Thermoelectric properties of Ca3Co4O9–Co3O4 composites

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CERAMICS INTERNATIONAL

Ceramics International 41 (2015) 10038–10043 www.elsevier.com/locate/ceramint

Thermoelectric properties of Ca3Co4O9–Co3O4 composites F. Delormea,n, P. Diaz-Chaob, E. Guilmeaub, F. Giovannellia a

Université François Rabelais de Tours, CNRS, CEA, INSA CVL, GREMAN UMR 7347, IUT de Blois, 15 rue de la chocolaterie, CS 2903, 41029 Blois Cedex, France b CRISMAT, UMR CNRS-ENSICAEN 6508, 14050 Caen, France Received 26 February 2015; received in revised form 9 April 2015; accepted 17 April 2015 Available online 24 April 2015

Abstract (1
x)Ca3Co4O9/xCo3O4 composites samples with x ¼0, 10, 20 and 50 vol% have been prepared by solid state reaction and sintered by spark plasma sintering. Their thermoelectric properties have been studied in the range 323–1000 K. No reaction occurs between Co3O4 and Ca3Co4O9 particles during the high temperature treatment. The addition of Co3O4 particles within a Ca3Co4O9 matrix strongly influences the thermoelectric properties. The powerfactor decreases as the Co3O4 content increases. As the thermal conductivity increases, this leads to lower ZT values. These modifications of the thermoelectric properties are consistent with the compressive strain on Ca3Co4O9 originating from the mismatch of thermal expansion coefficients of Ca3Co4O9 and Co3O4. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: B. Composites; C. Electrical properties; C. Thermal conductivity; D. Transition metal oxides

1. Introduction Thermoelectric materials can convert exhaust heat energy directly into electrical energy. Nevertheless, they have occupied only niche markets because of their low energy conversion efficiency η. The efficiency η is a function of the thermoelectric figure of merit ZT ¼ S2σT/κ, where T is the absolute temperature, σ is the electrical conductivity, κ is the thermal conductivity and S is the Seebeck coefficient. As a result, the optimum material for thermoelectric generation should simultaneously exhibit large S, large σ and small κ. However, these three parameters are strongly interconnected through more fundamental physical parameters. As a result, improvements achieved on one parameter have usually negative impacts on the others. Many theoretical studies predict that it is possible to increase thermoelectric properties in composites: some models predict an increase of the powerfactor [1–4], whereas other models seem to indicate that the main

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Corresponding author. Tel.: þ33 2 54 55 21 10; fax: þ 33 2 54 55 21 09. E-mail address: [email protected] (F. Delorme).

http://dx.doi.org/10.1016/j.ceramint.2015.04.091 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

effect will be related to a decrease of the thermal conductivity due to phonon scattering by interfaces [3,5]. First experimental results have been reported. Kim et al. [6] have observed an increased ZT value by a factor of two, mostly due to a reduced thermal conductivity in an ErAs incorporating alloy of In0.53Ga0.47As. Xiong et al. [7] have added 10–15 nm TiO2 particles to a Ba0.22Co4Sb12 skutterudite. They have found that for low amounts of nanoparticles added, the ZT value is improved from 1 to 1.1 at 850 K due to a lower thermal conductivity whereas Seebeck coefficient and electrical conductivity remain unaffected. The lower thermal conductivity is attributed to phonon scattering at the interfaces skutterudites-nanoparticles. In a more recent paper, they have studied the effects of 5–20 nm GaSb particles additions to a YbxCo4Sb12 skutterudite [8]. Similarly, they have observed an improved ZT value from 1.3 to 1.4 at 850 K due to a lower thermal conductivity. However a part of the ZT improvement is also due to a higher powerfactor. This higher powerfactor is correlated to a lower electrical conductivity and a higher Seebeck coefficient that is interpreted as a low energy carrier filtering by the interfaces. Kim et al. [9] have shown that Al2O3 nanoparticles dispersed in a Bi2Te3 matrix lead to increased ZT

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values, from 0.16 to 0.20 at 300 K. This increase is also correlated to both a higher powerfactor and a lower thermal conductivity. First reports [10–13] on particles additions in thermoelectric doped SrTiO3 oxides seems also to show lower thermal conductivities and improved ZT values. However, low density of the samples, grain growth and reactions between the thermoelectric oxide and the added oxides make the results difficult to interpret. Oxides are considered as potential candidates for thermoelectric applications since the discovery of a large thermopower in the metallic oxide NaxCoO2 [14]. Other layered cobalt oxides have been shown to present excellent thermoelectric properties such as Ca3Co4O9 [15]. This compound belongs to the family of misfit cobalt oxides [16–19]. Its crystal structure consists of CdI2-type CoO2 layers and triple rocksalt [Ca2CoO3] layers stacking alternately along the c-axis. These two kind of layers have similar a, c, and β lattice parameters but different b parameters. To emphasize the incommensurate nature of the structure, Ca3Co4O9 can be written as [Ca2CoO3](b2/b1)[CoO2], where b1 and b2 are two different lattice parameters for the rocksalt subsystem and the CoO2 subsystem, respectively. The edge sharing CoO2 octahedra layers are considered to be responsible for the electrical conduction, whereas the triple rocksalt layers can be regarded as a charge reservoir to supply charge carriers into the CoO2 layers. However, if single crystals have shown ZT values close to 1 at 1000 K [15], polycrystalline ceramics present lower ZT values close to 0.1 at 1000 K [20] mainly due to their larger resistivity, which depends greatly on grain size, electrical properties of grain boundary, bulk density, and grain orientation. Many efforts have been made to improve the ZT values of polycrystalline ceramics of Ca3Co4O9 by improving the grain alignment in the ceramics [21–26], reducing the average grain size [27–29] or by chemical substitutions [21,22,30–37]. However, if noticeable ZT improvements have been demonstrated by these techniques, the ZT values are still far lower than ZT values of single crystals: indeed, the best reported ZT value (ZT (1000 K)=0.5) has been obtained for a silver doped silver composite sample (Ca2.7Ag0.3Co4O9/Ag 10 wt%) [38]. As far as the authors know, only Ca3Co4O9/silver [38,39], Ca3Co4O9/SiO2 [40] and recently Ca3Co4O9/Na0.77CoO2 [41] composites have been studied. The aim of this paper is to study the influence of the amount of a Co3O4 secondary phase on the thermoelectric properties of the Ca3Co4O9 compound.

2. Material and methods Ca3Co4O9 samples were synthesized from CaCO3 (Sigma Aldrich, Z 99% purity) and Co3O4 (Sigma Aldrich, no specified purity) precursors. Stoichiometric amounts of the precursors were thoroughly mixed 5 min at 400 rpm in an agate ball mill (Retsch PM 100). The resulting black powder has been heated at 850 1C for 8 h at a rate of 5 1C/min, in an alumina crucible and slowly cooled down. Then, Co3O4 has been added to Ca3Co4O9 to produce (1
x)Ca3Co4O9/xCo3O4 composites samples with x ¼ 0, 10,

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20 and 50 vol% by thoroughly mixing the powders 5 min at 400 rpm in an agate ball mill (Retsch PM 100). Sintering was performed by Spark Plasma Sintering (SPS, FCT Systeme GmbH HP D 25). The synthesized powders were placed in a 15 mm diameter graphite die. A pressure of 70 MPa was applied whereas the temperature was raised at 100 1C/min up to 850 1C for 5 min. Then the sample was cooled at 100 1C/min to room temperature. The obtained pellets were then polished to remove the graphite foils used during the SPS process and cut for thermoelectric properties measurements. Differential thermal analysis (DTA) measurements were recorded on a Diamond TG/DTA (Perkin-Elmer), using alumina crucibles. The powdered samples were analyzed using a heating rate of 10 1C min
1 in air. Thermoelectric properties of the sintered samples were determined from simultaneous measurement of resistivity and Seebeck coefficient in a ZEM III equipment (ULVAC Technologies) and thermal conductivity. The thermal diffusivity was measured using the laser flash diffusivity technique (Netzsch LFA 457) from room temperature to 800 1C in air atmosphere. The thermal diffusivity measurement of all specimens was carried out three times at each temperature. The heat capacity of the materials was measured from room temperature to 800 1C, with a heating rate of 10 1C min
1 in platinum crucibles and in air atmosphere, using differential scanning calorimetry (Netzsch DSC 404C pegasus). X-ray diffraction (XRD) patterns have been performed on a BRUKER D8 Advance θ/2θ diffractometer equipped with a Linxeye energy-dispersive one-dimensional detector, using Cu-Kα radiation and operating at 40 kV and 40 mA at room temperature. The scans have been recorded from 5 to 851(2θ) with a step of 0.021 and a counting time of 0.5 s per step. Apparent density of the samples was calculated from the weight and dimensions of the bars cut from the pellets for ZEM III characterization. The scanning electron microscopy (SEM) observations have been performed using FEI Quanta 200 microscope coupled with an Energy Dispersive Spectrometer (EDS Oxford INCA X-Act) without prior coating of the samples. 3. Results and discussion Fig. 1 shows the DTA and TG curves of Co3O4 and Ca3Co4O9 reference samples and of the (1
x)Ca3Co4O9/ xCo3O4 composite with x ¼ 0.5 up to 1000 1C. All the samples present endothermic peaks related to weight losses. The Ca3Co4O9 reference sample exhibits two peaks at 911 and 965 1C, the first peak being less intense than the second one. According to Sedmidubsky et al. [42] the second peak can be attributed to the decomposition of Ca3Co4O9 into Ca3Co2O6 and Ca1
xCoxO whereas the small first one can be attributed to the same reaction but for a Ca3Co4O9 compound with a slightly different stoichiometry. The (1
x)Ca3Co4O9/xCo3O4 composite sample with x ¼ 0.5 exhibit also two similar peaks at 915 and 964 1C but the first peak is more intense than the second one. However, as demonstrated by the Co3O4 reference sample, an important contribution to the first peak comes from the transformation of Co3O4 into CoO (peak at 920 1C). This

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implies that no reaction occurs between Co3O4 and Ca3Co4O9 particles during the high temperature treatment. However, a possible transformation of Co3O4 into CoO can be expected. SEM images show that Co3O4 is composed of aggregates (circa 5 µm) of micrometer size octahedral crystals (Fig. 2a) whereas Ca3Co4O9 particles present an average size between 1 and 2 mm (Fig. 2b). Fig. 3 exhibits the XRD patterns measured on the synthesized Ca3Co4O9 powder and the ceramics pellets for the samples with x ¼ 0 and 0.5. For the synthesized Ca3Co4O9 powder and sample with x ¼ 0, the peaks correspond to Ca3Co4O9 as described by Masset et al. [17]. Sample with x ¼ 0.5 shows small peaks corresponding to Ca3Co4O9 at 16.5, Temperature (°C) 0

700

800

900

1000

Heat Flow (a.u.)

-50 Co3O4

-100

x = 0.5 Ca3Co4O9

-150 -200 -250 -300

33.3, the shoulder at 37.1, 39.9 and 43.2 12θ and peaks corresponding to Co3O4. No trace of CoO has been detected. This confirms the thermal analysis data: no reaction occurs between Co3O4 and Ca3Co4O9 particles during the high temperature treatment. Relative densities of the samples are 0.98, 0.93, 0.96 and 0.96 for x ¼ 0, 10, 20 and 50 vol%, respectively [17]. The theoretical densities of (1
x)Ca3Co4O9/xCo3O4 composites were calculated by the sum of the products of the theoretical densities of each phase and their respective volume fractions. Fig. 4 exhibits the temperature dependence of the Seebeck coefficient (S) of the (1
x)Ca3Co4O9/xCo3O4 composite samples with x ¼ 0, 10, 20 and 50 vol% from 323 to 1000 K. The Seebeck coefficient of all the samples shows a positive value over the measured temperature range, indicating p-type conduction. For all the samples, the value of the Seebeck coefficient increases with increasing temperature. The Seebeck coefficient of samples with x ¼ 0.1 and 0.2 is similar and lower than the one of the pure Ca3Co4O9 sample (x ¼ 0), especially at low temperatures. For sample with x¼ 0.5, the Seebeck coefficient is higher than for samples with x ¼ 0.1 or 0.2 but lower than for the pure Ca3Co4O9 sample (x ¼ 0) except for temperatures higher than 900 K where it becomes higher (S ¼ 181 and 193 mV K
1 at 1000 K for x ¼ 0 and 0.5 respectively). 100

Temperature (°C)

98 96

800

900

90 (002)

1000

80

Co3O4 x = 0.5 Ca3Co4O9

94

Intensity (a.u.)

Weight loss (%)

700 100

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(003)

60 50

(004) (20-1) (110) (020) (202) (203) (11-2) (006) (220) (22-3)

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40

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c)

10 0

92

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20

25

30

35

40

45

50

55

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2 Theta (Degrees)

90

Fig. 1. DTA (a) and TG (b) curves for Co3O4 reference sample, Ca3Co4O9 reference sample and (1
x)Ca3Co4O9/xCo3O4 composite with x¼ 0.5.

Fig. 3. XRD patterns of (a) Ca3Co4O9 powder after synthesis, (b) (1
x) Ca3Co4O9/xCo3O4 composite sample with x ¼0, (b) (1
x)Ca3Co4O9/xCo3O4 composite sample with x¼ 0.5.

Fig. 2. SEM images (secondary electrons) of (a) Co3O4 and (b) Ca3Co4O9.

200

3.5

190

3

180 170

x=

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150

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140

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130 120

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Power Factor (10-4W/m.K2)

Seebeck Coefficient (µV/K)

F. Delorme et al. / Ceramics International 41 (2015) 10038–10043

2.5 x

2

1

x=

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400

500

12000

3

10000 8000 6000 4000

x=

0

x=

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900

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Thermal Diffusivity (mm2/s)

Conductivity (S/m)

3.5

700

600

700

800

900

1000

1100

Fig. 6. Temperature dependence of the power factor of (1-x)Ca3Co4O9/ xCo3O4 composite samples.

14000

500

0.5

0.5

Temperature (K)

Fig. 4. Temperature dependence of the Seebeck coefficient of (1
x) Ca3Co4O9/xCo3O4 composite samples.

300

0.1

x = 0.2

Temperature (K)

0

=0

x=

1.5

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1100

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2.5 x=0

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x = 0.1 x = 0.2

1.5

x = 0.5

1 0.5 300

400

500

600

700

800

900

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1100

Temperature (K)

Temperature (K)

Fig. 5. Temperature dependence of the electrical conductivity of (1
x) Ca3Co4O9/xCo3O4 composite samples.

Fig. 7. Temperature dependence of the thermal diffusivity as a function of the Co3O4 content.

The temperature dependence of the electrical conductivity (σ) of the (1
x)Ca3Co4O9/xCo3O4 composite samples with x ¼ 0, 10, 20 and 50 vol% from 323 to 1000 K is shown in Fig. 5. All the samples exhibit the same behavior: the electrical conductivity tends to increase when temperature increases, which is characteristic of a semiconducting-like behavior (dρ/ dT r 0). The electrical conductivity of the samples with x¼ 0.1 and 0.2 is higher at low temperatures but lower at high temperatures compared to the one of the pure Ca3Co4O9 sample (x ¼ 0). However, the electrical conductivity of the sample with x ¼ 0.5 is divided by approximately a factor of 2 independently of the temperature. This is consistent with the fact that Co3O4 exhibits a lower electrical conductivity than Ca3Co4O9 [43]. The temperature dependence of the power factor (¼ S2σ) of the (1
x)Ca3Co4O9/xCo3O4 composite samples with x¼ 0, 10, 20 and 50 vol% from 323 to 1000 K is shown in Fig. 6. All the samples exhibit the same behavior: the power factor increases when temperature increases. Samples with x¼ 0.1, 0.2 and 0.5 exhibit lower power factor than the pure Ca3Co4O9 sample (x ¼ 0). The higher the Co3O4 content, the lower the power factor: at 1000 K the power factor value is 3.06, 2.61, 2.56 and 1.93  10–4 W m
1 K
2 for x ¼ 0, 0.1, 0.2 and 0.5 respectively. Temperature dependence of the thermal diffusivity from 323 to 1000 K shows the same trend for all the (1
x)Ca3Co4O9/ xCo3O4 composite samples with x ¼ 0, 10, 20 and 50 vol%

(Fig. 7). Indeed, thermal diffusivity slightly decreases from room temperature to 1000 K, from 1.09 to 0.67 mm2 s
1 for the unsubstituted sample (x¼ 0) and from 2.89 to 0.84 mm2 s
1 for the sample with x ¼ 0.5 for instance. Thermal diffusivity of the composites containing Co3O4 content is higher than the thermal diffusivity of the pure Ca3Co4O9 sample (x ¼ 0) on the whole temperature range. Moreover, thermal diffusivity increases on the whole temperature range when the Co3O4 content increases. The specific heat capacity at 1000 K of Co3O4 [44] is higher than Ca3Co4O9 one, 0.96 and 0.61 J g
1 K
1 respectively. Therefore, at 1000 K, with higher thermal diffusivity, higher density and higher specific heat capacity, the thermal conductivity will be higher. Assuming κ ¼ a ρ Cp, where κ is thermal conductivity, a is thermal diffusivity, ρ is density and Cp is specific heat capacity, the thermal conductivity values at 1000 K for samples with x ¼ 0, 0.1, 0.2 and 0.5 will be 1.9, 2, 2.5 and 3.4 W m
1 K
1, respectively. These values will lead to ZT values at 1000 K of 0.16, 0.13, 0.10 and 0.06 for x ¼ 0, 0.1, 0.2 and 0.5, respectively. The addition of Co3O4 within a Ca3Co4O9 matrix leads to decreased thermoelectric properties. Moreover, the higher the Co3O4 amount, the lower the thermoelectric properties. These results do not seem to be in accordance with the theoretical papers that predict an increase of the thermoelectric properties mainly due to phonon scattering by interfaces and the results reported in Ca3Co4O9/silver [38,39] and Ca3Co4O9/SiO2 [40]

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composites. However, silver as well as SiO2 [45] react with Ca3Co4O9 and therefore the reported properties cannot only be interpreted as the effect of the secondary phase but are due at least partially to substitution of calcium for silver or cobalt for silicon. Chen et al. [41] have attributed the resistivity and Seebeck coefficient increase in their samples to the compressive strain on Ca3Co4O9 originating from the mismatch of thermal expansion coefficients. This hypothesis is consistent with the results presented in this study as thermal expansion coefficient (dL/L0) of Ca3Co4O9 [41] is higher than that of Co3O4 [46]: indeed, at 1000 K the Seebeck coefficient tend to increase (S ¼ 181 and 193 mV K
1 at 1000 K for x¼ 0 and 0.5 respectively) whereas the electrical conductivity is reduced (σ ¼ 9300 and 5160 Sm
1 at 1000 K for x¼ 0 and 0.5 respectively), i.e. the resistivity increases. 4. Conclusion (1
x)Ca3Co4O9/xCo3O4 composites samples with x¼ 0, 10, 20 and 50 vol% have been prepared by solid state reaction, sintered by SPS and their thermoelectric properties at high temperature (323–1000 K) have been studied. All the sintered samples present a high bulk density with an apparent density value larger than 93% of the theoretical density. DTA study shows that no reaction occurs between Co3O4 and Ca3Co4O9 particles during the high temperature treatment. The addition of Co3O4 particles within a Ca3Co4O9 matrix strongly influences the thermoelectric properties. At high temperatures, the Seebeck coefficient is increased for the sample with x¼ 0.5. However, the electrical conductivity of the composite samples is reduced by a factor of two for x¼ 0.5 at 1000 K. Moreover, thermal diffusivity is increased. This leads to a reduction of ZT values by a factor close to 2.5 for the sample with x¼ 0.5. These modifications of the thermoelectric properties could be attributed to the compressive strain on Ca3Co4O9 originating from the mismatch of thermal expansion coefficients of Ca3Co4O9 and Co3O4. Acknowledgments The authors acknowledge ADEME (Agence de l’Environnement et de la Maîtrise de l’Energie) and TOTAL for the financial support (6th AMI ADEME-TOTAL, Project SONATE (DS2748)). The authors also acknowledge Dr. M.A. Bousnina for its technical support for SEM images. References [1] D.J. Bergman, L.G. Fel, Enhancement of thermoelectric power factor in composite thermoelectric, J. Appl. Phys. 85 (1999) 8205–8216. [2] H. Odahara, O. Yamashita, K. Satou, S. Tomiyoshi, J.-I. Tani, H. Kido, Increase of the thermoelectric power factor in Cu/Bi/Cu, Ni/Bi/Ni, and Cu/Bi/Ni composite materials, J. Appl. Phys. 97 (2005) 103722. [3] S.V. Faleev, F. Leonard, Theory of enhancement of thermoelectric properties of materials with nanoinclusions, Phys. Rev. B 77 (2008) 214304. [4] M. Zebarjadi, K. Esfarjani, A. Shakouri, J.-H. Bahk, Z. Bian, G. Zeng, J. Bowers, H. Lu, J. Zide, A. Gossard, Effect of nanoparticle scattering on thermoelectric power factor, Appl. Phys. Lett. 94 (2009) 202105. [5] J.W. Sharp, S.J. Poon, H.J. Goldsmid, Boundary scattering and the thermoelectric figure of merit, Phys. Status Solidi A 187 (2001) 507–516.

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