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Ceramics International

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Microwave heating synthesis and thermoelectric property characterizationof highly dense Ca3Co4O9 bulk

A. Raja Annamalaia, P. Ravi Tejab, Dinesh K. Agrawalc, A. Muthuchamyb,∗

a Centre for Innovative Manufacturing Research, VIT, Vellore, Indiab School of Mechanical Engineering, VIT, Vellore, IndiacMaterial Research Institute, Pennsylvania State University, USA

A R T I C L E I N F O

Keywords:Solid state synthesisCalcium cobaltiteElectrical propertiesReactive sintering

A B S T R A C T

The present study is focused on synthesis and sintering of calcium cobaltite (Ca3Co4O9) by solid-state reaction ofcalcium carbonate (CaCO3) and cobalt oxide (Co3O4) using microwave heating at 850, 900, and 950 °C.Thermogravimetric Analysis (TGA) and Differential Thermal Analysis (DTA) results show a weight loss of ~26%due to the decomposition of CaCO3 in the temperature range of 600–820 °C, resulting into single phase ofCa3Co4O9 which remains stable up to 920 °C. The Ca3Co4O9 phase is observed in samples heated at 850 °C and900 °C, whereas at 950 °C, Ca3Co4O9 is transformed to Ca3Co2O6 as identified by X-Ray Diffractometry. 98.76%of relative density is obtained by microwave sintered sample at 950 °C with 1hr holding time. Changes insynthesis temperature did not show any effect on the Seebeck coefficient (119–155 μV/K). The specimens sin-tered at 900 °C exhibit a low resistivity (150-160 μΩ-m) and better power factor (94–145 μW/mK2) in the35–500 °C temperature range.

1. Introduction

Metal oxides have gained attention as an important thermoelectric(TE) candidate over metallic alloys because of their chemical stability athigh temperatures and thermal stability. The TE performance of oxidesis not very high when compared to non-oxide materials, but they aremore abundant in nature, cheaper and less toxic as compared to non-oxides. Metal oxides possess unique properties, including optimizedfigure of merit, high thermal power and longer service life at hightemperatures [1,2]. Specifically, cobaltites with layered structure havegained attention over the past few years. They possess a wide variety ofstructural, magnetic, and electronic properties that are of interest for awide range of potential applications. Examples include lithium cobaltite(LiCoO2), mostly used as cathodes in lithium batteries, sodium cobaltite(NaxCoO2), Bi2Sr3Co2O8, and calcium cobaltite (Ca3Co4O9), widelystudied for their thermo-electric properties [3]. NaxCoO2 layered withCa3Co4O9 was reported as a promising thermoelectric oxide material[4] because of its good thermal, thermoelectrical properties and che-mical stability at high temperatures [5]. NaCo2O4 exhibits good ther-moelectric performance comparable to the performance of semi-conducting materials; however, greater attention has been given toother layered cobalt oxides like Ca3Co4O9, which possesses intrinsicmerits of low thermal conductivity and high oxidation resistance in air

at elevated temperatures. Chateigner et al. obtained highly textured andhighly densified Ca3Co4O9 ceramics through hot press and spark plasmasintering techniques and reported a distinct enhancement in mechan-ical and thermoelectric properties over conventionally sintered samples[6]. Matsukevich et al. prepared Ca3Co4O9 ceramics using solutionmethods and reported denser, fine grained ceramics with optimal per-formance at high temperature (T~1000 K) [7]. Agilandeswari et al.developed Ca3Co4O9 ceramics using starch assisted sol-gel combustionand reported a grain size of 150–300 nm, lower electrical resistivity of0.0012 mΩ-cm at 473 k of Ca3Co4O9 sample sintered at 1073 K [8].Kahraman et al. reported the synthesis of polycrystalline Ca3Co4O9

using sol-gel method followed by thermal treatment twice to ensure thedecomposition of calcium carbonate [9]. Luxiang Xu et al. synthesizedpolycrystalline Ca3Co4O9 by solid-state reaction and sintered at 900 °Cfor 48 h under flowing O2, pulverised, reground and sintered again atthe same temperatures for 12 h [10]. Driss Kenfaui et al. preparedCa3Co4O9 by solid state reaction at 900 °C for 24 h; subsequently thepowder was reground and sintered by SPS technique at optimizedprocessing parameters of 50 MPa and 900 °C for better thermoelectricperformance [11]. Zhang Feipeng et al. produced Ca3Co4O9 by citrateacid Sol-gel method followed by SPS to form bulk samples. Analyses ofbulk samples were performed to ascertain thermoelectric transportproperties [12]. Soteloa et al. studied the addition of a liquid phase

https://doi.org/10.1016/j.ceramint.2020.04.105Received 2 January 2020; Received in revised form 8 April 2020; Accepted 9 April 2020

∗ Corresponding author.E-mail addresses: raja.annamalai@vit.ac.in (A. Raja Annamalai), dxa4@psu.edu (D.K. Agrawal), muthuchamymt@gmail.com (A. Muthuchamy).

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Available online 10 April 20200272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

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promoting compound (K2CO3) during solid-state sintering and reportedcontrolled porosity of Ca3Co4O9, which helped in enhancing its per-formance [13]. Sophie Bresch reported the influence of the calcinationprocedure on TE properties of Ca3Co4O9 and concluded that Ca3Co4O9

can efficiently be synthesized by direct solid-state reactive sintering,which results in more interconnected Ca3Co4O9 microstructure thansintering of any calcined Ca3Co4O9 powder, thus giving higher elec-trical conductivity and better thermoelectric performance [14]. MuratGunes et al. studied the effect of Ca3Co4O9 grain size (18 nm–2 μm)prepared by sol-gel method and how the resulting porosity affected itsphonon scattering capability [15]. Yuheng Liu et al. produced singlephase Ca3Co4O9 using sol-gel method at 800 °C. A highly dense sample(~99% theoretical density) was prepared by SPS, resulting in improvedthermoelectric properties of the material [16,17]. Similarly, many otherresearchers have also reported the single-phase formation of Ca3Co4O9

by solid state or Sol gel method [18–26]. K. Miyazawa et al. preparedthe Ca3Co4O9 by Sol-gel method followed by hybrid microwave heatingat 800 °C. Single phase formation of Ca3Co4O9 reported by microwaveheating at dwell time of 60 min [27]. With the above cited reports, it isobvious that the reaction-sintering process remains a simple and ef-fective process to synthesize calcium cobaltite. To the best of ourknowledge, there is no report on microwave reactive sintering ofCa3Co4O9 thermoelectric material at high temperatures where the othermethods showed better results. In this study we have made an attemptto synthesize and sinter Ca3Co4O9 using microwave heating method atrelatively higher temperatures.

2. Experimental procedure

Precursor powders of calcium carbonate (CaCO3) and cobalt oxide(Co3O4) procured from SIGMA-ALDRICH, Bangalore, India with 99+%purity,< 10 μm particle sizes were used for solid-state reactive sin-tering. (See Fig. 1). Stoichiometric amounts of the precursors weremixed according to the following reaction [14,18,19]:

9 CaCO3 + 4 Co3O4 + O2 → 3 Ca3Co4O9 + 9 CO2 (1)

The precursor powders were mixed in a molar ratio correspondingto the nominal composition of Ca3Co4O9. Mixing was performed in aplanetary ball mill for 20min with steel ball (20 No's) and ethanol as agrinding medium (1:1 gm/ml). The mixture was first dried in air for24hrs and subsequently heated at 60 °C for 15min. The dried powderwas uni-axially pressed using hydraulic press at about 400 MPa andspecimens of 16 mm × 4 mm were produced. Microwave reactivesintering was performed in a multimode 2.45 GH microwave furnace(VB Ceramics, Chennai). Sintering was carried out at various tem-peratures (850, 900, 950 °C) with a heating rate of 45 °C/min and 1hrholding time.

Thermogravimetric Analysis (TGA) and Differential ThermalAnalysis (DTA) were performed in the temperature of range RT-1200 °Cusing Thermal Analyser (SDT Q600, TA instruments, USA). The phasecomposition of bulk sintered sample was identified using XRD (BRUKERD8 Advanced, Yokohama, Japan, Cu Kα, λ = 1.5405 Å) and recordedin 2THETA range of 20–70°. For SEM examination the sample surfacewas polished using diamond paste in disc polisher and followed bythermal etching at 150 °C below the sintering temperature for 1hr.Ceramics are insensitive to electron-beam transmission; therefore, thesample was gold-palladium (Au–Pd) coated by vapor depositionmethod for one hour. Microstructure of bulk solid sample was observedunder Scanning Electron Microscopy (SEM) (Zeiss Penta FET precision,Model: 51‐ADD0048, Carl Zeiss Pvt Ltd, Bangalore, India). EDAX wasperformed along with SEM on selected areas of interest to determineelemental composition. The electrical resistivity(ρ) and Seebeck coef-ficient (S) were determined from room temperature (RT) to 500 °C. Thesamples were cut into required dimensions (4*4*12 mm rectangle) tomeasure the electrical resistivity (ρ) and Seebeck coefficient (S)

Fig. 1. SEM image of (a) calcium carbonate (b) cobalt oxide.

0 100 200 300 400 500 600 700 800 900 1000 1100 120065

70

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105 TGA DTA

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% thgieW

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0.8

Tem

pera

ture

Diff

eren

ce (°

C/m

g)

Fig. 2. TGA/DTA curves of precursor powder mixture to synthesize Ca3Co4O9.

Table 1Comparison of Relative Density of Microwave synthesized Ca3Co4O9 samplessintered at different temperatures.

Sintering Temperature (°C) Microwave sintering (1 h holding time)

850 95.81900 96.71950 98.76

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simultaneously by using Ulvac ZEM-3 in He atmosphere.For electrical measurement, the electrode material is made of

copper (equipment Ulvac ZEM-3) used in the present study. Ohmiccontact self-check function (V–I plot) is standard in the equipment.

3. Results and discussion

3.1. TGA/DTA characterization of powder mixture

2.51 mg of powder mixture was used for characterization andweight loss of 5.712% was observed during the first stage (RT - 200 °C)which is attributed to desorption/drying. The next stage of weight loss(23.05% + 2.99%) depicts the decomposition of calcium carbonatethat started around 600 °C and ended at 820 °C as per the following tworeactions [19]. It is reported that single phase Ca3Co4O9 is formed at800 °C/24 h which is also confirmed with TGA/DTA results [14,19].CaCO3 decomposes into CaO and CO2 starting at 600 °C and Co3O4 intoCoO and O2 at about 800 °C [14]. The three exothermic peaks in theDTA curve indicate the decomposition of CaCO3, formation ofCa3Co4O9 and Ca3Co2O6, respectively [21]. This has also been con-firmed by XRD results.

CaCO3 → CaO + CO2 (2)

2Co3O4 → 6CoO + O2 (3)

It was observed that the Ca3Co4O9 phase starts appearing at 600 °C.During this reaction, CO2 is released during the decomposition ofCaCO3. Ca3Co4O9 is formed at around 800 °C and remains stable up to926 °C [14,15]. In our study Ca3Co4O9 forms at 820 °C and remainsstable up to around 920 °C. On further heating to 1200 °C, small weightgain is observed due to the oxidation of Ca3Co4O9 into Ca3Co2O6..

The DTA curve is divided into three parts. First part of curve

represents the evaporation of water and ethanol from room tempera-ture to ~120 °C. The water content is due to hygroscopic nature ofprecursor calcium carbonate and the little amount of ethanol, whichwas added as grinding media. An increase in ΔT is observed in DTAwhich shows the absorption of heat taking place up to 600 °C. Later, it isreduced due to the decomposition of calcium carbonate and is con-sistent with the TGA curve. This exothermic reaction is observed until820 °C. The two exothermic peaks in the DTA curve between thetemperature 600–900 °C correspond to the decomposition of calciumcarbonate during the formation Ca3Co4O9.

3.2. Relative density measurement

The relative densities of sintered samples were calculated by com-paring the data with the theoretical density of 4.567 gm/cc and shownin Table 1. A max 98.76% of relative density is obtained by microwavesintered sample at 950 °C and 1hr holding time. Improved density from95.81 to 96.71% density observed with increase in sintering tempera-ture from 850 °C to 900 °C. Samples prepared by Sotelo et al. throughsolid-state sintering method at 800 °C for 12 h, with subsequent manualmilling before pressed into desired form and sintered at 900 °C for 24 hachieved a relative density of only 73–74% even after several steps ofheating for hours at elevated temperatures [13,25]. High densificationobserved in the microwave sintered sample is attributed to the micro-wave effect that enhances the reaction kinetics and the material diffu-sion in much shorter holding time.

3.3. XRD: phase composition analysis

XRD of sintered Ca3Co4O9 at various sintering temperatures (850,900 and 950 °C) for 1 h were recorded in 2θ range of 20–70°. Theobtained diffractograms matched with the standard Ca3Co4O9 pattern

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

# 22

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Co3O4

CaCO3

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*207 * 1

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44041

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311

111* 0

01

* 205 51

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131

012

* Ca3Co4O9

Ca3Co2O6

# CaCO3

Co3O4

Fig. 3. X-Ray Diffraction patterns of Microwave sintered Ca3Co4O9 samples.

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(JCPDS – 023–0110). It was observed that a small amount of calciumcarbonate remained unreacted in all samples. The diffraction peak at43.1° is characteristic of CaCO3. Formation of single-phase calciumcobaltite (Ca3Co4O9) depends on temperature and holding time. In thisstudy Ca3Co4O9 was obtained at 900 °C/1hr. Other researchers reportedthe formation of Ca3Co4O9 at 900 °C/24 h [21], and found it to bestable between 800 and 900 °C [15,18]. The unreacted Co3O4 andCaCO3 were detected even after repeated calcinations at 760 °C, or at

860 °C [26]. The diffraction peaks at 20.5°, 42.3° and 60.2° are char-acteristic peaks of Ca3Co2O6 (JCPDS 051–0311). There is a small shiftin peaks observed due to induced strain in lattice because of the pre-sence of Ca3Co2O6 in the sample sintered at 950 °C. The XRD of sinteredCa3Co4O9 at various sintering temperatures (850, 900 and 950 °C) for 1h were recorded in 2θ range of 5–90° were shown in Fig. 3.

3.4. Microstructure

The platelets like grains were observed in sintered samples at dif-ferent temperatures in the SEM images shown in Fig. 4. This is a typicalfeature of layered crystal structure of Ca3Co4O9 [14]. Porosity wasobserved in samples sintered at 850 °C temperature and reduced withincrease in temperature to 900 °C which is consistent with the densityvalues. The presence of calcium carbonate in the XRD patterns is sup-ported with the elemental analysis determined by EDAX. The samplesintered at 850, 900 and 950 °C exhibited carbon content of 1.64, 2.11and 1.62 wt%, respectively (Table 2).

Fig. 4. SEM images of Microwave sintered Ca3Co4O9 samples.

Table 2Elemental analysis of Microwave sintered Ca3Co4O9 using EDAX.

Element Weight %

850°C-1 hr 900°C-1 hr 950°C-1 hr

C 1.64 2.11 1.62O 31.64 28.75 33.09Ca 21.50 21.69 25.93Co 45.22 47.45 39.36

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3.5. Electrical properties characterization

3.5.1. Seebeck coefficientFig. 5 shows the positive seebeck coefficient of the samples at dif-

ferent sintering temperatures, indicating that Ca3Co4O9 ceramics are P-type semiconductor TE material. The positive seebeck coefficient is dueto the dominating hole conduction mechanism. At room temperature aminimum value of 119 μV/K was obtained for specimens sintered at900 °C and a maximum value of 124 μV/K for sample sintered at 850 °Cwhich is closer to the value 123 μV/K (at 900 °C for 24hrs) [14], ASeebeck value of 125 μV/K was reported in literature at room tem-perature [25].

As the working temperature of TE material increases the Seebeckcoefficient also increases linearly and reaches a maximum value of156 μV/K at temperature 500 °C which is higher than 137.5 μV/K [22],and lower (~170 μV/K) reported in Ref. [17,25] at the same tem-perature. The literature survey supports our results that seebeck coef-ficient is insensitive to the sintering technique, temperature and thebulk density [14]. Samples processed by conventional sintering, SparkPlasma Sintering and hot press methods (solid state sintering) of thesame composition reported nearly same values of Seebeck coefficientwith a maximum value of 170 μV/K at 527 °C [6].

3.5.2. Electrical resistivityIt is observed that microwave sintered sample at 900 °C shows

minimum electrical resistivity of 150 μΩ-m which is lower than thevalue 190 μΩ-m [25] at room temperature in literature calcined at sametemperature. A slight increase to 166 μΩ-m of resistivity was noted withtemperature rise to 400 °C which is in the range reported in literature~131 μΩ-m [22], 180 μΩ-m [25] at 500 °C. Sample sintered at 950 °Cshows maximum resistivity of 480 μΩ-m at room temperature whichreduced to 400 μΩ-m with rise in temperature to 350 °C. The increase inresistivity is due to the formation of Ca3Co2O6 upon decompositionCa3Co4O9 beyond 926 °C. A relatively constant value of 420 μΩ-m hasbeen reported by conventionally sintered sample at 920 °C for 24hrs(RT-527 °C) [6]. Working temperature beyond 900 °C in formation ofCa3Co4O9 will drastically increase the resistivity as reported in Ref. [6].

3.5.3. Power factorThe following numerical expression is used in calculating the per-

formance of the synthesized thermoelectric material by different sin-tering methods at different temperatures. Material having the highestpower factor possesses highest thermoelectric performance.

Power factor (α) = S2 / ρ (μW/mK2) (4)

S - Seebeck coefficient (μV/K); ρ – Electrical resistivity (μΩ-m).Power factor was calculated for the sample sintered at 900 °C

(Fig. 6), and the values at room temperature and 500 °C were found tobe 94 and 145 μW/mK2 respectively. This is consistent with the re-ported values in the previous work [25] at same sintering temperaturethrough solid-state sintering. An appreciable increase in power factorwas observed at 900 °C, because of the formation of Ca3Co4O9. How-ever, a max value of 30 μW/mK2 has been reported for solid-state re-action sintered sample [14]. As per numerical expression, the ceramicswhich possess lower electrical resistivity will have a greater powerfactor. The sample sintered at 900 °C having higher power factor (30.7μW/mk2) obtained for sample sintered at 950 °C and formation ofCa3Co2O6 drastically reduces the thermoelectric properties.

4. Conclusions

Calcium cobaltite, Ca3Co4O9 was synthesized and sintered to highdensity using microwave heating method. Thermogravimetric Analysis(TGA) and Differential Thermal Analysis (DTA) showed a weight loss of26.04% due to decomposition and reaction between precursors in thetemperature range of 600°C-820 °C and formed Ca3Co4O9 phase whichremains stable up to 920 °C. Ca3Co4O9 phase was observed in samplessintered at 850 °C and 900 °C, whereas at 950 °C Ca3Co4O9 partiallydecomposed in to Ca3Co2O6. Small amount of unreacted CaCO3 was

50 100 150 200 250 300 350 400 450 500115

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Fig. 5. High temperature electrical properties of Microwave sintered Ca3Co4O9 samples.

50 100 150 200 250 300 350 400 450 50020

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Pow

er fa

ctor

(μW

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Fig. 6. High temperature Power factor of Microwave sintered Ca3Co4O9 sam-ples.

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found in all samples as determined by XRD analyses. Change in sin-tering temperature during microwave heating does not show any effecton Seebeck coefficient (119–155 μV/K). The sample sintered at 900 °Cshowed a low resistivity (150-160 μΩ-m) and good power factor(94–145 μW/mK2) at 35–500 °C.

Declaration of competing interest

The authors declare that they have no known competing financialinterests or personal relationships that could have appeared to influ-ence the work reported in this paper.

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