Effect of synthetic methods on the thermoelectrical properties of textured Bi 2. Ca 2. O x. Co 1.7

Transcription

1 BOLETIN DE LA SOCIEDAD ESPAÑOLA DE Cerámica y Vidrio A R T I C U L O Effect of synthetic methods on the thermoelectrical properties of textured Bi 2 ceramics Sh. Rasekh 1, M.A. Madre 1, A. Sotelo 1, E. Guilmeau 2, S. Marinel 2, J.C.Diez 1 1 ICMA (Universidad de Zaragoza-CSIC), Depto. de Ciencia y Tecnología de Materiales y Fluidos, C/María de Luna 3, E-50018, Zaragoza (Spain) 2 CRISMAT Laboratory, UMR 6508 CNRS-ENSICAEN, 6 Bld. Maréchal Juin, Caen cedex (France) Thermoelectric performances on cobaltite ceramics can be changed by grain orientation. This can be performed by directional solidification on samples prepared from different synthetic methods. Three synthetic methods, conventional solid state reaction, sol-gel and polymer solution were used for preparation of Bi 2 powders. The powders have been directionally grown, at a rate of 30 mm/h, by the laser floating zone method. In all the cases, the microstructure shows alternated layers, of Bi 2 and a Bi-Ca-O solid solution, with small CoO inclusions. It has been found an improvement on the power factor at 50 ºC for the solution methods with respect to the conventional solid sate sintered samples. Keywords: Processing, Melting, Sol-gel, Electrical properties, Thermopower, misfit cobaltites. Efecto del método de síntesis en las propiedades de cerámicas termoeléctricas texturadas Bi 2 Las prestaciones de las cerámicas basadas en óxidos de cobalto pueden modificarse por medio de la orientación de los granos. Este proceso puede llevarse a cabo mediante solidificación direccional de las muestras, que previamente han podido ser preparadas por diferentes rutas. En este trabajo se han utilizado tres métodos de síntesis, estado sólido, sol-gel y por vía polimérica, para preparar cerámicas con composición Bi 2. Estos polvos se han crecido direccionalmente a 30 mm/h, por el método de zona flotante inducida por láser. En todos los casos la microestructura consiste en capas alternas, de Bi 2 y una solución sólida Bi-Ca-O, con pequeñas inclusiones de CoO. Se ha encontrado que los métodos de disolución mejoran el factor de potencia a 50 ºC, con respecto a los valores que se obtienen a través de los métodos de sinterizado en estado sólido. Palabras clave: Procesamiento, Fusión, Sol-gel, Propiedades eléctricas, Poder termoeléctrico, cobaltitas. 1. Introduction Thermoelectric (TE) materials are extensively studied due to their ability to transform heat energy into electrical one as a consequence of the well-known Seebeck effect. This important characteristic has focused attention on this type of materials in order to be applied in waste heat recovery devices (1). Conversion efficiency of TE materials is described as a dimensionless factor, the figure of merit (ZT =TS 2 /ρκ) were T is the absolute temperature and S, ρ and κ are the Seebeck coefficient (or thermopower, TEP), electrical resistivity and thermal conductivity, respectively (2). A high performance TE material must have a figure of merit as high as possible. It can be obtained with a high Seebeck coefficient, low electrical resistivity and a low thermal conductivity. On the other hand, ZT improves when temperature increases. For this reason it is important to obtain TE materials with the ability to work at high temperature for long periods of time, usually under air. When thermal conductivity measurements are not available, the Power Factor (PF = S 2 / ρ), is usually used as a TE quality parameter. Bol. Soc. Esp. Ceram. V. 49, 1, (2010) Nowadays, TE devices based on semiconductor materials (Bi 2 Te 3, Sb 2 Te 3,...), with high ZT values, have been industrially applied, e.g. in vehicles (3,4). Due to the intrinsic characteristics of this type of compounds (chemical degradation, evaporation of heavy metals, etc.), they should not be used in devices working at relatively high temperatures. Such limitations have been overwhelmed by the discovery of large thermoelectrical properties in Na O 4 (5). This new thermoelectrical ceramic has attracted much attention on the CoO-based ceramic family. The intense research work performed on this ceramic family led to the discovery of high thermoelectrical performances in Ca 3 Co 4, Bi 2 Sr 3, LaCoO 3 and Bi 2 (6-9). Crystallographic studies on the CoO-based TE families showed that their structure is composed of two different layers: a common conductive CdI 2 -type CoO 2 layer, and an insulating rock-salt-type (RS) layer. The two sublattices possess common a- and c-axis lattice parameters and β angles, but different b-axis length, causing a misfit along the b-direction (10-12). 89

2 Sh. Rasekh, M.A. Madre, A. Sotelo, E. Guilmeau, S. Marinel, J.C.Diez The high structural anisotropy of this type of materials is reflected in their physical properties, as well as their preferential growth plane. This growth anisotropy leads to the formation of plate-like grains which can be aligned using physical, mechanical and/or chemical processes in order to attain macroscopic properties comparable to those obtained on single crystals. Numerous methods have been reported to be useful to obtain a good grain alignment, such as hot uniaxial pressing (13), spark plasma sintering (14), laser zone melting (LFZ) (15), template grain growth (TGG)(16), etc. In this work, the effect of three different synthetic routes (classical solid state route, sol-gel via nitrates (17) and polymer solution method (18)) on the final microstructure and thermoelectric properties (Seebeck coefficient, electrical resistivity, and power factor) of textured Bi 2 samples directionally grown by the laser floating melting zone method, has been studied. 2. Experimental The initial Bi 2 powders used in this work have been prepared by the three following methods: (i) Solid-state reaction: Bi 2 O 3 (98%, Panreac), CaCO 3 (98.5%, Panreac), and Co 3 O 4 (98%, Panreac) were ball-milled at 300 rpm for 1 h. The resulting mixture was placed in a furnace and heated slowly to 750 C, where it was kept for c.a. 12 h, followed by furnace cooling. After cooling, the remaining powder was ground and heated again at 800 ºC another 12 h, milled and isostatically pressed at c.a. 200 MPa in form of cylinders ( 3 mm diameter). (ii) Sol-gel: Bi(NO 3 ) 3 5H 2 O ( 98%, Aldrich), CaCO 3 (98.5%, Panreac), and Co(NO 3 ) 2 6H 2 O (98%, Panreac) were dissolved in a mixture of distilled water and concentrated HNO 3 (analysis grade, Panreac). Citric acid (99.5%, Panreac), and ethylene glycol (99%, Panreac), were added to this solution in the adequate proportions. Evaporation of the solvent was performed slowly in order to decompose the nitric acid excess, which allows the polymerization reaction between ethylene glycol and citric acid, forming a pink gel. The dried product was then decomposed (slow self combustion) by heating at 350 ºC for 1 h. The decomposed solid was mechanically ground for 30 minutes and calcined at 750 and 800 C for 12 h, with an intermediate grinding. As it was made for the solid state samples, the powders were isostatically pressed to obtain green ceramic cylinders. (iii) Polymer solution synthesis: Bi(CH 3 CO 2 ) 2 ( %, Aldrich), Ca(CH 3 CO 2 ) 2H 2 O (99%, Panreac), and Co(CH 3 CO 2 ) 2 4H 2 O (98 %, Panreac) commercial powders were dissolved in a mixture of distilled H 2 O and glacial acetic acid (ACS Reagent, Panreac), obtaining a dark-pink clear solution. Polyethylenimine (PEI) (50 % aqueous, Aldrich) was added to the above solution, which turned darker immediately due to the nitrogen-metal coordination. After partial evaporation ( 80 vol. %) of the solvent in a rotary evaporator, concentrated solution was placed onto a hot plate until a very dark pink paste appeared. Further heating turn this paste to violet color, followed by a slow combustion with the release of brown fumes (nitrogen oxides). The resulting powder was milled and calcined at 750 and 800 ºC for c.a. 12 h, with an intermediate milling, isostatically pressed, as in the previous methods. The thermal treatment used for the powders before pressing has been designed to decompose the carbonates before the LFZ process, otherwise their presence in the green bodies would lead to bubble formation inside the melt and disturb the crystallization front. The cylindrical ceramic bars, were subsequently used as feed in a LFZ device equipped with a continuous power Nd:YAG solid sate laser (1.06 μm), used successfully to texture other plate-like ceramic systems (19). The texturing process has been the same for all the samples, a growth rate of 30 mm/h has been used. In order to assure compositional homogeneity of the molten zone, seed and feed have been rotated, in opposite directions, at 3 and 15 rpm, respectively. After the directional growth process long, and very homogeneous dimensionally, textured cylindrical rods were obtained. Powder X-ray diffraction (XRD) patterns have been recorded in order to identify the main phases in the thermoelectric textured materials. Data have been collected at room temperature, with 2θ ranging between 10 and 35 degrees, using a Siemens Kristalloflex diffractometer working with Kα Cu radiation. Thermogravimetrical (TGA) studies have been performed on textured samples, under air, at a heating rate of 10 ºC/min, in order to study the evolution of the oxygen content with temperature. Microstructures have been observed by scanning electron microscopy JEOL 6000 equipped with an energy dispersive spectroscopy (EDS) device. On the one hand, micrographs of longitudinal fractured and polished sections have been recorded to observe the grain orientation and to analyze the different phases. The electrical resistivity (ρ) and the Seebeck Figure 1. Powder X-ray diffraction patterns of the textured Bi 2 samples, obtained by the different synthetic methods: a) solid state; b) sol-gel; and c) polymer solution. 90 Bol. Soc. Esp. Ceram. V. 49, 1, (2010)

3 Effect of synthetic methods on the thermoelectrical properties of textured Bi 2 ceramics coefficient (S) were simultaneously determined by the standard dc four-probe technique in a ZEM-3 measurement system (Ulvac-Riko), in the steady state mode and at temperatures ranging from 50 to 650 ºC. With the electrical resistivity and thermopower data, the power factor (PF) has been calculated (PF = S 2 /ρ) in order to determine the samples performances. 3. Results and discussion Powder XRD plots for samples obtained for the different synthetic methods are represented in Fig. 1. They show very similar patterns, where the most intense peaks correspond to the (002l) planes of the misfit cobaltite Bi 2, in agreement with previously reported data (20). Minor differences are found in weak peaks, corresponding to other planes of the structure, which are decreasing mostly in the case of the polymer solution method. In any case, cobaltite phase is the major one, independently of the synthetic route. The peak appearing at º, marked with a square, corresponds to the (111) peak of the Si, used as reference. A typical fracture of the textured samples is displayed in Fig. 2. It shows that all of them are well textured, posses a high degree of alignment of the ab planes along the growth direction, and formed by agglomerates of well-stacked platelike grains with dimensions exceeding 0.4 x 60 x 60 µm 3, in all the grown bars. When studying polished sections of the textured bars, it has been found that samples are composed by several phases, as it can be easily seen in Fig. 3, where SEM micrographs of longitudinal sections are displayed. This multiphasic system is found for all the samples, and it is composed by two major phases, a thermoelectric one, identified, by EDX, as Bi 2 (dark gray contrast) and a Bi-Ca-O solid solution (light grey contrast), which are growing as alternated layers. Moreover, in all the samples it is also found a third phase, corresponding Figure 2. Typical SEM micrograph of a fractured Bi 2 directionally grown sample. The arrow indicates the growth direction. Bol. Soc. Esp. Ceram. V. 49, 1, (2010) Figure 3. SEM micrographs of longitudinal polished sections, of the Bi 2 samples, for the different synthetic methods: a) solid state; b) sol-gel; and c) polymer solution. The arrows indicate the CaCoO black phase. 91

4 Sh. Rasekh, M.A. Madre, A. Sotelo, E. Guilmeau, S. Marinel, J.C.Diez to CoO with dendritic-like aspect (black contrast). Finally, it has also been found a fourth phase CaCoO, appearing only in samples derived from the solid state precursors. It can be clearly seen in Fig. 3a as long and thin black grains, indicated by arrows for clarity. In addition to the above observations, all the samples show a compact microstructure without appreciable porosity at this magnification level. When comparing samples obtained with the different synthetic methods, it is clear that using the solution methods allows the reduction of the amount and size of the secondary phases and, as a consequence, increases the thermoelectric phase. From image analysis performed on several SEM micrographs, it has been determined a decrease of the amount of secondary phases of about 5% for the sol-gel method and 15% for the polymer solution one, when compared with the solid state precursors. Another important difference, which can be observed in Fig. 3, is related with the thermoelectric grain orientation for each of the samples. The deviation from the growth axis has been measured over different micrographs. From these data, mean values have been calculated, corresponding to misalignments of about 10º for the solid state samples, 6º for the sol-gel, and 4º for the polymer solution method derived ones. In Fig. 4, the evolution of electrical resistivity (ρ), as a function of the temperature, is represented for the different samples. All the samples show similar semiconductor behaviour from room temperature to about 550ºC and a metallic-like one for higher temperatures. Because of the structural evolution described above, the resistivity values, at room temperature, decrease when the amount of secondary phases is reduced and the grain alignment is increased. As a consequence, the lowest values are measured in samples obtained from the polymer solution method, which are of the same order of the best reported values for sintered materials (10,12). The evolution of the thermopower (TEP) with temperature is presented in Fig. 5. In all the cases, the entire measurement is positive, indicating a hole-conduction mechanism. All samples show very similar values, which increase with temperature in all the measured range, reaching a maximun value of about 265 µv/k at 650 ºC. When comparing the TEP values, at room temperature, with the best results reported in the literature for sintered specimens (about 150 µv/k at 300K)(9), an important increase is obtained for all the samples. These high values of the TEP are not common in this system, but it can be explained by a variation of holes concentration. The LFZ growth can probably generate oxygen vacancies in a larger content than in bulk sintered ceramics. The holes concentration is consequently decreased due to the reduction of Co 4+ to Co 3+. It has already been evidenced that, under reduced conditions, the misfit phase [ CoO 3 ][CoO 2 ] 1.62 (close to the present misfit system) contains considerable amounts of oxygen vacancies (21). In order to confirm this hypothesis, thermogravimetrical analysis of the materials has been performed, under air, between room temperature and 1000 ºC. The results for a representative sample are displayed in Fig. 6. In this plot, it can be clearly seen that from about 850 to 1000 ºC, a weight reduction of about 1.5 % is produced. Taking into account that the texturing process implies a rapid solidification and cooling from the liquid, the oxygen content in this type of samples will be lower than for those obtained after sintering at ºC, increasing the amount of Co 3+ and, consequently, explaining the high measured values of the TEP for laser grown samples (9), interpreted from the Koshibae s expression (22). From the resistivity and TEP data, Power Factor (PF) has been calculated and represented in Fig. 7, as a function of the temperature, for the different samples. It can be observed that the power factor at low temperatures is depending on the synthetic route, while at high temperatures, all samples Figure 4. Temperature dependence of the electrical resistivity, ρ, as a function of the synthetic method; solid state; sol-gel; and polymer solution. Figure 5. Temperature dependence of the Thermopower, TEP, as a function of the synthetic method; solid state; sol-gel; and polymer solution. 92 Bol. Soc. Esp. Ceram. V. 49, 1, (2010)

5 Effect of synthetic methods on the thermoelectrical properties of textured Bi 2 ceramics display similar values, with a maximum of about mw/k 2 m at 650ºC. The highest increase on the PF (at room temperature) has been obtained with polymer solution method samples, which raise the PF in about 2.5 times, compared with the classical solid state route. When comparing with the best results reported in the literature for sintered materials, at room temperature ( 0.02 mw/k 2 m), all samples increase substantially these values between, approximately, 2.0 and 2.5 times. 4. Conclusions This paper demonstrates that Bi 2 thermoelectric materials can be directionally grown successfully by the laser floating zone method (LFZ). This process leads to well textured samples with the Bi-Ca-Co-O cobaltite phase as the main one, which is alternatively stacked with the Bi-Ca-O solid solution phase. Low amounts of CoO secondary phases are found in any case. The texture is improved, and the amount of secondary phases is reduced, when solution methods are used to obtain the ceramic precursors for the LFZ directional growth. Thermoelectrical performances are also increased by solution synthetic methods. The best results for all the measured properties have been obtained for samples grown from precursors synthesized by the polymer solution route. This method has been shown to be very promising for the synthesis of thermoelectrical ceramics with improved properties. 5. Acknowledgements The authors wish to thank the Gobierno de Aragón (Research Groups T12 and T74, and project PI154/08), the Spanish Ministry for Education and Science (Spanish-French Integrated Action HF ), and the Spanish Ministry of Science and Innovation (Project MAT ) for financial support. The technical contributions of C. Estepa, J. A. Gómez and C. Gallego are also acknowledged. 6. References 1. G. Mahan, B. Sales, J. Sharp, Thermoelectric materials: New approaches to an old problem, Phys. Today, 50, 3, (1997). 2. Thermoelectrics Handbook: Macro to Nano; D.M. Rowe, Ed.; CRC Press Taylor & Francis Group publications:boca Raton, Fl. 2006; pp 1-3 to John Fairbanks, Thermoelectric applications in vehicles status 2008, U.S. Department of Energy. 4. Bhatti, M.S., A Critical Look at R-744 and R-134a Mobile Air-Conditioning Systems SAE Technical Paper, SAE I. Terasaki, Y. Sasago, K. Uchinokura, Large thermoelectric power in Na O 4 single crystals, Phys. Rev. B, 56, 20, (1997). 6. R. Funahashi, I. Matsubara, H. Ikuta, T. Takeuchi, U. Mizutani, S. Sodeoka, An oxide single crystal with high thermoelectric performance in air, Jpn. J. Appl. Phys., 39, 11B, L1127-L1129 (2000). 7. A. C. Masset, C. Michel, A. Maignan, M. Hervieu, O. Toulemonde, F. Studer, B. Raveau, J. Hejtmanek, Misfit-layered cobaltite with an anisotropic giant magnetoresistance Ca 3 Co 4, Phys. Rev. B, 62, 1, (2000). 8. H. Leligny, D. Grebille, O. Perez, A. C. Masset, M. Hervieu, B. Raveau, A five-dimensional structural investigation of the misfit layer compound [Bi 0.87 SrO 2 ] 2 [CoO 2 ] 1.82, Acta Cryst. B, 56, Part 2, (2000). 9. A. Maignan, D. Pelloquin, S. Hebert, Y. Klein, M. Hervieu, Thermoelectric Power In Misfit Cobaltites Ceramics: Optimization By Chemical Substitutions, Bol. Soc. Esp. Ceram. V., 45, 3, (2006). 10. A. Maignan, S. Hébert, M. Hervieu, C. Michel, D. Pelloquin, D. Khomskii, Magnetoresistance and magnetothermopower properties of Bi/Ca/Co/O and Bi(Pb)/Ca/Co/O misfit layer cobaltites, J. Phys.: Condens. Matter., 15, 17, (2003). 11. H. Itahara, C. Xia, J. Sugiyama, T. Tani, Fabrication of textured thermoelectric layered cobaltites with various rock salt-type layers by using beta-co(oh) 2 platelets as reactive templates, J. Mater. Chem., 14, 1, (2004). 12. E. Guilmeau, M. Mikami, R. Funahashi, D. Chateigner, Synthesis and thermoelectric properties of Bi layered cobaltites, J. Mater. Res., 20, 4, (2005). 13. V. Garnier, R. Caillard, A. Sotelo, G. Desgardin, Relationship among synthesis, microstructure and properties in sinter-forged Bi-2212 ceramics, Physica C, 319, (1999). 14. Yanfeng Zhang, Jiuxing Zhang, Qingmei Lu, Synthesis of highly textured Ca 3 Co 4 ceramics by spark plasma sintering, Ceram. Int., 33, 7, , (2007). Figure 6. Thermogravimetrical analysis plot vs. temperature of a representative sample. Bol. Soc. Esp. Ceram. V. 49, 1, (2010) Figure 7. Temperature dependence of the power factor, PF, as a function of the synthetic method; solid state; sol-gel; and polymer solution. 93

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