INSTITUTE OF PHYSICS PUBLISHING

SUPERCONDUCTOR SCIENCE AND TECHNOLOGYSupercond. Sci. Technol. 19 (2006) S447–S450

doi:10.1088/0953-2048/19/7/S05

Thermal conductivity and thermoelectric
power of DyBaCuO bulk superconductors
Hiroyuki Fujishiro1 , Shinya Nariki2 and Masato Murakami2,3
1

Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan
Superconductivity Research Laboratory, ISTEC, 1-10-13 Shinonome, Tokyo 135-0062,
Japan
3Shibaura Institute of Technology, 3-9-14 Shibaura, Minato-ku, Tokyo 108-8548, Japan2

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

Received 28 December 2005
Published 11 April 2006
Online at stacks.iop.org/SUST/19/S447
Abstract
The ab-plane thermal conductivity κ(T ) and thermoelectric power S(T )have been investigated for DyBaCuO bulk superconductors for various
contents of the Dy2 BaCuO5 (Dy211) phase X . The absolute κ(T ) value ispretty small compared with that of other REBaCuO bulks (RE = Y and rareearth elements). The S(T ) value above the superconducting transition
temperature Tc changes from negative to positive and κ(T ) slightly decreases
with increasing oxygen deficiency in the DyBa2 Cu3 O7−δ (Dy123)superconducting phase. The point-defect-type phonon scattering due tooxygen deficiency affects the κ(T ) value, but is not a main origin for the low
κ(T ) even for the fully oxidized DyBaCuO bulk. Another type of powerfulphonon scattering centre governs the thermal transport properties in theDyBaCuO bulk.

1. Introduction
For practical applications and fundamental research of meltprocessed REBaCuO bulk superconductors (RE: Y and rareearth elements), thermal properties such as the thermal
conductivity κ , thermal diffusivity α and thermal dilatationd L/L are valuable parameters, besides the electro-magnetic
properties such as the superconducting transition temperature
Tc and the critical current density Jc . The REBaCuO
bulk is an anisotropic and complex material, which consistsof the REBa2 Cu3 O7−δ (RE123) superconducting phase, theRE2 BaCuO5 (RE211) non-superconducting phase and Ag andPt metals. The thermal properties depend on the quality andalignment of the RE123 phase, and the content, size anddispersion of the RE211 phase and added metals. Defects suchas voids and cracks are also influencing factors.
We have measured the thermal and mechanical properties
at low temperatures (4–300 K) and under a high magnetic
field (0–10 T) for the REBaCuO bulks, which were fabricatedby several companies and institutes. We have constructed a
database of these properties, which is available online [1, 2].
The absolute value and temperature dependence of the thermal
conductivity κ(T ) changes depending on the species of theRE ions; κ(T ) of RE = Y, Gd shows a high value and the0953-2048/06/070447+04$30.00

dκ/dT  0 behaviour above Tc [3, 4]. On the other hand,
κ(T ) of RE = Sm, Nd shows a somewhat low value andthe dκ/dT  0 behaviour above Tc due to the light RE(LRE) element substitution for the Ba site [5]. The increased
migration of LRE and Ba enhances the phonon scattering in thesuperconducting phase, working as strong phonon scattering
centres. Recently, we reported that the DyBaCuO bulk showeda lower κ(T ) of about a half or one-third value than that of theYBaCuO bulk, but had a comparable high- Jc value [6]. In theRE = Y and Dy systems, however, it has been considered thatthe migration does not take place during the crystal growth dueto the non-LRE element. The origin of the low κ(T ) in theDyBaCuO system is still an open question.
In this paper, we study the temperature dependence of theab-plane thermal conductivity κ(T ) and thermoelectric power
S(T ) of the DyBaCuO bulks with various Dy211 contents
X . The samples with different oxygen content in the Dy123
phase are prepared and the relation between the κ(T ) andthe oxygen deficiency is investigated. The S(T ) has proved
to be a sensitive probe to estimate the oxygen deficiency inthe REBaCuO superconductor. The origin of the low thermal
conductivity for the DyBaCuO bulk is discussed from theviewpoint of the oxygen deficiency.

© 2006 IOP Publishing Ltd Printed in the UKS447

H Fujishiro et al

Figure 1. The temperature dependence of the ab-plane electricalresistivity ρ(T ) for the DyBaCuO bulk for various Dy211
contents X .

2. Experimental details
The DyBaCuO bulk samples were fabricated at the International Superconductivity Technology Center, Superconductivity Research Laboratory (ISTEC-SRL). The content X of theDy211 phase in the Dy123 phase was changed from 5 to 40
in the molar ratio of Dy123:Dy211 = 100:X . 10 and 0.5 wt%
Ag2 O and Pt powders were added in all the samples [7, 8]. The
mixed powders were uniaxially pressed into pellets 40 mm indiameter and 25 mm in thickness. Melt-processing was performed by the hot-seeding method in air. The melt-grown
Nd123(100) crystal was used as a seed. The pellets wereheated to 1100 ◦ C and held for 0.5 h, cooled down to 1025 ◦ C,
then cooled at a rate of 0.15–0.5 ◦ C h−1 to 950 ◦ C and finallycooled down at a rate of 100 ◦ C h−1 to room temperature. The
grown bulks were heat treated in flowing oxygen gas at temperature THT = 400 ◦ C for 100 h, which seems to be an optimum heat treatment temperature to obtain the highest Tc . For
the measurements, rectangular pieces with dimensions of about
2.5 × 2.5 × 10.0 mm3 were cut from the heat-treated bulks.
In order to clarify the influence of the oxygen deficiency in
the Dy123 phase on the κ(T ) and S(T ) behaviours, the asgrown bulks ( X = 5) were heat treated in flowing oxygen
at THT = 500 and 550 ◦ C for 100 h. The heat treatment at
higher THT is expected to increase the oxygen deficiency δ inthe Dy123 phase. The ab-plane thermal conductivity κ(T )and thermoelectric power S(T ) were simultaneously measured
by a steady-state heat flow method using a Gifford–McMahon(GM) cycle helium refrigerator as a cryostat between 10 and300 K [9]. The electrical resistivity ρ(T ) was measured by afour-terminal method.

3. Results and discussion
3.1. Dy211 content X dependence of κ(T ) and S(T)
Figure 1 shows the temperature dependence of the ab-planeelectrical resistivity ρ(T ) for the DyBaCuO bulk for various
Dy211 contents X . The ρ value increases with increasing X ,which may result from the increase of the insulating Dy211
fraction. The superconducting transition temperature Tc isalmost identical (=91.5 K) for X = 5–20 but Tc of the X = 40
sample is somewhat higher at Tc = 93 K.
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Figure 2. The ab-plane thermal conductivity κ(T ) for the DyBaCuO
bulk for various Dy211 contents X .

Figure 2 presents the temperature dependence of the abplane thermal conductivity κ(T ) for the DyBaCuO bulk forvarious Dy211 content X . κ(T ) gradually decreases withdecreasing T , shows a characteristic peak below Tc and thendecreases to zero at low temperatures. The absolute κ(T ) value
decreases with increasing X . For the X = 5 sample, whichcontains the maximum amount of Dy123 superconducting
phase, the absolute κ value above Tc is larger and the κ(T )enhancement below Tc is prominent. For the X = 10sample, the κ(T ) enhancement decreases slightly and it isfurther reduced with further increase of X . However, it is
to be noticed that absolute κ(T ) values of X = 20 and40 samples almost coincide with each other over the entire
temperature range. κ(T ) of high-Tc superconductors (HTSCs)
is enhanced below Tc by both the electronic contribution κeand the phonon contribution κph [10, 11]. In impure HTSCs
with sizable residual electron scattering by impurities, the κ(T )enhancement mainly comes from the phonon component κph ,which is caused by the reduction in the phonon scattering byquasi-particles with the progress of their condensation into
Cooper pairs. Because the present DyBaCuO samples belong
to the impure superconductor containing the Dy211 and Ag
phases, the κ enhancement is due to the κph contribution.
The electronic thermal conductivity κe (T ) above Tc , estimatedby the Wiedemann–Franz law using the ρ value, decreases
with increasing X . The ratio of κe to the total κ at 200 K
is 24% for all the X values. The decrease of κ(T ) withincreasing X above Tc may come mainly from the reductionof κe , the increase of low thermal conductive Dy211 phase andthe enhanced phonon scattering in the Dy123 phase. The κphenhancement is masked out if the phonon scattering strength byother mechanisms becomes stronger. The slight suppression ofthe κ enhancement with increasing X may reflect the enhanced
phonon scattering.
Figure 3 shows the temperature dependence of the abplane thermoelectric power S(T ) for the DyBaCuO bulk
for various Dy211 contents X . In the normal state, theS(T ) value is negative and approaches zero with decreasing
temperature T . The absolute S value decreases with increaseof X . Quantitatively, the absolute S value is expected toincrease with some kinds of defects because of increasing
resistivity ρ values. It was reported that, in the YBa2 Cu3 O7−δ
polycrystals, the S value continuously changes from negative
to positive with increasing oxygen deficiency δ , which also

Thermal conductivity and thermoelectric power of DyBaCuO bulk superconductors

Figure 3. The ab-plane thermoelectric power S(T ) for theDyBaCuO bulk for various Dy211 contents X .

Figure 4. The temperature dependence of the S(T ) for the X = 5
samples with different heat treatment temperatures THT .

reflects the ρ values [12]. The observed S(T ) behaviour inthe present DyBaCuO bulks may partly be attributable to theρ(T ) increase in the Dy123 phase caused by the crystal defectand/or the increase of δ . The S(T ) of the SmBaCuO bulk
system with Sm123:Sm211 = 100:X is positive and increases
with increasing X , which may partly be attributable to theρ(T ) increase in the Sm123 phase caused by the migration
effect [5]. If the δ value is nearly identical for all thepresent DyBaCuO samples due to the identical heat treatment
condition in flowing oxygen, the approach of absolute S value
to zero with increasing X results from the crystal defect.
3.2. Influence of oxygen deficiency on κ(T ) and S(T)The X = 5 samples which were heat treated in flowing oxygenat THT = 500 and 550 ◦ C for 100 h were prepared besidethe X = 5 sample heat treated at THT = 400 ◦ C shown insection 3.1. Figure 4 shows the temperature dependence of theS(T ) for the X = 5 samples with different THT . The sign ofS(T ) changes from negative to positive with increasing THT .
Since the heat-treated bulk at higher THT seems to have thelarger oxygen deficiency δ , the sign change in S(T ) comesfrom oxygen deficiency δ in the Dy123 phase, similarly to theYBaCuO [12]. The δ value for each THT cannot be estimatedquantitatively because the bulk sample is a mixture of Dy123
and Dy211.

Figure 5. κ(T ) for the X = 5 samples with different THT . The inset
shows the resistivity ρ(T ) for the same samples.

Figure 5 shows κ(T ) for the X = 5 samples with different
THT . The inset shows the resistivity ρ(T ) for the same samples,
which show the identical Tc value. For the heat-treated sample
at THT = 500 ◦ C, κ(T ) hardly changes compared with thesample commonly heat-treated at THT = 400 ◦ C. On theother hand, for the heat-treated sample at THT = 550 ◦ C, theκ(T ) enhancement is reduced and the magnitude of the κvalue decreases. These results suggest that a small amountof oxygen deficiency has a fair influence on the magnitudeof κ(T ), but is not the main origin for the low κ(T ) even in
the fully oxidized DyBaCuO bulk. We have measured κ(T )for REBa2 Cu3 O7−δ polycrystals (RE = Y, Dy, Sm, Nd) andanalysed the phonon scattering mechanisms using the phonon
conduction model [13]. The estimated phonon mean free pathlb for the DyBaCuO system is pretty short compared with otherREBaCuO systems, which suggests that a sizable crystal defectexists. A small amount of migration of Dy and Ba may takeplace, although there has not been the experimental obtained
evidence to date for the migration of the Dy ion and the Ba ion.

4. Summary
The ab-plane thermal conductivity κ(T ) and thermoelectric
power S(T ) have been investigated for the DyBaCuO bulk
superconductors for various contents of the Dy2 BaCuO5(Dy211) phase X and for various oxygen contents 7-δ in theDyBa2 Cu3 O7−δ (Dy123) phase. The absolute κ(T ) value ispretty small even for the X = 5 sample. The oxygen deficiency
δ slightly affects the absolute κ(T ) value within the δ value
where the superconducting transition temperature Tc is kept at
91 K. However, the oxygen deficiency is not the main origin of
the low κ(T ) behaviour for the fully oxidized DyBaCuO bulk.
Powerful phonon scattering centres, which drastically reducethe κ(T ) value, must exist in the DyBaCuO bulk, comparedwith other REBaCuO bulks (RE = Y, Gd). A crystal defectwhich is inherent for the DyBaCuO bulk may exist. A detailed
investigation is in progress.

Acknowledgments
This work is supported in part by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
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H Fujishiro et al

Science and Technology, Japan (No 17560001) and from Iwate
Prefecture, Japan.

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