Influence of Sr substitution on thermoelectric properties of La1−xSrxFeO3 ceramics

Current Applied Physics 10 (2010) 866–870

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Influence of Sr substitution on thermoelectric properties of La1xSrxFeO3 ceramics H.C. Wang *, C.L. Wang, J.L. Zhang, W.B. Su, J. Liu, M.L. Zhao, N. Yin, Y.G. Lv, L.M. Mei School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China

a r t i c l e

i n f o

Article history: Received 2 May 2009 Received in revised form 30 September 2009 Accepted 8 October 2009 Available online 13 October 2009 Keywords: Oxide materials Ceramics Thermoelectric properties

a b s t r a c t Perovskite La1xSrxFeO3 (0.10 6 x 6 0.20) ceramics have been synthesized by the conventional solid-state reaction technique. Their electrical resistivity, Seebeck coefficient and thermal conductivity have been measured. It has been found that the increase of Sr content reduces significantly both the electrical resistivity and the Seebeck coefficient, but slightly increases the high-temperature thermal conductivity. An adiabatic hopping conduction mechanism of small polaron is suggested from the analysis of the temperature dependence of the electrical resistivity. Seebeck coefficients decrease with increasing temperature, and saturate at temperature above 573 K. The saturated value of Seebeck coefficient decreases with increasing of Sr contents, from 200 lV/K for x = 0.10 to 100 lV/K for x = 0.20. All samples exhibit lower thermal conductivity with values around 2.6 W/m K. The highest dimensionless figure of merit is 0.031 at temperature 973 K in La0.88Sr0.12FeO3. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction Oxides thermoelectric materials have many advantages, such as high temperature chemical stability, low content of toxic elements and economical raw materials. They have received much attention since the report of high thermoelectric performance in NaCo2O4, which shows a large Seebeck coefficient (S  100 lV/K at 300 K) and a relatively high conductivity around 2  104 S/cm at room temperature [1]. Perovskite-type oxides form a large family of metal oxides with versatile physical properties, such as ferroelectricity, superconductivity, magnetoresistance, as well as thermoelectricity. For potential applications as thermoelectric materials, early works has been focused on the decreasing of their electrical resistivity. Kobayashi et al. reported that the lanthanide elements substitution in CaMnO3 can increase the electrical resistivity below room temperature, resulting in an increase of the semiconductor-to-metal transition temperature [2]. Intensive work has been done by Ohtaki et al. [3] on the electric transport properties and high-temperature thermoelectric performance of this perovskites system. They found that the substitution at the Ca site causes a marked decrease in electrical resistivity along with a moderated decrease in the absolute value of the Seebeck coefficient. In later years, thermoelectric performance of strontium titanate has drawn much attention. A larger power factor about 28– 36 lW/K2 cm has been found in single crystal Sr1xLaxTiO3 with 0 6 x 6 0.1 [4]. The observed power factor was comparable to that * Corresponding author. Tel.: +86 531 88377035; fax: +86 531 88377031. E-mail address: [email protected] (H.C. Wang). 1567-1739/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2009.10.009

of Bi2Te3. Ohta et al. [5] clarified the intrinsic thermoelectric performance of heavily La- and Nb-doped SrTiO3 bulk crystal and effect of La- and Nb-doping upon carrier effective mass. Study of polycrystalline La doped BaTiO3–SrTiO3 solid solution demonstrated that the figure of merit has a peak around 400–600 K and the value is about 3.0  104 K1 for Ba0.3Sr0.6La0.1TiO3 [6]. Measurement results of thermoelectric property of LaCoO3 with composition La0.95Sr0.05CoO3 showed that this compound exhibits a very respectable room-temperature thermoelectric figure of merit value of 0.18 [7]. LaFeO3-based perovskites have low electrical resistivity, and have been used as high temperature cathodes in solid oxide fuel cells [8,9]. For good thermoelectrics, relatively low electrical resistivity is required, together with larger Seebeck coefficient and low thermal conductivity. From this point of view, LaFeO3-based ceramics could exhibit good thermoelectric performance if their Seebeck coefficients are reasonable high and thermal conductivities are relatively low. Therefore, a concept has been proposed for design of thermoelectric oxides. The method is to decrease the thermal conductivity without increasing the electrical resistivity using complex oxide systems [10]. To confirm this concept, properties of Sr-doped LaFeO3–NdFeO3 solid solution have been investigated. From the measurement of the electrical resistivity, thermoelectric power, and thermal conductivity of La0.45Nb0.45Sr0.1FeO3d [10], it has been found that their electrical conductive behavior shows a p-type behavior and the thermal transport behavior can be explained using the small polaron mechanism. From investigation of the transportation properties in the ceramic specimen of La0.7Sr0.3FeO3 with orthorhombic structure, Jung [11] found that the charge carrier responsible for conduction

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could be strongly localized, and experimental results of resistivity and the thermo-power can be interpreted in terms of a hopping process involving small polaron. A more intensive study has been carried out on the thermoelectric properties, defect structure and electronic structure of Ln0.9Sr0.1FeO3d (Ln = La, Nb) [12]. It has been revealed that the values of holes mobility, the equilibrium constant of the annihilation reaction for oxide ion vacancy, thermal conductivity of La0.9Sr0.1FeO3d are larger than the values of Nd0.9Sr0.1FeO3d, while the thermoelectric power of La0.9Sr0.1FeO3d is smaller than that of Nd0.9Sr0.1FeO3d at the same hole concentration and temperature. From the X-ray absorption spectroscopy measurement, they found that the thermoelectric properties of La0.9Sr0.1FeO3d and Nd0.9Sr0.1FeO3d are affected by the covalency of iron ions [12]. Recent study of the thermoelectric properties of La1xSrxFeO3 with 0 6 x 6 0.4 and LaFe1yNiyO3 with 0 6 y 6 0.6 [13] has shown that the substitutions of Sr and Ni could effectively enhance the power factor. It can be found that the electrical resistivity is relative large when x < 0.1 and the Seebeck coefficient is low when x > 0.2 for La1xSrxFeO3. Therefore, samples with 0.1 6 x 6 0.2 of La1xSrxFeO3 are expected to have a better thermoelectric performance. Therefore, ceramic samples of La1xSrxFeO3 with 0.1 6 x 6 0.2 are prepared in this work, and their electrical resistivity and the Seebeck coefficient as well as thermal conductivity are measured. The dimensionless figures of merit ZT are calculated finally.

obtained from the XRD data. The theoretical density ratio is the measured density over the theoretical density, the values are 95.4%, 96.6%, 96.9%, 97.2%, 97.0% for x = 0.10, 0.12, 0.15, 0.17, 0.20, respectively. The temperature dependence of electrical resistivity of La1xSrxFeO3 ceramics is shown in Fig. 1 for different Sr-doping amount x. The electrical resistivity decreases with increasing temperature up to 873 K, and then slightly increases with further increase of temperature. This general trend of temperature dependence is not changed by the Sr doping content. The temperature at which the electrical resistivity reaches the lowest values shifts slightly to a lower temperature when more Sr is doped. The slight increase of the electrical resistivity at high temperatures is mainly attributable to the increase scattering of the carriers at high temperatures and the decrease in holes density caused by the increase in oxygen defect concentration [13,14]. The reduction of the electrical resistivity by doping Sr is indicative of the carrier density increased by the increase of Sr2+ [15]. To explain the DC conductivity mechanism in transition metal oxides, many theoretical models have been proposed [16–20]. Among them, the pair correlation-type model of Mott et al. [16] based on a strong electron–photon coupling approximation has been extensively used. In the case of the charge carriers responsible for conduction are small polarons, the temperature dependence of the DC resistivity due to a process of adiabatic hopping of small polarons takes the formula [16–20]:

2. Experimental details

q ¼ AT expðDEa =kB TÞ;

Ceramic samples of La1xSrxFeO3 with x = 0.10, 0.12, 0.15, 0.17, 0.20, were prepared by the conventional solid-state reaction technique. The starting materials were La2O3 with purity of 99.99%, SrCO3 of 99.00%, and Fe2O3 of 99.50%.These raw materials were weighted in stoichiometric proportions and were mixed by ballmilled in ethanol with zirconia balls for 12 h. After the wet mixtures were dried, they were pressed into pellets, and calcined at 1000 °C for 12 h. The pellets were smashed and ball-milled for 12 h. Then the power was repressed into pellets, which were sintered in air at 1350 °C for 12 h to obtain ceramic discs. Bar-shaped specimens with the dimension of 15  2.5  2.5 mm3 were cut from the inner portion of sintered discs. These bar-shaped specimens were coated with four electrodes, two on each ends and two on sides, with silver paint annealed at 850 °C for 30 min. For electric measurements, four-probe method was used in the measurement of electrical resistivity q. A direct current I of 100 mA was set to pass through the two end electrodes and the potential difference V across the two side electrodes was read. For measurement of Seebeck coefficient S, two NiCr–NiSi thermal couples were attached on each side electrodes of the bar-shaped specimen with a separation of 8.0 mm. A temperature difference DT = T2  T1 about 3 °C was built up between the two thermal couples using an auxiliary heater during thermoelectric measurement. Seebeck coefficient S was determined from the slope of DV versus DT relation by the least-square method, where DV is the thermoelectromotive force produced by DT between the two electrodes. The thermal conductivity was measured using a laser flash apparatus (ULVAC-RIKO TC-7000).

where A is a pre-exponential factor, DEa is the activation energy for small polaron hopping, and jB is the Boltzmann constant. In fitting the electrical resistivity curves in Fig. 1 with Eq. (1), we found that this formula can not be fitted with the experimental data in the whole temperature range. However, the electrical resistivity can be fitted with Eq. (1) in temperature range of 300–573 K and 573–873 K fairly well respectively. This suggests that the electronic charge transport in La1xSrxFeO3 ceramics still possesses the feature of adiabatic hopping conduction of small polaron, but with different activation energy in different temperature regions. Fig. 2a and b shows the relation of ln(q/T) versus 1/T for the temperature range of 300–573 and 573–873 K respectively. The solid lines are the fitting curves of Eq. (1), and the points represent the experimental values from Fig. 1. The values of activation energy obtained from fitting curves of Fig. 2 are plotted in Fig. 3. The activation energy DEa is about 0.3 eV at low temperature range, and about 0.1–0.2 eV at high temperature range. In other words, the activation energy is larger at low temperature range than at high temperature range. From Fig. 3,

ð1Þ

x=0.10 x=0.12 x=0.15 x=0.17 x=0.20

log (ρ,Ω •cm)

1

0.1

3. Results and discussion The crystal structure is characterized by powder X-ray diffraction with Cu Ka radiation (k = 0.154056 nm) utilizing a Bruker AXS D8 ADVANCE diffractometer. Referred to the PDF card of No. 37-1493, all samples are of single phase in orthorhombic, and belongs to the Pnma space group. The theoretical density of the sintered samples is calculated from the lattice constants, which are

0.01 300

400

500

600

700

800

900 1000 1100

Temperature (K) Fig. 1. Temperature dependence of electrical resistivity of La1xSrxFeO3 with x = 0.10, 0.12, 0.15, 0.17, 0.20.

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-4

500 x=0.10 x=0.12 x=0.15 x=0.17 x=0.20

-6 -7 -8 -9

-10

(a)

-11

1.8

2.0

x=0.10 x=0.12 x=0.15 x=0.17 x=0.20

400

Seebeck ( μV/K)

ln (ρ/T,Ω•cm/K)

-5

2.2

2.4

2.6

2.8

3.0

300

200

100 300

3.2

400

ln(ρ/T,Ω•cm/K)

x=0.10 x=0.12 x=0.15 x=0.17 x=0.20

-10.5 -11.0

S¼

(b) 1.2

1.3

1.4

1.5

1.6

1.7

1.8

-1

1000/T (K ) Fig. 2. Fitting of electrical resistivity with small polaron formula Eq. (1) in low temperature range 300–573 K (a) and high temperature range 573–873 K (b). The solid lines are the least-square fitting curves.

0.31 300--573K 573--873K

0.28 0.24

0.29

0.20

0.28

0.16

ΔEα(eV)

ΔEα(eV)

0.30

0.27

0.12 0.10

800

900 1000 1100

ers are holes. The Seebeck coefficients decrease with increasing of temperature from room temperature to 573 K, and then nearly levels off up to 1100 K of our measurement limit. With increasing of Sr content, the room temperature Seebeck coefficient decreases from 487 lV/K for x = 0.10–217 lV/K for x = 0.20. The Seebeck coefficient S represents the transport entropy per carrier charge and its value can be expressed as [21]

-10.0

-11.5 1.1

700

Fig. 4. Temperature dependence of the Seebeck coefficient of La1xSrxFeO3.

-8.5

-9.5

600

Temperature (K)

-1

1000/T (K )

-9.0

500

0.12

0.14

0.16

0.18

0.20

Sr contents (x) Fig. 3. Activation energies for La1xSrxFeO3 samples derived from data on the electrical resistivity.

we can also see that the activation energy decreases with increasing Sr content slightly in both temperature ranges. This is probably caused by increasing of hopping in energy states of neighboring polarons [16]. The same explanation can be applied to the change in activation energy in the two temperature ranges. Fig. 4 shows the temperature dependence of Seebeck coefficient of the La1xSrxFeO3 ceramics with different Sr content. The Seebeck coefficients show positive values over the whole measured temperature range of 300–1100 K, confirming that the majority carri-

  KB 1c þ S0 ; ln c e

ð2Þ

where S0 represents the vibrational entropy and is estimated to be on the order of less than 10 lV/K, e is the charge of the carrier, and c is the charge carrier fraction. Eq. (2) relates the Seebeck coefficient S to the fractional charge-carrier concentration c. For small polaron systems, Eq. (2) is particularly successful in calculating the charge-carrier concentration [22,23]. The carrier fraction c calculated from Seebeck coefficient data using Eq. (2) for La0.9Sr0.1FeO3 at temperature range between 300 and 873 K is shown in Fig. 5. The carrier fraction increases from 3.9  104 at room temperature to 9.2  102 at 873 K. The increase of the carrier fraction with increasing of temperature may be caused by enhancement of hopping in energy states of neighboring polarons when temperature is increased. The positive slope in temperature dependence of the carrier fraction implies that the carrier formation is thermally activated. The carrier density n, which is related the carrier fraction c, follows Arrhenius law [14]:

N ¼ n0 expðDEn =K B TÞ

ð3Þ

where n0 is a pre-exponential factor, DEn is the activation energy for carrier formation. By fitting the carrier fraction c with Eq. (3) if we take c is proportional to n, the activation energy obtained for carrier formation of La0.9Sr0.1FeO3 is 0.099 eV in the temperature range of 300–573 K, and 0.053 eV in 573–873 K respectively. In the oxides where the hopping motion of small polarons dominates electrical transport, active energy DEn is the potential difference between identical lattice distortions with and without a hole and/or electron, while DEa = DWH + DEn, where DWH is the hopping energy of small polaron [11,16]. Then a large difference in the magnitudes between DEa and DEn is a hallmark of the polaronic transport [16] in our La1xSrxFeO3 ceramics. The temperature dependence of the power factor of La1xSrxFeO3 is shown in Fig. 6. The power factors increase with increasing temperature, reach a maxima around 900 K, and then drop slightly when temperature further increases. The slightly drop-down of the power factor is mainly due to the increase of the electrical resistivity at high temperatures. The highest power factor is in La0.88Sr0.12FeO3 with value of 147 lW/K2 m at 892 K.

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0.08

0.04

300 0.02

250 200 300

400

500

600

700

800

2.8

-1

-1

κ(W•m •K )

0.06

350

3.0

Carrier Fraction

400

2.6 2.4

0.00 900

2.2 300

400

Temperature (K) 0.30 0.25

120 100

x=0.10 x=0.12 x=0.15 x=0.17 x=0.20

-1 -1

Power factor ( μW/K m)

140

κe(W•m •K )

160

500

600

700

800

900

1000

900

1000

Temperature (K)

Fig. 5. The Seebeck coefficient and the carrier fraction calculated from Eq.(2) as a function of temperature for La0.9Sr0.1FeO3.

2

x=0.10 x=0.12 x=0.15 x=0.17 x=0.20

(a)

450

Seebeck (μV/K)

3.2

0.10

500

0.20

(b)

x=0.10 x=0.12 x=0.15 x=0.17 x=0.20

0.15 0.10

80 0.05

60 40

0.00 300

20 0 300

400

500

600

700

800

Temperature (K) 400

500

600

700

800

900 1000 1100

3.2

(c)

Temperature (K) 3.0 2.8

-1

-1

κL(W• m • K )

Fig. 6. Temperature dependence of the power factor of La1xSrxFeO3.

The temperature dependence of the thermal conductivity is shown in Fig. 7a for total thermal conductivity, and Fig. 7b and c for calculated electronic thermal conductivity and the lattice thermal conductivity respectively. It can be seen from Fig. 7a that the total thermal conductivity of all samples exhibits lower than 3.2 W m1 K1 from room temperature to 1000 K. For x = 0.10, 0.12, 0.15 samples, the thermal conductivity decreases with increasing temperature between 300 and 1000 K, which is indicative of the lattice contribution being dominated. This result is similar with that of previously reported [11]. For more Sr-doped samples, i.e., La0.83Sr0.17FeO3 and La0.8Sr0.2FeO3 samples, the thermal conductivity first decreases, then increases with increasing temperature after reaches the minimum around temperature of 400–500 K. However, the thermal conductivity at high temperature range increases with the increase of Sr content. Fig. 7b shows the electronic thermal conductivity, which is calculated from Wiedemann–Franz law as je = LT/q, with Lorentz constant L = 2.44  108 V2 K2 for free electrons. The most obvious feature of Fig. 7b is that the electronic thermal conductivity increases with the increase of Sr content. By compared with Fig. 7a, we can easily find that the electrons make a very smaller part of contribution to the total thermal conductivity. In other words, the lattice thermal conductivity plays major role in the total thermal conductivity. With increasing of temperature, the electronic thermal conductivity firstly increases and reaches a maximum, and then decreases slightly with further increasing of temperature. The lattice thermal conductivity is obtained by deductive electronic the thermal con-

x=0.10 x=0.12 x=0.15 x=0.17 x=0.20

2.6 2.4 2.2 300

400

500

600

700

800

900

1000

Temperature (K) Fig. 7. Temperature dependence of the total thermal conductivity of La1xSrxFeO3 (a); Electronic thermal conductivity calculated using Wiedemann–Franz law (b); Lattice thermal conductivity obtained by deductive of electronic thermal conductivity from total thermal conductivity (c).

ductivity from total thermal conductivity, and present in Fig. 7c. The lattice thermal conductivity decreases with increasing temperature, except for curve with Sr content x = 0.2. This means that the increase of total thermal conductivity at high temperature for sample of x = 0.17 is due to the electronic thermal conductivity completely. For the x = 0.2 sample, the electronic thermal conductivity plays major role in the increase of total thermal conductivity at high temperature, with minor contribution from lattice thermal conductivity. The dimensionless figure of merit ZT, which is calculated from ZT = S2T/qj, is shown in Fig. 8. The ZT values of all samples increase

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H.C. Wang et al. / Current Applied Physics 10 (2010) 866–870

0.035 0.030

ZT

0.025

in La0.88Sr0.12FeO3 is found to be 147 lW/K2 m at temperature of 892 K, and the highest dimensionless figure merit is 0.031 at 973 K. Overall, the thermal conductivity of LaFeO3-based ceramic has a reasonable low value and is less affected by Sr-doping. Further work is needed to enhance their Seebeck coefficient and keep a relatively low value of electrical resistivity.

x=0.10 x=0.12 x=0.15 x=0.17 x=0.20

0.020 Acknowledgements

0.015

The work is financially supported by National Basic Research Program of China of 2007CB607504 and Natural Science Fund of China under Grant Nos. 50902086 and 50572052.

0.010 0.005 0.000 300

400

500

600

700

800

900

1000

Temperature (K) Fig. 8. Temperature dependence of the dimensionless figure merit ZT of La1xSrxFeO3.

with temperature monotonically. Among these samples, La0.88Sr0.12FeO3 shows a slightly higher ZT value of 0.031 at 973 K, since it has a relatively large power factor. However, all ZT values are relatively low compared with that of NaCo2O4. Therefore, further improvement of the thermoelectric performance of this kind of material need to keep the Seebeck coefficient at a relative larger value, while decrease their electrical resistivity.

4. Conclusion From the measurement of electrical resistivity, Seebeck coefficient and thermal conductivity of La1xSrxFeO3 ceramics for 0.10 6 x 6 0.20, we found that the increase of Sr content can effectively reduce the electrical resistivity, but the Seebeck coefficient is reduced either and the high-temperature thermal conductivity is also slightly increased. Adiabatic polaron hopping mechanism is found responsible for the conductivity in La1xSrxFeO3, and the conductive carrier is thermally activated. The highest power factor

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