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Available online at www.sciencedirect.com

ScienceDirect

Journal of the European Ceramic Society 34 (2014) 925–931

High temperature thermoelectric properties of Ca3Co4O9+δ byauto-combustion synthesis and spark plasma sintering

NingYu Wu ∗, Tim C. Holgate, Ngo Van Nong, Nini Pryds, Søren LinderothDepartment of Energy Conversion and Storage, Technical University of Denmark, Frederiksborgvej 399, 4000 Roskilde, Denmark

Received 12 August 2013; received in revised form 10 October 2013; accepted 15 October 2013Available online 9 November 2013

bstract

rapid method for the synthesis of Ca3Co4O9+δ powder is introduced. The procedure is a modification of the conventional citric-nitrate sol–gelethod where an auto-combustion process is initiated by a controlled thermal oxidation–reduction reaction. The resulting powders inherit the

dvantages of a wet chemical synthesis, such as morphological and compositional homogeneity, and fine, well-defined particle sizes comingrom the controlled nature of the auto-combustion. Optimized spark plasma sintering (SPS) processing conditions were determined and used toabricate dense and highly c-axis oriented samples. The microstructure and thermoelectric transport properties were determined both parallel (||)

◦

nd perpendicular (⊥) to the SPS pressure axis in order to investigate any possible anisotropy variations in the transport properties. At 800 C,ower factors of 506 �W/m K2 (⊥) and 147 �W/m K2 (||), thermal conductivities values of 2.53 W/m K (⊥) and 1.25 W/m K (||), and resultinggures-of-merit, ZT, of 0.21 (⊥) and 0.13 (||) were observed.

2013 Elsevier Ltd. All rights reserved.

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eywords: Thermoelectric; Ca3Co4O9; Auto-combustion; Sol–gel; Citrate–nit

. Introduction

Thermoelectric materials provide a mean to convert ther-al energy into electrical energy via the Seebeck effect. The

erformance of a thermoelectric material can be evaluated byhe dimensionless figure-of-merit ZT = S2T/ρκ, which consistsf the Seebeck coefficient (S), electrical resistivity (ρ), ther-al conductivity (κ) and absolute temperature (T). Oxides are

ne of candidates of high interest for thermoelectric applica-ions due to their low cost, high-temperature stability in airnd chemical inertness. Shikano et al. first brought attentiono Ca3Co4O9+δ as a high temperature thermoelectric p-typexide candidate by reporting a ZT of 0.83 (at 800 ◦C) for a sin-le crystalline sample.1 Ca3Co4O9+δ possesses a misfit-layeredtructure with a CdI2-type hexagonal CoO2 subsystem and aock salt-type Ca2CoO3 subsystem that are alternately stacked

long the c-axis with identical a, c and β parameters but differentnd incommensurate b1 and b2. Therefore, this misfit-layeredxide can be described with the formula [Ca2CoO3]b1/b2[CoO2]

∗ Corresponding author. Tel.: +45 46774942.E-mail address: niwu@dtu.dk (N. Wu).

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955-2219/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.jeurceramsoc.2013.10.022

park plasma sintering

ith a b1 to b2 ratio of approximately 1.62.2,3 The electroniconduction takes place mainly within the CoO2 layer with thea2CoO3 layer serving as a charge reservoir, and while it isifficult to determine in which layer the lattice thermal conduc-ivity is higher, the misfit between layers is expected to hinderhe cross-plane phonon transport.4 Therefore, anisotropy in thehermal and electrical transport properties is expected due to thewo-dimensional character of this layered structure.2,5–7

At present, the fabrication of pure or doped Ca3Co4O9+δ pow-ers has mainly followed one of two routes: solid-state reactionr wet-chemical synthesis.8–17 The advantages of the solid-stateeaction are simplicity and high yield, but the purity and theomogeneity are two major issues of concern. By contrast, wet-hemical synthesis eliminates the drawbacks of the solid-stateeaction by atomic scale mixing, but the low yield associatedith this process limits the scalability. To retain the advantagesf the wet-chemical synthesis, Song et al. reported the use of

polyacrylamide gel synthesis involving fast polymerizationn calcium and cobalt nitrate aqueous solution, increasing the

8

ield. Zhang et al. also introduced a co-precipitation methodo ensure the homogeneity of the powder.9 Recently Wangt al. exhibited that via hydrothermal treatment the Ca3Co4O9+δ

article size can be reduced further for better thermoelectric

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26 N. Wu et al. / Journal of the Europe

roperties.16 Except for these unique methods, the Pechiniethod is still widely employed for Ca3Co4O9+δ synthesis,hich involves the chelation between citrate and the metal

ons originating from dissolved metal nitrates.18 Following theecomposition of the vaporization of the organic constituentsnd citrate complexes at elevated temperatures (400–500 ◦C),ighly homogenous and sub-micron scale oxide powders can bebtained.10 By this method, not only the atomic scale mixingor chemical homogeneity can be achieved, but also the feasi-ility of doping is provided by the introduction of dopant cationitrates.14 Despite the fact that this method is adaptable to manyxide systems, it still suffers from some practical difficulties, fornstance, the resulting gel after drying is very adhesive and diffi-ult to handle. Additionally, the gel swells considerably duringhe pyrolysis resulting in a drawn out and uneven reaction ratherhan a favoured rapid single step reaction. The present workought to overcome these problems by using calcium and cobaltitrates as the source of the metal ions, which were dissolved in

citric acid aqueous solution according to the Pechini method,ut with a modified citrate-nitrate ratio that resulted in a rapiduto-combustion process induced by an exothermal oxidation-eduction reaction. The introduction of the auto-combustion stepot only promotes the complete decomposition of the citrateomplexes but also suppresses the grain growth by shorteninghe total reaction time at high temperature. The auto-combustioneaction was followed by the consolidation of the powders usingpark plasma sintering (SPS) under different processing condi-ions. Due to the nature of the processing and the layered crystaltructure of the material, the properties along both the parallel||) and perpendicular (⊥) directions relative to the SPS pressurexis were studied and are discussed in this paper.

. Experimental procedure

A stoichiometric ratio of analytical regent gradea(NO3)2·4H2O (99+ %) and Co(NO3)2·6H2O (99+ %)as dissolved in an appropriate amount of distilled water,

nd a specific amount of citric acid (99+ %) was added toeep the citrate–metal cation molar ratio at 1:1. Additionally,he citrate-to-nitrate molar ratio was tuned by introducingifferent relative amounts of NH4NO3 (98+ %) ranging from.28 to 0.50. After initially drying the gel while stirringontinuously at 75 ◦C until a uniform viscous gel was obtained,he hot plate temperature was raised to about 250 ◦C until theuto-combustion process was initiated, which was completedithin a minute. The resulting powders were calcined at 750 ◦C

or 2 h to obtain single phase Ca3Co4O9+δ.Subsequently, the powders were consolidated by spark

lasma sintering (SPS, Dr. Sinter SPS-515S, Fuji Electronicndustrial Co., Ltd.). Powders were poured into a graphite die toabricate 10 mm thick pellets with a diameter of 18 mm. A pulsed

lectric current was then passed through the powders while underacuum (10−3 bar) to consolidate the material. Based on our pre-ious results, a range of the sintering temperatures were chosenrom 700 to 850 ◦C, with a fixed uniaxial pressure of 50 MPa

caas

eramic Society 34 (2014) 925–931

nd a ramping rate of 100 ◦C/min. The sintering time was keptonstant at 5 min.16

The SPS samples were cut in different directions as shown inig. 1 depending on the type of characterizations. The symbols||” and “⊥” are used to denote the properties measured along theirections parallel and perpendicular to the SPS pressure axis,espectively.

Simultaneous measurements of thermogravimetry (TG) andifferential thermal analysis (DTA) were performed in oxy-en with a heating rate of 10 ◦C/min on a Netzsch STA-449C.dditionally, the formation of the Ca3Co4O9+δ phase from

ol–gel produced precursors was analyzed by differential scan-ing calorimetry (DSC) using a Netzsch DSC-404C. Afteruto-igniting the sol–gel, the resulting powders were heated to25 ◦C to remove any residual organics. A few milligrams ofhe product were lightly packed into an alumina-lined platinumrucible and loaded into the DSC. The measurement was per-ormed under flowing air with a heating rate of 10 ◦C/min up to25 ◦C.

X-ray diffraction (XRD) patterns were obtained using aruker D8 diffractometer with Cu K� radiation. Microstructuralnalysis of the synthesized powders and the fracture surfaces ofPS samples was conducted using a ZEISS Supra 35 scanninglectron microscope (SEM). The Seebeck coefficient (S) andlectrical resistivity (ρ) measurements were carried out simulta-eously with an ULVAC-RIKO ZEM3 from room temperaturep to 800 ◦C under a low-pressure helium atmosphere.

The thermal conductivity was calculated using the equation = ρ·D·cv (where ρ, D and cv are the density, thermal diffusivitynd specific heat capacity, respectively). The thermal diffusiv-ty was obtained under vacuum in a NETZSCH LFA-457 laserash system. The cv in this work was taken to be the temperature

ndependent Dulong–Petit value of 0.798 J/g K. The bulk densi-ies of the samples were measured by the Archimedes’ methodnd compared with the theoretical density of Ca3Co4O9, whichs 4.68 g/cm3 according to Masset et al.2

. Result and discussion

According to Pederson’ reaction model, the thermalxidation-reduction reaction in a citrate-nitrate system can beepresented by the following19:

M(NO3)x + (5/2)xC6H8O7 → 15xCO2 + (9/2)xN2

+ 10xH2O + 9MOx/2

here M = Ca2+ or Co2+, x = 2 for Ca3Co4O9+δ, and NO3−

cts as the oxidant and citric acid as the fuel. The idealizedombustion reaction occurs with a citrate-to-nitrate molar ratiof about 0.28. Through tuning this ratio, the auto-combustioname propagation and the heat evolution can be controlled. In

his work, a series of citrate-to-nitrate molar ratios were chosenrom 0.50 to the ideal value 0.28. With a ratio of 0.50, no auto-

ombustion occurred. In contrast, the flame propagated rapidlynd vigorously at the ideal ratio of 0.28. These observationsre consistent with the conclusion reported by Roy et al. whotudied the YBa2Cu3O7−x system20; the excessive amounts of

N. Wu et al. / Journal of the European Ceramic Society 34 (2014) 925–931 927

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ig. 1. Schematics of samples cut parallel (||) and perpendicular (⊥) to the Sroperties S, ρ and κ with subscript || and ⊥ denote the properties measured alo

itric acid cannot induce the auto-combustion and the maximumeat evolution happens when the citric acid amount is slightlyigher than the stoichiometric nitrate requirement. To promoteomplete chelation, the Ca3Co4O9+δ powders were synthesizedrom gels with an excess amount of citric acid so that the citrate-o-metal cation molar ratio was 1, and the necessary amount ofitrate required to activate the auto-combustion was provided byH4NO3 to give a citrate-to-nitrate molar ratio of 0.40.Fig. 2(a) shows the TG/DTA curve of a dry Ca3Co4O9+δ

el with a citrate-to-nitrate molar ratio of 0.40. The DTA curvean be divided into three main regions. Below 200 ◦C therere three peaks, one endothermic at 100 ◦C and two exother-ic, at 150 and 190 ◦C, accompanied by about 75% weight

oss. These peaks can be ascribed to the evaporation of resid-al water, the partial decomposition of nitrate and the initiationf the auto-combustion reaction, respectively.20 In the second

p

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ig. 2. (a) TG/DTA curves of the dry Ca3Co4O9+� citric-nitrate gel, and (b) the enlaeasurement exhibiting a complex peak originating from the thermal decomposition

ressure axis for the anisotropic thermoelectric properties measurements. Theections parallel and perpendicular to the SPS pressure axis.

egion from 250 to 350 ◦C one broad exothermic peak can bebserved, which may be attributed to the decomposition of resid-al citric acid and results in an additional ∼2% weight loss. Noxothermic peaks were observed between 400 and 500 ◦C indi-ating no residual citrate complexes. With the incorporation ofhe auto-combustion reaction, the starting precursors – CaCO3nd Co3O4 – can be obtained at relatively low temperatures.he endothermic peak in the DSC curve displayed in Fig. 2(b)

s a composite peak originating from the breakdown of CaCO3nto CaO followed by the formation of the Ca3Co4O9+δ phase.lthough not presented here, comparison of the XRD patterns of

he precursors after the initial furnace treatment at 425 ◦C withhose of the sample reacted in the DSC confirm the origin of the

eak to be a result of the formation of Ca3Co4O9+δ.

XRD patterns of the Ca3Co4O9+δ powders as-synthesized andfter SPS consolidation at 850 ◦C are shown in Fig. 3. All the

rged region of the TGA curve from (a) overlaid with the independently DSC of CaCO3 to CaO and the formation of the Ca3Co4O9+� phase.

928 N. Wu et al. / Journal of the European Ceramic Society 34 (2014) 925–931

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ig. 3. XRD patterns of the as-synthesized Ca3Co4O9+� powders and the SPa3Co4O9+� phase identification. The inset is the SEM micrograph of the as-syn

he reader is referred to the web version of this article.)

eaks could be indexed as Ca3Co4O9 according to the JCPDSDF # 21–0139 of Ca9Co12O28 and the XRD patterns reported

n the literature, indicating the purity and obvious texturing ofhese samples.2 The inset in Fig. 3 displays the SEM micro-raphs of as-synthesized Ca3Co4O9+δ particles with the averagearticle size of ∼300 to 400 nm and a plate-like morphology.ig. 4 shows a set of SEM micrographs of the sintered frac-

ure surfaces after SPS processing at 850 ◦C for 5 min, whichere observed along directions parallel and perpendicular to

he SPS pressure axis. The platelet morphology and texturedicrostructure is observable in these two micrographs, which

s also consistent with the anisotropic XRD results. All samplesxhibited densities more than 99% of the theoretical density, andhese values agree with Liu et al., Kenfaui et al. and our previousorks.16,21,22

Due to the layered structure of Ca3Co4O9+δ and the mor-hological anisotropy observed in this study, the thermoelectricroperties were measured along both ⊥ and || directions. Amongamples which were densified by SPS at 700, 750, 800 and50 ◦C, the maximum deviation from the averages in the ⊥ direc-ion electrical resistivity, the Seebeck coefficient and the thermal

onductivity are 2.5%, 1.1% and 2.2%, respectively, as measuredt 700 ◦C (detailed data can be found in the appendix). These arell within the error of the measurements, and so to be consistentith our previous work on optimizing the SPS processing

α

Fig. 4. SPS sample microstructure micrographs observed along direction

solidated samples. The red line pattern is from JCPDS PDF # 21-0139 fored powders. (For interpretation of the references to colour in this figure legend,

onditions for solid-state prepared Ca3Co4O9+δ. For simplicity,ig. 5 shows only the results of the measurements performedn the sample processed by SPS at 850 ◦C for 5 minutesnder 50 MPa of pressure.16 The electrical resistivity in thelane perpendicular to the SPS pressure axis (ρ⊥) variedetween 5.24 m� cm and 5.72 m� cm throughout the measuredemperature regime with a minimum value of 5.24 m� cm at50 ◦C. These results are lower than those observed by Zhangt al. on a sample synthesized by the conventional citrate-nitrateethod – 6.13 m� cm at 700 ◦C.6 To our knowledge, so far this

s the lowest electrical resistivity value for pure polycrystallinea3Co4O9+δ with the normal SPS processing. The electrical

esistivity measured along the || direction (ρ||) was foundo be 15.32 m� cm at 700 ◦C, which is below the previouseported value of about 18 m� cm at that temperature.6 Theow magnitude and high anisotropy of the electrical resistivityan be explained as a result of the enhanced morphologicalexturing of the sample. A quantitative analysis of the texturingan be obtained by assessing the volume fraction, α(0 0 l), of-axis oriented grains in the samples after SPS processing usinghe following equation23:

(0 0 l) =∑

(I0 0 l/I∗0 0 l)∑

(Ihkl/I∗hkl)

s parallel (left) and perpendicular (right) to the SPS pressure axis.

N. Wu et al. / Journal of the European Ceramic Society 34 (2014) 925–931 929

lectric

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where Ihkl stands for the measured intensity of the (hkl)eaks from the XRD spectrum of the surface perpendicularo the SPS pressure axis of the consolidated samples and I∗

hkl

or those of the as-synthesized Ca3Co4O9+δ powders. The auto-ombustion powders exhibit a significant �(0 0 l) value of up to1%, which is consistent with high degree of texturing indicatedy the SEM observations. Due to the plate-like morphology ofhe as-synthesized Ca3Co4O9+δ powders shown in the inset ofig. 3, asymmetric necking during sintering occurs resulting innisotropic surface energies that in turn produce torques to drivehe grain rotation.24 Therefore, sintering with small plate-likea3Co4O9+δ particles may enhance microstructural anisotropys the higher density of grain boundaries results in a higher con-entration of grain rotation driving forces and therefore a higheregree of grain alignment. A benefit of the auto-combustioneaction is the rapid formation of the starting precursors atelatively low temperatures. This allows for the subsequentalcination period needed to achieve the Ca3Co4O9+δ phase for-ation to be kept relatively short (2 h in this work). This limits

he high temperature (above 700 ◦C) particle size growth andherefore results in the enhancement of the anisotropy of theicrostructure and the thermoelectric properties of the consol-

dated bulk samples. This conclusion corresponds well to Chent al. work on the relationship between varied Ca3Co4O9+δ cal-ination temperatures and particle sizes, which points out thathe smallest Ca3Co4O9+δ particle size calcinated at lower tem-

erature after sintering exhibited the better grain alignment andigher thermoelectric performance.25 The similar electrical con-uctivity improvement resulted from enhancing texturing also

Swu

properties measured along the ⊥ and || directions.

an be observed from other works. Both of Liu et al. and Kenfauit al. works agreed that the SPS densification could improve thehermoelectric properties with grain orientation and higher bulkensity.21,26 With the assistance of the magnetic alignment tech-ique and SPS processing, Zhou et al. reported a �⊥ of about

m� cm at 700 ◦C – exhibiting an improvement of about 30%ompared with the less textured sample.27 Lin et al. later pro-osed that fabricating Ca3Co4O9+δ by the SPS and a dynamicorging process can increase grain orientation, also increasinghe electrical conductivity by about 30%.28 Recently, Noudemt al. enhanced the texturing by a modified SPS processing –o-called “edge free” spark plasma sintering, and at 550 ◦C a0% improvement in the electrical resistivity (7 m� cm) and aower factor of 400 �W/m K2 was be obtained.29 Except SPSrocessing, the reactive-templated grain growth (RTGG) methods a means to achieve highly texturing Ca3Co4O9+δ and it alsoemonstrated that the lowest electrical resistivity was from theost textures sample, yielding a resistance of 3.8 m� cm at

bout 800 ◦C.30–33

The temperature dependence of the Seebeck coefficient andower factor (PF = S2/ρ) along both directions can also be seen inig. 5. The Seebeck coefficient increases with temperature, and

he anisotropy between ⊥ and || directions is not as pronounceds much as the electrical resistivity, which is consistent with theesults of Tani et al. and Zhang et al. reporting the Seebeck coef-cient to be insensitive to the grain orientation.6,30 The highest

eebeck coefficient value of 165 �V/K was measured at 800 ◦Cithin the ⊥ direction, which is comparable with reported val-es of samples synthesized by wet-chemical methods.6,8,9 Due

930 N. Wu et al. / Journal of the European C

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ig. 6. The temperature dependence of the ZT measured along the ⊥ and ||irections.

o the lower ρ and higher S within the ⊥ plane, PF⊥ remainsigher than PF|| throughout the whole temperature range with aaximum value of PF⊥ ≈ 500 �W/m K2 at 800 ◦C. At 700 ◦C

he value is ∼490 �W/m K2 – about 10% higher than reportedor samples produced by the conventional citrate-nitrate reac-ion (about 445 �W/m K2) with similar SPS condition.6 Thisnhanced PF⊥, which was obtained by the auto-combustionynthesis and normal SPS processing, approaches that of sam-les prepared by more time costly processes such as the RTGGrocessing and repeated hot-pressing.7,30,31,33

The thermal conductivities (κ) measured along ⊥ and || direc-ions shown also in Fig. 5 decrease with the temperature, whichs comparable with the reported results from Ca3Co4O9+δ singlerystal and polycrystalline samples.1,7 κ contains both the elec-ronic (κe) and the lattice (κL) contributions (i.e., κ = κe + κL),ut by applying the Wiedemann–Franz relationship, κe = L0 T/ρ,here the Lorenz factor is taken as L0 = 2.45 × 10−8 V2/K2,

he lattice contribution was calculated and accounts for morehan 80% of the total in both directions.1,4,34 Since Ca3Co4O9+δ

onsists of alternating misfit-layers of CoO2 and Ca2CoO3,nisotropy in the thermal and electrical transport properties isxpected. Although achieving lower ρ⊥ by enhancing grainlignment is a promising and effective route to obtaining higherF⊥, it will lead to the increase of the κe contribution in total κ

nd compromise the thermoelectric performance in ZT⊥, whichay be observed from works with the RTGG processing and

epeated hot-pressing.7,31 The relationship of the degree of tex-uring and the anisotropy in the transport properties will be theocus of future studies.

Fig. 6 displays the temperature dependence of ZT calculatedor both directions. At 800 ◦C, ZT⊥ and ZT|| were found to be.21 and 0.13, respectively, and these values are among the high-st ZT values reported for pure bulk polycrystalline Ca3Co4O9+δ

comparable to those synthesized via the hydrothermal treat-ent (ZT⊥ ≈ 0.27 at 750 ◦C,Ref. 16), or processed with the

TGG (ZT⊥ ≈ 0.26 at 800 ◦C,Refs. 30,33) and repeated hot-ressing (ZT⊥ ≈ 0.24 at 800 ◦C,Ref. 7). However, it shoulde noted that the in-plane thermal conductivity used for the

eramic Society 34 (2014) 925–931

alculation of ZT⊥ of the sample produced by RTGG was mea-ured on a sample exhibiting a lower electrical conductivityand therefore a lower electronic contribution to the thermalonductivity) than the sample from which the power factor wasaken.31 Also, the value of ZT⊥ at 800 ◦C of the sample preparedy repeated hot-pressing was extrapolated from a fit equationrovided in the reference – the measured data only extendso 650 ◦C.7 When considering that the ratio of ZT⊥/ZT|| andF⊥/PF|| are 1.62 and 3.44, the ⊥ plane can be expected to be

preferred direction for preparing thermoelectric legs.

. Conclusion

In this work, a new rapid synthesis method of producinga3Co4O9+δ fine powders using an auto-combustion synthesisethod is demonstrated. The auto-combustion is induced by

arefully tuning the ratio of citrate to nitrate based on the conven-ional sol–gel process. The synthesized Ca3Co4O9+δ powdersere consolidated by SPS and the anisotropic thermoelectricroperties were measured parallel and perpendicular to thePS pressure axis. The auto-combustion synthesis provides anfficient and rapid method for producing pure polycrystallinea3Co4O9+δ with a high power factor and ZT in the plane perpen-icular to the SPS pressure axis of about 500 �W/m K2 and 0.21t 800 ◦C, respectively. Moreover, the auto-combustion synthe-is can be applied not only to pure but also doped Ca3Co4O9+δ

or further thermoelectric performance improvement.

cknowledgement

The authors would like to thank Xiao Tang for the TG/DTAeasurements and the Programme Commission on Energy andnvironment (EnMi), which is part of the Danish Council fortrategic Research (Contract No. 10-093971) for sponsoring theTE-POWER research work.

ppendix A. Supplementary data

Supplementary material related to this article can be found, inhe online version, at http://dx.doi.org/10.1016/j.jeurceramsoc.013.10.022.

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