Double-filled Skutterudites Research Articles

Investigation of the Effect of Double-Filler Atoms on the Thermoelectric Properties of Ce-YbCo4Sb12.

Skutterudite compounds have been studied as potential thermoelectric materials due to their high thermoelectric efficiency, which makes them attractive candidates for applications in thermoelectric power generation. In this study, the effects of double-filling on the thermoelectric properties of the CexYb0.2-xCo4Sb12 skutterudite material system were investigated through the process of melt spinning and spark plasma sintering (SPS). By replacing Yb with Ce, the carrier concentration was compensated for by the extra electron from Ce donors, leading to optimized electrical conductivity, Seebeck coefficient, and power factor of the CexYb0.2-xCo4Sb12 system. However, at high temperatures, the power factor showed a downturn due to bipolar conduction in the intrinsic conduction regime. The lattice thermal conductivity of the CexYb0.2-xCo4Sb12 skutterudite system was clearly suppressed in the range between 0.025 and 0.1 for Ce content, due to the introduction of the dual phonon scattering center from Ce and Yb fillers. The highest ZT value of 1.15 at 750 K was achieved for the Ce0.05Yb0.15Co4Sb12 sample. The thermoelectric properties could be further improved by controlling the secondary phase formation of CoSb2 in this double-filled skutterudite system.

Enhanced thermoelectric figure of merit in indium and ytterbium double-filled skutterudite bulk materials through simultaneously optimising power factor and reducing thermal conductivity

In this study, YbxCo4Sb12 and InxYb0.3Co4Sb12 bulk materials were fabricated via microwave synthesis combined with spark plasma sintering. Based on ytterbium single filling, indium was further filled into the voids of CoSb3. Carrier concentration and mobility were regulated as 1.6–1.8 × 1020 cm-³ and ∼30 cm2V−1S−1, respectively, resulting in an improved power factor of ∼4900 μWm−1K−2. The phonon-resonant scattering, caused by indium and ytterbium double filling, was combined with other phonon scattering mechanisms such as nano-inclusion, dislocation, and grain boundary segregation, which resulted in a significantly decreased lattice thermal conductivity of 0.72–2.08 Wm−1K−1. Owing to the improvements in carrier concentration and phonon transport, excellent thermoelectric performance, reflected by ZT = 1.38 at 773 K, was achieved in In0.4Yb0.3Co4Sb12.

Thermal Conductivity Reduction by Tuning the Rattler Fraction in a p-type CeyYb1−yFe3CoSb12 Double-filled Skutterudite

The rattling effect has been a major research topic in attempts to reduce high-frequency phonon conduction, especially for the skutterudite system. Filling skutterudite with rattler atoms with different localized frequencies (double-filling) has also been reported as an effective method to decrease the thermal conductivity. Optimization research for the double rattler effect is required, especially in the p-type R1−yR′yFexCo4−xSb12 double-filled skutterudite system (R, R′ = rare earth rattlers) because these need larger numbers of rattlers (≈ 1.0) than are needed in the n-type skutterudite case (≈ 0.3). We attempted to optimize the rattler fraction in a p-type CeyYb1−yFe3CoSb12 skutterudite system in this research. We combined a melt spinning (MS) process with a subsequent spark plasma sintering (SPS) technique to fabricate bulk nanocrystalline skutterudites. The resulting low thermal conductivity (2.20 Wm−1K−1 at 723 K) and higher electrical conductivity (912 Ω−1cm−2 at 723 K) of this Ce0.6Yb0.4Fe3CoSb12 double-filled system makes the maximum dimensionless figure-of-merit (ZT, 0.90 at 723 K) in the double-filled specimen higher than those in the single-filled skutterudite systems (YbyFe3CoSb12, and CeyFe3CoSb12) previously reported by us. We found that filling with the Ce4+-rattler was more effective than filling with the Yb2+ for reducing the lattice thermal conductivity in the Cey Yb1−yFe3CoSb12 double-filler system. The Yb2+-filler effectively increased the electrical conductivity of the p-type Fe3CoSb12 filled skutterudite by modifying the carrier concentration.

Solubility study of Y in n-type YxCe0.15Co4Sb12 skutterudites and its effect on thermoelectric properties

Low-cost Ce has been proved to be an effective auxiliary filler for performance tuning in CoSb3-based skutterudite, one of the most promising thermoelectric materials at medium temperature. However, the low solubility of Ce in CoSb3 makes its overall thermoelectric properties so far inferior to that with expensive and scarce Yb filler. Herein, using non-equilibrium processing, we successfully synthesized a series of double-filled skutterudites with Ce as the dominating filling element and Y as the auxiliary filler. Our investigation shows that Y filling not only optimizes the carrier concentration and thus enhances the overall power factor but also introduces additional phonon scattering and hence leads to significantly suppressed lattice thermal conductivity. The concurrently optimized electrical and thermal properties realize the maximum ZT of ~1.37 at 873 K in the Y0.05Ce0.15Co4Sb12 sample, prevailing over the other involved Ce single- or even double-filled skutterudites, which will potentially facilitate a widespread application of filled skutterudites.

High thermoelectric performance from high carrier mobility and reduced lattice thermal conductivity in Ba, Yb double-filled Skutterudites

The semicoherent low-angle grain boundaries with dense dislocation arrays have been confirmed as an effective way to improve the thermoelectric properties for Yb single-filled Skutterudites using liquid-phase compaction method in our previous work. It is known that fillers with different rattling frequencies can scatter a wider spectrum of phonons, leading to further reduced lattice thermal conductivity. Thus, on the basis of liquid-phase compaction method, Ba, Yb double-filled Skutterudites were successfully synthesized. Ba, Yb double filling and liquid-phase compaction method synergistically optimize carrier concentration and reduce lattice thermal conductivity, resulting in exceptional high PF of 57 μW cm−1 K−2 at 773 K and excellent thermoelectric figure of merit ZT = 1.53 at 823 K. Correspondingly, the high theoretical output power density ∼15.6 W cm−2 and leg efficiency ≈14% can be obtained by calculation assuming the leg length of 2 mm, hot-side temperature Th = 800 K, and cold-side temperature Tc = 300 K. This work highlights that the obtained Skutterudites display great promise for harvesting electricity from waste heat with heat source at the medium temperature range.

Interaction between filler species in double-filled skutterudites

Filled skutterudites are workhorse materials in thermoelectric research. In recent years, double- or even triple-filling has emerged as a promising strategy to improve the thermoelectric figure of merit in skutterudites. For example, one type of filler is used to reduce thermal conductivity while the other filler is used to control the electron filling. However, in these studies, each filler atom is considered independent of each other and the interaction between the two filler atoms is completely ignored. Here we present our detailed investigation of the local structure of filler atoms in (${\mathrm{Yb},\mathrm{In})}_{x}{\mathrm{Co}}_{4}{\mathrm{Sb}}_{12}$ and show that significant interaction does exist between filler species. While Yb or In filler atoms at low concentration occupy the usual $2a$ filler site on their own, when Yb filler concentration goes above a critical value $x\ensuremath{\approx}0.15$, Yb pushes In atoms from the $2a$ site and into the $24g$ substitutional site replacing Sb atoms. This behavior is in stark contrast to that of Ga, which forms a dual-site defect even in the absence of Yb fillers. The temperature-dependent, extended x-ray absorption fine structure (EXAFS) data further reveal distinct lattice dynamical properties around In, Ga, and Yb filler atoms. Our findings point out that the filler interaction should be an important design consideration for new thermoelectric skutterudites.

High Temperature Transport Properties of Yb and In Double-Filled p-Type Skutterudites

Yb and In double-filled and Fe substituted polycrystalline p-type skutterudite antimonides were synthesized by direct reaction of high-purity elements, followed by solid-state annealing and densification by hot pressing. The stoichiometry and filling fraction were determined by both Rietveld refinement of the X-ray diffraction data and energy dispersive spectroscopic analyses. The transport properties were measured between 300 K and 830 K, and basically indicate that the resistivity and Seebeck coefficient both increase with increasing temperature. In both specimens, the thermal conductivity decreased with increasing temperature up to approximately 700 K, where the onset of bipolar conduction was observed. A maximum ZT value of 0.6 at 760 K was obtained for the Yb0.39In0.018Co2.4Fe1.6Sb12 specimen.

Enhancing the thermoelectric properties of single and double filled p-type skutterudites synthesized by an up-scaled ball-milling process

The single and double filled p-type skutterudites Ce0.8Fe3CoSb12 and Ce0.5Yb0.5Fe3.25Co0.75Sb12 have been prepared by mechanical alloying. This offers a rapid method for the preparation of skutterudites that could be scaled up for adoption at industrial level. The large-scale samples prepared by ball-milling exhibit enhanced figures of merit ZT, compared with materials prepared by conventional solid-state reaction. At room temperature ZT is increased by ca. 19% for both single and double filled skutterudites. Maximum figures of merit, ZT = 0.68 and ZT = 0.93 are attained for Ce0.8Fe3CoSb12 at 773 K and Ce0.5Yb0.5Fe3.25Co0.75Sb12 at 823 K respectively. The improvement in thermoelectric values at room temperature may be traced to a reduction in thermal conductivity in the ball-milled samples arising from the reduced grain size. The influence of the microstructure on the thermoelectric properties, together with the stability in air and the performance of the materials after several heating and cooling cycles has been studied and are detailed in this work. The densified samples prepared by ball-milling also show a higher resistance to oxidation, which starts at 694 K for Ce0.8Fe3CoSb12 and at 783 K for Ce0.5Yb0.5Fe3.25Co0.75Sb12.

Transport and mechanical properties of the double-filled p-type skutterudites La0.68Ce0.22Fe4−xCoxSb12

Till date, the dimensionless figure of merit (ZT) of p-type skutterudites lags its n-type counterpart. In this work, we report improved thermoelectric performance of La/Ce double filled p-type skutterudites. The samples were synthesized by hot pressing nano-powder made by ball milling annealed ingots of La0.68Ce0.22Fe4−xCoxSb12 with varying Co concentration. By tuning the Fe/Co ratio, we achieved a higher power factor (∼35 μW cm−1 K−2 at 475 °C), leading to a peak ZT of 1.15 at ∼475 °C in La0.68Ce0.22Fe3.5Co0.5Sb12. Nanoindentation experiment carried on fine surface of the best sample reveals hardness of 5.4 GPa and modulus of elasticity of 126 GPa, which is much higher than those of Bi2Te3 and PbTe based materials, indicating skutterudites are mechanically strong, good for some practical applications.

On the relevance between fine structure and enhanced performance of skutterudite thermoelectric materials: X-ray spectroscopy studies

The relevance between the fine structure and transport performance of thermoelectric materials can be revealed by X-ray spectroscopy including X-ray absorption and emission spectra as an effective tool. In this paper, the experimental spectra of extended X-ray absorption fine structure (EXAFS), X-ray absorption near-edge structure (XANES), and valence-band X-ray photoelectron spectroscopy (XPS) of Ba and In double-filled skutterudites (BaxInyCo4Sb12) were analyzed via the first-principles calculation and spectrum simulation. The atomic-scale fine structures indicate that the rectangle Sb4 rings become square when the total filing fraction of Ba and In increases. The transition of Sb4 rings leads to the band convergence and density of states (DOS) increase of the SbSb ppσ bonding and ppπ∗ antibonding states. The enhanced TE performance of BaxInyCo4Sb12 is essentially attributed to the band convergence, the increased DOS near the Fermi level, and the resonant phonon scattering of Ba and In fillers.

Synthesis and Thermoelectric Properties of p-Type Double-Filled Ce1−z YbzFe4−x Co x Sb12 Skutterudites

p-Type Ce1−z YbzFe4−x Co x Sb12 skutterudites were prepared by encapsulated melting and hot pressing. The thermoelectric and transport properties were examined as a function of Ce/Yb double filling fraction and amount of Co substitution. The Hall coefficients and Seebeck coefficients were positive, implying that all specimens had p-type conduction. Carrier concentration decreased with increasing Co and Yb content. Electrical conductivity decreased and Seebeck coefficients increased with increasing Co content, because of charge compensation. In contrast, electrical conductivity increased and Seebeck coefficients decreased with increasing Yb filling fraction, because of the valence difference between Yb2 or 3+ and Ce3 or 4+. The lattice thermal conductivity of the double-filled skutterudites was lower than that of the single-filled skutterudites. The thermoelectric properties of the double-filled skutterudites were improved substantially; the maximum power factor, PF = 3.2 mW m−1 K−2, was achieved at 823 K for Ce0.25Yb0.75Fe3.5Co0.5Sb12 and the maximum dimensionless figure of merit ZT = 0.87 was achieved at 723 K for Ce0.75Yb0.25Fe3.5Co0.5Sb12.

Improved thermoelectric performance of n-type Ca and Ca-Ce filled skutterudites

Thermoelectric (TE) technology for use in automotive waste heat recovery is being advanced by General Motors with support from the US Department of Energy. Skutterudites are a very promising material for this application of TE technology due to their superior mechanical properties and good TE performance. Double-filled YbxBayCo4Sb12 with ZT values around 1.1 at 750 K are the best performing n-type skutterudites produced on a large scale using an economically viable approach of melt spinning (MS) in conjunction with spark plasma sintering (SPS). Another economical production method on the tons scale, the melt quench annealing (MQA) technique, has been recently claimed by Treibacher Industrie AG, further information is available [G. Rogl et al., Acta Mater. 76, 434–448 (2014)]. A possible hurdle to commercial implementation of these materials is the use of rare earths as the fillers to reduce thermal conductivity and improve the electrical transport properties. It will be shown herein that skutterudites double-filled with Ca and Ce, both of which are lower-cost fillers, display markedly different TE properties depending on whether they are produced by MQA or MS + SPS synthesis techniques. Ca and Ce double-filled skutterudites prepared by MS + SPS have TE properties that are superior to the same compositions prepared by MQA and that are comparable to the best performing Yb and Ba filled materials. Furthermore, the results of this study suggest that the unusually poor transport properties of MQA Ca-filled skutterudites can be ascribed to deleterious secondary phases, which is contrary to reports in the literature attempting to explain these irregularities via band structure features.

Thermoelectric Properties of Double-Filled p-Type La1−z Yb z Fe4−x Co x Sb12 Skutterudites

La and Yb double-filled p-type skutterudites (La1−z Yb z Fe4−x Co x Sb12; 0.25 ≤ z ≤ 0.75 and 0.5 ≤ x ≤ 1) were synthesized by encapsulated melting and homogenized by use of heat treatment. It was apparent from the positive signs of the Seebeck coefficient and the Hall coefficient that all specimens had p-type characteristics. The carrier concentration decreased with charge compensation. The thermal conductivity and the electrical conductivity decreased and the Seebeck coefficient increased with increasing substitution of Co for Fe. However, the carrier concentration was increased by increasing the Yb filling ratio. Electrical conductivity increased and the Seebeck coefficient decreased with increasing Yb filling, because the electron valence of Yb2+ was lower than that of La3+. The thermal conductivity decreased with charge compensation, and the lattice thermal conductivity decreased with increasing Yb filling. Yb was more effective than La at reducing lattice thermal conductivity. The power factor (PF) and dimensionless figure of merit (ZT) increased with increasing temperature up to a specific temperature. The maximum PF = 2.81 mW/mK2 at 823 K was obtained for La0.75Yb0.25Fe3.5Co0.5Sb12 and the maximum ZT = 0.74 at 723 K was achieved for La0.5Yb0.5Fe3CoSb12.

Structures and thermoelectric properties of double-filled (CaxCe1−x)Fe4Sb12 skutterudites

The structures and thermoelectric properties of the double-filled (CaxCe1−x)Fe4Sb12 series (x=0, 0.25, 0.5, 0.75, and 1) have been studied using a combined experimental and computational methods. Compounds of (CaxCe1−x)Fe4Sb12 were obtained only for x=0, 0.5, and 1. Composition with x=0.25 was found to be a mixture of x=0 and 0.5 compounds, and composition with x=0.75 was found to be a mixture of x=1 and 0.5 compounds, respectively. Our conclusions on phase formation are supported by density functional theory (DFT) calculations. In Ca0.5Ce0.5Fe4Sb12, Ca substitution in the Ce site of CeFe4Sb12 leads to high hole concentrations, resulting in stronger semimetal transport as compared to CeFe4Sb12. Ca0.5Ce0.5Fe4Sb12 yields a slightly higher ZT value than that of CeFe4Sb12, which is attributed to its lower lattice thermal conductivity. Phonon mode calculations adopting a three-particle bending model suggest that thermal conductivity is reduced upon Ca substitution because of an additional vibration mode which involves both Ca and Ce atoms.

Qualifying the Role of Indium in the Multiple-Filled Ce0.1InxYb0.2Co4Sb12 Skutterudite

Literature confirms an improvement in the overall TE properties due to the in situ InSb nano-dispersed phases located along the grain boundaries in several double-filled InxYzCo4Sb12 skutterudites. However, the single-filled InxCo4Sb12 reports contribute enhancement in TE properties solely on the nature of In as a void filler. To qualify the effect of In on the TE properties on multiple-filled skutterudites several multiple-filled Ce0.1InxYb0.2Co4Sb12 skutterudite samples, with nominal composition Ce0.1InyYb0.2Co4Sb12 (0 ≤ y ≤ 0.2), were synthesized. A double-filled base-line sample Ce0.1Yb0.2Co4Sb12 was also synthesized and characterized to create a much fuller depiction of the nature of In and its impact on the TE properties of the filled Co4Sb12-based skutterudite materials. Our results confirm that small amounts of In can be effective at increasing electrical conductivity in the multiple-filled Ce0.1InyYb0.2Co4Sb12 skutterudite. An increased mobility and thus electrical conductivity result in a 15% increase in the dimensionless Figure of Merit, ZT, in the nominal sample composition, Ce0.1In0.05Yb0.2Co4Sb12, which exhibits a state of the art ZT > 1.4 at T = 820 K.

Enhanced thermoelectric performance of (Ba,In) double-filled skutterudites via randomly arranged micropores

Porous (Ba,In) double-filled skutterudite materials with pore diameter about 1–4 μm were prepared by the decomposition of metastable ZnSb inclusions induced by the Zn sublimation. Transport measurements revealed that the Seebeck coefficient was increased due to the electron filtering effect induced by nanostructures in the surfaces of pores, the electrical conductivity was almost unchanged because of the percolation effect of conducted network composed of filled skutterudites, and the lattice thermal conductivity was dramatically suppressed due to the enhanced pore-edge boundary scattering of long-wavelength phonons. As a result, a maximum ZT of 1.36 was obtained, increased by 22.5% as compared to that of the bulk material with same chemical composition. This work demonstrates that by introducing porous structures is thought to be an efficient approach to improve the thermoelectric performance of bulk materials.

Synthesis and Thermoelectric Properties of In and Pr Double-Filled Skutterudites In x Pr y Co4Sb12

Double-filled skutterudites InxPryCo4Sb12, which are currently being investigated for potential applications as thermoelectric materials, have been successfully prepared by inductive melting and annealing. Our results showed that In and Pr double filling effectively improves both electrical conductivity and Seebeck coefficient compared with pristine or single-filled CoSb3, giving rise to a respectable power factor. The largest power factor, 2.33 m Wm−1 K−2, was achieved at 609 K for In0.05Pr0.05Co4Sb12; this value is approximately three times that for InxCo4Sb12 (x ≤ 0.3) skutterudites. These results imply that In and Pr double filling are better than In single filling for efficient improvement of the thermoelectric properties of CoSb3 skutterudite.

Enhanced thermoelectric performance of optimized Ba, Yb filled and Fe substituted skutterudite compounds

Abstract It is well known that Ba and Yb are effective pair-filling species in skutterudites. They offer a large mass and size contrast therefore yielding widely different phonon resonant scattering frequencies. These features of Ba and Yb fillers inspired our investigation of the high temperature thermoelectric properties of Fe substituted and Ba/Yb double-filled skutterudite compounds of compositions BaxYbyFe0.4Co3.6Sb12 (0.25 ⩽ x, y ⩽ 0.4 nominal). We measured the Seebeck coefficient, electrical conductivity, thermal conductivity, and the Hall coefficient from 300 K to 800 K. All samples were prepared using the traditional method consisting of melting-annealing followed by spark plasma sintering. The Hall coefficient and the Seebeck coefficient signs indicate that all samples are n-type conductors. The electrical conductivity increases and the Seebeck coefficient decreases with the increasing Ba content, likely due to the increasing carrier density as the amount of Ba increases. It is seen that Ba is more effective than Yb in tuning electronic transport properties while Yb has a greater effect on the lattice thermal conductivity. EMPA and PXRD analyses indicate that increasing the Ba content reduces the filling fraction of Yb, therefore the solubility limit of combined Ba + Yb filling remains about 0.4 (actual) for BaxYbyFe0.4Co3.6Sb12 (0.25 ⩽ x, y ⩽ 0.4 nominal) compositions. The combination of a high power factor (47 μW/K2 cm) and a reasonably low thermal conductivity (2.79 W/m K) of Ba0.3Yb0.3Fe0.4Co3.6Sb12 (nominal) leads to a value of the thermoelectric figure of merit ZT∼1.35 at 800 K, one of the highest values of ZT for double-filled n-type skutterudite compounds.

Lattice thermal transport in double-filled skutterudites In0.1Yb y Co4Sb12

In order to investigate the phonon scattering mechanisms in double-filled skutterudites, the low-temperature lattice thermal conductivities of In0.1Yb y Co4Sb12 were measured and analyzed based on the Debye model. The eingenmode frequencies of In and Yb were obtained from low-temperature specific heat capacity analysis. It is found that filling these two types of guest atoms with different eingenmode frequencies into the voids in skutterudites could introduce strong point defect and resonant scattering to lattice phonons, thus lead to significant decrease in the lattice thermal conductivity.

The thermoelectric properties of InxM0.2Co4Sb12 (M=Ba and Pb) double-filled skutterudites

Skutterudite compounds InxM0.2Co4Sb12 (M=Ba and Pb) with bcc crystal structure have been prepared by high pressure and high temperature (HPHT) method. Through this method, the processing time can be reduced from a few days to half an hour. Remarkably the combination of In and Ba was much more effective in reducing lattice thermal conductivity κL than In and Pb. The structure of InxM0.2Co4Sb12 skutterudites was evaluated by means of x-ray diffraction, and Seebeck coefficient, electrical resistivity, power factor and thermal conductivity of InxM0.2Co4Sb12 were all presented in the temperature range of 314.9–673K. The transport properties measured on the synthesized samples have been compared with each other. The figures of merit, ZT, of samples all increased with the increasing temperature. Among all the samples, In0.5Ba0.2Co4Sb12 showed the highest power factor of 30.3μWcm−1K−2 at 643.5K, the lowest lattice thermal conductivity 1.128Wm−1K−1 at 643.5K and the maximum ZT value 0.80 at 673K.