The effect of nanostructure on the thermoelectric figure-of-merit of La0.875Sr0.125CoO3

Accepted Manuscript The effect of nanostructure on the thermoelectric figure-of-merit of La0.875Sr0.125CoO3 O.J. Dura, R. Andujar, M. Falmbigl, P. Rogl, M.A. López de la Torre, E. Bauer PII:

S0925-8388(17)31137-4

DOI:

10.1016/j.jallcom.2017.03.335

Reference:

JALCOM 41373

To appear in:

Journal of Alloys and Compounds

Received Date: 19 January 2017 Revised Date:

27 March 2017

Accepted Date: 29 March 2017

Please cite this article as: O.J. Dura, R. Andujar, M. Falmbigl, P. Rogl, M.A. López de la Torre, E. Bauer, The effect of nanostructure on the thermoelectric figure-of-merit of La0.875Sr0.125CoO3, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.03.335. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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The effect of nanostructure on the thermoelectric figure-of-merit of La0.875 Sr0.125 CoO3 O. J. Duraa,∗, R. Andujara , M. Falmbiglb , P. Roglb , M. A. L´opez de la Torrea , E. Bauerc a Departamento

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de F´ısica Aplicada and INEI, Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain. b Institute of Materials Chemistry and Research, University of Vienna, Waeringerstrasse 42, A-1090 Vienna, Austria. c Institute of Solid State Physics, TuVienna, Wiedner Hauptstrasse 810, A-1040 Vienna, Austria.

Abstract

Nanostructured thermoelectric materials prepared by mechanical alloying and sintering have already shown enhanced values of the dimensionless thermoelectric figure-of-merit (ZT) for different families of materials. Oxide materials, however, are still far from reaching competitive values of thermoelectric effi-

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ciency. In this work we report results regarding a nanostructuring approach on thermoelectric transport properties of La0.875 Sr0.125 CoO3 obtained by solidstate reaction combined with ball milling and sintering. This simple method allows us to obtain a set of samples with very different nanostructures, with grain sizes ranging from 19 nm to 0.7 µm. Both thermal and electrical properties

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were found to be dependent on the grain size. The grain size reduction results in an enhancement of the electrical resistivity whereas the thermal conductivity is strongly reduced. Interestingly the thermoelectric power of the sample with

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the smallest grain size increases significantly compared to the microcrystalline sample over the whole temperature range. As a result the thermoelectric figure of merit of the sample with the smallest grain sizes is enhanced by 30 % compared to the microcrystalline sample above room temperature. This finding I Fully

documented templates are available in the elsarticle package on CTAN. author Email address: [email protected] (O. J. Dura)

∗ Corresponding

Preprint submitted to Journal of LATEX Templates

29th March 2017

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confirms the validity of the nanostructuring approach on thermoelectric oxides

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in order to improve their thermoelectric efficiency. Keywords: Thermoelectric oxides, Cobaltates, Nanostructure

1. Introduction

Thermoelectric materials have attracted attention due to their applications

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in waste heat recovery and refrigeration. Several material families have been proposed as candidates for thermoelectric applications including skutterudites 5

[1], half-Heusler [2] and chalcogenide compounds among others [3]. Since the

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discovery of excellent properties in the layered cobalt oxide Nax CoO2 [4] many oxide thermoelectrics have been extensively investigated as candidates for thermoelectric applications [5, 6, 7, 8, 9, 10]. Oxide materials display several advantages such as high thermal and chemical stability in air, low cost of raw materials 10

and reduced environmental impact [11], whereas the main disadvantage is the poor electrical conductivity due to their low mobility and ionic bonding.

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The efficiency of a thermoelectric material depends on the thermoelectric figure of merit, ZT [12, 13]:

ZT =

σS 2 T κ

(1)

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Therefore efficient thermoelectric materials must exhibit high values of thermopower (S) and electric conductivity (σ), combined with a low thermal conductivity (κ). To enhance ZT, different approaches pursue to decouple the

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relationship between the thermal and electrical transport. On one hand, the electrical conductivity and Seebeck coefficient can be enhanced by modifying the electronic band structure and, on the other hand the nanostructure and nano-

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composite approaches increase the phonon scattering so reducing the thermal conductivity [14, 15, 16, 17]. Many research groups have attempted to control the nanostructure of bulk materials by introducing phonon-scattering interfaces and, consequently, reducing κ while avoiding a reduction in power factor (S 2 σ). Although the tuning of nanostructure has been reported to be successful in some 2

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cases [18, 19] this is not an easy task as a substantial reduction in κ is usually

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accompanied by the reduction of σ. One promising approach is to modify the thermoelectric properties of traditional materials by applying, for example, a mechanical milling treatment [20].

Cobalt oxide based materials result interesting due to their wide variety of 30

applications in gas sensing or electrochromic devices amongh others. In spite

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of pure cobalt oxide is not appropriate as termoelectric material due to its high resistivity, doped cobalt oxides have been proposed as the best transition metal oxides for different thermoelectric applications [15, 21].

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LaCoO3 is a charge transfer insulator without magnetic order. The substitution of Sr2+ for La3+ converts Co3+ to Co4+ ions and supplies holes, which drives the system towards a semiconducting state. Cobaltites have received attention due to the existence of spin-state transitions [22] as well as to the unusual magnetic ground state that they display [23]. These effects are due to the fact that the crystal-field splitting and Hund’s rule exchange energy are compar40

able. Thus, the t2g electrons can be easily thermally excited into the eg states

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resulting in a high spin configuration. However, up to date, the many studies concerning LaCoO3 have still not solved the controversy regarding the configuration of the Co spins. In fact, this complex spin configuration was proposed as the source of the large thermopower displayed by cobalt oxides such as NaCo2 O4 and La1−x Srx CoO3 [24]. In contrast to most other oxides, the La1−x Srx CoO3

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system is particularly appealing for applications in the operating temperature range of thermoelectric cooling and refrigeration [15].

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In this paper we present a study of the effect of the nanostructuring approach on the thermoelectric transport properties of La0.875 Sr0.125 CoO3 . For this aim

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we applied different sintering treatments on mechanically milled samples, which allow us to obtain samples with very different nanostructures, resulting in grain sizes ranging from 19 nm to 0.7 microns. In this set of samples we have performed a comparative study of thermoelectric transport properties; S(T ), σ(T ) and κ(T ) between 5 and 400 K. Whereas the nanostructuring approach increases

55

the electrical resistivity and strongly reduces the thermal conductivity, the ther3

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mopower is significantly improved. The thermopower of the sample with the

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smallest grain size reaches a maximun of 125 µV/K compared to 95 µV/K for the microcrystalline sample at the same temperature. From these results the evolution of ZT with grain size in La0.875 Sr0.125 CoO3 has been determined. The 60

values of ZT are decreased with grain size for low temperatures. However, at room temperature there is a crossover of the ZT where the sample with the

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smallest grain size exhibits the highest values in the temperature range relevant for applications. At 400 K an enhancement of 30 % compared o the microcrys-

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2. Material and methods

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talline sample is achieved.

The microcrystalline and nanocrystalline samples of La0.875 Sr0.125 CoO3 studied in this work were prepared by means of conventional solid-state ceramic method [25, 26, 27] from La2 O3 , SrCO3 and Co3 O4 starting powders, fired several times at 1100◦ C with a final treatment at 1300◦ C over 24 h. The powders obtained after the solid-state reaction were subjected to mechanical milling for

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16 h in order to reduce the grain size. The milling process was carried out using hardened steels vessels and balls, in which some sacrifice powder was added at the beginning of the process to avoid any contamination. The ball to powder weight ratio was 4:1. To monitor the milling process, x-ray diffraction patterns were recorded for intermediate times using a Siemens D5000 diffractometer.

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Powders achieved after milling shown a grain size of 11 nm, which is a typical stationary value for milled oxide powders. These powders were uniaxially cold

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pressed into pellets (10 mm diameter and ∼1.5 mm thick) by applying a pressure of 0.6 GPa. Then, three samples with significantly different grain sizes were

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prepared by means of different sintering treatments. The temperature and time were chosen between 590 and 950◦ C and 2 and 12 h respectively. As a final treatment, the three samples were annealed at 400◦ C for 8 h in flowing oxygen to ensure equivalent oxygen content for samples sintered at very different temperatures and times. By following this procedure we are able to obtain samples

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with grain sizes of 19, 28 nm and 0.7 µm. The Warren-Averbach method [28, 29]

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was used to estimate the grain size of the two samples with the smallest grain size from the x-ray diffraction patterns, whereas the grain size for the sample

with 0.7 µm, herein refereed to as microcrystalline sample, was obtained from

scanning electron microscope (SEM) images. The density of the samples was de90

termined using the Archimedes method with water as an immersion fluid. The

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chemical composition and microstructure of the samples were determined via

electron probe microanalyses (EPMA-EDX) using a Buehler Simplimet 3 and a JSM 6400 SEM equipment. SEM images of the surface were taken with a Philips

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XL 30 field emission environmental scanning electron microscope (ESEM-FEG). Electrical resistivity measurements on bar shaped samples were carried out in the temperature range from 4.2 to 300 K in a conventional 4 He cryostat. The electrical resistance was measured via a dc four-point technique employing a Lake Shore Resistance Bridge 370 AC, which measures voltage and temperature. The Seebeck coefficient measurements in the temperature range of 100

4.2 to 300 K were measured with a differential method and seesaw heating.

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The absolute thermopower Sx (T ) was obtained from the expression: Sx (T ) = SCh −VCh (SCh −SCon )/(VCh −VCon ) with SCh and SCon being the absolute thermopowers of chromel (Ch) and constantan (Con), VCh and VCon are the voltages along the chromel and constantan circuits, respectively. The spot-welded junctions of the thermocouple pairs, chromel and constantan, were soldered on to the

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sample, which was glued (G.E.-varnish) to two heating panels. The measured voltages are averaged over both temperature gradient directions. The sample

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temperature is determined using a Pt100 sensor from 30 K up to room temperature and a Ge resistive sensor for T< 30 K. Thermal conductivity measurements

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between 4.3 and 300 K were performed in a flow cryostat on cuboid-shaped samples (length ∼1 cm, cross section ∼2 mm2 ), which where kept cold by an-

choring one end of the sample onto a thick copper panel mounted on the heat exchanger of the cryostat. The temperature difference along the sample, established by electrical heating, was determined by means of a differential thermo-

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couple (Au+0.07Fe/Chromel). The measurement was carried out under high 5

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vacuum and three shields mounted around the sample reduced the heat losses

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due to radiation. The innermost shield was kept at the temperature of the sample via an extra heater. The electrical resistivity and the Seebeck coefficient

above room temperature were obtained using a commercial MMR-Technologies 120

system. The electrical resistivity measurements were performed on flat plates with contacts in the Van der Pauw configuration. The Seebeck coefficient was

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obtained on bar shaped samples by comparison with a reference constantan wire.

Thermal conductivity above room temperature was determined from separate measurements of the thermal diffusivity , specific heat and density using the relationship κ = αCp ρ. Thermal diffusivity was measured with a commercial

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Linseis LFA system. All samples were covered with a graphite screen to ensure full absorption of the flash light at the front surface and highest emissivity from the backside. Specific heat was measured between 200 and 773 K with a Netzsch Jupiter 404 differential scanning calorimeter (DSC), using platinum crucibles in 130

helium atmosphere. A single-crystalline sapphire sample was used as reference. Different measurements were performed several times, and different samples, in

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order to ensure the reproducibility of values discussed here. Therefore all properties measured in this work, i.e., the electrical resistivity, the Seebeck coefficient and the thermal conductivity, reveal a good agreement between the values obtained for both temperature ranges, above and below room temperature.

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3. Results and discussion The x-ray diffraction (XRD) patterns of the samples studied in this work

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with grain sizes between 19 nm and 0.7 µm are shown in Fig. 1(a). The three patterns show the characteristic peaks corresponding to a rhombohedral struc-

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ture with the R ¯ 3c space group symmetry. As expected, the patterns show

sharper peaks for the sample sintered at higher temperature. The evolution of the XRD patterns with the milling time (inset of Fig. 1 (a)) shows a characteristic progressive peak broadening, in agreement with the grain-size reduction during the milling process. While the lattice parameter a monotonically de-

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creases when reducing the grain size, the parameter b follows the opposite trend;

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thus no systematic dependence of the volume on the grain size is observed. The density obtained through Archimedes’ method and the lattice parameters

(in hexagonal setting) calculated from XRD patterns are summarized in table 1. The quantitative Rietveld analysis corresponding to the solid-state reaction 150

powder, displayed in Fig. 1(b), indicates a small presence (∼3%) of CoO (cubic

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space group Fm ¯ 3m). The grain sizes for the samples with nanocrystalline microstructure were determined from a Warren-Averbach line profile analysis of

the XRD patterns [28, 29]. In the case of the microcrystalline sample the grain

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size was estimated from the scanning electron microscope (SEM) images. In Fig. 2 we display the electron microscope micrograph showing the morphology of the microcrystalline sample.

Table 1 Grain size obtained from XRD analysis, density, sintering conditions and structural

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parameters for the powder obtained in the solid-state reaction (SSR) before milling and for the samples sintered at different temperatures (Tsint ) after milling.

Sample SSR

Tsint (◦ C)

tsint (h)

a (˚ A)

b (˚ A)

V (˚ A)3

5.4479

13.155

338.139

96.4

950

12

5.4473

13.142

337.681

28nm

95.3

650

2

5.4417

13.176

337.939

19nm

94.8

590

2

5.4337

13.211

337.810

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0.7µm

Density(%th.)

The results of dc electrical resistivity (ρ) measurements performed on sintered

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La0.875 Sr0.125 CoO3 samples with different grain sizes (19 nm≤ dg ≤ 0.7 µm) are shown as a function of temperature in Fig. 3. All samples reveal a thermally activated behavior, typical for these type of materials. As one expects, the sample with the smallest grain size exhibits the largest electrical resistivity values. However, comparing the effect of the nanostructuring approach on the 7

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(a ) I (a .u .)

1 6 h 8 h

3 6 2 θ(d e g .)

1 9 n m 3 0

3 5

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2 8 n m

4 0

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In te n s ity ( a r b . u n its )

0 h

3 2

M ic r o

4 0

4 5

5 0

5 5

6 0

6 5

2 θ( d e g .)

(b )

L a

0 .8 7 5

C o O

S r

0 .1 2 5

C o O

3

↑

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2 0

↑

4 0

Y o b Y c a Y o b B ra

(R -3 c )

(F m -3 m )

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In te n s ity ( a r b . u n its )

↑

↑

6 0 2 θ(d e g .)

↑↑

↑↑

8 0

s lc s - Y c a lc g g P o s itio n

↑↑

↑↑

↑↑

1 0 0

Fig. 1. (Color online) (a) Comparison of the X-ray diffraction patterns for the 19 nm, 28

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nm and microcrystalline samples. The evolution of the milling process for 0 h, 8 h and 16 h is shown in the inset. (b) X-ray diffraction pattern for the powder obtained by means of solid-state synthesis. The solid line corresponds to the profile calculated from the Rietveld refinement, and Yobs-Ycalc is the intensity difference between experimental data and Rietveld calculations.

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165

electrical resistivity with similar systems, mainly complex oxides, the increase

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compared to the resistivity values of the microcrystalline sample is just moderate. In addition, the electrical resistivity of all samples can be analyzed in terms

of polaron-dominated electron transport [30]. Thus, there is a smooth crossover

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follows the law

ρ(T ) = ρ0 T e(Eρ /kB T )

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from the high-temperature activated behavior, where the electrical resistivity

(2)

to the low-temperature variable-range hopping regime (VRH) for which

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Mott’s law predicts the well-know dependence: ρ(T ) ∝ e(T0 /T )

1/4

. Both charac-

teristic behaviors are displayed in Fig. 4. From the fits above room temperature to ρ(T ) = ρ0 T e(Eρ /kB T ) we obtained values of the activation energies, Eρ , which 175

are summarized in table 2. For the low-temperature range the observed VRH

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behavior is shown in Fig. 4 (b).

Fig. 2. Scanning electron microscope image of the microcrystalline sample.

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Table 2 Activation energies Eρ , ES and WH for 19 nm, 28 nm and 0.7 µm samples.

Eρ (meV)

ES (meV)

WH (meV)

0.7µm

104

28.6

75.3

28nm

85.0

29.5

55.5

19nm

70.0

24.3

45.7

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Sample

The thermopower S(T ), is, on the one hand, the predominant factor of the 180

thermoelectric figure of merit, while on the other hand it provides information

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on the nature of electronic transport. The values of S(T ) measurements on the 19 nm, 28 nm and microcrystalline sample as a function of the temperature are shown in Fig.5. Data corresponding to the microcrystalline sample display a similar behavior to that previously reported for this compound [31, 32, 33]. 185

S(T ) is positive for all the samples over the whole temperature range, implying

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that the electronic transport for every sample is dominated by holes. For the

6 5 4

2

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ρ Ω

· c m

)

3

1 9 n m 2 8 n m M ic r o c r y s ta llin e

(

/

1

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L

o

g

1

0

0

-1 -2 -3

0

1 0 0

2 0 0

3 0 0

4 0 0

T e m p e ra tu re (K )

Fig. 3. (Color online)Temperature dependence of electrical resistivity for the 19 nm (green, open circles), 28 nm (blue, open triangles) and microcrystalline (red, open diamonds) samples.

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6

1

9

8

n

n

m

m

i c r o

c r y s t a

- 1

K

M

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)

2

0 1

g o L 3 2

3

l l i n

e

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4

(

ρ

- 1

· T

/

Ω

- 1

· c m

5

4

5

6

7

- 1

1

3

2

0

0

/ T

( K

)

c m

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- 1

)

1

0

- 2

(

ρ

- 1

/

Ω

- 1

0

- 1

9

n

m

2

8

n

m

L

o

g

1

0

1

- 3

M

i c r o

c r y s t a

l l i n

e

- 4

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- 5

0

. 2

5

0

. 3

0

0

. 3

5

- 1

0

/ 4

T

. 4

- 1

( K

0

0

. 4

5

0

. 5

0

0

. 5

5

/ 4

)

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Fig. 4. (Color online)(a) Temperature dependence of Log10 (ρ−1 T ) for the 19 nm (green, open circles), 28 nm (blue, open triangles) and microcrystalline (red, open diamonds) samples. Solid lines represent linear fits discussed in the text. (b) The same data plotted as Log10 (ρ−1 ) vs T −1/4 .

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three samples, S(T ) increases with temperature from low temperature up to a

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maximum value, which is achieved around 150 K. This temperature, Tmax , is nearly constant for all the samples. Above Tmax the values of S(T ) decrease 190

with increasing in temperature up to the highest experimental temperature (410

K). It is important to note that as grain size decreases the S(T ) curves are shifted towards higher values. Thus, S(T ) of the sample with the smallest grain size

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reaches a maximum roughly 30 % higher than that corresponding to the microcrystalline sample. Considering that thermopower is closely related to details of 195

the density of states near the Fermi level, and the crucial role of grain boundaries

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to transport properties of nanomaterials, the evolution of the thermopower with grain size reflects that the electronic structure at grain boundaries dominates the overall behavior observed in the present La0.875 Sr0.125 CoO3 nanostructured samples. Assuming polarons as predominant charge carriers, as inferred from 200

the S(T ) behaviour at elevated temperatures, the thermopower should follow the adiabatic small polaron theory in this temperature range, i.e.,

κB e



ES +α κB T

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 (3)

where ES is the thermopower activation energy and α is a constant related to the spin and mixing entropy [34]. The fits of the S(T ) data to Eq.: 3 were

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performed in the same temperature range as those used to obtain Eρ . From these fits the corresponding ES values are obtained and summarized in table 2. Very similar values (∼ 30 meV) of ES were obtained for all the samples,

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whereas the electrical resistivity activation energy, Eρ , decreases when the grain size diminishes. Within the small polaron framework [30, 35], the estimated activation energy Eρ is the sum of the energy required to jump from one ion

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to a neibouring one (WH ) and the energy needed to create the carrier (ES ). Thus, the value of WH is obtained as Eρ - ES [34]. Therefore, the estimated

values for the hopping energy, WH , also decrease as grain size is reduced. This lowering of WH excludes an enhancement of the charge localization when the grain size reduces. Consequently, the enhancement of the thermopower can be 12

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related with a carrier energy filtering at grain boundaries observed on different

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nanostructured systems [36, 37].

1 6 0

1 9 n m 2 8 n m M ic r o c r y s ta llin e

1 4 0

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1 2 0

8 0 6 0 4 0 2 0 0 0

5 0

1 0 0

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S ( µV / K )

1 0 0

1 5 0

2 0 0

2 5 0

3 0 0

3 5 0

4 0 0

4 5 0

T e m p e ra tu re (K )

Fig. 5. Temperature dependence of thermopower for the 19 nm (green, open circles), 28

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nm (blue, open triangles) and microcrystalline (red, open diamonds) samples. Solid lines   represent fits to the expression S(T ) = κeB κEST + α (see text for details). B

The temperature dependence of the total and the lattice (phonon) thermal

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conductivity, κ and κL , of 19 nm, 28 nm and microcrystalline samples is shown in Fig. 6. To obtain κL from the total thermal conductivity, the Wiedemann-

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Franz law allows to estimate the thermal conductivity due to the charge carriers, κe = L0 T /ρ, where L0 is the Lorentz number and ρ the electrical resistivity. The radiation losses were considered for the thermal conductivity values obtained by the steady-state method, i.e. below room temperature. For this purpose the

225

Callaway model, which includes different scattering processes, and the corresponding T 3 radiation term were used to perform a fit with the experimental values below room temperature. For more details about this common procedure see [26, 38]. It is observed that the reduction of the grain size leads to

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depressed lattice thermal conductivity values in the whole temperature range. κL at 300 K is reduced from 2.0 Wm−1 K−1 for the microcrystalline sample to

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230

0.6 Wm−1 K−1 for the 19 nm sample. This effect has been observed in differ-

ent nanocrystalline materials and interpreted as an intrinsic effect of the grain

boundaries over the phonon transport [39, 40]. Hence, the extremely reduced values obtained in thermal conductivity with decreasing grain size are mainly

due to the mid-to-long wavelength phonons which get scattered by boundaries

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235

3

. 5

3

. 0

. 5

2

. 0

1

. 5

1

. 0

0

. 5

i c r o

2

8

1

9

c r y s t a

n

m

n

m

l l i n

e

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κ

L

+

κ

e

( W

/ m

· K

)

2

M

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[37, 41].

0

. 0

0

1

0

0

T

3

. 5

3

. 0

. 5

i c r o

c r y s t a

2

8

n

m

1

9

n

m

m

2

p

l l i n

0

0

e

3

r a

t u

r e

( K

r a

t u

r e

( K

0

0

4

0

0

4

0

0

)

e

2

. 0

1

. 5

1

. 0

0

. 5

0

. 0

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κ

L

( W

/ m

· k )

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2

M

e

0

1

0

0

2

T

e

m

p

0

e

0

3

0

0

)

Fig. 6. (Color online) (a) Temperature dependence of thermal conductivity for the 19 nm (green, open circles), 28 nm (blue, open triangles) and microcrystalline (red, open diamonds) samples. (b) thermal conductivity due to lattice (phonon) contribution, κL .

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Finally, figure 7 shows the temperature dependence of the dimensionless figure-of-merit ZT for the samples studied here. As a consequence of the rather 240

strong reduction of the thermal conductivity and the enhancement of the thermopower with decreasing grain size, a slight improvement of ZT above room

temperature is obtained for the nanostructured sample with the smallest grain

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size. Our results clearly demonstrate that nanostructuring enhances the thermoelectric efficiency in cobaltates above room temperature. However, further 245

improvements are required to drive the thermoelectric figure of merit into a

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range interesting for applications in thermoelectric generators

0 .0 3 5

M ic r o c r y s ta llin e 2 8 n m 1 9 n m

0 .0 3 0 0 .0 2 5

0 .0 1 5

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Z T

0 .0 2 0

0 .0 1 0 0 .0 0 5 0 .0 0 0

EP

0

1 0 0

2 0 0

3 0 0

4 0 0

T e m p e ra tu re (K )

Fig. 7. (Color online) Figure of merit for the 19 nm (green, solid circles), 28 nm (blue, solid

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triangles) and microcrystalline (red, solid diamonds) samples. Error bars included represent an 15 % estimated uncertainty.

4. Conclusions Bulk nanostructured La0.875 Sr0.125 CoO3 samples were prepared by conven-

250

tional solid-state reaction combined with subsequent mechanical milling and sin15

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tering treatments. By this simple method we obtain samples with very different

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nanostructure, with grain sizes ranging between 19 nm and 0.7 µm. The experimental results reveal a strong influence of the nanostructuring on the thermal and electronic transport properties and hence on the thermoelectric figure-of255

merit. As a consequence of the grain size reduction the thermal conductivity decreases strongly whereas the electrical resistivity increases. Interestingly the

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effect of nanostructuring enhances the thermopower by 30 % compared to the microcrystalline sample. Therefore, these results clearly demonstrate that this simple nanostructuring approach can be applied to thermoelectric oxides to significantly enhance their efficiency.

Acknowledgments

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This work was partially supported by JCCM through project N PPII-2014019-P and the Spanish MINECO department through grant N MAT2012-34655.

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References

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Parts of the work were supported by the Austrian FWF P24380.

[1] B. C. Sales, D. Mandrus, R. K. Williams, Filled skutterudite antimonides: A new class of thermoelectric materials, Science 272 (1996) 1325.

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[2] G. Schierning, R. Chavez, R. Schmechel, B. Balke, G. Rogl, P. Rogl, Concepts for medium-high to high temperature thermoelectric heat-to270

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Highlights We obtain nanostructured samples of the cobalt oxide with very different grain size. Transport properties are analyzed in terms of polarons and phonon localization.

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The strong influence of the nanostructure on the thermoelectric efficiency is proved.

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An enhancement of ZT at the most appropriate temperature range is achieved.