Enhanced thermoelectric properties of Ga and In Co added CoSb3 based skutterudites with optimized chemical composition and microstructure Enhanced thermoelectric properties of Ga and In Co added CoSb3[.]
Enhanced thermoelectric properties of Ga and In Co-added CoSb3-based skutterudites with optimized chemical composition and microstructure Seongho Choi, Ken Kurosaki, Guanghe Li, Yuji Ohishi, Hiroaki Muta, Shinsuke Yamanaka, and Satoshi Maeshima Citation: AIP Advances 6, 125015 (2016); doi: 10.1063/1.4971819 View online: http://dx.doi.org/10.1063/1.4971819 View Table of Contents: http://aip.scitation.org/toc/adv/6/12 Published by the American Institute of Physics Articles you may be interested in Vibrational mean free paths and thermal conductivity of amorphous silicon from non-equilibrium molecular dynamics simulations AIP Advances 6, 121904121904 (2016); 10.1063/1.4968617 Role of fringing field on the electrical characteristics of metal-oxide-semiconductor capacitors with co-planar and edge-removed oxides AIP Advances 6, 125017125017 (2016); 10.1063/1.4971845 Control of Rayleigh-like waves in thick plate Willis metamaterials AIP Advances 6, 121707121707 (2016); 10.1063/1.4972280 Nonlinear resonance converse magnetoelectric effect modulated by voltage for the symmetrical magnetoelectric laminates under magnetic and thermal loadings AIP Advances 6, 125016125016 (2016); 10.1063/1.4971821 AIP ADVANCES 6, 125015 (2016) Enhanced thermoelectric properties of Ga and In Co-added CoSb3 -based skutterudites with optimized chemical composition and microstructure Seongho Choi,1,a Ken Kurosaki,1,2,b Guanghe Li,1 Yuji Ohishi,1 Hiroaki Muta,1 Shinsuke Yamanaka,1,3 and Satoshi Maeshima4 Graduate School of Engineering, Osaka University, Suita, Japan PRESTO, 4 1 8 Honcho, Kawaguchi, Saitama, Japan Research Institute of Nuclear Engineering, University of Fukui, Tsuruga, Japan Business Unit, Panasonic Semiconductor Solutions Co., Kameoka 621 0018, Japan JST, (Received 14 October 2016; accepted 23 November 2016; published online 15 December 2016) Skutterudite compounds such as Co antimonite (CoSb3 ) contain cage-like voids inside crystal structure, which can be completely or partially filled with various different atoms, including group 13 elements The multiple filling approach is known as an effective way of reducing lattice thermal conductivity (κ lat ), which results in a high value of the thermoelectric dimensionless figure of merit (zT ) In this work, enhanced zT was achieved for the Ga and In co-added CoSb3 samples with a preferable microstructure and the nominal composition (Ga0.8 In0.2 )x Co4 Sb12 (x = 0.05 0.45) Although all added In atoms occupied exclusively the void sites, the Ga species filled both the void and Sb sites of CoSb3 Moreover, Ga atoms added in the quantities exceeding the solubility limit precipitated as GaSb nanoparticles The sample with x = 0.45 was characterized by the largest filling factions of Ga and In as well as the unique microstructure, consisting of microscale grains of the skutterudite phase and corresponding amounts of the GaSb nanoparticles The Ga and In co-added skutterudite samples with optimized chemical composition and microstructure maintained high carrier mobility and sufficiently low κ lat values, resulting in zT > 1.1, one of the best values for the skutterudites filled with group 13 elements © 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4971819] I INTRODUCTION Thermoelectric (TE) devices, which can convert waste heat into electrical power, have been considered clean and sustainable power sources.1 The efficiency of TE devices is determined by the properties of TE materials and represented by the dimensionless figure of merit, zT = S T ρ−1 κ −1 , where S is the Seebeck coefficient, T is the absolute temperature, ρ is the electrical resistivity, and κ is the total thermal conductivity (κ = κ lat + κ el , the lattice and electronic contributions, respectively) The reduction of κ lat plays an important role in the enhancement of zT because S, ρ, and κ el are closely related to each other as functions of the carrier concentration In order to achieve low κ lat values, it is imperative to select materials with complex structures (such as large unit cells) containing disorders and various atomic types.2–4 Many bulk materials have been investigated as possible advanced TE materials,5–10 including filled skutterudites Skutterudites have the composition MX , where M is a metal atom (such as Co, Rh, or Ir), and X represents a pnicogen atom such as P, As, or Sb These compounds are characterized by a body-centered cubic lattice with 32 atoms in the unit cell and space group Im-3 The skutterudite structure contains two large voids that can be filled by other a Electronic mail: shchoi@see.eng.osaka-u.ac.jp Electronic mail: kurosaki@see.eng.osaka-u.ac.jp b 2158-3226/2016/6(12)/125015/9 6, 125015-1 © Author(s) 2016 125015-2 Choi et al AIP Advances 6, 125015 (2016) atoms When additional atoms R are incorporated into the voids, the compound formula becomes RM X 12 , corresponding to filled skutterudites The filler element R is bonded weakly to other atoms and vibrates independently of them, producing an effect called rattling Therefore, the behavior of R introduced into the voids contributes to the phonon scattering, which leads to a reduction of κ lat In particular, CoSb3 -based filled skutterudites are considered promising TE materials Alkali metals,11,12 alkaline earth elements,13–15 lanthanides,16–24 electronegative elements,25,26 and group 13 elements27–41 are known as good candidates for filler elements Among group 13 elements, indium (In), as a co-dopant in multiple filled skutterudites, show excellent TE properties of maximum zT values of 1.2-1.4, in which co-dopants such as Barium, Cerium and Ytterbium easily react with air and thus the special process is required to apply for mass production.38,41–44 In contrast, gallium (Ga), as a same group element with In, is nontoxic and not reactive with both air and water at room temperature Furthermore, since Ga indicates the unique behavior in CoSb3 such as dual-site occupancy and the desirable electronic band engineering, it is worth studying as a co-dopant with In in CoSb3 -based skutterudites.29,33,34,45 Our group has previously investigated the TE properties of Ga and In co-added CoSb3 -based skutterudites27 and found that (1) Ga and In could be simultaneously introduced into the voids of the skutterudite structure; (2) Ga could also occupy Sb sites; and (3) Ga and In added in the amounts exceeding the filling limit precipitated as (Ga,In)Sb nanoinclusions These three phenomena contributed to the κ lat reduction significantly, and thus the maximum value of zT close to unity was obtained for the nominal composition Ga0.20 In0.30 Co4 Sb12 27 In the present study, we have attempted to improve the zT of Ga and In co-added CoSb3 by optimizing both its chemical composition and microstructure After characterizing the TE properties of (Ga0.8 In0.2 )x Co4 Sb12 (x = 0.05 0.45), we found that the sample corresponding to the stoichiometric composition of x = 0.45 has an optimized carrier concentration together with a preferable microstructure, leading to the maximum enhancement of zT equal to 14% (> 1.1) II EXPERIMENT Polycrystalline samples were prepared from the proper amounts of Ga In alloy (99.999%), Co (99.9%), and Sb (99.99%) chunks via direct reactions inside sealed silica tubes Since Ga In alloy is characterized by around the eutectic composition, the nominal compositions studied in this work were defined as (Ga0.8 In0.2 )x Co4 Sb12 (x = 0.05, 0.15, 0.25, 0.35, and 0.45) The sealed silica tubes were heated slowly to 1323 K followed by quenching in a water bath and annealing at 873 K for one week The obtained ingots were roughly crushed by hand into powders and then subjected to spark plasma sintering in graphite dies under Ar flow atmosphere at a pressure of 50 MPa and temperature of 923 K for 15 Bulk sample densities were calculated from the measured weights and dimensions The obtained samples were characterized by powder XRD using Cu Kα radiation at room temperature Lattice parameters were estimated via least-squares fitting to the indexed 2θ values (using Si as an external standard) by employing PDXL, Rigaku’s integrated X-ray powder diffraction software Filling fractions were determined for the obtained samples from the Rietveld refinements performed by utilizing the Rietan FP program.46 Samples’ microstructures were observed using field-emission scanning electron microscopy (FE SEM), while their chemical compositions were determined by EDX analysis under vacuum at room temperature The magnitudes of ρ and S were obtained using a commercially available apparatus for simultaneous measurement of the Seebeck coefficient and electrical resistivity of thermoelectric materials (ULVAC, ZEM 3) under He atmosphere The Hall coefficient (RH ) was measured at room temperature and applied magnetic field of 0.5 T using the van der Pauw method The Hall carrier concentration (nH ) and Hall mobility (µH ) were calculated from the obtained RH values based on the assumptions of a single-band model and a Hall factor of (represented by the relations nH = 1/(e·RH ) and µH = RH /ρ, where e was the elementary electric charge) The magnitudes of κ were evaluated from the thermal diffusivity (α), heat capacity (C p ), and sample density (d) values based on the relationship κ = αCp d C p values were estimated via the Dulong–Petit model, C p = 3nR, where n was the number of atoms per formula unit, and R was the gas constant The values of α were measured under Ar flow using a commercially available laser flash apparatus (NETZSCH, LFA457) Sample TE properties were evaluated in the temperature range between the room temperature and 773 K 125015-3 Choi et al AIP Advances 6, 125015 (2016) FIG Powder XRD patterns for the polycrystalline samples with the nominal compositions (Ga0.8 In0.2 )x Co4 Sb12 (x = 0.05, 0.15, 0.25, 0.35, and 0.45) III RESULTS AND DISCUSSION The powder X-ray diffraction (XRD) patterns of the studied samples are shown in Figure They indicate that all samples exhibit the skutterudite structure with negligible amounts of CoSb2 impurities (especially the sample with x = 0.45) The lattice parameters of the skutterudite phase and sample densities are listed in TABLE I, which shows that the lattice parameter slightly increases with increasing x in (Ga0.8 In0.2 )x Co4 Sb12 Generally, the presence of Ga and In atoms inside the voids expands the skutterudite lattice,37,39 while substituting the Sb sites with Ga atoms tends to contract it.27 Thus, the increase in the lattice parameter observed in the present study can be caused by the increase in the In filling fraction (a detailed analysis of the lattice parameter will be presented later) As a result, high-density bulk samples with densities exceeding 98% of the theoretical value were obtained Figure shows the temperature dependences of the Seebeck coefficient S, electrical resistivity ρ, and power factor S ρ−1 for the samples of (Ga0.8 In0.2 )x Co4 Sb12 (x = 0.05, 0.15, 0.25, 0.35, and 0.45) and In0.12 Co4 Sb12 47 The In0.12 Co4 Sb12 sample, as a reference for comparison, was taken with consideration for In content close to the sample with x = 0.45.47 The sample with nominal composition In0.12 Co4 Sb12 , as a reference sample, was taken with consideration for In content of the sample with x = 0.45.47 The S values for all samples (except for x = 0.05) were negative across the entire temperature range between room temperature and 773 K For the samples with x = 0.15 0.35, the absolute values of both S and ρ decreased with x, mainly due to increase in the carrier concentration caused by the introduction of Ga and In species In particular, the co-adding of Ga and In effectively gave rise to the enhancement of the absolute S compared to In single filling for CoSb3 , which led to high S ρ−1 Furthermore, the maximum values of the absolute S for these samples shifted to higher temperatures with increasing x, mainly due to the onset of a bipolar conduction caused by the increased electron concentration.35 On the other hand, when x reaches 0.45, the absolute values of S and ρ start increasing As a result, the samples with x = 0.35 and 0.45 exhibited similar characteristics in terms of S ρ−1 , corresponding to high values in the temperature range of 300 700 K (above mW m-1 K-2 ) with a relatively flat temperature dependence (the origin of such a high power factor will be discussed later) TABLE I Lattice parameter a, sample bulk density d, relative density %T.D measured for the polycrystalline samples with a nominal composition of (Ga0.8 In0.2 )x Co4 Sb12 (x = 0.05, 0.15, 0.25, 0.35, and 0.45) at room temperature x a (nm) d (g cm-3 ) %T.D 0.05 0.15 0.25 0.35 0.45 0.9033 0.9039 0.9040 0.9044 0.9045 7.54 7.55 7.61 7.58 7.61 98 98 99 98 98 125015-4 Choi et al AIP Advances 6, 125015 (2016) FIG Temperature dependences of the (a) Seebeck coefficient S, (b) electrical resistivity ρ, and (c) power factor S ρ−1 for the samples with the nominal compositions (Ga0.8 In0.2 )x Co4 Sb12 (x = 0.05, 0.15, 0.25, 0.35, and 0.45) and In0.12 Co4 Sb12 47 Since the samples with x = 0.35 and 0.45 produced the best values of S ρ−1 , the magnitudes of κ for these two samples were investigated Figure shows the temperature dependences of the κ, κ lat , and zT for the samples of (Ga0.8 In0.2 )x Co4 Sb12 (x = 0.35 and 0.45) and In0.12 Co4 Sb12 47 The value of κ lat was obtained by subtracting κ el (= LT ρ−1 , where L was the Lorentz number = 2.45×10-8 W Ω K-2 ) from the measured κ, resulting in the formula κ lat = κ−LT ρ−1 , which indicated that κ lat contained a bipolar contribution It was clearly confirmed that the κ lat values for the samples with x = 0.35 and 0.45 were systematically lower than those obtained at In single filled CoSb3 across the entire temperature range, the co-adding of Ga and In for CoSb3 effectively scatters heat carrying phonon Additionally, compared with x = 0.35, the lower κ lat values of x = 0.45 suggest that the increase in the added amount of Ga and In led to effective phonon scattering Owing to the combination of a high S ρ−1 magnitude and significantly reduced κ lat , the sample with x = 0.45 exhibited the maximum of zT > 1.1 at 675 K, corresponding to the enhancement of the best value obtained in the previous study (equal to approximately 14%).27 To confirm the origin of high zT achieved at x = 0.45, Ga and In site occupancies in the skutterudite phase, the existing states of Ga and In atoms added in the amounts over the filling limits, and sample’s microstructure were investigated in detail Figure shows the observed and calculated powder XRD patterns and the difference profile obtained for the samples with x = 0.35 and 0.45 The detailed results of the Rietveld refinement are provided in TABLE II CoSb2 peaks corresponding to the secondary phase of the XRD pattern were excluded during the refinement The Rietveld refinement was performed in accordance with the procedure described in our previous study.27 The site occupancy refinement was conducted for each element to check the degree of disorder As shown in TABLE II, the x =0.35 and 0.45 samples had the skutterudite phase with an actual composition of Ga0.11 In0.06 Co4 Sb11.94 Ga0.06 FIG Temperature dependences of the (a) thermal conductivity κ, (b) lattice thermal conductivity κlat , and (c) dimensionless figure of merit zT for the samples with the nominal compositions (Ga0.8 In0.2 )x Co4 Sb12 (x = 0.35 and 0.45) and In0.12 Co4 Sb12 47 125015-5 Choi et al AIP Advances 6, 125015 (2016) FIG Powder XRD patterns for the samples with (a) x = 0.35 and (b) x = 0.45 recorded at room temperature, which contain the observed, calculated, difference, and background curves The expected peak positions are marked with vertical ticks and Ga0.05 In0.09 Co4 Sb11.98 Ga0.02 , respectively For the x = 0.45 sample, by comparing this refined composition with the nominal composition Ga0.36 In0.09 Co4 Sb12 , it was concluded that all In atoms filled the voids, while the Ga species added over the solubility limit would precipitate as GaSb Based on the actual sample composition determined by the Rietveld analysis, the lattice parameter of the sample was estimated by using the Vegard rule.38 The lattice parameter a of Ga0.05 In0.09 Co4 Sb11.98 Ga0.02 can be written as: a of Ga0.05 Inx Co4 Sb11.98 Ga0.02 = a of Co4 Sb12 + ∆a1 + ∆a2 , (1) where a of Co4 Sb12 ∆a1 = a of Ga0.05 Co4 Sb11.98 Ga0.02 a of Co4 Sb12 , and ∆a2 = a of Inx Co4 Sb12 a of Co4 Sb12 The values of ∆a1 and ∆a2 can be calculated from the changes in lattice parameters of Ga and In single-filled single-filled CoSb3 , respectively.21,33 By fitting the calculated lattice parameter to the experimental value, we obtained x ≈ 0.09, which was in good agreement with x = 0.09 of the actual composition determined by the Rietveld analysis This result implies that the assumptions utilized for the Rietveld analysis are valid; in other words, in the case of Ga and In co-coped CoSb3 , In easier occupies the void sites as compared to Ga, which occupies not only the void sites, but also the Sb sites at a ratio of 2:1 Figure shows the scanning electron microscopy (SEM) images obtained for the x = 0.45 sample (the image for the Ga0.2 In0.15 Co4 Sb12 sample27 is shown for comparison) The sample with x = 0.45 was composed of dense grains with sizes over approximately 30 µm and nanoscale precipitates with is 0.9034 nm,40 TABLE II Rietveld refinement results for the (Ga0.8 In0.2 )x Co4 Sb12 samples with x = 0.35 and 0.45 Atomic positions: Ga/In, 2a (0, 0, 0); Co, 8c (0.25, 0.25, 0.25); Sb, 24g (0, y, z) Nominal composition (Ga0.8 In0.2 )0.35 Co4 Sb12 (Ga0.8 In0.2 )0.45 Co4 Sb12 Actual composition (GaVF )/(GaSb ) Space group Radiation 2θ range (deg.) Step width (deg.) Counting time (s/step) Uiso (Å) for GaVF and In Uiso (Å) for Co Uiso (Å) for Sb and GaSb y (Sb) z (Sb) RB (%) RF (%) S Ga0.17 In0.06 Co4 Sb11.94 0.110/0.055 Ga0.07 In0.09 Co4 Sb11.98 0.045/0.023 Im-3 (#204) Cu Kα 10 100 0.02 0.0199 0.0050 0.0058 0.8429 0.6651 3.48 2.71 1.45 0.0199 0.0081 0.0068 0.8421 0.6644 4.96 3.72 1.24 125015-6 Choi et al AIP Advances 6, 125015 (2016) FIG FE SEM images of the polycrystalline samples Panels (a), (b), and (c) correspond to the sample with x = 0.45, and panel (d) represents the sample with the nominal composition Ga0.20 In0.15 Co4 Sb12 21 The square area depicted in Fig 5(d) has been described in the previous study.21 sizes of below 100 nm, which were formed inside the grains and at the boundaries In the present case, the precipitates correspond to the GaSb phase, because Ga and In added in the quantities exceeding the filling limit react with Sb and form GaSb and InSb species, respectively.15,41 Furthermore, Figures 5(a) and (d) reveal that the sample with x = 0.45 contains larger grains and smaller amounts of the nanoscale precipitates as compared with those observed for the Ga0.2 In0.15 Co4 Sb12 sample The nanoscale precipitates were widely dispersed in the present sample in contrast to the agglomerations formed inside the previously studied sample,27 which could originate from the differences in nominal compositions and synthesis conditions It should be also noted that the Ga0.2 In0.15 Co4 Sb12 sample was prepared by hot-pressing from ball-milled fine powders,27 while the x = 0.45 sample was obtained by spark plasma sintering from roughly crushed powders Figures 6(a) and (b) show the dependences of the reduced effective mass m*/m0 and Hall mobility µH on the Hall carrier concentration nH respectively (all the plotted parameters were obtained at room temperature) Assuming the single parabolic band model dominated by acoustic phonon scattering, 125015-7 Choi et al AIP Advances 6, 125015 (2016) FIG Relationships between (a) the reduced effective mass m*/m0 and Hall carrier concentration nH and (b) the Hall mobility µH and Hall carrier concentration nH for Yb and Ga co-added CoSb3 ,29 Yb single filled CoSb3 ,17,29 three band model,17 and the samples with nominal compositions of (Ga0.8 In0.2 )x Co4 Sb12 (x = 0.15, 0.25, 0.35, and 0.45), (GaVF )0.06 Co4 Sb11.97 (GaSb )0.03 ,34 (GaVF )0.10 Co4 Sb11.95 (GaSb )0.05 ,34 (GaVF )0.15 Co4 Sb11.925 (GaSb )0.075 ,34 and Ga0.2 Inx Co4 Sb12 (x = 0.15, 0.20, 0.25, and 0.30)27 obtained at room temperature we can express S and the true carrier concentration n as follows:48 ( ) ke 2F1 (η) s= η− , e F0 (η) r n= m ∗ kB T π ~2 ∞ Fi (η) = (2) ! 3/ F1/2 (η), x i dx , + exp(x − η) (3) (4) where k B is the Boltzmann constant, F i is the Fermi integral of order i, η = EF /kB T is the reduced chemical potential, m* is the effective carrier mass, and ~ is the reduced Planck constant The measured Hall carrier concentration nH is connected to n via nH = n/r H with the Hall scattering factor determined by r H = (1.5F 0.5 F -0.5 )/2F As shown in Figure 6(a), the m*/m0 values gradually increased for all samples with increasing nH , which could be explained by the band convergence at high carrier concentration region Further increasing in the m*/m0 was observed for Ga and Yb co-filled CoSb3 at high nH values, which could be explained by the change in the band structure observed in the vicinity of the Fermi level through the first-principles calculations.29 Furthermore, m*/m0 is much larger for the x = 0.45 sample than for the Ga and Yb co-filled CoSb3 samples, which is related to the combined effect of both the degeneracy of the band structure and the energy filtering caused by the presence of evenly dispersed GaSb nanoscale precipitates According to Ref 23, the nanoparticles of p-type GaSb in the n-type host phase form an interfacial energy barrier, which impedes the diffusion of electrons with low energies, thus contributing to the enhancement of S On the other hand, Figure 6(b) shows that the µH values for the samples with x ≥ 0.15 are systematically higher than those obtained for the Ga0.20 Inx Co4 Sb12 (x = 0.15, 0.20, 0.25, 0.30) samples studied previously.27 As shown in Figure 5, the x = 0.45 sample contains larger grains as compared to those for the previously studied one, which could lead to higher µH magnitudes Figure 7(a) and (b) show the temperature dependences of κ lat and zT, respectively, for the samples with the nominal compositions Ga0.20 Co4 Sb12 ,39 (GaVF )0.10 Co4 Sb11.95 (GaSb )0.05 ,34 Ga0.20 In0.15 Co4 Sb12 ,27 and (Ga0.8 In0.2 )x Co4 Sb12 (the x = 0.45 sample), while Figure 7(c) displays the maximum zT values for these samples As indicated by Figure 7(a), the κ lat value for the sample with x = 0.45 is much lower than those for the other samples, despite its larger grain sizes The introduction of both Ga and In into the CoSb3 lattice leads to the effective phonon scattering caused by the following reasons: (1) rattling of both Ga and In atoms inside the voids, (2) the replacement of Sb with a small amount of Ga, (3) the formation of dispersed nanoscale precipitates, and (4) CoSb2 compounds were formed to preserve the mass balance In the Ga single-filled system,34,39 Ga species added over 125015-8 Choi et al AIP Advances 6, 125015 (2016) FIG Temperature dependences of the (a) thermal conductivity κ and (b) dimensionless figure of merit zT for the samples with nominal compositions of Ga0.20 Co4 Sb12 ,38 (GaVF )0.10 Co4 Sb11.95 (GaSb )0.05 ,34 and Ga0.20 In0.30 Co4 Sb12 27 as well as for the sample with x = 0.45 (c) Maximum zT values and schematic microstructural views for these samples the filling limit normally exist as microscale Ga metal precipitates, which maintain low values of zT.39 However, the optimization of the chemical composition in terms of the void filling and charge compensation results in a large decrease in the κ lat magnitude for (GaVF )0.10 Co4 Sb11.95 (GaSb )0.05 , which significantly enhances zT.34 The obtained results suggest that the dual-site occupancy of Ga is an effective way for enhancing the zT of Ga single-filled CoSb3 In addition, further decreases in κ lat and the related zT enhancement are achieved for Ga0.20 In0.15 Co4 Sb12 by introducing In and Ga atoms into the CoSb3 lattice.27 In the present study, the x = 0.45 sample exhibited lower κ lat than that of Ga0.20 In0.15 Co4 Sb12 , mainly due to the presence of large Ga contents not only inside the voids, but also at the Sb sites of the skutterudite structure The actual compositions of the x = 0.45 sample and the Ga0.20 In0.15 Co4 Sb12 sample were Ga0.05 In0.09 Co4 Sb11.98 Ga0.02 and Ga0.02 In0.11 Co4 Sb11.99 Ga0.01 ,27 respectively Further zT enhancement was achieved for the x = 0.45 sample, which was approximately 14% higher than that obtained for the Ga0.20 In0.30 Co4 Sb12 sample due to the optimization of both the chemical composition and microstructure The studied sample contained moderately large grains with small amounts of nanoscale precipitates, which maintained high carrier mobility while preserving low values of κ lat IV SUMMARY In this work, polycrystalline samples with nominal compositions of (Ga0.8 In0.2 )x Co4 Sb12 (x = 0.05, 0.15, 0.25, 0.35, and 0.45) were synthesized, and their TE properties were investigated The obtained TE data were compared with those for other skutterudites filled with Ga and/or In atoms The sample with x = 0.45 was mainly composed of two phases: a skutterudite phase with a composition of Ga0.05 In0.09 Co4 Sb11.98 Ga0.02 (in which Ga atoms occupied both the voids and Sb sites) and GaSb nanoscale precipitates The x = 0.45 sample contained larger grains with smaller amounts of nanoscale precipitates as compared with other previously investigated skutterudite samples A significant increase in the effective mass was observed, which resulted from the combined effect of both the change in electronic band structure and scattering of low-energy 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the GaSb nanoparticles The Ga and In co- added skutterudite samples with optimized chemical composition and microstructure maintained high carrier mobility and. ..AIP ADVANCES 6, 125015 (2016) Enhanced thermoelectric properties of Ga and In Co- added CoSb3 -based skutterudites with optimized chemical composition and microstructure Seongho Choi,1,a Ken