Thermoelectric properties of Ca0.8Dy0.2MnO3synthesized by solution combustion processKyeongsoon Park* and Ga Won Lee
Abstract
High-quality Ca0.8Dy0.2MnO3 nano-powders were synthesized by the solution combustion process. The size of thesynthesized Ca0.8Dy0.2MnO3 powders was approximately 23 nm. The green pellets were sintered at 1150-1300°C ata step size of 50°C. Sintered Ca0.8Dy0.2MnO3 bodies crystallized in the perovskite structure with an orthorhombicsymmetry. The sintering temperature did not affect the Seebeck coefficient, but significantly affected the electricalconductivity. The electrical conductivity of Ca0.8Dy0.2MnO3 increased with increasing temperature, indicating asemiconducting behavior. The absolute value of the Seebeck coefficient gradually increased with an increase intemperature. The highest power factor (3.7 × 10-5 Wm-1 K-2 at 800°C) was obtained for Ca0.8Dy0.2MnO3 sintered at1,250°C. In this study, we investigated the microstructure and thermoelectric properties of Ca0.8Dy0.2MnO3,depending on sintering temperature.
1. IntroductionSolid-state thermoelectric power generation based onSeebeck effects has potential applications in waste-heatrecovery. Thermoelectric generation is thermodynami-cally similar to conventional vapor power generation orheat pumping cycles [1]. Thermoelectric devices are notcomplicate, have no moving parts, and use electrons asworking fluid instead of physical gases or liquids [1,2].The efficiency of thermoelectric devices is determinedby the materials’ dimensionless figure-of-merit, definedas ZT = sa2/�T, where s, a, �, and T are the electricalconductivity, Seebeck coefficient, thermal conductivity,and absolute temperature, respectively. To be a goodthermoelectric material, it is required to have a largeelectrical conductivity and Seebeck coefficient as well asa low thermal conductivity. The three parametersdepend on each other since they are closely related tothe scattering of charge carriers and lattice vibrations. Itis thus necessary to compromise among them for opti-mizing the thermoelectric properties [3].Kobayashi et al. [4] proposed the possibility of (R1-
xCax)MnO-δ (R: Tb, Ho, and Y) with the orthorhombicperovskite-type structure as n-type thermoelectric
materials. Since then, the electrical transport propertiesof (Ca0.9M0.1)MnO3 (M = Y, La, Ce, Sm, In, Sn, Sb, Pb,and Bi) have been studied, and reported that partial sub-stitution for the Ca led to a significant increase in theelectrical conductivity, along with a moderate decreasein the absolute value of the Seebeck coefficient, therebyimproving the dimensionless figure-of-merit [3].It is well known that controlling the microstructure
and processing, especially sintering, is a feasible route toimprove the thermoelectric performance. Therefore, inthis study, to improve the thermoelectric properties,nano-sized Ca0.8Dy0.2MnO3 powders were synthesizedby the solution combustion process. The solution com-bustion process is favorable for synthesizing pure andnano-sized high-quality oxide powders in a short timeand is cost-effective [5,6]. Subsequently, we sintered theCa0.8Dy0.2MnO3 green pellets at 1150-1300°C and theninvestigated the microstructure and thermoelectric prop-erties, depending on sintering temperature.
2. ExperimentalCa0.8Dy0.2MnO3 powders were synthesized by the solu-tion combustion process. The process involved theexothermic reaction initiated by metal nitrates (oxidizer)and an organic fuel (reductant). Ca(NO3)2 · 6H2O, Mn(NO3)2 · 6H2O, Dy(NO3)3 · 5H2O were used as oxidizers
* Correspondence: [email protected] of Nanotechnology and Advanced Materials Engineering, SejongUniversity, Seoul 143-747, Korea
Park and Lee Nanoscale Research Letters 2011, 6:548http://www.nanoscalereslett.com/content/6/1/548
and glutamic acid (C5H9NO4) as combustion fuel. Themolar ratio of the metal nitrates to the fuel in the pre-cursor solution was adjusted to be 1:1. The appropriateproportions of the metal nitrates were separately dis-solved in distilled water to prepare homogeneous solu-tions. The glutamic acid was separately dissolved in thesolutions. The resulting solution was heated slowly on ahot plate, boiled, and dehydrated, forming a highly vis-cous gel. Subsequently, the gel frothed and swelled withevolution of huge volume of gases. The reaction lastedfor 3-4 min and produced a foam that readily crumbledinto powder. The size and morphology of the resultingpowders were characterized with a transmission electronmicroscope (TEM; JEOL JEM-2100F) operating at 200kV. Subsequently, the synthesized powders were cal-cined at 900 and 1,000°C for 12 h with intermediategrinding. The calcined nanopowders were cold-pressedunder 137 MPa to prepare green pellets. The pelletswere sintered at 1150-1300°C at a step of 50°C in air.The porosity of as-sintered Ca0.8Dy0.2MnO3 was mea-
sured by the Archimedes’ principle. The crystal struc-ture of as-sintered samples was analyzed with an X-raydiffractometer (XRD; Rigaku DMAX-2500) using Cu Karadiation at 40 kV and 100 mA. The microstructure ofas-sintered samples was investigated with a field emis-sion scanning electron microscope (FESEM; HitachiS4700). To measure the thermoelectric properties as afunction of temperature, the electrical conductivity sand the Seebeck coefficient a were simultaneously mea-sured over a temperature range of 500-800°C.Samples for the measurements of thermoelectric prop-
erties were cut out of the sintered bodies in the form ofrectangular bars of 2 × 2 × 15 mm3 with a diamond sawand polished with SiC emery paper. The electrical con-ductivity s was measured by the direct current (dc)four-probe method. For thermopower measurements, atemperature difference ΔT in the sample was generatedby passing cool Ar gas over one end of the sampleplaced inside a quartz protection tube. The temperaturedifference ΔT between the two ends of each sample wascontrolled at 4-6°C by varying the flowing rate of Argas. The thermoelectric voltage ΔE measured as a func-tion of the temperature difference ΔT gave a straightline. The Seebeck coefficient a was calculated from therelation a = ΔE/ΔT.
3. Results and discussionFigure 1 shows a TEM bright-field image of the synthe-sized Ca0.8Dy0.2MnO3 powders. The synthesizedCa0.8Dy0.2MnO3 powders show spherical and regularmorphologies, and smooth surfaces. The average size ofthe synthesized powders is in nano-scale, i.e., approxi-mately 23 nm. Obviously, this combustion processing isan extremely simple and cost-effective method for
the surfaces of Ca0.8Dy0.2MnO3 sintered at 1150, 1200,1250, and 1300°C, respectively. Most pores are locatedat the grain boundaries. As the sintering temperatureincreases, the average grain size of the samplesincreases, i.e., 399, 430, 545, and 590 nm for 1150, 1200,1250, and 1300°C, respectively. In addition, the densityof the samples escalates with an increase in sinteringtemperature up to 1250°C, and then decreases with a
Figure 1 TEM bright-field image of synthesized Ca0.8Dy0.2MnO3
powders.
Figure 2 FESEM images obtained from the surfaces ofCa0.8Dy0.2MnO3 sintered at (a) 1150, (b) 1200, (c) 1250, and (d)1300°C.
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further rise in sintering temperature. The densities ofCa0.8Dy0.2MnO3 sintered at 1150, 1200, 1250, and 1300°C are 81.5, 87.2, 98.5, and 96.3% of the theoretical den-sity, respectively. A fine-grain size and high density areobtained even at a low sintering temperature of 1250and 1300°C. This indicates that nano-sized powderssynthesized by the glutamic acid-assisted combustionmethod allow for dense and fine-grained pellets at muchlower sintering temperature, compared to conventionalsolid-state reaction processed powders. The finer pow-der has a larger surface energy, thus giving rise to largerdensification and grain growth rates because of a highdiffusivity near the surface and grain boundary duringsintering [7].The XRD patterns of the Ca0.8Dy0.2MnO3 sintered at
various temperatures are shown in Figure 3. The sin-tered Ca0.8Dy0.2MnO3 has an orthorhombic perovskite-type structure, belonging to the Pnma space group [8].The added Dy3+ does not affect the crystal structure ofCaMnO3. The crystallite size D of the Ca0.8Dy0.2MnO3
pellets can be calculated from the Scherrer formula: D =(0.9l)/(bcosθ), where l is the wavelength of radiation, θis the angle of the diffraction peak, and b is the fullwidth at half maximum of the diffraction peak (inradian) [9]. The calculated crystallite sizes of the sin-tered Ca0.8Dy0.2MnO3 are in the range of 20.0-24.5 nm.The electrical conductivity of Ca0.8Dy0.2MnO3 sintered
at various temperatures is shown in Figure 4. The elec-trical conductivity increases with increasing temperature,indicating a typical semiconducting behavior characteris-tic. In addition, the electrical conductivity increases withincreasing sintering temperature, reaching a maximumat 1250°C, and then decreases with further increasingsintering temperature. The electrical conductivities at800°C for the Ca0.8Dy0.2MnO3 samples sintered at 1150,
1200, 1250, and 1300°C are 82.8, 88.3, 120.5, and 96.6Ω-1 cm-1, respectively. The electrical conductivity of theCa0.8Dy0.2MnO3 sintered at 1300°C is lower than that ofthe Ca0.8Dy0.2MnO3 sintered at 1250°C. This result indi-cates that the porosity strongly affects the electrical con-ductivity of Ca0.8Dy0.2MnO3. Pores act as scatteringcenters for conduction, decreasing the time betweenelectron scattering events of charge carriers. The highestelectrical conductivity (120.5 Ω-1 cm-1) is obtained forthe Ca0.8Dy0.2MnO3 sintered at 1250°C.A relationship between the log(sT) and 1000/T for
Ca0.8Dy0.2MnO3 as a function of sintering temperatureis shown in Figure 5. We can find a nearly linear rela-tionship between log(sT) and 1000/T over the measuredtemperature range. The activation energy (Ea) for con-duction at high temperatures (500-800°C) is calculatedfrom the slope of the log(sT) and 1000/T. The
Figure 3 XRD patterns of Ca0.8Dy0.2MnO3 sintered at varioustemperatures.
Figure 4 Electrical conductivity of Ca0.8Dy0.2MnO3 sintered atvarious temperatures.
Figure 5 A relationship between the log(sT) and 1000/T forCa0.8Dy0.2MnO3 as a function of sintering temperature.
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calculated activation energies of the Ca0.8Dy0.2MnO3
sintered at 1150, 1200, 1250, and 1300°C are 0.096,0.126, 0.115, and 0.104 eV, respectively. This means thatthe conduction of these samples is caused by a ther-mally activated small polaron hopping [10]. A smallpolaron is formed when the effective mass of the rigidlattice hole is large and coupling to optical phonons isstrong [11].In the polaron hopping conduction, an electron moves
by a thermally activated hopping process from one loca-lized state to another with the activation energy Eh [12].The electrical conductivity s is written as s = (C/T)exp(-Eh/kBT), where C, T, Eh, and kB are the charge carrierconcentration, the absolute temperature, the activationenergy, and the Boltzmann constant, respectively [3].The electrical conductivity of the small polaron hoppingconduction in the adiabatic case is given as s = neμ =nea2(A/T)exp(-Eh/kBT), where n is the carrier concentra-tion, e is the electrical charge of the carrier, μ is the car-rier mobility, a is the intersite distance of hopping, Eh isthe activation energy for hopping, and A is the pre-exponential tern related to the carrier scatteringmechanism, respectively [3,13].The Seebeck coefficient of Ca0.8Dy0.2MnO3 as a func-
tion of temperature is shown in Figure 6, depending onsintering temperature. The absolute value of the Seebeckcoefficient for Ca0.8Dy0.2MnO3 gradually increases withan increase in temperature. The sign of the Seebeckcoefficient is negative over the measured temperaturerange, indicating n-type conduction. The absolute valuesof the Seebeck coefficients at 800°C for theCa0.8Dy0.2MnO3 sintered at 1150, 1200, 1250, and 1300°C are 55.0, 54.7, 55.1, and 54.9 μV K-1, respectively,indicating sintering temperature has no significant influ-ence on the Seebeck coefficient.
The power factor sa2 is calculated using the electricalconductivity s and the Seebeck coefficient a. The powerfactor obtained from the data in Figures 4 and 6 isplotted in Figure 7. At a given sintering temperature,the power factor increases with an increase in tempera-ture. In addition, the power factor increases with sinter-ing temperature up to 1250°C and then decreases forhigher sintering temperature. The highest power factor(3.7 × 10-5 Wm-1 K-2 at 800°C) is obtained for theCa0.8Dy0.2MnO3 sintered at 1250°C. From the aboveresults, it is believed that controlling the sintering tem-perature of Ca0.8Dy0.2MnO3 is important for improvingits thermoelectric properties.
4. ConclusionWe synthesized Ca0.8Dy0.2MnO3 nanopowders (approxi-mately 23 nm in size), which showed spherical and reg-ular morphologies, and smooth surfaces, by the glutamicacid-assisted combustion method. The nano-sized pow-ders led to dense and fine-grained pellets at low sinter-ing temperature. The average grain sizes of theCa0.8Dy0.2MnO3 sintered at 1150, 1200, 1250, and 1300°C were 399, 430, 545, and 590 nm, respectively. In addi-tion, the densities of the Ca0.8Dy0.2MnO3 sintered at1150, 1200, 1250, and 1300°C were 81.5, 87.2, 98.5, and96.3% of the theoretical density, respectively. TheCa0.8Dy0.2MnO3 sintered had an orthorhombic perovs-kite-type structure, belonging to the Pnma space group.The electrical conductivity increased with increasing sin-tering temperature, reaching a maximum at 1250°C, andthen decreased with further increasing sintering tem-perature. However, a noticeable change in the Seebeckcoefficient of Ca0.8Dy0.2MnO3 sintered at various tem-peratures was not evident. The Ca0.8Dy0.2MnO3 sinteredat 1250°C showed the highest power factor (3.7 × 10-5
Figure 6 Seebeck coefficient of Ca0.8Dy0.2MnO3 as a function oftemperature.
Figure 7 Power factor of Ca0.8Dy0.2MnO3 sintered at varioustemperatures.
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Wm-1 K-2) at 800°C. It is necessary to control the sinter-ing temperature of Ca0.8Dy0.2MnO3 for improving thethermoelectric properties.
AcknowledgementsThis study is the outcome of a Manpower Development Program for Energy& Resources supported by the Ministry of Knowledge and Economy (MKE),Republic of Korea.
Authors’ contributionsKP conceived of the study, participated in its design and coordination, anddrafted the manuscript. GWL carried out the synthesis, microstructureanalysis, and thermoelectric studies. All authors read and approved the finalmanuscript.
Competing interestsThe authors declare that they have no competing interests.
Received: 25 May 2011 Accepted: 5 October 2011Published: 5 October 2011
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doi:10.1186/1556-276X-6-548Cite this article as: Park and Lee: Thermoelectric properties ofCa0.8Dy0.2MnO3 synthesized by solution combustion process. NanoscaleResearch Letters 2011 6:548.
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