2016 Synthesis, Characterization and Thermoelectric Properties of Sm Filled

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Journal of Alloys and Compounds 655 (2016) 321e326

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Journal of Alloys and Compounds

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Synthesis, characterization and thermoelectric properties of Sm filledFe4�xNixSb12 skutterudites

Riccardo Carlini a, b, *, Atta Ullah Khan c, Riccardo Ricciardi a, Takao Mori c, d,Gilda Zanicchi a, b

a Chemistry and Industrial Chemistry Department, University of Genoa, Via Dodecaneso 31, Genova, Italyb INSTM e Interuniversitary Consortium of Science and Technology of Materials, UoR Genoa, Italyc International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba 305-0044, Japand Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba 305-8671, Japan

a r t i c l e i n f o

Article history:Received 6 March 2015Received in revised form9 September 2015Accepted 15 September 2015Available online 25 September 2015

Keywords:Filled skutteruditesAdvanced thermoelectric materialsIntermetallic compoundsSynthesis routes

* Corresponding author. Chemistry and Industrialversity of Genoa, Via Dodecaneso 31, Genova, Italy.

E-mail address: [email protected] (R. Carlin

http://dx.doi.org/10.1016/j.jallcom.2015.09.1410925-8388/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

High figure of merit of n-type skutterudites have encouraged the researchers to focus on its p-typecounterpart. In the following study, we report a rather simple synthesis route for Sm filled and Ni-dopedp-type skutterudites (SmyFe4�xNixSb12.8, y ¼ 0.7e0.8, x ¼ 0.4e1.2, nominal composition), along withhigh temperature thermoelectric properties. Sample were prepared by melting, quenching, followed byannealing and the resulted chunks were cut into disks form to measure the thermoelectric propertieswithout applying high pressure sintering techniques. Ni addition helped in optimizing the carrier con-centration and resulted in a final ZT of 0.55 at 560 K.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Today, the general energy shortage leads us to investigatedifferent fields in order to search for new energy sources or energysaving pathways. Thermoelectricity is an attractive technology forgenerating electrical power directly from waste heat. The devicesbased on thermoelectric technology could play an important role inthe future global requirement of energy due to their dual role inpower generation and in refrigeration [1e3]. This thermoelectricenergy approach relating to the fundamental issues of sustain-ability and eco-environmental compatibility can be employed indifferent areas: aerospace applications, automotive field, andnautical freight etc.

The efficiency of a thermoelectric material is related to adimensionless quantity, the figure of merit (ZT)

ZT ¼ S2sk

T

Chemistry Department, Uni-

i).

where T denotes the absolute temperature, S is the Seebeck coef-ficient, s the electrical conductivity and k the total thermal con-ductivity. A good thermoelectric material is characterized by a highSeebeck coefficient combined with a high electrical conductivityand a low thermal conductivity. k is constituted by an electroniccontribution ke, directly related to s via the Wiedemann-Franz law,and a lattice contribution kl: a decrease of kl could lead to adecrease of k without reducing S2s term [4e7].

Numerous intermetallic compounds could be considered asgood candidates for thermoelectric applications; a particular in-terest has been devoted to the transition metal antimonides (Zintlphases, clathrates, skutterudites, etc.) [8e16].

Skutterudites attracted great interest for decades due to theirrelative low-cost and promising thermoelectric performance aspossible candidates for thermoelectric power generation at middletemperature range [17]. Both n- and p-type skutterudites arenecessary in the design of TE modules. N-type skutterudites showhigher ZTs than the p-type ones due to their large effective massesand high power factors. Therefore it is very important to improvethe TE performances of p-type skutterudites above all, in terms ofSeebeck coefficient and thermal conductivity. Typically p-typeskutterudites originate from the binary compound CoSb3 afterpartial/total substitution of Co with other transition metals such as

Fig. 1. Diffraction patterns of samples belonging to the SKUT series compared to theFe2Ni2Sb12 [29].

R. Carlini et al. / Journal of Alloys and Compounds 655 (2016) 321e326322

Fe [18,19]. Rare earth elements are employed as fillers which in-crease the electrical conductivity by donating 2 or 3 electrons peratom with a consequent increase of structure stability, and alsosuppress the thermal conductivity due to rattling. But the limitedfilling fraction in Fe/Co compounds cannot effectively lower thehole fraction: addition of Ni atoms could overcome this problem.Different researches indicate that the Ni substitution could help inoptimizing hole concentration leading to higher ZTs [20e22].Theoretical studies reveal that the valence band of these skutter-udites consists of 5p-state for Sb and 3d-state for the transitionmetal: this condition could be influenced by either Co or Ni sub-stitution. Recently this behaviour has been the subject of deepenedstudies, but its understanding is still far from the answer.

In this work, Fe and Ni based skutterudites are obtained as bulksamples by quick and cheap synthesis and a chemical-physicalcharacterization is carried out. Sm was chosen as the filler due toits dual valence: Sm2þ and Sm3þ. This property could justify thepresence of divalent or trivalent cations in skutterudites leading toan increasing electronic stability together with an improvement ofthe TE performances [22e24].

2. Materials and methods

A very similar route to the state of the art synthesis reported forFeeCo and FeeNi based skutterudites was carried out [25e28]. Inparticular, the proposed synthesis provided an unfilled skutterudite(Fe, Ni)4Sb12 which then was filled with Sm.

Starting with iron (rod by Alfa-Aesar: 99.99 mass %), antimony(ingot by Mateck: 99.999 mass %), nickel (foil by Mateck: 99.99mass %) and samarium (rod by NewMet: 99.9 mass %), small piecesof the stoichiometric quantities of elements were sealed in silicaampoules under Ar flow.

Then, the desired amount of samarium was added in the ob-tained ternary skutterudites and subjected to a thermal cycle: asecond melting in a muffle furnace at 1273 K, followed by a quickmelting in the induction furnace, to promote the sample homoge-nization, and an annealing at 903 K for 7 days. The samples thusobtained are labelled as SKUT series.

The preliminary analysis carried out revealed that the key pointsto get filled single phase skutterudites were the rapid cooling andannealing temperature.

In order to reduce time and costs of synthesis, according to thefirst results, a different synthesis route was proposed. Startingdirectly from the four elements, the samples were heated at 1273 Kin a muffle furnace for 3 h and rapidly quenched in salt water andice.

In literature, there is no information about the peritectic tem-perature for the quaternary SmyFe4�xNixSb12; nevertheless,assessing the phase diagrams of similar skutterudites, 873 K wasreasonably adopted as annealing temperature to stay for surebelow the peritectic temperature.

Therefore, the samples were annealed for 60 h at 873 K and airquenched. This synthesis provided a significant reduction in termsof number of steps, annealing times and, consequently, productioncosts.

The samples obtained through this route are labelled as SKTseries.

The nominal content of Ni (x) in each series was x ¼ 0.4, 0.8 and1.2 and such number was used labelling the series, so that SKT 0.4 isreferred to the SKT sample which Ni content is x ¼ 0.4, and so on.

Scanning Electron Microscopy (SEM) equipped with EnergyDispersive X-ray Spectroscopy (EDXS) and X-ray diffraction (XRD)analyses were used to examine microstructures and determinephase composition of bulk samples. A scanning electron micro-scope EVO 40 (Carl Zeiss) was employed, equipped with a Pentafet

Link (Oxford Instruments) detector for EDXS analysis. Smoothsurfaces for microscopic observation were prepared by using SiCpapers and diamond pastes with grain size down to 1 mm. For thequantitative analysis, an acceleration voltage of 20 kV was applied;a cobalt standard was used for calibration. The X-ray spectra wereprocessed by the software package Inca Energy (Oxford In-struments). X-ray diffraction analysis was used to identify thephase crystal structures and determine the lattice parameters. Themeasurements were performed on powdered samples (obtained bygrinding parts of chunks) mounted on a zero background Si sup-port, by means of a vertical diffractometer (Philip X'Pert model). X-ray patterns were recorded using the Cu Ka radiations in the19e100 2q angular range, with a step of 0.013� and a counting timeper step of 30s. A 40 kV voltage and a 30mA current were applied tothe X-ray tube. Lattice parameters were calculated and refined by aleast-squares routine, and compared with literature values. Datarefinement was carried out using Rietveld method by FullProf Suitesoftware. To determine thermal conductivity, obtained chunks aftersynthesis, having 12mm diameter and 20mm length roughly, werecut into round discs with diameter close to 10 mm by using dia-mond saw. Platinum wire based thermocouples were attached onthese discs with silver paste to measure 1/Dt (where t stands fortemperature). The thermal diffusivity coefficient and relative spe-cific heat were measured by the laser flash method (ULVACTC7000). As the absolute specific heat value at 300 K, the valuefrom a reference sample measured by Physical Properties Mea-surement System (PPMS, Quantum Design) was used. Rectangularrods after cutting these discs with size of 2*2*8 mm3 were used forelectrical transport measurement. Resistivity and the thermoelec-tric power were measured simultaneously with an ULVAC ZEM-2under a low pressure inert gas (He) by using the four probemethod. Measurements temperature ranges from 300 K to 750 Kwith some unavoidable differences between the resistivity andthermoelectric power measured temperatures and the thermalconductivity measurements. To estimate the ZT, the thermal con-ductivity data was fitted to obtain the values matching the re-sistivity and thermoelectric power measurements.

3. Results and discussion

The skutterudites belong to the body-centred-cubic Im3 spacegroup; only six of the available 8 small cubes are filled by a ring ofSb, meanwhile the other two could host different fillers.

Fig. 2. Plot of lattice parameters with respect to Ni content (x). Increasing Ni contentdecreases lattice parameters.

Table 1Nominal and SEM-EDXS samples compositions.

Code Sample Composition (at.%)

Nominal SEM-EDXS

Fe Ni Sb Sm Fe Ni Sb Sm

SKUT 1.2 Sm0.7Fe2.8Ni1.2Sb12.8 16.0 6.9 73.1 4 16.5 6.9 73.5 3.1SKUT 0.8 Sm0.7Fe3.2Ni0.8Sb12.8 18.3 4.6 73.1 4.0 18.5 4.6 73.4 3.5SKUT 0.4 Sm0.8Fe3.6Ni0.4Sb12.8 20.4 2.3 72.7 4.5 20.5 3.9 72.2 3.4SKT 1.2 Sm0.7Fe2.8Ni1.2Sb12.8 16.0 6.9 73.1 4 16.3 7.1 73.5 3.1SKT 0.8 Sm0.7Fe3.2Ni0.8Sb12.8 18.3 4.6 73.1 4.0 18.7 4.4 73.2 3.7SKT 0.4 Sm0.8Fe3.6Ni0.4Sb12.8 20.4 2.3 72.7 4.5 20.3 2.2 73.1 4.4

R. Carlini et al. / Journal of Alloys and Compounds 655 (2016) 321e326 323

Powder X-ray diffraction was carried out on the prepared Smy-Fe4�xNixSb12 samples at room temperature to obtain lattice pa-rameters and their dependence on x. All diffraction patterns showalmost single phase samples where the prevalent phase is theskutterudite together with a very low amount of Sb. Furthermore,no significant differences can be observed between the two seriesstudied in terms of lattice parameter values and number of phases.In Fig. 1, the diffraction patterns of the SKUT series are displayed.

Diffraction patterns reveal a good agreement of the experi-mental lattice parameters with those reported in literature for thesimilar compounds [4,21,29e31].

Fig. 2 shows the trend of lattice parameters depending on x: alinear dependence could be observed, according to the Vegard'slaw. Small size Ni (as compared to Fe) results in shrinkage of thelattice.

Morphological and compositional data of the investigatedskutterudites, obtained by SEM-EDXS, indicate that all “as cast”samples can be considered as almost single-phase. Fig. 3a and bdisplaysmicrophotographs of SKUT 1.2 and SKT 1.2, obtained by theBack Scattered Electrons (BSE), as a representative of all the sam-ples. The dominant phase is the skutterudite; a little amount ofantimony can be observed in Fig. 3a. The dark numerous dots showthe porosity of bulk samples, which is understandable in theabsence of any high pressure technique. Within the largest of theseholes, well-formed crystals of quaternary skutterudite can beobserved. A beautiful cubic crystal is shown in Fig. 3c obtained by

Fig. 3. a) BSE-SEM micrograph of SKUT 1.2; b) BSE-SEM micrograph SKT 1.2. Skutterudite phbe observed in SKUT 1.2; c) SE-SEM micrograph of a cubic crystal of SKT 1.2.

using Secondary Electrons (SE).The compositions of samples are reported in Table 1. Both series

show a good agreement between the EDXS compositions and thenominal ones.

Table 2 shows the measured densities of the samples and thosecalculated by the Rietveld refinement of the XRD patterns. It can beobserved that all samples have similar densities but smaller thanthe theoretical ones; confirming that the synthesis carried out inthe absence of high pressures favour the formation of cavities witha consequent lowering of the density.

The temperature dependent electrical and thermal conductivityfor SmyFe4�xNixSb12 samples are shown in Figs. 4 and 5 respec-tively. For the samples having the lowest Ni amount, electricalconductivity declines with increasing temperature in accordancewith a typical metallic behaviour. Only in the case of theSm0.7Fe2.8Ni1.2Sb12.8 samples, they show a very small decrease ofthe electrical conductivity till 370 K and a stronger increase withincreasing temperature thereafter. A similar behaviour is reportedby Tan et al. and Goldsmid [21,33] and justified by the intrinsicexcitation of electron-hole pairs across the energy gap. In general,increasing Ni content degrades the electrical conductivity byneutralizing some of the holes, hence decreasing carrier concen-tration [21]. Although, SKT 0.8 differs, it could be due to the highporosity of the sample (Table 1). The electrical conductivity valuesin general, are in good agreement with the literature data[17,21,25,26,32].

All samples show an overall rise in thermal conductivity withincreasing temperature. However, there is a drastic change atelevated temperatures most probably due to the bipolar diffusion[21]. In general, Ni addition suppresses the thermal conductivity,due to two reasons, (1) by phonon scattering from lattice distor-tions caused by the different atomic size of Ni from Co, and (2), dueto the lowering electronic contribution of thermal conductivitybecause of degradation in electrical conductivity. The thermalconductivity values (Fig. 5) are comparable with the literature data

ase is the dominant phase (dark grey), a very little amount of antimony (light grey) can

Table 2Measured and calculated densities of the samples.

Sample Measured density Calculated density Densities ratio

SKUT 1.2 6.25 7.63 0.82SKUT 0.8 6.12 7.77 0.79SKUT 0.4 6.47 7.54 0.86SKT 1.2 7.05 7.68 0.92SKT 0.8 6.14 7.78 0.79SKT 0.4 6.27 7.70 0.81

R. Carlini et al. / Journal of Alloys and Compounds 655 (2016) 321e326324

[17,21,25,26,32]. Although, a bit higher slope could be observed inthe present study as compared to the similar ones reported inliterature.

Positive Seebeck coefficients (S) for all samples (Fig. 6) in theentire temperature range indicates holes as the majority chargecarriers. These values are slightly higher than the ones reported inliterature [17,21,25e27,32]. Furthermore, the samples having a lowNi content show a continuous increase of S roughly up to 700 K,where a peak value (Smax) is observed. Increasing Ni content affects

Fig. 4. Temperature dependence of the electrical cond

Fig. 5. Temperature dependence of the thermal cond

the Smax and it is shifted towards lower temperatures; thereafter astrong slope is shown by the x ¼ 1.2 curves. This behaviour is inagreement with Kaltzoglou et al. [28] and to some extent with Tanet al. [21] and is justified by the electrical conductivity trend pre-viously discussed. Overall, Ni addition plays a positive role inenhancing the Seebeck Coefficient at lower and middle tempera-tures but the difference diminishes at higher temperatures, mostlikely due to the intrinsic electron excitations and quite similar tothe one reported in literature [21].

Fig. 7 displays the figure of merit ZT values as a function oftemperature for all the investigated samples. Higher Ni contentseems to enhance the ZT. However, increasing Ni amount shifts themaxima of ZT towards lower temperatures. This behaviour could beconsidered positive owing to the highest efficiency of these skut-terudites observed at lower temperature than the ones reported inliterature [17,21,25e27,32]. However, these values are rather lowerthan the ones reported by Rogl et al. and Zhang et al. [25,32] butquite similar to the ones reported by Tan et al. [17,21]. Highest ZT of0.55 was observed with x ¼ 1.2 at ~560 K. A more important aspectof the present work is the high average ZTof SKUT 1.2 sample below

uctivity for different values of Ni concentration.

uctivity for different values of Ni concentration.

Fig. 6. Temperature dependence of the Seebeck coefficient for different values of Ni concentration.

Fig. 7. Temperature dependence of the figure of merit (ZT) for the samples with different values of Ni concentration.

R. Carlini et al. / Journal of Alloys and Compounds 655 (2016) 321e326 325

600 K with a room temperature value exceeding 0.3. It is evidentfrom these results that we can tune themaxima of ZT by controllingNi content and it is possible to alter this material according to therequired temperature range.

The samples obtained through the cheaper and faster syntheticroute (SKUT series) show a better performance than the other ones(SKT series). A processing such as Spark Plasma Sintering or OpenDie Pressing could lead to higher values than the ones observed inthis study by improving the overall density of the samples.

4. Conclusions

A fast and cheap synthesis route has been developed for Fe, Nibased - skutterudites filled with Sm. A simple melting annealingsolid state synthesis give nearly single phase bulk samples, asshown by SEM-EDXS investigation, having compositions quiteconsistent with the nominal ones. The lattice parameters reveal agood agreement with the ones of similar compounds reported inliterature; their trendwith the Ni content respects the Vegard's law.

The increasing Ni content leads to a lower electrical conductivitytypical of semiconductive materials, while an improvement inSeebeck coefficient is observed. Thermal conductivity is hardlyinfluenced by the Ni content. The resulting figure of merit ZT couldbe considered slightly lower than the ones reported in literature.The best performance is observed for Sm0.7Fe2.8Ni1.2Sb12.8 samplehaving the highest ZT ¼ 0.55 at ~560 K. This study shows that aselection of appropriate fillers, and optimization of carrier con-centration through adjustment of Ni content could further improvethe thermoelectric properties of these p-type skutterudites.

References

[1] D.M. Rowe, CRC Handbook of Thermoelectrics, CRC, Press Inc., Boca Raton, FL,1995.

[2] D.M. Rowe, Thermoelectrics Handbookdmacro to Nano, CRC Press Inc., FL,2005.

[3] K. Koumoto, T. Mori, Thermoelectric Nanomaterials, Springer, Heidelberg,2013.

[4] L. Chapon, D. Ravot, J.C. Tedenac, Nickel-substituted skutterudites: synthesis,structural and electrical properties, J. Alloy Compd. 282(199) 58e63.

R. Carlini et al. / Journal of Alloys and Compounds 655 (2016) 321e326326

[5] G.S. Nolas, J. Poon, M. Kanatzidis, Structural and transport properties ofBa8Ga16SixGe30�x clathrates, Mater. Res. Soc. Bull. 31 (2006) 199e205.

[6] R. Amatya, R.J. Ram, Trend for thermoelectric materials and their earthabundance, J. Electron. Mater. 41 6 (2012) 1011e1019.

[7] M.G. Kanatzidis, S.D. Mahanti, T.P. Hogan, Chemistry, Physics, and MaterialsScience of Thermoelectric Materials, Kluver Academic Plenum Publishers,New York, 2002.

[8] K. Niedziolka, R. Pothin, F. Rouessac, R.M. Ayral, P. Jund, Theoretical andexperimental search for ZnSb-based thermoelectric materials, J. Phys. Con-dens. Matter 26 (2014) 365401.

[9] L. Bjerg, G.K.H. Madsen, B.B. Iversen, Enhanced thermoelectric properties inzinc antimonides, Chem. Mater. 23 (17) (2011) 3907e3914.

[10] P.H.M. B€ottger, S. Diplas, E. Flage-Larsen, Ø. Prytz, T.G. Finstad, Electronicstructure of thermoelectric ZneSb, J. Phys. Condens. Matter 23 (2011) 265502.

[11] J.M. Cameron, R.W. Hughes, Y. Zhao, D.H. Gregory, Ternary and higher pnic-tides; prospects for new materials and applications, Chem. Soc. Rev. 40 (2011)4099e4118.

[12] J. Yang, P. Qiu, R. Liu, L. Xi, S. Zheng, W. Zhang, L. Chen, D.J. Singh, J. Yang,Trends in electrical transport of p-type skutterudites RFe4Sb12 (R ¼ Na, K, Ca,Sr, Ba, La, Ce, Pr, Yb) from first-principles calculations and Boltzmann trans-port theory, Phys. Rev. B 84 (2011) 235205.

[13] R. Carlini, C. Artini, G. Borzone, R. Masini, G. Zanicchi, G.A. Costa, Synthesis andcharacterisation of the compound CoSbS, J. Therm. Anal. Calorim. 103 (2011)23e27.

[14] R. Carlini, G. Zanicchi, G. Borzone, N. Parodi, G.A. Costa, Synthesis and char-acterization of the intermetallic compound NiSbS, J. Therm. Anal. Calorim. 108(2012) 793e797.

[15] R. Carlini, D. Marr�e, I. Pallecchi, R. Ricciardi, G. Zanicchi, Thermoelectricproperties of Zn4Sb3 intermetallic compound doped with aluminum and sil-ver, Intermet 45 (2014) 60e64.

[16] A. Shankar, D.P. Rai, R. Khenata, J. Maibam, R.K. Sandeep, Thapa Study of 5felectron based filled skutterudite compound EuFe4Sb12, a thermoelectric (TE)material: FP-LAPW method, J. Alloy Compd. 619 (2015) 621e626.

[17] G. Tan, S. Wang, Y. Yan, H. Li, X. Tang, Enhanced thermoelectric performancein p-type Ca0.5Ce0.5Fe4�xNixSb12 skutterudites by adjusting the carrier con-centration, J. Alloy Compd. 513 (2012) 328e333.

[18] W. Liu, Q. Jie, Q. Li, Z. Chen, B. Li, Synchrotron X-ray study of filled skutter-udites CeFe4Sb12 Ce0.8Fe3CoSb12, Phys. B 406 (2011) 52e55.

[19] M. Ueda, Y. Kawahito, K. Tanaka, D. Kikuchi, H. Aoki, H. Sugawara, Y. Aoki,H. Sato, Synthesis and basic properties of the filled skutterudite SmFe4Sb12,

Phys. B 403 (2008) 881e883.[20] M. Hasaka, T. Morimura, S. Tabata, in: 22th Int. Conf. Thermoelectricity, 2003.[21] G. Tan, Y. Zheng, Y. Yan, X. Tang, Preparation and thermoelectric properties of

p-type filled skutterudites CeyFe4�xNixSb12, J. Alloy Compd. 584 (2014)216e221.

[22] R. Liu, J.Y. Cho, J. Yang, W. Zhang, L. Chen, Thermoelectric Transport Propertiesof RyFe4NiSb12 (R ¼ Ba, Nd and Yb), J. Mater. Sci. Technol. 30 11 (2014)1134e1140.

[23] J.Y. Cho, Z. Ye, M.M. Tessema, R.A. Waldo, J.R. Salvador, J. Yang, W. Cai,H. Wang, Thermoelectric properties of p-type skutterudites YbxFe3.5Ni0.5Sb12(0.8 � x � 1), Acta Mater. 60 (2012) 2104e2110.

[24] M. Mizumaki, S. Tsutsui, H. Tanida, T. Uruga, D. Kikuchi, H. Sugawara, H. Sato,Determination of the valence in Sm-based filled skutterudite compounds,Phys. B Condens. Matter 383 (2006) 144e145.

[25] G. Rogl, A. Grytsiv, E. Bauer, M. Hochenhofer, R. Anbalagan, R.C. Mallik,E. Schafler, Nanostructuring of p- and n-type skutterudites reaching figures ofmerit of approximately 1.3 and 1.6, respectively, Acta Mater. 76 (2014)434e448.

[26] G. Rogl, A. Grytsiv, E. Bauer, P. Rogl, A. Zehetbauer, Multifilled nanocrystallinep-type didymium - skutterudites with ZT > 1.2, Intermet 18 (2010) 57e64.

[27] G. Tan, Y. Zheng, X. Tang, High thermoelectric performance of nonequilibriumsynthesized CeFe4Sb12 composite with multi-scaled nanostructures, Appl.Phys. Lett. 103 (2013) 183904.

[28] A. Kaltzoglou, P. Vaqueiro, K.S. Knight, A.V. Powell, Synthesis, characterizationand physical properties of the skutterudites YbxFe2Ni2Sb12 (0 � x � 0.4),J. Solid State Chem. 193 (2012) 36e41.

[29] G. Rogl, A. Grytsiv, M. Falmbigl, E. Bauer, P. Rogl, A. Zehetbauer, Y. Gelbstein,Effect of HPT processing on the structure, thermoelectric and mechanicalproperties of Sr0.07Ba0.07Yb0.07Co4Sb12, J. Alloy Compd. 537 (2012) 242e249.

[30] D. Berardan, E. Alleno, C. Godart, O. Roleau, J. Rodriguez Carvajal, Preparationand chemical properties of the skutterudites (Ce-Yb)(y)Fe4�x(Co/Ni)(x)Sb12,Mater. Res. Bull. 40 (2005) 537e551.

[31] L. Chapon, L. Girard, A. Haidoux, R.I. Smith, D. Ravot, Structural changesinduced by Ce filling in partially filled skutterudites, J. Phys. Condens. Matter17 (2005) 3525e3535.

[32] L. Zhang, A. Grytsiv, B. Bonarski, M. Kerber, D. Setman, E. Schafler, P. Rogl,E. Bauer, G. Hilscher, M. Zehetbauer, Impact of high pressure torsion on themicrostructure and physical properties of Pr0.67Fe3CoSb12, Pr0.71Fe3.5Ni0.5Sb12,and Ba0.06Co4Sb12, J. Alloy Compd. 494 (2010) 78e83.

[33] H. Goldsmid, Applications of Thermoelectricity, Methuen, London, 1960.