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A new approach to obtain calcium cobalt oxide by microwave-assisted hydrothermal synthesis
Ceramics International xxx (xxxx) xxx–xxx
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A new approach to obtain calcium cobalt oxide by microwave-assisted hydrothermal synthesis M.S. Medina∗,1, J.C. Bernardi, A. Zenatti, M.T. Escote Center for Engineering, Modeling and Applied Social Sciences, Federal University of ABC, UFABC, Santo Andre, Brazil
A R T I C LE I N FO
A B S T R A C T
Keywords: Ca3Co4O9 Thermoelectric materials Thermoelectric oxides Microwave-assisted hydrothermal synthesis Thermoelectricity
This study describes the effect of microwave-assisted hydrothermal synthesis on the crystalline phase-formation, microstructural characteristics, and thermoelectric performance of Ca3Co4O9. Synthesis parameters, such as time, temperature, KOH concentration, and oxidation states of Co ions, to produce this compound, were studied. X-ray diffraction and X-ray photoelectron spectroscopy showed that the most critical parameters for the phase formation are the oxidation states of Co ions. Ca3Co4O9 can be obtained by microwave-assisted hydrothermal synthesis at 503 K for 30 min and thermal treatment at 1173 K for 120 min. Scanning electron microscopy images showed that the sample exhibits randomly oriented particles with plate-like morphology and range size between 0.4 and 1.2 μm. From measurements of electrical resistivity, thermal conductivity, and Seebeck coefficient, we obtained a figure of merit of ~0.008 at 300 K.
1. Introduction
and high thermoelectric performance. Layers cobaltates with general
Devices based on thermoelectric (TE) materials can convert the waste heat from conventional sources to electrical energy [1]. Currently, a great effort has been made to find new energies sources that are less polluting and more efficient, TE devices can be employed to reach this objective. The conversion efficiency of TE materials can be evaluated using the dimensionless figure-of-merit (ZT), which is related to the interactions between electrical and thermal properties and observed through electrical measurements induced by thermal gradients [2]. ZT is defined as ZT = T(S2σ / k ) , where S is the Seebeck coefficient, σ is the electrical conductivity, and k is the thermal conductivity. Highperformance thermoelectric materials present a high Seebeck coefficient and electrical conductivity and low thermal conductivity [2]. Nowadays semiconductors alloys are used in TE devices, compounds like Bi2 Te3 and Sb2 Te3 with isoelectronic substitutions being the most promising TE materials. They present a high TE performance and high ZT values [3]. However, practical applications with these materials are restricted because they work in a low and narrow range temperature, and some of them are toxics and scarce [4–6]. Also, devices built requires TE materials with high working temperatures to reach higher power and to simplify the devices building [3,7]. In this context, thermoelectric oxides, such as ZnO, CaMnO3 , and layer-based Co, have been studied for high-temperature applications, due to their stability
formula [Mm A2O(m+2) ]q ⎡CoO2 (M= Co , Bi, Tl, etc.; A= Ca , Sr Ba, etc.; ⎢ ⎣ m= 0 , 1, 2; and q ≥ 0 , is defined as the misfit between the two layers), the compounds Bi2 Sr3Co2 O9 , NaCoO2 , and [Ca2CoO3 + δ ]0.614 [CoO2] have attracted a lot of attention, mainly due to their high efficiency (ZT~1) in the monocrystalline samples [8–10]. Although oxides are more suitable for applications in high-temperature, polycrystalline samples still present a low performance (ZT < 1), but their chemical and thermal stability in air and high temperatures, and low cost of raw materials are attractive for TE devices. Among these compounds, the most promising is the [Ca2CoO3 + δ ) , known as Ca3Co4 O9 (CCO), which presents a Seebeck coefficient of ~125 μV/K, a low electrical resistivity (ρ between 1 − 20 mΩcm at 300 K), and low thermal conductivity (k varying from 2 to 5 Wm−1 K−1 at 300 K) [11]. This compound presents ZT values from ~2 × 10−4 to 0.01 at 300 K and of ~0.2 at 800 K [12–14]. CCO crystallize in a modulated incommensurate crystalline structure, built from two different interpenetrating subsystems. The subsystem 1 is a rock salt structure [Ca2CoO3] that consists of a triple layer CaO-CoO-CaO and subsystem 2 is a hexagonal structure [CoO2] consisting of edge-sharing octahedra of CoO6 . These layers are stacked alternating along the c-axis. The subsystems crystallize in a monoclinic structure, with lattice parameters values: a = 4.8376(7) Å, c = 10.833(1)
∗
Corresponding author. Travessa R, 400, Cidade Universitária, 05508-900, Sǎo Paulo, SP, Brazil. E-mail address: [email protected] (M.S. Medina). 1 Center of Science and Technology Materials, Energy and Nuclear Research Institute, São Paulo, SP, Brazil. https://doi.org/10.1016/j.ceramint.2019.09.130 Received 2 March 2019; Received in revised form 13 September 2019; Accepted 13 September 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: M.S. Medina, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.130
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Å, β = 98.06(1)∘ , b1 = 4.5565(6) Å for the subsystem Ca2CoO3 , whereas for subsystem 2 the lattice parameter is b2 = 2.8189(4) Å and q = 0.62 ± 3 [15]. Such features give to CCO the incommensurate character. In this structure, the CoO2 subsystem contributes to the electronic, and magnetic properties; and the Ca2CoO3 subsystem acts as a phonon scattering region and can interfere in the thermal conductivity. Several works described the effect of chemical substitution and the influence of the synthesis procedure in the TE properties of CCO [16–20]. Investigations have shown that the characteristics like microstructure, oxygen content, Co3 +/Co4 + ratio, grain size, porosity, and secondary phases can influence the ZT value [16,18,19,21–29]. It should be noticed that most of these parameters are affected by the synthesis conditions, like the size and font of starting salts, the temperature and time for thermal treatment [16,17,21–29]. Commonly the CCO is synthesized by solid-state reaction and sol-gel, and these methods give a variety of values for ZT. Other synthesis methods such as co-precipitation [28], slurry [29], and thermal decomposition [30] also showed the influence of the synthesis on Ca3Co4 O9 TE properties. Thermoelectric and mechanical properties of CCO compounds were improved using different preparation methods and controlling the sintering through spark-plasma or applying high-magnetic-fields [17,21,30,31]. This work proposed to study the effect of the microwave-assisted hydrothermal synthesis on phase formation and thermoelectric properties of Ca3Co4 O9 . The chosen of the hydrothermal method was based on the possibility to produce homogeneous particles with controlled size and shape. Additionally, to the best of our knowledge, this is the first time that the cobalt calcium oxide is obtained using the microwave-assisted hydrothermal synthesis.
were taken on an ARS closed-cycle system using a Lakeshore temperature controller 331 and a Keithley 2400C source meter. These measurements were taken in the temperature range of 10–300 K. The steady-state thermal conductivity was measured using a homemade device based in the parallel heat flow technique [32]. Two platinum thermometers were accoupled in the upper and lower edges of a rectangular shape sample (2x2x7 mm3) to measure the temperature flow. The sample was fixed in the hot and cold terminals of the sample holder with silver epoxy (H2EEPO-TEK). The same set up was used to measure the Seebeck coefficient. The measurements were performed at 300 K.
3. Results In the hydrothermal method, several parameters can influence the phase formation and morphology. Thus, the hydrothermal synthesis was performed at different times, temperatures, KOH concentration, and amounts of H2 O2 . For samples obtained in the absence of the H2 O2 , the microwave-assisted hydrothermal synthesis was performed at 453, 483 and 503 K during 30, 60 and 120 min for each temperature, with KOH concentration varying from 0.5 to 2 mol/L. XRD results showed that the CCO phase formation is not influenced significantly by the conditions used during the hydrothermal synthesis (time, temperature, and KOH concentration). Fig. 1 shows an example of the result obtained for the sample as prepared, synthesized without H2 O2 at 503 K for 120 min and heat-treated at 1173 K for 20 h. The X-ray pattern of these samples revealed a few peaks of the Ca3Co4 O9 phase and several peaks belonging to the Co3 O4 phase, which remained even after the heat treatment during 20 h. The influence of oxidation states of Co ions in the phase formation was verified by adding different amounts of H2 O2 (% in vol.) in the reaction medium [33]. The addition of H2 O2 intended to change the proportions of Co2 +/Co3 + in the precursor solutions. From the study with samples without H2 O2 , the conditions of the hydrothermal synthesis were set up at 503 K during 30 min and ~0.7 mol/L of KOH. Fig. 2 displays the XRD patterns for the sample hydrothermally treated at these conditions with different amounts of H2 O2 (10, 20 and 50%); these samples were named CCO10, CCO20, and CCO50, respectively. Xray results revealed that all Bragg reflections belong to the Ca3Co4 O9 phase and these patterns agreed with the standard ICDD card no. 21–139. Few low intense peaks identified as a residual phase of Co3 O4 (ICDD card no. 43–1003) were observed in these X-ray patterns, which relatives intensities seemed to increase with the increase of H2 O2 amounts. The H2 O2 works as an oxidant agent to change the oxidation of the cobalt (II) hydroxide to cobalt (III) hydroxide. In addition, partial oxidation of the cobalt (III) to cobalt (IV) may occur [33].
2. Experimental procedure The syntheses of Ca3Co4 O9 were performed using cobalt acetate (Co(CH3 COO)2 4H2 O, 98% Sigma Aldrich) and calcium acetate (Ca(CH3 CO)2 H2 O, 99% Sigma Aldrich) as metal precursors and potassium hydroxide (KOH, 99% Sigma Aldrich) as a mineralizer agent. Firstly, stoichiometric amounts of the metal salts were dissolved in deionized water at 373 K under constant stirring for 30 min. Then, this solution was added to a KOH solution with pH adjusted by varying the KOH amounts from 0.5 to 2 mol/L. The synthesis was carried out with and without hydrogen peroxide (H2 O2 , 29% Sigma Aldrich) (% vol.). For syntheses with the oxidizing agent, the H2 O2 was slowly added in the solution after the hydroxides co-precipitation. In both cases, the final solution was transferred to a Teflon® vessel, which was placed in a multimode microwave reactor (Anton Paar, Synthos 3000). The resulting product was collected, washed with deionized water and ethanol, dried at 373 K and heat-treated at 1173 K for 2 h or 20 h. To thermal and electrical characterization, the final powder was pressed into pellets and sintered at 1173 K during 20 h. The structure was analyzed by X-ray powder diffraction (XRD), using two different diffractometers: (a) a Bruker AXS diffractometer model D8 focus with CuKα (λ = 1.54056 Å) radiation, in an angular range of 5 − 70∘ , an angular step of 0.05∘ and time of exposure of 3 s; and (b) a STADI-P diffractometer model Stoe, operating in a transmission mode, with MoKα (λ = 0.7093 Å) radiation, angular range of 2 − 50∘, an angular step of 0.015∘ and integration time of 150 s. Additionally, the structure was analyzed by selected area electron diffraction (SAED), using an electronic transmission microscope model Libra 120. The Raman spectroscopy measurements were performed from 200 to 800 cm−1 on the CCO pellets. The spectra were taken on a Raman spectrometer Horiba JobinYvon with 532 nm laser excitation. Powder morphology and size were examined by Field Emission Scanning Electron Microscopy (FESEM), on a JEOL microscope model JSM 6701F. The valence state of Co ions was determined by X-ray photoelectron spectroscopy (XPS) on a ThermoFisher scientific spectrophotometer with Al-Kα radiation at room temperature. The transport properties measurements were carried out in pellets samples using the 4-probe method; these measurements
Fig. 1. XRD pattern for sample synthesized by microwave-assisted hydrothermal synthesis at 503 K during 120 min and heat-treated at 1173 K during 20 h. Measurements realized with Cu radiation. 2
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Table 1 Percentages of different ions Co obtained from deconvolution of XPS spectra of samples CCO10, CCO20, and CCO50.
Co2+ Co3+ Co4+
CCO10
CCO20
CCO50
44.83 28.43 26.74
30.63 43.05 26.32
28.21 45.21 26.58
of this sample, which was indexed using the program Gatan Microscope Suite® [35], associated with the DIFFTOOLS plugin, the structural data obtained by Miyazaki et al. [15] was used in this analysis. Fig. 4(C) shows the hkl indices obtained through SAED data analysis. From this data, we estimated the interplanar distance, and the values found are in good agreement with the literature [15,36]. Fig. 5 shows the Raman spectrum for the CCO10 sample. In general, the CCO spectrum can present three main vibrational modes at Raman shift at 300, 540, and 630 cm−1 [22]. In this work, the CCO Raman spectrum reveals ten vibrational modes locating at positions A1 (290 cm−1), A2 (419 cm−1), A3 (455 cm−1), A4 (474 cm−1), A5 (518 cm−1), A6 (545 cm−1), A7 (584 cm−1), A8 (618 cm−1), A9 (660 cm−1), and A10 (685 cm−1). The assignment of the vibrational modes of [Ca2CoO3 + δ ]q [CoO2] is rather difficult due to the complex crystalline structure of this compound; then, in this work, the assignment was made by comparison with Raman results reported in the literature [27,37,38]. The peak at approximately 290 cm−1 is assigned to the vibration of O atoms in the Ca-O planes in [Ca2CoO3] layers. A6 and A8 peaks are assigned to E1g and A1g phonon, respectively. In the literature, the E1g and A1g modes represent in-plane and out-of-plane vibrations of oxygen present in Co-O planes of [CoO2] layers [34]. Fig. 6 shows the SEM images obtained for the CCO10 sample. These images revealed plate-like shape particles with a grain size varying from ~ 0.4–1.2 μm. A similar shape was observed in the TEM images of this sample (see Fig. 4); Ca3Co4 O9 prepared through other methods reported in the literature presents a similar plate-like shape [12,16,30,36,37,39,40]. Temperature dependence electrical resistivity, ρ (T ) , was analyzed through 4-probe electrical measurements. Fig. 7 shows the ρ (T ) curve of this samples, where above of ~100 K, the curve presents a metallic behavior. At temperatures below Tmin (see Fig. 7), there is a metal-insulator transition in which the ρ (T ) values increase rapidly with the decrease in temperature. Such behavior has been described due to the emergence of an incommensurate spin-density-wave (SDW) ordering close to Tmin in CCO compound, which promotes the appearance of partial charge carrier localization and leads to the observed metal-
Fig. 2. XRD pattern for samples synthesized by microwave-assisted hydrothermal synthesis at 503 K with different amounts of H2 O2 for 30 min and heattreated at 1173 K during 2 h. The asterisks indicate Co3 O4 . Measurements realized with Mo radiation.
XPS measurements were performed to confirm the charges states of the Co ions and the composition for samples CCO10, CCO20, and CCO50. The XPS survey spectrum (Fig. 3(A)) revealed the composition of the CCO compound. Fig. 3(B) shows the XPS spectrum close to the Co 2p band; there are two main peaks at ~780 and ~796 eV attributed to Co 2p3/2 and Co 2p1/2 , respectively. The spectrum was deconvoluted into two spin-orbital doublets (D) and three low intense satellites peaks (S). The doublet D1 and the S2 peak can be assigned to Co3 +, whereas the doublet D2 and S3 peak identified as Co2 +. The small satellite peak S1 can be attributed to Co4 + [27,34]. The quantitative analysis revealed a reduction in Co2 + and an increase in the Co3 + ions quantities with the increase in H2 O2 amounts. The amounts of Co4 + ions remain almost constant for all samples (see Table 1). These results indicated that for samples synthesized with large quantities of H2 O2 , the reduction in the amounts of Co2 + ions could be related to the formation of the small amounts of residual Co3 O4 phase. Also, the results indicated that only 10% of H2 O2 (%vol.) is enough to promote the CCO phase formation. The crystalline structure of the CCO10 sample was analyzed through SAED results (see Fig. 4). Fig. 4(A) shows the plate-like shape particles with an average size of ~0.4 μm. Fig. 4(B) shows the electron diffraction image. The ring patterns in Fig. 4(C) revealed the polycrystalline nature
Fig. 3. (A) XPS spectrum and (B) Co 2p peaks adjustment for sample CCO10.
3
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Fig. 4. (A) TEM images of the sample CCO10; (B) and (C) The bright field and SAED pattern with the hkl indices, respectively.
Fig. 7. Temperature dependence of electric resistivity between 10 − 300 K for sample CCO10.
Fig. 5. Raman spectrum of CCO10. The blue line indicates de experimental data, red and black lines indicate the adjust. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
[Ca2CoO3.34 ][CoO2] obtained through solid-state reaction [29] and to the compound named Ca3Co4 O9 obtained by sol-gel and solid-state reaction or other methods, such as thermal decomposition and co-precipitation [12,17,26]. In the region between 200 − 100 K, ρ (T ) presents a Fermi-liquid behavior that can be adjusted by the equation ρ (T ) = ρ0 + AT 2 , where ρ0 is the residual resistivity, and A is the transport coefficient for Fermiliquid [13,43]. The A value and the temperature T * calculated from the ρ xT 2 curves (Fig. 7) were 4.64x10−5 mΩcm/T 2 and 149 K, respectively [18,43]. Through the measurements of the thermal conductivity and Seebeck coefficient, the k value of ~4.8 W/mK and the Seebeck coefficient value around 127 μV/K were obtained. These values are similar to the data found in the literature for Ca3Co4 O9 pure, and with partial substitution of the Ca or Co [44]. From k, S, and ρ we calculate the ZT value of ~0.008 at 300 K. Results showed that from microwave-assisted hydrothermal synthesis is possible to obtain the calcium cobalt oxide with characteristics similar to that found for this compound synthesized through solid-state reaction and sol-gel method.
Fig. 6. SEM images of CCO10 obtained after the microwave-assisted hydrothermal synthesis at 503 K for 30 min and a heat-treatment at 1173 K for 2 h.
insulator transition [41,42]. The electrical resistivity at 300 K is ~14 mΩcm and the obtained value for Tmin , estimated from the change in the curve behavior, is ~72 K. This value is close to the compound with the formula 4
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4. Conclusions
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The CCO was synthesized via microwave-assisted hydrothermal synthesis. Several parameters that can influence the phase formation and morphology were studied. The XRD, Raman spectroscopy, electron diffraction images, and XPS techniques confirmed phase formation and composition, and it was observed that the most critical condition for phase formation is the oxidation states of the Co ions. The analyses of the transport properties, ρ, S, and k, showed that it is possible to use the hydrothermal synthesis to obtain samples with characteristics comparable to that listed for samples synthesized by solid-state reaction and sol-gel, with ZT value at 300 K similar to that found in the literature for the Ca3Co4 O9 . Declaration of competing interest The authors declare that they have no conflict of interest. Acknowledgments The authors gratefully acknowledge the Multiuser Central Facilities at UFABC for experimental support (XRD, SEM, Raman, XPS). Additionally, the authors acknowledge the financial support provided by CAPES and FAPESP (Proc. no. 16/03575-2). References [1] H.J. Goldsmid, The thermoelectric and related effects, Introduction to Thermoelectricity, Springer, 2016, pp. 1–7. [2] K. Behnia, Fundamentals of Thermoelectricity, OUP, Oxford, 2015. [3] B. Orr, A. Akbarzadeh, M. Mochizuki, R. Singh, A review of car waste heat recovery systems utilising thermoelectric generators and heat pipes, Appl. Therm. Eng. 101 (2016) 490–495. [4] G.J. Snyder, E.S. Toberer, Complex thermoelectric materials, Materials for Sustainable Energy: A Collection of Peer-Reviewed Research and Review Articles from Nature Publishing Group, World Scientific, 2011, pp. 101–110. [5] M.G. Kanatzidis, T. Hogan, S. Mahanti, Chemistry, Physics, and Materials Science of Thermoelectric Materials: beyond Bismuth Telluride, Springer Science & Business Media, 2012. [6] H. Goldsmid, Bismuth telluride and its alloys as materials for thermoelectric generation, Materials 7 (4) (2014) 2577–2592. [7] J. Yang, T. Caillat, Thermoelectric materials for space and automotive power generation, MRS ulletin 31 (3) (2006) 224–229. [8] J.W. Fergus, Oxide materials for high temperature thermoelectric energy conversion, J. Eur. Ceram. Soc. 32 (3) (2012) 525–540. [9] W. Koshibae, K. Tsutsui, S. Maekawa, Thermopower in cobalt oxides, Phys. Rev. B 62 (11) (2000) 6869. [10] D. Moser, L. Karvonen, S. Populoh, M. Trottmann, A. Weidenkaff, Influence of the oxygen content on thermoelectric properties of Ca3−xBixCo4O9+δsystem, Solid State Sci. 13 (12) (2011) 2160–2164. [11] M. Shikano, R. Funahashi, Electrical and thermal properties of single-crystalline (Ca2CoO3)0.7CoO2with a sCa3Co4O9tructure, Appl. Phys. Lett. 82 (12) (2003) 1851–1853. [12] S. Demirel, E. Altin, E. Oz, S. Altin, A. Bayri, An enhancement ZT and spin state transition of Ca3Co4O9with Pb doping, J. Alloy. Comp. 627 (2015) 430–437. [13] Y. Miyazaki, K. Kudo, M. Akoshima, Y. Ono, Y. Koike, T. Kajitani, Low-temperature thermoelectric properties of the composite crystal [Ca2CoO3.34]0.614[CoO2], Jpn. J. Appl. Phys. 39 (6A) (2000) L531. [14] Y. Wang, Y. Sui, H. Fan, X. Wang, Y. Su, W. Su, X. Liu, High temperature thermoelectric response of electron-doped CaMnO3, Chem. Mater. 21 (19) (2009) 4653–4660. [15] Y. Miyazaki, M. Onoda, T. Oku, M. Kikuchi, Y. Ishii, Y. Ono, Y. Morii, T. Kajitani, Modulated structure of the thermoelectric compound [Ca2CoO3]0.62CoO2, J. Phys. Soc. Jpn. 71 (2) (2002) 491–497. [16] G. Tang, W. Yang, Y. He, Z. Wang, Enhanced thermoelectric properties of Ca3Co4O9+δby Ni, Ce co-doping, Ceram. Int. 41 (5) (2015) 7115–7118 Part B. [17] C. Chen, T. Zhang, R. Donelson, D. Chu, R. Tian, T.T. Tan, S. Li, Thermopower and chemical stability of Na0.77CoO2/Ca3Co4O9 composites, Acta Mater. 63 (2014) 99–106.
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
A new approach to obtain calcium cobalt oxide by microwave-assisted hydrothermal synthesis M.S. Medina∗,1, J.C. Bernardi, A. Zenatti, M.T. Escote Center for Engineering, Modeling and Applied Social Sciences, Federal University of ABC, UFABC, Santo Andre, Brazil
A R T I C LE I N FO
A B S T R A C T
Keywords: Ca3Co4O9 Thermoelectric materials Thermoelectric oxides Microwave-assisted hydrothermal synthesis Thermoelectricity
This study describes the effect of microwave-assisted hydrothermal synthesis on the crystalline phase-formation, microstructural characteristics, and thermoelectric performance of Ca3Co4O9. Synthesis parameters, such as time, temperature, KOH concentration, and oxidation states of Co ions, to produce this compound, were studied. X-ray diffraction and X-ray photoelectron spectroscopy showed that the most critical parameters for the phase formation are the oxidation states of Co ions. Ca3Co4O9 can be obtained by microwave-assisted hydrothermal synthesis at 503 K for 30 min and thermal treatment at 1173 K for 120 min. Scanning electron microscopy images showed that the sample exhibits randomly oriented particles with plate-like morphology and range size between 0.4 and 1.2 μm. From measurements of electrical resistivity, thermal conductivity, and Seebeck coefficient, we obtained a figure of merit of ~0.008 at 300 K.
1. Introduction
and high thermoelectric performance. Layers cobaltates with general
Devices based on thermoelectric (TE) materials can convert the waste heat from conventional sources to electrical energy [1]. Currently, a great effort has been made to find new energies sources that are less polluting and more efficient, TE devices can be employed to reach this objective. The conversion efficiency of TE materials can be evaluated using the dimensionless figure-of-merit (ZT), which is related to the interactions between electrical and thermal properties and observed through electrical measurements induced by thermal gradients [2]. ZT is defined as ZT = T(S2σ / k ) , where S is the Seebeck coefficient, σ is the electrical conductivity, and k is the thermal conductivity. Highperformance thermoelectric materials present a high Seebeck coefficient and electrical conductivity and low thermal conductivity [2]. Nowadays semiconductors alloys are used in TE devices, compounds like Bi2 Te3 and Sb2 Te3 with isoelectronic substitutions being the most promising TE materials. They present a high TE performance and high ZT values [3]. However, practical applications with these materials are restricted because they work in a low and narrow range temperature, and some of them are toxics and scarce [4–6]. Also, devices built requires TE materials with high working temperatures to reach higher power and to simplify the devices building [3,7]. In this context, thermoelectric oxides, such as ZnO, CaMnO3 , and layer-based Co, have been studied for high-temperature applications, due to their stability
formula [Mm A2O(m+2) ]q ⎡CoO2 (M= Co , Bi, Tl, etc.; A= Ca , Sr Ba, etc.; ⎢ ⎣ m= 0 , 1, 2; and q ≥ 0 , is defined as the misfit between the two layers), the compounds Bi2 Sr3Co2 O9 , NaCoO2 , and [Ca2CoO3 + δ ]0.614 [CoO2] have attracted a lot of attention, mainly due to their high efficiency (ZT~1) in the monocrystalline samples [8–10]. Although oxides are more suitable for applications in high-temperature, polycrystalline samples still present a low performance (ZT < 1), but their chemical and thermal stability in air and high temperatures, and low cost of raw materials are attractive for TE devices. Among these compounds, the most promising is the [Ca2CoO3 + δ ) , known as Ca3Co4 O9 (CCO), which presents a Seebeck coefficient of ~125 μV/K, a low electrical resistivity (ρ between 1 − 20 mΩcm at 300 K), and low thermal conductivity (k varying from 2 to 5 Wm−1 K−1 at 300 K) [11]. This compound presents ZT values from ~2 × 10−4 to 0.01 at 300 K and of ~0.2 at 800 K [12–14]. CCO crystallize in a modulated incommensurate crystalline structure, built from two different interpenetrating subsystems. The subsystem 1 is a rock salt structure [Ca2CoO3] that consists of a triple layer CaO-CoO-CaO and subsystem 2 is a hexagonal structure [CoO2] consisting of edge-sharing octahedra of CoO6 . These layers are stacked alternating along the c-axis. The subsystems crystallize in a monoclinic structure, with lattice parameters values: a = 4.8376(7) Å, c = 10.833(1)
∗
Corresponding author. Travessa R, 400, Cidade Universitária, 05508-900, Sǎo Paulo, SP, Brazil. E-mail address: [email protected] (M.S. Medina). 1 Center of Science and Technology Materials, Energy and Nuclear Research Institute, São Paulo, SP, Brazil. https://doi.org/10.1016/j.ceramint.2019.09.130 Received 2 March 2019; Received in revised form 13 September 2019; Accepted 13 September 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: M.S. Medina, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.130
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Å, β = 98.06(1)∘ , b1 = 4.5565(6) Å for the subsystem Ca2CoO3 , whereas for subsystem 2 the lattice parameter is b2 = 2.8189(4) Å and q = 0.62 ± 3 [15]. Such features give to CCO the incommensurate character. In this structure, the CoO2 subsystem contributes to the electronic, and magnetic properties; and the Ca2CoO3 subsystem acts as a phonon scattering region and can interfere in the thermal conductivity. Several works described the effect of chemical substitution and the influence of the synthesis procedure in the TE properties of CCO [16–20]. Investigations have shown that the characteristics like microstructure, oxygen content, Co3 +/Co4 + ratio, grain size, porosity, and secondary phases can influence the ZT value [16,18,19,21–29]. It should be noticed that most of these parameters are affected by the synthesis conditions, like the size and font of starting salts, the temperature and time for thermal treatment [16,17,21–29]. Commonly the CCO is synthesized by solid-state reaction and sol-gel, and these methods give a variety of values for ZT. Other synthesis methods such as co-precipitation [28], slurry [29], and thermal decomposition [30] also showed the influence of the synthesis on Ca3Co4 O9 TE properties. Thermoelectric and mechanical properties of CCO compounds were improved using different preparation methods and controlling the sintering through spark-plasma or applying high-magnetic-fields [17,21,30,31]. This work proposed to study the effect of the microwave-assisted hydrothermal synthesis on phase formation and thermoelectric properties of Ca3Co4 O9 . The chosen of the hydrothermal method was based on the possibility to produce homogeneous particles with controlled size and shape. Additionally, to the best of our knowledge, this is the first time that the cobalt calcium oxide is obtained using the microwave-assisted hydrothermal synthesis.
were taken on an ARS closed-cycle system using a Lakeshore temperature controller 331 and a Keithley 2400C source meter. These measurements were taken in the temperature range of 10–300 K. The steady-state thermal conductivity was measured using a homemade device based in the parallel heat flow technique [32]. Two platinum thermometers were accoupled in the upper and lower edges of a rectangular shape sample (2x2x7 mm3) to measure the temperature flow. The sample was fixed in the hot and cold terminals of the sample holder with silver epoxy (H2EEPO-TEK). The same set up was used to measure the Seebeck coefficient. The measurements were performed at 300 K.
3. Results In the hydrothermal method, several parameters can influence the phase formation and morphology. Thus, the hydrothermal synthesis was performed at different times, temperatures, KOH concentration, and amounts of H2 O2 . For samples obtained in the absence of the H2 O2 , the microwave-assisted hydrothermal synthesis was performed at 453, 483 and 503 K during 30, 60 and 120 min for each temperature, with KOH concentration varying from 0.5 to 2 mol/L. XRD results showed that the CCO phase formation is not influenced significantly by the conditions used during the hydrothermal synthesis (time, temperature, and KOH concentration). Fig. 1 shows an example of the result obtained for the sample as prepared, synthesized without H2 O2 at 503 K for 120 min and heat-treated at 1173 K for 20 h. The X-ray pattern of these samples revealed a few peaks of the Ca3Co4 O9 phase and several peaks belonging to the Co3 O4 phase, which remained even after the heat treatment during 20 h. The influence of oxidation states of Co ions in the phase formation was verified by adding different amounts of H2 O2 (% in vol.) in the reaction medium [33]. The addition of H2 O2 intended to change the proportions of Co2 +/Co3 + in the precursor solutions. From the study with samples without H2 O2 , the conditions of the hydrothermal synthesis were set up at 503 K during 30 min and ~0.7 mol/L of KOH. Fig. 2 displays the XRD patterns for the sample hydrothermally treated at these conditions with different amounts of H2 O2 (10, 20 and 50%); these samples were named CCO10, CCO20, and CCO50, respectively. Xray results revealed that all Bragg reflections belong to the Ca3Co4 O9 phase and these patterns agreed with the standard ICDD card no. 21–139. Few low intense peaks identified as a residual phase of Co3 O4 (ICDD card no. 43–1003) were observed in these X-ray patterns, which relatives intensities seemed to increase with the increase of H2 O2 amounts. The H2 O2 works as an oxidant agent to change the oxidation of the cobalt (II) hydroxide to cobalt (III) hydroxide. In addition, partial oxidation of the cobalt (III) to cobalt (IV) may occur [33].
2. Experimental procedure The syntheses of Ca3Co4 O9 were performed using cobalt acetate (Co(CH3 COO)2 4H2 O, 98% Sigma Aldrich) and calcium acetate (Ca(CH3 CO)2 H2 O, 99% Sigma Aldrich) as metal precursors and potassium hydroxide (KOH, 99% Sigma Aldrich) as a mineralizer agent. Firstly, stoichiometric amounts of the metal salts were dissolved in deionized water at 373 K under constant stirring for 30 min. Then, this solution was added to a KOH solution with pH adjusted by varying the KOH amounts from 0.5 to 2 mol/L. The synthesis was carried out with and without hydrogen peroxide (H2 O2 , 29% Sigma Aldrich) (% vol.). For syntheses with the oxidizing agent, the H2 O2 was slowly added in the solution after the hydroxides co-precipitation. In both cases, the final solution was transferred to a Teflon® vessel, which was placed in a multimode microwave reactor (Anton Paar, Synthos 3000). The resulting product was collected, washed with deionized water and ethanol, dried at 373 K and heat-treated at 1173 K for 2 h or 20 h. To thermal and electrical characterization, the final powder was pressed into pellets and sintered at 1173 K during 20 h. The structure was analyzed by X-ray powder diffraction (XRD), using two different diffractometers: (a) a Bruker AXS diffractometer model D8 focus with CuKα (λ = 1.54056 Å) radiation, in an angular range of 5 − 70∘ , an angular step of 0.05∘ and time of exposure of 3 s; and (b) a STADI-P diffractometer model Stoe, operating in a transmission mode, with MoKα (λ = 0.7093 Å) radiation, angular range of 2 − 50∘, an angular step of 0.015∘ and integration time of 150 s. Additionally, the structure was analyzed by selected area electron diffraction (SAED), using an electronic transmission microscope model Libra 120. The Raman spectroscopy measurements were performed from 200 to 800 cm−1 on the CCO pellets. The spectra were taken on a Raman spectrometer Horiba JobinYvon with 532 nm laser excitation. Powder morphology and size were examined by Field Emission Scanning Electron Microscopy (FESEM), on a JEOL microscope model JSM 6701F. The valence state of Co ions was determined by X-ray photoelectron spectroscopy (XPS) on a ThermoFisher scientific spectrophotometer with Al-Kα radiation at room temperature. The transport properties measurements were carried out in pellets samples using the 4-probe method; these measurements
Fig. 1. XRD pattern for sample synthesized by microwave-assisted hydrothermal synthesis at 503 K during 120 min and heat-treated at 1173 K during 20 h. Measurements realized with Cu radiation. 2
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Table 1 Percentages of different ions Co obtained from deconvolution of XPS spectra of samples CCO10, CCO20, and CCO50.
Co2+ Co3+ Co4+
CCO10
CCO20
CCO50
44.83 28.43 26.74
30.63 43.05 26.32
28.21 45.21 26.58
of this sample, which was indexed using the program Gatan Microscope Suite® [35], associated with the DIFFTOOLS plugin, the structural data obtained by Miyazaki et al. [15] was used in this analysis. Fig. 4(C) shows the hkl indices obtained through SAED data analysis. From this data, we estimated the interplanar distance, and the values found are in good agreement with the literature [15,36]. Fig. 5 shows the Raman spectrum for the CCO10 sample. In general, the CCO spectrum can present three main vibrational modes at Raman shift at 300, 540, and 630 cm−1 [22]. In this work, the CCO Raman spectrum reveals ten vibrational modes locating at positions A1 (290 cm−1), A2 (419 cm−1), A3 (455 cm−1), A4 (474 cm−1), A5 (518 cm−1), A6 (545 cm−1), A7 (584 cm−1), A8 (618 cm−1), A9 (660 cm−1), and A10 (685 cm−1). The assignment of the vibrational modes of [Ca2CoO3 + δ ]q [CoO2] is rather difficult due to the complex crystalline structure of this compound; then, in this work, the assignment was made by comparison with Raman results reported in the literature [27,37,38]. The peak at approximately 290 cm−1 is assigned to the vibration of O atoms in the Ca-O planes in [Ca2CoO3] layers. A6 and A8 peaks are assigned to E1g and A1g phonon, respectively. In the literature, the E1g and A1g modes represent in-plane and out-of-plane vibrations of oxygen present in Co-O planes of [CoO2] layers [34]. Fig. 6 shows the SEM images obtained for the CCO10 sample. These images revealed plate-like shape particles with a grain size varying from ~ 0.4–1.2 μm. A similar shape was observed in the TEM images of this sample (see Fig. 4); Ca3Co4 O9 prepared through other methods reported in the literature presents a similar plate-like shape [12,16,30,36,37,39,40]. Temperature dependence electrical resistivity, ρ (T ) , was analyzed through 4-probe electrical measurements. Fig. 7 shows the ρ (T ) curve of this samples, where above of ~100 K, the curve presents a metallic behavior. At temperatures below Tmin (see Fig. 7), there is a metal-insulator transition in which the ρ (T ) values increase rapidly with the decrease in temperature. Such behavior has been described due to the emergence of an incommensurate spin-density-wave (SDW) ordering close to Tmin in CCO compound, which promotes the appearance of partial charge carrier localization and leads to the observed metal-
Fig. 2. XRD pattern for samples synthesized by microwave-assisted hydrothermal synthesis at 503 K with different amounts of H2 O2 for 30 min and heattreated at 1173 K during 2 h. The asterisks indicate Co3 O4 . Measurements realized with Mo radiation.
XPS measurements were performed to confirm the charges states of the Co ions and the composition for samples CCO10, CCO20, and CCO50. The XPS survey spectrum (Fig. 3(A)) revealed the composition of the CCO compound. Fig. 3(B) shows the XPS spectrum close to the Co 2p band; there are two main peaks at ~780 and ~796 eV attributed to Co 2p3/2 and Co 2p1/2 , respectively. The spectrum was deconvoluted into two spin-orbital doublets (D) and three low intense satellites peaks (S). The doublet D1 and the S2 peak can be assigned to Co3 +, whereas the doublet D2 and S3 peak identified as Co2 +. The small satellite peak S1 can be attributed to Co4 + [27,34]. The quantitative analysis revealed a reduction in Co2 + and an increase in the Co3 + ions quantities with the increase in H2 O2 amounts. The amounts of Co4 + ions remain almost constant for all samples (see Table 1). These results indicated that for samples synthesized with large quantities of H2 O2 , the reduction in the amounts of Co2 + ions could be related to the formation of the small amounts of residual Co3 O4 phase. Also, the results indicated that only 10% of H2 O2 (%vol.) is enough to promote the CCO phase formation. The crystalline structure of the CCO10 sample was analyzed through SAED results (see Fig. 4). Fig. 4(A) shows the plate-like shape particles with an average size of ~0.4 μm. Fig. 4(B) shows the electron diffraction image. The ring patterns in Fig. 4(C) revealed the polycrystalline nature
Fig. 3. (A) XPS spectrum and (B) Co 2p peaks adjustment for sample CCO10.
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Fig. 4. (A) TEM images of the sample CCO10; (B) and (C) The bright field and SAED pattern with the hkl indices, respectively.
Fig. 7. Temperature dependence of electric resistivity between 10 − 300 K for sample CCO10.
Fig. 5. Raman spectrum of CCO10. The blue line indicates de experimental data, red and black lines indicate the adjust. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
[Ca2CoO3.34 ][CoO2] obtained through solid-state reaction [29] and to the compound named Ca3Co4 O9 obtained by sol-gel and solid-state reaction or other methods, such as thermal decomposition and co-precipitation [12,17,26]. In the region between 200 − 100 K, ρ (T ) presents a Fermi-liquid behavior that can be adjusted by the equation ρ (T ) = ρ0 + AT 2 , where ρ0 is the residual resistivity, and A is the transport coefficient for Fermiliquid [13,43]. The A value and the temperature T * calculated from the ρ xT 2 curves (Fig. 7) were 4.64x10−5 mΩcm/T 2 and 149 K, respectively [18,43]. Through the measurements of the thermal conductivity and Seebeck coefficient, the k value of ~4.8 W/mK and the Seebeck coefficient value around 127 μV/K were obtained. These values are similar to the data found in the literature for Ca3Co4 O9 pure, and with partial substitution of the Ca or Co [44]. From k, S, and ρ we calculate the ZT value of ~0.008 at 300 K. Results showed that from microwave-assisted hydrothermal synthesis is possible to obtain the calcium cobalt oxide with characteristics similar to that found for this compound synthesized through solid-state reaction and sol-gel method.
Fig. 6. SEM images of CCO10 obtained after the microwave-assisted hydrothermal synthesis at 503 K for 30 min and a heat-treatment at 1173 K for 2 h.
insulator transition [41,42]. The electrical resistivity at 300 K is ~14 mΩcm and the obtained value for Tmin , estimated from the change in the curve behavior, is ~72 K. This value is close to the compound with the formula 4
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4. Conclusions
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The CCO was synthesized via microwave-assisted hydrothermal synthesis. Several parameters that can influence the phase formation and morphology were studied. The XRD, Raman spectroscopy, electron diffraction images, and XPS techniques confirmed phase formation and composition, and it was observed that the most critical condition for phase formation is the oxidation states of the Co ions. The analyses of the transport properties, ρ, S, and k, showed that it is possible to use the hydrothermal synthesis to obtain samples with characteristics comparable to that listed for samples synthesized by solid-state reaction and sol-gel, with ZT value at 300 K similar to that found in the literature for the Ca3Co4 O9 . Declaration of competing interest The authors declare that they have no conflict of interest. Acknowledgments The authors gratefully acknowledge the Multiuser Central Facilities at UFABC for experimental support (XRD, SEM, Raman, XPS). Additionally, the authors acknowledge the financial support provided by CAPES and FAPESP (Proc. no. 16/03575-2). References [1] H.J. Goldsmid, The thermoelectric and related effects, Introduction to Thermoelectricity, Springer, 2016, pp. 1–7. [2] K. Behnia, Fundamentals of Thermoelectricity, OUP, Oxford, 2015. [3] B. Orr, A. Akbarzadeh, M. Mochizuki, R. Singh, A review of car waste heat recovery systems utilising thermoelectric generators and heat pipes, Appl. Therm. Eng. 101 (2016) 490–495. [4] G.J. Snyder, E.S. Toberer, Complex thermoelectric materials, Materials for Sustainable Energy: A Collection of Peer-Reviewed Research and Review Articles from Nature Publishing Group, World Scientific, 2011, pp. 101–110. [5] M.G. Kanatzidis, T. Hogan, S. Mahanti, Chemistry, Physics, and Materials Science of Thermoelectric Materials: beyond Bismuth Telluride, Springer Science & Business Media, 2012. [6] H. Goldsmid, Bismuth telluride and its alloys as materials for thermoelectric generation, Materials 7 (4) (2014) 2577–2592. [7] J. Yang, T. Caillat, Thermoelectric materials for space and automotive power generation, MRS ulletin 31 (3) (2006) 224–229. [8] J.W. Fergus, Oxide materials for high temperature thermoelectric energy conversion, J. Eur. Ceram. Soc. 32 (3) (2012) 525–540. [9] W. Koshibae, K. Tsutsui, S. Maekawa, Thermopower in cobalt oxides, Phys. Rev. B 62 (11) (2000) 6869. [10] D. Moser, L. Karvonen, S. Populoh, M. Trottmann, A. Weidenkaff, Influence of the oxygen content on thermoelectric properties of Ca3−xBixCo4O9+δsystem, Solid State Sci. 13 (12) (2011) 2160–2164. [11] M. Shikano, R. Funahashi, Electrical and thermal properties of single-crystalline (Ca2CoO3)0.7CoO2with a sCa3Co4O9tructure, Appl. Phys. Lett. 82 (12) (2003) 1851–1853. [12] S. Demirel, E. Altin, E. Oz, S. Altin, A. Bayri, An enhancement ZT and spin state transition of Ca3Co4O9with Pb doping, J. Alloy. Comp. 627 (2015) 430–437. [13] Y. Miyazaki, K. Kudo, M. Akoshima, Y. Ono, Y. Koike, T. Kajitani, Low-temperature thermoelectric properties of the composite crystal [Ca2CoO3.34]0.614[CoO2], Jpn. J. Appl. Phys. 39 (6A) (2000) L531. [14] Y. Wang, Y. Sui, H. Fan, X. Wang, Y. Su, W. Su, X. Liu, High temperature thermoelectric response of electron-doped CaMnO3, Chem. Mater. 21 (19) (2009) 4653–4660. [15] Y. Miyazaki, M. Onoda, T. Oku, M. Kikuchi, Y. Ishii, Y. Ono, Y. Morii, T. Kajitani, Modulated structure of the thermoelectric compound [Ca2CoO3]0.62CoO2, J. Phys. Soc. Jpn. 71 (2) (2002) 491–497. [16] G. Tang, W. Yang, Y. He, Z. Wang, Enhanced thermoelectric properties of Ca3Co4O9+δby Ni, Ce co-doping, Ceram. Int. 41 (5) (2015) 7115–7118 Part B. [17] C. Chen, T. Zhang, R. Donelson, D. Chu, R. Tian, T.T. Tan, S. Li, Thermopower and chemical stability of Na0.77CoO2/Ca3Co4O9 composites, Acta Mater. 63 (2014) 99–106.
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