Unveiling the Correlation between the Crystalline Structure of M‐Filled CoSb3 (M = Y,K, Sr) Skutterudites and Their Thermoelectric Transport Properties

Skutterudite‐type pnictides based on CoSb₃ are promising semiconductor materials for thermoelectric applications. An exhaustive structural characterization by synchrotron X‐ray powder diffraction of different M‐filled CoSb₃ (M = Y, K, Sr, La, Ce, Yb) skutterudites, with a panoply of M atoms with very different chemical nature, allows to better understand the effects of filling from a crystallo‐chemical point of view. These analyses focus on the correlation of chemical and structural features with the enhanced thermoelectric properties displayed by certain families of filled‐CoSb₃ skutterudites. These are mainly determined by Sb positional parameters, yielding Oftedal plots that depend on the filling fraction, ionic state, and atomic radius of the filler. Together with the distortion of [Sb₄] rings and [CoSb₆] octahedra present in the skutterudite structure, these results are linked to the band‐convergence concept and its influence on the thermoelectric transport properties. Here, the structural changes observed in the different chemical compositions are relevant to understand the improved thermoelectric performance of single partially filled n‐type skutterudites.     

Thermoelectric Properties of Ca-Filled CoSb<Subscript>3</Subscript>-Based Skutterudites Synthesized by Mechanical Alloying

Ca _z Co_4−x (Fe/Mn) _x Sb₁₂ skutterudites were prepared by mechanical alloying and hot pressing. The phases of mechanically alloyed powders were identified as γ-CoSb₂ and Sb, but they were transformed to δ-CoSb₃ by annealing at 873 K for 100 h. All specimens had a positive Hall coefficient and Seebeck coefficient, indicating p-type conduction by holes as majority carriers. For the binary CoSb₃, the electrical conductivity behaved like a nondegenerate semiconductor, but Ca-filled and Fe/Mn-doped CoSb₃ showed a temperature dependence of a degenerate semiconductor. While the Seebeck coefficient of intrinsic CoSb₃ increased with temperature and reached a maximum at 623 K, the Seebeck coefficient increased with increasing temperature for the Ca-filled and Fe/Mn-doped specimens. Relatively low thermal conductivity was obtained because fine particles prepared by mechanical alloying lead to phonon scattering. The thermal conductivity was reduced by Ca filling and Fe/Mn doping. The electronic thermal conductivity was increased by Fe/Mn doping, but the lattice thermal conductivity was decreased by Ca filling. Reasonable thermoelectric figure-of-merit values were obtained for Ca-filled Co-rich p-type skutterudites.     

Effects of Double Substitution with Ge and Te on Thermoelectric Properties of a Skutterudite Compound

Studies have shown that the thermoelectric properties of CoSb₃ could be improved by the substitution of group IVB or VIB elements for Sb. However, the substitution volume is limited. To get a better picture of the substitution volume in view of thermoelectric properties, Ge and Te double-substituted skutterudite materials were prepared with the nominal composition of Co₄Sb _x Ge_5.9−0.5x Te_6.1−0.5x (x = 11, 10, 9, 8) by the traditional solid-state reaction method and spark plasma sintering, and Rietveld analysis was employed to refine the crystal structure. The results showed that the lattice parameter decreased linearly and the solubility limitations of group IVB and VIB elements were greatly alleviated by the Ge and Te codoping. Besides, the thermoelectric properties were analyzed through measurements of electrical and thermal conductivities as well as room-temperature electrical transport properties. The results showed that the substitution volume of Ge and Te could play an important role in the thermoelectric properties, and a minimum lattice thermal conductivity value of 1.56 W m⁻¹ K⁻¹ was obtained at around 673 K for Co₄Sb₈Ge_1.9Te_2.1. Co₄Sb₁₁Ge_0.4Te_0.6 achieved the best figure of merit of 0.89 at around 773 K, which was remarkably improved over that of untreated CoSb₃.     

Search for Resonant-Like Impurity in Ag-Doped CoSb3 Skutterudite: Theoretical and Experimental Study

Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) calculations of Ag-doped CoSb₃ point to the presence of either an extra sharp peak of s-symmetry Ag density of states near the valence-band edge when filling the void (2a) or to conventional p-type doping when substituting Sb site (24g). These results suggest a resonant-like impurity level in the former or nearly rigid-band behavior in the latter. To confirm the theoretical predictions, a series of samples with nominal composition Co₈Sb₂₄:Ag _x (x = 0, 0.1, 0.3, 0.4, 0.5) were prepared. Structural and phase composition analyses were carried out by x-ray diffraction, scanning electron microscopy, and scanning thermoelectric microprobe. Investigations of the influence of Ag impurity on the electrical conductivity and Seebeck coefficient were performed over the temperature range from 300 K to 560 K. It was found that doping CoSb₃ with Ag leads to an increase of the thermoelectric power factor α ² σ in the temperature range from 300 K to 475 K of about an order of magnitude for all doped samples. However, electron probe microanalysis revealed accumulation of Ag mainly in grain boundaries while the presence of Ag in CoSb₃ crystallites was not confirmed. This observation corroborates the results of KKR-CPA calculations concerning the formation energy of the Ag _x Co₄Sb₁₂ system, which is much lower than values calculated for A _x Co₄Sb₁₂ (A = Ca, Ba).     

Low Thermal Conductivity and Enhanced Thermoelectric Performance in In and Lu Double-Filled CoSb<Subscript>3</Subscript> Skutterudites

The thermoelectric properties of indium (In) and lutetium (Lu) double-filled skutterudites In _x Lu _y Co₄Sb₁₂ prepared by high-pressure synthesis were investigated in detail from 4 K to 365 K. Our results indicate that In and Lu double filling can remarkably reduce the thermal conductivity, and substantially improve the thermoelectric performance. A thermoelectric figure of merit of ZT = 0.27 for In_0.13Lu_0.05Co_4.02Sb₁₂ was achieved at 365 K, being larger by one order of magnitude than that for CoSb₃. It is thought that the large difference in resonance frequencies of the In and Lu elements broadens the range of normal phonon scattering in the multifilled skutterudites, helping to achieve an even lower lattice thermal conductivity. This investigation suggests that an effective way to improve the thermoelectric performance of skutterudite materials is to use In and Lu double filling.     

On Intensifying Carrier Impurity Scattering to Enhance Thermoelectric Performance in Cr‐Doped CeyCo4Sb12

The beneficial effect of impurity scattering on thermoelectric properties has long been disregarded even though possible improvements in power factor have been suggested by Ioffe more than a half century ago. Here it is theoretically and experimentally demonstrated that proper intensification of ionized impurity scattering to charge carriers can benefit the thermoelectric figure of merit (ZT) by increasing the Seebeck coefficient and decreasing the electronic thermal conductivity. The optimal strength of ionized impurity scattering for maximum ZT depends on the Fermi level and the density of states effective mass. Cr‐doping in Ce_yCo₄Sb₁₂ progressively increases the strength of ionized impurity scattering, and significantly improves the Seebeck coefficient, resulting in high power factors of 45 μW cm^?1 K^?2 with relatively low electrical conductivity. This effect, combined with the increased Ce‐filling fraction and thus decreased lattice thermal conductivity by charge compensation of Cr‐dopant, gives rise to a maximum ZT of 1.3 at 800 K and a large average ZT of 1.1 between 500 and 850 K, ≈30% and ≈20% enhancements as compared with those of Cr‐free sample, respectively. Furthermore, this study also reveals that carrier scattering parameter can be another fundamental degree of freedom to optimize electrical properties and improve thermal‐to‐electricity conversion efficiencies of thermoelectric materials.     

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

Double-filled skutterudites In _x Pr _y Co₄Sb₁₂, 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 In_0.05Pr_0.05Co₄Sb₁₂; this value is approximately three times that for In _x Co₄Sb₁₂ (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.     

In y Co4Sb12 Skutterudite: Phase Equilibria and Crystal Structure

Phase relations were investigated for the In-Co-Sb system in the temperature range from 375°C to 800°C using as-cast and annealed alloys. Phase equilibria in the CoSb-InSb-(Sb) composition triangle are presented by a series of isothermal sections and solidus and liquidus surfaces, accompanied by a Schulz–Scheil reaction scheme. The indium-filled skutterudite In _y Co₄Sb₁₂ already forms an equilibrium with liquid at 484°C, which might limit high-temperature applications of In-Co-Sb-based skutterudites. The maximal solubility of indium in In _y Co₄Sb₁₂ (y = 0.22) remains almost constant in the temperature range from 475°C to 700°C and corresponds to the equilibrium with CoSb₂ and InSb. The solubility of indium in the skutterudite phase is reduced to y = 0.09 when it coexists in equilibrium with InSb and (Sb), and this decrease of the solubility might be responsible for the formation of InSb precipitates. Temperature-dependent x-ray single-crystal and specific heat data for In _y Co₄Sb₁₂ were employed to determine the rattling behavior of In atoms in the skutterudite lattice.     

Low-Temperature Thermoelectric Properties of Co<Subscript>0.9</Subscript>Fe<Subscript>0.1</Subscript>Sb<Subscript>3</Subscript>-Based Skutterudite Nanocomposites with FeSb<Subscript>2</Subscript> Nanoinclusions

Bulk thermoelectric nanocomposite materials have great potential to exhibit higher ZT due to effects arising from their nanostructure. Herein, we report low-temperature thermoelectric properties of Co_0.9Fe_0.1Sb₃-based skutterudite nanocomposites containing FeSb₂ nanoinclusions. These nanocomposites can be easily synthesized by melting and rapid water quenching. The nanoscale FeSb₂ precipitates are well dispersed in the skutterudite matrix and reduce the lattice thermal conductivity due to additional phonon scattering from nanoscopic interfaces. Moreover, the nanocomposite samples also exhibit enhanced Seebeck coefficients relative to regular iron-substituted skutterudite samples. As a result, our best nanocomposite sample boasts a ZT = 0.041 at 300 K, which is nearly three times as large as that for Co_0.9Fe_0.1Sb₃ previously reported.     

Effects of Excess Sb on Thermoelectric Properties of Barium and Indium Double-Filled Iron-Based p-Type Skutterudite Materials

A series of Ba and In double-filled iron-based p-type skutterudite thermoelectric (TE) materials with nominal composition BaInFe_3.7Co_0.3Sb_12+m (0.72????m????2.4) have been prepared by melting, quenching, annealing, and spark plasma sintering (SPS) methods. The effects of excess Sb on the phase composition, microstructure, and TE transport properties of these materials were investigated in this work. All the SPS bulk materials are composed of the main skutterudite phase and trace InSb and FeSb₂. The content of FeSb₂ in the SPS bulk materials gradually decreased and that of InSb remained nearly invariable with increasing m. The impurities InSb and metallic Sb are found at grain boundaries. The amount of metallic Sb at grain boundaries gradually increased with increasing m. The excess Sb had no effect on the growth of grains. The dependence of the TE properties on m indicates that preventing the formation of FeSb₂ by adjusting the excess Sb value may significantly improve the TE properties of Ba and In double-filled iron-based p-type skutterudite materials. The significant increases in the carrier concentration and electrical conductivity as well as the remarkable reduction in the lattice thermal conductivity of the sample with m?=?0.96 are due to the significant reduction in the FeSb₂ content induced by the excess Sb. The gradual increase in ZT with increasing m from 0.72 to 1.44 is attributed to the gradual decrease of the FeSb₂ content, and the gradual decrease in ZT in the m range of 1.44 to 2.4 is due to the gradual increase of the Sb content in the Sb-In alloy impurity occurring at grain boundaries. The lowest lattice thermal conductivity of 0.31?W?m^?1?K^?1 and the highest ZT value of 0.63 were obtained at 800?K for the sample with m?=?1.44.     

Thermoelectric Properties of Co<Subscript>4</Subscript>Sn<Subscript>6</Subscript>Se<Subscript>6</Subscript> Ternary Skutterudites

A ternary ordered variant of the skutterudite structure, the Co₄Sn₆Se₆ compound, was prepared. Polycrystalline samples were prepared by a modified ceramic method. The electrical conductivity, the Seebeck coefficient and the thermal conductivity were measured over a temperature range of 300–800 K. The undoped Co₄Sn₆Se₆ compound was of p-type electrical conductivity and had a band gap E _g of approximately 0.6 eV. The influence of transition metal (Ni and Ru) doping on the thermoelectric properties was studied. While the thermal conductivity was significantly lowered both for the undoped Co₄Sn₆Se₆ compound and for the doped compounds, as compared with the Co₄Sb₁₂ binary skutterudite, the calculated ZT values were improved only slightly.     

Effect of In Impurity on Thermoelectric Properties of Ba and In Double-Filled n-Type Skutterudite Materials

A series of (Ba,In) double-filled n-type skutterudite materials with nominal composition Ba_0.4In _m Co₄Sb₁₂ (m?=?0 to 0.4, ??m?=?0.1) has been prepared by melt quenching, annealing, and spark plasma sintering (SPS). The presence of In impurity and its effect on the thermoelectric properties of the filled skutterudite materials have been precisely investigated in this work. All samples consisted of skutterudite phase, while traces of In-containing impurity were detected in samples with m????0.3. The electrical conductivity and thermal conductivity decreased, and the absolute value of the Seebeck coefficient increased with increasing m in the range 0 to 0.2; however, the inverse behavior of the electrical conductivity, thermal conductivity, and Seebeck coefficient was observed in the samples with m????0.3. The thermoelectric properties of Ba_0.4In _m Co₄Sb₁₂ in the m range of 0 to 0.2 were changed because of carrier concentration degradation and strong lattice scattering induced by the In filler, while they were intensively affected by the In-containing impurity for m????0.3. Compared with the Ba single-filled skutterudite material, the power factors of all (Ba,In) double-filled skutterudite materials significantly increased and the lattice thermal conductivity dramatically decreased. As a result, two large ZT values for the samples with m?=?0.2 and 0.4 reached 1.19 and 1.25 at 800?K, which is an enhancement of 52% and 60%, respectively.     

Thermoelectric Properties of Indium-Added Skutterudites In x Co4Sb12

The high-temperature thermoelectric properties of In _x Co₄Sb₁₂ (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 In _x Co₄Sb₁₂ was in the range 0.05 < x < 0.10. The electrical resistivity, Seebeck coefficient, and thermal conductivity of the In _x Co₄Sb₁₂ 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 In_0.35Co₄Sb₁₂ at 600 K.     

Electronic Structures and Transport Properties of Single-Filled CoSb<Subscript>3</Subscript>

Band structure and density of states (DOS) of CoSb₃ single-filled by seven kinds of atoms (R_0.125Co₄Sb₁₂) are calculated by the density functional method. The results for the electronic structures in turn determine the electrical transport and thermal performance. It is found that the band structure of R_0.125Co₄Sb₁₂ shows no significant changes compared with that of CoSb₃, and the results indicate that void filling with a small quantity of R atoms does not change the bond formation in CoSb₃. However, the partial DOS reveals that there could be interaction of Sn, Tl, In, and Yb atoms with CoSb₃. The results for the electrical transport properties and thermal properties show that Sn, Tl, and In atoms increase the Seebeck coefficient and La, Eu, and Yb atoms are helpful for increasing the electron concentration and decreasing the thermal conductivity further. According to our calculations and Yang’s principle, double-filled CoSb₃ with atomic combinations of (In, Ca), (In, Ba), (Sn, Eu), and (Sn, La) may exhibit good thermoelectric performance.     

Thermoelectric Properties of Multifilled Skutterudites with La as the Main Filler

Bulk multifilled n- and p-type skutterudites with La as the main filler were fabricated using the spark plasma sintering (SPS) method. The thermoelectric properties and thermal stability of these skutterudites were investigated. It was found that the interactions among the filling atoms also play a vital role in reducing the lattice thermal conductivity of the multifilled skutterudites. ZT = 0.76 for p-type La_0.8Ba_0.01Ga_0.1Ti_0.1Fe₃CoSb₁₂ and ZT = 1.0 for n-type La_0.3Ca_0.1Al_0.1Ga_0.1In_0.2Co_3.75Fe_0.25Sb₁₂ skutterudites have been achieved. Furthermore, the differential scanning calorimetry (DSC) results show that there is no skutterudite phase decomposition till 750°C for the La_0.8Ba_0.01Ga_0.1Ti_0.1Fe₃CoSb₁₂ sample. The thermal stability of the La_0.8Ba_0.01Ga_0.1Ti_0.1Fe₃CoSb₁₂ skutterudite is greatly improved. Using the developed multifilled skutterudites, the fabricated module with size of 50 mm × 50 mm × 7.6 mm possesses maximum output power of 32 W under the condition of hot/cold sides = 600°C/50°C.     

Effect of High-Pressure Torsion on Texture,Microstructure, and Raman Spectroscopy: Case Study of Fe- and Te-Substituted CoSb3

Fe_0.05Co_0.95Sb_2.875Te_0.125, a double-element-substituted skutterudite, was prepared by induction melting, annealing, and hot pressing (HP). The hot-pressed sample was subjected to high-pressure torsion (HPT) with 4 GPa pressure at 673 K. X-ray diffraction was performed before and after HPT processing of the sample; the skutterudite phase was observed as a main phase, but an additional impurity phase (CoSb₂) was observed in the HPT-processed sample. Surface morphology was determined by high-resolution scanning electron microscopy. In the HP sample, coarse grains with sizes in the range of approximately 100 nm to 300 nm were obtained. They changed to fine grains with a reduction in grain size to 75 nm to 125 nm after HPT due to severe plastic deformation. Crystallographic texture, as measured by x-ray diffraction, indicated strengthening of (112), (102) poles and weakening of the (123) pole of the HPT-processed sample. Raman-active vibrational modes showed a peak position shift towards the lower energy side, indicating softening of the modes after HPT. The distortion of the rectangular Sb–Sb rings leads to broadening of Sb–Sb vibrational modes due to local strain fluctuation. In the HPT process, a significant effect on the shorter Sb–Sb bond was observed as compared with the longer Sb–Sb bond.     

Thermoelectric Properties of <Emphasis Type="Italic">n</Emphasis>-Type Multiple-Filled Skutterudites

Filled skutterudites are prospective intermediate temperature materials for␣thermoelectric power generation. CoSb₃-based n-type filled skutterudites have good electrical transport properties with power factor values over 40 μW/cm K² at elevated temperatures. Filling multiple fillers into the crystallographic voids of skutterudites would help scatter a broad range of lattice phonons, thus resulting in lower lattice thermal conductivity values. We report the thermoelectric properties of n-type multiple-filled skutterudites between 5 K and 800 K. The combination of different fillers inside the voids of the skutterudite structure shows enhanced phonon scattering, and consequently a strong suppression of the lattice thermal conductivity. Very good power factor values are achieved in multiple-filled skutterudite compared with single-element-filled materials. The dimensionless thermoelectric figure of merit for n-type filled skutterudites is improved through multiple-filling in a wide temperature range.     

Effects of doping on the high-temperature thermoelectric properties of IrSb3 skutterudite compounds

The IrSb₃-based skutterudite compounds have a potential for thermoelectric applications because of high Hall mobility, Seebeck coefficient, and relatively low thermal conductivity. In the present study, polycrystalline p- and n-type IrSb₃ compounds are prepared by powder metallurgy techniques. The effect of doping on thermoelectric properties has been investigated in binary and ternary IrSb₃ compounds using Ru, Ge, Pd, or Pt as a dopant. It is shown that the electrical properties depend strongly not only on the kinds of doping impurities but also their levels. Our theoretical analysis suggests that the effective mass is significantly affected by doping impurities and the levels.     

Thermal Stability of Barium and Indium Double-Filled Skutterudite Ba<Subscript>0.3</Subscript>In<Subscript>0.2</Subscript>Co<Subscript>3.95</Subscript>Ni<Subscript>0.05</Subscript>Sb<Subscript>12</Subscript> Coated by SiO<Subscript>2</Subscript> Nanoparticles

Filled skutterudite thermoelectric (TE) materials have been extensively studied to search for better TE materials in the past decade. However, there is no detailed investigation about the thermal stability of filled skutterudite TE materials. The evolution of microstructure and TE properties of nanostructured skutterudite materials fabricated with Ba_0.3In_0.2Co_3.95Ni_0.05Sb₁₂/SiO₂ core–shell composite particles with 3 nm thickness shell was investigated during periodic thermal cycling from room temperature to 723 K in this work. Scanning electronic microscopy and electron probe microscopy analysis were used to investigate the microstructure and chemical composition of the nanostructured skutterudite materials. TE properties of the nanostructured skutterudite materials were measured after every 200 cycles of quenching in the temperature range from 300 K to 800 K. The results show that the microstructure and composition of Ba_0.3In_0.2Co_3.95Ni_0.05Sb₁₂/SiO₂ nanostructured skutterudite materials were more stable than those of single-phase Ba_0.3In_0.2Co_3.95Ni_0.05Sb₁₂ bulk materials. The evolution of TE properties indicates that the electrical and thermal conductivity decrease along with an increase in the Seebeck coefficient with increasing quenching up to 2000 cycles. As a result, the dimensionless TE figure of merit (ZT) of the nanostructured skutterudite materials remains almost constant. It can be concluded that these nanostructured skutterudite materials have good thermal stability and are suitable for use in solar power generation systems.     

Thermoelectric Performance of Multiple-Doped Co4Sb12−x−y−z Ge x Te y S z Skutterudite Compounds

CoSb₃ skutterudites multiply doped with Ge, Te, and S were synthesized by solid-state reaction and spark plasma sintering. x-Ray diffraction studies revealed that Ge, Te, and S entered the lattice of the CoSb₃ compounds, and while Te increased the lattice volume, Ge and S decreased it. Compared with the undoped and single-doped CoSb₃ compounds, the thermal conductivity and lattice thermal conductivity are significantly suppressed due to greatly increased point defect scattering. It is found that S is more effective for decreasing the lattice thermal conductivity than Te and Ge. The highest thermoelectric figure of merit, ZT, exceeds 1.1 for the Co₄Sb_11.25Ge_0.05Te_0.63S_0.07 compound at 800 K.