Physica B 406 (2011) 2919–2923

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Physica B

0921-45

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/physb

High temperature transport properties of Ag-added(Ca0.975La0.025)3Co4O9 ceramics

Ying Song a,b, Ce-Wen Nan a,n

a State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR Chinab School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, PR China

a r t i c l e i n f o

Article history:

Received 29 September 2009

Received in revised form

15 January 2011

Accepted 3 May 2011Available online 7 May 2011

Keywords:

Cobalt oxides

Polyacrylamide gel method

Spark plasma sintering

Transport properties

26/$ - see front matter & 2011 Elsevier B.V. A

016/j.physb.2011.05.006

esponding author.

ail address: cwnan@tsinghua.edu.cn (C.-W. N

a b s t r a c t

Ag-added (Ca0.975La0.025)3Co4O9 ceramics were fabricated using spark plasma sintering from the

precursor powder synthesized by a polyacrylamide gel method. The results indicated that Ag

precipitated as a second phase in Ca3Co4O9 matrix. The addition of Ag was effective in enhancing the

electrical conductivity and had a slight effect on Seebeck coefficient. In addition, the temperature

dependence of electrical conductivity showed that the hole hopping conduction mechanism was

dominant for the Ag-added (Ca0.975La0.025)3Co4O9 ceramics. The activation energy remained unchanged

with the increasing Ag content. The thermoelectric power factor of Ag-added (Ca0.975La0.025)3Co4O9

ceramics reached about 5�10�4 Wm�1 K�2 at 700 1C, suggesting a promising thermoelectric oxide

candidate at high temperatures.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Layered cobalt oxides have attracted broad interests in recentyears due to their notably potential applications in environmen-tally friendly thermoelectric (TE) power generation, which candirectly convert heat into electrical energy and vise versa withoutproducing any detrimental emissions [1]. For practical TE materi-als, a large TE figure of merit Z (Z¼sS2/k) is necessary, whichrequires large Seebeck coefficient S and electrical conductivity s,and a small thermal conductivity k.

Ca3Co4O9 (CCO), which exhibits a good chemical stability andcan be used at high temperature, is regarded as one of the mostpromising TE materials among the layered cobalt oxides. Manyefforts have been made to improve the properties of CCO and oneeffective approach is partial substitution of the Ca site by rare-earth elements [2,3]. For example, the substitution of Ca with Lamay increase S by changing the Co valence and simultaneouslyincrease s due to the increase of carrier mobility m [4]. Moreover,it was reported that the TE performance of CCO could beoptimized with the substitution of Ag for Ca or adding Ag toCCO polycrystalline matrix [5–7]. In this paper, we study theTE transport properties of (Ca0.975La0.025)3Co4O9, by furtherintroducing Ag into the CCO ceramics.

Powders of Ag-added (Ca0.975La0.025)3Co4O9 were synthesizedby a polyacrylamide gel (PG) method, which is a fast and cheap

ll rights reserved.

an).

wet-chemical route to synthesize the powder of layered cobaltoxides [8]. It is a free radical polymerization reaction, which usesthe acrylamide gelification to form an organic three-dimensionaltangled network soaking the respective cations. The artificial gelprepared by the PG method could form in aqueous solutions atlow temperature within a few minutes, which is rapid in compar-ison with the progressive transformation from viscous to resin inthe citric acid complex method and the polymerized complexprocess [9,10]. Then the dense ceramic samples were prepared viaspark plasma sintering (SPS), a fast densification technique,from the PG-derived powders. Electrical transport properties ofthe Ag-added (Ca0.975La0.025)3Co4O9 ceramics were measured toinvestigate the effect of Ag on TE properties of the ceramics.

2. Experimental

The polycrystalline samples with the nominal composition of(Ca0.975La0.025)3Co4O9–xAg ( x¼0.0, 0.01, 0.025, and 0.05 ) weresynthesized from the precursor powder prepared by the PGmethod. Reagent grade Ca(CH3COO)2 �H2O, Co(CH3COO)2 �4H2O,La(NO3)2 �6H2O, and AgNO3 were used as the starting materials.Citric acid was used as the chelating agent and it was firstlydissolved in distilled water. Subsequently, stoichiometric mix-tures of Ca(CH3COO)2 �H2O, Co(CH3COO)2 �4H2O, La(NO3)2 �6H2O,and AgNO3 were dissolved to this citric acid solution withcontinuous stirring. Then acrylamide monomers (99%), thecross-linker N, N0-methylenebisacrylamide (99%) and a thermo-chemical initiator a, a0-azoisobutyronitrile (AIBN, 98%) were

Fig. 1. XRD patterns of the pressed surfaces of the SPS sintered (Ca0.975�xLa0.025)3

Co4O9– xAg ceramics.

Table 1Lattice parameters and activation energy of (Ca0.975-xLa0.025)3Co4O9–xAg ceramics.

b1 is the b-axis length of the rock salt-type Ca2CoO3 subsystem.

Samples a (nm) b1 (nm) c (nm) b (1) Eg (eV)

CCO 0.4836 0.4540 1.081 98.12 0.087

x¼0.0 0.4841 0.4555 1.084 98.08 0.094

x¼0.01 0.4840 0.4556 1.084 98.06 0.094

x¼0.025 0.4839 0.4558 1.084 98.03 0.095

x¼0.05 0.4838 0.4559 1.084 98.02 0.096

Y. Song, C.-W. Nan / Physica B 406 (2011) 2919–29232920

added under heating and vigorous stirring. The polymerizationthrough the AIBN took place within only a few seconds when thesolution was heated up to 80 1C, resulting in a homogenous gel.The hydrous gel was dried in a domestic microwave oven for afew minutes to form a dry sponge. The loose solid precursor waspulverized and then calcined at 800 1C for 4 h in air. For compar-ison, CCO powder was synthesized by the same PG method, whichwas reported in detail in Ref. [8].

The obtained powder was placed into a graphite die with adiameter of 20 mm, and a pulsed electric current of 400–1000 Awas passed through it under an uniaxial pressure of 50 MPa.During this procedure, temperature was raised to 750 1C at a rateof 100 1C/min, and the dense ceramics were obtained after beingheld at 750 1C for 5 min.

The XRD analysis was performed on the pressed plane of thesintered samples using a Rigaku diffractometer with Cu Karadiation. Microstructure of the sample was examined by scan-ning electron microscopy (SEM, Cam Scan 2600 FE). The chemicalstate of Co on the surface of the samples was investigated by theX-ray photoelectron spectroscopy (XPS), which was carried out ina PHI Quantera, equipped with a small spot (9–1500 mm2) Al–Kasource (hn¼1486.6 eV). The XPS overall energy resolution was0.5 eV. The binding energy (B.E.) reference was taken at 284.8 eVfor the C 1s level of the spurious carbon contaminating thesamples.

For electrical measurement, the ceramic samples werepolished, and then cut into rectangular bars of 15�3�3 mm3.S and s were measured along the pressed plane of the SPS-sintered sample from room temperature to about 700 1C in aflowing Ar gas atmosphere using a computer-controlled equip-ment. First, s was measured by the standard four-probe directcurrent method. Subsequently, a small (o5 1C) and steady statetemperature gradient was applied by a heater located near oneend of the sample, and then S was obtained from the slope of thethermoelectromotive force as a function of the temperaturedifference. The hole carrier concentration n, and Hall mobility mof the samples were measured at room temperature using aphysical property measurement system (PPMS-9 T, QuantumDesign Inc., San Diego, CA). The applied magnetic field was variedfrom �2 to 2 T.

Fig. 2. SEM micrograph with back-scattered electron mode of the fractured

surface of the sample with x¼0.05. White spots correspond to Ag particles.

3. Results and discussion

The XRD patterns for the pressed surfaces of the SPS sinteredsamples are shown in Fig. 1. It can be seen that the(Ca0.975La0.025)3Co4O9 sample contains single CCO phase and nodetectable impurity phase appears, indicating that La is dopedinto the lattice of CCO without obvious change in the crystalstructure. In addition, the Ag-added samples are composed of CCOand metallic Ag. The weak diffraction peaks of Ag become morevisible with increase in Ag content, indicating that Ag precipitatesas a second phase in the samples.

CCO is a misfit-layered oxide consisting of two monoclinicsubsystems with identical a, c, and b parameters, but differentb parameter [11]. The two subsystems, i.e., triple rocksalt typeCa2CoO3 layers and single CdI2-type CoO2 layers, are stackedalternately along the c-axis direction. The lattice parameters ofthe Ag-added samples are calculated in monoclinic symmetrybased on the XRD patterns by a standard least-square refinementmethod, as listed in Table 1, where b1 corresponds to the b-axislength of Ca2CoO3 layers. The estimated lattice parameters of CCOare consistent with those reported in Ref. [11]. The a, c, and b1

parameters of (Ca0.975La0.025)3Co4O9 increase compared withthose of the undoped CCO sample, indicating that the largerLa ions are doped into the lattice. However, the lattice parameters

for the Ag-added (Ca0.975La0.025)3Co4O9 ceramic samples almostdo not change with the Ag addition, suggesting that the Agprecipitates as the second phase in the samples rather than beingdoped into the crystal lattice of CCO.

Fig. 2 shows the SEM micrograph with back-scattered electronmode of the fractured cross-section perpendicular to the pressedplane of the sample with x¼0.05. It can be seen that Ag particles

Y. Song, C.-W. Nan / Physica B 406 (2011) 2919–2923 2921

(white spots phase) are around 0.1–0.5 mm in size, which is muchsmaller than that of CCO. In addition, the Ag particles dispersedevenly in the CCO matrix (dark gray phase).

The average valence of Co is an important factor for the TEproperties of CCO. The high-resolution XPS spectra of Co 2p are usedto study the valence of Co, as shown in Fig. 3. For the sample CCO,the binding energy of the Co 2p3/2 main peak is �780.0 eV and thatof Co 2p1/2 peak is �795.0 eV. There are charge transfer satellites forthe Co 2p3/2 peak and Co 2p3/2 peak at higher binding energy. The(Ca0.975-xLa0.025)3Co4O9�xAg samples with x¼0.0 and 0.025 presentsimilar XPS spectra to the sample CCO. To investigate Co chemicalvalence, Co 2p spectra were computer-fitted using a XPS Peak FittingProgram (Version 4.1), which was performed after Shirley back-ground subtraction by a non-linear least-square fitting methodusing a Gaussian–Lorentzian product function. From the curve fitof the peaks (not shown in this paper), it could been seen that Co 2pregion can be decomposed into two contributions corresponding toCo3þ and Co4þ , which indicates that the Co–O rock-salt layers in theCCO structure have the similar low spin Co3þ and Co4þ configura-tion as that of the Co–O triangular layers [12]. From the area undereach respective peak, the relative atomic concentrations for Co3þ

and Co4þ were determined. The peak position used to fit the Co 2ppeaks and the relative contribution of the two components to thespectra are listed in Table 2. The content of Co4þ decreases with theaddition of La because the substitution of trivalent La3þ for divalentCa2þ would decrease the hole concentrations. However, the content

Fig. 3. High-resolution XPS spectra of Co 2p in the (Ca0.975�xLa0.025)3Co4O9–xAg

samples.

Table 2Co 2P3/2 peak position and amount in the (Ca0.975�xLa0.025)3Co4O9–xAg ceramics

by XPS.

Samples Co 2P3/2 peak position(eV)

Co 2P1/2 peak position(eV)

Amount(%)

CCO

Co3þ 779.82 794.93 55.9

Co4þ 781.16 796.06 44.1

x¼0.0

Co3þ 779.66 794.76 65.3

Co4þ 781.12 796.02 34.7

x¼0.025

Co3þ 779.71 794.82 64.6

Co4þ 781.13 796.03 35.4

of Co4þ keeps almost unchanged after further Ag addition, alsosuggesting that Ag does not substitute for the Ca or Co sites in CCO.

The hole concentrations measured at room temperature for theCCO, La-doped CCO, and La-doped CCO with x¼0.025 are6.18�1020 cm�3, 4.49�1020 cm�3, and 4.17�1020 cm�3, respec-tively. It could be seen that the hole concentration decreasedremarkably from 6.18�1020 cm�3 for CCO to 4.49�1020 cm�3

for La-doped CCO due to the substitution of La3þ for Ca2þ , andthen to 4.17�1020 cm�3 for La-doped CCO samples with Agaddition because the electron carrier provided by Ag could neutra-lize a part of hole carrier. However, Hall mobility increased from0.97 cm2 V�1 s�1 for CCO to 1.28 cm2 V�1 s�1 for La-doped CCO,which indicated that the substitution of La3þ for Ca2þ can increasethe carrier mobility. Moreover, the hall mobility further increased to1.69 cm2 V�1 s�1 for La-doped CCO samples with Ag additionbecause Ag contributes a well connection between cobaltite grains,which makes the carrier scattering at the grain boundary reducedeffectively.

Fig. 4 shows the temperature dependence of the Seebeckcoefficient S of the samples. The positive S of all the samples alsoindicates hole conduction, and the S values increase with increasein temperature over the measured temperature range. The S valueof (Ca0.975La0.025)3Co4O9 is slightly larger than that of the sampleCCO because La substitution for Ca decreases the carrier concen-tration and results in an increase in the thermoelectric power [13].For the Ag-added samples, the S values are almost the same withthat of the (Ca0.975La0.025)3Co4O9 sample without Ag. The reasonmay be that the amount of Ag added in the samples is low, and thewell dispersed Ag particles are quite small compared to the CCOgrain. This kind of Ag particles could act as electrical connectionsbetween cobaltite grains, which has little influence on S [14]. Thesame phenomenon was observed in the case of the NaxCo2O4

ceramics with Ag addition [15]. Further work remains to be done toexplain it.

The electrical conductivity s as a function of temperature forthe samples is shown in Fig. 5. All the samples show a similartransport behavior, i.e., ds/dTo0 at low temperature andds/dT40 at high temperature. There exists a critical temperaturepoint T*. As the temperature is below T*, ds/dTo0, and it isreverse as T is above T*. T* can be regarded as a metallic-to-semiconductive transition temperature. T* is about 150 1C for theLa-substituted and Ag-added CCO based ceramics, which is about100 1C lower than that for the single phase CCO. It is noted that sof the (Ca0.975La0.025)3Co4O9 sample is larger than that of CCO

Fig. 4. Temperature dependence of Seebeck coefficient S of the (Ca0.975�xLa0.025)3

Co4O9– xAg ceramics.

Fig. 5. Temperature dependence of electrical conductivity s of the (Ca0.975�xLa0.025)3

Co4O9–xAg ceramics.

Fig. 6. ln(sT)–1/T plot for the (Ca0.975�xLa0.025)3Co4O9–xAg ceramics.

Fig. 7. Temperature dependence of power factor P of the (Ca0.975�xLa0.025)3Co4O9– xAg

ceramics.

Y. Song, C.-W. Nan / Physica B 406 (2011) 2919–29232922

sample above 200 1C. Normally, the substitution of La3þ for Ca2þ

will decrease the hole concentration, thus leading to a decreasein s. However, previous experiment results have indicated that sis related to the energy-correlated carrier concentration andcarrier mobility. In the La-doped CCO samples, the substitutioncan increase the carrier mobility, and compensates the effect ofcarrier concentration on the conductivity, and thus leads to anincrease in s [4,13]. For this reason, the s of the (Ca0.975La0.025)3-

Co4O9 sample is smaller than that of CCO sample below 200 1Cdue to the decreasing hole concentration, while it is larger above200 1C because of the increasing carrier mobility. In addition, s ofthe Ag-added samples significantly increases over the measuredtemperature range by addition of a good electrical conductor, Ag.The conductivity s of the sample with x¼0.05 reaches about146 S/cm at 700 1C, which is 16.8% larger than that of the samplewith x¼0.0 without Ag, and 28.1% larger than that of the undopedCCO sample.

Fig. 6 shows the linear relationship between ln(sT) and 1000/T,which suggests that the carriers obey the small polaron hoppingconduction mechanism above 300 1C [16]. The activation energyEg can be obtained from the plot of ln(sT) versus 1000/T [17], asshown in Table 1. For all the samples, Eg is in the range of0.087–0.096 eV from 300 to 700 1C and almost independent of the

La substitution and Ag addition, which indicates that the Lasubstitution and Ag addition do not change the activation energyof the hopping process and the transport mechanism.

As shown in Fig. 7, the power factor P(¼s S2) for all samplesincreases with increase in temperature, and Ag addition results inthe increase of P due to the obvious increase in s. The P value forthe sample with x¼0.05 reaches 5.03�10�4 W m�1 K�2 at700 1C, which is up to 19% larger than that of the (Ca0.975La0.025)3-

Co4O9 sample without Ag and 32% larger than that of the undopedCCO sample. It is noted that in CCO ceramics with Ag additionreported in Ref. [14], the P value increased nearly 25% at 700 1Cafter 28% atomic ratio of Ag was added. However, in this work, toimprove the P of CCO sample at the similar percentage, onlynearly one-sixth amount of Ag was needed. Thus, the La substitu-tion and Ag addition at the same time is effective on improvingthe electrical transport properties of layered cobalt oxides.

4. Conclusions

Ag-added (Ca0.975La0.025)3Co4O9 powder has been synthesizedby the polyacrylamide gel method, and their dense ceramicsamples have been prepared via SPS. The results show that Agprecipitates as the second phase in Ca3Co4O9 matrix. Introductionof small amount of Ag into La-substituted CCO ceramics results inan increase in the electrical conductivity and has a slight effect onSeebeck coefficient, thus leading to an enhanced thermoelectricpower factor, suggesting the La substitution and Ag addition atthe same time is a feasible route to optimize the thermoelectricperformance of Ca3Co4O9 based oxide ceramics.

Acknowledgments

This work was supported by the Ministry of Science and Techno-logy of China through a 973-Project (Grant no. 2007CB607504), theNational Nature Science Foundation of China (Grant no. 50772026),and Nature Science Key Fund of Hei-longjiang Province in China(Grant no. ZJG0605-01).

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