Skutterudite Compounds Research Articles

Superconducting parameters of BaPt4−xAuxGe12filled skutterudite

We report on a study of the superconducting properties for a series of polycrystalline BaPt(4-x)Au(x)Ge12 filled skutterudite compounds for x = 0, 0.5, 0.75, and 1. Muon spin rotation (muSR) spectroscopy as well as magnetization, specific heat, and electrical resistivity measurements were performed. The magnetic penetration depth \lambda, the coherence length \xi, and the Ginzburg-Landau parameter \kappa are evaluated. The temperature dependence of the superfluid density is well described by an s-wave superconducting gap and this classical scenario is supported by the field-independent \lambda. The gap-to-Tc ratio \Delta/kTc increases with the Au content from 1.70 for x = 0 to 2.1(1) for x = 1. By combining muSR, magnetization, and specific heat data, we find that BaPt(4-x)Au(x)Ge12 compounds are in between the dirty and clean limits with mean free paths of the carriers l ~ \xi. Interestingly, resistivity data for BaPt4Ge12 indicate a much higher upper critical field, which is probably due to defects or impurities close to the surface of the crystallites.

Unusual local disorder in NdOs4Sb12and PrOs4Sb12skutterudites

We present a temperature-dependent, extended x-ray absorption fine structure (EXAFS) analysis of the filled skutterudite compounds NdOs${}_{4}$Sb${}_{12}$, PrOs${}_{4}$Sb${}_{12}$, and EuOs${}_{4}$Sb${}_{12}$. Although the interpretation of recent ultrasonic measurements suggested off-center displacements for Nd and Pr in NdOs${}_{4}$Sb${}_{12}$ and PrOs${}_{4}$Sb${}_{12}$, the EXAFS analysis shows that the Nd-Sb, Pr-Sb, and Sb-Sb peaks are well ordered. Surprisingly, however, the second-neighbor Nd-Os and Pr-Os peaks are highly disordered, even at low temperatures, and the Os-Os peak also has some disorder in the Nd and Pr systems. In contrast to the anomalous results for the Pr and Nd samples, neither the Eu-Sb, Eu-Os, nor the Os-Os peak is disordered at low temperature for EuOs${}_{4}$Sb${}_{12}$. For all three systems within estimated errors, the rare-earth atom is on-center inside the Sb cage. We propose that for the Nd and Pr compounds, the Os cage distorts, with some of the Os atoms moving either towards or away from the Nd or Pr atoms, such that the Nd-Sb, Pr-Sb, and Sb-Sb pair distances have very little disorder. Some possible distortion models are suggested and the possible ramifications for thermoelectric and transport properties are discussed.

Thermoelectric Properties of Indium-Added Skutterudites In x Co4Sb12

The high-temperature thermoelectric properties of InxCo4Sb12 (0.05 ≤ x ≤ 0.40) skutterudite compounds were investigated in this study. The phase states of the samples were identified by x-ray diffraction analysis and field-emission scanning electron microscopy at room temperature. InSb and CoSb2 were found as secondary phases in samples with x = 0.10 to 0.40. The filling limit of In into the CoSb3 cages of InxCo4Sb12 was in the range 0.05 < x < 0.10. The electrical resistivity, Seebeck coefficient, and thermal conductivity of the InxCo4Sb12 samples were measured from room temperature to 773 K. The Seebeck coefficient of all samples was negative. Reduction of the thermal conductivity by In addition resulted in a high thermoelectric figure of merit (ZT) of 0.67 for In0.35Co4Sb12 at 600 K.

Structural Instability of Unfilled Skutterudite CompoundsMX3(M= Co, Rh, and Ir;X= As and Sb) under High Pressure

Powder X-ray diffraction patterns of the binary unfilled skutterudite compounds MX 3 ( M = Co, Rh, and Ir; X = As and Sb) have been studied under high pressures up to 60 GPa, using synchrotron radiation and a diamond anvil cell. The X-ray diffraction patterns of MX 3 above a critical pressure show anomalous reductions in the intensities of several Bragg peaks. Furthermore, the pressure-volume curves of MX 3 reveal a saturation behavior of the volume reduction with pressure. Upon pressure release, MX 3 exhibit a volume greater than those of pristine compounds. This anomalous behavior could be interpreted as a pressure-induced self-insertion reaction. Furthermore, the high-temperature and high-pressure phase diagram of RhSb 3 was obtained. The structural instability of MX 3 under high pressures is discussed.

Thermal to Electrical Energy Conversion of Skutterudite-Based Thermoelectric Modules

The performance of thermoelectric (TE) materials has improved tremendously over the past decade. The intrinsic thermal and electrical properties of state-of-the-art TE materials demonstrate that the potential for widespread practical TE applications is very large and includes TE generators (TEGs) for automotive waste heat recovery. TE materials for automotive TEG applications must have good intrinsic performance, be thermomechanically compatible, and be chemically stable in the 400 K to 850 K temperature range. Both n-type and p-type varieties must be available at low cost, easily fabricated, and durable. They must also form robust junctions and develop good interfaces with other materials to permit efficient flows of electrical and thermal energy. Among the TE materials of interest for automotive waste heat recovery systems are the skutterudite compounds, which are the antimony-based transition-metal compounds RTE4Sb12, where R can be an alkali metal (e.g., Na, K), alkaline earth (e.g., Ba), or rare earth (e.g., La, Ce, Yb), and TE can be a transition metal (e.g., Co, Fe). We synthesized a considerable quantity of n-type and p-type skutterudites, fabricated TE modules, incorporated these modules into a prototype TEG, and tested the TEG on a production General Motors (GM) vehicle. We discuss our progress on skutterudite TE module fabrication and present module performance data for electrical power output under simulated operating conditions for automotive waste heat recovery systems. We also present preliminary durability results on our skutterudite modules.

ChemInform Abstract: High Thermoelectric Performance of In, Yb, Ce Multiple Filled CoSb3 Based Skutterudite Compounds.

AbstractThe thermoelectric properties of Yb0.2In0.2Co4Sb12, Yb0.2Ce0.15Co4Sb12, Yb0.2Ce0.15In0.2Co4 Sb12, and Yb0.3Ce0.15In0.2Co4 Sb12 (obtained from the elements by melting—annealing followed by spark plasma sintering) are examined by electrical and thermal conductivity, Seebeck coefficient and Hall coefficient measurements.

Evidence for multiple superconducting gaps in the filled skutterudite compound LaRu4As12

Superconducting properties of the filled skutterudite compound LaRu${}_{4}$As${}_{12}$ were investigated both as a function of temperature and magnetic field. At $T\ensuremath{\ll}{T}_{\mathrm{c}}=10.45$ K, the electronic contribution to the specific heat was found to be at variance with the predictions for an $s$-wave superconductor with one energy gap. In constant applied fields up to ${H}_{\mathrm{c}2}(0)\ensuremath{\approx}10.2$ T, multiple gaps have been further inferred from both a nonlinear magnetic-field dependence of the specific heat in the zero-temperature limit and a positive curvature of the upper critical field in the vicinity of ${T}_{\mathrm{c}}$. Thus, LaRu${}_{4}$As${}_{12}$ appears to be a rare multiple-gap superconductor with cubic symmetry. Intriguing evidence for multiband effects is observed in the compound with enhanced superconducting properties as compared to other skutterudite superconductors.

Electrical and thermal properties of Fe substituted double-filled BaxYbyFezCo4−zSb12 skutterudites

Fe-substituted double-filled BaxYbyFezCo4−zSb12 (x=0.1, y=0.2 and z=0.0–0.4 nominal) compounds were synthesized using a melting-annealing-spark plasma sintering (SPS) method. Their thermoelectric properties were assessed by measuring the Seebeck coefficient, electrical conductivity, thermal conductivity and the Hall coefficient. The sign of the Hall coefficient indicates that electrons are the dominant carriers in all compounds except Ba0.1Yb0.2Fe0.4Co3.6Sb12. The temperature dependence of the electrical conductivity and the carrier concentration reflect the transition from extrinsic to intrinsic behavior depending on the amount of Fe substituted for Co. Jonker and Ioffe analyses are applied to Fe-substituted double-filled BaxYbyFezCo4−zSb12 compounds in order to evaluate the range of minimum and maximum power factors achievable in n-type filled skutterudite compounds at room temperature (300K). The predicted maximum room temperature power factor values in the range of 15–45μW/K2cm are comparable to experimentally reported values of n-type skutterudite compounds.

Preparation and thermoelectric properties of Ga-substituted p-type fully filled skutterudites CeFe4−xGaxSb12

We demonstrate a successful substitution of Ga at the Fe sites with a solubility limit of −0.11 in the p-type filled skutterudite compounds CeFe4−xGaxSb12. With increasing Ga content, the electrical conductivity declines while the Seebeck coefficient improves gradually, consistent with the expected decrease in hole concentration due to the extra electrons introduced by Ga. Moreover, the resemblances in electrical transport properties between Ga- and Co-substituted systems with similar composition indicate that Ga doping exerts little influence on the electronic structure near the top of the valence band. The phonon transport is scarcely affected by the introduction of Ga because of negligible differences in atomic masses and sizes of Ga and Fe. The thermoelectric performance of Ga-substituted samples is slightly improved in the temperature range of 600K to 800K with respect to that of Ga-free sample, revealing a favorable effect of Ga-substitution on the intermediate temperature power generation application of this promising p-type material.

Topological insulators in filled skutterudites

We propose new topological insulators in cerium filled skutterudite (FS) compounds based on ab initio calculations. We find that two compounds CeOs4As12 and CeOs4Sb12 are zero gap materials with band inversion between Os-d and Ce-f orbitals, which are thus parent compounds of two and three-dimensional topological insulators just like bulk HgTe. At low temperature, both compounds become topological Kondo insulators, which are Kondo insulators in the bulk, but have robust Dirac surface states on the boundary. This new family of topological insulators has two advantages compared to previous ones. First, they can have good proximity effect with other superconducting FS compounds to realize Majarona fermions. Second, the antiferromagnetism of CeOs4Sb12 at low temperature provides a way to realize the massive Dirac fermion with novel topological phenomena.

Microstructure and thermoelectric properties of CoSb2.75Ge0.25−xTex prepared by rapid solidification

Since the vibration modes of the pnicogen rings in CoSb3-based skutterudites fall within the range of frequencies of heat-carrying phonons, disruption of the rings by doping should have a strong influence on heat transport in this material. To test the premise, single-phase double-doped CoSb2.75Ge0.25−xTex (x=0.125–0.20) compounds were synthesized by combining melt spinning with a spark plasma sintering method. Following the melt-spinning process, the side of the ribbons contacting the copper drum is featureless and reflects its amorphous nature while the free surface of the ribbons is composed of 30–80nm grains. After spark plasma processing the average grain size of the bulk samples is about 200nm. High-resolution transmission electron microscopy images show an in situ nanostructure consisting of circular, 15nm diameter dots of Te- and Ge-enriched skutterudite phase embedded in the skutterudite matrix. Transport properties were measured from 2 to 800K as a function of Te and Ge content on the pnicogen (Sb) rings and the results were correlated with the structural data. Double-doping on pnicogen rings with Ge and Te, and using melt-spinning processing, results in binary skutterudite compounds that possess an impressive figure of merit of ZT∼1.1 at 750K.

Enhanced thermoelectric performance in sulfur-doped Co4Sb11.9 − xTexS0.1 skutterudites

Te and S co-doped Co4Sb11.9−xTexS0.1 skutterudite compounds have been synthesized by solid state reaction and spark plasma sintering, and the effects of S-addition on its the structure and thermoelectric properties have been investigated. The results indicate that small amount of sulfur addition diminishes the electrical conductivity, while suppresses the thermal conductivity and lattice thermal conductivity significantly due to the increased point-defect scattering. As a result, the dimensionless figure of merit ZT of Co4Sb11.9−xTexS0.1 is enhanced over the whole measured temperature range, and the optimal value ~1.08 is obtained at 800K for Co4Sb11.3Te0.6S0.1 compound, which is about a 13% improvement over that of Co4Sb11.4Te0.6 at the same temperature.

High thermoelectric performance of In, Yb, Ce multiple filled CoSb3 based skutterudite compounds

Filling voids with rare earth atoms is an effective way to lowering thermal conductivity which necessarily enhances thermoelectric properties of skutterudite compounds. Yb atom is one of the most effective species among the rare earth atoms for filling the voids in the skutterudite structure due to a large atomic mass, radius and it is intermediate valence state. In this work, we aim to find the best filling partners for Yb using different combinations of Ce and In as well as to optimize actual filling fraction in order to achieve high values of ZT. The traditional method of synthesis relying on melting–annealing and followed by spark plasma sintering was used to prepare all samples. The thermoelectric properties of four samples of Yb0.2In0.2Co4Sb12, Yb0.2Ce0.15Co4Sb12, Yb0.2Ce0.15In0.2Co4Sb12, and Yb0.3Ce0.15In0.2Co4Sb12 (nominal) were examined based on the Seebeck coefficient, electrical conductivity, thermal conductivity, and Hall coefficient. Hall coefficient and Seebeck coefficient signs confirm that all samples are n-type skutterudite compounds. Carrier density increases with the increasing Yb+Ce content. A high power factor value of 57.7μW/K2/cm for Yb0.2Ce0.15Co4Sb12 and a lower thermal conductivity value of 2.82W/m/K for Yb0.2Ce0.15In0.2Co4Sb12 indicate that small quantities of Ce with In may be a good partner to Yb to reduce the thermal conductivity further and thus enhance the thermoelectric performance of skutterudites. The highest ZT value of 1.43 was achieved for Yb0.2Ce0.15In0.2Co4Sb12 triple-filled skutterudite at 800K.

Residual Strength Degradation of CoSb3 Skutterudite Compounds Under Low-Cycle Fatigue Loading

This paper reports an experimental and theoretical study on the residual strength degradation of CoSb3-based skutterudites subjected to compression–compression low-cycle fatigue loading. Based on the experimental results and findings of other researchers, a residual strength degradation model using a normal distribution function is proposed. The model can predict the residual strength at a given number of fatigue loading cycles with given probability. Lastly, the validity of the model is confirmed by experimental results for CoSb3.

Thermal Cycling Effects on the Thermoelectric Properties of n-Type In,Ce-Based Skutterudite Compounds

n-Type In-filled CoSb3 is a known skutterudite compound that has shown promising thermoelectric (TE) properties resulting in high dimensionless figure of merit values at elevated temperatures. Use in various waste heat recovery applications will require survival and operation after exposure to harsh thermal cycling environments. This research focused on uncovering the thermal cycling effects on TE properties of n-type In0.2Co4Sb12 and In0.2Ce0.15Co4Sb12 skutterudite compositions as well as quantifying their temperature-dependent structural properties (elastic modulus, shear modulus, and Poisson’s ratio). It was observed that the Seebeck coefficient and resistivity increased only slightly in the double-filled In,Ce skutterudite materials upon thermal cycling. In the In-filled skutterudites the Seebeck coefficient remained approximately the same on thermal cycling, while the electrical resistivity increased significantly after thermal cycling. Results also show that the thermal conductivity marginally decreases in the case of In-filled skutterudites, whereas the reduction is more pronounced in In,Ce-based skutterudite compounds. The possible reason for this kind of reduction can be attributed to grain pinning effects due to formation of nanoinclusions. High-temperature structural property measurements (i.e., Young’s modulus and shear modulus) are also reported. The results show that these structural properties decrease slowly as temperature increases and that the compounds are structurally stable after numerous (up to 200) thermal cycles.

Synthesis and high temperature transport properties of Te-doped skutterudite compounds

Te-doped skutterudite compounds Co4Sb12−x Te x (x = 0.4–0.7) have been fabricated by solid state reaction method and spark plasma sintering. The scanning electron microscope images indicate all samples are compact and the average particle size increases with the Te doping fraction. The carrier concentration and electrical conductivity exhibit positive doping fraction dependence, and a maximum electrical conductivity of 16.29 × 104 Sm−1 is obtained at 300 K for Co4Sb11.3Te0.7. The values of the power factor (x = 0.4–0.6) are greater than 4.0 × 10−3 Wm−1 K−2 at the temperature range of 650–800 K, larger than previous literature reports. The lattice thermal conductivity decreases monotonously over the whole investigated temperature range and exhibits a negative doping fraction dependence except for Co4Sb11.3Te0.7. The resultant dimensionless figure of merit of all the samples increases monotonously over the whole investigated temperature range, and a maximum value of 0.95 is achieved at 800 K for Co4Sb11.4Te0.6.

Multipolar Interaction Derived from d–f Hybridization in Pr-Based Filled Skutterudite Compounds

The conduction bands of the skutterudite compounds are composed of p orbitals of the pnictogen cages and d orbitals of the transition metal ions. In this paper, we address the effect of hybridization between the d bands and localized 4 f states in relation with ordering and fluctuation of multipole moments in Pr-based filled skutterudites. First, we study the properties of a non-degenerate d band around the zone center in the framework of a tight-binding band model. Introducing a realistic form of the d – f hybridization, we show that the a g band can be hybridized only with the Γ 8 orbitals of f electrons. Then, we derive d – f multipolar interactions from the hybridization in the localized limit of f electrons in which the basis states are restricted to the singlet–triplet crystal field states. It is found that the quadrupolar interaction of the O x y -type becomes predominant in the case of the singlet ground state, whereas the triplet ground state apparently favors the magnetic interaction. The relati...

The role of Ga in Ba0.30GaxCo4Sb12+x filled skutterudites

Ba0.30GaxCo4Sb12+x (x = 0–0.30) skutterudite compounds were synthesized using a melt–quench–anneal–SPS method and the effect of the content of Ga on the structure and thermoelectric properties was investigated. In samples with Ga content x ≤ 0.15, Ga enters the skutterudite voids and its presence seems to stimulate a more homogeneous distribution of the filler species. Samples with Ga content x ≥ 0.20 possess uniformly dispersed nanoinclusions consisting of circular domains of GaSb with a diameter of 20 nm. As the content of Ga increases, the carrier concentration decreases, the Seebeck coefficient increases, and the heat transport is progressively more impeded. The thermoelectric figure of merit of Ba0.30GaxCo4Sb12+x is strongly enhanced in comparison to that of Ba0.30Co4Sb12 and reaches values in excess of 1.35 at 850 K for Ba0.30Ga0.30Co4Sb12.30, approximately twice the value of the Ba0.30Co4Sb12 sample.

Enhanced thermoelectric properties of Ba-filled skutterudites by grain size reduction and Ag nanoparticle inclusion

Ba-filled skutterudite compounds, Ba0.3Co4Sb12, with dispersed Ag nanoparticles have been synthesized by ball milling followed by hot pressing. The influence of Ag nanoparticles and their size distribution on electrical and thermal transport properties has been investigated in the temperature range from room temperature to 823 K. It was found that Ag nanoparticles in the Ba0.3Co4Sb12 matrix drastically enhance the electric conductivity and slightly increase the Seebeck coefficient. Surprisingly, Ag nanoparticles do not alter the carrier density of Ag/Ba0.3Co4Sb12 nanocomposites. This large improvement in the electrical conductivity as well as the corresponding power factor is assumed to come primarily from the enhanced mobility in Ag/Ba0.3Co4Sb12 nanocomposites. In addition, a large reduction in thermal conductivity is achieved by grain size reduction resulting from ball milling processing and as a result of the embedded Ag nanoparticles that scatter a wider range of phonon frequencies. These concomitant effects result in an enhanced thermoelectric performance with the dimensionless figure of merit ZT some 30% higher in comparison to the parent Ba0.3Co4Sb12. Moreover, we found it is advantageous to employ a wider size distribution of Ag nanoparticles to reduce the thermal conductivity to a large degree by enhancing phonon scattering. These observations demonstrate an exciting scientific opportunity to raise the figure-of-merit of filled skutterudites.

Thermoelectric Properties of Trisubstituted Skutterudite Co4Sb11Ge1−x−y Te x Se y Compounds

Trisubstituted skutterudite compounds Co4Sb11Ge1−x−y Te x Se y (x = 0.70 to 0.85, y = 0; x = 0.60 to 0.75, y = 0.1) have been fabricated by the solid-state reaction method combined with a subsequent spark plasma sintering process, and their thermoelectric properties measured in the temperature range from 300 K to 800 K. As the Te fraction increases, the electrical and thermal conductivity of the samples increase gradually, while the absolute Seebeck coefficient falls. The electrical and thermal conductivity of the samples doped with Se (y = 0.1) decrease significantly compared with the Se-free samples (y = 0). Particularly, the lattice thermal conductivity of the y = 0.1 samples drop by 20% to 31% compared with that of the y = 0 samples with the same Ge fraction at 300 K. In comparison with the double-substituted compounds Co4Sb11Ge1−x Te x (x = 0.70 to 0.85), the trisubstituted compounds Co4Sb11- Ge0.9−x Te x Se0.1 (x = 0.60 to 0.75) exhibit better thermoelectric performance. A maximum dimensionless figure of merit of 0.99 is obtained at 775 K for Co4Sb11Ge0.2Te0.70Se0.1, which is about 16% higher than that of Co4Sb11- Ge0.2Te0.80 at the same temperature.