1124

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 12, NO. 1, MARCH 2002

Thermal Conductivity, Thermal Diffusivity
and Thermoelectric Power of Sm-Based
Bulk Superconductors
Hiroyuki Fujishiro and Shuichi Kohayashi

Abstract—The -plane thermal conductivity( ), thermal
diffusivity( ) and thermoelectric power( ) were measured for the highly-oriented Ag-doped superconducting materials
Sm1+2 Ba2+ Cu3+ O7+5 ( = 0 1
0 4) prepared by amelt process. The absolute value ofand its characteristic enhancement below
were suppressed with increasing content
of the Sm2 BaCuO5 (Sm211) secondary phase, while they were enhanced with increasing content of doped-Ag. The application of themagnetic field up to 10 T clarified that the phonon scattering byvortex was most enhanced with the applied field perpendicular to
the -plane. The trapped magnetic also seriously suppressed the( ) enhancement.
Index Terms—Phonon scattering, Sm-based superconductors,
thermal conductivity, thermal diffusivity.

I. INTRODUCTIONV

ARIOUS kinds of practical applications using the meltprocessed REBaCuO-system superconductors (RE: rareearth ions), such as magnetic bearings, levitation systems, flywheels, bulk magnets, current leads and so on have been recently proposed and developed [1], [2]. The critical current denof REBaCuO-system in high magnetic fields has beensity
improved by employing smaller RE ions such as Sm and Nd, exceeding those of the YBaCuO-system. The melt-processed RE(RE123)
BaCuO-crystals consist of both the REBa Cu O
superconducting phase and the RE BaCuO (RE211) impurity
phase. Silver (Ag) metal is added in order to improve the mechanical strength and to increase . A small amount of platinum (Pt) metal is also added to disperse finely and uniformly
the RE211 particles, which act as pinning centers. For practicalapplications, thermal properties of superconducting materials
such as the thermal conductivity , thermal diffusivity , thermoelectric power , specific heat are very important. The low
thermal conductivity, for example, is essential for the application of bulk superconducting materials as power current leads.
We have reported and analyzed the anisotropy for the bulk
Y123 and Y211 composite system [3].

In the present paper, we reportandof the highly-oriented Ag-doped Sm
Ba
Cu
O
(
) in the temperature range of 10 300 K and in
the magnetic field of 0 10 T. We analyze the effect of additionsof Sm BaCuO (Sm211) and Ag metal, on the thermal properis also investities. The magnetic field dependence ofgated and the phonon scattering by the vortex is discussed.
II. EXPERIMENTAL
The preparation of Ag-doped Sm
Ba
Cu
O
(
) bulk superconductors was described elsewhere [4]. The content of the added Ag was
10 or 15 wt.% and that of the Pt was 0.42 wt.% to theBaCuO
weight. The sizes of the Sm211
Smand Ag particles were 2 5 and 20 30 m, respectively. From
the X-ray diffraction analyses, the grown crystals were crystallographycally highly oriented, where the bulk surface is parallelto the -plane of Sm123. The slender-shaped samples (4.0 mm
4.0 mm 25.0 mm) were cut parallel to the -plane for themeasurement. The Sm211 polycrystal was also fabricated by
the solid state reaction at 1150 C in air in order to compare thethermal properties with those of the bulk crystals.
and the therThe -plane thermal conductivity
were automatically measured by amoelectric power
steady-state heat flow method between 10 and 300 K and themeasurement was performed by anthermal diffusivity
arbitrary heating method under an identical experimental setup
andmeasurements [5]. A Gifford–McMahon
with the(GM) cycle helium refrigerator was used as a cryostat. The
chromel-constantan thermocouples with 76 m in diameterand measurements.
were used as thermometers for the
The magnetic field of up to 10 T was applied using a cryocooler-cooled superconducting magnet.
III. RESULTS AND DISCUSSION
A. Volume Fraction of the Constituent Phases

Manuscript received September 24, 2001. This work was supported in part by
Japan Science and Technology Corporation under the Joint-Research Projectfor Regional Intensive in the Iwate Prefecture on “Development of practicalapplications of magnetic field technology for use in the region and in everyday
living.”H. Fujishiro is with the Faculty of Engineering, Iwate University, Ueda,
Morioka 020-8551 Japan (e-mail: fujishiro @iwate-u.ac.jp).
S. Kohayashi is with the Central Research Laboratory, DOWA Mining Co.,
Ltd., Hachioji 192-0001 Japan (e-mail: kohayass@dowa.co.jp).
Publisher Item Identifier S 1051-8223(02)04138-6.

The present Ag-doped SmBaCuO superconductors consistof the Sm123 superconducting phase, the Sm211 insulatingphase, and Ag and Pt particles. The constituent phases andparticles contribute the thermal properties of this system. Fig. 1
shows the calculated volume fraction of each component for
Ba
Cu
O
(
). The measuredSmdensity and the ideal densities of the Sm123, Sm211, Ag and
Pt (6.87, 7.46, 10.25 and 21.37 g/cm , respectively) were used1051-8223/02$17.00 © 2002 IEEE

FUJISHIRO AND KOHAYASHI: Sm-BASED BULK SUPERCONDUCTORS

Fig.
1. ThevolumeBa
CuO
Sm
wt.% (X = 0:2) samples.

fractionof
eachcomponentfor(
0:1 0:4) for Ag = 10 wt.% and 15

X=



1125

Fig. 3.



The temperature dependence of the ab-plane thermal conductivity(T ) for various X under the zero magnetic field.

B. Thermal Conduction in Zero Magnetic Field

Fig. 2.

The temperature dependence of the ab-plane resistivity  (T ).

for the calculation. The volume fraction of Pt was omitted in
the figure because it is very small ( 0.12 vol.%). The volume
fraction of Sm123 decreases and that of Sm211 increases withincreasing . It should be noticed that the fraction of void also
increases with increasing . In Fig. 1, the volume fractions) sample. The
are also shown for the Ag 15 wt.% (increase of the Ag content to 15 wt.% markedly reduces voids
possibly through the intrusion of Ag particles into the grainboundaries.
Fig. 2 shows the temperature dependence of the -plane. The
values for the Ag
10electrical resistivity
wt.% samples increase with increasing , which may result
from the increase of the insulating Sm211 fraction. The superconducting transition temperature slightly decreases with insample, thecreasing . As can be seen for thedecreases by increasing the Ag content from 10 to 15 wt.%. For
and Ag 15 wt.% sample,
recovers to 94 K,
thethe maximum value of the present samples. The dependencecannot be interpreted byand the Ag content dependence of
only the changes of the volume fraction of the Sm123 phase. It
is suggested that the superconductivity of the Sm123 phase isimproved by the increase of Ag content up to 15 wt.%.

for varFig. 3 shows the temperature dependence ofwith Ag 10 wt.%. For the
sample which
ious
consists of the maximum amount of Sm123 superconducting
value above ( 93 K) is large and thephase, the absoluteenhancement below
is prominent. For the
sample, theenhancement becomes less prominent and it
is further reduced with further increase of . At the same time,in the normal state decreases. However, it is to be novalues ofandsamples
ticed that
almost coincide each other over the entire temperature range.
of the Sm211 polycrystals is also presented in Fig. 3,
which decreases with decreasing temperature and is very small,of
making a contrast with that of the Y211 polycrystal [3].
50 K
Y211 was somewhat larger, took a faint maximum atand decreased with further increase of . The electronic thermal, estimated by the Wiedemann–Franz law
conductivity
values, decreases with increasing . The ratio ofusing theto the total at 300 K is 31% and 16% for
0.1 and 0.4,
with increasing above
respectively. The decrease of
may come mainly from the reduction of , the increase of
low thermal conductive Sm211 phase and the enhanced phonon
scattering in the Sm123 phase.
It has been found that in REBaCuO-systems with thelight RE (LRE) element such as Sm and Nd, a part of the
LRE ions is substituted for the Ba site, thus forming theBa
Cu O -type solid solution [1]. ThenshouldLRE
actually
decrease with increasing RE ion substitution.
decreases with increasing as shown in Fig. 2, which suggests
increased substitution of Sm for Ba site with increasing .
The increased migration of Sm and Ba enhances the phonon
scattering in the Sm123 phase, working as a kind of point
defect type phonon scattering centers.
oxide superconductors
The thermal conductivity of highby both the electronic contribu(HTSC) is enhanced below
and the phonon contribution. In not ideally puretion
HTSC with sizable residual electron scattering by impurities,
the enhancement mainly comes from the phonon component[6]. The
enhancement below
is caused by the
reduction in the phonon scattering by quasiparticles with the1126IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 12, NO. 1, MARCH 2002

Fig. 4. The temperature dependence of the ab-plane thermal diffusivity(T ) for typical samples.

Fig. 5. The temperature dependence of the thermoelectric power S (T ) in
the ab-plane.

progress of their condensation into cooper pairs [5]. BecauseBa
CuO
samples belong to
the present Sm
the impure HTSC material containing a considerable amountof the Sm and Ba migration and the impurity Sm211 phase,contribution. The
the enhancement is due to theenhancement is masked out if the phonon scattering strength
by other mechanisms becomes stronger. The suppression of theenhancement with increasing
may reflect the enhanced
phonon scattering due to increased Sm and Ba migration. For
sample with Ag 15 wt.%,
recovers to 94 K
theenhancement also
and then the absolute value and therecover as shown in Fig. 3. This may result from the suppression of the migration and/or from the improved crystallinity in
the Sm123 phase.
. In theFig. 4 shows the temperature dependence ofof all the samples increases slightly withnormal state,increases more anddecreasing temperature. Below ,more rapidly with decreasing temperature. The absolute value of
decreases with the increase of and increases with increasing.
content of the doped-Ag, in accord with the behavior of. For theFig. 5 shows the temperature dependence ofabove
positively increases
Ag 10 wt.% samples,Fig. 6. The ab-plane thermal diffusivities  (T ) with the applied field B (a)perpendicular and (b) parallel to the ab-plane, respectively, for the X = 0:1(Ag = 10 wt.%) sample. (c) The magnetic field dependence of  at 40 K
where B in both parallel and perpendicular directions to the ab-plane.

with increasing
given by. The thermoelectric powerof a metal is(1)
where the electron energy. Quantitatively, is expected toincrease in impure metals because of increasing values. Acwith may partly be atcordingly, the observed increasing
increase in the Sm123 phase caused bytributable to thethe migration effect. Since the present Ag-doped Sm-based material is a composite system composing of Ag particle, Sm123. However, it is worth
and Sm211, it is difficult to analyze

FUJISHIRO AND KOHAYASHI: Sm-BASED BULK SUPERCONDUCTORS

while to note thatof thesample is clearly larger
sample in spite of almost indistinguishthan that of the. This fact indicates that the Sm211 phase also intrinsiable, enhancesofcally, i.e., not only through the increase ofthis composite system.
C. Thermal Conduction Under the Magnetic Fieldof the(Ag 10In this subsection, we presentwt.%) sample under the magnetic fields. Fig. 6(a) and (b) showsfor field cooling runs with the applied field perpendicularand parallel to the -plane, respectively. The solid lines are to
above
is indeguide the reader’s eyes. In both figures,
enpendent of the applied magnetic fields. In Fig. 6(a), thehancement below is drastically suppressed with increasing
perpendicular to the -plane. This reduction can be understoodas caused by the enhanced phonon scattering by the quasiparticles in the vortex cores. For the application of parallel toenhancement isthe -plane, however, the reduction of thebarely discernible as shown in Fig. 6(b). In this case, the vortices parallel to the -plane hardly hinders the heat flow. The
wassimilar anisotropic effect of the magnetic field on
observed for YBCO and Bi-2212 single crystals.
at 40 K
Fig. 6(c) shows the magnetic field dependence of-planewhere was swept up and down up to 10 T. For the(40 K) promptly decreases with increasing magnetic
setting,field and partly recovers with decreasing field; very clear hysis noticed. The(40 K) value at 0 T
teretic behavior ofafter applying field is 8% smaller than the initial one. This suggests that the trapped magnetic field scatters the phonon transport even after removing the field. The trapped magnetic field-plane setting is estimated to be about 2 T fromfor thethe data for the increasing virgin run. On the other hand, for-plane setting, the field dependence of(40 K) arethequite small up to the applied field of 10 T.
Lastly, we comment on the effect of the added-Ag particles
on the thermal transport. In the previous paper, we reported thethermal conductivity of the Ag YBCO mixed crystals for various Ag contents and determined the purity of Ag particles inthese crystals [7]. Using the previous data, the residual resistivity ratio (RRR) of the doped-Ag particles is estimated to be
15 in the present case. The measured thermal conductivity of
Ag 0.22 at.% Au alloy with RRR 15 does not show theenhancement around 40 K. The addition of 10 and 15 wt% Agover the entire temperaenhances the absolute value ofenhancement
ture range, but it cannot directly cause thebelow .
IV. SUMMARY
The-plane thermal conductivity, diffusivity
and thermoelectric power
were measuredBa
CuO
for highly oriented Ag-doped Sm
materials, which consisted of superconducting Sm Ba Cu O
(Sm123) and semiconductive Sm Ba Cu O (Sm211) phasesby the ratio 1 : . The obtained results are summarized asfollows.

1127i) For the Ag 10 wt.%-doped series, thevaluesenhancement below , characteristic ofand thesuperconductors, are largest for,
highup towhich are rapidly reduced with increasing.
values became almost identical forandsamples. An importanttheorigin for the reduction was suggested to come fromincreasing migration of Sm and Ba ions. The qualityof the Sm123 crystal was deteriorated with decreasing, as suggested also by thedependence on .
The increasing Ag-doping from 10 wt.% to 15 wt.%
crystal enhanced thevalues,for theincrease to 94 K. These results
accompanying theindicate that the deteriorated crystallinity of Sm123 isrecovered or improved by the increased Ag-doping. The
behaved consistently to.
diffusivity
enhanceii) Under the magnetic field up to 10 T, thement was markedly reduced for the applied field config-plane. In contrast, very weak suppression
uration,-plane configuration. These
was observed for theenresults are consistent with the explanation that thehancement below originates from the reduction in thephonon scattering by quasiparticles. The anisotropic suppression with respect to the field direction can be understood to come from the appearance of anisotropic vortex
scattering for phonons which originates from the layer
structure of Sm123 lattice. From the hysteretic behavior
under the applied fields, we could estimate theof
to be about 2 T after the 10
trapped magnetic flux
-plane configuration.
T field scan in thewas positiveiii) The -plane thermoelectric power
and increased with increasing , precisely reflecting
up to. It is worthwhile
the Sm211 content
andbehaved almost
to notice that both
0.3 and 0.4 samples, making a sharp
identically for.
contrast with the behavior of
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