Effect of Mixed Grain Sizes on the Thermoelectric Propertiesof Ca0.9Yb0.1MnO3

TSUBASA FUKUI,1 MIE MATSUZAWA,1 RYOJI FUNAHASHI,2,3

and ATSUKO KOSUGA1,4

1.—Nanoscience and Nanotechnology Research Center, Research Organization for the 21st Cen-tury, Osaka Prefecture University, Sakai 599-8570, Japan. 2.—Research Institute for UbiquitousEnergy Devices, National Institute of Advanced Industrial Science & Technology, Ikeda 563-8577,Osaka, Japan. 3.—CREST, Japan Science and Technology Agency, Chiyoda, Tokyo 102-0075,Japan. 4.—e-mail: a-kosuga@21c.osakafu-u.ac.jp

Ca0.9Yb0.1MnO3 has been identified as a material that might be suitable forthermoelectric applications. We fabricated micro/nanograined Ca0.9Yb0.1MnO3

composites, with the aim of controlling the passage of electrons and phononssimultaneously. Micro/nanograined Ca0.9Yb0.1MnO3 composites containingvarious fractions of nanosized powder were prepared by sintering mixtures ofmicroparticulate and nanoparticulate Ca0.9Yb0.1MnO3, obtained by solid-statereaction and by gas-phase reaction, respectively. The electrical resistivityincreased markedly when the weight fraction of nanosized powder exceeded50%, probably as a result of a percolation phenomenon. However, the thermalconductivity was considerably reduced when the weight fraction of nanosizedpowder exceeded 25%, but then remained almost constant. The absolute val-ues of the Seebeck coefficient of micro/nanograined Ca0.9Yb0.1MnO3 compos-ites were larger than those of monolithic micro- or nanograin Ca0.9Yb0.1MnO3,probably as a result of the effects of potential-barrier scattering. The highestdimensionless figure of merit ZT value of 0.09 at 973 K was achieved with asample containing 50% nanosized powder, and this value is 10% larger thanthat of monolithic micrograined Ca0.9Yb0.1MnO3.

Key words: Thermoelectric effect, calcium manganate, grain boundary,electrical property, thermal conductivity

INTRODUCTION

Thermoelectric (TE) energy conversion can beused to generate electricity from waste heat. Theefficiency of a TE material in this process is deter-mined by the dimensionless TE figure of merit,ZT = S2rT/j, where S is the Seebeck coefficient, r isthe electrical conductivity, T is the absolute tem-perature, and j is the thermal conductivity.1,2 Thelattice contribution jlat can be obtained by sub-tracting the electronic contribution jel from j byusing the Wiedemann–Franz relationship. Therealization of widespread large-scale use of TEtechnology is dependent on the development of bulk

TE materials that have low cost, are environmen-tally friendly, and are stable at high temperaturesin air. TE bulk oxides have been the subject ofconsiderable interest because they fulfill theserequirements.3–7

Calcium manganese perovskites have attractedmuch attention because of their interesting electri-cal and magnetic properties,8,9 which result fromtheir flexible crystal structures. Perovskite com-pounds have the general formula ABO3, where A isa rare-earth or alkaline-earth metal and B is atransition metal; A and B can be selected from awide variety of elements. The TE properties ofsamples of CaMnO3 substituted at Ca and/or Mnsites have been widely investigated.10–14 Generally,the valence state of Mn in CaMnO3 is 4+. On thebasis of valence equilibrium, substitution of RE3+

(Received June 28, 2013; accepted September 7, 2013)

Journal of ELECTRONIC MATERIALS

DOI: 10.1007/s11664-013-2778-6� 2013 TMS

(RE = rare-earth element) for Ca2+ or the presenceof an oxygen deficiency in the CaMnO3 systeminduces the creation of Mn3+ cations in the Mn4+

matrix, resulting in a low electrical resistivity (q),suitable for TE applications.11 Among previouslyevaluated calcium manganites prepared by con-ventional solid-state reactions, the highest ZTmax

value of �0.2 at 1000 K in air was found forCa0.9Yb0.1MnO3.11,12 We have tried to improve theTE and mechanical properties of Ca0.9Yb0.1MnO3 invarious ways.15–17

One general way which is often used to improveZT values is to reduce the value of j by grain-boundary scattering through refinement of grainsizes.18 However, refining the grain size leads to anincrease in q, which is not favorable for enhancingthe ZT values.19 To achieve a compromise betweenthese two effects of grain size, a high-performanceTE material needs to contain an optimal combi-nation of coarse and fine grains. We thereforehypothesized that micro/nanograined composite TEmaterials might show better TE performance thanmonolithic micro- or nanograined samples. Somestudies to confirm this hypothesis have already beenperformed on Bi2Te3 and Mg2Si compounds.20,21 Inthe present work, we fabricated micro/nanograinedCa0.9Yb0.1MnO3 composites and evaluated their TEproperties in the temperature range between 300 Kand 973 K.

EXPERIMENTAL PROCEDURES

Ca0.9Yb0.1MnO3 micro- and nanoparticles weresynthesized by means of a conventional solid-statereaction and a gas-phase reaction, respectively, asdescribed in our previous studies.16,17 Mixtures ofmicroparticles and nanoparticles were preparedwith weight fractions of 0:100, 25:75, 50:50, 75:25,and 100:0 by ball milling. These mixtures werenamed 100N, 75N, 50N, 25N, and 0N, respectively.The mixed powders were then pressed uniaxiallyinto pellets at pressure of 40 MPa, sintered in air at1343 K to 1633 K for 15 h, and slowly cooled toroom temperature at about 1 K/min. The sinteringtemperature was adjusted to obtain comparablerelative densities for the prepared samples. Thecrystal structure was analyzed by powder x-raydiffraction (XRD, SmartLab; Rigaku, Japan) atroom temperature with Cu Ka radiation. Themicrostructure of the samples was examined indetail by field-emission scanning electron micros-copy (FE-SEM, S-4500; Hitachi, Japan). The aver-age grain size of the samples was determined bychecking a set of 50 randomly selected grains. Thedensity (d) was measured by the standard Archi-medes method. The values of q and S were mea-sured by a standard four-probe method using acommercially available apparatus (ZEM-3; ULVACTechnologies, Japan) in the temperature rangebetween 300 K and 973 K in helium atmosphere.The value of j was also evaluated between 300 K

and 973 K as the product of the thermal diffusivitya, the heat capacity Cp, and the density (d) of pre-pared samples. The values of a and Cp were mea-sured simultaneously in vacuum by a laser-flashmethod and a differential laser-flash calorimeter,respectively,22 using commercially available appa-ratus (Flash TE; Pico Therm, Japan).

RESULTS AND DISCUSSION

FE-SEM observations showed that the averagesize of the Ca0.9Yb0.1MnO3 microparticles was about5 lm. The average particle size of the sample wasdetermined by checking a set of 50 randomlyselected particles. The nanoparticles were too smallto be observed by FE-SEM. In our previous study,17

the average size of the Ca0.9Yb0.1MnO3 nanoparti-cles estimated from the Brunauer–Emmett–Teller(BET) surface area and the theoretical densitywas in good agreement with the equivalent parti-cle diameter of transmission electron microscopy(TEM) observations. Therefore, we estimated theaverage size of nanoparticles in the same way. Theestimated size of the nanoparticles, calculated fromthe BET surface area (19.7 m2 g�1) and the theo-retical density (4.98 g cm�3) of the material, was62 nm. The XRD peaks of both the as-preparedmicro- and nanoparticulate Ca0.9Yb0.1MnO3 couldbe indexed almost completely to an orthorhombicCaMnO3 structure,23 although there were smallimpurity peaks corresponding to Ca2MnO4 andCaCO3 in the micro- and nanopowder, respectively,as shown in Fig. 1a–c. The Ca0.9Yb0.1MnO3 micro-and nanoparticles therefore consisted of almost asingle phase of orthorhombic Ca0.9Yb0.1MnO3. TheCa0.9Yb0.1MnO3 nanoparticles exhibited a broadermain peak than the corresponding microparticles,which is attributed to the smaller particle sizes ofthe nanoparticles.

Figure 2 shows the relative density of the micro/nanograined Ca0.9Yb0.1MnO3 composites (0N, 25N,50N, 75N, and 100N) as a function of sinteringtemperature. In this study, a sintering temperatureof 1633 K was required to convert monolithic mi-croparticulate Ca0.9Yb0.1MnO3 into a dense bulkmaterial with about 74% of the theoretical density(%TD), whereas sintering of Ca0.9Yb0.1MnO3 com-posites with higher contents of nanopowder pro-ceeded rapidly at lower temperatures; For instance,bulk material with relative density of approxi-mately 74%TD could be obtained from the mono-lithic nanopowder by sintering at 1343 K, whichis about 300 K lower than that required forthe monolithic micrograined Ca0.9Yb0.1MnO3. Onadjusting the sintering temperature, the relativedensity varied only within the range from 73%TDto 76%TD, independently of the ratio of the Ca0.9

Yb0.1MnO3 micro- and nanoparticles. Therefore, theeffects of pores on the transport properties of thecomposite materials can be ignored in all cases.

Fukui, Matsuzawa, Funahashi, and Kosuga

FE-SEM images of the surfaces of several sin-tered Ca0.9Yb0.1MnO3 composites (0N, 50N, and100N) are shown in Fig. 3a–c. The 100N sampleconsisted of small grains with average diameter of620 nm. In contrast, larger grains with diameters of5 lm to 20 lm were bonded together to form anetwork-like structure in the 0N sample. Themicrostructure of the 50N sample shows both anetwork-like structure of large grains and a gran-ular structure of smaller grains. Thus, differences inthe particle size before sintering led to marked dif-ferences in the morphology of the composite andcould therefore be expected to affect the TE prop-erties of the material.

The temperature dependence of q for each of theCa0.9Yb0.1MnO3 composites (0N, 25N, 50N, 75N,and 100N) is shown in Fig. 4. In electron-dopedmanganites, q exhibits n-type semiconductingbehavior below room temperature, and the trans-port properties can be well interpreted in theframework of variable-range hopping of elec-trons.24–26 At high temperatures, a metal–insulatortransition (MIT) occurs as a result of a change in thespin state of Mn3+, and as a result, the value of qincreases with increasing temperature. The valuesof q for the 0N, 25N, and 50N samples were in therange from 3 mX cm to 15 mX cm over the entirerange of temperatures that we examined, and theyincreased with increasing temperature. Thismetallic behavior is a typical behavior above theMIT temperature, which in the case of the 0N, 25N,and 50N samples might be below room temperature.On the other hand, the value of q for the 75N and100N samples showed semiconducting behavior,decreasing with increasing temperature below500 K to 600 K, but increasing with increasingtemperature at above 500 K to 600 K. In otherwords, at the point where the proportion of nano-particles exceeded 50 wt.%, a difference occurred inthe electrical resistivity behavior. One possibleexplanation for the difference in the MIT that weobserved in this study is that the increased quantityof grain boundaries in the 75N and 100N samplesenhances the insulating property.27,28 In this case,the MIT temperatures are expected to move tohigh temperatures for samples containing greater

Fig. 1. XRD patterns of (a) microparticulate and (b) nanoparticulate Ca0.9Yb0.1MnO3; (c) literature data.31

1300 1400 1500 1600 170072

73

74

75

76

77

78

0 N25 N50 N

75 N

Rel

ativ

e d

ensi

ty (

%)

Sintering temperature (K)

XRD Density: 4.98 g/cm3

100 N

Fig. 2. Relative density of micro/nanograined Ca0.9Yb0.1MnO3

composites (0N, 25N, 50N, 75N, and 100N) as a function of sinteringtemperature.

Effect of Mixed Grain Sizes on the Thermoelectric Properties of Ca0.9Yb0.1MnO3

numbers of small particles. The reason for the dif-ference between the 50N and 75N samples is proba-bly the result of a percolation phenomenon in whichthe electrical resistivity of a composite consisting ofan insulating matrix phase and a conducting dis-persed phase is reduced as the content of the dis-persed phase approaches the percolation threshold fc.A previous study reported29 that the value of fc isrelated to the diameters of the matrix insulatingphase R1 and dispersed metallic phase R2, and thecorresponding relationship can be described asfc = 0.64, when R1 > R2. This relationship suggeststhat, in the current study, the electrical resistivityshould be reduced when the fraction of microparticlesis more than 64%. This agrees well with the resultthat we obtained, in that the effects of the nanopar-ticles became apparent on increasing the fraction ofnanoparticles from 50% to 75%.

The temperature dependence of the value of S forCa0.9Yb0.1MnO3 composites (0N, 25N, 50N, 75N,and 100N) is shown in Fig. 5. The S values for allthe samples were negative, showing that the pre-dominant carriers are electrons over the entiretemperature range that we examined. The actualvalues varied from �70 lV K�1 to �130 lV K�1

within the temperature range examined. The abso-lute value of S for the 100N sample is slightly lowerthan that of the 0N sample, probably as a result of asubtle difference in carrier concentration, attributedto the existence of impurity phases and to a smalldifference in composition. If this difference betweenthe 0N and 100N samples arises exclusively fromthe effect of the carrier concentration, the S valuesfor other samples would be expected to fall betweenthe values for the 0N and 100N samples, becausethe other samples are mixtures of these two. The Svalues for the 75N, 50N, and 25N samples diddecrease in this expected order, but they wereshifted to values that were higher than expected.This fact suggests that there might be some phe-nomenon that enhances the S values besides theeffect of carrier concentration, for example, poten-tial-barrier scattering.30 The highest value of S2/qat 973 K (0.20 mW m�1 K�2) was obtained with the0N sample, mainly as a result of its low q value andits moderate S value, as shown in Fig. 6.

The temperature dependence of j of Ca0.9Yb0.1

MnO3 composites (0N, 25N, 50N, 75N, and 100N) is

shown in Fig. 7. The j values of all the samples werein the range from 1.5 W m�1 K�1 to 3.1 W m�1 K�1

over the entire temperature range examined, andthey decreased with increasing nanoparticle content,probably as a result of increased phonon scattering.To examine the values of j for our samples in moredetail, we estimated jlat and jel. jel accounts for 10%to 20% of jtotal for all the samples, so the lattice con-tribution is dominant in this system.

Figure 8 shows the normalized TE properties at973 K, comparing the TE properties of the 25N,50N, 75N, and 100N samples with those of the 0N

Fig. 3. Selected FE-SEM images of micro/nanograined Ca0.9Yb0.1MnO3 composites (a) 0N, (b) 50N, and (c) 100N. Microparticles and nano-particles are indicated by rectangular and circular lines, respectively.

300 400 500 600 700 800 900 10000

5

10

15

20

25

30

35

100N

25N75N

50N

Ele

ctri

cal r

esis

tivi

ty, ρ

(mΩ

cm)

Temperature, T (K)

0N

Fig. 4. Temperature dependence of the electrical resistivity of micro/nanograined Ca0.9Yb0.1MnO3 composites.

300 400 500 600 700 800 900 1000

-150

-140

-130

-120

-110

-100

-90

-80

-70

25N

50N 0N

75N

See

bec

k co

effi

cien

t, S

(μV

K-1)

Temperature, T(K)

100N

Fig. 5. Temperature dependence of the Seebeck coefficient ofmicro/nanograined Ca0.9Yb0.1MnO3 composites.

Fukui, Matsuzawa, Funahashi, and Kosuga

sample. Compared with the other properties, thevalue of q increased markedly with increasingfraction of nanoparticles. Additionally, the increasein q was larger than the decrease in j, implying thatelectrons were scattered more than phonons withincreasing fraction of nanoparticles. As shown in

Fig. 9, the maximum ZT value of 0.09 at 973 K wasobtained for the 50N sample, probably as a result ofa combination of relatively low values of q and jwith a high value of S. This ZT value is 10% higherthan that of monolithic Ca0.9Yb0.1MnO3.

CONCLUSIONS

Micro/nanograined Ca0.9Yb0.1MnO3 compositescontaining various fractions of nanosized powderwere prepared by sintering appropriate mixtures ofmicroparticles of Ca0.9Yb0.1MnO3 powder withaverage size of about 5 lm with nanoparticles of thesame material with average size of about 62 nm. Byadjusting the sintering temperature, the relativedensity could be varied within a range of 73% to76% of the theoretical XRD density, independentlyof the ratio of the micro- and nanoparticulateCa0.9Yb0.1MnO3 in the mixture. After sintering, the100N sample consisted of small grains with averagediameter of 620 nm. In contrast, larger grains withdiameters of 5 lm to 20 lm were bound together toform a network-like structure in the 0N sample. Themicrostructure of the 50N sample combined fea-tures of both the 100N and the 0N samples, such asa network-like structure of large grains and gran-ule-like smaller grains. The increase in q was largerthan the decrease in j, implying that electrons werescattered more than phonons with increasing frac-tion of nanoparticles. The absolute values of S forthe micro/nanograined Ca0.9Yb0.1MnO3 compositeswere larger than those of monolithic micro- ornanograined Ca0.9Yb0.1MnO3, probably as a resultof the effects of potential-barrier scattering. Themaximum ZT value of 0.09 at 973 K was obtainedfor the 50N sample, probably because of the rela-tively low values of q and j combined with the highvalue of S. This ZT value is 10% higher than that ofmonolithic Ca0.9Yb0.1MnO3.

ACKNOWLEDGEMENTS

This work was supported in part by the KansaiResearch Foundation for Technology Promotion.

300 400 500 600 700 800 900 10000.00

0.05

0.10

0.15

0.20

0.25

100N

75N

25N

50N

Po

wer

fac

tor,

S2 ρ-1

(mW

m-1K

-2)

Temperature, T (K)

0N

Fig. 6. Temperature dependence of the power factor of micro/nanograined Ca0.9Yb0.1MnO3 composites.

300 400 500 600 700 800 900 1000

1.61.82.02.22.42.62.83.03.23.4

100N

75N

50N

25N

Th

erm

al c

on

du

ctiv

ity,

κ(W

m-1K

-2)

Temperature, T(K)

0N

Fig. 7. Temperature dependence of the thermal conductivity ofmicro/nanograined Ca0.9Yb0.1MnO3 composites.

0 20 40 60 80 1000

1

2

3

4

5

6

No

rmal

ized

Th

erm

oel

ectr

ic P

rop

erty

Fraction of Nanopaticles (wt%)

ρSS2/ρκ

Fig. 8. Normalized TE properties at 973 K as a function of fraction ofnanoparticles.

300 400 500 600 700 800 900 10000.00

0.01

0.02

0.03

0.040.05

0.06

0.07

0.08

0.09

0.1050N

0N25N

75N

Dim

ensi

on

less

Fig

ure

of

Mer

it, Z

T

Temperature, T(K)

100N

Fig. 9. Temperature dependence of the dimensionless figure ofmerit of micro/nanograined Ca0.9Yb0.1MnO3 composites.

Effect of Mixed Grain Sizes on the Thermoelectric Properties of Ca0.9Yb0.1MnO3

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Fukui, Matsuzawa, Funahashi, and Kosuga