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Thermal conductivity of YBCO coated conductors fabricated by IBAD–PLD method

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2010 Supercond. Sci. Technol. 23 105013(http://iopscience.iop.org/0953-2048/23/10/105013)
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IOP PUBLISHINGSUPERCONDUCTOR SCIENCE AND TECHNOLOGYSupercond. Sci. Technol. 23 (2010) 105013 (4pp)

doi:10.1088/0953-2048/23/10/105013

Thermal conductivity of YBCO coatedconductors fabricated by IBAD–PLD
methodTomoyuki Naito1 , Hiroyuki Fujishiro1 , Hiroshi Okamoto2 ,Hidemi Hayashi2 , Noboru Fujiwara3 , Yoshihiro Gosho3 andYuh Shiohara3
1

Faculty of Engineering, Iwate University, Morioka 020-8551, Japan
Kyushu Electric Power Co., Incorporated, Fukuoka 815-8520, Japan
3
Superconductivity Research Laboratory, ISTEC, Tokyo 135-0062, Japan2

E-mail: tnaito@iwate-u.ac.jp

Received 28 June 2010, in final form 17 August 2010
Published 13 September 2010
Online at stacks.iop.org/SUST/23/105013
Abstract
We have measured the temperature dependence of the thermal conductivity κ(T ) of YBCO
coated conductor tapes with various thicknesses of stabilizing Ag layer and analyzed themeasured κ(T ) using the equivalent heat current circuit. κ(T ) of the tape increased withincreasing thickness of Ag layer. In the analysis, the dominant thermal carriers were found to be
the electrons in the Ag layer in the entire temperature range. The thermal conductivity of the Aglayer, κAg (T ), was estimated to decrease with increasing thickness of Ag layer, which suggested
that impurities and/or crystal defects were introduced during the long time deposition of Ag.
(Some figures in this article are in colour only in the electronic version)

tape [3] as follows,

1. Introduction

Qt =It is well known that YBa2 Cu3 O7−δ (YBCO) shows a highperformance at 77 K under a magnetic field parallel to thec-axis, B  c, in comparison with the Bi–Sr–Ca–Cu–O
system. The decrease of the critical current density Jc ofYBCO is not so large under the high magnetic field. Theabsolute value of Jc of the YBCO film has been improvedby the introduction of various artificial vortex pinning centers(AVPCs) using nanotechnology [1]. Recently, AVPCs werealso introduced to YBCO coated conductors (YCC) and Jc (B c) at 77 K exceeded that of NbTi at 4.2 K [1]. A longsuperconducting wire with higher Jc is required for practicalapplications, and numerous studies have been developed by anumber of researchers and companies [2]. Recently, YCC has
just started to be used for prototype power electric applications
such as power cables, motors, transformers, and fault current
limiters [2].
The thermal conductivity κ(T ) of the tape is one of theessential physical properties in the design of superconducting
equipment because it gives the heat flow Q t through the0953-2048/10/105013+04$30.00

SL

 T2

κ(T ) dT.

T1

In the equation, T1 and T2 are the temperatures of bothterminals of the tape, S the cross section and L the length ofthe tape. YCC has a layered structure which consists of a metal
substrate, buffer and cap layers, YBCO film, and a stabilizing
Ag layer. The absolute value and temperature dependence ofthe thermal conductivity of the YCC tapes should depend on
the species of each material, its purity and thickness. However,
κ(T ) of YCC tapes has not been reported as far as we know.
In this paper, we investigate the temperature dependence ofthe thermal conductivity κ(T ) of YCC tapes with various
thicknesses of the stabilizing Ag layer. The measured κ(T )was compared to the estimated one, which was calculated by
the parallel heat flow model.

2. Experimental details
YCC was fabricated by an ion beam assisted deposition(IBAD) and a pulsed laser deposition (PLD) method.
1© 2010 IOP Publishing Ltd Printed in the UK & the USA

Supercond. Sci. Technol. 23 (2010) 105013

T Naito et al

Figure 2. The temperature dependence of the electrical resistivity,
ρ(T ), for three YBCO coated conductors (YCCs). Inset shows theestimated electrical resistivity of the Ag layer, ρAg (T ), for three YCC
tapes.

Figure 1. The temperature dependence of the thermal conductivity,
κ(T ), for three YBCO coated conductors. The broken lines represent
the electronic thermal conductivity, κe (T ), which is estimated fromthe electrical resistivity shown in figure 2. Inset shows thedependence on the thickness of the Ag layer of the κ value at 100 K,
κ(100 K).

3. Results and discussion
Figure 1 shows the κ(T ) of three YCC tapes with various
thicknesses of Ag layer tAg . For YCC–Ag20, κ(T ) decreases
moderately with decreasing temperature from 200 K down to85 K, begins to increase below 85 K, and reaches a maximum
around 20 K. The absolute values of κ are 71 W m−1 K−1 at
200 K and 63 W m−1 K−1 at 77 K. The peak value of κ isapproximately 130 W m−1 K−1 . The shape of κ(T ) for YCC–
Ag35 and YCC–Ag55 is similar to that for YCC–Ag20. Theabsolute value of κ increases with increasing tAg because of
the high thermal conductivity of the Ag layer, κAg (T ). The
inset of figure 1 shows the κ value at 100 K, κ(100 K), as afunction of tAg , where the κ(100 K) value at tAg = 0 μm is thatof Hastelloy measured here. κ(100 K) is not proportional totAg . YCC–Ag35 and YCC–Ag55 reach peaks of approximately
158 W m−1 K−1 near 24 K and approximately 203 W m−1 K−1near 26 K, respectively. The observed peak is caused by thecompetition of the mean free path of the thermal carriers, l , andthe specific heat, C , because of the relation, κ = 13 Cvl , wherev is the mean velocity of the thermal carriers [6]. The shiftof the peak temperature suggests a change in the purity of Ag
layer, as observed in κ(T ) for various high-purity metals [7, 8].
Figure 2 shows the temperature dependence of theelectrical resistivity ρ(T ) of three YCC tapes.
Thesuperconducting transition temperature Tc defined at themiddle-point of the transition was approximately 92 K for
all the tapes. All the ρ(T ) curves depend linearly on thetemperature in the normal state. The absolute value of ρ(T )decreases with increasing tAg . Assuming that the applied
electric current flows through each conducting layer in parallel,we estimated the electrical resistivity of the Ag layer, ρAg ,above Tc by the reduced equation of the combined resistance,ρAg = ρ × (tAg /ttape ), where ttape is the total thickness of thetape. As found in the inset of figure 2, all the ρAg (T ) curves

The details of sample preparation have been described
elsewhere [4]. The substrate was Hastelloy C-276 (100 μm inthickness). Gd2 Zr2 O7 (GZO) film (about 1 μm in thickness)
was deposited on the Hastelloy as the buffer layer by theIBAD method. The cap layer of a CeO2 film (0.7 μmin thickness) and YBCO film were prepared on the IBADGZO film by the PLD method. The thickness of the YBCOlayer was approximately 1.5 μm. Finally, Ag was deposited
on YBCO/CeO2 /GZO/Hastelloy as the thermal stabilizer.
We prepared three YCC tapes with different Ag thickness
tAg , 20, 35 and 55 μm. These were named YCC–Ag20,YCC–Ag35 and YCC–Ag55, respectively. To examine thecontribution of the buffer and cap layers to the thermaltransport, plain Hastelloy tape and CeO2 /GZO/Hastelloy tapewere also prepared.
The thermal conductivity κ(T ) was measured by a steadystate heat flow method [5]. One end of the tape was soldered
to the copper-block as a heat sink. A small metal chip
resistor (1 k) was adhered to the other end of the tape as aheater using GE7031 varnish. κ is given as κ = (Q/T ) ·(L/S), where Q is the applied heat flow, T the temperature
difference, L the distance between the two thermocouples,and S the cross section of the tape. T was measured
using two chromel–constantan thermocouples and maintained
at about 1 K by controlling the electric current applied to thechip resistor. The electrical resistivity ρ(T ) was measured
by a four-probe method with a typical current density of 5–
7 A cm−2 . The temperature of the sample stage was controlledusing a Gifford–McMahon cycle helium refrigerator and a30 W heater. To eliminate the radiation loss, κ was measured
below 200 K.
2

Supercond. Sci. Technol. 23 (2010) 105013

T Naito et al

are scaled on the universal curve within the experimental error.
Therefore, the applied electric current flows through the Aglayer in the normal state. The electronic thermal conductivity
κe can be estimated roughly from ρ(T ) using the Wiedemann–Franz (WF) law, κe = L 0 T /ρ , where L 0 is the Lorenz number.
The estimated κe ’s for three YCC tapes at 200–100 K areabout 60–70 (YCC–Ag20), 90–105 (YCC–Ag35), and 130–
140 W m−1 K−1 (YCC–Ag55), respectively. The estimatedκe (T ) curves are shown in figure 1 and are roughly comparable
to the measured κ(T ), suggesting that the dominant thermalcarriers in the normal state are the electrons in the Ag layer for
all the YCC tapes.
We attempt to estimate κ(T ) of the YCC tapes
approxiamately using κ(T ) of each elemental material such as
CeO2 /GZO/Hastelloy, YBCO, and Ag. The schematic imageof the heat flow through a layered sample which consists of nkinds of materials is shown in figure 3(a). We assume that theheat flows through each layer in parallel. The total applied heat
Q tot is written asnQ tot =Qi .

(a)

(b)

i=1

Q i is the heat flow through the i th layer, which depends on thethermal conductance of each layer κi−1 . Since T and L are
the same for all the layers, the total thermal conductivity, κtot ,can be estimated bynκtot =(κi × Si )Stot .

(c)

i=1

9

Si and Stot are the cross section for the i th layer andthe tape, respectively.
Figures 3(b) and (c) show thetemperature dependence of the measured and calculated
thermal conductivity, κ(T ) and κcal (T ), of YCC–Ag20 andYCC–Ag55. The reported κ(T ) of Ag–Au alloy [7, 9] andthat in the ab -plane of YBCO superconducting bulk [5] arealso plotted. The measured κ(T ) curves of Hastelloy andCeO2 /GZO/Hastelloy tapes almost coincide with each other,
meaning that the contribution of both buffer and cap layers
to the thermal transport is negligibly small. The absolutevalue of κ(T ) for Hastelloy is about one tenth of that for
YCC–Ag20 at 100 K. As seen in figure 3(b), κ(T ) of YCC–
Ag20 is reproduced well by κcal (T ), when κAg (T ) of Ag–
0.09 at.%Au alloy was used. On the other hand, the measured
κ(T ) of YCC–Ag55 was between κcal (T ) using κAg (T ) of Ag–0.09 at.%Au alloy and that using κAg (T ) of Ag–0.22 at.%Aualloy. These results suggest that the absolute value of κAg (T )of the Ag layer in YCC–Ag55 is lower than that in YCC–
Ag20. The impurity content in the Ag layer increased for theYCC–Ag55 sample and/or crystal defects corresponding to theamount of impurity were introduced during the deposition ofthe Ag layer. Other residual substances in the chamber are
possible impurities, which will be examined using an electronprobe micro analyzer (EPMA) and inductively coupled plasma(ICP) spectroscopy. We plan to evaluate the crystal defects
using a scanning electron microscope (SEM). Consequently,
the dominant thermal carriers are the electrons in the Ag layer
above and below Tc for the YCC tapes.

Figure 3. (a) A schematic image of the heat flow through a layered
sample consisting of the n -kinds of layers. (b) and (c) show themeasured and calculated thermal conductivity, κ(T ) and κcal (T ), for
YCC–Ag20 and YCC–Ag55, respectively. κ(T ) for theAg–0.09 at.%Au [7], Ag–0.22 at.%Au [9] and YBCO bulk [5] arereported values. The measured κ(T ) of Hastelloy andCeO2 /GZO/Hastelloy tapes is also shown.

4. Summary
We have measured the temperature dependence of the thermal
conductivity κ(T ) of three YBCO coated conductors (YCCs)
with different thicknesses of the Ag layer. κ(T ) showed apeak around 20 K because of the κAg (T ) of the stabilizing Aglayer. The absolute value of κ(T ) increased with increasing
thickness of the Ag layer. The thermal conductivity κ(T )was compared to the calculated thermal conductivity κcal (T )using the equivalent heat current circuit. As a result, the3Supercond. Sci. Technol. 23 (2010) 105013

T Naito et al

dominant thermal carriers were the electrons in the Ag layer.
The magnitude of κAg (T ) of the Ag layer was suppressed inthicker Ag layers. A possible reason is that the purity of theAg layer decreased with increasing thickness or that crystal
defects were introduced during long time deposition. κ(T ) ofHastelloy and buffer and cap layers deposited on the Hastelloy
are less than one tenth of κ(T ) of YCC tapes stabilized withAg layers.

References[1] Matsumoto K and Mele P 2010 Supercond. Sci. Technol.
23 014001[2] Shiohara Y, Fujiwara N, Hayashi H, Nagaya S, Izumi T andYoshizumi M 2009 Physica C 469 863[3] Sasaoka T, Nomura K, Sato J, Kuma S, Fujishiro H,Ikebe M and Noto K 1994 Appl. Phys. Lett. 64 1304
[4] Kakimoto K et al 2010 Supercond. Sci. Technol. 23 014016[5] Fujishiro H, Ikebe M, Naito T, Noto K, Kohayashi S andYoshizawa S 1994 Jpn. J. Appl. Phys. 33 4965
[6] Berman R 1976 Thermal Conduction in Solids (Oxford:Clarendon)[7] Crisp R S and Rungis J 1970 Phil. Mag. 22 217[8] Ho C Y, Ackerman M W, Wu K Y, Oh S G and Havill T N 1978
J. Phys. Chem. Ref. Data 7 959[9] Fujishiro H, Ikebe M, Noto K, Sasaoka T and Nomura K 1993
Cryogenics 33 1086

Acknowledgments
We thank Dr Y Iijima at Fujikura Ltd for offering us theinformation on the measured samples. This work wassupported by the New Energy and Industrial Technology
Development Organization (NEDO).

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