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Nanocrystalline Thermoelectric Ca3Co4O9 Ceramics by Sol-Gel Based Electrospinning and Spark Plasma Sintering Tongfang Yin,† Dawei Liu,‡ Yun Ou,† Feiyue Ma,§ Shuhong Xie,†,§ Jing-Feng Li,‡ and Jiangyu Li*,§ Key Laboratory of Low Dimensional Materials and Application Technology of Ministry of Education, and Faculty of Materials Optoelectronics and Physics, Xiangtan UniVersity, Xiangtan, Hunan 411105, People’s Republic of China, State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua UniVersity, Beijing 100084, People’s Republic of China, and Department of Mechanical Engineering, UniVersity of Washington, Seattle, Washington 98105-2600 ReceiVed: March 19, 2010; ReVised Manuscript ReceiVed: April 23, 2010
A novel technique is developed to process nanocrystalline Ca3Co4O9 ceramics with much enhanced thermoelectric properties. Nanocrystalline Ca3Co4O9 nanofibers are synthesized first using sol-gel based electrospinning, and then consolidated into bulk ceramics by spark plasma sintering with preferred grain orientation distribution and without substantial grain growth. The nanofiber-sintered ceramic has a grain size much smaller than that sintered from sol-gel synthesized powders with improved texture, and has simultaneously enhanced Seebeck coefficient, electric conductivity, and thermal resistivity, resulting in substantial enhancement in thermoelectric figure of merit ZT. This technique is promising for high-efficiency thermoelectric conversion of waste heat directly into electricity. 1. Introduction High-performance thermoelectric materials are promising in converting waste heat directly into electricity in an efficient, economical, and environment friendly manner, and can be used for example to harvest a large portion of petroleum energy lost as waste heat in a typical gasoline-fueled internal combustion engine.1,2 High-efficiency thermoelectric conversion requires simultaneously high electric conductivity and thermal resistivity,3,4 and this turns out to be rather challenging. Since the electron contribution to thermal conductivity is proportional to electric conductivity according to the Wiedemann-Franz law, high electric conductivity is usually accompanied by high thermal conductivity in most materials. As a result, it is necessary to reduce phonon contribution to the thermal conductivity for the enhanced thermoelectric figure of merit, and this could be realized by increasing phonon scattering at grain boundaries and interfaces, for which engineered nanostructures are very effective due to their substantially increased interfaces.5 Indeed, many nanostructured thermoelectric materials have been developed, including low-dimensional materials such as nanowires, nanotubes, and superlattices,6-16 and bulk materials such as nanocrystalline alloys and nanocomposites.17-25 For example, it has recently been reported that the dimensionless thermoelectric figure of merit ZT can be improved to 1.4 in the bulk nanocrystalline BiSbTe alloy,18 which is believed to arise from its low thermal conductivity due to the enhanced phonon scattering at grain boundaries and defects. Encouraged by these developments and our recent works on nanocrystalline multiferroic nanofibers,26-28 we have developed a novel processing technique to synthesize nanocrystalline thermoelectric oxide ceramics, combing sol-gel based electro* To whom correspondence should be addressed. Tel: 1-206-543-6226. Fax: 1-206-685-8047. E-mail:
[email protected]. † Xiangtan University. ‡ Tsinghua University. § University of Washington.
spinning and spark plasma sintering (SPS). Thermoelectric oxide ceramics are particularly suitable for harvesting waste heat at high temperature, since they are nontoxic with good chemical stability in air and at temperature up to 800-1000 K,29,30 and they are much lighter than typical thermoelectric alloys such as SbTe. Among potential oxide materials, layered cobalt oxides Ca3Co4O9 are particularly promising, in which CoO2 nanosheets possessing a strongly correlated electron system serve as electronic transport layers, while calcium cobalt oxide misfit layers serve as phonon scattering regions. As a result, it is possible to control and optimize electron and phonon transports separately, and numerous studies confirmed that such layered cobalt oxides do exhibit low thermal conductivity and metalliclike electric conductivity, very attractive for thermoelectric applications.31-34 Indeed, a ZT value of 1.2 has been estimated for a single crystalline Ca2Co2O5 whisker.31 The figure of merit for polycrystalline Ca3Co4O9, however, is substantially lower,35,36 due to the random orientation distribution of grains in the ceramics and highly anisotropic thermoelectric properties of Ca3Co4O9 single crystal. This makes it imperative to control not only the grain growth during ceramic sintering process, but also the grain orientation for texturing, and we hope to accomplish both by combining electrospinning and SPS. Sol-gel based electrospinng is a versatile process that is capable of synthesizing a wide range of ceramic nanofibers, with a grain size as small as 10 nm.37-40 For the first time, this technique is combined with SPS to consolidate nanocrsytalline thermoelectric nanofibers into bulk ceramics without substantial grain growth, since SPS is a fast sintering process that takes only a few minutes. In addition, large compressive stress is applied during sintering, which would induce preferred grain orientation that is particularly effective for layered oxides such as Ca3Co4O9.41-43 Indeed, nanocrystalline Ca3Co4O9 nanofibers with grain size in the range of 40 nm have been synthesized in this work with use of sol-gel based electrospinning, and have been subsequently sintered into bulk ceramics with SPS. The
10.1021/jp1024872 2010 American Chemical Society Published on Web 05/12/2010
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Figure 1. SEM images of Ca3Co4O9 nanofibers under different calcinating temperatures: nanofibers before calcination (a), and nanofibers calcinated at 550 (b), 750 (c), and 850 °C (d).
nanocrystalline structure of the nanofiber has been largely preserved with preferred grain orientation distribution after SPS, though grains did grow to over 100 nm, suggesting the need for further optimization of the sintering process. A series of thermoelectric measurements reveal that nanofiber-sintered Ca3Co4O9 ceramics possess simultaneously enhanced Seebeck coefficient, electric conductivity, and thermal resistivity compared to powder-sintered ones, resulting in a substantially enhanced thermoelectric figure of merit ZT. 2. Experimental Methods 2.1. Syhthesis. The Ca3Co4O9 sol-gel precursor was prepared by dissolving calcium acetate monohydrate (C4H6O4Ca · H2O) and cobalt acetate tetrahydrate (CO(CH3COO)2 · 4H2O) with stoichiometric proportions into a hybrid solution of methanol (CH3OH) and propionic acid (CH3CH2COOH) with a volume ratio of 3/7. Poly(vinyl pyrrolidone) (PVP, Mw ) 1 300 000 g · mol-1) was then added to the sol-gel precursor with its concentration controlled to be 0.03 g · mL-1. The mixture was stirred continuously until a homogeneous electrospinning precursor of 0.2 mol · L-1 Ca3Co4O9 was formed. The solution was loaded into a plastic syringe equipped with a stainless steel needle connected to a high-voltage supply (SpellmanSL40P300), and then electrospun with the feed rate of the solution controlled at 0.015 mL · min-1 and the electric field set around 1.4 kV · cm-1. The as-spun nanofibers were collected by a glass flake or Pt/Ti/SiO2/Si wafer, dried at 120 °C for 4 h, and then calcinated at 550, 700, 750, or 850 °C for 2 h. For comparison, Ca3Co4O9 ceramic powders were also synthesized, using the conventional sol-gel process, by drying the Ca3Co4O9 precursor solution without PVP in an oven at 120 °C for 2 days to obtain xerogel, and then calcinating the xerogel at 850 °C for 2 h. Calcinated Ca3Co4O9 nanofibers and powders were consolidated by using spark plasma sintering (SPS1050, Sumitomo Coal Mining Co., Japan). Loose Ca3Co4O9 nanofibers or powders were loaded into a graphite die and placed in the SPS chamber. The sintering temperature was increased to 750 °C at a heating rate of 100 deg · min-1, and a constant pressure of 50 MPa was applied. The temperature was kept at 750 °C for 5 min under a high DC current, before the pressure was released and the
sample was cooled to room temperature. The SPS-sintered ceramics were annealed at 750 °C for 2 h in air to eliminate the carbon on the surfaces, and were cut into appropriate size for thermoelectric measurements. 2.2. Characterizations. Scanning electron microscopy (SEM) (LEO-1525) was used to examine the morphology of Ca3Co4O9 nanofibers and powders before and after SPS, and X-ray diffraction (XRD) (Rigaku D/max-rA) was used to identify their crystalline structures. Transmission electron microscopy (TEM) (JEM-2100) and high-resolution transmission electron microscopy (HRTEM) were used to further examine the grain size and lattice structure of Ca3Co4O9, with TEM samples obtained by ultrasonic dispersion of nanofibers or powders in ethanol solution for 30 min. Thermoelectric properties were measured along specimen surface perpendicular to the pressing direction of SPS. The Seebeck coefficient and electrical resistivity were measured with a Seebeck coefficient/electric resistance measuring system (ZEM-2, Ulvac-Riko, Japan). The thermal diffusivity was measured by the laser flash method (NETZSCH, LFA427, Germany). The specific heat was measured with a thermal analyzing apparatus (Dupont 1090B, USA). The density of the sample was measured by the Archimedes method. The thermal conductivity was calculated from the product of thermal diffusivity, specific heat, and density. 3. Results and Discussions The morphology of Ca3Co4O9 nanofibers before and after calcination was examined by SEM, as shown in Figure 1. It is observed that the as-spun nanofibers after drying are straight with smooth surface and uniform cross-section of approximately 350 nm in diameter, and there is no obvious indication of crystallization, as showing in Figure 1a. After calcination, the nanofibers are able to keep their continuous fibrous morphology, though the surface of nanofibers becomes much rougher, which is believed to be caused by crystallization. Furthermore, the diameter of the calcinated nanofibers is reduced to about 200 nm, mainly due to the burn off of PVP polymers. Ca3Co4O9 crystal has a layered lattice structure, from which lamellar type
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Figure 4. SEM images of Ca3Co4O9 ceramics sintered from (a) nanofibers and (b) sol-gel powders by SPS method. Figure 2. TEM and HRTEM images of Ca3Co4O9 nanofiber: (a) TEM image of a single nanofiber showing nanocrystalline structure and (b) HRTEM image with SAED pattern showing the lattice structure of Ca3Co4O9.
Figure 5. XRD pattern of Ca3Co4O9 nanofibers and powders before and after SPS. Figure 3. Images of sol-gel synthesized Ca3Co4O9 powders: (a) SEM and (b) TEM.
of grains are expected, and this is clearly visible in nanofibers calcinated at 750 and 850 °C as seen in Figure 1c,d. Calcination at 550 °C appears insufficient to crystallize Ca3Co4O9, as indicated by the much smaller surface roughness of nanofibers shown in Figure 1b, while calcination at 750 °C results in inhomogeneous grain distribution in the nanofibers, as seen in Figure 1c. As such, we identify 750-850 °C as the optimal calcination temperature range for Ca3Co4O9, which is also confirmed by XRD results to be discussed later. The crystalline structure of Ca3Co4O9 nanofiber was examined by TEM and HRTEM, as shown in Figure 2. It is observed that nanofiber is composed of nanocrystalline grains with grain size around 40 nm, as seen in Figure 2a, although larger grains are also present. From the HRTEM image in Figure 2b, the lattice space of Ca3Co4O9 nanofiber is measured to be 0.245 nm between (201) planes and 0.269 nm between (004) planes. The polycrystalline nature of Ca3Co4O9 nanofiber is evident from the selected area electron diffraction (SAED) ring pattern shown in the insert of Figure 2b, indicating the random grain orientation distribution in the nanofibers and the fine grain size. One of the major advantages of sol-gel based electrospinning is the nanoscrystalline structure it produces, which could result in enhanced thermoelectric properties. To demonstrate this, we also synthesized Ca3Co4O9 powders using the conventional sol-gel process, and the resulting structures are shown in Figure 3. From the SEM image, it is clear that the sol-gel synthesized powders have much larger grain size around 1 µm, as seen in Figure 3a, and the powders have lamellar crystalline morphology of Ca3Co4O9 expected from its layered lattice structure. Such large grain size is also confirmed by the TEM image in Figure 3b, showing a grain size well over 500 nm, 1 order of magnitude larger than that of nanofibers. For such large grains, we expect that the SAED pattern will show characteristics of single crystalline lattice instead of polycrystalline ring pattern, though
we have not examined it in detail in this work. These results suggest that the electrospinning is indeed a very effective method to synthesize nanocrystalline thermoelectric oxides, capable of producing extremely fine grains an order of magnitude smaller than the conventional sol-gel technique. Such fine grain size may be caused by the geometry confinement of nanofiber, which has a rather small diameter itself in the range of 200 nm that may constrain the grain growth. In addition, when polymers are burned out during calcination, local reductive atmosphere is induced, which may also limit the grain growth of oxide. Indeed, the calcinated nanofibers are not very dense, and pores are clearly visible in Figure 2a. Thermoelectric properties of Ca3Co4O9 nanofibers are extremely difficult to measure, and energy harvesting from waste heat requires bulk thermoelectric modules instead of individual nanofibers. As such, spark plasma sintering (SPS) is used to consolidate Ca3Co4O9 nanofibers and powders into bulk ceramics, and the resulting microstructures are examined by SEM, as shown in Figure 4. The ceramics are rather dense after SPS, and the lamellar grain structure is evident in both samples, especially in ceramics sintered from nanofibers. It appears that the ceramic sintered from nanofibers has a thinner structure with more interfaces, though both ceramics are sintered at 750 °C
Figure 6. TEM images of Ca3Co4O9 ceramics sintered by SPS from (a) electrospun nanofibers and (b) sol-gel synthesized powders.
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Figure 7. Thermoelectric properties of Ca3Co4O9 ceramics sintered from electrospun nanofibers and sol-gel synthesized powders: (a) Seebeck coefficient; (b) electric conductivity; (c) thermal conductivity; and (d) figure of merit ZT.
under identical conditions. This is because the nanofiber has much smaller grain size before sintering, and it may result in reduced thermal conductivity and enhanced thermoelectric figure of merit. The lamellar grains appear to be more or less aligned, and such texture would be advantageous for thermoelectric performance, since the thermoelectric properties of layered Ca3Co4O9 are highly anisotropic.35,36 The crystalline structures of both Ca3Co4O9 specimens before and after SPS are also examined by XRD, as shown in Figure 5, and excellent crystallinity is observed in nanofibers calcinated in 750-850 °C ranges with no detectable impurity phase. Lower calcinating temperature results in poorer crystallinity, and sometimes impurity phase, consistent with SEM observations in Figure 1. Before SPS, the diffraction peaks are very sharp, especially in Ca3Co4O9 nanofibers. After SPS, some diffraction peaks disappeared, indicating preferred grain orientation distribution resulting from mechanical pressing at high temperature, consistent with the SEM morphology shown in Figure 4. The XRD patterns also suggest that the ceramic sintered from nanofibers does have better texture, probably due to pores in the nanofibers that make the grain rotation under compression easier. This will also be advantageous for improved thermoelectric properties. The grain size of Ca3Co4O9 ceramics is also examined by TEM, as shown in Figure 6, and it is observed that grains do grow larger after SPS. For Ca3Co4O9 sintered from nanofibers, the grain size appears to be over 100 nm, while for ceramic sintered from Ca3Co4O9 powders, it is well over 500 nm. Such grain growth is disadvantageous for thermal conductivity reduction, and we are currently working on controlling the grain growth during the SPS process. The nanocrystalline structure and improved texture of Ca3Co4O9 ceramic sintered from electrospun nanofibers is expected to result in improved thermoelectric properties compared to those sintered from sol-gel synthesized powders, which is verified by thermoelectric measurements, as shown in Figure 7. The Seebeck coefficient (a), electric conductivity (b), thermal
conductivity (c), and thermoelectric figure of merit ZT (d) of both Ca3Co4O9 ceramics as a function of temperature are presented, and interestingly, the ceramic sintered from nanofibers shows simultaneously higher Seebeck coefficient and electric conductivity yet lower thermal conductivity. This confirms our hypothesis that the thermal conductivity of nanoscrystalline ceramics can be reduced by phonon scattering at grain boundaries without compromising its electric conductivity, which results in a thermoelectric figure of merit ZT approximately 33% higher than that of powder-sintered ceramics, as shown in Figure 7d. In fact, the electric conductivity of nanofiber-sintered ceramic is substantially higher, and the Seebeck coefficient is also enhanced modestly, despite smaller grain size, which is believed to be caused by improved texture. Furthermore, both Seebeck coefficient and electric conductivity increase with temperature (except for a few points of electric conductivity at low temperatures), while thermal conductivity decreases with temperature, resulting in substantially higher ZT at higher temperature, ideal for thermoelectric energy harvesting from waste heat at high temperature. Compared with Ca3Co4O9 ceramics sintered with other techniques, our specimen sintered from nanofibers has comparable Seebeck coefficient and electric conductivity, yet much smaller thermal conductivity,41-43 which is expected to result from phonon scattering at grain boundaries. For example, it has recently been reported that La-doped Ca3Co4O9 ceramics sintered by using SPS and dynamic forging has a thermal conductivity of 2.067 W · m-1 · K-1 at 975 K, much larger than our value of 1.235 W · m-1 · K-1 at 773 K.44 This thermal conductivity is expected to be reduced to around 0.970 W · m-1 · K-1 at 975 K by using linear extrapolation, which is quite reasonable according to Figure 7c. In contrast, our sample has a power density of 0.000254 W · m-1 · K-2 at 780 K, compared to 0.00035 W · m-1 · K-2 for La-doped Ca3Co4O9 ceramics at similar temperature. This would result in 55% enhancement in the thermoelectric figure of merit ZT, even though our ceramics was not doped. On the basis of reported
Nanocrystalline Ca3Co4O9 Ceramics ZT of 0.26 for La-doped Ca3Co4O9 at 975 K, our nanofibersintered Ca3Co4O9 is expected to have a ZT value around 0.40 at 975 K, and modification by doping can enhance the figure of merit even further.45-47 4. Conclusion In conclusion, nanocrystalline Ca3Co4O9 ceramics have been synthesized by using a novel technique combining sol-gel based electrospinning and SPS. Much smaller grain size and improved texture are observed in the nanofiber-sintered ceramics, resulting in simultaneously enhanced Seebeck coefficient, electric conductivity, and thermal resistivity. Compared with La-doped Ca3Co4O9 ceramics synthesized with solid state reaction and spark plasma sintering recently reported,44 our nanofiber-sintered Ca3Co4O9 ceramics without doping has substantially reduced thermal conductivity, resulting in 55% enhancement in the thermoelectric figure of merit ZT that is estimated to be around 0.40 at 975 K. Acknowledgment. We acknowledge support from National Science Foundation (CMMI-0969543) and Natural Science Foundation of China (Approval No. 10732100). References and Notes (1) Arunachalam, V. S.; Fleischer, E. L. MRS Bull. 2008, 33, 264. (2) Yang, J. H.; Caillat, T. MRS Bull. 2006, 31, 224. (3) DiSalvo, F. J. Science 1999, 285, 703. (4) Tritt, T. M.; Subramanian, M. A. MRS Bull. 2006, 31, 188. (5) Nowotny, J.; Bak, T.; Burg, T.; Nowotny, M. K.; Sheppard, L. R. J. Phys. Chem. C 2007, 111, 9769. (6) Rao, A. M.; Ji, X. H.; Tritt, T. M. MRS Bull. 2006, 31, 218. (7) Bottner, H.; Chen, G.; Venkatasubramanian, R. MRS Bull. 2006, 31, 211. (8) Venkatasubramanian, R.; Siivola, E.; Colpitts, T.; O’Quinn, B. Nature 2001, 413, 597. (9) Harman, T. C.; Taylor, P. J.; Walsh, M. P.; LaForge, B. E. Science 2002, 297, 2229. (10) Linchung, P. J.; Reinecke, T. L. Phys. ReV. B 1995, 51, 13244. (11) Broido, D. A.; Reinecke, T. L. Phys. ReV. B 1995, 51, 13797. (12) Dresselhaus, M. S.; Chen, G.; Tang, M. Y.; Yang, R. G.; Lee, H.; Wang, D. Z.; Ren, Z. F.; Fleurial, J. P.; Gogna, P. AdV. Mater. 2007, 19, 1043. (13) Yang, Y. A.; Taggart, D. K.; Brown, M. A.; Xiang, C. X.; Kung, S. C.; Yang, F.; Hemminger, J. C.; Penner, R. M. ACS Nano 2009, 3, 4144. (14) Liu, Y. S.; Chen, Y. R.; Chen, Y. C. ACS Nano 2009, 3, 3497. (15) Chen, C. L.; Chen, Y. Y.; Lin, S. J.; Ho, J. C.; Lee, P. C.; Chen, C. D.; Harutyunyan, S. R. J. Phys. Chem. C 2010, 114, 3385–3389. (16) Tai, G. A.; Zhou, B.; Guo, W. L. J. Phys. Chem. C 2008, 112, 11314. (17) Snyder, G. J.; Toberer, E. S. Nat. Mater. 2008, 7, 105.
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