Thermoelectric and magnetic properties of Ca3Co4-xCuxO9+ δ with x = 0.00, 0.05, 0.07, 0.10 and 0.15

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Thermoelectric and magnetic properties of Ca 3 Co 4–x Cu x O 9 + d with x = 0.00, 0.05, 0.07, 0.10 and 0.15 Ankam Bhaskar, Z.R. Lin, Chia-Jyi Liu * Department of Physics, National Changhua University of Education, Changhua 500, Taiwan 1. Introduction Thermoelectric (TE) generators are considered as energy conversion systems, which can convert heat energy into electric energy directly from vast amounts of waste heat emitted by automobiles, factories, and similar sources without using moving parts such as turbines and without producing CO 2 gas, radioactive substances, or other emissions [1]. Wide attention has been focused on the exploration of thermoelectric materials recently. Good thermoelectric materials require a large thermopower (S) for generating a large thermal voltage, a low electrical resistivity (r) for minimizing the Joule heating, and a low thermal conductivity (k) for retaining the heat at the junctions in order to obtain a high figure of merit ZT = S 2 T/rk [2]. Besides, thermoelectric materials are required to be stable at high temperatures. In recent years, layered cobalt oxides have gained great attention since NaCo 2 O 4 single crystal is found to exhibit good thermoelectric properties [3]. The misfit cobalt oxides (Ca 3 Co 4 O 9 + d ) have been investigated extensively as potential thermoelectric material because it has large S, low r, and low k [4–9]. The crystal structure of Ca 3 Co 4 O 9 + d system consists of two subsystems, viz., the distorted NaCl-type (Ca 2 CoO 3 ) sublattice and the CdI 2- type (CoO 2 ) sublattice, alterna- tively stacking along the c-axis [10]. The polycrystalline bulk Ca 3 Co 4 O 9 + d samples are still at a relatively low level for industrial applications. Many attempts have been made to optimize the thermoelectric performance of Ca 3 Co 4 O 9 + d by either partially substituting cations or using appropriate fabrication methods such as hot pressing (HP) or spark plasma sintering (SPS) techniques. Partial replacement of cations in the Ca 3 Co 4 O 9 + d has been carried out on either the Ca site [11–22] or the Co sites [5,8,23–26]. Many groups have attempted to prepare Ca 3 Co 4–x Cu x O 9 + d system with lower concentration (x = 0.00, 0.05, 0.10) of dopants [21,24], and with higher concentration (x 0.2) of dopants [27–29], reporting remarkable changes in the thermoelectric properties. In this paper, we report the low-temperature (<300 K) thermoelectric and magnetic properties of Ca 3 Co 4–x Cu x O 9 + d (x = 0.00, 0.05, 0.07, 0.10 and 0.15) samples. 2. Experimental Polycrystalline samples of Ca 3 Co 4–x Cu x O 9 + d with x = 0.00, 0.05, 0.07, 0.10 and 0.15 were synthesized by conventional solid state reaction from CaCO 3 , Co 3 O 4 , and CuO powders. The powders were heated at 900 8C for 24 h with intermediate grinding. The resulting powders were then pressed into parallelepiped and sintered in air at 900 8C for 24 h. The phase purity of resulting powders was examined by a Shimadzu XRD-6000 powder X-ray diffractometer equipped with Fe Ka radiation. The electrical resistance measure- ments were carried out using standard four-probe techniques. The thermopower measurements were performed between 80 K and 300 K using steady-state techniques with a temperature gradient of 0.5–1 K across the sample. A type E differential thermocouple Materials Research Bulletin 48 (2013) 4884–4888 A R T I C L E I N F O Article history: Received 28 March 2013 Received in revised form 13 May 2013 Accepted 5 July 2013 Available online 13 July 2013 Keywords: Oxides X- ray diffraction Electrical properties Magnetic properties Thermal conductivity. A B S T R A C T Ca 3 Co 4–x Cu x O 9 + d (x = 0.00, 0.05, 0.07, 0.10 and 0.15) samples were prepared by conventional solid-state synthesis and their thermoelectric properties were systematically investigated. The thermopower of all the samples was positive, indicating that the predominant carriers are holes over the entire temperature range. Ca 3 Co 3.85 Cu 0.15 O 9 + d had the highest power factor of 2.17 mW cm 1 K 2 at 141 K, representing an improvement of about 64.4% compared to undoped Ca 3 Co 4 O 9 + d . Magnetization measurements indicated that all the samples exhibit a low-spin state of cobalt ions. The observed effective magnetic moments decreased with increasing copper content. ß 2013 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: +886 4 723 2105x3337; fax: +886 4 728 0698. E-mail address: [email protected] (C.-J. Liu). Contents lists available at SciVerse ScienceDirect Materials Research Bulletin jo u rn al h om ep age: ww w.els evier.c o m/lo c ate/mat res b u 0025-5408/$ see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.07.004

Transcript of Thermoelectric and magnetic properties of Ca3Co4-xCuxO9+ δ with x = 0.00, 0.05, 0.07, 0.10 and 0.15

Page 1: Thermoelectric and magnetic properties of Ca3Co4-xCuxO9+ δ with x = 0.00, 0.05, 0.07, 0.10 and 0.15

Materials Research Bulletin 48 (2013) 4884–4888

Thermoelectric and magnetic properties of Ca3Co4–xCuxO9 + d withx = 0.00, 0.05, 0.07, 0.10 and 0.15

Ankam Bhaskar, Z.R. Lin, Chia-Jyi Liu *

Department of Physics, National Changhua University of Education, Changhua 500, Taiwan

A R T I C L E I N F O

Article history:

Received 28 March 2013

Received in revised form 13 May 2013

Accepted 5 July 2013

Available online 13 July 2013

Keywords:

Oxides

X- ray diffraction

Electrical properties

Magnetic properties

Thermal conductivity.

A B S T R A C T

Ca3Co4–xCuxO9 + d (x = 0.00, 0.05, 0.07, 0.10 and 0.15) samples were prepared by conventional solid-state

synthesis and their thermoelectric properties were systematically investigated. The thermopower of all

the samples was positive, indicating that the predominant carriers are holes over the entire temperature

range. Ca3Co3.85Cu0.15O9 + d had the highest power factor of 2.17 mW cm�1 K�2 at 141 K, representing an

improvement of about 64.4% compared to undoped Ca3Co4O9 + d. Magnetization measurements

indicated that all the samples exhibit a low-spin state of cobalt ions. The observed effective magnetic

moments decreased with increasing copper content.

� 2013 Elsevier Ltd. All rights reserved.

Contents lists available at SciVerse ScienceDirect

Materials Research Bulletin

jo u rn al h om ep age: ww w.els evier .c o m/lo c ate /mat res b u

1. Introduction

Thermoelectric (TE) generators are considered as energyconversion systems, which can convert heat energy into electricenergy directly from vast amounts of waste heat emitted byautomobiles, factories, and similar sources without using movingparts such as turbines and without producing CO2 gas, radioactivesubstances, or other emissions [1]. Wide attention has beenfocused on the exploration of thermoelectric materials recently.Good thermoelectric materials require a large thermopower (S) forgenerating a large thermal voltage, a low electrical resistivity (r)for minimizing the Joule heating, and a low thermal conductivity(k) for retaining the heat at the junctions in order to obtain a highfigure of merit ZT = S2T/rk [2]. Besides, thermoelectric materialsare required to be stable at high temperatures. In recent years,layered cobalt oxides have gained great attention since NaCo2O4

single crystal is found to exhibit good thermoelectric properties[3]. The misfit cobalt oxides (Ca3Co4O9 + d) have been investigatedextensively as potential thermoelectric material because it haslarge S, low r, and low k [4–9]. The crystal structure of Ca3Co4O9 + d

system consists of two subsystems, viz., the distorted NaCl-type(Ca2CoO3) sublattice and the CdI2-type (CoO2) sublattice, alterna-tively stacking along the c-axis [10]. The polycrystalline bulkCa3Co4O9 + d samples are still at a relatively low level for industrial

* Corresponding author. Tel.: +886 4 723 2105x3337; fax: +886 4 728 0698.

E-mail address: [email protected] (C.-J. Liu).

0025-5408/$ – see front matter � 2013 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.materresbull.2013.07.004

applications. Many attempts have been made to optimize thethermoelectric performance of Ca3Co4O9 + d by either partiallysubstituting cations or using appropriate fabrication methods suchas hot pressing (HP) or spark plasma sintering (SPS) techniques.Partial replacement of cations in the Ca3Co4O9 + d has been carriedout on either the Ca site [11–22] or the Co sites [5,8,23–26]. Manygroups have attempted to prepare Ca3Co4–xCuxO9 + d system withlower concentration (x = 0.00, 0.05, 0.10) of dopants [21,24], andwith higher concentration (x � 0.2) of dopants [27–29], reportingremarkable changes in the thermoelectric properties. In this paper,we report the low-temperature (<300 K) thermoelectric andmagnetic properties of Ca3Co4–xCuxO9 + d (x = 0.00, 0.05, 0.07,0.10 and 0.15) samples.

2. Experimental

Polycrystalline samples of Ca3Co4–xCuxO9 + d with x = 0.00, 0.05,0.07, 0.10 and 0.15 were synthesized by conventional solid statereaction from CaCO3, Co3O4, and CuO powders. The powders wereheated at 900 8C for 24 h with intermediate grinding. The resultingpowders were then pressed into parallelepiped and sintered in airat 900 8C for 24 h. The phase purity of resulting powders wasexamined by a Shimadzu XRD-6000 powder X-ray diffractometerequipped with Fe Ka radiation. The electrical resistance measure-ments were carried out using standard four-probe techniques. Thethermopower measurements were performed between 80 K and300 K using steady-state techniques with a temperature gradientof 0.5–1 K across the sample. A type E differential thermocouple

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Fig. 1. XRD patterns of Ca3Co4–xCuxO9 + d (x = 0.00, 0.05, 0.07, 0.10 and 0.15)

samples.

Table 2Carrier concentration and mobility of Ca3Co4–xCuxO9 + d (x = 0.00, 0.05, 0.07, 0.10.

and 0.15) samples.

x Carrier concentration (1020 cm�3) Mobility (cm2/Vs)

0.00 1.88 1.88

0.05 2.56 1.56

0.07 2.78 1.49

0.10 2.99 1.46

0.15 3.11 1.70

A. Bhaskar et al. / Materials Research Bulletin 48 (2013) 4884–4888 4885

was used to measure the temperature difference between hot andcold ends of sample, which was measured using a Keithley 2000multimeter [30]. The thermopower of the sample was obtained bysubtracting the thermopower of Cu Seebeck probe. The carrierconcentration and mobility were determined by Hall measure-ments using the van der Pauw method under an applied field of0.55 T (ECOPIA: HMS-3000) [31]. The thermal conductivitymeasurements were carried out using transient plane sourcetechniques with very small temperature perturbations of samplematerial using a hot disk thermal constants analyzer. The transientplane source technique makes use of a thin sensor element in theshape of a double spiral. The hot disk sensor acts both as a heatsource for generating temperature gradient in the sample and aresistance thermometer for recording the time dependenttemperature increase. The encapsulated sensor was sandwichedbetween two pieces of samples. During a preset time, 200resistance recordings were taken and from these a relationbetween temperature and time was established [31]. A commercialsuperconducting quantum interference device magnetometer(quantum design) was used to characterize the magnetic proper-ties of samples. The oxygen content and valence state of cobaltwere determined using iodometric titration [32].

3. Results and discussion

Fig. 1a shows the XRD patterns of Ca3Co4–xCuxO9 + d (x = 0.00,0.05, 0.07, 0.10 and 0.15) samples. The XRD patterns reveal that allthe samples are single phase, and no secondary phase is detected.The diffraction peaks are matched with earlier reports of Ca3Co4–

xCuxO9 + d system [24,27–29]. The crystal structure of Ca3Co4O9 + d

consists of two subsystems; these are triple-layered NaCl-typerocksalt Ca2CoO3 block (subsystem 1) and a CdI2-type hexagonalCoO2 layer (subsystem 2) [10]. Therefore, the Cu ion may occupyeither Ca2CoO3 subsystem or CoO2 subsystem. The Co cation in the

Table 1Room temperature characterization and properties of Ca3Co4–xCuxO9 + d (x = 0.00, 0.05,

x Cov+ d r (V-cm) TMI (K) T* (K) S (mV/K) k

0.00 3.168 0.336 0.0176 89 235 130 0

0.05 3.210 0.390 0.0150 86 228 140 0

0.07 3.212 0.390 0.0149 82 224 114 1

0.10 3.249 0.436 0.0142 79 222 125 1

0.15 3.261 0.428 0.0117 70 227 139 0

CoO2 layer are the mixture of Co3+ and Co4+ and their ionic radii insix-coordination are 0.54 A and 0.53 A, respectively, whereas theCo cation in the rocksalt structure (Ca2CoO3) is Co2+ with the ionicradius in six-coordination of 0.74 A [29], while the ionic radius ofCu2+ is 0.73 A and Cu3+ is 0.54 A.

Table 1 summarizes the characterization and properties ofCa3Co4–xCuxO9 + d (x = 0.00, 0.05, 0.07, 0.10, and 0.15) samples. Theundoped sample shows the highest resistivity (0.0176 V-cm at300 K) among the samples. The size of r for all the doped samples isin the range of 0.0150 V-cm to 0.0117 V-cm; decreasing withincreasing Cu content due to an increase in charge carrierconcentration (Table 2). The substitution of Cu ion creates holecarriers into the system because the valence state of Cu ion is lowerthan the average valence state of Co ion (Table 1). The averagevalence state of Co ion is between 3+ and 4+, while the Cu ions: Cu2+

or Cu3+. The Hall-effect measurements reveal that hole carrierconcentration increases with increasing Cu content, as shown inTable 2. Similar results were also reported for Ca3Co4–xCuxO9 + d

system [24,29,33]. On the other hand, the excess of oxygen contentalso creates the hole carriers into the system. Karppinen et al. [34]have reported that the electrical resistivity is affected by the excessof oxygen content for calcium cobalt oxide ((CoCa2O3)qCoO2)system. Iodometric titration results show that the excess of oxygencontent increases with increasing Cu content, expect the one withx = 0.15, indicates an increase in hole carrier concentration andhence reducing the resistivity of samples. Karppinen et al. [34]have reported that the resistivity of samples decreases withincreasing excess of oxygen content for Ca3Co3.95O9 + d (d = 0.00,0.24 and 0.29) system. Therefore, these results suggest that theexcess of oxygen content and Cu content contribute to decreasingthe resistivity of samples. According to the previous reports [35–37] the doping at Co-site in CoO2 layers can bring notably variationof electronic correlation in the system, because the carriertransport mainly occurs in the CoO2 layers and these layers playa crucial role in determining the electronic structure of system asrevealed by the band calculation.

Fig. 2 shows the temperature dependence of resistivity forCa3Co4–xCuxO9+d (x = 0.00, 0.05, 0.07, 0.10, and 0.15) samples. Forall the samples, the electrical resistivity decreases with increasingtemperature, a typical characteristic of nonmetallic-like tempera-ture dependence (dr/dT <0), then increases with increasingtemperature, a typical characteristic of metallic-like temperaturedependence (dr/dT >0). The electrical resistivity exhibits thenonmetallic to metallic transition (TIM) occurs at below 90 K for allthe samples, which is similar to other elements doped in the

0.07, 0.10 and 0.15) samples.

total (W/mK) kcar (W/mK) kph (W/mK) PF (mW/cm-K2) ZT

.73 0.04 0.69 0.96 0.038

.79 0.05 0.74 1.30 0.047

.17 0.05 1.12 0.87 0.021

.09 0.05 1.04 1.10 0.029

.99 0.06 0.93 1.65 0.050

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Fig. 2. The temperature dependence of electrical resistivity for Ca3Co4–xCuxO9 + d

(x = 0.00, 0.05, 0.07, 0.10 and 0.15) samples.

A. Bhaskar et al. / Materials Research Bulletin 48 (2013) 4884–48884886

Ca3Co4O9 + d system [5,7]. This may be due to the incommensuratespin-density-wave (IC-SDW) [35,38,39]. Similar results was alsoobserved by Huang et al. [29]. Sugiyama et al. [35,38,39] havereported that Ca3Co4O9 system exhibits the various magneticstates. These magnetic states: ferromagnetic insulating state(�19 K), short-range order IC-SDW insulating state (�100 K),and paramagnetic semiconducting state to a paramagneticmetallic state (�380 K). The appearance of IC-SDW localizes thecharge carrier and results in the nonmetallic behavior of systembelow TIM. The broad minimum is observed around TIM in the r–T

curve with the nonmetallic to metallic transition (�90 K), which isconsistent the IC-SDW transition (�100 K). Sugiyama et al.[35,38,39] have reported that there is no significant effect onthe IC-SDW transition and an average valence state of Co ion doesnot change for Sr2+ doped in the Ca3–xSrxCo4O9 system. In addition,they have found that the IC-SDW transition is increased for Y3+ orBi3+ doped in the Ca3–xMxCo4O9 system (M = Y or Bi), and Y3+ or Bi3+

doping decreases the average valence of Co ion. These resultssuggest that the IC-SDW transition associate with the averagevalence of Co ion. In our case, the nonmetallic to metallic transition

Fig. 3. Variation of r versus T2 for Ca3Co4–xCuxO9 + d (x = 0.00, 0.05, 0.07, 0.10 and

0.15) samples. The solid lines are linear fitting using r = r0 + AT2. TIM: transition

temperature of nonmetallic to metallic, T*: strongly correlated Fermi-liquid regime

up to the temperature.

temperature (TIM) decreases with increasing average valence of Coion for all the samples.

In general, the transport behavior of a Fermi-liquid system canbe expressed by the following equation:

r ¼ r0 þ AT2 (1)

where r0 is the residual resistivity owing to the domain boundariesand other temperature-independent scattering mechanisms[29,40–43], and A is the Fermi-liquid transport coefficient [40–43]. Limelette et al. [40–43] have reported that Ca3Co4O9 systemexhibits two resistivity characteristic temperatures (TIM, T*)between 5 K and 300 K, where TIM is the nonmetallic to metallictransition and T* is the transition temperature from Fermi-liquidmetal to incoherent metal. The curves are fitted using Eq. (1) atabove TIM in Fig. 3, and T* is presented in Table 1. The transitiontemperature from Fermi-liquid metal to incoherent metal (T*) isobtained at the end temperature of linear dependence in metallicrange. Table 1 shows the T* decreases with increasing Cu contentup to x = 0.10, which is similar to the previous report [29].However, the T* slight increases for the x = 0.15. According to thedynamical mean field theory [29], a key role of effective mass m* ofa Fermi liquid is predicated as T* � 1/m*. The decrease of T*

indicates an increase in m* with increasing Cu content due to adecrease in bandwidth and enhance the electronic correlation inthese system.

The temperature dependence of resistivity behavior graduallyvaries with Cu content. It is well known that nonmetallic behavioris obtained for Ca3Co4–xCuxO9 + d at low temperature range, whichobey the variable range hopping (VRH) theory [44,45],

r ¼ r00expðT0=TÞ1=4 (2)

where r00 is weakly temperature dependent, and T = 24/[pkBN(eF)lv

3] is the VRH characteristic temperature associatedwith the density of localized states at Fermi energy N(eF), kB isBoltzman constant, and lv is localization length. By the fitting ofexperimental data using Eq. (2), the Fig. 2 displays nonmetallic-likebehavior. The plots of ln versus T�1/4 for all the samples lie onstraight lines in the nonmetallic-like behavior, as shown in Fig. 4.This behavior might be associated with the positional disorderinvolved in the incommensurate structure of the misfit layeredtitle system.

The positive thermopower confirms that dominant chargecarriers are holes for all the samples. The undoped sample exhibitsa larger absolute S values 130 mV/K�1 at 300 K. Lin et al. [47,48],

Fig. 4. Plot of In r versus T�1/4(K�1/4) for Ca3Co4–xCuxO9 + d (x = 0.00, 0.05, 0.07, 0.10

and 0.15) samples.

Page 4: Thermoelectric and magnetic properties of Ca3Co4-xCuxO9+ δ with x = 0.00, 0.05, 0.07, 0.10 and 0.15

Fig. 5. The temperature dependence of thermopower (S) for Ca3Co4–xCuxO9 + d (x =

0.00, 0.05, 0.07, 0.10 and 0.15) samples.

A. Bhaskar et al. / Materials Research Bulletin 48 (2013) 4884–4888 4887

Chen et al. [4], and Nong et al. [8,9] have also reported that theundoped (Ca3Co4O9 + d) sample exhibits a large room-temperaturethermopower of 132 mV/K at 300 K. The Hall-effect measurementsconfirm that the majority carriers are p-type, which is consistentwith the thermopower data. The thermopower is also affect by theexcess of oxygen content. The excess of oxygen content decreases,expect the one with x = 0.15, with increasing Cu content, indicatingthat the hole carrier concentration decreases and thermopowerincreases. Karppinen et al. [34] have reported that the thermo-power slightly decreases with increasing excess of oxygen contentfor Ca3Co3.95O9 + d (d = 0.07, 0.24 and 0.29) system. Fig. 5 shows thetemperature dependence of thermopower (S) for Ca3Co4–xCuxO9 + d

(x = 0.00, 0.05, 0.07, 0.10 and 0.15) samples. It can observe that allthe curves of thermopower are similar, but the absolute thermo-power values are different.

In general the thermoelectric power can be expressed by theMott formula [35,37,45].

SðTÞ ¼ 1

eT

R1�1 sðeÞðe � mÞ @ f ðeÞ

@e deR1�1 sðeÞ @ f ðeÞ

@e de(3)

where s(e) and f(e) represent electrical conductivity and Fermi–Dirac distribution function at energy e. The product of thethermopower coefficient and temperature can therefore beunderstood as the mean energy flow carried by a conductionelectron. Using the condition of @f(e)/@e = d (e�EF), and s = nem(e)

Fig. 6. The temperature dependence of power factor for Ca3Co4–xCuxO9 + d (x = 0.00,

0.05, 0.07, 0.10 and 0.15) samples.

[34,36] Eq. (3) can be written as:

SðTÞ ¼ Ce

nþ p2k2

BT

3e

@lnmðeÞ@e

� �e¼EF

(4)

where m(e), Ce, and kB are energy correlated carrier mobility,electronic specific heat, and Boltzmann constant, respectively. If S

is inversely proportional to the n, it is usually interpret as thepredominance of first term in Eq. (4). If the second term in Eq. (4)dominant then S closely related to the electronic correlation[28,29]. The thermopower values of doped samples are random;this may be related to the Cu occupying at different subsystem(Ca2CoO3 or CoO2). Huang et al. [29] have reported that thethermopower of doped samples does not show a monotonic trenddue to Cu occupying at different subsystem (Ca2CoO3 or CoO2) forCa3Co4–xCuxO9 + d single crystals. Furthermore, they have foundthat the electronic correlation plays a key role in determining thethermopower. Therefore, further studies are required (ex: specificheat) to know which term is dominant in our case.

Fig. 6 shows the temperature dependence of power factor (S2s)for Ca3Co4–xCuxO9 + d (x = 0.00, 0.05, 0.07, 0.10 and 0.15) samples. Itcan be seen that the power factor initially rises, reaches amaximum value (<141 K) and then continuously falls withincreasing temperature for all the samples. The highest value ofS2s = 2.17 mW cm�1 K�2 at 141 K for Ca3Co3.85Cu0.15O9 + d sampleas a result of its low r value combined with its moderate S value.This represents a 64.4% increase when compared to undopedsample at 141 K. It should be noted that higher thermoelectricpower factor leads to higher efficiency of the thermoelectricgenerator.

The thermal conductivity is measured at room temperature andvalues are presented in Table 1. Total thermal conductivity (ktotal)can be expressed as ktotal = kcar + kph, where kcar and kph representthe carrier and the lattice thermal conductivity, respectively. kcar

can be calculated using the Wiedemann–Franz–Lorenz relation-ship, kcar = LsT, where L = p2k2/3e2 = 2.45 � 10�8 W V K�2 is theLorenz number and T is the absolute temperature. kph is obtainedby subtracting kcar from ktotal. It can be clearly seen from Table 1that the total thermal conductivity slightly increases, thendecreases with increasing Cu content; this may be related to thestructure distortion [29]. For materials with r > 1 V-cm, kcar isnegligible. But in our case, the resistivity is lower than 1 V-cm, afact which leads us to determine the kcar using the Wiedemann–Franz law. The calculated value of kcar is 0.04 and 0.06 W m�1 K�1

at 300 K for Ca3Co4O9 + d and Ca3Co3.85Cu0.15O9 + d, respectively. For

Fig. 7. The temperature dependence of magnetic susceptibility for

Ca3Co3.93Cu0.07O9 + din an applied field of 50,000 Oe. The solid line is a fit to the

Curie–Weiss law.

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A. Bhaskar et al. / Materials Research Bulletin 48 (2013) 4884–48884888

all the samples, the lattice contribution is more important than thecarrier one. Due to the small kcar, ktotal is mainly attributed to thelattice contribution. The figure of merit (ZT = S2T/rk) is calculatedfor all the samples. The calculated values are tabulated in Table 1.Among the samples, Ca3Co3.85Cu0.15O9 + d has the highest dimen-sionless figure of merit (ZT) of 0.050 at 300 K. This value representsan improvement of about 31% compared to undoped sample. Theseresults suggest that there is scope for further improvement ofthermoelectric properties.

Fig. 7 shows the temperature dependence of magneticproperties for Ca3Co3.93Cu0.07O9 + d sample. The observed effectivemagnetic moment is derived by fitting the magnetization versustemperature using the Curie–Weiss law. The observed effectivemagnetic moments are 1.37 mB/Co for x = 0.00, 1.36 mB/Co forx = 0.03, 1.30 mB/Co for x = 0.05, and 1.11 mB/Co for x = 0.07,respectively. The observed effective magnetic moments decreasewith increasing Cu content. These results suggest that theferromagnetic is suppressed by the Cu content. According to theprevious reports [35,38,39], the ferromagnetic of Ca3Co4O9 isoriginated by the interlayer coupling between CoO2 and Ca2CoO3

sublayers. The Ca2CoO3 layer consists of two Ca-O planes and oneCo–O plane, where the Co–O plane is sandwiched by the two Ca–Oplanes, and the Ca–O planes are located between Co–O plane andCaO2 sublayers [10]. The Cu ions may disturb the interlayercoupling between CoO2 and Ca2CoO3 sublayers, which woulddecrease the magnetic moments. The observed effective magneticmoment of undoped is good agreement with the earlier report [48].Zhao et al. [48] have reported that the undoped sample exhibits theobserved effective magnetic moment is 1.3 mB/Co. The Co3+ andCo4+ ions with a low-spin configuration have a theoretical effectivemagnetic moment of 0 mB/Co and 1.73 mB/Co, respectively. TheCo3+ and Co4+ ions with a high-spin configuration have atheoretical effective magnetic moment of 4.89 mB/Co and5.91 mB/Co, respectively. The observed effective magnetic momentis close to the low-spin configuration of cobalt ion. Chen et al. [4]and Liu et al. [6] have also observed the low-spin state of cobaltions at low temperature range (5 K–300 K) for Ca3Co4–xMxO9 + d

(0.00–0.15) system.

4. Conclusions

The thermoelectric and magnetic properties of Ca3Co4–

xCuxO9 + d (x = 0.00, 0.05, 0.07, 0.10, and 0.15) have beeninvestigated systematically. The XRD patterns show that all thesamples are single phase. The positive thermopower confirms thatthe dominant carriers are holes for all the samples. A maximumpower factor of 2.17 mW cm�1 K�2 is reached at 141 K forCa3Co3.85Cu0.15O9 + d, representing an improvement of about64.4% compared to undoped sample. The highest figure of merit(0.050) is obtained for Ca3Co3.85Cu0.15O9 + d as compared toundoped sample. Magnetization measurements show a low-spinstate of cobalt ion for all the samples. The observed effectivemagnetic moments decrease with increasing Cu content.

Acknowledgement

This work was supported by National Science Council ofRepublic of China, Taiwan under the Grant No. 101-2112-M-018-003-MY3. Ankam Bhaskar would like to express thanks to thepostdoctoral fellowship sponsored by NSC of Taiwan.

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