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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 19, NO. 3, JUNE 2009

Thermal Conductivity and Thermal Dilatation ofCommercial BSCCO (DI-BSCCO) Tapes
Tomoyuki Naito, Hiroyuki Fujishiro, and Yuichi Yamada

Abstract—We have measured the thermal conductivity,( ) (300 K), as afunction of the temperature for four types of commercial(Bi Pb)2 Sr2 Ca2 Cu3 O10+ (Bi2223) tapes sheathed withsilver-alloy (DI-BSCCO). ( ) of three Bi2223/Ag-alloy tapes[Type H, Type HT, and Type AC] shows a similar temperaturedependence, but the absolute value of the ( ) depends on the
structure and volume ratio of sheath part. The sheath materialof types H and AC is a silver-alloy and that of type HT consists of both silver-alloy and stainless steel tapes. Type AC has
the twisted Bi2223 filaments. On the other hand, ( ) of theBi2223/Ag-5.4wt%Au-alloy tape [Type G] shows very low value
and decreases monotonically with decreasing temperature. Near
77 K, the absolute values of the type H and type G tapes wereabout 260 W m 1 K 1 and 80 W m 1 K 1 , respectively.
( ) (300 K) of four tapes decreases with decreasing temperature, and approaches 0 31
0 38% below 25 K for all
the tapes.
( ), and thermal dilatation,

Index Terms—DI-BSCCO, thermal conductivity, thermal dilatation.

I. INTRODUCTION

When the superconducting materials are used in the practicaldevices, we need to estimate correctly the amount of heat intru, in the materials
sion through them [3]. The heat intrusion,
between a low and high temperature sides,and , is in general given by

where is the thermal conductivity, the length and the crosssection of the material. In the case of the Bi2223/Ag-alloy tapes,
the absolute value and temperature dependence of are mainly
dominated by the sheath material, the volume ratio of sheath material to the Bi2223 and fabrication process [3]–[5]. However,of DI-BSCCO has not been measured yet. On the other, is also the important physhand, the thermal dilatation,ical property, because the change in the length of the tape would
give itself a mechanical stress which suppresses the superconducting properties. In this paper, we report the temperature de, and thermal dilatapendence of the thermal conductivity,, for four types of DI-BSCCO tapes.
tion,

T

HE
(Bi2223) tapes sheathedwith silver-based alloy, Bi2223/Ag-alloy, have beendeveloped by many researchers and companies. Recently, Sumitomo Electric Industries, Ltd. (SEI) developed a new sintering
technique with the controlled overpressure (CT-OP) [1], whichenables us to fabricate the Bi2223/Ag-alloy tape including
very small amount of voids and nonsuperconducting region
and having a high Young’s modulus in comparison with theconventional Bi2223/Ag-alloy tapes. Using this technique, SEI
succeeded in making a Bi2223/Ag-alloy tape with high criticalcurrent, , over 200 A at 77 K and under the self-field [2]. Thisvalue gives us the possibility that the superconducting applications such as high-field coil magnet, power cable are practicably
operated at 77 K. Since the Bi2223/Ag-alloy tapes made by
CT-OP technique surpass other tapes in all properties, SEI
named them Dynamically Innovative BSCCO (DI-BSCCO).

II. SAMPLE
DI-BSCCO tapes were fabricated by the powder in tube (PIT)
method with the CT-OP sintering technique. The details of thesample preparation were described elsewhere [1]. Table I summarizes the specifications of four types of DI-BSCCO tapes(types H, HT, G, and AC) investigated in this study. Type H,
whose sheath is a silver-alloy, is a standard tape. The silver-alloy
consists of the silver and a small amount of other metal element.
In order to achieve a high yield stress, type HT was prepared, inwhich the type H tape was laminated from both surfaces by thestainless steel (SUS) tapes 0.02 mm in thickness. The sheath material of type G is an Ag-5.4wt%Au alloy. Type AC is designed
for AC applications, in which the twisted Bi2223 filaments aresheathed by the silver-alloy.
III. EXPERIMENTAL PROCEDURE

Manuscript received August 25, 2008. First published June 30, 2009; current version published July 15, 2009. Corresponding author: T. Naito (e-mail:tnaito@iwate-u.ac.jp).
T. Naito and H. Fujishiro are with the Faculty of Engineering, Iwate
University, Morioka 020-8551, Japan (e-mail: tnaito@iwate-u.ac.jp; fujishiro@iwate-u.ac.jp).
Y. Yamada is with the Superconductivity and Energy Technology Department
and Electric Power and Energy Research Laboratories, Sumitomo Electric Industries, Ltd., Osaka 554-0024, Japan (e-mail: yamada-yuichi@sei.co.jp).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TASC.2009.2017850

Thermal conductivity,, along the tape surface, i.e. the-planes of the Bi2223 filaments, was measured by a steadystate heat flow method [6]. The experimental setup is schematically shown in the inset of Fig. 2. One end of the tape was soldered to the copper-block (sample stage). We use a small metal
chip resistor of 1 k as a heater, which was adhered to the otherend of the tape by GE7031 varnish. is obtained by the relation,

1051-8223/$25.00 © 2009 IEEE
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NAITO et al.: THERMAL CONDUCTIVITY AND THERMAL DILATATION OF COMMERCIAL BSCCO (DI-BSCCO)

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TABLE ISPECIFICATIONS OF THE DI-BSCCO TAPES

including the thickness of SUS parts (0.02 mm

2 2) and solder layers (N.A.).

where
is applied heat flow,
is temperature difference
and is the distance between two chromel-constantan thermowas
couples, and is the cross sectional area of sample.
maintained to be 0.5–0.8 K by controlling the heater power.
Thermal dilatation normalized by the sample length, , at 300K,, wasmeasured by a strain-gauge method using a commercial straingauge (CFLA-1-350-11 [the gauge length is 1 mm, the gauge
resistance is 350 , and the gauge factor is 2.09]; Tokyo Sokki
Kenkyujo Co., Ltd.). Electrical resistivity, , was measured bya four-probe method with a typical current density of about 3. The temperature of sample stage was controlled between 5 and 300 K using a Gifford-McMahon cycle helium refrigerator. To minimize a radiation loss, was measured below
was measured above 10 K because of
250 K.
the sensitivity of strain gauge.

Fig. 1. Temperature dependence of the electrical resistivity for four types of
DI-BSCCO tapes. The sheath material is different; Ag-alloy (types H and AC),
Ag-alloy stainless steel (type HT), and Ag-5.4wt%Au-alloy (type G).

+

IV. RESULTS AND DISCUSSION
A. Electrical Resistivity
Fig. 1 shows the temperature dependence of the electrical re, for various DI-BSCCO tapes. The critical temsistivity,
perature, , is about 111 K for all the samples, indicating goodquality of the superconducting part of Bi2223 filaments. All thecurves show a good -linear dependence in the normalof the metalstate. In this regime, the absolute value of
sheaths are rather smaller than that of Bi2223 filaments, therefore the applied current mainly flows through the part of themetal sheath; for instance, the value for the Bi2223 polycrystal
at 295 K [6] and that for
was reported to be about 900
at room temperature [7]. The absolute
pure silver 1.6
value offor type G is rather larger than those for otherthree types, because the sheath material of type G is the solid solution of Ag-Au alloy (Ag-5.4wt%Au alloy). On the other hand,the value for type AC is slightly smaller than that for type
H, although both sheath materials are the same. These results
originate from the fact that type AC contains twisted Bi2223 filaments with large and the volume ratio of the low-resistive
sheath material to the Bi2223 filament of the type AC is larger
than that of the type H.
B. Thermal Conductivity
Fig. 2 shows the temperature dependence of the thermal con, for various DI-BSCCO tapes.
of type H
ductivity,
decreases quite slightly with decreasing temperature from 250
K to 100 K. It increases moderately between 100 K and 60 K,
starts to increase rapidly below 60 K, and takes a peak of 780
around 17 K. The steep increase and peak strucof a high-purity silver [9], which
ture is often observed inoriginates from the fact that the mean free path of the electrons
becomes large in the low temperatures. The behavior offor type HT is basically similar to that of type H, but the absolute value of for type HT is about 20%–25% smaller than thatfor type H. Assuming that the thickness of the “type H” tape in
the type HT is the same as that of the type H tape measured inthis study, the ratio of the cross section between a couple of SUS
with solder layers and the whole tape is estimated to be 0.29:1.
of type HT should be smaller about 29% than
Therefore,that of type H, if the applied heat thoroughly flows through thepart of Ag-alloy. SUS is a poor heat conductor compared with
Ag. For instance, the absolute values of of a standard sampleof the austenitic stainless steel (SRM1460) provided by NIST
(the National Institute of Standards and Technology) are about
at 200 K and 1.5
at 20 K [8].
12On the other hand, those of pure Ag are approximately 600
at 200 K and 20000
at 20 K [9].
Therefore, we can consider that the most of applied heat flows
through the part of Ag-alloy sheath also for type HT, giving rise.
of type G decreases monotonto the reduction of
ically with decreasing temperature. The absolute value of oftype G near 77 K is about one third of that of type H. No peakfor type G is often observed inand a small value in thethe low-purity Ag-Au alloys [9], which originates from the fact’s havethat the impurities scatter the electrons. The similar
been observed in the Bi2223 tapes sheathed with other metalof type AC behaves similarly to that of types
alloy [3]–[5].
H and HT, but the peak value of type AC near 10 K is extremely
larger than that for other two types. This large peak suggests thatthe volume ratio of sheath material to the Bi2223 is large in type

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3036

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 19, NO. 3, JUNE 2009

Fig. 2. Temperature dependence of the thermal conductivity, (T ), for fourtypes of DI-BSCCO tapes. The sheath material is different; Ag-alloy (types H
and AC), Ag-alloy + stainless steel (type HT), and Ag-5.4wt%Au-alloy (typeG). The experimental setup is schematically illustrated in the inset, where thecoordinate axes represent the crystallographic axes of Bi2223. T.C. is thermocouple. Q, 1T and l are notated in the text.

AC. Finally, we note that no anomaly is observed atin anyof the samples.
In a conducting solid, the measured thermal conductivity
comprises both contributions due to phonons and electrons,
and . The
component can be estimated from the electrical,resistivity, , using the Wiedemann–Franz law,is the Lorenz number.
whereSince thecurves show the good -linear dependence’s are expected to be
above , as found in Fig. 1, the-independent. The obtained values ofat 150 K for eachsample are about 244 (type H), 198 (type HT), 110 (type G),(type AC), respectively. These values
and 282
values in the normal
are comparable with the measured
state. On the other hand, the value of the sintered Bi2223
at 150polycrystal was reported to be about 3.3
K [6]. Therefore, the contribution of the Bi2223 filament to theheat transport is negligibly small and the most of the applied
heat is carried by electrons in the metal sheath.
C. Thermal Dilatation
Fig. 3 shows the temperature dependence of the thermal
for various DI-BSCCO tapes.
dilatationof all the samples shows a similar temperature dependence; from 300 K to 25 K, thedecreases monotonically with decreasing temperature, andshows almost constant value of
below 25
K. Recently, Weiss et al. reported that the thermal dilatation oftypes H and HT at 5 K are approximately 0.27% and 0.3%,
respectively [5], which are about 10% smaller than the present
results. However, we cannot discuss the discrepancy, because
they did not described the measuring method and accuracy.
V. SUMMARY
We have measured the thermal conductivity,, and, as a function of thethermal dilatation,
temperature for four types of DI-BSCCO tapes. The

Fig. 3. Temperature dependence of the thermal dilatation, L=dL(300 K), forfour types of DI-BSCCO tapes. The sheath material is different; Ag-alloy (types
H and AC), Ag-alloy + stainless steel (type HT), and Ag-5.4wt%Au-alloy (typeG).

of both Bi2223/Ag-alloy (type H) and Bi2223/Ag-alloy
laminated by stainless steel (type HT) show a similar temof type
perature dependence, but the absolute values of
HT is approximately 20%–25% smaller than that of type H,of type HT is dominated bywhich indicates that thethe sheath material with low-thermal-conductive SUS. For
decreases
the Bi2223/Ag-5.4wt%Au-alloy (type G), themonotonically with decreasing temperature and its magnitudeof type AC
is rather smaller than that of type H. The
shows a very large peak near 10 K, which might originate
from the larger volume ratio of Ag compared with that ofshows a similar temperature
type H. The
dependence for all the samples. It reaches nearly constant value
below 25 K.
of
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