HighTemperature Thermoelectric Behaviors of FineGrained GdDoped

0 downloads 0 Views 558KB Size Report
grain size, which indicates that fine-grained ceramics can main- tain good electrical property. However, thermal conductivity values of the fine-grained sample ...
J. Am. Ceram. Soc., 93 [8] 2121–2124 (2010) DOI: 10.1111/j.1551-2916.2010.03673.x r 2010 The American Ceramic Society

Journal High-Temperature Thermoelectric Behaviors of Fine-Grained Gd-Doped CaMnO3 Ceramics Jinle Lan,z Yuan-Hua Lin,w,z Hui Fang,z Ao Mei,z Ce-Wen Nan,z Yong Liu,y Shaoliang Xu,y and Matthew Petersz z

State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

y

School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China z

School of Science and Engineering, Tulane University, New Orleans, Louisiana 70118

pare with that of the state-of-the-art thermoelectric material Bi2Te3. The poor performance is due to the high thermal conductivity, and hence it is desirable to reduce the thermal conductivity, thereby increasing the thermoelectric dimensionless figure of merit ZT. One of the possible ways to lower the lattice thermal conductivity is enhancing the grain boundary scattering through refining the grain size. Many experimental results have shown that the reduction of thermal conductivity by grain boundary scattering is more significant than the enhancement of S2s.8,9 Thus, fine-grained materials are expected to be effective to enhance the thermoelectric properties. However, a systematic investigation of fine-grained thermoelectric bulk oxides has yet to be carried out. Some previous studies on the CaMnO3 perovskite system suggested that Ca MnO3 could be a potential candidate as an n-type thermoelectric material.10,11 For example, Bocher et al.12 used a sol–gel synthesis method to obtain the sub micrometer-size grains and observed that their TE performance could be improved greatly. In the present work, we attempted to improve the thermoelectric properties of Gd-doped CaMnO3 by fine-grained structuring. Fine-grained (200–400 nm) and coarse (3–5 mm) ceramics were prepared by the coprecipitation method and the solid-state reaction, respectively. Low thermal conductivity and enhanced ZT values have been observed in the fine-grained samples.

Gd-doped CaMnO3 were synthesized by two methods: a coprecipitation route and a conventional solid-state reaction. The ceramic samples obtained by the coprecipitation method possess nanometer-scale grains but the grain sizes by the solid-state reaction route are several micrometers. Our results show that the electrical conductivity is slightly decreased with a decreasing grain size, which indicates that fine-grained ceramics can maintain good electrical property. However, thermal conductivity values of the fine-grained sample are relatively low due to the enhancement of grain boundary scattering. The highest dimensionless figure of merit ZT 5 0.24 has been obtained at 973 K in the air for fine-grained Ca0.96Gd0.04MnO3, suggesting that they can be a promising candidate of n-type material for high-temperature thermoelectric application.

I. Introduction

T

HERMOELECTRIC materials can directly convert heat into electric energy and vice versa through the thermoelectric phenomena in solids. This makes it a potential method for clean energy generation by transforming heat into electricity.1 The conversion efficiency can be well characterized by the dimensionless figure of merit ZT 5 S 2sT/k, where T is the absolute temperature, S the thermoelectric power, s the electrical conductivity, and k the thermal conductivity, respectively. Therefore, a high power factor (PF) S2s, and a low thermal conductivity k are necessary for practical applications of thermoelectric materials and related devices. Oxide semiconductors, which are thermally and chemically stable in air at high temperature, are easy to manufacture at low cost, and thus have attracted considerable attention recently. Since Terasaki et al.2 have found that NaCo2O4 single crystals showed good TE performance with a high Seebeck coefficient (100 mV/K at 300 K) and a low resistivity (200 mO  cm at 300 K), various oxides such as Ca3Co4O9,3–5ZnO,6 and SrTiO3,7 were found to exhibit intriguing thermoelectric properties. Although the properties of oxides can be modified and optimized by different cationic substitutions, the ZT value is too low to com-

II. Experimental Section Polycrystalline ceramic samples of Ca1xGdxMnO3 (x 5 0.02, 0.04, and 0.06) were synthesized via a chemical coprecipitation method to obtain nanometer powders. Additionally, coarse Ca0.96Gd0.04MnO3 powders were prepared by a solid-state reaction method (SSR) for comparison. For the coprecipitation method, a mixture of Ca(NO3)2  4H2O, Mn(NO3)2, Gd(NO3)3  6H2O, and NH4HCO3 (all agents are of analytical purity) were used as starting materials, and dissolved in distilled water to make the nitrate stock solution. The pH value was controlled in the range of 7.5–9.0 to make the metal ions precipitate completely. The resultant suspension was subjected to suction filtration. The filtration cake was washed several times with distilled water before drying at 343 K for 2 h. The precursor powder was calcined at 1073 K for 6 h in air. The power compact was sintered at 1373 K for 6 h in air, and then slowly cooled to room temperature. For the SSR method, analytical purity CaCO3, Mn2O3, and Gd2O3 were used as raw materials. These were weighed in the stoichiometric ratio of Ca0.96Gd0.04MnO3 and mixed using a ball mill for 24 h. The mixed powders were heated at 1373 K for 12 h and at 1423 K for

D. Chateigner—contributing editor

Manuscript No. 27088. Received November 30, 2009; approved January 10, 2010. This work was financially supported by the Ministry of Sci & Tech of China through a 973-Project, under grant No. 2007CB607504, and the National High Technology Research Project, under grant No. 2009AA03Z216. w Author to whom correspondence should be addressed. e-mail: [email protected]

2121

2122

Rapid Communications of the American Ceramic Society

Vol. 93, No. 8

III. Results and Discussion

Fig. 1. X-ray diffraction patterns of various CaMnO3 samples.

12 h with intermediate grinding. Finally, all the ceramics samples were obtained as sintered at 1573 K for 12 h. X-ray diffraction (XRD) with a Rigaku D/MAX-2550 V diffractometer (Rigaku, Tokyo, Japan; CuKa radiation) and scanning electron microscopy (SEM, JSM-6460LV, JEOL, Tokyo, Japan) were used to reveal the phase composition and microstructure of the as-synthesized samples, respectively. The particle morphology was observed by transmission electron microscopy (TEM, JEOL-2011). The temperature dependence of electric conductivity was measured from room temperature to 1000 K by a four-probe method. Thermoelectric power was obtained from the slope of the linear relation between DV and DT, where DV is the thermoelectromotive force produced by a temperature difference DT. The high-temperature thermal conductivity k was determined from measurements of the thermal diffusivity (a), the heat capacity (Cp), and the density (r), using the relationship: k 5 a  Cp  r. The relative bulk density was measured by the Archimedes method, and a Netzsch LFA 457 (Selb, Germany) laser flash apparatus measured the thermal diffusivity. The specific heat was determined by differential scanning calorimetry (DSC) using a Netzsch DSC 404 C Pegasus.

Four kinds of Gd-substituted CaMnO3 ceramic samples with the nominal composition Ca0.98Gd0.02MnO3 (CMO-1), Ca0.96Gd0.04MnO3 (CMO-2), Ca0.94Gd0.06MnO3 (CMO-3) were synthesized by the chemical coprecipitation method and Ca0.96Gd0.04MnO3 (CMO-2s) by the SSR. Figure 1 indicates that all samples are single CaMnO3 phase with an orthorhombic perovskite-type structure. The average crystallite sizes for samples fabricated by chemical coprecipitation, estimated from XRD data by means of the Scherer equation, are about 50–70 nm and the size of CMO-2s is about 1–2 mm. The inset figure shows the XRD patterns of the enlarged (112) peaks for Ca0.96Gd0.04MnO3 powders by the two methods. It can be observed that the peak width for the coprecipitation method is narrower than that of the SSR one. Figure 2(a) shows the TEM photograph of the powder synthesized by the chemical coprecipitation method. Average particle size determined from the TEM observation is 65 nm. Therefore, both XRD and TEM data confirm that the size of powders prepared by the coprecipitation method is in the range of 50–70 nm. Similar nanoparticles were reported by Sanmathi et al.13 Generally, a solid-state reaction for synthesizing perovskite-type CaMnO3 requires temperatures 41473 K and time longer than 12 h. However, the coprecipitation technique can fabricate the powder at a low temperature, which maybe ascribed to the high specific surface area and high reactivity of nanometer precursor powders. Figures 2(b) and (c) exhibit the typical morphology of sintered pellets obtained by the coprecipitation method and SSR, respectively. The grain size of the samples prepared by SSR is about 3–5 mm, while those fabricated by coprecipitation are about 200–400 nm. The relative density of samples prepared through both two methods is about 80%–85%, indicating that compact ceramics can be obtained by a low sintering temperature by nanopowders. The temperature dependence of electrical conductivity of these Gd-substituted CaMnO3 samples is shown in Fig. 3. As shown in Fig. 3, with an increasing doping concentration, s gradually increases and reaches the largest value (113.4 S/cm) at x 5 0.06 at a high temperature. The equivalent Gd substitution ceramics synthesized by coprecipitation is slightly smaller than by the SSR method. This indicates that the grain size has little

Fig. 2. Typical transmission electron microscopic and scanning electron microscopic micrograph of the Ca0.96Gd0.04MnO3 sample, (a) calcined powder by the coprecipitation route, (b) the surface of sintered pellets synthesized with the coprecipitation method, (c) the surface of sintered pellets synthesized by the solid-state reaction method method.

August 2010

Fig. 3. Temperature dependence of electrical conductivity (filled symbols) and Seebeck coefficient (open symbols). Inset, the activation energy for samples.

effect on the electrical conductivity. The temperature dependence of the conductivity is generally described using the small polaron model given by Mott as the following equation14:   Ea s ¼ CT 1 exp kB T

2123

Rapid Communications of the American Ceramic Society

(1)

where C, Ea, kB, and T are the preexponential terms related to the scattering mechanism, the activation energy, Boltzmann constant, and the absolute temperature, respectively. The inset in Fig. 3 shows the activation energy increasing with an increasing concentration of Gd (x). This indicates that the increasing of Mn31 concentration is favorable for the formation of polarons in this temperature interval. The activation energy of 0.053 eV for Ca0.96Gd0.04MnO3 by the SSR method increases to 0.057 eV obtained by the coprecipitation method. Because of the slight difference in activation energies, we can conclude that the finegrained sample exhibits good interface connection between the grains, and grain boundaries in fine-grained have a weak influence on the electrical conductivity. The temperature dependence of thermoelectric power S for the Gd-doped CaMnO3 samples are shown in Fig. 3. The thermoelectric power S values are all negative, indicating an n-type conduction. The absolute value of thermoelectric power for all specimens increases with a rising temperature ranging from room temperature to 973 K. Samples containing equal amounts of Gd also share the same thermopower, regardless of the fab-

rication method. In addition, because the thermopower depends on the electronic structure of the material, we can conclude that we have obtained high-quality compounds. Figure 4 displays the total thermal conductivity and the electronic contribution (300 KoTo1000 K) for all of the studied compounds. The total thermal conductivity k can be expressed by the formula k 5 kL1ke, where kL is the lattice component and ke is the electronic component. Because ke (ke 5 LoTs, where Lo 5 2.45  108 W  (O  K2))1 is the Lorenz number and s is the electrical conductivity) does not exceed 20% of the total thermal conductivity, heat conduction is predominantly represented by the lattice component kL. The k of the samples prepared by the coprecipitation method have lower values (1.3– 1.5 W  (m  K)1, 973 K) than the samples prepared by the solidstate reaction (2.0–2.4 W (m  K)1, 973 K).10,11 The reduction in thermal conductivity maybe related to the enhancement of grain boundary scattering.15 Strong phonon scattering at the interfaces of fine-grained samples yields a significant reduction in the thermal conductivity compared with raw samples. The nanopores in the fine-grained (as seen in Fig. 2(b)) samples may play an important role in reducing the thermal conductivity. Although the samples synthesized by the two methods have almost a similar porosity, the fine ceramics have much more nanopores. These can greatly enhance the phonon scattering in the interface between the pores and ceramics medium. The ZT of the studied compounds are shown in the inset of Fig. 4. The maximum ZT achieved for the Ca0.96Gd0.04MnO3 sample prepared by coprecipitation at 973 K is ZT 5 0.24, while the ZT value of the Ca0.96Gd0.04MnO3 sample prepared by SSR is only 0.15. This implies that the grain boundaries can act as scattering centers effective for phonons but ineffective for charge carriers. Further in-depth physical analysis on the correlation between grain size and thermal or electrical conductivity are required.

IV. Conclusion We investigated the effect of two synthesis methods on the microstructure and thermoelectric properties of Ca1xGdxMnO3 (0rxr0.06) compounds. The 50–70 nm nanopowders were prepared by a coprecipitation method and the bulk samples have a fine grain size (200–400 nm) due to the low sintering temperature. The different grain sizes influenced the electrical and thermal transport properties. The electrical conductivity of the Ca0.96Gd0.04MnO3 ceramics synthesized by coprecipitation is slightly smaller than by the SSR method, while the thermal conductivity of the samples prepared by the coprecipitation method have much lower values (1.3–1.5 W  (m K)1) than the samples prepared by the solid-state reaction (2.0–2.4 W (m  K)1). The enhanced scattering of phonons in fine-grained samples is due to the numerous interfaces. The highest dimensionless ZT of 0.24 has been obtained at 973 K in the air for Ca0.96Gd0.04MnO3 by the coprecipitation method and it is 1.5 times larger than that by the SSR method. Accordingly, it should be necessary to improve the thermoelectric properties not only by different doping elements or doping concentration but also by fabricating fine-grained nanostructures.

References 1

Fig. 4. Temperature dependence of thermal conductivity. Inset, temperature dependence of the ZT value.

R. Venkatasubramanian, E. Siivola, T. Colpitts, and B. O’Quinn, ‘‘Thin-Film Thermoelectric Devices with High Room-Temperature Figures of Merit,’’ Nature, 413, 597–602 (2001). 2 I. Terasaki, Y. Sasago, and K. Uchinokura, ‘‘Large Thermoelectric Power in NaCo2O4 Single Crystals,’’ Phys. Rev. B, 56, 12685–7 (1997). 3 Y. H. Liu, Y. H. Lin, Z. Shi, C. W. Nan, and Z. J. Shen, ‘‘Preparation of Ca3Co4O9 and Improvement of its Thermoelectric Properties by Spark Plasma Sintering,’’ J. Am. Ceram. Soc., 88, 1337–40 (2005). 4 E. Guilmeau, H. Itahara, T. Tani, D. Chateigner, and D. Grebille, ‘‘Quantitative Texture Analysis of Grain-Aligned [Ca2CoO3]0.62[CoO2] Ceramics Processed by the Reactive-Templated Grain Growth Method,’’ J. Appl. Phys., 97, 064902–4 (2005). 5 M. Prevel, S. Lemonnier, Y. Klein, S. Hebert, D. Chateigner, B. Ouladdiaf, and J. G. Noudem, ‘‘Textured Ca3Co4O9 Thermoelectric Oxides by Thermoforging Process,’’ J. Appl. Phys., 98, 093706–7 (2005).

2124 6

Rapid Communications of the American Ceramic Society

H. Ohta, W. S. Seo, and K. Koumoto, ‘‘Thermoelectric Properties of Homologous Compounds in the ZnO–In2O3 System,’’ J. Am. Ceram. Soc., 79, 2193–6 (1996). 7 H. Ohta, S. Kim, Y. Mune, T. Mizoguchi, K. Nomura, S. Ohta, T. Nomura, Y. Nakanishi, Y. Ikuhara, M. Hirano, H. Hosono, and K. Koumoto, ‘‘Giant Thermoelectric Seebeck Coefficient of Two-Dimensional Electron Gas in SrTiO3,’’ Nat. Mater., 6, 129–34 (2007). 8 B. Poudel, Q. Hao, Y. Ma, Y. C. Lan, A. Minnich, B. Yu, X. Yan, D. Z. Wang, A. Muto, D. Vashaee, X. Y. Chen, J. M. Liu, M. S. Dresselhaus, G. Chen, and Z. Ren, ‘‘High-Thermoelectric Performance of Nanostructured Bismuth Antimony Telluride Bulk Alloys,’’ Science, 320, 634–8 (2008). 9 K. F. Hsu, S. Loo, F. Guo, W. Chen, J. S. Dyck, C. Uher, T. Hogan, E. K. Polychroniadis, and M. G. Kanatzidis, ‘‘Cubic AgPbmSbTe21m: Bulk Thermoelectric Materials with High Figure of Merit,’’ Science, 303, 818–21 (2004). 10 D. Flahaut, T. Mihara, R. Funahashi, N. Nabeshima, K. Lee, H. Ohta, and K. Koumoto, ‘‘Thermoelectrical Properties of A-Site Substituted Ca1xRexMnO3 System,’’ J. Appl. Phys., 100, 84911–4 (2006).

11

Vol. 93, No. 8

Y. Wang, Y. Sui, and W. H. Su, ‘‘High Temperature Thermoelectric Characteristics of Ca0.9R0.1MnO3 (R 5 La, Pr, y , Yb),’’ J. Appl. Phys., 104, 93703–7 (2008). 12 L. Bocher, M. H. Aguirre, D. Logvinovich, A. Shkabko, R. Robert, M. Trottmann, and A. Weidenkaff, ‘‘CaMn1xNbxO3 (xo 5 0.08) Perovskite-Type Phases as Promising New High-Temperature n-Type Thermoelectric Materials,’’ Inorg. Chem., 47, 8077–85 (2008). 13 C. S. Sanmathi, R. Retoux, M. P. Singh, and J. Noudem, ‘‘Structure and Properties of Electron-Doped Ca1xSmxMnO3 Nanoparticles,’’ Mater. Chem. Phys., 114, 676–80 (2009). 14 A. J. Bosman and H. J. Vandaal, ‘‘Small-Polaron Versus Band Conduction in Some Transition-Metal Oxides,’’ Adv. Phys., 19, 1–17 (1970). 15 M. S. Dresselhaus, G. Chen, M. Y. Tang, R. G. Yang, H. Lee, D. Z. Wang, Z. F. Ren, J. P. Fleurial, and P. Gogna, ‘‘New Directions for Low-Dimensional Thermoelectric Materials,’’ Adv. Mater., 19, 1043–53 (2007). &