Enhanced high temperature thermoelectric characteristics of transitionmetals doped Ca3Co4O9+δ by cold high-pressure fabricationYang Wang, Yu Sui, Xianjie Wang, Wenhui Su, and Xiaoyang Liu Citation: J. Appl. Phys. 107, 033708 (2010); doi: 10.1063/1.3291125 View online: http://dx.doi.org/10.1063/1.3291125 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v107/i3 Published by the American Institute of Physics. Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

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Enhanced high temperature thermoelectric characteristics of transitionmetals doped Ca3Co4O9+� by cold high-pressure fabrication

Yang Wang,1 Yu Sui,1,2,a� Xianjie Wang,1 Wenhui Su,1 and Xiaoyang Liu3

1Department of Physics, Center for Condensed Matter Science and Technology (CCMST), Harbin Instituteof Technology, Harbin 150001, People’s Republic of China2International Center for Materials Physics, Academia Sinica, Shenyang 110015, People’s Republic ofChina3State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry,Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China

�Received 10 October 2009; accepted 13 December 2009; published online 2 February 2010�

A series of Fe, Mn, and Cu doped Ca3Co4O9+� samples, Ca3�Co,M�4O9+� �M=Fe, Mn, and Cu�,were fabricated by cold high-pressure compacting technique. Their thermoelectric properties wereinvestigated from room temperature up to 1000 K. The cold high-pressure compacting method isadvantageous to increasing density and texture, in favor of the improvement of thermoelectricperformance. The electrical transport measurements indicate that Fe/Mn substitutes for Co mainlyin �CoO2� layers whereas the substitution of Cu for Co takes place in �Ca2CoO3� layers. Thethermoelectric properties as well as electronic correlations depend not only on the substitution ionbut also the Co site that is replaced. Thermopower can be well calculated by the carrier effectivemass according to Boltzmann transport model, indicating that the electronic correlation plays acrucial role in the unusual thermoelectric characteristics of this system. From the changes inthermopower, resistivity, and thermal conductivity, thermoelectric performance of Ca3Co4O9+� isefficiently improved by these transition metals doping. Fe doped samples possess the highest ZTvalues. Combining cold high-pressure technique, ZT of Ca3Co3.9Fe0.1O9+� can reach �0.4 at 1000K, which is quite large among ceramic oxides, suggesting that Fe doped Ca3Co4O9+� could be apromising candidate for thermoelectric applications at elevated temperatures. © 2010 AmericanInstitute of Physics. �doi:10.1063/1.3291125�

I. INTRODUCTION

As clean energy source and device cooler, environment-friendly thermoelectric �TE� materials, which can be used toconvert heat energy into electrical energy directly via See-beck effect and vice versa by Peltier effect, have attractedextensive attention.1,2 The energy conversion performance ofa TE material is usually characterized by the dimensionlessfigure of merit ZT=S2T /��, where S, T, �, and � are ther-mopower, absolute temperature, resistivity, and thermal con-ductivity, respectively, and ZT�1 is required for practicalapplications. Apparently, good TE materials should havelarge S along with low � and �. Nevertheless, the three pa-rameters are not independent; they all strongly depend on thecrystalline structure, energy band, carrier concentration, etc.,of a system.2 For instance, with the increase in carrier con-centration, � will decrease but simultaneously S will alsolessen. Consequently, the TE properties are usually opti-mized for materials with typical carrier concentration of1019 cm−3,2 so only a few intermetallic compounds such asBi2Te3 alloys, chalcogenides, and skutterudites have beenknown to fulfill the requirement,3–7 and these alloys remainthe state-of-the-art TE materials to date.

Metal oxides, due to their structural and chemical stabil-ity at high temperatures in air, oxidation resistance, easy

manufacture, and low cost, are more suitable for high tem-perature TE applications than the conventional TE alloys.Nevertheless, the strong ionic characteristic with narrow con-duction bandwidth in metal oxides usually causes the local-ization of electrons with low carrier mobilities, which thusleads to poor TE performance.8,9 However, the discoveries oflarge TE response in some transition metal oxides such asmanganites,10–14 titanates,15–17 and cobaltites18–20 give rise toa renewed interest in TE oxides recently. Among these tran-sition metal oxides, misfit-layered Ca3Co4O9+� exhibits un-usual TE properties: coexistence of a large thermopower anda low resistivity, and thus attracts much attention in recentyears.18,19 The crystal structure of Ca3Co4O9+� consists oftwo subsystems, viz., the distorted NaCl-type �Ca2CoO3�sublattice and the CdI2-type �CoO2� sublattice alternativelystacking along the c axis.19 Due to its large TE figure ofmerit �ZT�1 of the single crystal at 1000 K�,21,22

Ca3Co4O9+� is considered to be a promising candidate for ap-type material component of high temperature TE powergeneration systems. For practical industrial exploitation, ce-ramic oxide materials are needed but the ZT of the polycrys-talline bulk Ca3Co4O9+� is still at a relatively low level. Tooptimize the TE performance of Ca3Co4O9+� ceramics, manyattempts have been made by choosing kinds of ions dopingor improving fabrication methods, and the ZT values are in-deed enhanced by these studies.23–29 From the previous re-ports, most investigations mainly concentrated on the effectsof substitutions for Ca ion but very few research was per-

a�Author to whom correspondence should be addressed. Electronic mail:suiyu@hit.edu.cn.

JOURNAL OF APPLIED PHYSICS 107, 033708 �2010�

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formed on substitutions for Co ion. Actually, doping in Cosites, especially in �CoO2� layers, may induce more notableeffects on the transport and TE properties, because thecharge carrier transport mostly occurs within �CoO2�planes,2,30 and the electronic structure calculation revealsthat the Fermi level lies in the crystal-field gap of the d statesin �CoO2� subsystem.31 For example, it was reported that thesubstitution of Mn or Ti for Co results in the change intransport mechanism and enhanced spin fluctuations.32–34

Moreover, it is interesting that the slight substitution of Fefor Co causes the simultaneous increase in thermopower andelectrical conductivity, and hence a remarkable improvementof TE characteristics26 but the nature is not very clear. There-fore, it is significant to investigate various transition metalions doping in Co sites and explore the mechanism of TEcharacteristic variations, which is helpful for the further im-provement of TE performance in Ca3Co4O9+� and relevantsystems.

In this paper, the effects of the substitutions of Fe, Mn,and Cu for Co on the transport and TE properties ofCa3Co4O9+� were investigated from room temperature up to1000 K. We adopted cold high-pressure technique to fabri-cate samples since the cold high pressure has been confirmedto be a useful method for improving TE properties in layeredNaxCoO2 ceramic.35 The results reveal that the TE perfor-mance of Ca3Co4O9+� is efficiently enhanced by these tran-sition metal ions doping particularly by Fe doping.

II. EXPERIMENTS

Fe, Mn, and Cu doped Ca3Co4O9+� ceramics �dopinglevel x=0–0.1� were synthesized by a cold high-pressuremethod. High purity powders of CaCO3, Co2O3, Fe2O3,MnO2, and Cu2O in the stoichiometric ratio were mixed,pressed into pellets and sintered at 1173 K for 48 h under O2

flow with intermediate grindings. Then the pellets were thor-oughly pulverized and cold pressed into disk-shaped pelletsunder a high pressure of 3 GPa with a special steel die.Finally, the pellets were annealed under O2 flow at 1173 Kfor 12 h and slowly cooled to room temperature. X-ray dif-fraction �XRD� data at room temperature were collected us-ing the XRD diffractometer �D8 Advanced� with Cu K� ��=0.154 06 nm� radiation. The microstructure was observedwith a JEOL JSM-6700F scanning electron microscope�SEM�. Oxygen content was determined by iodometric titra-tion. The excess oxygen � is �0.35 for all samples. Resis-tivity from room temperature to 1000 K was measured by astandard four-probe method using the 2400 SourceMeter�Keithley�. Thermopower was calculated from the TE volt-age and the temperature gradient across the samples�ULVAC-RIKO: ZEM-3�. Thermal conductivity was calcu-lated by �=DCpd, where D is thermal diffusivity �measuredby a laser flash method, Netzsch LFA-457�, Cp is specificheat capacity �determined by Netzsch DSC-404�, and d is thematerial density. The room-temperature Hall effect measure-ments were conducted by the commercial Quantum Designphysical property measurement system, using a standardfive-wire method.

III. RESULTS AND DISCUSSION

The XRD patterns for the crushed powder of the speci-mens are shown in Fig. 1. The XRD patterns of all thesamples are identical to the standard Ref. 36. The sites andrelative intensities of the XRD diffraction peaks are also con-sistent with those of reported results of Ca3Co4O9+� with Cm�1p0� superspace group,19,31 indicating that the single-phasecompounds were successfully synthesized. It was reportedthat in Ca3Co4O9+� system the substitution for Co ions cantake place in either �Ca2CoO3� sublattice or �CoO2�sublattice.25,33,34 Based on the following analysis, we will seethat Fe or Mn ions mainly replace Co ions in �CoO2� sublat-tice, while Cu ions essentially occupy Co sites in �Ca2CoO3�sublattice. Additionally, it can be found that the 2� angles ofdiffraction peaks of the samples increase slightly, mainly re-sulting from the different radii of Co ions and these transitionmetal ions.

The SEM photographs of the specimens are shown inFig. 2. For comparison, a SEM micrograph of the sampleprepared by conventional solid-state reaction without coldhigh-pressure process is also presented �Fig. 2�a��. Due to thestructural characteristic of Ca3Co4O9+� that two subsystemsalternatively stack along the c-axis to form a layeredstructure,19 there exists a highly crystallographic anisotropy.From the SEM micrograph, it is clear that compared withconventional ceramic specimen, c-axis of the grains withinthe sample is preferred to align along the pressing directionafter compacting under high pressure and sintering treat-ment. Most grains in Fig. 2�b� exhibit a platelike shape andare preferentially aligned along the pressing direction, viz.,highly textured polycrystalline samples were successfullyfabricated by means of the cold high-pressure compactingtechnique. Also, the average grain size ��2 �m� of the coldhigh-pressed samples is larger than that of conventionalsamples ��1 �m�, which suggests that the cold high-pressure compacting technique is helpful for the graingrowth. Furthermore, it can be found that the cold high-

FIG. 1. �Color online� X-ray powder diffraction patterns for Ca3Co4O9+�

parent and Ca3Co3.9M0.1O9+� �M=Fe, Mn, and Cu�. The inset shows theXRD patterns for the pressed surface of Ca3Co4O9+�, in which only the �00l�diffraction peaks are detectable.

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pressed samples are much denser than the conventionalsamples. The densities of the cold high-pressed samples are�4.5–4.6 g /cm3, very close to the theoretical density�4.94 g /cm3� of Ca3Co4O9+�.37 High texture, large grainsize, and large density are all helpful for reducing resistivityand thus improving TE performance.

To further clarify the texture characteristic of thesamples, XRD measurements for the pressed surface of thespecimens were conducted �see the inset of Fig. 1�. Com-pared with the patterns of the crushed powder and Ref. 36,the XRD profile for the pressed surface of samples exhibitsstrong diffraction peaks from �00l� planes, but other diffrac-tion peaks are quite weak, also indicating the high-texturedstructure in these ceramic samples, which is consistent withthe SEM results. Generally, the degree of crystallographicanisotropy of a material can be evaluated in terms of theLotgering factor f ,38 f = �p− p0� / �1− p0�, where p=�I�00l� /�I�hkl�, and p0= p for a crystallographic isotropic�randomly oriented� sample that can be calculated from thepeak data of Ref. 36. When the c-axis directions of grains arecompletely aligned along the pressing direction, the f valueis calculated to be 1. The values of f of �00l� planes for thesecold high-pressed samples are near 0.9, implying their largecrystallographic anisotropy. It should be noted that when thesamples were pulverized, the diffraction peaks from other

planes appear, but the relative intensity of the peaks for thepowder XRD is still different from that of Ref. 36. The Lot-gering factor f of the powder was calculated to be around0.6, suggesting that the high texture was partly destroyedafter pulverizing, but the crystallographic anisotropy of thegrains still exists in the samples. The f values and densitiesobtained in our samples are comparable to those texturedCa3Co4O9+� ceramics prepared by reactive templated graingrowth method, sol-gel spin coating method, hot-pressingmethod, etc,39–42 indicating that a polycrystalline sample ofCa3Co4O9+� system with a large crystallographic anisotropyand high density can be easily fabricated by adopting a coldhigh-pressure compacting process before sintering. Com-pared with those above-mentioned techniques, this method issimple and effortless, and can be applied to more systems.

Figure 3�a� presents the temperature dependence of re-sistivity � of the samples from room temperature to 1000 K.Several characteristics can be found: �1� compared with theparent material, Fe/Cu doping reduces �, while Mn dopingincreases �; �2� the �-T curve for undoped sample shows asudden metal-semiconductor �MS� transition around 380 K,a metalliclike behavior in the temperature range from 300 to�380 K and then a semiconducting-like regime at highertemperatures; �3� with doping, the obvious MS transition dis-appears gradually, and only broad humps in the range of

FIG. 2. SEM micrographs for �a� conventional fabricated Ca3Co3.9Fe0.1O9+� and �b� cross section of cold high-pressure fabricated Ca3Co3.9Fe0.1O9+�. The insetof �b� shows the pressed surface of cold high-pressure fabricated Ca3Co3.9Fe0.1O9+�.

FIG. 3. �Color online� �a� Temperature dependence of resistivity � for all samples. �b� Magnetic field H dependence of Hall resistivity �H of Ca3Co4O9+�,Ca3Co3.9Fe0.1O9+�, and Ca3Co3.9Mn0.1O9+� at 300 K.

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300–500 K in �-T curves are exhibited; �4� Fe/Cu dopingincreases the MS transition temperature TMS, whereas Mndoping shifts TMS to lower temperatures. The change in �magnitude with doping should be attributed to the variationin carrier concentration in the system, since the majoritycharge carriers of Ca3Co4O9+� system are holes, as con-firmed by the positive values of Hall coefficient andthermopower.19,43 In the Hall effect measurements, Hall re-sistivity �H of samples was measured under variable mag-netic field; subsequently Hall coefficient RH can be obtainedfrom the slope of �H versus magnetic field, and carrier con-centration n is calculated by n=1 /eRH. Figure 3�b� presentsthe relationship between �H and magnetic field for the threesamples at 300 K. Table I lists the obtained room-temperature carrier concentration n according to the Hall co-efficient measurements, from which one can see that Fe orCu doping increases n, but Mn doping lessens n. InCa3Co4O9+� system, there are three different valences of Coions: Co2+, Co3+, and Co4+. The average valence of Co isbetween +3 and +4.19 Fe ions can be Fe2+ or Fe3+, and Cuions are usually Cu2+, so their valences are all lower than theaverage valence of Co in Ca3Co4O9+�. Therefore, the substi-tution of Fe or Cu for Co will introduce hole carriers �i.e.,

hole-doping� and decrease � as a result. It is also noteworthythat because the valence of Cu ions is lower than that of Fe,which means Cu doping can introduce more hole carriersthan Fe in the case of same doping level, Cu doped sampleshave lower � than Fe doped samples. The change in � withMn doping is similar with previous reports.25,32 Carrier con-centration decreasing with Mn doping suggests that the av-erage valence of Mn is higher than that of Co �i.e., electron-doping�. Therefore, although Mn ions may be Mn2+, Mn3+,or Mn4+, the fact that the substitution of Mn for Co decreasesn of the system suggests that Mn4+ should be dominant, so �increases consequently.

The sharp MS transition at TMS of Ca3Co4O9+� has beenobserved in some previous reports.19 This transition is con-sidered to be associated with a spin-state transition of Coions.19,44,45 However, recently we found that the MS transi-tion has evident first-order phase transition characteristic,and suggested this transition may be a structural phasetransition.46 Fe, Mn, or Cu doping does not change the crys-talline structure of system, only induces some structural dis-tortions, so the MS transition still exists, but TMS varies bydoping �see Table I, herein we define TMS as the temperatureat which the slope d� /dT is zero in �-T curves�. As men-

FIG. 4. �a� Temperature dependence of thermopower S for undoped Ca3Co4O9+� and Ca3Co3.9M0.1O9+� �M=Fe, Mn, and Cu�. �b� and �c� Temperaturedependence of � and S for cold high-pressure fabricated and conventional fabricated Ca3Co3.9Fe0.1O9+�.

TABLE I. TE characteristics, MS transition temperature TMS, activation energy Ea, electronic effective massm�, and the room-temperature transport parameters �carrier concentration n, mobility �, and relaxation time �for all the samples.

Ca3Co4O9+�

Ca3Co4−xFexO9+� Ca3Co4−xMnxO9+� Ca3Co4−xCuxO9+�

x=0.05 x=0.1 x=0.05 x=0.1 x=0.05 x=0.1

�300 K �m cm� 15.5 12.8 10.9 16.1 18.7 10.0 7.98�1000 K �m cm� 13.2 10.9 9.68 14.3 14.8 9.44 7.76S300 K ��V /K� 121.5 145.7 171.4 138.2 162.1 110.8 93.5S1000 K ��V /K� 202.1 223.6 257.3 218.6 240.5 192.7 174.9

�300 K �Wm−1 K−1� 2.87 2.53 2.23 2.51 2.37 2.32 1.96�1000 K �Wm−1 K−1� 2.20 1.95 1.77 1.92 1.79 1.86 1.68

ZT1000 K 0.13 0.24 0.39 0.17 0.22 0.21 0.24TMS �K� 380 392 426 360 342 421 447

Ea �meV� 77.1 88.6 95.3 82.8 86.6 79.0 80.2m� ��m0� 12.6 14.3 15.2 13.5 14.1 12.4 12.7

n ��1020 cm−3� 4.30 4.52 4.66 4.21 4.08 4.59 4.72� �cm2 V−1 s−1� 0.877 1.080 1.230 0.862 0.819 1.361 1.659

��10−15 s� 6.29 8.78 10.63 6.61 6.57 9.59 11.98

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tioned above, Mn doping decreases carrier concentration,which can increase the average distance between holes, sothe localization of system increases, suggesting that semi-conducting state is more stable. Therefore, TMS decreaseswith Mn doping. In contrast, Fe or Cu doping increases holeconcentration, which will stabilize the metallic state. Accord-ingly, the MS transition takes place at higher temperatures.In addition, since Cu doping brings on more holes than Fe,TMS of Cu doped series is higher than Fe doped series. Thevariations of carrier localization are also evidenced by thechanges in carrier mobility � �=1 /�ne� and carrier relaxationtime �=�m� /e, where m� is carrier effective mass�, as listedin Table I, both of which observably increase by Fe/Cu dop-ing but keep unchanged or slightly decrease by Mn doping.

Figure 4�a� shows the temperature dependence of ther-mopower S for the samples. Compared with undopedCa3Co4O9+�, doping Fe, Mn, or Cu does not change theshapes of S-T curves on the whole but changes the values ofS obviously. It can be found that Fe/Mn doping enhances S,while Cu doping reduces S. Figures 4�b� and 4�c� display �and S for cold high-pressure fabricated and conventional fab-ricated Ca3Co3.9Fe0.1O9+�. It is obvious that cold high-pressure method efficiently decreases � but has tiny influenceon S, and thus is in favor of improving TE performance. InCa3Co4O9+� system, S can be expressed by the Mottformula,47,48 viz.,

S�T� =1

eT

�−�� ����� − ��

� f�����

d�

�−�� ���

� f�����

d�

, �1�

where ��� and f��� represent electrical conductivity andFermi–Dirac distribution function at energy �. By using thecondition of �f��� /��=���=�F� and =en����,48,49 Eq. �1�can turn into

S�T� =ce

n+

�2kB2T

3e� � ln ����

��

�=�F

, �2�

where ����, ce, and kB are energy correlated carrier mobility,electronic specific heat, and Boltzmann constant, respec-tively. The first term of Eq. �2� is dominant, similar with thesimple Drude picture S�ce /n,50 so the change in S can usu-ally be explained by the variation in carrier concentrationn.34,51 Herein this scenario can account for the variations of Swith Mn or Cu doping, however, it is obviously incompatiblewith the S behavior of Fe doped series because of the simul-taneous increases of n and S. To illuminate this unusual be-havior, next we focus on the transport mechanism and elec-tronic correlations in the system.

The plots of ln�� /T� versus 1 /T for all the samples lie onstraight lines above approximately 600 K, as shown in Fig. 5,indicating the hopping conduction behavior at high tempera-ture. The substitution of Fe, Mn, or Cu does not change thetransport mechanism but the hopping activation energy Ea,obtained from the slopes of the plots, is changed. As can beseen from Fig. 6�a�, Ea observably increases for Fe and Mndoped series but only shows a slight increase for Cu dopedseries. Compared with the reports for Fe, Mn, Cu etc doped

NaxCoO2 that has the same �CoO2� sublattice as that inCa3Co4O9+�,52,53 the different effects of Fe/Mn doping andCu doping on the transport behavior should be attributed tothe different substitution sites, because the alterations of thephysical properties in Fe or Mn doped Ca3Co4O9+� are simi-lar with those of Co-site doped NaxCoO2 but the alterationsof Cu doped Ca3Co4O9 are highly incompatible with thosereported in NaCo1−xCuxO2. That is to say, Fe/Mn replaces Coions in �CoO2� layers, whereas the substitution of Cu for Comainly occurs in �Ca2CoO3� layers. Because the carriertransport is mostly restricted in �CoO2� layers, the substitu-tion of Fe/Mn for Co in �CoO2� disturbs the conduction pathand these doped ions can act as scatter centers. Conse-quently, Ea increases obviously with doping. In contrast, Cudoping does not disturb conduction path and thus has lessinfluence on Ea of the system. A possible reason why Fe/Mnand Cu substitute for different Co ions may stem from theirdifferent ionic radii. Considering the usual spin states ofthese cations,19,44,45,54,55 the radii in six-coordination ofFe2+ /Fe3+ �0.61 Å/0.55 Å, both low-spin state�, andMn3+ /Mn4+ �0.645 Å/0.53 Å, both high-spin state� are veryclose to those of Co3+ �0.545 Å, low-spin state� and Co4+

�0.53 Å, high-spin state�, so Fe or Mn ions can be easilysubstituted for Co ions in �CoO2� layers. For Cu2+, its radius�0.73 Å� is much larger than the radii of Co3+ /Co4+ but close

FIG. 5. �Color online� Plot of ln�� /T� vs 1000 /T for all samples; the redlines denote the linear fitting.

FIG. 6. �a� Activation energy Ea obtained from the fitting of ln�� /T�-1000 /Tcurves. �b� Temperature dependence of specific heat C as C /T vs T2 with thefitting lines for Ca3Co4O9+� and Ca3Co3.9M0.1O9+� �M=Fe, Mn, and Cu�.

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to Co2+ �0.745 Å, high-spin state� in �Ca2CoO3� layers, so itseems that Cu ions are easier to replace Co ions in�Ca2CoO3� layers but hardly doped in �CoO2� layers. More-over, as mentioned above, most doped Mn ions are Mn4+

whose radius is nearly same as the average Co3+ /Co4+; incontrast, the average radius of Fe ions are less close to that ofCo ions in �CoO2� layers than Mn. Therefore, the substitu-tion of Fe for Co will cause larger structural distortions com-pared with Mn and hence have more notable effects on car-rier transport, so Ea in Fe doped series is larger than Mndoped series.

According to the band calculation and on-resonant spec-tra of x-ray photoemission spectroscopy for Ca3Co4O9+�,31,49

Fermi level of Ca3Co4O9+� lies in the crystal-field gap of thed states in �CoO2� layers, whereas Co 3d partial density ofstates in �Ca2CoO3� layers hardly reaches the Fermi level.Therefore, substitutions for Co in �CoO2� layers cannot onlyaffect transport behavior but also change electronic structureand then influence effective mass and electronic correlations.The low temperature specific heat measurements confirmedthe variations in electronic correlations by doping. ForCa3Co4O9+� system, the specific heat from the magnetic con-tribution is negligible,43 so the specific heat C in low tem-perature range only involve electronic specific heat and pho-non specific heat terms, and thus the temperature dependenceof C can be described as C /T��+�T2, where � is electronicspecific heat coefficient and � denotes phonon specific heatterm. One can see from Fig. 6�b� that Fe/Mn doping mark-edly increases �, indicating the enhancement in effectivemass m� and electronic correlations, while Cu doping onlygives rise to a slight augmentation of �. This result alsoconfirms the different doping sites of Fe/Mn ions and Cuions. Fe doping indeed increases electronic specific heat, andaccording to Eq. �2�, one can find that the effect of increasingce on S overcomes the effect of increasing n on S, so finallyS is essentially enhanced with Fe doping. Similarly, we maysuggest that the increase of S with Mn doping originatesfrom both the decrease of n and the enhancement of elec-tronic correlations, and the decrease of S with Cu dopingonly results from the increase of n. As mentioned already, Fedoping induces more lattice distortions than Mn, which maylead to a smaller Co–O–Co band angle and hopping integraland hereby a narrower bandwidth, so Fe doping causes stron-ger electronic correlations. Therefore, Fe doped series ex-hibit more pronounced increase of S. This result not only

indicates �CoO2� layers are more important for electroniccorrelations in this system but also unambiguous reveals thecrucial role of electronic correlation in determining S in suchelectronic strongly correlated systems, and may also eluci-date why Ca3Co4O9+� and NaxCoO2 with the same �CoO2�layers both exhibit large effective mass along with unusualhigh thermopower. To further validate this scenario, Boltz-mann transport model is used to calculate S according to theobtained effective mass m�. In this model,56 S can be ex-pressed by

S =kB

e� �r + 5/2�Fr+3/2����

�r + 3/2�Fr+1/2����− �� , �3�

where r is scattering parameter, �� is reduced chemical po-tential �=� /kBT�, and Fr���� is a Fermi integral written as

Fr���� = 0

� xr

1 + ex−��dx . �4�

Effective mass m� is expressed by

m� =h2

2kBT� n

4�F1/2����2/3

, �5�

where h is Planck constant. The scattering parameter r is 3/2in perovskite cobaltites, namely ionized impurityscattering.57 However, it is reported that r=2 is more suitablein titanates which have approximately m� withCa3Co4O9+�.15 In the present Ca3Co4−xMxO9+� system, whenthe magnitude of r between 3 /2�2 is adopted, the calcu-lated S values are very consistent with the measured values,which clearly indicates that the large m� �strongly electroniccorrelation� is the source of unusual large S in these high-carrier-concentration compounds, and the enhancement ofelectronic correlation plays a key role in the increase of S.Besides, we should emphasize that all the samples have large� values, implying that the strong electronic correlations al-ways exist in this system; doping influences the strongly cor-related characteristic, but does not change their strongly cor-related nature.

Combining S and �, TE power factor P �=S2 /�� for thesecold high-pressure fabricated specimens is calculated andshown in Fig. 7. As temperature rises, P increases gradually,due to decreasing � along with increasing S with T at el-evated temperatures. Although the substitutions for Co maycause the simultaneous increase or simultaneous decrease of

FIG. 7. �a� Temperature dependence of power factor P for Ca3Co4O9+� and Ca3Co3.9M0.1O9+� �M=Fe, Mn, and Cu�. �b� P at 1000 K as a function of x.

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� and S, the values of P in the three series are all enhancedwith doping. Especially for Fe doped samples, because of theincrease of S together with the decreases of �, P is muchhigher than all the others. The P value for Ca3Co3.9Fe0.1O9+�

reaches �700 �W m−1 K−2 at 1000 K, which is quite siz-able among oxide ceramics and nearly three times larger thanthat of the undoped sample.

Figure 8 presents the temperature dependence of thermalconductivity � of the samples. � monotonously decreaseswith temperature, typical thermal conduction behavior forcrystalline semiconductors. Doping at Co sites reduces � inthe whole measured temperature range. � in paramagneticstate �above approximately 19 K, Ca3Co4O9+� system isparamagnetic� can be expressed by the sum of phonon ther-mal conductivity term �ph and hole carrier thermal conduc-tivity term �hole, viz., �=�ph+�hole. �hole can be calculatedfrom Wiedemann–Franz’s law as �hole=L0T /�, where L0

=�2kB2 /3e2 is the Lorentz constant. As shown in Fig. 9�a�,

the contribution of �hole to total � is quite small, so �ph is thepredominant component in � of the system. Therefore, thereduction in � with doping mainly results from the change in�ph. Due to the structural distortions and size disorder in-duced by doping, which can shorten the average free path ofphonons, the suppression of � with doping can be antici-

pated. Cu doped series show a more pronounced suppressionof �, mainly because of the largest weight of Cu ions that canlead to strong lattice disharmony.

The ZT values of Ca3�Co,M�O9+� �M=Fe, Mn, and Cu�are shown in Fig. 10. It is clear that ZT is enhanced bydoping. Particularly for Fe doped series, the TE performanceis efficiently improved. It is noteworthy that the ZT values ofFe doped series increase dramatically with temperature ris-ing, so the improvement of ZT is more visible at high tem-peratures. For the Ca3Co3.9Fe0.1O9+� sample, the ZT is near0.4 at 1000 K, suggesting that it could be a good candidate asp-type materials for high temperature TE applications. Addi-tionally, the obtained ZT values in the cold high-pressureprepared samples are much higher than those synthesized bythe conventional method, which makes clear that the coldhigh-pressure compacting is helpful for the improvement ofTE performance.

IV. CONCLUSIONS

In summary, the substitution of Fe, Mn, or Cu for Co hasa noticeable influence on the high temperature transport andTE characteristics in Ca3Co4O9+�. The strongly electroniccorrelation plays a crucial role in the unusual large S of thissystem. Fe/Mn replaces the Co ions in �CoO2� layers, whichaffects the electronic structure and enhances the electroniccorrelations, so S increases observably in Fe and Mn dopedseries. Cu ions mainly occupy the Co sites in �Ca2CoO3�layers, so S “normally” varies with carrier concentration. Onaccount of the simultaneous increase in thermopower andelectrical conductivity, TE performance of Ca3Co4O9+� is re-markably improved with Fe doping. The ZT value of coldhigh-pressure fabricated Ca3Co3.9Fe0.1O9+� is close to 0.4 at1000 K, suggesting that Fe doped Ca3Co4O9+� is promisingin high temperature TE applications. This research points outthat TE characteristics of Ca3Co4O9+� can be pronouncedlyimproved by choosing suitable ions as dopant. To explore theeffects of other elements doping on electronic correlationsand TE response is highly desired, and it is expected that TE

FIG. 8. Temperature dependence of thermal conductivity � for Ca3Co4O9+�

and Ca3Co3.9M0.1O9+� �M=Fe, Mn, and Cu�.

FIG. 9. �a� Temperature dependence of total �, phonon thermal conductivity �ph, and hole carrier thermal conductivity �hole for Ca3Co4O9+� �solid symbols�and Ca3Co3.9Fe0.1O9+� �open symbols�, respectively. �b� � at 300 K �solid symbols� and 1000 K �open symbols� as a function of x.

033708-7 Wang et al. J. Appl. Phys. 107, 033708 �2010�

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performance can be further improved by co-doping or suit-able element adding in this family. The cold high-pressurecompacting method adopted in this study efficiently in-creases the density and texture, and is very advantageous tothe improvement of TE performance. Therefore, such simpletechnique is helpful for fabrications of TE materials and canbe applied to more systems.

ACKNOWLEDGMENTS

This work is supported by the National Natural ScienceFoundation of China �Grant Nos. 50672019 and 10804024�and the Scientific Research Foundation for the ReturnedOverseas Chinese Scholars, State Education Ministry.

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FIG. 10. �a� Temperature dependence of ZT for Ca3Co4O9+� and Ca3Co3.9M0.1O9+� �M=Fe, Mn, and Cu�; �b� ZT at 1000 K as a function of x. For comparison,ZT at 1000 K for the conventional fabricated Ca3Co4−xFexO9+� specimens is also shown in �b�.

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