Grain alignment modulation and observed electrical transport properties of Ca3Co4O9 ceramics

Results in Physics 12 (2019) 321–326

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Grain alignment modulation and observed electrical transport properties of Ca3Co4O9 ceramics

T

⁎

F.P. Zhanga,b, , J.L. Shic, J.W. Zhangc, X.Y. Yangc, J.X. Zhanga,b,c a

Henan Provincial Engineering Laboratory of Building-Photovoltaics, Institute of Physics, Henan University of Urban Construction, 467036 Henan, People’s Republic of China b National Key Laboratory of Advanced Functional Materials, Chinese Ministry of Education, The College of Materials Science and Engineering, Beijing University of Technology, 100124 Beijing, People’s Republic of China c Anhui Provincial Key Laboratory of Advanced Functional Materials and Devices, School of Materials Science and Engineering, Hefei University of Technology, 230009 Hefei, People’s Republic of China

A R T I C LE I N FO

A B S T R A C T

Keywords: Ca3Co4O9 ceramics Preparation Grain alignments Electrical transport properties

The polycrystalline Ca3Co4O9 oxide powders were synthesized by citrate acid sol-gel method, and the Ca3Co4O9 oxide ceramics with diverse grain alignments were prepared by sintering the pressed pellet bulks through controlling the procedures, respectively. The phase compositions, grain alignments, bulk microstructures, textures as well as the electrical transport properties were investigated by means of X-ray diffraction pattern XRD, scanning electron microscopy SEM, transmission electron microscopy TEM and electrical constant measurements apparatus. The results showed that the obtained Ca3Co4O9 ceramics with uniformly single phases and diverse grain alignments can be fabricated by modulating the preparing parameters; moreover, the grain alignment can be enhanced by applying the pressure of the sintering procedure. The Ca3Co4O9 ceramics with uniformly single phase and strongest grain alignment could be fabricated with formation pressure of 500 MPa and sintering pressure of 30 MPa. The sintering pressure is a key factor enhancing the grain alignment for this type of ceramics. The ceramic sample with highest grain alignment was found to have the highest electrical properties, which showed the maximum power factor value of 3.83 μW cm−1 K−2 at 700 °C.

Introduction Thermoelectric (TE) device affords a way converting energies between thermal energy and electrical energy directly. The thermoelectric materials are regarded as a type of high-tech key functional material that can be used in autos, electronic devices, air-craft, space satellite, factories heat energy collection fields and so on [1–5]. The conversion efficiency of a TE material depends on the dimensionless figure of merit ZT, which is formulated by:

ZT = a2T / ρk

(1)

where the α is Seebeck coefficient, T is absolute temperature, ρ is electrical resistivity and κ is the total thermal conductivity. An applicable TE material requires simultaneously high Seebeck coefficient α, low resistivity ρ and thermal conductivity κ [3]. The research attention has been focused on alloys based semiconductor materials for a long time until Terasaki et al. found the high Seebeck coefficient and distinctive thermoelectric properties of cobaltite oxides [6]. The oxides-

based TE materials such as the Cobalt oxides have several advantages comparing with alloys-based TE materials, for instance, resistance to decomposition, easy fabrication, cheapness, high serving temperature and so on [1,6]. Among these Cobalt oxides, the Ca3Co4O9 exhibits moderately high TE properties, and the single crystal shows very high TE performance with nearly unit dimensionless figure of merit ZT700°C = 0.87 [7]. This is comparable to the TE performance of the Bismuth telluride based alloys which are the best TE materials near ambient temperature region. The Ca3Co4O9 has received much attention during the past decade for its potential high temperature applications within energy conversion areas. However, the single crystals are not practical for applications due to the high cost; the way is to fabricate polycrystalline ceramics. Unfortunately, the pessimistic situation of the TE performance for the polycrystalline ceramics makes a great deal of work to do, and the relatively low TE performance is mainly as a result of the relatively low electrical properties. Several feasible kinds of methods have been exploited to enhance its electrical transport properties till now [1,8–11].

⁎ Corresponding author at: Henan Provincial Engineering Laboratory of Building-Photovoltaics, Institute of Physics, Henan University of Urban Construction, 467036 Henan, People’s Republic of China. E-mail addresses: [email protected], [email protected] (F.P. Zhang).

https://doi.org/10.1016/j.rinp.2018.11.071 Received 1 November 2018; Received in revised form 21 November 2018; Accepted 21 November 2018 Available online 28 November 2018 2211-3797/ © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).

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The Ca3Co4O9 single crystal is composed of rock salt like Ca2CoO3 layer and CdI2-type CoO2 layer along c axis, the two sub-layers have the same lattice parameters along a and b axis and different lattice parameters along c axis [6,7]. The rock salt like Ca2CoO3 layer is regarded as carrier reservoir layer and the CdI2-type CoO2 layer is regarded as carrier transport layer [7,8]. The transport anisotropy of Ca3Co4O9 single crystal means that higher grain alignment and texture should be needed for enhancing electrical properties of polycrystalline ceramics. Some works have reported the synthesis of Ca3Co4O9 polycrystalline powders and ceramics by means of solid state reaction method with adjusting the processing parameters. Several reports have revealed the fabrication of Ca3Co4O9 by spark plasma sintering (SPS) method for the bulk consolidation and grain alignment [8]. Among these methods, sintering with pressure is effective for fabricating high textured Ca3Co4O9-based polycrystalline ceramics [11]. The polymerized complex and flux method is also effective for fabricating textured materials [12]. The reactive templated grain growth method is thirdly effective way for preparing grain aligned ceramics [13]. The SPS is reported indeed very much effective for fabricating grain aligned ceramics. But there were no further reports in terms of effects of grain alignments on electrical properties of Ca3Co4O9 ceramics, and there were no further reports on modulation of grain alignment by the preparation procedures to date, either. The grain alignment should play a key role in determining the carrier transport for this type of ceramic material; however, the driving force for the grain alignment still needs investigation. In this paper, the polycrystalline single phase Ca3Co4O9 oxide powders are synthesized by citrate acid sol-gel method, the Ca3Co4O9 oxide ceramics are prepared by sintering the pellets with adjusting the processing procedures. The phase compositions, grain alignments, microstructures as well as the bulk textures are investigated by means of X-ray diffraction pattern XRD, scanning electron microscopy SEM and transmission electron microscopy TEM. The modulation mechanism of grain alignment of this type of ceramics is investigated and the electrical properties of Ca3Co4O9 ceramics are studied.

Fig. 1. XRD pattern for the polycrystalline Ca3Co4O9 power sample.

presents the sample index and the preparation parameters. The phase compositions of ceramic samples were analyzed by X-ray diffraction (XRD) at room temperature on a Rigaku diffractometor with CuKα radiation in a 2 theta range of 5° ∼ 75°, with steps of 0.02°(2θ) and a time per step of 1 s. The microscopic images of the materials were obtained with the scanning electron microscope (SEM) using secondary electron mode by Carl Zeiss SUPRA 40 operated at 10KV and highresolution transmission electron microscopy (HRTEM, JEM-2100F). The electrical resistivity and Seebeck coefficient for the bulk samples were measured perpendicular to pressure direction in He atmosphere from room temperature up to 700 °C using a conventional dc standard four-probe method on ULVAC ZEM-2 system. Results and discussion Fig. 1 presents the XRD pattern for the polycrystalline Ca3Co4O9 power sample. As shown in the Figure, the diffraction peaks of the XRD pattern could be indexed as the Ca3Co4O9-type compound by comparing with the standard JCPDS card (No. 23-0110) and there were no second phase that can be found. The wet citric acid method affords a convenient way synthesizing polycrystalline ceramic powders such as cobaltite oxides and manganese oxides with pure phase composition [1,8,15]. Fig. 2(A) (B) presents the SEM image and TEM image for the polycrystalline Ca3Co4O9 power sample. It can be seen in figure (A) that the Ca3Co4O9 particles are of several hundreds of nanometers in magnitude, with uniform size distribution and shape of polyhedron. Secondly, layered microstructure could be observed for the particles, this phenomenon indicates that the Ca3Co4O9 seeds should grow firstly along a and b axis, and then the Ca3Co4O9 slices stack along c axis. It can also be inferred that the bond between these slices is much weaker than that within the slices. The Ca3Co4O9 single crystal is composed of rock salt like Ca2CoO3 layer and CdI2-type CoO2 layer along c axis. In other words, it can be inferred that the bond between Ca2CoO3 layer and CdI2-type CoO2 layer is much weaker than the bong within these layers. It can be seen by TEM image of the single grain of Ca3Co4O9 from Fig. 2(B) that the grain is 3 hundreds nanometers in magnitude with layered structure and diverse thickness, that could be distinctively observed by the dark field image of the figure. This is in accordance with the SEM results. Fig. 3 presents the XRD patterns for all the sintered polycrystalline Ca3Co4O9 ceramics, noting that the X-ray diffraction surfaces of the bulk samples are perpendicular to the formation pressure axis. As shown in the figure, the XRD patterns of all ceramic samples are in good agreement with the standard JCPDS card (No. 23-0110), this exemplifies that all ceramic samples are in the Ca3Co4O9-type structure. It is also confirmed that the Ca3Co4O9 ceramics can be prepared by sintering the powder pellets at 800 °C, with various processing pressures, respectively. Thirdly, it can be inferred that the holding time during the

Experimental details The polycrystalline Ca3Co4O9 oxide ceramic powders were synthesized by the citrate acid sol-gel reaction method. Stoichiometric ratios of highly pure nitrates of Ca and Co were dissolved in distilled water; the citric acid was added in the aqueous solution. The solution was continuously mixed at 80 °C in order to form the precursor gel. The obtained gel was dried at 120 °C for 12 h in air to evaporate the excessive water. Then the dried gel was ground and calcined at 800 °C for 8 h to remove excess organics and to get the Ca3Co4O9 oxide ceramic powders. Then the powder was finely ground for bulk platelet samples preparation. The powers were pressed into bulk pellets with uniaxial formation pressure of 100, 200, 300, 400 and 500 MPa for further processing, respectively. The ceramic samples were obtained by sintering the bulk pellets at 800 °C, via conventional annealing furnace and SPS with certain pressures. For bulk platelet samples preparation via conventional annealing furnace, the pellet samples were subjected to furnace sintering at 800 °C for 12 h with a heating rate of 20 °C /min. For bulk platelet samples preparation via SPS, the obtained pellets pressed at 500 MPa were sintered by SPS at 800 °C with a heating rate of 100 K/min at pressure of 30 MPa and holding time of 5 min. Table 1 Table 1 Sample index, formation pressures and sintering pressures for Ca3Co4O9 ceramics. Samples

A

B

C

D

E

F

Formation pressure, / MPa Sintering pressure, / MPa

100 0

200 0

300 0

400 0

500 0

500 30

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the ceramic samples prepared by sintering the powder pellets at 800 °C with changed processing pressures. Moreover, the quantities of [00l] diffraction peaks detected for the ceramic samples can be adjusted by changing the formation pressure and sintering pressure. It is observed that more [00l] diffraction peaks can be detected for ceramic samples with higher formation pressure. This means that the ceramic samples within this work is not totally c-axis aligned. As estimation, it is thereafter possible from the present work that Ca3Co4O9 ceramic with totally c axis grain alignment could be obtained by further optimizing the sintering procedures. As is discussed, the Ca3Co4O9 seeds grow along a and b axis, and then the Ca3Co4O9 slices stack along c axis during the powder synthesis process. It could also be inferred here that the situation is the same during the solidification process of the ceramics. The bond between these slices are much weaker than the bond within the slices, the Ca3Co4O9 seeds align along a and b axis, then the Ca3Co4O9 slices stack along c axis under formation pressure, and finally the aligned Ca3Co4O9 seeds tend to grow larger in the sintering atmosphere. There are more [00l] diffraction peaks for these ceramic samples comparing with that of the polycrystalline powder sample, this is an indication that grain alignments are formed and ceramic textures are obtained. To study the effect of preparing pressure and pressure strategy on grain alignments and ceramic textures of the title oxide materials, the Lotgering method is used [14]. The Lotgering factor L can be formulated as: (2)

F = (P − P0)/(1 − P0) P=

∑ I{00l} / ∑ I{hkl}

(3)

where P is calculated from grain aligned ceramic samples. P0 is calculated from grain randomly aligned powder sample. Table 2 presents the deduced Lotgering factor L for all ceramic samples. The ceramic samples were prepared in parallel, thus the comparative analysis of the effects of formation pressure on ceramics grain alignment is reasonable. Higher L can be found for bulk ceramic samples as a result of higher formation pressure. For example, the sintered ceramic with formation pressure of 100 MPa has L value of 0.58; the sintered ceramics with formation pressure of 300 MPa and 500 MPa have L values of 0.64 and 0.67, respectively. It is seen that the formation pressure is moderately a key important factor in order to facilitate the grain alignment. Fig. 4 presents the illustration for the grain growth and grain alignment of slice-like Ca3Co4O9 seeds during the formation pressing process and sintering process. The flows in the upper figure demonstrate the ceramic formation with higher formation pressure of 500 MPa, while the flows in the lower figure demonstrate the ceramic formation with lower formation pressure of 200 MPa. The grains of powder samples under high pressure should tent to align more regularly, however the grains under lower pressure should align hard. So the ceramic samples prepared with higher pressure show higher Lotgering factor L. It is also confirmed here that the slice like Ca3Co4O9 grains align along a and b axis, then the Ca3Co4O9 slices stack along c axis under formation pressure, and in the end the aligned Ca3Co4O9 grains tend to grow larger in the sintering atmosphere. It is worth noting that the sintered ceramic sample indexed as F shows largest L value, which indicates its strongest grain alignment and ceramic texture. Comparing with other ceramics, the ceramic sample F via SPS is prepared under sintering pressure of 30 MPa from the pressed bulk pellets with formation pressure of 500 MPa. It can be inferred that

Fig. 2. SEM image a) and TEM image b) for the polycrystalline Ca3Co4O9 power sample.

Fig. 3. XRD patterns for all the sintered polycrystalline Ca3Co4O9 ceramic samples.

sintering procedure has no impact on the phase composition of this type of ceramic materials. However, fourthly, one can see that more [00l] diffraction peaks could be found within these ceramic samples comparing with that of the polycrystalline powder sample, grain alignment and orientation can be inferred. The [00l] diffraction peaks are indexed in the figure to guide the eyes. This means that more Ca3Co4O9 slices are stacked perpendicular to the formation pressure axis, naming that the Ca3Co4O9 slices are tempted to align along ab plane. It is also indicated and inferred from this work that the pressing procedure plays important role in grain alignment for this kind of ceramics [11]. It is true that the non-[00l] diffraction peaks could also be found for

Table 2 Density d and Lotgering factor L for Ca3Co4O9 ceramics.

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Samples

A

B

C

D

E

F

Absolute density, d / g cm−3 Lotgering factor, L

2.97 0.58

3.21 0.58

3.50 0.64

3.73 0.65

3.87 0.67

4.54 0.78

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Fig. 4. Illustration flow for the growing and grain aligning of slice-like Ca3Co4O9 seeds for different samples, a) for E and b) for B.

the as pressure during the sintering procedure is moderately the most important factor for forming well grain alignment for this type of oxide materials. It is inferred that the grain aligned Ca3Co4O9 ceramics are formed as follow. First of all, the Ca3Co4O9 slice-like grains with several hundred nanometers stack along c axis under formation pressure, then the packed Ca3Co4O9 grains tend to grow larger in the sintering atmosphere. For the ceramic samples sintered by conventional furnace, the Ca3Co4O9 grains grow larger under equalized atmospheric pressure; it should be a little bit spherical shaped and the porosity should be greater. The Ca3Co4O9 grains should grow larger under mono-axial pressure in the sintering, too; however, the grains should be a little bit flat shaped. The ceramic should have lower porosity simultaneously. This make the resulting ceramic samples with more c-axis aligned Ca3Co4O9 grains, and therefore there are more and stronger [00l] diffraction peaks than that of the ceramic samples sintered by conventional furnace. In the end, the density and the Lotgering factor L for the ceramic sintered by SPS with sintering pressure have both largest values. It is also true within literatures that the ceramics have better grain alignment and bulk texture for samples prepared with sintering pressure [8–11]. Fig. 5 shows the fractured cross-section SEM images for Ca3Co4O9 ceramic samples E (A) and F (B), respectively, the pressure direction is indicated by arrows within the figures. As shown in Fig. 5, flake like particles can be found with several microns in magnitude. By comparing Fig. 5(A) with Fig. 5(B), one can see that the layered structure is much more weaker for samples prepared without sintering pressure, and the ceramic sample sintered with mono-axis sintering pressure shows strong grain alignment and body texture. This is in accordance with the Lotgering factor results shown in Table 2. Furthermore, it is observed that the ceramic sample prepared by SPS with formation pressure and sintering pressure have consolidated bulk microstructure with lower porosity, this is in agreement with measured density within Table 2. It could also be found that the Lotgering factor L and the density d show positive relationship, inferring that texture formation of bulk is favorable for formation of condensed bulk material. Fig. 6 shows the electrical resistivity for all Ca3Co4O9 ceramic samples. It can be seen from Fig. 6 that the obtained Ca3Co4O9 ceramic samples have wide variety range of resistivity within the measuring temperature region; this means that the titled oxide ceramics could be tuned in terms of the electrical transport by modulating the grain alignment and bulk texture. It is seen from the figure that the ceramics prepared by sintering without sintering pressure have high resistivities, ranging from tens of mΩ cm to several mΩ cm. Furthermore, the resitivities of ceramic samples with lower L values are all moderately higher than that of the ceramic samples with higher L values. This could

Fig. 5. Cross-section SEM images for Ca3Co4O9 ceramic materials, a) for E and b) for F.

Fig. 6. Electrical resistivity for Ca3Co4O9 ceramic samples.

be attributed to the grain alignment and ceramic texture of the obtained samples, and the resistivity ρ of the titled oxide ceramics could be expressed as composing of three parts:

ρ = ρab + ρc − c + ρg − g

(4)

where ρab , ρc − c and ρg − g stand for resistivities along ab plane of the Ca3Co4O9 crystal, across ab plane of the Ca3Co4O9 crystal and beyond Ca3Co4O9 grains, respectively. Fig. 7 presents the SEM image of the Ca3Co4O9 ceramic sample, the ρab , ρc − c and ρg − g are marked out by arrows to give a schematic illustration of these kinds of resistivities to guide the eyes. For the samples with lower grain alignments and bulk textures, scattering should be stronger. As far as those ceramics with lower grain 324

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Fig. 7. SEM image of the Ca3Co4O9 ceramic sample with marked ρab , ρc − c and ρg − g . Fig. 8. Seebeck coefficient α for all Ca3Co4O9 ceramic samples.

alignment, the grain boundary between Ca3Co4O9 grains is larger; at the same time the potential for carriers to cross between ab planes of the Ca3Co4O9 crystals is greater, too. Carriers transport is deteriorated by scattering, thus the ρc − c and ρg − g should be larger than that of the samples with better grain alignment and bulk texture. The F sample with best grain alignment and bulk texture exhibits lowest resitivity because of the minimum ρc − c and ρg − g among all ceramics. The resistivity ρ for the titled oxide ceramics can be expressed as a funcion of carrier concentration and mobility:

ρ = 1/ σ ∝ 1/ pqμ

The analysis of Seebeck coefficient can shed light on the scattering mechanism, so special attention is paid. The Seebeck coefficient for a polycrystalline ceramic can be regarded as a function of the total scattering factor and carrier concentration expressed by:

α ∝ γ − ln nc

where γ is the scattering factor, nc is the carrier concentration [5]. The scattering factor for the titled ceramics should be considered as composing of several kinds of mechanism such as in-layer scattering γab that is originated from the Ca2CoO3 layer and CoO2 layer, the out-layer scattering γc that is originated across these layers and the scattering between grains γg . The total γ can be expressed as:

(5)

where σ is conductivity, p is carrier concentration, q is the charge of the carriers and μ is the carrier mobility[5]. The mobility μ could be fumulated as:

μ=

qτ m

γ = γab + γc + γg

(6)

(9)

The scattering can be intensified by elevating temperature because of the thermally activated lattice vibration and increased phonon quantities expressed as follow:

where τ is mean free time of the carriers, m is effective mass of the carriers. Then the resistivity ρ for the titled ceramics can be deduced as:

ρ = 1/ σ ∝ m / pq2 τ

(8)

(7)

1 ħω ∝ exp( )−1 n¯ kT

It could be considered as hole carrier gas system for the Ca3Co4O9 ceramics. The carrier mean free time τ for single crystal consists of different scattering mechanism such as acoustic phonons scattering, impurities scattering and non-polar optical phonon scattering [15–17]. However the most important mechanism for polycrystalline ceramics should be the scattering between Ca3Co4O9 grains. The mean free time τ should be very small for those samples with low grain alignment and texture. Thus those ceramics with low Lotgering factor L values show larger resistivities, and the resistivity is reversely related to the Lotgering factor L values. It is reported that the Ca3Co4O9 has semiconductor resistivity along ab plane [7]. The F sample has highest grain alignment, most slice like grains tend to align along ab plane of Ca3Co4O9, the ρc − c and ρg − g should be neglected comparing with ρab , the ρab should accounts for the total resistivity ρ . Thus the whole ceramic sample exhibits semiconductor resistivity. Ceramic samples with very low grain alignment and bulk texture exhibit metallic resistivity, that is because of the enlarged ρc − c and ρg − g . The ρc − c and ρg − g should account for the total resistivity ρ , the whole conduction behavior of the ceramic should be the manifestation both of conduction across ab plane of the Ca3Co4O9 crystal and conduction beyond Ca3Co4O9 grains. It has been reported that another Cobalt oxide NaCoO2 has metallic carrier conduction along c axis, too [6]. Although further work should be done, it is inferred that the single crystal for the titled oxide should have metallic carrier conduction along c axis. Fig. 8 shows the Seebeck coefficient α for all Ca3Co4O9 ceramic samples. The α increases with increasing temperature and all samples exhibit positive α values, which indicate their hole carrier conduction.

(10)

where n¯ is phonon quantity, ħ is reduced Plank constant and ω is vibration frequency of the lattice. The phonon quantity n¯ can be increased by increasing temperature; thereby the scattering factor is enhanced. So the Seebeck coefficients for all ceramics are enhanced by increasing temperature. The ceramic samples with lower grain alignment should have larger γc and γg , and accordingly they should have larger Seebeck coefficients. Fig. 9 shows the power factor value P (P = α2/ρ) of all ceramic samples. For all ceramic samples, the values of P are enhanced with increasing temperature, indicating their enhanced electrical performance and potential application within high temperature fields. The ceramic sample with highest grain alignment and texture is found to have the highest power factor value reaching 3.83 μW cm−1 K−2 at 700 °C, which is very much higher than that of the ceramic samples with lower grain alignment and texture, this is chiefly on account of the low resistivity. The electrical conduction of the titled oxide ceramic can be tuned by modulating the grain alignment and bulk texture, through adjusting the preparing procedures. Conclusion To be concluded, the polycrystalline Ca3Co4O9 powders were synthesized by sol-gel method, and the Ca3Co4O9 ceramic samples with diverse grain alignments and bulk textures were prepared, respectively. The Ca3Co4O9 ceramics with uniformly single phases and diverse grain alignment can be fabricated, the grain alignment and bulk texture can 325

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μW cm−1 K−2 at 700 °C. The electrical conduction of the titled ceramic can be tuned by modulating the grain alignment and bulk texture, through adjusting the preparing procedures. Acknowledgments The authors would like to gratefully thank the support provided by National Natural Science Foundation of China (51572066), Henan Natural Science Foundation (162300410007), the science funds for young scholar of Henan Normal University (5101029279071), the doctoral research fund of Henan Normal University (5101026500275) and Basic and Advanced Technology Research Project of Henan Province (132300410071). References [1] Fergus JF. J Euro Ceram Soc 2012;32:525. [2] Peng JY, Liu XY, Fu LW, Xu W, Liu QZ, Yang JY. J Alloys Compds 2012;521:141. [3] Zhu TJ, Xiao K, Yu C, Shen JJ, Yang SH, Zhou AJ, et al. J Appl Phys 2010;108:044903. [4] Zhao LD, Tan G, Hao S, He J, Pei Y, Chi H, et al. Science 2016;351:411. [5] Zhang X, Liu HL, Lu QM, Zhang JX, Zhang FP. Appl Phys Lett 2013;103:063901. [6] Terasaki I, Sasago Y, Uchinokura K. Phys Rev B 1997;56:12685. [7] Shikano M, Funahashi R. Appl Phys Lett 2003;82:1581. [8] Zhang FP, Lu QM, Zhang JX. J Alloy Compd 2009;484:550. [9] Prevel M, Perez O, Noudem JG. Solid State Sci 2007;9:231. [10] Nong NV, Pryds N, Linderoth S, Ohtaki M. Adv Mater 2011;23:2484. [11] Kenfaui D, Chateigner D, Gomina M, Noudem JG. J Alloy Compd 2010;490:472. [12] Itahara H, Xia C, Sugiyama J, Tani T. J Mater Chem 2004;14:61. [13] Tani T, Itahara H, Xia C, Sugiyama J. J Mater Chem 2003;13:1865. [14] Lotgering FK. J Inorg Nucl Chem 1959;9:113. [15] Zhang FP, Zhang X, Lu QM, Zhang JX, Liu YQ, Zhang GZ. Solid State Ionics 2011;201:1. [16] Li JC, Wang CL, Wang MX, Peng H, Zhang RZ, Zhao ML, et al. J Appl Phys 2009;105:043503. [17] Lu ND, Li L, Liu M. Phys Rev B 2015;91:195205.

Fig. 9. Power factor for Ca3Co4O9 ceramic samples.

be enhanced by increasing the formation pressure and by applying the sintering pressure. The slice like Ca3Co4O9 grains tend to align along a and b axis, then the Ca3Co4O9 slices stack along c axis under formation pressure, and the aligned Ca3Co4O9 grains tend to grow larger in the sintering atmosphere. The pressing procedure plays important role in grain alignment for this kind of ceramics, the formation of bulk texture is favorable for condensed bulk material formation. The ceramic samples with lower grain alignment have larger resistivities, this is due to the deteriorated carrier conduction across the sub-layers and between the grains. The out-layer scattering as well as the scattering between grains are responsible for larger Seebeck coefficients of ceramic samples with lower grain alignment. The ceramic sample with highest grain alignment and texture has the highest power factor value reaching 3.83

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