Effects of praseodymium doping on thermoelectric transport properties of CaMnO3 compound system

JOURNAL OF RARE EARTHS, Vol. 31, No. 9, Sep. 2013, P. 885

Effects of praseodymium doping on thermoelectric transport properties of CaMnO3 compound system ZHANG Feipeng (ᓴ亲吣)1,2,*, NIU Baocheng (⠯ֱ៤)1, ZHANG Kunshu (ᓴസк)1, ZHANG Xin (ᓴ ᗏ)2, LU Qingmei (䏃⏙ṙ)2, ZHANG Jiuxing (ᓴЙ݈)2 (1. Institute of Physics, Henan University of Urban Construction, Pingdingshan 467036, China; 2. Key Laboratory of Advanced Functional Materials, Chinese Ministry of Education, College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China) Received 17 May 2013; revised 25 June 2013

Abstract: The rare earth Pr doped Ca1–xPrxMnO3 (x=0, 0.06, 0.08, 0.1, 0.12, and 0.14) compound bulk samples were prepared to study the effect of Pr doping on thermoelectric transport properties of CaMnO3 compound system. The doped samples exhibited single phase composition within the experimental doping range, with condensed bulk microstructure and small porosities. The electrical resistivity was remarkably reduced for doped samples, on account of the enhanced carrier concentration; the absolute value of Seebeck coefficient was deteriorated mainly due to enhanced electron carrier concentration. The electrical performances of the doped samples reflected by resistivity and Seebeck coefficient fluctuations were optimistically tuned, with an optimized power factor value of 0.342 mW/(m·K2) at 873 K for x=0.08 sample, which was very much higher comparing with that of the un-doped sample. The lattice thermal conduction was really confined, leading to distinctly repressed total thermal conductivity. The thermoelectric performance was noticeably improved by Pr doping and the dimensionless figure of merit ZT for the Ca0.92Pr0.08MnO3 compound was favorably optimized with the maximum value 0.16 at 873 K. Keywords: CaMnO3 compound; Pr doping; thermoelectric properties; rare earths

More and more efforts have been paid to the field of thermoelectric (TE) materials in the past decades owing to their clean energy conversion between heat and electricity through Seebeck effect and Peltier effect. The efficiency of energy transformation is positively correlated to the materials’ dimensionless figure of merit ZT formulated by: ZT=2T/ (1) where , , T and  are the Seebeck coefficient, the electrical resistivity, the absolute temperature and the total thermal conductivity, respectively[1–3]. For crystal phase materials, the total thermal conductivity  is usually regarded as composing of the carrier thermal conductivity component c and the lattice thermal conductivity component L: =c+L (2) Good TE materials should have high Seebeck coefficient , low electrical resistivity  and total thermal conductivity  simultaneously. Nevertheless, these parameters are not independent of each other; they are closely correlated to transport mechanism, for instance, the sort of charge carriers, the carrier density, mobility, carrier effective mass, phonon mean free path, vibration and phonon modes. A good combination of transport parameters is needed in order to achieve a considerable ZT

value for applicable TE materials[4]. They are also sensitive to materials’ microstructures and textures, thereafter the materials preparation techniques. The hotspot systems are tellurides, silicides, sulphide, half-Heusler alloys, clathrates, skutterudites and oxides[4–10]. The oxides-based TE materials have many advantages over alloys-based materials such as atmospheric stability, easy fabrication, cheapness, high temperature stability, etc.[4]. The n-type CaMnO3 compound shows semiconductorlike conductivity (d/dT<0), high Seebeck coefficient  (|RT|350 V/K) and high temperature stability (1500 K)[4]. It has been regarded as one of the most promising n-type TE oxide materials, and it has also been receiving much interest in the past decade in terms of its structural, topological, physical, magnetic properties and TE performance[4,11,12]. The structure of the CaMnO3 is based on the framework of corner sharing O–Mn–O octahedron in which the Mn is surrounded by six O and the Ca is surrounded by twelve anions within the cavity. This framework allows it the ability of incorporating dopants with different sizes and valences[13]. It is verified from the theoretical investigation that the doping type, namely n-type or p-type, together with the dopant atomic mass, plays a key role in determining the optimization of TE performance

Foundation item: Project supported by National Natural Science Foundation of China (50801002), Beijing Municipal Natural Science Foundation (2112007) and Basic and Advanced Technology Research Project of Henan Province (132300410071) * Corresponding author: ZHANG Feipeng (E-mail: [email protected]; Tel.: +86-375-2089151) DOI: 10.1016/S1002-0721(12)60374-3

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of CaMnO3 [11–14]. There are many reports concerning doping-induced behaviors for CaMnO3 system, the doping categories include Ca site doping, Mn site doping and double doping for both Ca and Mn sites. The rare earth (RE) doping provides electron carriers in the system, the carrier concentration should be increased, and the carrier mean free path should not be intensively lowered due to the comparable ionic radii between Ca and RE, this is favorable for reducing electrical resistivity[13,14]. At the same time, the enhanced effective mass introduced by heavy dopants should contribute to the maintaining of Seebeck coefficient[11,12,14]. Thirdly, the heavy elements RE doping affords confined phonon mean free path, thereby this is favorable for lattice thermal conductivity suppression[14]. It is thus mostly hopeful to tune the TE transport parameters by RE elements doping, to obtain a good combination of these parameters. In this paper, the rare earth Pr doped Ca1–xPrxMnO3 (x=0, 0.06, 0.08, 0.1, 0.12, and 0.14) compound bulk samples were fabricated; the effect of Pr doping on the TE transport properties of n-type CaMnO3 compound from 373 K up to 973 K was investigated in detail.

1 Experimental As reported within our former works[15], the CaMnO3based compound powder with grain size in nanometer magnitude order was synthesized by citrate acid sol-gel reaction method. In the present paper, the Pr doped Ca1–xPrxMnO3 (x=0, 0.06, 0.08, 0.1, 0.12, and 0.14) compound powders were synthesized by the same method, and then the bulk samples were prepared by ceramic preparation technique. Stoichiometric ratios of highly pure nitrates of Ca, Pr, and Mn were dissolved in distilled water; the citric acid was added in the aqueous solution. The solution was continuously mixed at 360 K in order to form the precursor gel. The gel was dried at 473 K for 12 h in air to evaporate the excessive water. Then the dried gel was ground and calcined at 1173 K for 8 h to remove excess organic compounds and to get the Ca1–xPrxMnO3 (0x0.14) powder. Then the powder was finely ground and pressed into platelets. Finally the pressed platelets were heated slowly to the temperature of 1473 K in air at the heating rate of 10 K/min, and the samples were maintained at 1473 K for 12 h, then the samples were subjected to furnace cooling from that temperature to room temperature. The phase constitutions of bulk samples were analyzed by X-ray diffraction (XRD) at room temperature on a Rigaku diffractometer with Cu K radiation in a 2 range of 20º–85º, with steps of 0.02°(2) and a time per step of 1 s. The microscopic image of the bulk samples was obtained with the scanning electron microscopy (SEM) using secondary electron mode by Nova NanoSEM operated at 18 kV. The electrical resistivity and Seebeck co-

JOURNAL OF RARE EARTHS, Vol. 31, No. 9, Sep. 2013

efficient were measured in He atmosphere from room temperature up to 1000 K using a conventional DC standard four-probe method on ULVAC ZEM-2 system. The specific heat capacity Cp and thermal diffusivity  were measured in Ar atmosphere by the laser flash technique on ULVAC-RIKO TC-7000 system. The total thermal conductivity was then calculated by =dCp, the density d was measured by Archimedes method. The electron carrier concentration n and the mobility  were measured via n=1/RHe on Accent HL5500 Hall System and the RH is Hall coefficient.

2 Results and discussion 2.1 Phase composition and bulk microstructure Fig. 1 presents the XRD patterns for rare earth Pr doped Ca1–xPrxMnO3 (x=0, 0.06, 0.08, 0.1, 0.12, and 0.14) compound bulk samples. All diffraction peaks are indexed by comparing with standard JCPDS card No. 50-1746 for orthorhombic CaMnO3, no impurity phases are found. This confirms the formation of single phase CaMnO3 compound. In addition, 2 shift to higher angles could be observed (Fig. 2), indicating the decreased lattice parameters with increasing Pr doping content. The rare earth element Pr exhibits trivalent Pr3+ or tetravalent Pr4+ state within the compound; this phenomenon is con-

Fig. 1 XRD patterns for Ca1–xPrxMnO3 (0x0.14) compounds

Fig. 2 2 shift for Ca1–xPrxMnO3 (0x0.14) compounds

ZHANG Feipeng et al., Effects of praseodymium doping on thermoelectric transport properties of CaMnO3 …

sidered to be caused by ionic radii differences between Pr3+/Pr4+ and Ca2+. Moreover, the width of the diffraction peaks for Pr doped Ca1–xPrxMnO3 tends to get slightly broader as the doping content increases (Fig. 2), which indicates the crystalline grain size decreasing. Fig. 3 shows the cross-section SEM images for the Ca1–xPrxMnO3 (x=0, 0.08, 0.1, 0.12) compound bulk samples, and condensed bulk samples are formed as seen in the figures. Table 1 presents the measured relative density of the Pr doped bulk samples, increased bulk density with increasing the doping content x can be observed. It can also be observed from the SEM images that the bulk samples have enhanced inter-grain connections with increasing the Pr doping content x. Furthermore, decreased crystalline grain size could also be seen with increasing the Pr doping concentration, this is in agreement with the above estimations. This phenomenon is in accordance with that of Yb and Fe doped Ca1–xMxMnO3 (M=Yb, Fe) system[15,16]. Although further work should be done, it is estimated that the grain nucleation rate can be enhanced, while the grain growth rate remained moderately invariable with increasing the doping concentration, so the crystalline grain size is decreased. The enhanced grain connection should contribute to carrier transport, while the modified grain size would be positive for increasing the phonon scattering, likewise decreasing the thermal conductivity. 2.2 Electrical transport properties

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The electrical resistivity behavior is a combination phenomenon of carrier transport parameters. The analyzing of resistivity would shed light on carrier transport process. The electrical resistivity  as a function of temperature and doping content x for rare earth Pr doped Ca1–xPrxMnO3 (x=0, 0.06, 0.08, 0.1, 0.12, and 0.14) compound bulk samples is presented in Fig. 4. The un-doped CaMnO3 compound shows a high resistivity , in agreement with its anti-ferromagnetic insulator nature. Favorably, the  of rare earth Pr doped samples is remarkably reduced along with increasing doping concentration x, and the  is moderately saturated at x=0.14. The conductivity enhancement of the titled system can be mainly ascribed to the increased carrier concentration owing to the bivalent Ca2+ doped by trivalent Pr3+ or tetravalent Pr4+ that provides electron carriers to the system[17]. The electrical resistivity  of CaMnO3 system could be approximately expressed as a function of carrier density n and mobility : 1/nq (3) where q is the elementary charge of a carrier[18]. Table 1 shows the measured room temperature carrier concentration n and mobility  for several samples. It is obvious that the carrier concentration n is increased with increasing the doping content x, the mobility  is also enhanced with increasing the doping content x. For instance, the n is increased from 1.02×1019 cm–3 of the un-doped sample to 2.82×1019 cm–3 of x=0.14 sample. The increased electron carrier concentration and the en-

Fig. 3 Cross-section SEM images for Ca1–xPrxMnO3 compounds (a) x=0; (b) x=0.08; (c) x=0.1; (d) x=0.12

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Fig. 4 Resistivity  as a function of temperature for Ca1–xPrxMnO3 (0x0.14) compounds

Fig. 5 ln(/T) vs. 1/T for Ca1–xPrxMnO3 (0x0.14) compounds

Table 1 Measured carrier density (n) and mobility () for Ca1–xPrxMnO3 (x=0, 0.06, 0.1, 0.14) compounds x

n/(1019 cm–3)

/(cm2/Vs)

0.14

2.82

16.9

96.7

0.1

1.68

10.1

96.0

0.06

1.29

7.2

95.2

0

1.02

2.8

95.1

Relative density/%

hanced mobility are regarded to be responsible for the resistivity decreasing. According to transport theory proposed by Mott[12,19], the electrical resistivity  as a function of temperature for the titled compound system could be simulated by the small polaron model expressed as: (T)=CT exp(Ea/kT) (4) where Ea is the activation energy of the polarons, k the Boltzman constant, T the absolute temperature, and the structure dependent variable C can be given by: C

C0

exp(2 R ) Ne 2 a 2 f 1  f p

(5)

where N the number of ion sites per unit cell volume (Mn sites), e the electron charge, a an average inter-site distance for polaron hopping, the electron wave function decay constant, p the optical phonon frequency, f the fraction of available sites occupied by small polarons and the Co a constant. For this transport model, the activation energy of polaron carriers Ea is regarded as the determining factor influencing carrier transport process and the fluctuations of resistivity, the structure dependent variable C can be neglected in analyzing the carrier transport process. By calculating the equation, the Ea can be obtained. Fig. 5 shows the plots of ln(/T) and 1/T for all samples. It can be seen that the plots for all samples lie on the straight lines in the whole temperature region; this verifies the applicability of the transport model. By deducing the slopes of the linear fit of ln(/T) and 1/T, the activation energy Ea of the polarons can be obtained. As calculated, the Ea is decreased from 0.09 eV of the un-doped CaMnO3 to 0.03 eV

of the Ca0.86Pr0.14MnO3 sample. It is true that the carriers are more easily activated to surpass the band gap with increasing Pr doping content, and thus the conduction capability is ultimately enhanced[13]. Fig. 6 presents the Seebeck coefficient D as a function of temperature for rare earth Pr doped Ca1–xPrxMnO3 (x=0, 0.06, 0.08, 0.1, 0.12, and 0.14) compound bulk samples. The D values for all samples are negative, confirming that the electron carriers are dominant for the titled compound. The dependence of  on Pr doping content x for all the samples shows the same behaviors with that of the electrical resistivity , namely the  value is decreased by increasing the doping content x. The fluctuation of Seebeck coefficients D for CaMnO3 system in the studied temperature range can be explained by employing Marsh and Parris’s theory for strong coupling systems[20]: =–(kB/q)·ln[(3–r–y)/(r–1+y)] (6) where q the elementary charge of an electron, kB the Boltzman constant, r the average number of eg electrons per trivalent Mn3+ and y the electron concentration. According to the simplified formula, the absolute Seebeck coefficient  can be reduced by increasing the electron concentration y value. The electron concentration y would be increased by increasing the doping content x. Therefore the Seebeck coefficient is decreased by

Fig. 6 Seebeck coefficient () as a function of temperature for Ca1–xPrxMnO3 (0x0.14) compounds

ZHANG Feipeng et al., Effects of praseodymium doping on thermoelectric transport properties of CaMnO3 …

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increasing x. It can also be seen that the Seebeck coefficient of the Pr doped samples is not linearly dependent of doping content x. Since the Seebeck coefficient D is also related to the carrier effective mass and carrier scattering mechanism[1,14], although further work is needed, it is estimated that the modulated scattering effects as well as the carrier effective mass are responsible for the phenomenon. 2.3 Thermal properties Fig. 7 presents the total thermal conductivity  as a function of temperature for rare earth Pr doped Ca1–xPrxMnO3 (x=0, 0.06, 0.08, 0.1, 0.12, and 0.14) compound bulk samples. It can be seen that the total thermal conductivity  for all samples is weakly dependent of the temperature, indicating that the main thermal transport process is undertaken by lattice thermal conduction. In order to evaluate the influence of rare earth Pr doping on lattice thermal conductivity, the lattice thermal conductivity kL is deduced. The lattice thermal conductivity kL is obtained by subtracting carrier thermal conductivity component kc from total thermal conductivity k. The carrier contribution kc is calculated by applying the Wiedemann-Franz[18] law: kc=LT/ (7) –8 2 2 where L is the Lorenz constant 2.45×10 V /K and T is absolute temperature. Fig. 8 gives the calculated lattice thermal conductivity for all samples. It can be seen that the total thermal conductivity depression is mainly due to the lattice thermal conductivity reduction. The suppression of total thermal conductivity can be explained by the reason that the phonon scattering is made much more effective through heavy element Pr doping, the vibration modes that carry heat efficiently is lowered by the enlarged number of species in the unit cell[21], therefore the lattice thermal conductivity and the total thermal conductivity are reduced. This is in accordance with the theoretical study[14,21]. Additionally, the grain boundary area and quantity should be increased as a result of the reduced crystalline grain size; this would be in favor of

Fig. 8 Lattice thermal conductivity as a function of temperature for Ca1–xPrxMnO3 (0x0.14) compounds

the grain boundary phonon scattering enhancement which is partially responsible for the thermal conductivity suppression. 2.4 Thermoelectric figure of merit, ZT Fig. 9 presents the TE dimensionless figure of merit ZT as a function of temperature for rare earth Pr doped Ca1–xPrxMnO3 (x=0, 0.06, 0.08, 0.1, 0.12, and 0.14) compound bulk samples. The ZT values increase with increasing temperature indicating that the titled compound is more suitable for high temperature energy conversion fields. The slopes of temperature dependence curves for doped sample are all larger than that of the un-doped samples, showing rapidly enhanced TE properties with elevating the applied temperature. Secondly, the ZT value of doped sample are remarkably higher than that of the un-doped sample, these confirm that the TE transport property could be really improved by rare earth elements doping at a lower doping content. It is worth noting that the x=0.08 sample has the highest ZT value with the peak 0.16 at 873 K which is much higher than that of the un-doped compound system.

Fig. 9 Dimensionless figure of merit ZT as a function of temperature for Ca1–xPrxMnO3 (0x0.14) compounds Fig. 7 Total thermal conductivity as a function of temperature for Ca1–xPrxMnO3 (0x0.14) compounds

3 Conclusions In summary, the effects of rare earth Pr doping within

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low concentration on phase compositions and TE transport properties of the Ca1–xPrxMnO3 (x=0, 0.06, 0.08, 0.1, 0.12, and 0.14) compound systems were studied. All samples were found to be single phased within the experimental doping range and they exhibited condensed bulk microstructure accompanied by enhanced inter-grain connection and decreased grain size. The electrical resistivity was remarkably decreased along with increasing the Pr doping content, mainly due to carrier density enhancement. All the samples followed the polaron transport model well and the energy for polarons to hop was decreased. The Seebeck coefficient experienced a decreasing trend, in accordance with the electrical resistivity behavior as a function of doping content x. The total thermal conductivity was reduced because of noticeable lattice thermal conductivity suppression. On account of tuning TE transport parameters independently, the Ca0.92Pr0.08MnO3 sample showed the largest dimensionless figure of merit ZT with a peak value of 0.16 at 873 K, greatly higher than that of the un-doped CaMnO3 compound. The present investigation suggested that the thermoelectric properties of the titled compound system could be effectively improved via tuning the transport parameters independently, by rare earth doping at a lower content through simple sample preparation procedure.

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