Journal of Computer and Communications, 2013, 1, 1-4
Published Online December 2013 (http://www.scirp.org/journal/jcc)
http://dx.doi.org/10.4236/jcc.2013.17001
Open Access JCC
1
Thermoelectric Properties of Ce0.09Fe0.67Co3.33Sb12/FeSb2Te
Multi-Layered Structures
R. Zeipl1, M. Jelínek1,2, J. Walachová1, T. Kocourek1, M. Vlček3
1Institute of Physics, Academy of Sciences of the Czech Republic, v.v.i., Na Slovance 2, 18221 Prague, Czech Republic; 2Czech
Technical University, Fa culty of Biomedical Engineering, Nám. Sítná 3105, 27201 Kladno, Czech Republic; 3Institute of Macromo-
lecular Chemistry, Academy of Sciences of the Czech Republic, v.v.i., Heyrovského nám. 2, 16206 Prague, Czech Republic.
Received August 2013
ABSTRACT
Thermoelectric properties of Ce0.09Fe0.67Co3.33Sb12/FeSb2.1Te multi-layered structures with period of 5 nm were studied
in temperature ranging from 300 K to 500 K. Structures were prepared by Pulsed Laser Deposition (PLD) on fused sili-
ca quartz glass substrates at the substrate temperature during the deposition Ts = 230˚C and Ts = 250˚C with the laser
beam energy density Ds = 3 Jcm−2. In the contribution temperature dependencies of the in-plane electrical conductivity,
the Seebeck coefficient and the resultant power factor together with room temperature value of thermoelectric figure of
merit are presented.
Keywords: Thermoelectrics; PLD Deposition; Thin Lay ers; Multi-Layered Systems
1. Introduction
Skutterudites have been of high interest as a promising
candidate for thermoelectric applications. The key advan-
tage of skutterudites is their possible high figure of merit
ZT [1-3]. ZT is the essential material property for ther-
moelectric energy conversion. It is proportional to the
electrical conductivity, temperature and to the square of
the Seebeck coefficient and it is disproportional to the
thermal conductivity of a material.
The lattice thermal conductivity can be reduced by subs-
tituting Co with Fe as in our case.
A great improvement of thermoelectric properties was
mathematically and also experimentally proved by pre-
paring materials in the form of a low dimensional system
[4-13] such as thin layer, superlattice or multi-layered
structure. Su ch improvement in comparison with bulk
materials was published for the skutterudite superlattices
[14].
Earlier we published results on thin skutterudite layers
prepared by PLD in Ar atmosphere from the
Ce0.1Fe0.7Co3.3Sb12 hot pressed target [15]. The best ther-
moelectric properties were obtained on the layers pre-
par ed at Ts = 250˚C. All layers were of P-type electrical
conductivity [15].
Recently, we prepared thin skutterudite layers by PLD
in Ar atmosphere from the FeSb2Te hot pressed target.
The best thermoelectric properties were obtained on the
layers prepared at Ts = 250˚C and Ts = 230˚C with Ds =
3 J cm−2. Such layers were also of P-typ e electrical con-
ductivity. Bulk ternary skutterud ite FeSb2Te had been
examined and published in details before [16,17] and was
proved to be a good thermoelectric material.
In this contribution, we examine thin thermoelectric
multi-layered Ce0.09Fe0.67Co3.33Sb12/FeSb2.1Te system com-
posed of thin equidistant layers Ce0.09Fe0.67Co3.33Sb12 and
FeSb2.1Te each 5 nm in thickness (5 nm period) prepared
by PLD on a fused silica quartz glass substrate. The
structures were prepared at Ts = 230˚C and Ts = 250˚C
with Ds = 3 Jcm−2. It is expected that the preparation of
Ce0.09Fe0.67Co3.33Sb12/FeSb2.1Te multi-layered structure can
be successful because of the similar lattice constant of
both materials [16-18] and that an improvement of ther-
moelectric properties in comparison to the thin single
layers might be achieved.
2. Methods
PLD targets of FeSb2Te and Ce0.1Fe0.7Co3.3Sb12 composi-
tion where synthesized from individual elements by high-
temperature solid-state reactions. Stoichiometric amounts
of Fe (99.9%), Sb (99.999%), Te (99.999%) and Ce
(99.9%), Fe (99.9%), Co (99.9%) and Sb (99.999%) were
sealed into evacuated carbon-coated silica glass tubes
and heated up to 1050˚C for 48 hrs in a furnace. After
quenching into a water bath, the same ampoule was placed

Thermoelectric Properties of Ce0.09Fe0.67Co3.33Sb12/FeSb2Te M ul ti-Layered Structures
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into the furnace and annealed at 550˚C for 120 hrs. The
resultant material was then ground under acetone, pelle-
tized and heated again at 550˚C for 120 hrs. The comple-
tion of the solid-state reaction of obtained powder sam-
ples was verified by powder XRD.
The final targets for PLD deposition 20 mm in diame-
ter and 2 mm in height were prepared by the hot pressing
method (temperature 500˚C, pressure ~60 MPa for 1 hr).
The measured density of pressed targets was found about
96% - 98% of theoretical density.
The basic schema of the experimental apparatus for
PLD is depicted in Figure 1. Conceptually and experi-
mentally, PLD is an extremely simple method, probably
the simplest of all thin film growth techniques. A high
power pulsed excimer KrF laser (COMPexProTM 205 F)
radiation (1) is used as an external energy source to va-
porize materials of target (5) and to deposit a thin film. A
set of optical components is used to focus the laser beam
to the target surface (2, 3). After the laser pulse irradia-
tion the temperature rises very rapidly (101 1 Ks−1) and
the evaporation becomes non-equilibristic. In our expe-
riment substrates were cleaned from the mechanical dirt
in an ultrasonic cleaner. After that the substrates were
subsequently cleaned in acetone, toluene and in ethanol.
Cleaning in the vapours of boiling ethanol then completed
this process. Fused silica subs trates were finally annealed
in an oven at a temperature around 250 ˚C. The layers and
multi-layered structures were deposited on fused silica
quartz glass substrate 10 × 10 mm. The deposition took
place at Ar atmosphere (13 Pa). The distance of the sub-
strate from the target was set to 40 mm.
The Ce0.09Fe0.67Co3.33Sb12/FeSb2.1Te multi-layered stru c-
tures composed of thin equidistant Ce0.09Fe0.67Co3.33Sb12
and FeSb2.1Te layers of 5 nm in thickness and total
thickness of about 60 nm were prepared by PLD at Ts =
230˚C and Ts = 250˚C with Ds = 3 Jcm−2. The deposition
conditions were chosen based on previous results taken
on single Ce0.09Fe0.67Co3.33Sb12 [15] and FeSb2.1Te layers
as the conditions giving the best thermoelectric proper-
ties.
Figure 1. The basic scheme of the experimental apparatus
for PLD: (1) laser beam, (2) mirrors, (3) focusing lens, (4)
quartz window, (5) target holder, (6) substrate holder, (7)
vacuum pump, (8, 9) Pirani and Penning vacuum gauges,
respectively.
Transport properties, such as the in-plain electrical re-
sistivity and the Seebeck coefficient, were measured on
each multi-layered structure and on single layers in the
temperature range from 300 K up to 500 K. The power
factor was then calculated. Four square shaped contacts
for the measurements were prepared by evaporating Ti.
Pressed Pt/PtRh thermocouples with diameter of 0.07 mm
were used as leads. A conventional DC van der Pauw ’s
method was used for the electrical conductivity mea-
surement. The experimental error of this method is about
10% for the conductivity measurement.
The Seebeck coefficient was determined from the var-
iation of the electromotive force for different temperature
gradients across the layer. The both sides of the sample
were in the thermal contact with an independent wire
resistant sub-heater that supplies the heat and induces the
sample temperature gradient. The thermocouple junctions
were bonded to each corner of the square shaped sample.
The exper imental error of the Seebeck coefficient mea-
surement is about 20%.
3. Results and Discussion
The multi-layered Ce0.09Fe0.67Co3.33Sb12/FeSb2.1Te struc-
tures were prepared by PLD at Ts = 250˚C and Ts =
230˚C with Ds = 3 Jcm−2.
The in-plain temperature dependencies of the electrical
resistivity of Ce0.09Fe0.67Co3.33Sb12/FeSb2.1Te multi-layered
structures are given in Figure 2. Both multi-layered struc-
tures showed semi-conducting P-typ e behavior—the de-
crease of electrical resistivity with the increase of tem-
perature. The measured electrical resistivity of multi-
layered structures was lo wer than the earlier published
values obtained on single thin layers of
Ce0.09Fe0.67Co3.33Sb12 [15] and FeSb2.1Te in the whole
studied temperature range.
Figure 2. Temperature dependency of electrical resistivity
for Ce0.09Fe0.67Co3.33Sb12/FeSb2.1Te multi-layered structures
prepared at Ts = 230˚C (empty squares) and at Ts = 250˚C
(black fil led squares).

Thermoelectric Properties of Ce0.09Fe0.67Co3.33Sb12/FeSb2Te M ul ti-Layered Structures
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The temperature dependencies of the Seebeck coeffi-
cient of Ce0.09Fe0.67Co3.33Sb12/FeSb2.1Te multi-layered
structures are presented in Figure 3. The multi-layered
structures showed much lower in-plain Seebeck coeffi-
cient in the whole measured temperature range than the
previously presented results on single layers [15]. Due to
the low Seebeck coefficient, the resultant power factor of
all prepared multi-layered structure s is lower than the
power factor of the best prepared single layers. The in-
plain temperature dependencies of the power factor of
Ce0.09Fe0.67Co3.33Sb12/FeSb2.1Te multi-layered structures
are depicted in Figure 4.
If we compare the measured values of power factor of
the Ce0.09Fe 0.67Co3.33Sb12/FeSb2.1Te multi-layered struc-
tures with publishe d bulk Ce0.12Fe0.71Co3.29Sb12 material
Figure 3. Temperature dependency of Seebeck coefficient
for Ce0.09Fe0.67Co3.33Sb12/FeSb2.1Te multi-layered structures
prepared at Ts = 230˚C (empty squares) and at Ts = 250˚C
(black fil led squares).
Figure 4. Temperature dependency of the power factor for
Ce0.09Fe0.67Co3.33Sb12/FeSb2.1Te multi-layered structures pre-
pared at Ts = 230˚C (empty squares) and at Ts = 250˚C
(black fil led squares).
[1], we found that multi-layered system values are quite
worse—roughly four times lower at room temperature
and roughly ten times lower at about 500 K. If the de-
crease of thermal conductivity on multi-layered structure
in comparison to bulk material is taken into account, we
may speculate that even better values of ZT for prepared
multi-layered systems may be achieved in comparison
with the bulk Ce0.12Fe0.71Co3. 29Sb12 material. It is assumed
that cross-sectional electrical conduc tivity and Seebeck
coefficient are not much influenced by interfaces in the
multi-layered stru ctures. Temperature dependenc y of power
factor of bulk FeSb2Te has never been published, so the
power factor of multiple-structures can not be compared.
The room temperature value of ZT of the two multi-
layered Ce0.09Fe0.67Co3.33Sb12/FeSb2.1Te structures (60 nm
and 124 nm in thickness) with period 2 nm prepare d at
Ts = 230˚C with Ds = 3 Jcm−2 were measured by Harman
method. We found ZT ~ 0.12 for thinner multi-layered
structure (60 nm in thickness). This value is more than
two times larger than room temperature ZT ~ 0.05 pub-
lished for bulk Ce0.12Fe0.71Co3.29Sb12 [1] and much larger
than the room temperatu re ZT ~ 0.024 published for the
bulk FeSb2Te [16,17]. The room temperature ZT ~ 0.25
measured on thicker structure (124 nm in thickness) is
about two times bigger. It means that the improvement of
ZT in our case depends mainly on the number of the in-
terfaces in multi-layered structure.
4. Conclusions
Multi-layered Ce0.09Fe0.67Co3.33Sb12/FeSb2.1Te structures
composed of equidistant 5 nm Ce0.09Fe0.67Co3.33Sb12 and
FeSb2.1Te layers were successfully prepared by PLD at
Ts = 250˚C and Ts = 230˚C with Ds = 3 Jcm−2. The
measured thermoelectric properties were wor se than pre-
viously published results on the single thin layers. But to
make an overall evaluation of thermoelectric properties,
the Harman measurement of thermoelectric figure merit
and a measurement of the thermal conductivity, which is
expected to decrease due to number of interfaces in the
structure, are necessary.
The roo m temperature ZT of multi-layered structures
with period of 2 nm measured by Harman method are
promising (ZT ~ 0.12 and ZT ~ 0.25 for 60 nm and 124
nm thick multi-layered structures, respectively) and ex-
ceed ZT of both bulk materials. The further ZT improve-
ment is expected by using flatter layers, optimization of
deposition conditions and using Ce0.29Fe1.5Co2.5Sb12 ma-
terial instead of Ce0.09Fe0.67Co3.33Sb12.
5. Acknowledgements
The project has been suppor te d by Czech Grant Agency
under GAČR P108/10/1315 and P108/13-33056S.

Thermoelectric Properties of Ce0.09Fe0.67Co3.33Sb12/FeSb2Te M ul ti-Layered Structures
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