IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 21, NO. 3, JUNE 2011

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Thermal Conductivity of YBCO Coated Conductors
Reinforced by Metal Tape
Tomoyuki Naito, Hiroyuki Fujishiro, Yasuhisa Yamamura, Kazuya Saito, Hiroshi Okamoto, Hidemi Hayashi,
Noboru Fujiwara, Yoshihiro Gosho, and Yuh Shiohara

Abstract—We have measured the thermal conductivity, ( ),of YBCO coated conductors (YCCs) reinforced by Cu or CuNitape. ( ) of YCC reinforced by Cu tape with 100–300 m inthickness, YCC-Cu, was 250–400 W m 1 K 1 at 77 K, which
depended on the thickness of Cu tape and was roughly one orderof magnitude larger than that of the bare YCC. On the other
hand, ( ) of YCC reinforced by CuNi tape of 300 m thickness,
YCC-CuNi, was comparable to that of the bare YCC except
for the low temperatures. The contribution of each reinforcing
material to the thermal transport was estimated by analyzing
the measured ( ) using an equivalent heat current circuit. The
applied heat flows mainly through the Cu tape for the YCC-Cu
tapes and through both CuNi tape and stabilizing Ag layer for
the YCC-CuNi tapes, respectively. Phonons cannot be ignored asthermal carriers in YCC-CuNi tapes.
Index Terms—Specific heat, thermal conductivity, YBCO coatedconductor.

value of over 0.1
at 77 K
film showed a highin magnetic fields parallel to the -axis up to about 8 T. This
exceeds that of NbTi andat 4.2 K [5]. Therefore,
YBCO film is one of the most promising materials that canbe used practically in power applications operated at 77 K.
YBCO coated conductor (YCC) tape, which consists of metal
substrate, buffer and cap layers, YBCO film, and stabilizing Ag
layer, has been developed. Recently, the YCC tapes have juststarted to be used for prototype of power electric applications
such as power cables, motors, transformers, and fault current
limiters [6].
, of the tape is one of the imThermal conductivity,
portant physical properties in the design of a superconducting
equipment, because we can estimate the amount of the heat flow,, through the tape as follows:

I. INTRODUCTION
Bi2223 superconducting tape sheathed with silver,
Bi2223/Ag, has already been commercially provided byseveral companies. Recently, a high critical current over 200
A at 77 K under the self-field was achieved [1], [2], which
has a high potential for the practical applications. However,
the operating temperature of the Bi2223/Ag tapes is limited
to temperatures lower than 20 K, because their critical current
density, , decreases rapidly with increasing magnetic field attemperatures as high as 77 K [3]. On the other hand, the degra(YBCO) is not as large even at
dation of of
77 K under a high magnetic field as that of Bi2223/Ag tapes
value of YBCO film has been recently enhanced[4]. The
by introduction of artificial vortex pinning centers (AVPCs)
employing nanotechnology [5]. The AVPC-introduced YBCO

A

Manuscript received August 02, 2010; accepted October 16, 2010. Date of
publication November 15, 2010; date of current version May 27, 2011. This
work was supported by the New Energy and Industrial Technology Development
Organization (NEDO).
T. Naito and H. Fujishiro are with Faculty of Engineering, Iwate University,
Morioka, 020-8551, Japan (e-mail: tnaito@iwate-u.ac.jp; fujishiro@iwate-u.ac.
jp).
Y. Yamamura and K. Saito are with Department of Chemistry, Graduate
School of Pure and Applied Sciences, University of Tsukuba, Tsukuba,
305-8571, Japan.
H. Okamoto and H. Hayashi are with Kyushu Electric Power Co., Inc.,
Fukuoka, 815-8520, Japan.
N. Fujiwara, Y. Gosho, and Y. Shiohara are with Superconductivity ResearchLaboratory, ISTEC, Tokyo, 135-0062, Japan.
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.2010.2089481

In the equation,andare the temperatures of the terminals of the tape; the cross section; and the length of thetape. Quite recently, we measured the temperature dependence, of YCC tapes and the domiof the thermal conductivity,
nant thermal carriers were found to be the electrons in the stabilizing Ag layer [7]. The volume fraction of the Ag layer to the, is roughly estimated to be 10–30% for YCC,
whole tape,where the thickness of the Ag layer is 10–30
and that of. On the other hand,of a conHastelloy substrate is 100
ventional Bi2223/Ag tape is approximately 60% [8], [9]. The
value of YCC is preferable because of the cost reducsmalltion; however it might cause poor thermal stability. To improve
the thermal stability and mechanical strength simultaneously,
the YCC tape is usually reinforced by a metal tape. In this paper,of the YCC
we measured the temperature dependence oftapes reinforced by Cu or CuNi tape along the length direction,and discuss the effect of the reinforcing metal on thermal transwith that estimated fromport by comparing the measured
a parallel heat flow model. The temperature dependence of the, of the bare YCC tape was also measured.
specific heat,
This is an important parameter in view of the estimation of thethermal stability.
II. EXPERIMENTAL
A. Sample
YCC tapes were fabricated by an ion beam assisted deposition (IBAD) and a pulsed laser deposition (PLD) method.

1051-8223/$26.00 © 2010 IEEE

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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 21, NO. 3, JUNE 2011

TABLE ISPECIFICATIONS OF THE YCC TAPESThe details of sample preparation have been described elsewhere [10]. The substrate material was Hastelloy C-276 (5 mmthick).
(GZO) film (about 1
wide and 100
thick) was deposited on the Hastelloy as a buffer layer by thefilm (0.7
thick) and
IBAD method. The cap layer ofthick) were prepared on thethe YBCO film (about 1.5
IBAD-GZO film by the PLD method. Finally, Ag was deposited/GZO/Hastelloy as the thermal stabilizer. We
on YBCO/prepared YCC tapes reinforced by Cu or CuNi tape, which werenamed YCC-Cu or YCC-CN. In YCC-Cu tapes, the thickness
and that of Cu tape at 100 orof Ag layer was fixed at 10. In YCC-CN tapes, the thickness of Ag layer was 20300
and that of CuNi tape was fixed at 300
. The
or 30
measured sample names and the thicknesses of Ag and of thereinforcing metal are described in Table I.

Fig. 1. Temperature dependence of the thermal conductivity (T ) of YCC andYCC-Cu tapes.

B. Measurements
Thermal conductivity,, was measured by a steady-stateheat flow method. One end of the tape was attached toa copper-block as a heat sink using In solder. A small) was adhered to the other end of
metal chip resistor (1is given as
the tape as a heater using GE7031 varnish.
, whereis the applied heat flow,
the temperature difference; (typically 10 mm) the distance
between the two thermocouples; andthe cross section of
was measured using two chromel-constantan
the tape.
thermocouples and was maintained at about 0.5–0.7 K by automatically controlling the heater power. Electrical resistivity,, was measured by a four-probe method. The temperatureof the sample stage was controlled using a Gifford-McMahon
cycle helium refrigerator and 30 W heater. To eliminate theradiation loss, was measured below 200 K. Specific heat,, was measured by a relaxation technique using the Physical Properties Measurement System (PPMS, Quantum DesignInc.).
III. RESULTS AND DISCUSSION
A. Thermal Conductivityof the YCC-Cu tapes. The reported
Fig. 1 showsof bare YCC tape with a 20
thick Ag layer [7], denoted by
of YCC-Cu tapes increases
YCC-Ag20, is also plotted.
slightly with decreasing temperature, begins to increase rapidly
below 90 K, and takes a maximum of approximately 2000–2400
around 20 K. The absolute value ofat 77 K
for YCC-Ag10Cu100 and about
is about 250
for YCC-Ag10Cu300. The absolute value of400
of YCC-Cu tapes increases with increasing thickness ofFig. 2. Temperature dependence of the thermal conductivity (T ) of YCC andYCC-CuNi tapes.

Cu tape.
value of YCC-Ag20 at 77 K is about one orderof magnitude smaller than that of YCC-Cu tapes.
of two YCC-CN and YCC-Ag20 tapes.
Fig. 2 presentsof the YCC-CN tapes monotonically decreases with decreasing temperature and shows a bump structure around 30 K.
of the YCC-CN tapes at
The absolute value ofis comparable to that of the YCC-Ag20 tape and increases withincreasing thickness of the Ag layer.
of all the YCC-Cu and YCC-CuNi tapes.
Fig. 3 showsof the YCC-Ag20 is also presented. The superconducting
transition occurs at around 90 K for all the tapes. The temis almost linear for the YCC-Cu
perature dependence oftapes and its magnitude of
decreases with increasing thickfor YCC-CN tapes
ness of Cu tape. On the other hand,, and the magnitude
shows a positive curvature,
decreases with increasing thickness of Ag layer. We
ofNAITO et al.: THERMAL CONDUCTIVITY OF YBCO COATED CONDUCTORS REINFORCED BY METAL TAPE

Fig. 3. Temperature dependence of the electrical resistivity (T ) of YCC,
YCC-Cu, and YCC-CuNi tapes.

can estimate the electronic thermal conductivity,, fromusing the Wiedemann-Franz law,
, whereis the Lorenz number. The
values at 200 K,’s,for YCC-Ag10Cu100 and YCC-Ag10Cu300 are estimated to, respectively, which are roughly
be 180 and 300
value. The
comparable to the measured
values are about 40 and 52
for YCC-Ag20CN300
and YCC-Ag30CN300 tapes, respectively, which are about 70%
value. These results suggest that theof the measured
applied heat is transported dominantly by electrons in both the
Ag layer and the reinforcing metal tape and that the contribution of phonons to thermal transport cannot be ignored for the
YCC-CN tapes.
The amount of the contribution of each layer to thermal transport can be estimated by an equivalent heat current circuit asfollows. In a multi-layered sample consisting of layers, the, is assumed to flow through each layer
total applied heat,,in parallel. If heat flow through the th layer is written as
is given by:

Since
and
conductivity,

are the same for all layers, the total thermal, can be estimated from:

where is the thermal conductivity of the -th layer and andare the cross section of the -th layer and of the sample, respectively. Fig. 4 shows the temperature dependence of the meaand,
sured and calculated thermal conductivities,
was calfor YCC-Ag10Cu300 and YCC-Ag20CN300.
culated usingof Hastelloy, Cu and Ag, because the contributions of the buffer and YBCO layers can be ignored due to[7] and a slight thicknesses. In the calculation,
their smallof the Cu tape. Since the purity of
we used the measured

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Fig. 4. Temperature dependence of the measured and calculated thermal conductivity (T ) of the YCC-Ag10Cu300 and YCC-Ag20CN300 tapes.

Fig. 5. Temperature dependence of the specific heat C (T ) of the YBCO coatedconductor tape.

the stabilizing Ag layer in the present YCC was estimated tomeasurement [7], the reported
be about 99.9% from theof Ag-0.09at%Au alloy [11] was used in this calculation.
for YCC-Ag20CN300 was also estimated usingof YCC-Ag20 and CuNi tapes. The measured
for both. Althoughtapes is roughly reproduced by the calculateddifferences were observed above 80 K, the reinforcing Cu tapedominates thermal transport for the YCC-Cu tapes. On the other
hand, the applied heat flows through both the CuNi tape and the
Ag layer for YCC-CN tapes. Hastelloy tape can be ignored as
the heat current path in each tape.
B. Specific Heat
Fig. 5 shows the temperature dependence of the specific heat,, of the YCC-Ag20 tape.
of YCC-Ag20 decreases
monotonically with decreasing temperature. The temperatureroughly correspond todependence and absolute value of
those of Hastelloy [12]. Using the reported
value for Cu
value of the YCC reinand CuNi, we can estimate thevalues of the YCC andforced by the metal tape using thethe reinforcing metal tape.

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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 21, NO. 3, JUNE 2011

IV. SUMMARY
We have measured the temperature dependence of the thermal, of various YBCO coated conductor (YCC)
conductivity,tapes with 10–30
Ag layer reinforced by Cu (100 or 300
thick) or CuNi (300thick) tape.
of YCC-Cu tape increased slightly with decreasing temperature, started to increase
rapidly below 90 K, and took a maximum around 20 K. The
at 77 K was 250–400, deabsolute value ofpending on the thickness of Cu tape and was about 5–8 times
larger than that of the bare YCC tape. On the other hand,of YCC-CuNi tapes decreased monotonically with decreasing
temperature and showed a bump structure around 30 K. Theirvalues were about 50–60
at 77 K, whichis comparable to that of bare YCC tape. By assuming that theapplied heat flowed through each layer of the tape in parallel,using the calculated. The
we analysed the measured
dominant thermal carriers were the electrons in the reinforcing
Cu region for the YCC-Cu tapes, which originated from the highthermal conductivity of Cu. For the YCC-CuNi tapes, although
the applied heat mainly flows through both Ag layer and CuNi
layer in parallel, the contribution of phonons to the thermaltransport cannot be ignored. We also measured the temperature, of the bare YCC tapedependence of the specific heat,with 20
Ag layer, which was roughly comparable to that ofvalues reported for the metals,
Hastelloy. Using theof the YCC tape reinforced by various kinds of metal tape canvalues of YCC and reinbe estimated by summing bothforcing metal tapes.

ACKNOWLEDGMENT
The authors would like to thank Dr. Y. Iijima at Fujikura Ltd.
for offering the information of the measured tapes.

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