Available online at ScienceDirect ScienceDirect J Mater Sci Technol., 2014, 30(8), 821e825 High Temperature Thermoelectric Properties of Dy-doped CaMnO3 Ceramics Bin Zhan1), Jinle Lan1), Yaochun Liu2), Yuanhua Lin1)*, Yang Shen1), Cewen Nan1) 1) State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China 2) School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China [Manuscript received May 22, 2013, in revised form June 26, 2013, Available online 25 January 2014] Dy-doped CaMnO3 ceramics have been synthesized by co-precipitation method combined with the solid-state reaction Phase composition and microstructure analysis indicate that high density and pure CaMnO3 phase can be achieved The electric conductivity can be enhanced by Dy doping, and result in a slight increase of the thermal conductivity The highest dimensionless figure of merit ZT of 0.15 has been obtained at 973 K for x ¼ 0.02 sample, which is about times larger than that of the pure CaMnO3, which indicate that CaMnO3 can be a promising candidate for n-type thermoelectric material at high temperature KEY WORDS: CaMnO3; Thermoelectric properties; Thermal conductivity Introduction Environment friendly thermoelectric materials have attracted widespread interests for potential applications in space exploration, exhaust recycling, and clean cooling[1e3] The energy conversion efficiency is related to the materials intrinsic properties which can be characterized by the dimensionless figure of merit ZT ¼ S2sT/k, where T is the absolute temperature, S is the Seebeck coefficient, s is the electrical conductivity, and k is the thermal conductivity The three parameters S, s, and k are correlated to each other Therefore, it is difficult to optimize all parameters simultaneously The good thermoelectric performance needs high power factor (PF, S2s), and low thermal conductivity k Normally, alloy semiconductors, such as Bi2Te3, PbTe and SiGe[4e7], their ZT values can exceed 1.0, and show a good practical prospects For oxides-based thermoelectrics, their high chemical and thermal stability makes them to be promising candidates at high temperature application Recently, various oxides such as Ca3Co4O9[8,9], SrTiO3[10], ZnO[11] and BiCuSeO[12,13] have been investigated in detail to enhance the ZT value CaMnO3 (CMO) is a typically n-type oxide thermoelectric materials[14e18] However, low electric conductivity leads to Corresponding author Prof., Ph.D.; Tel.: þ86 10 62773741; Fax: þ86 10 62771160; E-mail address: linyh@tsinghua.edu.cn (Y Lin) 1005-0302/$ e see front matter Copyright Ó 2014, The editorial office of Journal of Materials Science & Technology Published by Elsevier Limited All rights reserved http://dx.doi.org/10.1016/j.jmst.2014.01.002 * poor performance in CMO system The s of pure CMO is just w10 S/cm and ZT is less than 0.04 at 900 K In the previous work, the polycrystalline ceramics of La-doped CaMnO3 were synthesized by conventional solid-state reaction (SSR)[14], and showed a porous structure Lan et al.[19] reported that the ZT value can been improved to 0.24 at 973 K though reducing the thermal conductivity in fine grain size (200e400 nm) at a low sintering temperature Wang et al.[20] studied the electron-doped CaMnO3 by rare earth, which shows that the Seebeck coefficient is determined by the carrier concentration, while the electric conductivity and thermal conductivity can be tuned by the ion radius and ion mass, respectively In this work, we attempted to control the microstructure and optimize thermoelectric properties The cold isostatic pressing (CIP) was used to achieve high density sample and Dy as a dopant to optimize the electrical properties Our results indicate CIP is available to enhance the density and Dy dopant is effectively to improve the ZT value Experimental Polycrystalline ceramic samples of Ca1ÀxDyxMnO3 (x ¼ 0, 0.02, 0.04, 0.06, 0.08, 0.10) were synthesized via a chemical coprecipitation method and solid-state reaction CIP was used to control the porosity For the co-precipitation method, Dy2O3 (99.90%) was carefully dissolved in nitric acid as a kind of starting material, and then mixed together with Ca(NO3)2$4H2O, Mn(NO3)2 aqueous solution in deionized water to make the nitrate stock solution NH4HCO3 and ammonia solution were used to control the reaction pH value in the range of 7.5e9.0 to make 822 B Zhan et al.: J Mater Sci Technol., 2014, 30(8), 821e825 the metal ions precipitate completely The resultant suspension was aged to remove supernatant fluid and then was subjected to suction filtration The precursor powder was calcined at 1073 K for h in air, and then pressed to pellets The bulks sealed in glove and compacted by CIP at 200 MPa in oil to enhance the density (LDJ-100/320-300 Sichuan Airlines Industry Chuanxi Machinery Factory, Yaan, China) The samples were sintered at 1473 K for 12 h X-ray diffraction (XRD) with a Rigaku D/MAX-2550V diffractometer (Rigaku, Tokyo, Japan; CuKa radiation) and scanning electron microscopy (SEM, JSM-6460LV, JEOL, Tokyo, Japan) were used to investigate the phase composition and microstructure of CMO bulks, respectively The temperature dependence of electric conductivity was measured from room temperature to 973 K by a four-probe method Seebeck coefficient was obtained from the slope of the linear relation between DV and DT, where DV is the thermoelectromotive force produced by the temperature gradient DT The thermal conductivity k was determined by the following parameters: the thermal diffusivity (a), the heat capacity (Cp), and the density (r), using the relationship k ¼ aCpr The relative bulk density was measured by the Archimedes method, and a Netzsch LFA 457 (Selb, Germany) laser flash apparatus was used to measure the thermal diffusivity and the specific heat Results and Discussion 3.1 Morphology and structure Fig 1(a) shows the XRD patterns for Dy-doped CMO samples All the samples are corresponded to CaMnO3 phase with an orthorhombic perovskite-type structure in Pnma space group The lattice parameters have been calculated by the XRD data, as shown in Fig 1(b) The lattice constant a and cell volume are increased as dopant concentration increases This behavior is independent of the ionic radius of Dy[21,22], in spite of the radius of Dy3þ ions (1.08 nm, C.N IX) is slightly smaller than that of Ca2þ ions (1.18 nm, C.N IX) The electron doping will induce the presence of Mn3þ within the Mn4þ matrix for charge balance The ionic radius of Mn3þ is larger than that of Mn4þ (r [Mn3þ] ¼ 0.64 nm, C.N VI and r[Mn4þ] ¼ 0.53 nm, C.N VI), which leads to the MnO6 octahedra to be distorted in CMO structure Fig shows the SEM images of microstructure of CMO samples The precursor powder is typical spherical and diameter changes from to 10 mm as seen in Fig 2(a), which are composed of many small grains of w100 nm in size as shown in the insert of Fig 2(a) Fig 2(b) shows the surface of sample without CIP, and some pores with diameter of 1e2 mm appeared As shown in Fig 2(c), the macroporous pores disappeared by CIP technology and the density of samples can increase to 95% after CIP as compared with the density 80% of samples without CIP It can be observed that the high density will deteriorate the thermal conductivity, which can reduce thermoelectric properties Compared Fig 2(c) and (d), pores will further decrease with more dopant And the binding between grains are more closely, which indicates that the addition of Dy can act as the sintering aids and contribute to the formation of grains, which is helpful to make CMO ceramic be densified 3.2 Electric properties Fig 3(a) shows the temperature dependence of electric conductivity s of samples from 300 to 973 K With increasing doping concentration, s gradually increases and reaches the largest value (184 S/cm) at x ¼ 0.10 at 973 K As compared with undoped CMO (sw10 S/cm), a significant increase of electric conductivity can be observed by Dy doping, which is mainly derived from the variation of carrier concentration The substitution of Dy3þ for Ca2þ will import a large number of electron carriers and induce Mn3þ appeared in Mn4þ matrix The increased carrier concentration is directly affect the s, and the presence of Mn3þ is beneficial to electron hopping in perovskite manganites Therefore, Dy doping can facilitate the transport of carriers by hopping mechanism and then enhance the electric conductivity The slope of seT curve is positive relationship (ds/dT > 0) at low temperature, which indicates semiconducting behavior And then it shows a metallic behavior (ds/dT < 0) as the temperature further increases The insulator-metal (IM) transition temperature TIM increases from 400 to 550 K with the Dy content increasing For semiconductor part, the temperature dependence of the conductivity is generally described using the small polaron model given by Mott as the following equation[23] s¼ Fig XRD patterns (a) and the lattice constant a and cell volume (b) of Ca1ÀxDyxMnO3 samples C ÀEa exp ; kB T T where C, kB, and Ea are the pre-exponential terms, Boltzmann constant, and activation energy, respectively Fig 3(b) shows the activation energy increases with increasing content of Dy at low temperature interval, which raising from 0.048 to 0.070 eV With more dopant, the densifying of ceramic may be beneficial to electronic transport The carrier concentration increases and more Mn3þ ions can be formed, and then a higher s and TIM can be obtained This indicates that the increase in Mn3þ concentration is favorable for the formation of polaron in this temperature range[19] Fig 3(c) displays the temperature dependence of Seebeck coefficient All samples exhibit negative Seebeck coefficient, B Zhan et al.: J Mater Sci Technol., 2014, 30(8), 821e825 823 Fig Typical SEM images of CaMnO3 samples: (a) x ¼ 0.06 powder by co-precipitation route; (b) CMO ceramic without CIP; (c) x ¼ 0.02 ceramic by CIP; (d) x ¼ 0.06 ceramic by CIP which indicates that electrons are the predominant charge carriers (n-type conduction) The x ¼ 0.02 sample has a very large S value, being about À370 mV KÀ1 at 300 K The absolute value of S decreased obviously as Dy content increasing, which arises from the increase of carrier concentration For x ¼ 0.02, absolute value of Seebeck coefficient decreases with increasing temperature, which shows a typical characteristic of nonmetallike temperature dependence With more Dy doping, the absolute value of S decreases with increasing temperature and exhibit metallic behavior This difference should be attributed to the contribution of the oxygen deficiency[18,24] The turning point came in the x ¼ 0.04 sample, which did not appear in s Fig Temperature dependence of electric conductivity (a), Seebeck coefficient (c), and power factor (d); and (b) activation energy plotted vs Dy content 824 B Zhan et al.: J Mater Sci Technol., 2014, 30(8), 821e825 sum of lattice thermal conductivity (kl) and electronic thermal conductivity (ke) as k ¼ kl þ ke The ke can be calculated by the WiedemanneFranz law ke ¼ LTs, where L is the Lorentz constant In order to simplify the calculation, the Lorentz constant Lo (Lo ¼ p2kB2/(3e2) ¼ 2.44 Â 10À8 W U KÀ2) was used The calculated ke in the entire temperature range is quite small as compared to kl (less than 15%), which ranges from 0.168 to 0.436 W mÀ1 KÀ1 at 973 K kl is the predominant component in thermal conductivity, and the variation of k is mainly caused by the change of kl as shown in the insert of Fig 4(a) The lowest value of k is 2.48 W mÀ1 KÀ1 at 973 K for x ¼ 0.02 As we mentioned before, CIP process can increase the density, which is an important reason for high k The variation of thermal diffusion coefficient is the dominant factor for this result, which indicates that CIP not only makes the ceramic be densified, but also changes some intrinsic properties of materials As shown in Fig 4(b), the dimensionless ZT of Ca1ÀxDyxMnO3 was calculated The optimal ZT is 0.15 at 973 K when x ¼ 0.02, which is about times larger than that of the pure CaMnO3 (w0.038) More detailed further work is desirable to further enhance the thermoelectric performance of CaMnO3 Conclusion Fig Temperature dependence of k (a) and ZT (b) value for Dy-doped CMO For materials with more than one type of charge carrier, the diffusion Seebeck coefficient can be expressed as S ¼ Xsi i s Si where si and Si are the partial electrical conductivity and the partial Seebeck coefficient associated with the ith group of carriers, respectively We can rewrite S of CMO as S ¼ sin sex;defact S þ S sin þ sex;defact in sin þ sex;defact ex;defact where sin and Sin are the contribution from intrinsic carriers; sex,defect and Sex,defect are the contribution from extrinsic carriers due to the oxygen defects Since the increase of electrical conductivity ðweÀEa =ðKB T Þ Þ is faster than the decrease of S (wÀEa/(KBT )) for semiconductors, one could expect the second term in above equation would increase and therefore the absolute value of Seebeck coefficient for CMO would increase, which should be responsible for the simultaneous increase of the electrical conductivity and absolute value of S with increasing temperature at low temperature interval It indicates that the existence of oxygen deficiency is important for Seebeck coefficient 3.3 Thermoelectric properties The temperature dependence of the power factor (PF) is shown in Fig 3(d) At 973 K, the maximum of PF can reach 3.82 mW cmÀ1 KÀ2 when x ¼ 0.02, which is a high value for a kind of n-type oxide thermoelectric material Fig 4(a) shows the temperature dependence of thermal conductivity of CMO samples Thermal conductivity k can be expressed generally by the x 0.10) In summary, high density Ca1ÀxDyxMnO3 (0 ceramic has been prepared and the microstructure and thermoelectric properties have been investigated Nanostructured precursor powders were obtained by co-precipitation method and the bulks can reach a high density with CIP The electrical conductivity can be obviously improved by Dy doping The maximum power factor can reach 3.82 mW cmÀ1 KÀ2 at 973 K in sample x ¼ 0.02 The highest dimensionless figure of merit ZT of 0.15 has been obtained at 973 K in the air for Ca0.98Dy0.02MnO3 Acknowledgments This work was financially supported by the Ministry of Sci & Tech of China through a 973 Project, under grant No 2013CB632506, the National Natural Science Foundation of China under Grant Nos 51025205 and 11234012, and the Specialized Research Fund for the Doctoral Program of Higher Education, under grant No 20120002110006 REFERENCES [1] R Venkatasubramanian, E Siivola, T Colpitts, B O’Quinn, Nature 413 (2001) 597e602 [2] A.J Minnich, M.S Dresselhaus, Z.F Ren, G Chen, Energy Environ Sci (2009) 466e479 [3] J.W Fergus, J Eur Ceram Soc 32 (2012) 525e540 [4] J.R Sootsman, D.Y Chung, M.G Kanatzidis, Angew Chem Int Edit 48 (2009) 8616e8639 [5] 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, Z.F Ren, Science 320 (2008) 634e638 [6] J.P Heremans, V Jovovic, E.S Toberer, A Saramat, K Kurosaki, A Charoenphakdee, S Yamanaka, G.J Snyder, Science 321 (2008) 554e557 [7] X.W Wang, H Lee, Y.C Lan, G.H Zhu, G Joshi, D.Z Wang, J Yang, A.J Muto, M.Y Tang, J Klatsky, S Song, M.S Dresselhaus, G Chen, Z.F Ren, Appl Phys Lett 93 (2008) 193121 [8] Y Wang, Y Shi, J.G Cheng, X.J Wang, W.H Su, J Alloy Compd 477 (2009) 817e821 B Zhan et al.: J Mater Sci Technol., 2014, 30(8), 821e825 [9] Y.H Lin, J Lan, Z.J Shen, Y.H Liu, C.W Nan, J.F Li, Appl Phys Lett 94 (2009) 072107 [10] A Kikuchi, N Okinaka, T Akiyama, Scripta Mater 63 (2010) 407e410 [11] M Ohtaki, K Araki, K Yamamoto, J Electron Mater 38 (2009) 1234e1238 [12] J Li, J.H Sui, Y.L Pei, C Barreteau, D Berardan, N Dragoe, W Cai, J.Q He, L.D Zhao, Energy Environ Sci (2012) 8543e8547 [13] Y Liu, L.D Zhao, Y.C Liu, J.L Lan, W Xu, F Li, B.P Zhang, D Berardan, N Dragoe, Y.H Lin, C.W Nan, J.F Li, H.M Zhu, J Am Chem Soc 133 (2011) 20112e20115 [14] J.L Lan, Y.H Lin, A Mei, C.W Nan, Y Liu, B.P Zhang, J.F Li, J Mater Sci Technol 25 (2009) 535e538 [15] S Populoh, M Trottmann, M.H Aguire, A Weidenkaff, J Mater Res 26 (2011) 1947e1952 825 [16] L Bocher, M.H Aguirre, D Logvinovich, A Shkabko, R Robert, M Trottmann, A Weidenkaff, Inorg Chem 47 (2008) 8077e8085 [17] C.L Wang, L Shi, X.M Xu, S.M Zhou, J.Y Zhao, Y.Q Guo, H.F Liu, L.F He, X Cai, G.J Xu, Appl Phys A 112 (2013) 1003e1009 [18] A Bhaskar, C.J Liu, J.J Yuan, C.L Chang, J Alloy Compd 552 (2013) 236e239 [19] J.L Lan, Y.H Lin, H Fang, A Mei, C.W Nan, Y Liu, S.L Xu, M Peters, J Am Ceram Soc 93 (2010) 2121e2124 [20] Y Wang, Y Sui, W Su, J Appl Phys 104 (2008) 093703 [21] H Muguerra, B Rivas-Murias, M Traianidis, C Marchal, Ph Vanderbemden, B Vertruyen, C Henrist, R Cloots, J Alloy Compd 509 (2011) 7710e7716 [22] R.D Shannon, Acta Crystallogr A 32 (1976) 751e767 [23] A.J Bosmana, H.J van Daala, Adv Phys 19 (1970) 1e117 [24] A Bhaskar, J.J Yuan, C.J Liu, J Electroceram 31 (2013) 124e128