IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 16, NO. 2, JUNE 2006

1007

Low-Thermal-Conductive DyBaCuO Bulk
Superconductor for Current Lead Application
Hiroyuki Fujishiro, Manabu Ikebe, Hidekazu Teshima, and Hosei Hirano

Abstract—The thermal conductivity( ) and thermal dilatation( )( ), thermal diffusivityof the DyBaCuO-planeofbulk superconductor have been measured. The
the bulk at 50 K is about one third of that of the YBaCuO
bulk, and moreover, the superconducting characteristics (criticalcurrent density c and superconducting transition temperaturec ) compare favorably with those of the YBaCuO bulk. The
current-voltage (I-V) characteristics of the current lead fabricatedfrom DyBaCuO bulk and the durability against the thermal
quenching are as good as those of the YBaCuO lead. The
DyBaCuO bulk is a promising material for the power current
lead use to be replaced for the YBaCuO bulk.

,
In this paper, we study the thermal properties such as
and the thermal dilatationof thebulk in the -plane and along the -axis. The -planeof thebulk is fairly lower than that of thebulk. We fabricated the power current leads using
bulk and measured the current-voltage (I-V) characteristics andthe durability against the heat cycles between 77 K and room
bulk has proved to be atemperature. As a result, thepromising material for the power current lead application to be
replaced for the YBCO bulk usually used.

Index Terms—DyBaCuO bulk, heat intrusion, low thermal
conductivity, power current lead, thermal dilatation.

II. EXPERIMENTAL PROCEDURE
A. Sample Preparation and Thermal Property Measurements

I. INTRODUCTIONTHE SUPERCONDUCTING current lead using thesuperconducting
bulk
melt-textured
rare earth elements and ) is one of the exemplary(
superconducmodels for practical applications of hightors, in which both the low thermal conductivity and theare indispensable conditions.
high critical current density
bulk is a composite material, which conThe( 123) superconducting matrix,
sists of( 211) nonsuperconducting secondary phase,and Pt metals. The temperature dependence ofde123 phase, the content, size
pends on the quality of the
211 phase andparticles. For theand dispersion of the,,andsystems, the crystal growth method.
has been optimized to attain the higher and trapped field
and the thermal diffusivity
The thermal properties such aswere also reported for each system [1]–[3]. Recently,
a higherof 1.4–1.9 T at 77 K has been obtained for thewith 32–48 mm in diameters,
large single domainofbulk [4]–[6]. This system has
far exceedinga merit of possible crystal growth in air. However, there is no
detailed report for the thermal properties for thebulk.

bulk superconductor
A highly -axis oriented
was prepared by a modified quench and melt growth (QMG)
method at Nippon Steel Co., Ltd. [7]. The bulk was composed( 123) and( 211) withof123: 211 = 1.0:0.33 and 0.5 wt% Pt
a molar ratio of
powder. The precursor disk was partially melted at 1150 andthen cooled down to 1040 , keeping a temperature gradientor
between the top and bottom of the disk. A
seed crystal was attached to the top-center of thedisk to promote highly oriented crystal growth. The bulk was
to 970 . After the paralslowly cooled down from 1040
lelepiped shaped specimens designed for along the -planeand -axis measurements were cut from the grown disk, thefor 100 h in flowing
specimens were heat treated at 450
oxygen to materialize the superconductivity.
was measured by a steadyThe thermal conductivity
meastate heat flow method and the thermal diffusivity
surement was performed by an arbitrary heating method under
an identical experimental setup with the measurement [8]. The
thermal dilatation
was measured by a strain gauge method, which was normalized to the length at 300 K [1]. The electrical resistivity
was measured by a four terminal method. A Gifford-McMahon(GM) cycle helium refrigerator was used as a cryostat for themeasurements between 10 and 300 K.
B. Design of Power Current Lead Using

Manuscript received September 18, 2005. This work was supported in part by
Grant-in-Aid for Scientific Research 17560001 from the Ministry of the Education, Culture, Sports, Science and Technology, Japan and from Iwate Prefecture,
Japan.
H. Fujishiro and M. Ikebe are with the Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan (e-mail: fujishiro@iwate-u.ac.jp;
ikebe@iwate-u.ac.jp).
H. Teshima and H. Hirano are with the Advanced Technology Research Laboratories, Nippon Steel Corporation, 20-1 Shintomi, Futtsu 293-8551, Japan(e-mail: hteshima@re.nsc.co.jp; hirano@re.nsc.co.jp).
Digital Object Identifier 10.1109/TASC.2006.871305

Bulk

Two types of the current leads were fabricated and tested.
Fig. 1 presents the photograph which shows a schematic viewof the current leads; one is referred as “B-type” and the otheras “1 K-type”. For the “B-type” lead, a thin plate parallel to the-plane of thebulk (
in cross section and 40 mm in length) was used. In the “1 K-type” lead,bar (in cross section andan -shaped
58 mm in length) with length direction parallel to the -plane1051-8223/$20.00 © 2006 IEEE

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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 16, NO. 2, JUNE 2006

Fig. 3. The temperature dependence of the ab-plane and c-axis thermal conductivity  (T ),  (T ) of the DyBaCuO bulk. For comparison, those of the
YBaCuO bulk with the same composition are also shown.

Fig. 1. Photograph (upper panel) and schematic view (lower panel) of the two
types of power current leads fabricated by use of the ab-plane DyBaCuO bulk.

Fig. 4. The temperature dependence of the ab-plane thermal diffusivity(T ) of the DyBaCuO and YBaCuO bulks. The inset shows the specific(T ) .
heat C (T ) estimated using the relation C (T ) =  (T )=Fig. 2. The temperature dependence of the ab-plane and c-axis resistivity
 (T ),  (T ) of the DyBaCuO and YBaCuO bulks.

was used. The
bulk was connected with theterminals and was covered with fiber reinforced plastics (FRPs).
The current-voltage (I-V) characteristics were measured at 77 K
under the magnetic field of 0.5 T by attaching thepermanent magnet onto the lead. The sweep rate of the current
was 12 A/s and 36 A/s for the “B-type” and “1 K-type”, respectively. The endurance test against the heat cycle between 77 K
and room temperature was performed by measuring the change
terminals.
of resistance between
III. RESULTS AND DISCUSSION
A. Thermal Properties of

Bulk

Fig. 2 shows the temperature dependence of the -plane and-axis electrical resistivity,
for thebulk. For comparison,ofbulk with the same,,
composition (without
metal) is also given.
ofbulk isat 200 K and is smaller than that of
bulk,
0.37
bulk.
suggesting a good crystalline state of theis 91.5 K and slightly higher than that ofbulk.
of thebulk is about two orders of magnitude.
higher thanFig. 3 shows the temperature dependence of the -plane and-axis thermal conductivity,
for thebulk, comparing with those of the
bulk.
of

both
and
exhibits a broad peak below ,superconductors. It should be nocharacteristic of the highof thebulk is fairly lower than thatticed thatbulk;ofis about one halfof the(at ) and one third (at 50 K) of that of thebulk. Theofbulk is as small as that of themagnitude ofbulk [2]. It has been found that in thesystem with the light RE (LRE) element such as
and,a part of the LRE ions is substituted for thesite and forms-type solid solution [9], resulting intheis notthe strong phonon scattering centers [2]. Sinceis small comparable
LRE element and the ionic radius of
with
, the
ion has been believed to be the nonsubstituted
site. From the thermal conductivity measurement,
one for theandmay take placea sizable amount of the migration ofor other powerful phonon scattering centers may be introduced.
[10] sugOur previous data for sintered
gested the existence of a strong phonon scattering mechanism
other than point defect type, whose scattering strength is nearly
independent of temperature. In order to clarify the relevant physical mechanisms of low thermal conductivity, detailed studiesof theare in progress. The -axis thermal conductivity
bulk is also fairly smaller than that of thebulk with no characteristic broad peak at low temperature.
ofFig. 4 presents the -plane thermal diffusivity
andbulks as a function of . The inset
estimated using the relationshows the specific heat
. In the normal state,FUJISHIRO et al.: LOW-THERMAL-CONDUCTIVE

BULK SUPERCONDUCTOR

1009

TABLE IVALUES FOR THE CURRENT LEADS USING
DyBaCuO AND YBaCuO BULKSESTIMATED Q

Fig. 5. The temperature dependence of the thermal dilatation dL(T )=L of the
DyBaCuO bulk in the ab-plane and along the c-axis. For comparison, those of
the YBaCuO bulk with the same composition are also shown.

of both samples increases slightly with decreasing temincreases more and more rapidly
perature. Below ,of thewith decreasing temperature. The absolute value ofbulk is smaller than that of the
bulk.
for
These results are reasonable because the specific heatbulk should hardly depend on the species oftheions [11], [12].
Fig. 5 shows the temperature dependence of the thermal diof thebulk in the -plane andlatationbulk. For bothalong the -axis compared with that ofdecreases (thein the -plane and along the -axis,
absolute value
increases) with decreasing temperature, showing a tendency for saturation at low temperatures. The-axis
is nearly twice as large as that of -plane. Itof thebulk is simshould be noticed thatilar to that of thebulk for both -plane and -axis.
data are a valuable physical quantity in the caseTheof the current lead design.

Fig. 6. The I-V characteristics of two types of current leads using the
DyBaCuO bulk at 77 K. The additional voltage generation takes place above
due to the excess current.
the threshold I

B. Calculation of Heat Intrusion Through the Current Leadthrough the current lead was calculated
The heat intrusion
data shown in Fig. 3. As the current lead conusing thesists of the bulk and the FRP support, the heat flows in parallelvalue of the current
through both the bulk and the FRP. The
and , is given
lead in the area between the temperatures,byFig. 7. The results of the durability against the 100 heat cycles.

C. Characteristics of(1)

andare thewhere is the length of the lead, andcross sections of the bulk and FRP, respectively. The
data from the literature were used. Table I shows the results ofvalues in cases of the [
,
] and
the
[
,
] for both theandbulks, respectively.
in (1) is far larger than
. The
using
bulk decreases to about aheat intrusion
bulk current lead.
half of that of theCurrent Lead

Fig. 6 presents the current-voltage (I-V) characteristics forboth types of current leads at 77 K. For the low current region,
the I-V characteristics are linear. In current region higher than
, the I-V curve deviates from the linear
the threshold valueline and the additional voltage is generated due to the excess
bulk. The
current above the critical current of thevalue is roughly 600 1000 A for “B-type” and 2700 3600
A for “1 K-type”, respectively. These results are similar to thebulk [13].
current lead using the
Fig. 7 shows the results of the endurance test against theandheat cycle. The current leads were immersed in liquid

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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 16, NO. 2, JUNE 2006

then were warmed in air to the room temperature. This thermal
cycle was repeated by 100 times. The current of 300 A for“B-type” and 1200 A for “1 K-type” was flown through the leadfor every 10 cycles and the resistance between
in liquid
the voltage terminals are given in Fig. 7. It was found that theresistance does not change after the 100 times heat cycling andno burning-out of the lead was observed. The durability of thebulk shows a high performance simlead using thebulk [13].
ilar to that using the

REFERENCES[1] H. Fujishiro, M. Ikebe, S. Kohayashi, K. Noto, and K. Yokoyama,and 211 phase on thermal expansion of“Additional effect ofbulk superconductor (, Y),” Cryogenics, vol.
43, pp. 477–481, 2003.
[2] H. Fujishiro and S. Kohayashi, “Thermal conductivity, thermal diffusivity and thermoelectric power of-based bulk superconductors,”
IEEE Trans. Appl. Supercond., vol. 12, pp. 1124–1127, 2002.
[3] H. Fujishiro, H. Teshima, M. Ikebe, and K. Noto, “Thermal conducbulk superconductors under applied field: effect oftivity ofcontent and size of Y211 phase,” Phys. C, vol. 392–396, pp. 171–174,
2003.
[4] S. Nariki and M. Murakami, “Superconducting properties ofbulk superconductors,” Supercond. Sci. Technol., vol.
15, pp. 786–790, 2002.
[5] S. Nariki, N. Sakai, M. Murakami, and I. Hirabayashi, “Effects ofcontents on microstructure and mechanical strength of-addedbulk superconductors,” Phys. C, vol.
412–414, pp. 651–656, 2004.
[6] S. Yoshizawa, T. Sato, H. Ohta, S. Hirano, Y. Hishinuma, S. Kohayashi,
H. Fujimoto, and H. Nakane, “D-211 content dependence of superconductivity in single-grain-123 system,” Supercond. Sci. Technol.,vol. 18, pp. S1–S4, 2005.
[7] M. Morita, M. Sawamura, S. Takebayashi, K. Kimura, H. Teshima, M.
Tanaka, K. Miyamoto, and M. Hashimoto, “Processing and properties
of QMG materials,” Phys. C, vol. 235–240, pp. 209–212, 1994.
[8] M. Ikebe, H. Fujishiro, T. Naito, and K. Noto, “Simultaneous measurement of thermal diffusivity and conductivity applied to Bi-2223
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[10] M. Ikebe, H. Fujishiro, K. Nakasato, and K. Noto, “Thermal conduc() superconducting,,,
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[11] H. Fujishiro, K. Yokoyama, M. Kaneyama, T. Oka, and K. Noto, “Estimation of generated heat in pulse field magnetizing forbulk superconductor,” Phys. C, vol. 412–414, pp. 646–650, 2004.
[12] H. Fujishiro, M. Ikebe, K. Noto, H. Teshima, and M. Sawamura,bulk superconductors -compar“Thermal properties ofbulk crystals-,” (in Japanese) J. Cryogenic Soc.
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Jpn., vol. 37, pp. 659–664, 2002.
[13] H. Teshima, H. Hirano, and M. Morita, “Critical current ofbulk current leads,” IEEE Trans. Appl. Supercond., vol. 14,
pp. 1778–1781, 2004.

REBaCuOAg

RE = SmSm

YBaCuO

Dy 0

Ba 0 Cu 0 O

IV. SUMMARY, thermal diffusivity
1) The thermal conductivity
and thermal dilatationof thebulk
superconductor have been measured in the -plane andat 50 K is very small
along the -axis. The -planebulk. Theand is about one third of that of thesuperconducting characteristics compare rather favorablywith those for YBCO bulk. These results strongly suggest
bulk is a promising material for thethat thepower current lead application to be replaced for the YBCO
bulk.
2) The power current leads were fabricated using thebulk. The heat intrusion
through thebulk decreased to about a half
lead usingbulk. The current-voltage (I-V)
of that using
characteristics and the durability against the thermal
quenching for the current lead fabricated byshowed the desirable behavior. The power current leadbulk superconductor provides an exusingcellent electrical properties and the mechanical strength
lead.
similarly to those of the

Dy BaCuOAg
Dy 0 Ba 0 Cu 0 O
Dy

Ba0Cu 0O

REBa Cu O RE = Y Dy Sm Nd

SmBaCuO

YBaCuO

Cu 0 O

GdBaCuO

Y 0 Ba 0