Effect of Synthesis Conditions on Formation, ElectricalProperties, and Seebeck Coefficient of p-Type Ca3Co4O9±d

Thermoelectric Ceramics

T. RADHIKA,1,4 N. RAGHU,1 N. POWRNAMI,1,2

R. JOTHI RAMALINGAM,3,5 and HAMAD A. AL-LOHEDAN3

1.—Centre for Materials for Electronics Technology [C-MET] (Scientific Society under M/o Com-munications and Information Technology, India), Athani P.O., Thrissur, Kerala 680581, India.2.—P.S.G.R. Krishnammal College for Women (Bharathiar University), Coimbatore, Tamilnadu,India. 3.—Surfactants Research Chair, Chemistry Department, College of Science, King SaudUniversity, Riyadh 11451, Kingdom of Saudi Arabia. 4.—e-mail: rads12@gmail.com. 5.—e-mail:rjothiram@gmail.com

Ca3Co4O9±d ceramic powders have been prepared by a solid-state method. Thecalcination and sintering temperatures and reaction conditions were varied toachieve highly dense materials for thermoelectric applications. The optimizedcalcination temperature and reaction conditions were derived. X-ray diffrac-tion patterns showed formation of secondary phases for longer calcinationduration. The density of the ceramics ranged from 3.2 g cm�3 to 3.4 g cm�3, notvarying greatly with the calcination/sintering conditions. The electrical prop-erties and Seebeck coefficient reveal that the density and nonstoichiometrygreatly influenced the achievement of good thermoelectric properties. Trans-mission electron microscopy (TEM) images showed fine particles with nanosize,strongly bound together to form metal-rich particle aggregates. Tubular mor-phology below 50 nm to 100 nm scale was observed in TEM images of as-pre-pared solid-state Ca3Co4O9±d. As-prepared samples showed improved electricalconductivity and Seebeck coefficient, suitable for thermoelectric applications.

Key words: Ca3Co4O9±d, thermoelectric, sintering, particle size, cobalt oxide

INTRODUCTION

The major energy sources worldwide are crude oil,wood, coal, etc., which face depletion in the future,leading to increased interest in and importance ofefforts to tap alternative energy sources. One of theways to achieve this is use of thermoelectric (TE)-based devices. High-efficiency TE materials areimportant for power generation devices due to theirability to generate an electrical potential directlyfrom a temperature difference through the Seebeckeffect.1,2 Such devices can also perform Peltiercooling by the reverse process, i.e., generate atemperature gradient when current is applied.3

Thermoelectric devices are used for microelectroniccooling, generation of electrical energy for vehiclesby collecting thermal energy from exhaust gases,and in gas sensor applications. To date, however,the results of materials and system technologieshave not reached a level that can support the TEpower generation market.4 Both p-type and n-typesemiconductor materials are required to fabricate aTE module.5

So far, tellurides such as Bi2Te3, PbTe, andrelated materials have been utilized to fabricateTE modules, especially for Peltier cooling.6 How-ever, these conventional TE materials as well asother classes of materials such as chalcogenides,skutterudites, half-Heuser alloys, and clathratescannot be employed for extensive applicationsbecause of their toxic nature and heavy constituent(Received November 16, 2016; accepted December 9, 2016;

published online January 5, 2017)

Journal of ELECTRONIC MATERIALS, Vol. 46, No. 3, 2017

DOI: 10.1007/s11664-016-5229-3� 2016 The Minerals, Metals & Materials Society

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elements, as well as oxidation problems at hightemperature in air.7 To overcome these problems,novel TE materials with high ZT values, composedof nontoxic, naturally abundant, lightweight, cheapelements are required. Mixed metal oxide thermo-electric ceramic materials have been predicted to begood candidates for thermoelectric energy conver-sion at high temperature as they are nontoxic,inexpensive, and stable at high temperature in air.Oxides have attracted much research attention asTE materials due to their high thermal stability andoxidation resistance at high temperature (700 K to1000 K).8 Inspired by the striking TE performanceof NaCo2O4 (ZT around 0.7 to 0.8 at 1000 K), mostcurrent studies focus on cobalt-based layered oxidessuch as Ca3Co4O9±d and Bi2Sr3Co2Oy, which crys-tallize in misfit (lattice-mismatched) layered struc-tures. Layered cobalt oxides such as NaxCoO2

and cobaltate series with general formula[MmA2Om+2]q[CoO2] have been found to be good p-type TE materials.9 The ZT values of single crystalsof these oxides are around unity at 700 K to 1000 Kin air. However, the ZT values of these ceramics arelow, which needs to be addressed. Literature surveyshows that identified n-type oxides such as dopedZnO, CaMnO3, and SrTiO3 fall short in terms of TEperformance compared with p-type oxides, despitecontinuing efforts to develop these materials. Lay-ered misfit cobaltates [MmA2Om+2]q[CoO2], andespecially Ca3Co4O9±d, have attracted much atten-tion as p-type TE materials due to their highSeebeck coefficient coexisting with low electricalresistivity as well as low thermal conductivity. Suchcompositions are suitable for use in automobile heat

recovery systems due to their stability even incontact with high-temperature exhaust gases(�850 K). Although the ZT values of single crystalsof layered cobalt oxides exceed unity, the perfor-mance of bulk ceramics is several times lower thanthat of single crystals.

Though misfit layered cobaltites show promisingproperties for use in high-temperature applications,sintering of these oxides under normal conditions isa major problem in reported research.9,10 Tech-niques such as spark plasma sintering (SPS) and

Fig. 1. DSC/TGA curve of Ca3Co4O9±d.

Fig. 2. XRD pattern of Ca3Co4O9±d sintered at 900�C.

Radhika, Raghu, Powrnami, Jothi Ramalingam, and Al-Lohedan1788

isostatic pressing have been adopted to achievehigher density, hence there is a need to understandwhether defects in the structure could help toimprove the density and thereby the thermoelectricperformance. Synthesis of pure spinel-phase mate-rial is a challenge, and the properties have to befine-tuned to make use of these ceramic oxides in TEapplications. Further improvement in ZT can beachieved either by engineering the structurethrough substitution/doping and correlating theresulting properties and/or by newer processmethodologies such as nanosizing, oriented graingrowth, etc. Hence, in this work, Ca3Co4O9±d

ceramics were prepared through a solid-statemethod. The influence of the calcination and sin-tering temperatures and duration was studied toobtain highly dense ceramics. The morphology,structure, and electrical properties of Ca3Co4O9±d

were also studied.

EXPERIMENTAL PROCEDURES

Ca3Co4O9±d ceramics were prepared by the fol-lowing simple solid-state method: Stoichiometricamounts of CaCO3 (Merck, purity 98.5%) andCo3O4 (Alfa Aesar) were mixed well with iso-propanol (Merck). After some time, the dried sam-ples were calcined at 850�C for different durationsof 4 h, 8 h, 12 h, and 16 h. After forming pelletsfrom calcined powder, sintering was carried out inthree steps at 900�C for duration of 4 h, 24 h, and24 h. After each sintering procedure, the sampleswere crushed and repelletized for the next sinteringstep. Material density was measured after sinter-ing, using the Archimedes method. Thermal analy-sis of dried powder was carried out by differentialscanning calorimetry/thermogravimetric analysis(DSC/TGA) (TA instrument SDT Q600 V8.3 build101) in the temperature range from 25�C to 825�C atheating rate of 10�C/min in air. Powder x-raydiffraction (XRD) analysis of the ceramic oxideswas carried out using an D5005 device (Bruker,Germany) with Cu Ka radiation at wavelength of1.54 A. The electrical properties of the materialswere determined using Hall measurements with anEcopia Hall measurement system (HMS -3000-VER3.51.5). The carrier concentration, mobility,conductivity, and average Hall coefficient of thesamples were measured at room temperature andfixed magnetic field of 0.54 T. The Seebeck coeffi-cient and electrical conductivity were measuredusing an SBA 458 Nemesis.

RESULTS AND DISCUSSION

To understand the weight changes on heat treat-ment, DSC/TGA results for the Ca3Co4O9±d ceramicpowders after solid-state processing were recordedin air and are shown in Fig. 1. The DSC/TGAresults show an extremely high endothermic peakat �687�C with weight loss of 22.3% due to decom-position of CaCO3. Formation of Ca3Co4O9±d takes

place at around 800�C. The plateau in the TG curvein the region of 700�C to 900�C also confirmsformation of single-phase Ca3Co4O9±d ceramics inthis temperature range. These results suggest thatthere is a remarkable change in properties aftercalcination at 850�C for 12 h. The powder XRDpattern of ceramics prepared by the solid-statemethod and calcined at 850�C for 12 h and 16 hare shown in Fig. 2. The XRD patterns wereindexed and found to match that for Ca3Co4O9±d

[Joint Committee on Powder Diffraction Standards(JCPDS) no. 21-0139] with monoclinic structure inspace group p21/m.9 However, as the calcinationduration was increased, secondary phase appeared,which may affect the material density. The latticeparameters a, b, c, and b calculated using Igor Pro

Table I. Lattice parameters and b of Ca3Co4O9±d

calcined at 850�C for 12 h and sintered at 900�C

Preparationmethod

Latticeparameters (A)

b (�)a b c

Solid state 4.80 4.50 10.70 98.50Reported values 4.83 4.56 10.83 98.06

Table II. Density of Ca3Co4O9±d ceramics sinteredat 900�C for 4 h + 24 h + 24 h

Calcinationcondition

Density(g cm23)

Relativedensity (%)

850�C, 12 h 3.3 67850�C, 16 h 3.3 67

10 20 30 40 50 60 70 80

CCo-9

Inte

nsity

(a.u

.)

2 theta (deg.)

CCO-9a

CCO-10

CCO-18

Fig. 3. Represents the XRD of CCO-9- 850�C, 12 h, CCO-9a–850�C, 16 h, CCO-10- CCO-9 sintered at 900�C 4 h + 24 h + 24 h,CCO-18- CCO-9a sintered at 900�C 4 h + 24 h + 24 h.

Effect of Synthesis Conditions on Formation, Electrical Properties, and Seebeck Coefficient ofp-Type Ca3Co4O9±d Thermoelectric Ceramics

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software are presented in Table I. The obtainedlattice parameter values match with reportedvalues, confirming formation of Ca3Co4O9±d underpreparation conditions. The density of the sintered(900�C, 4 h + 24 h + 24 h) ceramics measured bythe Archimedes method is reported in Table II. Thetheoretical density of Ca3Co4O9±d is 4.9 g cm�3.7

The density of the samples measured using theconventional method was found to be almost thesame, although the sample sintered at 850�C for12 h showed relatively higher density. Sampleswere calcined for various durations then sinteredat 900�C for durations of 4 h + 24 h + 24 h. Table IIpresents the density after the normal sinteringprocess. Figure 3 shows the XRD patterns of cal-cium cobaltite prepared using different reactiontemperatures and conditions. The XRD patternsshowed the formation of the same phases with fewchanges. Figure 4a, b, c, and d shows the structure

observed for the as-prepared calcium cobaltitesample, revealing fine spherically shapednanoparticles together with tubular structures.The small particle size is illustrated in the TEMimage in Fig. 4b. Darker contrast is observed dueto metal-rich cobalt ion aggregates, boundtogether by the sintering process. The red linesin Fig. 4b indicate the measured particle size ofbelow 10 nm to 15 nm for the spherical calciumcobaltite particles.

Electrical Properties

The electrical properties measured by the Hallmethod for the ceramic oxides prepared by the solid-state method are presented in Table III. The posi-tive average Hall coefficient of these samples con-firms their p-type nature. The carrier concentration,mobility, and conductivity values agree withreported results.8 The carrier concentration

Fig. 4. (a–d) TEM images of stoichiometric Ca2.99Co3.91Ox±d prepared by solid-state route.

Radhika, Raghu, Powrnami, Jothi Ramalingam, and Al-Lohedan1790

increased after 12 h of calcination. The mobility wasfound to increase up to 12 h, then started todecrease, following a trend opposite to that of thecarrier concentration (Fig. 5). The average Hallcoefficient increased up to 12 h but decreased forlonger calcination duration, following the sametrend as the mobility. The conductivity was main-tained up to 12 h, but decreased for longer calci-nation duration. Table III presents the electricalproperties of the as-prepared samples. The carrier

concentration shows a marked decrease aftercalculation at 850�C for 12 h, while the mobilityfollows the opposite trend. One can see that, after12 h at 850�C, there was a remarkable increase inmobility. The average Hall coefficient followed thesame trend as the mobility, exhibiting an increaseafter calcination at 850�C for 12 h. The conduc-tivity increased after 8 h at 850�C, then flattenedout. The conductivity followed the same trend asthe density.

Carrier concentration with calcination time Mobility with calcination time

Average Hall coefficient with calcination time. Conductivity with calcination duration.

4 hr 8 hr 12 hr 16 hr63

65

67

69

71

73

Con

duct

ivity

(Ωcm

-1)

Calcination duration (h)

4 hr 8 hr 12 hr 16 hr5.00E+019

1.00E+020

1.50E+020

2.00E+020

2.50E+020

3.00E+020

3.50E+020

Bul

k C

once

ntra

tion

(cm

-3)

Calcination duration (h)4 hr 8 hr 12 hr 16 hr

0.8

1.3

1.8

2.3

2.8

3.3

3.8

4.3

4.8

5.3

5.8

Mob

ility

(cm

2 (V

s)-1

)

Calcination duration (h)

4 6 8 10 12 14 160.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

Ave

rage

Hal

l Coe

ffici

ent (

cm-3

C-1

)

Calcination duration (h)

(a) (b)

(c)(d)

Fig. 5. (a–d) Measurements of conductivity and Hall coefficient of calcium cobaltite ceramics.

Table III. Electrical properties of Ca3Co4O9±d sintered at 900�C for 4 h + 24 h + 24 h

Calcinationtime (h)

Carrierconcentration(1020 cm23)

Mobility(cm2 V21 s21)

Conductivity(X cm21)

Resistivity(1022 X cm)

Average Hallcoefficient

(1022 cm23 C21)

4 2.299 1.933 71.21 1.404 2.7158 1.646 2.738 72.20 1.385 3.79212 8.894 4.934 70.67 1.415 6.98216 3.475 1.159 64.29 1.555 1.797

Effect of Synthesis Conditions on Formation, Electrical Properties, and Seebeck Coefficient ofp-Type Ca3Co4O9±d Thermoelectric Ceramics

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The density and physical properties of thesamples match reported values up to 850�C for12 h. At higher temperature, formation of sec-ondary phases was observed, resulting in adecreasing trend in the electrical properties. Sincethe samples are oxide ceramics, achieving highdensity is an important aspect, greatly influencingthe properties of the samples.11,12 The density wasfound to be around 67%, indicating that thesamples were highly porous. Literature reportssuggest that density can be improved by solid-state substitution, doping using a reaction sinter-ing process, or nonstoichiometry.10–18 To checkthe effect of nonstoichiometric composition,Ca2.95Co3.91O9±d ceramics were prepared andtheir properties studied. The density was foundto be higher for these compositions in comparisonwith Ca3Co4O9±d (Table IV). The density wasslightly improved for the nonstoichiometric com-position compared with Ca3Co4O9±d.

19,20 Fig-ures 6 and 7 show the temperature dependenceof the electrical conductivity (r) and Seebeckcoefficient (S), respectively, of as-prepared stoi-chiometric calcium cobaltite compared with pris-tine calcium cobaltite. The electrical conductivityof Ca3Co4O9±d was slightly higher compared withnonstoichiometric Ca2.99Co3.91O9±d material.Above the transition point (�100�C), the mobility

increased along with the electrical conductivity asthe temperature was increased. The positivevalue of S indicates that the materials are p-type semiconductors. The Seebeck coefficientincreased with increasing temperature in thetemperature range up to 700�C, with a slightdecrease observed as the temperature wasincreased further to 800�C. The highest S valueof 175 lV/K was obtained at temperature of700�C, being as large as that of typical layeredcobalt oxides.20–22 Similar values are observed forboth materials.17,23 Even though the nonstoichio-metric material exhibits slightly improved den-sity, not much difference was observed betweentheir S values.

CONCLUSIONS

The carrier concentration decreased up to 12 hof calcination, then increased. The mobility andHall coefficient sharply increased up to 12 h, thendecreased. The conductivity remained constant upto 12 h, then decreased. The density and physicalproperties match with reported values up to 12 hat 850�C, while secondary phase formation wasobserved for longer duration or higher tempera-ture, degrading the physical properties. The opti-mized calcination condition for calcium cobaltiteformation was 850�C for 12 h. Using normalceramic process, the density and properties werefound to be similar and close to reported values.TEM images revealed tubular morphology withmetal-rich particle aggregation. The presentstudy adopted a simplified normal ceramic processcompared with costly plasma or thin-film tech-niques, achieving good electrical conductivity andSeebeck coefficient by adjusting the reaction/sin-tering conditions.

Table IV. Density of Ca3Co4O9±d and Ca2.95Co3.91

O9±d ceramics sintered at 900�C for 4 h + 24 h + 24 h

Calcinationcondition Density (g cm23)

Relativedensity (%)

Ca3Co4O9±d 3.3 67Ca2.99Co3.91O9±d 3.5 71

Fig. 7. Seebeck coefficient of calcium cobaltite nanomaterials.

Fig. 6. Electrical conductivity of calcium cobaltite samples.

Radhika, Raghu, Powrnami, Jothi Ramalingam, and Al-Lohedan1792

ACKNOWLEDGEMENTS

R.J.R. and H.A.A. acknowledge financial supportfrom King Saud University, Vice Deanship of Sci-entific Research, Research Chairs.

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