- Email: [email protected]
Fabrication of highly textured Ca3Co4O9 ceramics with controlled density and high thermoelectric power factors
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Original Article
Fabrication of highly textured Ca3Co4O9 ceramics with controlled density and high thermoelectric power factors Rina Shimonishi, Manabu Hagiwara*, Shinobu Fujihara* Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, 223-8522, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Ca3Co4O9 Reactive templated grain growth Porous ceramics Textured ceramics Thermoelectricity
Ca3Co4O9 ceramics have been studied as an alternative p-type thermoelectric material. Thermoelectric properties of the ceramics would be improved by either orientation of grains or introduction of pores. In this study, we fabricated textured Ca3Co4O9 ceramics with controlled density by a reactive-templated grain growth method combined with a hot-forging technique. A powder precursor obtained by mixing β-Co(OH)2 as a template and CaCO3 as a matrix was uniaxially pressed into pellets and sintered under hot-forging pressures up to 5.0 MPa. The relative density of the resulting ceramics was varied between 41.0 and 83.8 % while all the ceramics showed excellent c-axis orientation. The in-plane electrical conductivity of our ceramics could be kept relatively higher than that ever reported previously due to the orientation. Because Seebeck coefficient did not depend on the relative density, the higher electrical conductivity of our ceramics led directly to improved thermoelectric power factors between 67.0 and 409 μW·m−1 K−2.
1. Introduction Thermoelectric materials have been attracting attention from the view point of effective utilization of waste heat. The performance of thermoelectric materials is characterized by the dimensionless figure of merit ZT = (S2σ/κ)T, where S, σ, κ, and T are Seebeck coefficient, electrical conductivity, thermal conductivity, and temperature, respectively. Materials with ZT > 1 is required for thermoelectric modules with a practical level of the energy conversion efficiency. The thermal conductivity of materials with a carrier concentration suitable for thermoelectric applications (1019–1021 cm−3) is often governed by the contribution from phonons (lattice thermal conductivity) [1]. Since phonons have a much longer mean free path compared to electrons (or holes), it is possible to design the microstructure of materials aiming at reducing their lattice thermal conductivity without significant degradation of the electrical conductivity, leading to enhancement of ZT. For example, the all-scale hierarchicalarchitectural design, containing lattice defects, nanoinclusions, and mesoscale grains, has been proposed as an effective way to scatter phonons with the various mean free path [2]. The formation of pores in thermoelectric materials is also known to be effective for enhancing ZT. Khan et al. [3] have recently reported that ZT of the polycrystalline unfilled skutterudite ceramic with nano- to micrometer-sized pores can be increased by more than 100 % compared to that of the dense
⁎
ceramics. Bi2Te3 and PbTe have long been used in practical thermoelectric modules due to their superior performance, but they cannot be operated at temperatures above around 400 and 700 K, respectively [1]. As an alternative material, Ca3Co4O9 (CCO) has been studied extensively because of its high operation temperature and high chemical stability at high temperatures in air. CCO has a layered crystal structure in which CdI2-type CoO2 layers and NaCl-type Ca2CoO3 layers are stacked alternatively along the c-axis. CCO shows a p-type semiconducting behavior due to hopping conduction from Co3+ to Co4+ in the CoO2 layer [4]. Therefore, the electrical conductivity of CCO along the ab plane is much higher than that along the c-axis direction [5]. It is also known that the thermal conductivity of CCO is intrinsically low (2.9 W·m–1 K–1 at 973 K [6]) due to the misfit structure between the CoO2 layer and the Ca2CoO3 layer. As a result, the CCO single crystal shows a high ZT value of 0.89 at an elevated temperature of 973 K [6]. Polycrystalline ceramic materials generally have some practical advantages over single crystals, such as a lower production cost, better formability, and a higher mechanical strength. With regard to the thermoelectric CCO ceramics, a direction for applying a temperature difference should be parallel to the ab plane to utilize their high electrical conductivity. Therefore, the spark plasma sintering (SPS) and the hot pressing (HP) technique have been widely used to produce textured and dense CCO ceramics [7–11]. However, the ZT values of such the
Corresponding authors. E-mail addresses: [email protected] (M. Hagiwara), [email protected] (S. Fujihara).
https://doi.org/10.1016/j.jeurceramsoc.2019.11.077 Received 29 August 2019; Received in revised form 25 November 2019; Accepted 26 November 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Rina Shimonishi, Manabu Hagiwara and Shinobu Fujihara, Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2019.11.077
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
R. Shimonishi, et al.
water for 5 min, and annealed at 873 K for 30 min at a heating rate of 10 K·min–1 before characterizations. Note that the thickness of the ceramic samples was reduced to 1.0–2.5 mm after the hot forging and the polishing treatment.
textured and dense ceramics are still much lower than that of the single crystals [6,12]. Recently, Bittner et al. [13] have reported that a porous CCO ceramic with a low relative density of 68 % shows ZT of 0.4, which is the highest ever reported for undoped CCO ceramics, due to the reduced thermal conductivity. Unfortunately, it seems that orientation of each grain is not enough in this porous ceramic judging from its structural analysis [13]. We consider that, if higher orientation is achieved in porous CCO ceramics, they should have further enhanced ZT. The aforementioned SPS and HP techniques are limited to the fabrication of textured and dense ceramics and hence we need to develop another method. In this study, we employed the reactive-templated grain growth (RTGG) method to fabricate well-textured and porous CCO ceramics. It has been reported by Itahara et al. [14] that β-Co(OH)2, which has CdI2type CoO2 layers similarly to CCO, can be used as a reactive template to fabricate textured and dense CCO ceramics by applying a high uniaxial pressure during sintering. Our challenge is the fabrication of highly textured and porous CCO ceramics with a wide range of the relative density utilizing the RTGG method. β-Co(OH)2 plate-like particles with a high aspect ratio were synthesized by a homogeneous precipitation method and used as the template. The normal uniaxial pressing of a βCo(OH)2–CaCO3 precursor powder resulted in a strong orientation of the β-Co(OH)2 particles in green compacts. The pressureless sintering of the green compact provided a highly textured CCO ceramic with a relative density as low as 41 %. The relative density of textured CCO ceramics could then be controlled in a wide range up to 84 % by applying a uniaxial pressure during sintering. We also demonstrate that the resulting textured CCO ceramics show higher thermoelectric power factors (S2σ), originating from their high electrical conductivity, compared to previously reported CCO ceramics with a similar relative density.
The crystal structure of the samples was identified by X-ray diffraction (XRD) analysis using Cu Kα radiation with a Bruker AXS D8 diffractometer. The degree of the c-axis orientation of the green compact and the ceramics was assessed using the Lotgering factor f [16]. The peak intensity for the random orientation was calculated according to the PDF cards (ICDD 00-030-0443 for β-Co(OH)2 and 00-021-0139 for CCO). The particle morphology and the microstructure of the samples were observed with a field emission scanning electron microscope (FE-SEM; Hitachi S-4700). The average size of particles or grains and its standard deviation were obtained from observed values for 40 particles/grains. Elemental mapping of the green compact was conducted by another FE-SEM (FEI INSPECT F50) equipped with an energy-dispersive X-ray (EDX) spectrometer (AMETEK EDAX). The relative density of the ceramics was calculated from their weight and dimensions. The in-plane electrical conductivity of the ceramics was measured by the van der Pauw method [17] from room temperature to 900 K in air. The in-plane Seebeck coefficient of the ceramics was also measured from room temperature to 900 K in air using an apparatus constructed according to a previous report [18]. Accuracy of the above measurement systems was evaluated with an Nb-doped SrTiO3 single crystal (doping level: 3.3 × 1020 cm−3). It was then estimated to be ± 8 % for the electrical conductivity and ± 11 % for the Seebeck coefficient at room temperature by comparing its measured and reported values [19].
2. Experimental
3. Results and discussion
2.1. Preparation of precursor powder
3.1. Crystalline phase and morphology of a green compact
β-Co(OH)2 plate-like particles were synthesized by a homogeneous precipitation method. 2.50 mmol of CoCl2·6H2O (99.0 %, Kanto Chemical Co., Inc.) and 150 mmol of hexamethylenetetramine (HMT) (99.0 %, FUJIFILM Wako Pure Chemical Corp.) were dissolved in 500 ml of deionized water. The solution was stirred at 363 K for 1 h in an oil bath. Then, a beaker containing the resulting pink suspension was put in ice water for cooling immediately down to about room temperature. The β-Co(OH)2 particles were collected from the suspension by suction filtration, followed by washing with deionized water and drying at 333 K, for observing their morphology. 1.91 mmol of calcite CaCO3 (> 99 %, Kojundo Chemical Laboratory Co., Ltd.), which had been ball-milled for 24 h beforehand, was added directly to the abovementioned suspension. The suspension was then stirred under ultrasonication at room temperature for 10 min. The β-Co(OH)2–CaCO3 precursor powder was collected by filtration, washed with deionized water, and dried at 333 K overnight.
The as-synthesized β-Co(OH)2 particles have a hexagonal plate-like morphology with an effective diameter (the length of the in-plane diagonal) of 7.1 ± 1.4 μm, a thickness of 180 ± 70 nm, and an aspect ratio (diameter/thickness) of about 40, as observed in Fig. 1(a) and (b). Such the hexagonal shape corresponds to the (001)-faceted β-Co(OH)2 particles formed during the homogeneous precipitation. The ball-milled calcite powder (Fig. 1(c)) has an average size below 1.5 μm and an irregular morphology. The β-Co(OH)2 particles can maintain their platelike morphology in the mixed β-Co(OH)2–CaCO3 precursor powder (Fig. 1(d)). The XRD pattern of the pressed green compact shows a strong (001) peak of β-Co(OH)2 (Fig. S1, Supplementary Information). The Lotgering factor f for the c-axis of β-Co(OH)2 is calculated to be as high as 0.97, indicating that the β-Co(OH)2 particles are strongly oriented by the normal uniaxial pressing, as we have reported recently for a similar Cobased hydroxide [20]. In the previous study on the RTGG method of CCO [21], the tape-casting technique was employed to orient β-Co (OH)2 plate-like particles with a low aspect ratio of 5 [22]. Our β-Co (OH)2 plate-like particles have the much higher aspect ratio of 40 so that they can be easily oriented by the normal uniaxial pressing. An SEM-EDX mapping image of a fractured surface of the green compact (Fig. S2) shows that the β-Co(OH)2 particles are stacked along the pressing direction and surrounded by the smaller CaCO3 particles, agreeing well with the high f value of the green compact.
2.3. Characterizations and thermoelectric measurements
2.2. Fabrication of green compacts and ceramics The precursor powder was uniaxially pressed under pressure of 500 MPa into green compacts with a diameter of 6.0–6.1 mm and a thickness of 2.9–3.0 mm. The green compacts were heated to 1173 K at a heating rate of 10 K·min–1 in air, and kept for 1 h to obtain ceramic samples. During heating, a uniaxial pressure of 0.0, 0.4, 2.1, or 5.0 MPa was applied to the green compacts along the thickness direction, while their side was kept mechanically free. In other words, the ceramic samples were prepared by a pressure sintering technique known as hot forging [15]. Hereafter a ceramic sample sintered under a uniaxial pressure of p MPa will be referred as CCO-p. The resulting ceramics were polished by abrasive papers, washed by ultrasonication in distilled
3.2. Crystalline phase and morphology of ceramics The XRD patterns of crushed powders of CCO-p ceramic samples (p = 0.0, 0.4, 2.1, 5.0) confirm that all the samples are the single phase of CCO (Fig. S3). The XRD patterns collected from the polished surface 2
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
R. Shimonishi, et al.
Fig. 1. FE-SEM images of (a, b) the as-synthesized β-Co(OH)2 powder, (c) the CaCO3 powder after ball milling, and (d) the β-Co(OH)2–CaCO3 precursor powder after ultrasonic dispersion.
densification, the highly oriented CCO ceramics with a wide range of relative density can be obtained. It is important to note that the hotforging pressure in our method is required just for promoting the densification and not necessary for achieving the high degree of orientation. Fig. 3 shows FE-SEM images of a cross-sectional fractured surface of CCO-0.4 and 5.0. The vertical direction of the images corresponds to the thickness direction of the ceramics. Plate-like grains are observed in the fractured surface of both samples. It is seen especially for CCO-5.0 that the plate-like grains are stacked along the thickness direction of the ceramic. This observation agrees with the high Lotgering factor of 0.97 of this sample. It is also seen that the morphology of the plate-like grains changes by the hot-forging pressure. The thin β-Co(OH)2 platelike particles as the template are surrounded by the low-density CaCO3 matrix at the low hot-forging pressure. The reactive grain growth then occurs separately for the respective templates, resulting in the individually grown plate-like grains (Fig. 3(a)). At the high hot-forging pressure, the grains are formed by the growth of several templates reacting together with the matrix and stacked along the hot-forging pressure direction (Fig. 3(b)). As summarized in Table 1, the average diameter of the plate-like grains does not change largely among the CCO-p samples. This indicates that the grain growth in the diameter direction is limited by the size of the template.
of the same CCO-p samples placed horizontally on the sample stage show diffraction peaks only from the (00l) lattice planes (Fig. 2(a)), clearly indicating that the CCO grains are c-axis-oriented in these ceramics. The f values and relative densities of the ceramics are summarized in Table 1. The relative density of the ceramics is found to be drastically increased from 41.0 % for CCO-0.0–83.8 % for CCO-5.0 by increasing the hot-forging pressure. On the other hand, the f value is as high as 0.89 even for the sample sintered without pressing (CCO-0.0) and increases further with increasing the hot-forging pressure. Fig. 2(b) shows the relationship between the relative density and the f value. For comparison, the data of previously reported CCO ceramics, which were prepared by SPS, HP, or hot forging [11,23–28], are also plotted in the figure. In the reported data, there appears a trend that the lowering of the relative density leads to the significant decrease in the f value (the shaded region), and thus a high f value over 0.9 can be obtained only in dense ceramics with a relative density over 90 %. On the other hand, our ceramics maintain high f values above 0.89 over a wide range of their relative density between 41.0 and 83.8 %. The different orientation behavior of our samples from the previously reported ones is explained mainly by the morphology of the particles used for compacting. In the previous reports, particles of CCO were once prepared by the solid-state reaction [11,23,25,27], the citric acid complex method [24,26], or the RTGG method [28] and then pressed into pellets for sintering by SPS, HP, or hot forging. Because of the low aspect ratio of the CCO particles (13 for the solid-state reaction and 5 for the citric acid complex method [23,26]), the additional sintering techniques like SPS and HP are required to obtain highly textured ceramics. Since the additional sintering promotes simultaneously the grain orientation and the densification, the f value of the resultant ceramics depends highly on their final density. Our β-Co(OH)2 template particles having the much higher aspect ratio of 40 are already oriented well in the green compact. They maintain their high orientation degree during both the pyrolysis into Co3O4 and the subsequent solid-state reaction with calcite, leading to the formation of highly textured CCO ceramics [21]. Since this process does not necessarily require the
3.3. Thermoelectric properties of ceramics We measured the temperature dependence of the in-plane electrical conductivity (σ) and Seebeck coefficient (S) of CCO-p from room temperature to 900 K, as shown in Fig. 4(a) and (b), respectively. All the samples show a similar temperature dependence of the conductivity with an indication of the metal–semiconductor transition around 380 K, which is consistent with a previous report [25]. It is found that the conductivity increases largely with the hot-forging pressure increasing. Consequently, CCO-5.0 shows the highest electrical conductivity among the samples in the whole temperature range. As discussed above, the 3
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
R. Shimonishi, et al.
regardless of their wide variety of the relative density. Fig. 4(c) shows the temperature dependence of the in-plane power factor (S2σ) from room temperature to 900 K. The power factor increases by both density and temperature, according to the temperature and density dependences of the electrical conductivity and the Seebeck coefficient. Especially, the power factor of CCO-5.0 reaches as high as 400 μW·m−1 K−2 at the maximum temperature of 900 K. The electrical conductivity, the Seebeck coefficient, and the power factor of the CCO-p samples at 900 K are plotted against their relative density in Fig. 4(d)–(f), together with the respective values reported in the literature [11,13,23–25,30–32]. Note that the electrical conductivity, the Seebeck coefficient, and the power factor taken from Ref. [32] are the values measured at 860 K. In Fig. 4(d), a positive correlation between the electrical conductivity and the relative density is found for the literature values. Our samples basically follow this trend, but each of them exhibits the highest electrical conductivity when compared to the previously reported ceramics with a similar relative density. The Lotgering factor f of the ceramics in the previous reports largely decreases with the relative density decreasing, as already shown in Fig. 2(b). In the less oriented ceramics with the lower f values, the path length of the holes is elongated because the hole-conducting CoO2 layers are not aligned in the in-plane direction. This leads to the significant degradation of the macroscopic electrical conductivity. On the other hand, the f value of our ceramics remains high even for CCO-0.0 with the lowest density of 41.0 % as shown in Fig. 2(b), guaranteeing the smooth transport of the holes. Therefore, our samples show higher electrical conductivity than those in the previous reports. The Seebeck coefficients of our ceramics exhibit slightly higher values compared with most of the reported values (Fig. 4(e)). Since the Seebeck coefficient can be affected by many microstructural parameters such as the grain size and orientation, a full explanation of the difference between CCO-p and the previous samples is quite difficult. As shown in Fig. 4(f), all the CCO-p samples exhibit a higher power factor compared to that of the previously reported ceramics with a similar relative density. This is due to the improved electrical conductivity originating from the high grain orientation, demonstrating that the RTGG method employed in this study is effective for fabricating CCO ceramics with a superior thermoelectric property. It is difficult to discuss the accurate ZT value of CCO-p at present because the measurement of the thermal conductivity along the in-plane direction has not been achieved due to the small thickness of the ceramic samples. Nevertheless the enhanced power factors of the CCO-p samples are suggestive of their high ZT values because the introduction of pores has been reported to reduce effectively the lattice thermal conductivity of a wide variety of materials, leading to an enhanced ZT value [3,33,34].
Fig. 2. (a) XRD patterns of the bulk ceramics of CCO-p (p = 0.0, 0.4, 2.1, and 5.0) samples. The samples were set on the sample stage in the same way with the green compact as shown in Fig. S1. (b) Relationship between the relative density and the Lotgering factor for (00l) of CCO. Previous data for CCO ceramics taken from the literature [11,23–28] are also plotted in (b) for comparison. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). Table 1 The Lotgering factor (f), relative density, and grain diameter for the CCO-p ceramics (p = 0.0, 0.4, 2.1, and 5.0). Sample
Lotgering factor (f)
Relative density(%)
Diameter/μm
CCO-0.0 CCO-0.4 CCO-2.1 CCO-5.0
0.89 0.93 0.95 0.97
41.0 61.5 75.1 83.8
2.0 1.9 1.8 2.3
± ± ± ±
0.2 0.3 0.3 1.2
± ± ± ±
4. Conclusions We fabricated the textured CCO ceramics with a high c-axis orientation by the RTGG method combined with the hot-forging technique. The β-Co(OH)2 plate-like template particles synthesized by the homogeneous precipitation method had a high aspect ratio of 40, and hence they were easily oriented in the green compact by the normal uniaxial pressing. It was found that the density of the final CCO ceramics could be controlled in a wide range between 41.0 and 83.8 % by changing the hot-forging pressure, while their high Lotgering factor over 0.89 was retained. Owing to the improved electrical conductivity, all the resulting porous CCO ceramics showed enhanced power factors between 67.0 and 409 μW·m−1 K−2 compared to the previously reported CCO ceramics having a similar density. The fabrication method developed here opens a new way of microstructure control of layered cobaltite thermoelectrics.
0.7 0.5 0.6 1.1
decrease in the density is associated with the individual growth of the grains. Insufficient connection between the grains narrows the effective cross section for holes to pass through the entire ceramics. On the other hand, the Seebeck coefficient is almost the same for the CCO-p samples within the standard deviation; it increases monotonically from about 120 (300 K) to about 180 μV·K−1 (900 K). The porous CCO ceramic can be regarded as a composite of CCO and air. According to the effectivemedium-theory [7,29], the Seebeck coefficient of a composite consisting of a conductor (CCO) and an insulator (air) is attributed only to that of the conductor, regardless of the volume ratio of the two phases. As a result, the CCO-p samples exhibit similar Seebeck coefficients
Declaration of Competing Interest The authors declare that they have no known competing financial 4
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
R. Shimonishi, et al.
Fig. 3. FE-SEM images of a fractured surface of (a) CCO-0.4 and (b) CCO-5.0.
Fig. 4. Temperature dependence of (a) the in-plane electrical conductivity, (b) the Seebeck coefficient, and (c) the power factor of CCO-p (p = 0.0, 0.4, 2.1, and 5.0) from room temperature to 900 K, and relationship between the relative density and (d) the electrical conductivity (σ900), (e) the Seebeck coefficient (S900), and (f) the power factor (S2σ900) at 900 K. Previous data for the CCO ceramics taken from the literature [11,13,23–25,30–32] are also plotted in (d–f) for comparison.
interests or personal relationships that could have appeared to influence the work reported in this paper
[3]
Acknowledgment [4]
One of the authors (R.S.) thanks Kato Foundation for Promotion of Science for its financial support.
[5]
Appendix A. Supplementary data
[6]
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jeurceramsoc.2019.11. 077.
[7]
[8] [9]
References
[10]
[1] G.J. Snyder, E.S. Toberer, Complex thermoelectric materials, Nat. Mater. 7 (2008) 105–114. [2] K. Biswas, J. He, I.D. Blum, C.-I. Wu, T.P. Hogan, D.N. Seidman, V.P. Dravid,
5
M.G. Kanatzidis, High-performance bulk thermoelectrics with all-scale hierarchical architectures, Nature 489 (2012) 414–418. A.U. Khan, K. Kobayashi, D.-M. Tang, Y. Yamauchi, K. Hasegawa, M. Mitome, Y. Xue, B. Jiang, K. Tsuchiya, D. Golberg, Y. Bando, T. Mori, Nano-micro-porous skutterudites with 100% enhancement in ZT for high performance thermoelectricity, Nano Energy 31 (2017) 152–159. W. Koshibae, K. Tsutsui, S. Maekawa, Thermopower in cobalt oxides, Phys. Rev. B 62 (2000) 6869–6872. B.C. Zhao, Y.P. Sun, W.J. Lu, X.B. Zhu, W.H. Song, Enhanced spin fluctuations in Ca3Co4−xTixO9 single crystals, Phys. Rev. B 74 (2006) 144417. M. Shikano, R. Funahashi, Electrical and thermal properties of single-crystalline (Ca2CoO3)0.7CoO2 with a Ca3Co4O9 structure, Appl. Phys. Lett. 82 (2003) 1851–1853. Y. Liu, Y. Lin, Z. Shi, C.-W. Nan, Z. Shen, Preparation of Ca3Co4O9 and improvement of its thermoelectric properties by spark plasma sintering, J. Am. Ceram. Soc. 88 (2005) 1337–1340. D. Wang, L. Chen, Q. Yao, J. Li, High-temperature thermoelectric properties of Ca3Co4O9+δ with Eu substitution, Solid State Commun. 129 (2004) 615–618. D. Kenfaui, D. Chateigner, M. Gomina, J.G. Noudem, Texture, mechanical and thermoelectric properties of Ca3Co4O9 ceramics, J. Alloys Compd. 490 (2010) 472–479. G. Xu, R. Funahashi, M. Shikano, I. Matsubara, Y. Zhou, Thermoelectric properties of the Bi- and Na- substituted Ca3Co4O9 system, Appl. Phys. Lett. 80 (2002) 3760–3762.
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
R. Shimonishi, et al.
[24] S. Katsuyama, M. Ito, Synthesis of Ca3Co4O9 ceramics by citric acid complex and hydrothermal hot-pressing processes and its thermoelectric properties, Proc. 25th International Conference on Thermoelectrics, IEEE, Piscataway, USA, 2006, pp. 543–547. [25] Y. Wang, Y. Sui, X. Wang, W. Su, X. Liu, Enhanced high temperature thermoelectric characteristics of transition metals doped Ca3Co4O9+δ by cold high-pressure fabrication, J. Appl. Phys. 107 (2010) 033708. [26] Q.M. Lu, J.X. Zhang, Q.Y. Zhang, Y.Q. Liu, D.M. Liu, Improved thermoelectric properties of Ca3-xBaxCo4O9 (x = 0∼0.4) bulks by sol-gel and SPS method, Proc. 25th International Conference on Thermoelectrics, IEEE, Piscataway, USA, 2006, pp. 66–69. [27] C.-H. Lim, S.-M. Choi, W.-S. Seo, K.H. Kim, J.-Y. Kim, H.-H. Park, Improvements of thermoelectric transport properties for a partially substituted Ca3Co4O9 system by spark plasma sintering, J. Ceram. Process. Res. 13 (2012) 197–201. [28] H. Itahara, J. Sugiyama, T. Tani, Enhancement of electrical conductivity in thermoelectric [Ca2CoO3]0.62[CoO2] ceramics by texture improvement, Jpn. J. Appl. Phys. 43 (2004) 5134–5139. [29] I. Webman, J. Jortner, M.H. Cohen, Thermoelectric power in inhomogeneous materials, Phys. Rev. B 16 (1977) 2959–2964. [30] M. Gunes, M. Ozenbas, Effect of grain size and porosity on phonon scattering enhancement of Ca3Co4O9, J. Alloys Compd. 626 (2015) 360–367. [31] K. Obata, Y. Chonan, T. Komiyama, T. Aoyama, H. Yamaguchi, S. Sugiyama, Grainoriented Ca3Co4O9 thermoelectric oxide ceramics prepared by solid-state reaction, J. Electron. Mater. 42 (2013) 2221–2226. [32] J.G. Noudem, R. Retoux, S. Lanfredi, Spark plasma texturing (SPT) of p-type [Ca2CoO3]0.62CoO2 thermoelectric oxide, J. Alloys Compd. 676 (2016) 499–504. [33] H. Ning, G.D. Mastrorillo, S. Grasso, B. Du, T. Mori, C. Hu, Y. Xu, K. Simpson, G. Maizza, M.J. Reece, Enhanced thermoelectric performance of porous magnesium tin silicide prepared using pressure-less spark plasma sintering, J. Mater. Chem. A 3 (2015) 17426–17432. [34] B. Xu, T. Feng, Z. Li, S.T. Pantelides, Y. Wu, Constructing highly porous thermoelectric monoliths with high-performance and improved portability from solutionsynthesized shape-controlled nanocrystals, Nano Lett. 18 (2018) 4034–4039.
[11] P.-H. Xiang, Y. Kinemuchi, H. Kaga, K. Watari, Fabrication and thermoelectric properties of Ca3Co4O9/Ag composites, J. Alloys Compd. 454 (2008) 364–369. [12] H. Ohta, K. Sugiura, K. Koumoto, Recent progress in oxide thermoelectric materials: p-type Ca3Co4O9 and n-type SrTiO3, Inorg. Chem. 47 (2008) 8429–8436. [13] M. Bittner, L. Helmich, F. Nietschke, B. Geppert, O. Oeckler, A. Feldhoff, Porous Ca3Co4O9 with enhanced thermoelectric properties derived from sol–gel synthesis, J. Eur. Ceram. Soc. 37 (2017) 3909–3915. [14] H. Itahara, C. Xia, J. Sugiyama, T. Tani, Fabrication of textured thermoelectric layered cobaltites with various rock salt-type layers by using β-Co(OH)2 platelets as reactive templates, J. Mater. Chem. 14 (2004) 61–66. [15] N. Murayama, J.B. Vander Sande, Hot forging with heat treatment of Bi-Pb-Sr-CaCu-O superconductors, Phys. C 241 (1995) 235–246. [16] F.K. Lotgering, Topotactical reactions with ferrimagnetic oxides having hexagonal crystal structures–I, J. Inorg. Nucl. Chem. 9 (1959) 113–123. [17] L.J. van der Pauw, A method of measuring specific resistivity and Hall effect of discs of arbitrary shape, Philips Res. Rep. 13 (1958) 1–9. [18] M. Schrade, H. Fjeld, T. Norby, T.G. Finstad, Versatile apparatus for thermoelectric characterization of oxides at high temperatures, Rev. Sci. Instrum. 85 (2014) 103906. [19] S. Ohta, T. Nomura, H. Ohta, K. Koumoto, High-temperature carrier transport and thermoelectric properties of heavily La- or Nb-doped SrTiO3 single crystals, J. Appl. Phys. 97 (2005) 034106. [20] R. Shimonishi, M. Hagiwara, S. Fujihara, Synthesis of Ca–Co hydroxides and their use in facile fabrication of textured CayCoO2 thermoelectric ceramics, Ceram. Int. 45 (2019) 3600–3607. [21] H. Itahara, W.-S. Seo, S. Lee, H. Nozaki, T. Tani, K. Koumoto, The formation mechanism of a textured ceramic of thermoelectric [Ca2CoO3]0.62[CoO2] on β-Co(OH)2 templates through in situ topotactic conversion, J. Am. Chem. Soc. 127 (2005) 6367–6373. [22] H. Itahara, S. Tajima, T. Tani, Synthesis of β-Co(OH)2 platelets by precipitation and hydrothermal methods, J. Ceram. Soc. Jpn. 110 (2002) 1048–1052. [23] M. Presečnik, J. de Boor, S. Bernik, Synthesis of single-phase Ca3Co4O9 ceramics and their processing for a microstructure-enhanced thermoelectric performance, Ceram. Int. 42 (2016) 7315–7327.
6
Contents lists available at ScienceDirect
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Original Article
Fabrication of highly textured Ca3Co4O9 ceramics with controlled density and high thermoelectric power factors Rina Shimonishi, Manabu Hagiwara*, Shinobu Fujihara* Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, 223-8522, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Ca3Co4O9 Reactive templated grain growth Porous ceramics Textured ceramics Thermoelectricity
Ca3Co4O9 ceramics have been studied as an alternative p-type thermoelectric material. Thermoelectric properties of the ceramics would be improved by either orientation of grains or introduction of pores. In this study, we fabricated textured Ca3Co4O9 ceramics with controlled density by a reactive-templated grain growth method combined with a hot-forging technique. A powder precursor obtained by mixing β-Co(OH)2 as a template and CaCO3 as a matrix was uniaxially pressed into pellets and sintered under hot-forging pressures up to 5.0 MPa. The relative density of the resulting ceramics was varied between 41.0 and 83.8 % while all the ceramics showed excellent c-axis orientation. The in-plane electrical conductivity of our ceramics could be kept relatively higher than that ever reported previously due to the orientation. Because Seebeck coefficient did not depend on the relative density, the higher electrical conductivity of our ceramics led directly to improved thermoelectric power factors between 67.0 and 409 μW·m−1 K−2.
1. Introduction Thermoelectric materials have been attracting attention from the view point of effective utilization of waste heat. The performance of thermoelectric materials is characterized by the dimensionless figure of merit ZT = (S2σ/κ)T, where S, σ, κ, and T are Seebeck coefficient, electrical conductivity, thermal conductivity, and temperature, respectively. Materials with ZT > 1 is required for thermoelectric modules with a practical level of the energy conversion efficiency. The thermal conductivity of materials with a carrier concentration suitable for thermoelectric applications (1019–1021 cm−3) is often governed by the contribution from phonons (lattice thermal conductivity) [1]. Since phonons have a much longer mean free path compared to electrons (or holes), it is possible to design the microstructure of materials aiming at reducing their lattice thermal conductivity without significant degradation of the electrical conductivity, leading to enhancement of ZT. For example, the all-scale hierarchicalarchitectural design, containing lattice defects, nanoinclusions, and mesoscale grains, has been proposed as an effective way to scatter phonons with the various mean free path [2]. The formation of pores in thermoelectric materials is also known to be effective for enhancing ZT. Khan et al. [3] have recently reported that ZT of the polycrystalline unfilled skutterudite ceramic with nano- to micrometer-sized pores can be increased by more than 100 % compared to that of the dense
⁎
ceramics. Bi2Te3 and PbTe have long been used in practical thermoelectric modules due to their superior performance, but they cannot be operated at temperatures above around 400 and 700 K, respectively [1]. As an alternative material, Ca3Co4O9 (CCO) has been studied extensively because of its high operation temperature and high chemical stability at high temperatures in air. CCO has a layered crystal structure in which CdI2-type CoO2 layers and NaCl-type Ca2CoO3 layers are stacked alternatively along the c-axis. CCO shows a p-type semiconducting behavior due to hopping conduction from Co3+ to Co4+ in the CoO2 layer [4]. Therefore, the electrical conductivity of CCO along the ab plane is much higher than that along the c-axis direction [5]. It is also known that the thermal conductivity of CCO is intrinsically low (2.9 W·m–1 K–1 at 973 K [6]) due to the misfit structure between the CoO2 layer and the Ca2CoO3 layer. As a result, the CCO single crystal shows a high ZT value of 0.89 at an elevated temperature of 973 K [6]. Polycrystalline ceramic materials generally have some practical advantages over single crystals, such as a lower production cost, better formability, and a higher mechanical strength. With regard to the thermoelectric CCO ceramics, a direction for applying a temperature difference should be parallel to the ab plane to utilize their high electrical conductivity. Therefore, the spark plasma sintering (SPS) and the hot pressing (HP) technique have been widely used to produce textured and dense CCO ceramics [7–11]. However, the ZT values of such the
Corresponding authors. E-mail addresses: [email protected] (M. Hagiwara), [email protected] (S. Fujihara).
https://doi.org/10.1016/j.jeurceramsoc.2019.11.077 Received 29 August 2019; Received in revised form 25 November 2019; Accepted 26 November 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Rina Shimonishi, Manabu Hagiwara and Shinobu Fujihara, Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2019.11.077
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
R. Shimonishi, et al.
water for 5 min, and annealed at 873 K for 30 min at a heating rate of 10 K·min–1 before characterizations. Note that the thickness of the ceramic samples was reduced to 1.0–2.5 mm after the hot forging and the polishing treatment.
textured and dense ceramics are still much lower than that of the single crystals [6,12]. Recently, Bittner et al. [13] have reported that a porous CCO ceramic with a low relative density of 68 % shows ZT of 0.4, which is the highest ever reported for undoped CCO ceramics, due to the reduced thermal conductivity. Unfortunately, it seems that orientation of each grain is not enough in this porous ceramic judging from its structural analysis [13]. We consider that, if higher orientation is achieved in porous CCO ceramics, they should have further enhanced ZT. The aforementioned SPS and HP techniques are limited to the fabrication of textured and dense ceramics and hence we need to develop another method. In this study, we employed the reactive-templated grain growth (RTGG) method to fabricate well-textured and porous CCO ceramics. It has been reported by Itahara et al. [14] that β-Co(OH)2, which has CdI2type CoO2 layers similarly to CCO, can be used as a reactive template to fabricate textured and dense CCO ceramics by applying a high uniaxial pressure during sintering. Our challenge is the fabrication of highly textured and porous CCO ceramics with a wide range of the relative density utilizing the RTGG method. β-Co(OH)2 plate-like particles with a high aspect ratio were synthesized by a homogeneous precipitation method and used as the template. The normal uniaxial pressing of a βCo(OH)2–CaCO3 precursor powder resulted in a strong orientation of the β-Co(OH)2 particles in green compacts. The pressureless sintering of the green compact provided a highly textured CCO ceramic with a relative density as low as 41 %. The relative density of textured CCO ceramics could then be controlled in a wide range up to 84 % by applying a uniaxial pressure during sintering. We also demonstrate that the resulting textured CCO ceramics show higher thermoelectric power factors (S2σ), originating from their high electrical conductivity, compared to previously reported CCO ceramics with a similar relative density.
The crystal structure of the samples was identified by X-ray diffraction (XRD) analysis using Cu Kα radiation with a Bruker AXS D8 diffractometer. The degree of the c-axis orientation of the green compact and the ceramics was assessed using the Lotgering factor f [16]. The peak intensity for the random orientation was calculated according to the PDF cards (ICDD 00-030-0443 for β-Co(OH)2 and 00-021-0139 for CCO). The particle morphology and the microstructure of the samples were observed with a field emission scanning electron microscope (FE-SEM; Hitachi S-4700). The average size of particles or grains and its standard deviation were obtained from observed values for 40 particles/grains. Elemental mapping of the green compact was conducted by another FE-SEM (FEI INSPECT F50) equipped with an energy-dispersive X-ray (EDX) spectrometer (AMETEK EDAX). The relative density of the ceramics was calculated from their weight and dimensions. The in-plane electrical conductivity of the ceramics was measured by the van der Pauw method [17] from room temperature to 900 K in air. The in-plane Seebeck coefficient of the ceramics was also measured from room temperature to 900 K in air using an apparatus constructed according to a previous report [18]. Accuracy of the above measurement systems was evaluated with an Nb-doped SrTiO3 single crystal (doping level: 3.3 × 1020 cm−3). It was then estimated to be ± 8 % for the electrical conductivity and ± 11 % for the Seebeck coefficient at room temperature by comparing its measured and reported values [19].
2. Experimental
3. Results and discussion
2.1. Preparation of precursor powder
3.1. Crystalline phase and morphology of a green compact
β-Co(OH)2 plate-like particles were synthesized by a homogeneous precipitation method. 2.50 mmol of CoCl2·6H2O (99.0 %, Kanto Chemical Co., Inc.) and 150 mmol of hexamethylenetetramine (HMT) (99.0 %, FUJIFILM Wako Pure Chemical Corp.) were dissolved in 500 ml of deionized water. The solution was stirred at 363 K for 1 h in an oil bath. Then, a beaker containing the resulting pink suspension was put in ice water for cooling immediately down to about room temperature. The β-Co(OH)2 particles were collected from the suspension by suction filtration, followed by washing with deionized water and drying at 333 K, for observing their morphology. 1.91 mmol of calcite CaCO3 (> 99 %, Kojundo Chemical Laboratory Co., Ltd.), which had been ball-milled for 24 h beforehand, was added directly to the abovementioned suspension. The suspension was then stirred under ultrasonication at room temperature for 10 min. The β-Co(OH)2–CaCO3 precursor powder was collected by filtration, washed with deionized water, and dried at 333 K overnight.
The as-synthesized β-Co(OH)2 particles have a hexagonal plate-like morphology with an effective diameter (the length of the in-plane diagonal) of 7.1 ± 1.4 μm, a thickness of 180 ± 70 nm, and an aspect ratio (diameter/thickness) of about 40, as observed in Fig. 1(a) and (b). Such the hexagonal shape corresponds to the (001)-faceted β-Co(OH)2 particles formed during the homogeneous precipitation. The ball-milled calcite powder (Fig. 1(c)) has an average size below 1.5 μm and an irregular morphology. The β-Co(OH)2 particles can maintain their platelike morphology in the mixed β-Co(OH)2–CaCO3 precursor powder (Fig. 1(d)). The XRD pattern of the pressed green compact shows a strong (001) peak of β-Co(OH)2 (Fig. S1, Supplementary Information). The Lotgering factor f for the c-axis of β-Co(OH)2 is calculated to be as high as 0.97, indicating that the β-Co(OH)2 particles are strongly oriented by the normal uniaxial pressing, as we have reported recently for a similar Cobased hydroxide [20]. In the previous study on the RTGG method of CCO [21], the tape-casting technique was employed to orient β-Co (OH)2 plate-like particles with a low aspect ratio of 5 [22]. Our β-Co (OH)2 plate-like particles have the much higher aspect ratio of 40 so that they can be easily oriented by the normal uniaxial pressing. An SEM-EDX mapping image of a fractured surface of the green compact (Fig. S2) shows that the β-Co(OH)2 particles are stacked along the pressing direction and surrounded by the smaller CaCO3 particles, agreeing well with the high f value of the green compact.
2.3. Characterizations and thermoelectric measurements
2.2. Fabrication of green compacts and ceramics The precursor powder was uniaxially pressed under pressure of 500 MPa into green compacts with a diameter of 6.0–6.1 mm and a thickness of 2.9–3.0 mm. The green compacts were heated to 1173 K at a heating rate of 10 K·min–1 in air, and kept for 1 h to obtain ceramic samples. During heating, a uniaxial pressure of 0.0, 0.4, 2.1, or 5.0 MPa was applied to the green compacts along the thickness direction, while their side was kept mechanically free. In other words, the ceramic samples were prepared by a pressure sintering technique known as hot forging [15]. Hereafter a ceramic sample sintered under a uniaxial pressure of p MPa will be referred as CCO-p. The resulting ceramics were polished by abrasive papers, washed by ultrasonication in distilled
3.2. Crystalline phase and morphology of ceramics The XRD patterns of crushed powders of CCO-p ceramic samples (p = 0.0, 0.4, 2.1, 5.0) confirm that all the samples are the single phase of CCO (Fig. S3). The XRD patterns collected from the polished surface 2
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
R. Shimonishi, et al.
Fig. 1. FE-SEM images of (a, b) the as-synthesized β-Co(OH)2 powder, (c) the CaCO3 powder after ball milling, and (d) the β-Co(OH)2–CaCO3 precursor powder after ultrasonic dispersion.
densification, the highly oriented CCO ceramics with a wide range of relative density can be obtained. It is important to note that the hotforging pressure in our method is required just for promoting the densification and not necessary for achieving the high degree of orientation. Fig. 3 shows FE-SEM images of a cross-sectional fractured surface of CCO-0.4 and 5.0. The vertical direction of the images corresponds to the thickness direction of the ceramics. Plate-like grains are observed in the fractured surface of both samples. It is seen especially for CCO-5.0 that the plate-like grains are stacked along the thickness direction of the ceramic. This observation agrees with the high Lotgering factor of 0.97 of this sample. It is also seen that the morphology of the plate-like grains changes by the hot-forging pressure. The thin β-Co(OH)2 platelike particles as the template are surrounded by the low-density CaCO3 matrix at the low hot-forging pressure. The reactive grain growth then occurs separately for the respective templates, resulting in the individually grown plate-like grains (Fig. 3(a)). At the high hot-forging pressure, the grains are formed by the growth of several templates reacting together with the matrix and stacked along the hot-forging pressure direction (Fig. 3(b)). As summarized in Table 1, the average diameter of the plate-like grains does not change largely among the CCO-p samples. This indicates that the grain growth in the diameter direction is limited by the size of the template.
of the same CCO-p samples placed horizontally on the sample stage show diffraction peaks only from the (00l) lattice planes (Fig. 2(a)), clearly indicating that the CCO grains are c-axis-oriented in these ceramics. The f values and relative densities of the ceramics are summarized in Table 1. The relative density of the ceramics is found to be drastically increased from 41.0 % for CCO-0.0–83.8 % for CCO-5.0 by increasing the hot-forging pressure. On the other hand, the f value is as high as 0.89 even for the sample sintered without pressing (CCO-0.0) and increases further with increasing the hot-forging pressure. Fig. 2(b) shows the relationship between the relative density and the f value. For comparison, the data of previously reported CCO ceramics, which were prepared by SPS, HP, or hot forging [11,23–28], are also plotted in the figure. In the reported data, there appears a trend that the lowering of the relative density leads to the significant decrease in the f value (the shaded region), and thus a high f value over 0.9 can be obtained only in dense ceramics with a relative density over 90 %. On the other hand, our ceramics maintain high f values above 0.89 over a wide range of their relative density between 41.0 and 83.8 %. The different orientation behavior of our samples from the previously reported ones is explained mainly by the morphology of the particles used for compacting. In the previous reports, particles of CCO were once prepared by the solid-state reaction [11,23,25,27], the citric acid complex method [24,26], or the RTGG method [28] and then pressed into pellets for sintering by SPS, HP, or hot forging. Because of the low aspect ratio of the CCO particles (13 for the solid-state reaction and 5 for the citric acid complex method [23,26]), the additional sintering techniques like SPS and HP are required to obtain highly textured ceramics. Since the additional sintering promotes simultaneously the grain orientation and the densification, the f value of the resultant ceramics depends highly on their final density. Our β-Co(OH)2 template particles having the much higher aspect ratio of 40 are already oriented well in the green compact. They maintain their high orientation degree during both the pyrolysis into Co3O4 and the subsequent solid-state reaction with calcite, leading to the formation of highly textured CCO ceramics [21]. Since this process does not necessarily require the
3.3. Thermoelectric properties of ceramics We measured the temperature dependence of the in-plane electrical conductivity (σ) and Seebeck coefficient (S) of CCO-p from room temperature to 900 K, as shown in Fig. 4(a) and (b), respectively. All the samples show a similar temperature dependence of the conductivity with an indication of the metal–semiconductor transition around 380 K, which is consistent with a previous report [25]. It is found that the conductivity increases largely with the hot-forging pressure increasing. Consequently, CCO-5.0 shows the highest electrical conductivity among the samples in the whole temperature range. As discussed above, the 3
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
R. Shimonishi, et al.
regardless of their wide variety of the relative density. Fig. 4(c) shows the temperature dependence of the in-plane power factor (S2σ) from room temperature to 900 K. The power factor increases by both density and temperature, according to the temperature and density dependences of the electrical conductivity and the Seebeck coefficient. Especially, the power factor of CCO-5.0 reaches as high as 400 μW·m−1 K−2 at the maximum temperature of 900 K. The electrical conductivity, the Seebeck coefficient, and the power factor of the CCO-p samples at 900 K are plotted against their relative density in Fig. 4(d)–(f), together with the respective values reported in the literature [11,13,23–25,30–32]. Note that the electrical conductivity, the Seebeck coefficient, and the power factor taken from Ref. [32] are the values measured at 860 K. In Fig. 4(d), a positive correlation between the electrical conductivity and the relative density is found for the literature values. Our samples basically follow this trend, but each of them exhibits the highest electrical conductivity when compared to the previously reported ceramics with a similar relative density. The Lotgering factor f of the ceramics in the previous reports largely decreases with the relative density decreasing, as already shown in Fig. 2(b). In the less oriented ceramics with the lower f values, the path length of the holes is elongated because the hole-conducting CoO2 layers are not aligned in the in-plane direction. This leads to the significant degradation of the macroscopic electrical conductivity. On the other hand, the f value of our ceramics remains high even for CCO-0.0 with the lowest density of 41.0 % as shown in Fig. 2(b), guaranteeing the smooth transport of the holes. Therefore, our samples show higher electrical conductivity than those in the previous reports. The Seebeck coefficients of our ceramics exhibit slightly higher values compared with most of the reported values (Fig. 4(e)). Since the Seebeck coefficient can be affected by many microstructural parameters such as the grain size and orientation, a full explanation of the difference between CCO-p and the previous samples is quite difficult. As shown in Fig. 4(f), all the CCO-p samples exhibit a higher power factor compared to that of the previously reported ceramics with a similar relative density. This is due to the improved electrical conductivity originating from the high grain orientation, demonstrating that the RTGG method employed in this study is effective for fabricating CCO ceramics with a superior thermoelectric property. It is difficult to discuss the accurate ZT value of CCO-p at present because the measurement of the thermal conductivity along the in-plane direction has not been achieved due to the small thickness of the ceramic samples. Nevertheless the enhanced power factors of the CCO-p samples are suggestive of their high ZT values because the introduction of pores has been reported to reduce effectively the lattice thermal conductivity of a wide variety of materials, leading to an enhanced ZT value [3,33,34].
Fig. 2. (a) XRD patterns of the bulk ceramics of CCO-p (p = 0.0, 0.4, 2.1, and 5.0) samples. The samples were set on the sample stage in the same way with the green compact as shown in Fig. S1. (b) Relationship between the relative density and the Lotgering factor for (00l) of CCO. Previous data for CCO ceramics taken from the literature [11,23–28] are also plotted in (b) for comparison. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). Table 1 The Lotgering factor (f), relative density, and grain diameter for the CCO-p ceramics (p = 0.0, 0.4, 2.1, and 5.0). Sample
Lotgering factor (f)
Relative density(%)
Diameter/μm
CCO-0.0 CCO-0.4 CCO-2.1 CCO-5.0
0.89 0.93 0.95 0.97
41.0 61.5 75.1 83.8
2.0 1.9 1.8 2.3
± ± ± ±
0.2 0.3 0.3 1.2
± ± ± ±
4. Conclusions We fabricated the textured CCO ceramics with a high c-axis orientation by the RTGG method combined with the hot-forging technique. The β-Co(OH)2 plate-like template particles synthesized by the homogeneous precipitation method had a high aspect ratio of 40, and hence they were easily oriented in the green compact by the normal uniaxial pressing. It was found that the density of the final CCO ceramics could be controlled in a wide range between 41.0 and 83.8 % by changing the hot-forging pressure, while their high Lotgering factor over 0.89 was retained. Owing to the improved electrical conductivity, all the resulting porous CCO ceramics showed enhanced power factors between 67.0 and 409 μW·m−1 K−2 compared to the previously reported CCO ceramics having a similar density. The fabrication method developed here opens a new way of microstructure control of layered cobaltite thermoelectrics.
0.7 0.5 0.6 1.1
decrease in the density is associated with the individual growth of the grains. Insufficient connection between the grains narrows the effective cross section for holes to pass through the entire ceramics. On the other hand, the Seebeck coefficient is almost the same for the CCO-p samples within the standard deviation; it increases monotonically from about 120 (300 K) to about 180 μV·K−1 (900 K). The porous CCO ceramic can be regarded as a composite of CCO and air. According to the effectivemedium-theory [7,29], the Seebeck coefficient of a composite consisting of a conductor (CCO) and an insulator (air) is attributed only to that of the conductor, regardless of the volume ratio of the two phases. As a result, the CCO-p samples exhibit similar Seebeck coefficients
Declaration of Competing Interest The authors declare that they have no known competing financial 4
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
R. Shimonishi, et al.
Fig. 3. FE-SEM images of a fractured surface of (a) CCO-0.4 and (b) CCO-5.0.
Fig. 4. Temperature dependence of (a) the in-plane electrical conductivity, (b) the Seebeck coefficient, and (c) the power factor of CCO-p (p = 0.0, 0.4, 2.1, and 5.0) from room temperature to 900 K, and relationship between the relative density and (d) the electrical conductivity (σ900), (e) the Seebeck coefficient (S900), and (f) the power factor (S2σ900) at 900 K. Previous data for the CCO ceramics taken from the literature [11,13,23–25,30–32] are also plotted in (d–f) for comparison.
interests or personal relationships that could have appeared to influence the work reported in this paper
[3]
Acknowledgment [4]
One of the authors (R.S.) thanks Kato Foundation for Promotion of Science for its financial support.
[5]
Appendix A. Supplementary data
[6]
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jeurceramsoc.2019.11. 077.
[7]
[8] [9]
References
[10]
[1] G.J. Snyder, E.S. Toberer, Complex thermoelectric materials, Nat. Mater. 7 (2008) 105–114. [2] K. Biswas, J. He, I.D. Blum, C.-I. Wu, T.P. Hogan, D.N. Seidman, V.P. Dravid,
5
M.G. Kanatzidis, High-performance bulk thermoelectrics with all-scale hierarchical architectures, Nature 489 (2012) 414–418. A.U. Khan, K. Kobayashi, D.-M. Tang, Y. Yamauchi, K. Hasegawa, M. Mitome, Y. Xue, B. Jiang, K. Tsuchiya, D. Golberg, Y. Bando, T. Mori, Nano-micro-porous skutterudites with 100% enhancement in ZT for high performance thermoelectricity, Nano Energy 31 (2017) 152–159. W. Koshibae, K. Tsutsui, S. Maekawa, Thermopower in cobalt oxides, Phys. Rev. B 62 (2000) 6869–6872. B.C. Zhao, Y.P. Sun, W.J. Lu, X.B. Zhu, W.H. Song, Enhanced spin fluctuations in Ca3Co4−xTixO9 single crystals, Phys. Rev. B 74 (2006) 144417. M. Shikano, R. Funahashi, Electrical and thermal properties of single-crystalline (Ca2CoO3)0.7CoO2 with a Ca3Co4O9 structure, Appl. Phys. Lett. 82 (2003) 1851–1853. Y. Liu, Y. Lin, Z. Shi, C.-W. Nan, Z. Shen, Preparation of Ca3Co4O9 and improvement of its thermoelectric properties by spark plasma sintering, J. Am. Ceram. Soc. 88 (2005) 1337–1340. D. Wang, L. Chen, Q. Yao, J. Li, High-temperature thermoelectric properties of Ca3Co4O9+δ with Eu substitution, Solid State Commun. 129 (2004) 615–618. D. Kenfaui, D. Chateigner, M. Gomina, J.G. Noudem, Texture, mechanical and thermoelectric properties of Ca3Co4O9 ceramics, J. Alloys Compd. 490 (2010) 472–479. G. Xu, R. Funahashi, M. Shikano, I. Matsubara, Y. Zhou, Thermoelectric properties of the Bi- and Na- substituted Ca3Co4O9 system, Appl. Phys. Lett. 80 (2002) 3760–3762.
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
R. Shimonishi, et al.
[24] S. Katsuyama, M. Ito, Synthesis of Ca3Co4O9 ceramics by citric acid complex and hydrothermal hot-pressing processes and its thermoelectric properties, Proc. 25th International Conference on Thermoelectrics, IEEE, Piscataway, USA, 2006, pp. 543–547. [25] Y. Wang, Y. Sui, X. Wang, W. Su, X. Liu, Enhanced high temperature thermoelectric characteristics of transition metals doped Ca3Co4O9+δ by cold high-pressure fabrication, J. Appl. Phys. 107 (2010) 033708. [26] Q.M. Lu, J.X. Zhang, Q.Y. Zhang, Y.Q. Liu, D.M. Liu, Improved thermoelectric properties of Ca3-xBaxCo4O9 (x = 0∼0.4) bulks by sol-gel and SPS method, Proc. 25th International Conference on Thermoelectrics, IEEE, Piscataway, USA, 2006, pp. 66–69. [27] C.-H. Lim, S.-M. Choi, W.-S. Seo, K.H. Kim, J.-Y. Kim, H.-H. Park, Improvements of thermoelectric transport properties for a partially substituted Ca3Co4O9 system by spark plasma sintering, J. Ceram. Process. Res. 13 (2012) 197–201. [28] H. Itahara, J. Sugiyama, T. Tani, Enhancement of electrical conductivity in thermoelectric [Ca2CoO3]0.62[CoO2] ceramics by texture improvement, Jpn. J. Appl. Phys. 43 (2004) 5134–5139. [29] I. Webman, J. Jortner, M.H. Cohen, Thermoelectric power in inhomogeneous materials, Phys. Rev. B 16 (1977) 2959–2964. [30] M. Gunes, M. Ozenbas, Effect of grain size and porosity on phonon scattering enhancement of Ca3Co4O9, J. Alloys Compd. 626 (2015) 360–367. [31] K. Obata, Y. Chonan, T. Komiyama, T. Aoyama, H. Yamaguchi, S. Sugiyama, Grainoriented Ca3Co4O9 thermoelectric oxide ceramics prepared by solid-state reaction, J. Electron. Mater. 42 (2013) 2221–2226. [32] J.G. Noudem, R. Retoux, S. Lanfredi, Spark plasma texturing (SPT) of p-type [Ca2CoO3]0.62CoO2 thermoelectric oxide, J. Alloys Compd. 676 (2016) 499–504. [33] H. Ning, G.D. Mastrorillo, S. Grasso, B. Du, T. Mori, C. Hu, Y. Xu, K. Simpson, G. Maizza, M.J. Reece, Enhanced thermoelectric performance of porous magnesium tin silicide prepared using pressure-less spark plasma sintering, J. Mater. Chem. A 3 (2015) 17426–17432. [34] B. Xu, T. Feng, Z. Li, S.T. Pantelides, Y. Wu, Constructing highly porous thermoelectric monoliths with high-performance and improved portability from solutionsynthesized shape-controlled nanocrystals, Nano Lett. 18 (2018) 4034–4039.
[11] P.-H. Xiang, Y. Kinemuchi, H. Kaga, K. Watari, Fabrication and thermoelectric properties of Ca3Co4O9/Ag composites, J. Alloys Compd. 454 (2008) 364–369. [12] H. Ohta, K. Sugiura, K. Koumoto, Recent progress in oxide thermoelectric materials: p-type Ca3Co4O9 and n-type SrTiO3, Inorg. Chem. 47 (2008) 8429–8436. [13] M. Bittner, L. Helmich, F. Nietschke, B. Geppert, O. Oeckler, A. Feldhoff, Porous Ca3Co4O9 with enhanced thermoelectric properties derived from sol–gel synthesis, J. Eur. Ceram. Soc. 37 (2017) 3909–3915. [14] H. Itahara, C. Xia, J. Sugiyama, T. Tani, Fabrication of textured thermoelectric layered cobaltites with various rock salt-type layers by using β-Co(OH)2 platelets as reactive templates, J. Mater. Chem. 14 (2004) 61–66. [15] N. Murayama, J.B. Vander Sande, Hot forging with heat treatment of Bi-Pb-Sr-CaCu-O superconductors, Phys. C 241 (1995) 235–246. [16] F.K. Lotgering, Topotactical reactions with ferrimagnetic oxides having hexagonal crystal structures–I, J. Inorg. Nucl. Chem. 9 (1959) 113–123. [17] L.J. van der Pauw, A method of measuring specific resistivity and Hall effect of discs of arbitrary shape, Philips Res. Rep. 13 (1958) 1–9. [18] M. Schrade, H. Fjeld, T. Norby, T.G. Finstad, Versatile apparatus for thermoelectric characterization of oxides at high temperatures, Rev. Sci. Instrum. 85 (2014) 103906. [19] S. Ohta, T. Nomura, H. Ohta, K. Koumoto, High-temperature carrier transport and thermoelectric properties of heavily La- or Nb-doped SrTiO3 single crystals, J. Appl. Phys. 97 (2005) 034106. [20] R. Shimonishi, M. Hagiwara, S. Fujihara, Synthesis of Ca–Co hydroxides and their use in facile fabrication of textured CayCoO2 thermoelectric ceramics, Ceram. Int. 45 (2019) 3600–3607. [21] H. Itahara, W.-S. Seo, S. Lee, H. Nozaki, T. Tani, K. Koumoto, The formation mechanism of a textured ceramic of thermoelectric [Ca2CoO3]0.62[CoO2] on β-Co(OH)2 templates through in situ topotactic conversion, J. Am. Chem. Soc. 127 (2005) 6367–6373. [22] H. Itahara, S. Tajima, T. Tani, Synthesis of β-Co(OH)2 platelets by precipitation and hydrothermal methods, J. Ceram. Soc. Jpn. 110 (2002) 1048–1052. [23] M. Presečnik, J. de Boor, S. Bernik, Synthesis of single-phase Ca3Co4O9 ceramics and their processing for a microstructure-enhanced thermoelectric performance, Ceram. Int. 42 (2016) 7315–7327.
6