape because of the low fsL
K~~ i sthetapes sheathed with Ag-Au alloys agreed with that of thefrom 40 to 200K. The thermal
almost the same as K

4,

AgT

1649

3000

77

at 77K and OT
(3)
A

FJ

$2

5
.
5.

2000

0

3 1000
7

0

>

0

0

0-

0.1

1

10

100

Au content (at.%)Fig. 7. Overall-J of the Bi-2223 superconducting tapes with var-ious concentrationsof Au at 77K and OT, in which thesuperconducting core ratio fs, is 0.65.

a t 77K, B l l T a p e s u r f a c e
f t= 0 . 6 5hFJsheathed wlth pure Ag

E 1500.

0 sheathed with Ag+0.085at%Au

0

A sheathed wilh A g i l l s t X A u :

4

0
.

f!
0

0
0

0.1

0.2

0.3

where Lis the length and S is the cross section of the lead,respectively. We used the Bi-2223 superconducting tapes
sheathed with Ag-Au alloy with fsc=0.65as shown in Fig.7,
and calculated the Q value for a lOOcm current lead of lOOOA
using Eqs. from (1) to (3).
Fig. 9 shows the results of the calculation of the relation
between the heat intrusion through the lead and the Au
content in the Ag-Au alloy sheath. The heat intrusion Q of
the current lead decreases with increased Au content. For
example, the Q value of a Superconductinglead sheathed with
the Ag+llat.%Au alloy is about two orders smaller than thatof a conventional copper lead without gas cooling or a Bi2223 superconductinglead sheathed with pure Ag.

E. Influence of magneticfield
In many cases power current leads for cryogenic equip
ment such as SCMs are used under magnetic fields. The
influence of magnetic fields on the electrical resistivity of
Ag-Au alloy tapes was investigated. Fig. 10 shows theelectrical resistivities of the Ag-Au alloy tapes as a functionof magnetic fields up to 12T. The electrical resistivity oftapes with higher Au content depends hardly at all on themagnetic field. This independence is a favorable condition forstable operation of superconducting power current leads withalloy sheathes. Although the intrinsic thermal stability ofsuperconducting tapes sheathed with alloys somewhat
deteriorates due to large electrical resistivity in the sheath,the electrical resistivity of bulk oxide superconductors in thenormal state is several m!2 cm, which is three orders larger
than that of the Ag-Au alloy tapes. It should still be

Magnetic Field (Tesla)

Fig. 8. Uvernll-J, vs. applied magnetic field curves with various Au contents in the Ag-Au alloy sheathes at 77K. The
magnetic field was applied parallel to the tape surface.

11h111I

I IIIlllI1111111,I

1 I 1

Cu lea\ wlthout gas'coollng

pure Ag sheath tapepure-Ag sheathed tape. It is recognized that the overall-Jccan
be somewhat increased by decreasing f,, below 0.65 becauseo f the resultant higher crystal orientation of the superconducting core. However, the heat intrusion through the tapeincreases at the same time because of the lower f,; I t isnecessary to optimize the Au content in the alloy sheath andthe f ,according to the purpose of each cryogenic apparatus.

L = lOOcm
optlmlzed lead(gas coollng)

sc

D.Calculation of heat intrusion through the current lead
As the thermal conductivity of the superconducting tapes
can be estimated using Eq.(l) as shown in Figs.4 and 5, theheat intrusion through the current lead using the superconducting tapes was calculated. The heat intrusion Q of thecurrent lead in thq area between 77K and 4.2K is given by

1

1 o-2

lo-'

1 oo

10'

1o2

A u content (at.%)

Fig.9. Relation between the heat intrusion through a lOOcm superconducting lead of lOOOA which is fsc=0.65and the
Au content in the Ag-Au alloy sheath.

1650

A

i l a t . % Auw

2.9at.% Au

3

3
0

5

10

15

Magnetic Field (Tesla)

Fig. 10. Magnetic field dependcnce of the electrical resistivity of
Ag-Au alloys with various concentrations of Au at 4.2K.
beneficial for the thermal stability of superconducting powerl e a d s t o u s e t h e A g - A u alloy sheath. The thermal
conductivity of the oxide superconductor is selisitive to themagnetic field and decreases with increasing applied field
below Tc [12], and so could cause unstable operation ofsuperconducting power current leads. Because the absolutev a l u e o f the thermal conductivity of the Bi-2223
superconducting core is about one fiftieth smaller than that of
Ag-Au alloy as shown in this investigation, the thermal
conductivity of the Bi-2223 superconducting tape is expected
to be nearly independent of the applied magnetic field.

IV. SUMMARY
Low thermal conductive Bi-2223 superconducling tapes
sheathed with Ag-Au alloys were developed for application to
power current leads of cryogenic equipment. The tapes
sheathed with the-alloy with Au concentration up to 11 at.%showed almost the same superconducting characteristics as
those sheathed with pure Ag. OweralZ-J, of about 1700
Ncmz at 77K and OT was obtained, independent of the Au
content in the Ag-Au alloy sheath. The thermal conductivityof Bi-2223 tape sheathed with Ag+llat.%Au alloy whose
superconductor cross-section ratio f,, was 0.65 was about
0.2WhmK at 77K. This value is almost as low as that of
Cu-Znalloy. The value of measured thermal conductivity canbe reproduced by calculatjon based on the core ratio and thethermal conductivity values of both the Ag-Au alloy and thesuperconducting core. The Ag-Au alloy sheathed superconducting tapes give full promise for application in power
current leads.

ACKNOWLEDGEMENTS
The authors wish to thank T.Kurihara and T.Naito of
Iwafe University for their assistance in the thermal conductivity and electrical resistivity measurements.

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