Filler Atoms Research Articles

Neutron and X-ray powder diffraction study of skutterudite thermoelectrics

N- and p-type filled-skutterudite materials prepared for thermoelectric power generation modules were analyzed by neutron diffraction at the POWGEN beam line of the Spallation Neutron Source (SNS) and X-ray diffraction (XRD). The skutterudite powders were processed by melt spinning, followed by ball milling and annealing. The n-type material consists of Ba–Yb–Co–Sb and the p-type material consists of Di–Fe–Ni–Sb or Di–Fe–Co–Sb (Di = didymium, an alloy of Pr and Nd). Powders for prototype module fabrication from General Motors and Marlow Industries were analyzed in this study. XRD and neutron diffraction studies confirm that both the n- and p-type materials have cubic symmetry. Structural Rietveld refinements determined the lattice parameters and atomic parameters of the framework and filler atoms. The cage filling fraction was found to depend linearly on the lattice parameter, which in turn depends on the average framework atom size. This knowledge may allow the filling fraction of these skutterudite materials to be purposefully adjusted, thereby tuning the thermoelectric properties.

Improved thermoelectric performance of (Fe,Co)Sb3-type skutterudites from first-principles

Skutterudite materials have been considered as promising thermoelectric candidates due to intrinsically good electrical conductivity and tailorable thermal conductivity. Options for improving thermal-to-electrical conversion efficiency include identifying novel materials, adding filler atoms, and substitutional dopants. Incorporating filler or substitutional dopant atoms in the skutterudite compounds can enhance phonon scattering, resulting in reduction of thermal conductivity, as well as improving electrical conductivity. The structures, electronic properties, and thermal properties of double-filled Ca0.5Ce0.5Fe4Sb12 and Co4Sb12−2xTexGex compounds (x = 0, 0.5, 1, 2, 3, and 6) have been studied using density functional theory-based calculations. Both Ca/Ce filler atoms in FeSb3 and Te/Ge substitution in CoSb3 cause a decrease in lattice constant for the compounds. As Te/Ge substitution concentration increases, lattice constant decreases and structural distortion of pnictogen rings in the compounds occurs. This indicates a break in cubic symmetry of the structure. The presence of fillers and substitutions cause an increase in electrical conductivity and a gradual decrease in electronic band gap. A transition from direct to indirect band-gap semiconducting behavior is found at x = 3. Phonon density of states for both compounds indicate phonon band broadening by the incorporation of fillers and substitutional atoms. Both systems are also assumed to have acoustic-mode-dominated lattice thermal conductivity. For the Co4Sb12−2xTexGex compounds, x = 3 has the lowest phonon dispersion gradient and lattice thermal conductivity, agreeing well with experimental measurements. Our results exhibit the improvement of thermoelectric properties of skutterudite compounds through fillers and substitutional doping.

Convergence of multi-valley bands as the electronic origin of high thermoelectric performance in CoSb3 skutterudites.

Filled skutterudites R(x)Co4Sb12 are excellent n-type thermoelectric materials owing to their high electronic mobility and high effective mass, combined with low thermal conductivity associated with the addition of filler atoms into the void site. The favourable electronic band structure in n-type CoSb3 is typically attributed to threefold degeneracy at the conduction band minimum accompanied by linear band behaviour at higher carrier concentrations, which is thought to be related to the increase in effective mass as the doping level increases. Using combined experimental and computational studies, we show instead that a secondary conduction band with 12 conducting carrier pockets (which converges with the primary band at high temperatures) is responsible for the extraordinary thermoelectric performance of n-type CoSb3 skutterudites. A theoretical explanation is also provided as to why the linear (or Kane-type) band feature is not beneficial for thermoelectrics.

Estimation of Van der Waals Interaction Using FTIR Spectroscopy

Van der Waals (vdW) interaction is one of the important properties in the composite. The usual measurement used to investigate the vdW interaction between filler and polymeric binder is FTIR spectroscopy where the result obtained is the band shift appearance, however, it has not been used yet to estimate the vdW magnitude. Using the Lennard-Jones potential, we developed a new analysis method to obtain approximately its magnitude and further the distance between filler atoms and polymeric group. We used our model proposed to several data reported by some authors, and interestingly we also found an appearance of anharmonic vibration of the atoms.

On the True Indium Content of In-Filled Skutterudites.

The incongruently melting single-filled skutterudite InxCo4Sb12 is known as a promising bulk thermoelectric material. However, the products of current bulk syntheses contain always impurities of InSb, Sb, CoSb, or CoSb2, which prevent an unbiased determination of its thermoelectric properties. We report a new two-step synthesis of high-purity InxCo4Sb12 with nominal compositions x = 0.12, 0.15, 0.18, and 0.20 that separates the kieftite (CoSb3) formation from the topotactic filler insertion. This approach allows conducting the reactions at lower temperatures with shorter reaction times and circumventing the formation of impurity phases. The synthesis can be extended to other filled skutterudites. High-density (>98%) pellets for thermoelectric characterization were prepared by current-assisted short-time sintering. Sample homogeneity was demonstrated by potential and Seebeck microprobe measurements of the complete pellet surfaces. Synchrotron X-ray diffraction showed a purity of 99.9% product with traces (≤0.1%) of InSb in samples of nominal composition In0.18Co4Sb12 and In0.20Co4Sb12. Rietveld refinements revealed a linear correlation between the true In occupancy and the lattice parameter a. This allows the determination of the true In filling in skutterudites and predicting the In content of unknown AxCo4Sb12. The high purity of InxCo4Sb12 allowed studying the transport properties without bias from side phases. A figure of merit close to unity at 420 °C was obtained for a sample of a true composition of In0.160(2)Co4Sb12 (nominal composition In0.18Co4Sb12). The lower degree of In filling has a dramatic effect on the thermoelectric properties as demonstrated by the sample of nominal composition In0.20Co4Sb12. The presence of InSb in amounts of ∼0.1 vol% led to a substantially lower degree of interstitial site filling of 0.144, and the figure of merit zT decreased by 18%, which demonstrates the significance of the true filler atom content in skutterudite materials.

Lattice-dynamical model for the filled skutteruditeLaFe4Sb12: Harmonic and anharmonic couplings

The filled skutterudite ${\mathrm{LaFe}}_{4}{\mathrm{Sb}}_{12}$ shows greatly reduced thermal conductivity compared to that of the related unfilled compound ${\mathrm{CoSb}}_{3}$, although the microscopic reasons for this are unclear. We calculate harmonic and anharmonic force constants for the interaction of the La filler atom with the framework atoms. We find that force constants show a general trend of decaying rapidly with distance and are very small for the interaction of the La with its next-nearest-neighbor Sb and nearest-neighbor La. However, a few rather long-range interactions, such as with the next-nearest-neighbor La and with the third neighbor Sb, are surprisingly strong, although still small. We test the central-force approximation and find significant deviations from it. Using our force constants we calculate a bare La mode Gruneisen parameter and find a value of 3--4, substantially higher than values associated with cage atom anharmonicity, i.e., a value of about 1 for ${\mathrm{CoSb}}_{3}$ but much smaller than a previous estimate [Bernstein et al., Phys. Rev. B 81, 134301 (2010)]. This latter difference is primarily due to the previously used overestimate of the La-Fe cubic force constants. We also find a substantial negative contribution to this bare La Gruneisen parameter from the aforementioned third-neighbor La-Sb interaction. Our results underscore the need for rather long-range interactions in describing the role of anharmonicity on the dynamics in this material.

Frequency of Filler Vibrations in CoSb3Skutterudites: A Mechanical Interpretation

A mechanical interpretation of the frequency trend observed in Ca-, Sr-, and Ba-filled CoSb3 skutterudites is presented. Relevant vibrational frequencies computed at the zone center are presented for fully filled, half-filled, and unfilled systems. The frequency of the filler vibrations increases as the mass of the filler atom increases, which is a counterintuitive trend that is difficult to explain within the classical “rattler” concept. As an alternative theory, we propose the interpretation of the filler vibrations as modified Sb ring vibrations instead. The energetically degenerate Sb ring vibrations in unfilled CoSb3 split into two separate groups of vibrations through the mechanical interaction introduced by fillers, and one of the group forms the filler vibrations. A one-dimensional mass-spring model is also presented for illustrative purposes. The frequency trend of the ab initio phonons at the zone center is reproduced by the model, substantiating our interpretation. The result suggests that engi...

An Optimization of Composition Ratio among Triple‐Filled Atoms in In0.3-x-yBaxCeyCo4Sb12 System

Bulk nanostructured materials are important as energy materials. Among thermoelectric materials, the skutterudite system of CoSb3 is a representative material of bulk nanostructured materials. Filling a skutterudite structure with atoms that have different localized frequencies (also known as triple filling) was reported to be effective for lowering thermal conductivity. Among studies representing superior power factors, In‐filled skutterudite systems showed higher Seebeck coefficients. This study sought to optimize the composition ratio among the triple‐filled atoms in an In0.3-x-yBaxCeyCo4Sb12 system. The composition dependence of the thermoelectric properties was investigated for specimens with different ratios among the three kinds of filler atoms in the In0.3-x-yBaxCeyCo4Sb12 system. In addition, the process variables were carefully optimized for filled skutterudite systems to obtain a maximum ZT value.

Fast phase formation of double-filled p-type skutterudites by ball-milling and hot-pressing

Filled-skutterudites are promising mid-temperature thermoelectric materials for heat-to-electricity conversion. Traditional preparation methods need a very long time annealing (7 to 14 days) to form the right skutterudite phase. Annealing is especially critical for p-type filled-skutterudites, since Fe4Sb12 needs filler atoms to enter the cage to form a stable phase. In this work, we prepared Ce and Nd double-filled p-type skutterudite materials by directly ball-milling the quenched ingot without annealing followed by hot-pressing. The results showed that with appropriate ball-milling time, a pure p-type filled-skutterudite phase can be obtained in just 5 minutes by hot-pressing. The samples prepared in this way have the same quality as those prepared by traditional long time annealing methods, and showed ZT values above 1 between 700 and 800 K. This simple and efficient method is very useful for the preparation of many other materials that are kinetically difficult to make.

Study on lattice dynamics of filled skutterudites InxYbyCo4Sb12

As a promising class of thermoelectric materials, skutterudites are featured by the naturally formed oversize cages in its crystal lattice. Cage-filling by guest atoms has thus become an important approach to reducing the lattice thermal conductivity and optimizing the thermoelectric performance. To probe the impact of filler atoms on lattice dynamics, we herein reported specific heat measurements in two single-filled skutterudite samples In0.2Co4Sb12 and Yb0.2Co4Sb12, and a double-filled skutterudite sample In0.2Yb0.2Co4Sb12. The low temperature specific heat data was analyzed in the context of combined electronic specific heat (the Sommerfeld term), the Debye mode (long wavelength acoustic phonon modes), and the Einstein modes (localized vibration modes). We found that the specific heat difference between the single and double-filled samples can be well accounted by one extra Einstein mode, as expected from the extra filler atom and confirmed by the results of inelastic neutron scattering measurements. Interestingly, two Einstein modes, in addition to the Sommerfeld term and the Debye term, are needed to satisfactorily account for the specific heat of the single-filled sample. The Einstein mode with lower frequency has the frequency close to the low-lying mode reported in La, Ce, Tl single-filled skutterudites, this mode is largely unaffected when the second type of filler atoms is introduced. The frequency of this mode has been verified by inelastic neutron scattering measurement. The other Einstein mode with higher frequency may be originated from the motion of Sb atoms.

Effect of HPT processing on the structure, thermoelectric and mechanical properties of Sr0.07Ba0.07Yb0.07Co4Sb12

N-type skutterudite Sr0.07Ba0.07Yb0.07Co4Sb12 with ZT=1.4 at 800K was processed by high pressure torsion (HPT), a technique of severe plastic deformation (SPD) to produce a nanocrystalline material with many deformation induced lattice defects like dislocations and vacancies. As already shown previously, after HPT processing ZT∼1.8 was reached mainly due to a significantly reduced thermal conductivity (the lattice thermal conductivity reached almost the theoretical calculated minimum) although the electrical resistivity was higher. In this paper, the microstructural changes after HPT leading to such high ZT values were investigated. X-ray line profile analysis (XPA) before and after HPT was used to detect a smaller crystallite size and a high number of defects (dislocations and vacancies) resulting in an increase of the electrical resistivity but a significant decrease of the thermal conductivity after HPT processing. The decrease of the crystallite size could also be identified as the reason for enhanced microhardness, which means that Hall–Petch strengthening applies. In addition, for the first time, energy filtered transmission electron microscopy (TEM) was employed for the investigation of HPT processed skutterudites. Dislocations as well as grain boundaries of two types (polarised dipole walls and polarised tilt walls) could be directly observed, confirming what so far was assumed. Also for the first time thermal expansion was measured below and above room temperature and compared with the results before HPT revealing a slightly lower thermal expansion coefficient, the same Debye temperature but an Einstein temperature only half of that before HPT, the latter indicating lower frequencies of the filler atoms after HPT processing. Furthermore it could be shown that the decrease of the electrical resistivity after reaching a maximum runs parallel with a shrinking of the sample during thermal expansion measurements, proving that annealing out and closing of microcracks are responsible for this behaviour.

Rattler-seeded InSb nanoinclusions from metastable indium-filled In0.1Co4Sb12 skutterudites for high-performance thermoelectrics

The synthesis and thermoelectric properties of In0.1−xCo4Sb12−x skutterudite-based nanocomposites with xInSb nanoinclusions are reported. The nanoinclusions reduce the thermal conductivity of the composites considerably compared with nanoinclusion-free In0.1Co4Sb12. Unequivocal evidence is provided demonstrating that the InSb nanoinclusions found in some of the highest reported thermoelectric figure of merit (ZT) skutterudites are synthesized in situ from filler atoms diffusing out of the icosahedral void-sites, and the kinetics of their synthesis is enhanced strongly by mechanical attrition. Moreover, a procedure designed to maximize the concentration of InSb nanoinclusions is reported, and can be employed to create void-site-filled and optimally doped skutterudite-based InSb-nanocomposites with exceptional ZT.

Thermoelectric properties of Spark Plasma Sintered In xYb yLa 0.3- x- yCo 4Sb 12 skutterudite system

A thermoelectric material with sufficient performance at high temperatures is a key factor for thermoelectric generation from solar-thermal energy (I. R. radiation). In this study, we analyzed CoSb 3 skutterudite compounds as a thermoelectric material at high temperatures and synthesized filled skutterudites with two types of filler atoms: 1) Yb- and La-fillers with a different electronegativity ( X) from the Sb atoms and 2) the In-filler with nearly the same electronegativity ( X) as the Sb atoms. All the samples were sintered with the spark plasma sintering method, and their thermoelectric properties were investigated. From the results, the Yb atoms were the most effective in decreasing the thermal conductivity of this system due to their very different electronegativity ( X) value from the Sb atoms and lower melting temperature than that of the La-filler. In addition, In-filled compositions had high power factor values while the In-free composition had the lowest power factor value due to its lower melting temperature. The addition of La atoms for filling was not effective in reducing the thermal conductivity of this filled skutterudite system due to its higher melting point. A maximum ZT value of 0.9 at 782 K was obtained in the In 0.1Yb 0.2Co 4Sb 12 specimen.

Effect of disorder on the magnetic properties of a partially filled skutterudite

The magnetic properties of the partially filled skutterudite Eu0.5Co4Sb12 are investigated by a model Hamiltonian, with special emphasis on the effect of ordering and disordering occupancy of the filler atoms Eu on the magnetic properties. The magnetization, magnetic specific heat and entropy are calculated within the mean-field approximation. By introducing the position disorder of the filler atoms, the critical temperature TC above which the magnetization disappears is changed. The magnetization curve near TC also becomes concave compared to the convex magnetization of the ordered system. The filler disordering also leads to a kink in the isothermal magnetization curve and a valley in the magnetic specific heat near TC. The effective magnetic field acting on the localized spin of the filler atoms has a competitive effect with the disorder and therefore makes the valley disappear.

Filled skutterudites: from single to multiple filling

Research on traditional thermoelectric (TE) materials with narrow bandgap have made great progress in recent years. Here, we primarily reviewed the study on CoSb3 skuttuerdites with intrinsic crystal voids that could be partially filled with various filler atoms for TE performance optimization. By combining experimental measurement of electrical/thermal transport properties and theoretical study of electronic structure and structural stability, we analyzed the fundamentals in understanding the filling behavior of fillers atoms in the voids, and provided a few guidance rules in simultaneously optimizing electrical and thermal transport properties for thermoelectric performance optimization. By following those rules, nearly independent tunning of electron and phonon transports can be achieved in these caged compounds. As a result, these partially filled skutterudites basically satisfy the picture of the ‘phonon glass-electron crystal’ paradigm, and TE figure of merit ( ZT ) was pushed up to reach 1.7@850 K, highest among all reported bulk TE materials. The general discussions could also provide guidance in searching for novel thermoelectric materials.

Phase Transformation and Thermoelectric Properties of Sn-Filled/Fe-Doped CoSb<sub>3</sub> Skutterudites

Sn-filled and Fe-doped CoSb3 skutterudites were synthesized by encapsulated induction melting. A single δ-phase was obtained by subsequent annealing, as confirmed by X-ray diffraction. The as-solidified ingot consisted of mixed phases of -CoSb, -CoSb2, δ-CoSb3 and elemental Sb. The phases could be transformed by annealing, and the phases of the as-solidified ingot annealed at 773 K for 24 h transformed to δ-CoSb3. The temperature dependence of the Seebeck coefficient, electrical resistivity and thermal conductivity were examined from 300 K to 700 K. The positive Seebeck coefficient confirmed p-type conduction. The electrical resistivity increased with increasing temperature, which showed that the SnzCo3FeSb12 skutterudite is highly degenerate. The thermal conductivity was reduced by Sn-filling because the filler atoms acted as phonon scattering centers in the skutterudite lattice. The thermoelectric figure of merit was enhanced by Sn filling and its optimum composition was considered to be Sn0.3Co3FeSb12.

High-Temperature Thermoelectric Properties of Co4Sb12-Based Skutterudites with Multiple Filler Atoms: Ce0.1In x Yb y Co4Sb12

Void-filling in the CoSb3 skutterudite lattice with different kinds of heavy elements has proven to be an effective mechanism to enhance thermoelectric performance due primarily to a reduction in lattice thermal conductivity. Specifically, our findings on the series In x Yb y Co4Sb12 [0 ≤ (x, y) ≤ 0.2] have further motivated an attempt to form triple-filled skutterudites Ce0.1In x Yb y Co4Sb12 with In and Yb concentrations [0 ≤ (x, y) ≤ 0.2] and with the Ce concentration held constant (Ce0.1). All of these samples have been prepared via a simplified melting–annealing–sintering procedure and were first characterized by means of x-ray powder diffraction and scanning electron microscopy, followed by measurements of the Hall coefficient, electrical and thermal conductivities, and Seebeck coefficient. Our aim is to further elucidate the roles of the three elements (Ce, In, and Yb) in these materials. Compared with the addition of just In or Yb, we found that simultaneous addition of both In and Yb reduced the lattice thermal conductivity without significantly degrading the power factor. Further addition of the third element (Ce), along with In and Yb, also produced a similar result. However, we noticed that some of the In and Yb were also observed in the form of secondary phases (InSb and Yb2O3), not entering entirely as filler atoms. As a result of our investigation, several compositions achieved increased sustainability and enhanced thermoelectric performance, with maximum ZT values of about 1.3 to 1.4 obtained at around 800 K.

Cage-Shaped Mo9 Chalcogenides: Promising Thermoelectric Materials with Significantly Low Thermal Conductivity

Thermoelectric properties of molybdenum selenides containing Mo9 clusters have been investigated between 300 K and 800 K. AgxMo9Se11 (x = 3.4 and 3.8) have been synthesized by solid-state reaction and spark plasma sintering. X-ray diffraction and scanning electron microscopy reveal high purity and good homogeneity of the samples. The thermoelectric power of the samples is positive over the whole investigated temperature range, indicating that the majority of charge carriers are holes. The Seebeck coefficient increases with temperature, and the temperature coefficient of the resistivity is positive. Significantly low thermal conductivity, comparable to values reported for state-of-the-art thermoelectric materials, is observed in this new system, and this is assumed to be associated with the rattling effect from the Ag filler atoms. It has been demonstrated that the electrical and thermal properties correlate to the Ag concentration. For x = 3.8, a promising dimensionless thermoelectric figure of merit of ∼0.7 is obtained at 800 K.

Multitemperature synchrotron powder diffraction and thermoelectric properties of the skutterudite La0.1Co4Sb12

Compounds with the skutterudite structure have potential use in thermoelectric power generation and the role of the filler atoms in filled skutterudites is still an open question regarding their effects on thermal conductivity. Partially filled skutterudite La0.1Co4Sb12 has been investigated by synchrotron powder x-ray diffraction between 90 and 700 K, as well as heat capacity, electrical and thermal transport property measurements, and Hall effect measurements. Anomalously large atomic displacement parameters (ADPs) are found for the La filler atoms, indicating that the La atoms are weakly bound in the structure. Analysis of the multitemperature ADPs of La using the Einstein model and the ADPs of the framework atoms using the Debye model gives an Einstein temperature of 79 K and a Debye temperature of 268 K. The heat capacity estimated from the Debye and Einstein temperatures is in good agreement with the measured experimental data. The ADP analysis also indicates that the La atoms are slightly displaced (0.06 Å) from the cage center possible due to covalent bonding with the host structure. Compared with unfilled CoSb3, the low energy La vibration modes significantly increase the heat capacity of La0.1Co4Sb12 at lower temperature despite the low La occupancy (refined occupancy of 4.9%). The small content of filling atoms results in a large reduction in the thermal conductivity.

Mechanical properties of filled antimonide skutterudites

Abstract Time-of-flight and resonant ultrasound spectroscopy techniques were employed for elastic moduli measurements on a set of various Fe 4 Sb 12 - and Co 4 Sb 12 -based skutterudites filled by mischmetal, didymium, or alkaline earths (Ca, Sr, Ba). A weak temperature influence on the longitudinal modulus C 11 indicates weak degradation of elastic properties within the thermoelectric working temperature range. Elastic moduli for Co 4 Sb 12 -based skutterudites are only slightly higher than for Fe 4 Sb 12 -based skutterudites, and the influence of various filler atoms or filling fractions on the elastic moduli is even smaller. Ball milled and hot pressed samples (grain size ∼250 nm) illustrate an obvious improvement of elastic properties in relation to those hot pressed from hand milled powders (grain size ∼50 μm). Debye temperatures calculated from sound velocity measurements are comparable to the values obtained from the parameters fitted to thermal expansion, which indicate that Co 4 Sb 12 -based skutterudites having slightly higher values than Fe 4 Sb 12 -based skutterudites. Vickers hardness is increased by Co or Ni substitution and demonstrates a linear dependence on density, Young's modulus, and shear modulus.