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

doi:10.4236/jcc.2013.17001

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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

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.

Email: [email protected]

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

Open Access JCC

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

Open Access JCC

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

Open Access JCC

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