J Mater Sci Technol., Vol.25 No.4, 2009 535 High-temperature Electric Properties of Polycrystalline La-doped CaMnO3 Ceramics Jinle Lan1) , Yuanhua Lin1)† , Ao Mei1) , Cewen Nan1)† , Yong Liu2) , Boping Zhang2) and Jingfeng Li1) 1) State Key Laboratory of New Ceramics and Fine Processing, Department 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 June 30, 2008, in revised form January 20, 2009] Polycrystalline La-doped CaMnO3 ceramics have been prepared by a solid-state sintering method Analysis of microstructure and phase composition indicates that the addition of La can prohibit the further growth of grain, and no impurity phase appears The results revealed that the La doping can lead to a large change of the activation energy (from 0.22 to 0.02 eV), and thus result in a marked increase in electric conductivity of 2–4 orders of magnitude The power factor can reach about 1.5×10−4 W·m−1 ·K−2 in a wide temperature range, which potentially make them attractive for n-type high-temperature thermoelectric materials KEY WORDS: CaMnO3 ; Thermoelectric; Activation energy; Seebeck coefficient Introduction Thermoelectric material can directly convert heat into electric energy and vice versa through the thermoelectric phenomena in solids, which make it a potential way for clean energy generation by transforming the heat into electricity[1] Normally, as for the thermoelectric materials, the conversion efficiency can be well characterized by the dimensionless figure ZT =S σT /κ (where T , S, σ, and κ are the absolute temperature, thermoelectric power, electrical conductivity, and thermal conductivity, respectively) Therefore, high electrical conductivity σ, large Seebeck coefficient S, and low thermal conductivity κ are highly expected for practical development of thermoelectric materials and related devices[2] As for the conventional non-oxides materials (e.g., Bi-Te, Bi-Se system)[3] , there exist some shortcomings such as surface oxidation and vaporization at high temperature, and limit further applications of these materials Oxides, owing to their low thermal conductivity and high-resistance of oxidation at high temperature, have recently attracted considerable attention Recently, oxides-based thermoelectric materials have been explored (e.g., SrTiO3 , NiO, TiO2 )[4–6] Some CoO2 -based oxides with layered structures (e.g., Ca3 Co4 O9 ) have been reported to show good p-type thermoelectric properties at high temperature in air[7,8] , the much-needed corresponding n-type oxides with good thermoelectric properties for power generation are still a challenge for high thermoelectric performance Some previous studies on the CaMnO3 perovskite system suggested that CaMnO3 could be a potential candidate as an n-type thermoelectric material[9,10] As we know, CaMnO3 is an electron-doped compound, which belongs to the perovskite structure with a=0.5278 nm, b=0.7448 nm, and c=0.5268 nm Some previous studies indicated that the CaMnO3 perovskite system can exhibit colossal magnetoresistance properties at † Corresponding author Tel.: + 86 10 62773741; E-mail address: linyh@tsinghua.edu.cn (Y.H Lin); cwnan@tsinghua.edu.cn (C.W Nan) low temperature[11] Funahash et al.[12] have investigated the structure and thermoelectric properties of polycrystalline samples Ca1−x Ax MnO3 (A=Yb, Tb, Nd, and Ho), and reported that the Ybsubstituted CaMnO3 showed good thermoelectric properties (ZT ≈0.16 at 1000 K) Although electron-doped CaMnO3 as a member of the large family of perovskite oxides has been suggested to be potential n-type thermoelectric materials, up to now, only a few systematical studies on the high-temperature thermoelectric properties have been reported In this work, we fabricated La-doped CaMnO3 -based ceramics, and reported the effect of La substitution on the phase composition, microstructure and high-temperature thermoelectric properties Experimental Polycrystalline ceramic samples of Ca1−x Lax MnO3 (x=0, 0.02, 0.04, 0.06, 0.08) were synthesized via a conventional solid-state reaction Analytical purity CaCO3 , MnCO3 , and La2 O3 were used as raw materials, which were weighted in stoichiometric ratio and mixed by ball mill for 24 h, and then the mixed powders were pre-calcined at 1373 K for h Finally, La-doped CaMnO3 ceramics can be obtained by sintering at 1473 K for 10 h X-ray diffraction (XRD) with a Rigaku D/MAX2550V diffractometer (CuKα radiation) and scanning electron microscopy (SEM) were employed to reveal the microstructure and phase composition of the assintered Ca1−x Lax MnO3 ceramic samples The samples for the measurements of thermoelectric properties were cut out of the sintered bodies in the form of rectangular bars of mm×4 mm×20 mm with a diamond saw, and silver paint electrodes were formed on both sides of the sintered ceramic discs for electrical measurements The temperature dependence of electric conductivity was measured in the temperature range from room temperature to 800◦ C by fourprobe method Thermoelectric power was obtained from the slope of the linear relation between ∆V and ∆T , where ∆V is the thermoelectromotive force pro- 536 (d) (c) (b) (a) 30 40 50 60 Lattice constants / A (422) (220) (e) 20 a b c 7.465 (211) (022) Intensity / a.u (110) (112) J Mater Sci Technol., Vol.25 No.4, 2009 7.460 5.32 5.30 5.28 5.26 70 0.00 / deg Fig XRD patterns of as-sintered samples: (a) CaMnO3 , (b) Ca0.98 La0.02 MnO3 , (c) Ca0.96 La0.04 MnO3 , (d) Ca0.94 La0.06 MnO3 , (e) Ca0.92 La0.08 MnO3 duced by temperature difference ∆T Results and Discussion The XRD patterns shown in Fig indicate that all the samples are single phase CaMnO3 with an orthorhombic perovskite-type structure (ICSD#82211) No impurity phase can be observed in these ceramic samples, which reveals that La ions can replace the Ca sites The substitution of the Ca sites with trivalent La ions will lead to the variation of electric properties, and the detailed results will be given in the following content Figure shows the lattice parameters a, b and c as a function for the changes with La doping The parameters a, b and c all increase monotonously This result 0.02 0.04 La content, 0.06 0.08 x / mol Fig Lattice parameters of the Ca1−x Lax MnO3 series as a function of La substitution content can be well understood based on the fact that the ionic radius of La3+ is larger than that of Ca2+ Figure shows the SEM images of the surface of the as-sintered La-doped CaMnO3 samples Obviously, in pure CaMnO3 ceramic sample, the grain size is about 3–5 µm, and larger than that in the La-doped samples, which indicates that the addition of La can act as the grain growth inhibitor and prohibit the further growth of CaMnO3 grain The similar behavior has also been observed in the La-doped Na0.5 Bi0.5 TiO3 ceramic samples[13] It can be observed that the samples sintered at 1473 K have small pores, and these pores can reduce the thermal conductivity, which can improve thermoelectric properties The effect of the pores will be investigated in future work Figure gives the temperature dependence of electrical conductivity of these La-doped CaMnO3 samples in the temperature range from room tempera- Fig SEM images of the surface of as-sintered samples: (a) CaMnO3 , (b) Ca0.96 La0.04 MnO3 , (c) Ca0.94 La0.06 MnO3 , (d) Ca0.92 La0.08 MnO3 537 J Mater Sci Technol., Vol.25 No.4, 2009 (a) 120 (b) 100 (d) Seebeck / ( v/K) - -100 (c) (e) / (S cm ) 140 80 60 40 20 -300 (a) -400 (b) (c) (d) -500 (e) 300 -200 400 500 600 700 T/K 800 -600 300 900 1000 1100 400 3.0 W/mK ) (b) (c) 2.0 1.5 Power factor / (10 2.5 -4 (e) Activation energy / eV K) -1 T) / (S cm Lg ( (d) 3.0 800 900 1000 1100 (a) (a) 3.5 700 Fig Temperature dependence of the Seebeck coefficient for: (a) CaMnO3 , (b) Ca0.98 La0.02 MnO3 , (c) Ca0.96 La0.04 MnO3 , (d) Ca0.94 La0.06 MnO3 , (e) Ca0.92 La0.08 MnO3 5.0 4.0 600 T/K Fig Electrical conductivity as a function of temperature for: (a) CaMnO3 , (b) Ca0.98 La0.02 MnO3 , (c) Ca0.96 La0.04 MnO3 , (d) Ca0.94 La0.06 MnO3 , (e) Ca0.92 La0.08 MnO3 4.5 500 0.20 0.15 0.10 0.05 (b) 2.5 (c) (d) 2.0 (e) 1.5 1.0 0.5 0.00 0.00 1.0 0.02 0.04 0.06 0.08 0.10 0.0 x / mol 300 0.5 1.0 1.5 2.0 (1000/ 2.5 3.0 3.5 500 600 700 800 900 1000 1100 T/K 4.0 T) / (1/K) Fig lg(σT ) vs 1000/T plots for: (a) CaMnO3 , (b) Ca0.98 La0.02 MnO3 , (c) Ca0.96 La0.04 MnO3 , (d) Ca0.94 La0.06 MnO3 , (e) Ca0.92 La0.08 MnO3 ture to 1075 K The undoped CaMnO3 is a typical n-type semiconductor, and shows a low conductivity (∼0.02 S/cm) at room temperature However, the La doping, even as low as x=0.02, can cause a marked increase in conductivity of 2–3 orders of magnitude, especially when x=0.08 (∼100 S/cm), and the value changes slightly with increasing the temperature This similar electrical behavior has also been observed in the low Pr-doped CaMnO3 ceramics[14] , which may be ascribed to the variation of the strength of the bending of the Mn–O–Mn bond and narrowing the conduction bandwidth For the interpretation of conducting phenomenon in Mn-based perovskite system, the temperature dependence of the conductivity is generally described using the small polaron model given by Mott and Davis[15] as follows, σ = (C/T )exp(−Ea /KB T ) 400 (1) where C, Ea , KB , and T are the pre-exponential terms related to the scattering mechanism, the activation energy, Boltzmann constant, and absolute temperature, respectively Figure illustrates the plots of lg(σT ) vs 1/T for various La-doped samples Obviously, when the temperature is below 600 K, all samples show a good linearity between lg(σT ) and 1/T Additionally, for La-doped CaMnO3 sample, this linear behavior can Fig Temperature dependence of the power factor for: (a) CaMnO3 , (b) Ca0.98 La0.02 MnO3 , (c) Ca0.96 La0.04 MnO3 , (d) Ca0.94 La0.06 MnO3 , (e) Ca0.92 La0.08 MnO3 be retained at higher temperature as the La-doping concentration increases The activation energy Ea obtained from Eq (1) (insert in Fig 5) for pure CaMnO3 is about 0.22 eV, which is in agreement with that reported in previous work (0.16 eV)[16] Moreover, for La-doped CaMnO3 samples, they yield smaller activation energies of ∼0.02–0.03 eV, which was also observed in Ce3+ -, Y3+ -, or Sm3+ -doped CaMnO3 samples (∼0.02–0.04 eV)[16] The variation of activation energy should be attributed to the doping effect of trivalent La ions on the Ca sites and the related hopping conduction mechanism in the CaMnO3 -based perovskite oxides To understand the nature of the conduction behavior in this system, further investigation is necessary and desirable Figure shows the temperature dependence of S for the La doped CaMnO3 samples The S values are all negative, indicating n-type conduction The pure CaMnO3 sample shows a large absolute value of S (∼550 µV/K at near 500 K), and decreases with increasing the temperature, which should be related to the carrier concentration and semiconductor behavior For these La-doped CaMnO3 samples, with the temperature increasing (from RT to 450 K), the absolute S values also increase It may be ascribed to the variation of Mn2+ /Mn3+ These various valence states of Mn ions will give contribution on the Seebeck coefficients like Co2+ /Co3+ in the CoO2 -based layered oxides as previously reported[7,8] As the tempera- 538 J Mater Sci Technol., Vol.25 No.4, 2009 ture further increases, the absolute S values intend to decrease, which may be mainly related to the variation of the corresponding electrical conductivity, and need further work to understand these behaviors It should be pointed out that though the S decrease, a maximum power factor S σ=1.5×10−4 W·m−1 ·K−2 appears for the La-doped sample with x=0.04 in a wide temperature range (see Fig 7), being ascribed to the improvement in the electric conductivity As shown in Fig 7, the power factors Sσ calculated from the electric conductivity and Seebeck coefficient indicate that the La substitution has a great influence on the power factor, which should be caused by the significant variation of the electric conductivity and Seebeck coefficient, which can be comparable to the other n-type La, Y, or Dy doped SrTiO3 and Al and Ti co-doped ZnO ceramic systems[17,18] The good thermoelectric performance and high temperature durability in air suggest that these oxides can be potential high temperature thermoelectric materials Conclusion We fabricated polycrystalline La-doped CaMnO3 ceramics by the conventional solid state reaction, and investigated the effect of La substitution on the hightemperature thermoelectric properties Our results indicate that all La-doped CaMnO3 samples show negative Seebeck coefficients, indicating n-type conduction The La doping has a remarkable effect on the electric transport properties of these CaMnO3 based ceramics, which may be related to the carrier concentration, defect structure, the spin-state of the electrons caused by the La3+ ions on the Ca2+ sites Acknowledgements This work was financially supported by the National Program on Key Basic Research Project (“973 Program”) under grant No 2007CB607505, and the National High Technology Research and Development Program of China under grant No 2009AAO3Z216 REFERENCES [1 ] D.M Rowe: CRC Handbook of Thermoelectrics, CRC, Boca Raton, FL, 1995 [2 ] G Chen, M.S Dresselhaus, G Dresselhaus, J.P Fleurial and T Caillat: Int Mater Rev., 2003, 48, [3 ] M Ito, W.S Seo and K Koumoto: J Mater Res., 1999, 14, 209 [4 ] S Ohta, T Nomura and H Ohta: Appl Phys Lett., 2005, 87, 092108 [5 ] W Shin and N Murayama: Jpn J Appl Phys., 1999, 38, L1336 [6 ] D Kurita, S Ohta, K Sugiura, H Ohta and K Koumoto: J Appl Phys., 2006, 100, 096105 [7 ] I Terasaki, Y Sasago and K Uchinokura: Phys Rev B, 1997, 56, R12685 [8 ] Y.H Lin, C.W Nan, Y.H Liu, J.F Li, T Mizokawa and Z Shen: J Am Ceram Soc., 2007, 90, 132 [9 ] L Sudheendra, A.R Raju and C.N 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