Modified texture and high temperature transport properties of doubly substituted BaxAgyCa2.8Co4O9 thermoelectric oxide

ARTICLE IN PRESS Physica B 404 (2009) 2142–2145

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Modified texture and high temperature transport properties of doubly substituted BaxAgyCa2.8Co4O9 thermoelectric oxide F.P. Zhang , Q.M. Lu, J.X. Zhang Key Laboratory of Advanced Functional Materials, Ministry of Education, College of Material Science and Engineering, Beijing University of Technology, 100124 Beijing, PR China

a r t i c l e in f o

a b s t r a c t

Article history: Received 5 November 2008 Received in revised form 22 February 2009 Accepted 1 April 2009

Doubly substituted polycrystalline compound bulk samples of BaxAgyCa2.8Co4O9 were prepared via citrate acid sol–gel method followed by spark plasma sintering. The phase composition, orientation, texture and high temperature electrical properties were systematically investigated. The results showed that the orientation and the texture could be modified by altering ratio of Ba to Ag. The resistivity and the Seebeck coefficient of substituted samples were decreased by decreasing Ba/Ag ratio except for that of Ba0.1Ag0.1Ca2.8Co4O9 sample with lowest electrical resistivity (7.2 mO cm at 973 K), moderately high Seebeck coefficient (172 mV/K at 973 K) and improved power factor (0.42 mW/mK2 at 973 K). & 2009 Elsevier B.V. All rights reserved.

PACS: 72.15.Jf 72.80.Ga Keywords: Ca3Co4O9 Doubly substitution Grain orientation Transport properties

1. Introduction Cobalt oxide Ca3Co4O9 with misfit layered structure composed of rock salt layer Ca2CoO3 and CdI2-type layer CoO2 subsystems along c axis has received continuous interests since the discovery of special thermoelectric (TE) properties of NaCo2O4 single crystals by Terasaki et al. in 1997 [1–7]. High Seebeck coefficient a, low resistivity r, low thermal conductivity k and high figure of merit Z (Z ¼ a2/rk) are needed for an applicable TE material. It is well known that Ca3Co4O9 is highly electrically anisotropic due to its layered structure [8,9]. It is indicated that texture has a great influence on the electrical resistivity suggesting that electrical performance of highly oriented Ca3Co4O9 would be much higher than randomly oriented ones with the same composition. Ca3Co4O9 is thought to be a strong electron correlated system and fit for substitution to improve its bulk TE properties. Substitution is also a conventional way to enhance properties of TE bulk material [10]. By now many kinds of ways have been exploited to enhance the TE properties and partial substitution for Ca site is just one of them [4,6]. Some reports revealed that the electrical resistivity could be restrained by substitution for Ca site by Ba and the electrical properties of Ca3Co4O9 could be enhanced by substituting for Ca site by Ag or adding Ag2O to Ca3Co4O9

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E-mail address: [email protected] (F.P. Zhang). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.04.002

polycrystalline matrix [11–14]. However, there were no reports about effects of double substitution for Ca by both Ba and Ag on texture and electrical properties of Ca3Co4O9. In this paper, polycrystalline BaxAgyCa2.8Co4O9 (x ¼ 0.05, 0.1, 0.15, 0.2; y ¼ 0.15, 0.1, 0.05, 0) bulk samples were prepared via citrate acid sol–gel and spark plasma sintering (SPS) method. Orientation, texture and high temperature electrical transport properties from room temperature up to 1000 K were investigated in detail for the first time to our knowledge. The methods of optimizing texture [15–17] such as sintering pressure were not studied.

2. Experimental Polycrystalline bulk samples of BaxAgyCa2.8Co4O9 (x ¼ 0.05, 0.1, 0.15, 0.2; y ¼ 0.15, 0.1, 0.05, 0) were prepared by citrate acid sol–gel and SPS method. Nominal ratios of nitrates of Ba, Ag, Ca and Co, all with purity above 99.95%, were dissolved in aqueous solution of citric acid. The solution was heated under continuous stirring at 353 K in order to form the BaxAgyCa2.8Co4O9 precursor gel. The gel was dried at 393 K in a muffle oven for 12 h. Then the dried gel was ground and sintered at 1073 K for 8 h to remove excess organic compounds and to get BaxAgyCa2.8Co4O9 powder. Finally the powder was placed into a graphite die with a diameter of 20 mm for SPS (SPS-3.20MK-V) processing (sintering temperature 1073 K, holding time 5 min) under a pressure of 30 MPa with a heating rate of 100–120 K/min from room temperature. The

ARTICLE IN PRESS F.P. Zhang et al. / Physica B 404 (2009) 2142–2145

0.9

1.06

0.8

1.04

0.7 1.02

Relative density, dr

Orientation degree, F

parallel sample Ca3Co4O9 was fabricated and the detail could be found in Ref. [11]. The phase composition was analyzed by X-ray diffraction (XRD) at room temperature on a Rigaku diffractometor with CuKa radiation in the 2y range of 5–751 at increments rate of 0.021/s (2y). The density of sample was measured using Archimedes method. The microscopic structure was observed with scanning electron microscope (SEM) using secondary electron by NANO SEM-200 (operated at 30 kV). The electrical resistivity and Seebeck coefficient were measured in He atmosphere from room temperature up to 1000 K using a conventional dc standard fourprobe method on ULVAC ZEM-2 system.

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0.6

3. Results and discussion Fig. 1 presents the XRD patterns at room temperature of the bulk sample surfaces perpendicular to the pressure direction of SPS process. The main XRD patterns of all samples were in good agreement with the standard JCPDS card (No. 23–0110), indicating the formation of Ca3Co4O9-type compound at room temperature. The 2y angles of BaxAgyCa2.8Co4O9 compounds were decreased by 0.241. Small quantities of Impurity phase of Ag was indexed within xpy compounds. Since the average ionic radius of Co and O are invariable and all samples crystallize in the main form of Ca3Co4O9-type structure, it is estimated that Ba and Ag substituted Ca site and changed the lattice parameters. The relative bulk densities of the samples are shown in Fig. 2. The relative densities of all substituted samples were higher than that of parallel sample. Additionally, the relative density reaches the peak value of 1.054 for Ba0.1Ag0.1Ca2.8Co4O9 sample. The sublayers of the BaxAgyCa2.8Co4O9 grains were aligned along direction parallel to sintering pressure axis since strong peaks in each XRD pattern can be assigned to {0 0 l} peak of Ca3Co4O9. The Lotgering factor [18] F used to evaluate the orientation degree of Ca3O4O9 polycrystalline bulk is expressed as

x = 0.2, y = 0

(008)

(203) (006)

pressure direction (20-1) (020) (005)

(002)

(003)

(111)

Intensity (a.u.)

(001)

(hkl) BaxAgyCa2.8Co4O9 * Ag

(004)

F ¼ ðP  P 0 Þ=ð1  P0 Þ (1) P P Here P ¼ If0 0 lg = Ifh k lg and the value was calculated from BaxAgyCa2.8Co4O9 samples. P0 was calculated from parallel Ca3O4O9 sample. The orientation degree as a function of ratio of Ba to Ag r (r ¼ x/y) is shown in Fig. 2. The F values of doubly substituted samples (0.61–0.72) were lower than that of parallel sample. The F value of substituted samples was increased with increasing r and the F

x = y = 0.1 x = 0.05, y = 0.15 Ca3Co4O9

5°

75° 2 theta (°)

Fig. 1. XRD patterns for BaxAgyCa3xyCo4O9 bulk samples perpendicular to pressure direction.

1 r, (Ba/Ag)

3

Fig. 2. Orientation degree, F and relative density, dr for BaxAgyCa3xyCo4O9 bulk samples as a function of Ba/Ag ratio.

value reached a maximum 0.72 for r ¼ 1 sample. The F value of Ba singly substituted sample was the largest, 0.88. The orientation could be modified by optimizing r, thus, grain orientation modified bulk samples could be fabricated by altering ratio of Ba to Ag in sol–gel process followed by SPS. Fig. 3 shows the SEM images of fractured cross-section for the BaxAgyCa2.8Co4O9 (x ¼ 0.15, y ¼ 0.05; x ¼ y ¼ 0.1; x ¼ 0.05, y ¼ 0.15) bulk samples. It can be stated that most of the particles with grain size of several microns were plate-like, most grains in the samples were preferentially aligned, and textured polycrystalline samples with different orientation degrees were fabricated by means of altering r in sol–gel process followed by SPS. In Fig. 3(a and c) no large area modified texture could be obviously observed while local area texture could be found. The r ¼ 1 sample showed mostly textured structure. This was in accordance with orientation degree shown in Fig. 2. It can also be seen that with increasing r from 1/3 to 3, the grain size was increased slightly. The grain size reached several microns for r ¼ 1 sample. Crystallite growth was favored by optimizing r as illustrated by the microstructures. In other words, crystallite growth was easier at r ¼ 1. The favored crystallite grain growth could lead to density enhancement. Fig. 4 shows the temperature dependence of the electrical resistivity (r) for all samples. It is apparent that the r of all samples was reduced with increasing temperature showing semiconductor transport characteristic. The average r of r41 sample was higher than that of ro1 sample. The r ¼ 1 sample exhibited lowest r among substituted samples (7.2 mO cm at 973 K). This is attributed to orientation optimizing and density enhancement discussed above. The electrical resistivity as a function of temperature for BaxAgyCa2.8Co4O9 samples can be expressed by

rðTÞ ¼ ðT=CÞ exp ðEa =kTÞ

x = 0.15, y = 0.05

*

1/3

8

1.00 0

(2)

where Ea is the activation energy, k is the Boltzman constant [19]. Fig. 5 shows the relationship between ln(r/T) and 1/T for all the samples. It was found that the relationship between r and T for all samples followed the SPHC model [20] very well. The plots of ln (r/T) versus 1/T for all samples were straight lines above 600 K characterizing the polaron hopping conduction mechanism in high temperature region. The Ca site within the Ca3CoO2 sub-layer were partially substituted by Ba and Ag, the conduction path CoO2 sub-layer would not be disturbed, so the Ea will keep constant in high temperature region (4650 K). The temperature dependence of Seebeck coefficient (a) for BaxAgyCa2.8Co4O9 bulk samples is shown in Fig. 6. All samples

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x, y 0, 0 0.2, 0 0.15, 0.05 0.1, 0.1 0.05, 0.15

Resistivity,  (mΩ cm)

13 12 11 10 9 8 7 400

600 Temperature (K)

800

1000

Fig. 4. Temperature dependence of resistivity for BaxAgyCa3xyCo4O9 bulk samples.

ln ( /T)

-3.6

x, y 0, 0 0.2, 0 0.15, 0.05 0.1, 0.1 0.05, 0.15

-4.2

-4.8 0.0014

0.0021 1/T (K-1)

0.0028

Fig. 5. ln(r/T) vs. 1/T for BaxAgyCa3xyCo4O9 bulk samples.

exhibited positive a indicating the hole carrier conduction. The a increases monotonically with increasing temperature and a thermally activated behavior was observed in all samples. The a was reduced for doubly substituted samples. The a of the doubly substituted BaxAgyCa2.8Co4O9 samples could be estimated using the extended Heikes formula [21] written as   kB d3 x ln a¼ (3) e d4 1  x where di is the number of the degenerated configuration of the Coi+ state in the CoO2 layers. The x (x ¼ Co4+/Co) is the concentration fraction of Co4+ holes on the Co sites in these layers. d3 ¼ 1 and d4 ¼ 6 are given based on magnetic susceptibility measurement of cobaltite layer structured oxides which have the same edge-shared CoO2 layers [22–24]. As a result, a is decided by x. For Ba and Ag doubly substituted samples, the x is negatively proportional to r expressed approximately as x ¼ 0:2=cðr þ 1Þ

(4) 4+

Fig. 3. Cross-section SEM for Ba0.05Ag0.15Ca2.8Co4O9 (a), Ba0.1Ag0.1Ca2.8Co4O9 and (b) Ba0.15Ag0.05Ca2.8Co4O9 (c) bulk samples.

where c is regarded as a constant. The Co concentration fraction can be increased by the substitution of bivalent Ca2+ site by univalent Ag+. According to the formulas and analyses above, the reduced a could be increased with r from 0 to 3. Thus the r ¼ 3 sample exhibited highest a value (176 mV/K at 973 K) and the r ¼ 1/3 sample exhibited lowest a value (162 mV/K at 973 K) among all doubly substituted samples. The power factor P (P ¼ a2/r, Fig. 7) was calculated from measured a and r values. The P of each sample increased with

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Seebeck coefficient, α (μV/K)

190 180

4. Conclusion

x, y 0, 0

Doubly substituted BaxAgyCa2.8Co4O9 bulk samples have been fabricated by sol–gel and SPS method. The effects of double substitution on phase composition, orientation and texture of resulting bulks have been studied for the first time. By reducing the electrical resistivity without restraining Seebeck coefficient severely for Ba0.1Ag0.1Ca2.8Co4O9 bulk sample, the power factor was improved by 13%.

0.2, 0 170

0.15, 0.05 0.1, 0.1

160

2145

0.05, 0.15

150 140

Acknowledgments

130 400

600

800

1000

Temperature (K) Fig. 6. Temperature dependence of Seebeck coefficient for BaxAgyCa3xyCo4O9 bulk samples.

The authors would like to thank financial supports both of National Natural Science Foundation of China (Grant no. 50702003) and Beijing Municipal Commission of Education Foundation (Grant no. JC009001200803). References

x, y 0, 0

Power factor, P (mW/mK2)

0.4

0.2, 0 0.15, 0.05 0.1, 0.1

0.3

0.05, 0.15

0.2

400

600 800 Temperature (K)

1000

Fig. 7. Temperature dependence of power factor for BaxAgyCa3xyCo4O9 bulk samples.

increasing temperature. Because of greatly reduced r and moderately maintained a for Ba0.1Ag0.1Ca2.8Co4O9 bulk sample, the P was improved by 13% (0.42 mW/mK2 at 973 K). It was true that the electrical properties could be further improved through double substitution of Ca by both Ba and Ag. Furthermore, it was indicated that double substitution would be a way enhancing electrical properties of Ca3Co4O9-based TE oxides.

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