Thermodynamic analysis of the filling fraction limits for impurities in CoSb3 based on ab initio calculations

The thermodynamic stabilities of alkaline earth (Ca, Sr and Ba), and rare earth (La, Ce and Yb), filled CoSb₃ skutterudites have been studied using a plane-wave density functional method. By combining the formation energy of inserting an impurity into the intrinsic void of CoSb₃ and that of secondary phases ISb₂ and CoSb₂, it is found that the filling fraction limit (FFL) or the maximum filling fraction of an impurity I corresponds to the minimum formation energy for a mixed chemical reaction route that results in the formation of filled skutterudite I_yCo₄Sb₁₂ at the maximum filling as well as the formation of secondary phases. Theoretically estimated FFLs of various impurities in the voids of CoSb₃ are in good agreement with the reported experimental data. A schematic phase diagram for filled CoSb₃ is given. Discussion on the effect of the ionic radius of a filler and the content of Sb on FFL is presented.     

Substitution of Antimony by Tin and Tellurium in n-Type Skutterudites CoSb2.8Sn x Te0.2−x

We have studied the substitution of antimony by tin and tellurium in n-type skutterudites CoSb_2.8Sn _x Te_0.2?x . The samples were made by ball milling ingots and hot pressing the ball-milled nanopowder. Rather than filling the cage of the structure, we aimed to use disorder in pnictogen rings by elemental substitution of Sb by Sn and Te. In skutterudite CoSb₃, the dominant heat-carrying phonons are associated with the vibrational modes of the Sb-rings; disorder in the rings can be an effective way to suppress the thermal transport. By suitably tuning the contents of Sn and Te in the skutterudites, we have suppressed the thermal conductivity and achieved a power factor of ~42 μW cm^?1 K^?2 at 530°C. A peak thermoelectric figure of merit (ZT) reaches ~1.1 at 530°C for CoSb_2.8Sn_0.005Te_0.195. This ZT value is comparable with that of some of the single-filled skutterudites.     

High-pressure synthesis of phonon-glass electron-crystal featured thermoelectric LixCo4Sb12

Li-filled CoSb₃, which is inaccessible under ambient pressure, was successfully synthesized with a high-pressure synthesis technique, demonstrating a fast and effective way to broaden elemental species that can be filled into voids of skutterudites. The optimized Li_0.36Co₄Sb₁₂, with a greatly enhanced thermal power factor and much reduced thermal conductivity, has a ZT value of 1.3 at 700 K, the highest among all single elemental filled CoSb₃ materials at this temperature. In addition, an instructive linear relationship between the Einstein temperatures of the distinct rattling fillers and their ionic radii is revealed, which as a reference can easily be applied to the multiple elemental filling strategy for selecting suitable filling elemental species to reduce the lattice thermal conductivity more effectively.     

p-Type skutterudites RxMyFe3CoSb12 (R,M = Ba,Ce, Nd,and Yb): Effectiveness of double-filling for the lattice thermal conductivity reduction

Ab-initio calculations of the resonant modes and frequencies for a number of possible fillers in p-type RFe₃CoSb₁₂ and RFe₄Sb₁₂ were carried out. The results indicate that, although the exact values of fillers’ resonant frequencies in p-type skutterudites are somewhat different from those in n-type Co-based skutterudites, the Einstein-like resonant modes of the fillers are similar to those in n-type materials. Experimentally, several pairs of the fillers were selected and double-filled p-type skutterudite compounds R_xM_yFe₃CoSb₁₂ (R, M = Ba, Ce, Nd, and Yb) were successfully synthesized. The reduction in the lattice thermal conductivity was realized by extending the range of resonant frequencies. As a result, enhanced ZT values above unity were achieved in these double-filled p-type skutterudites.     

Realization of high thermoelectric performance in p-type unfilled ternary skutterudites FeSb2+xTe1−x via band structure modification and significant point defect scattering

FeSb₂Te, a ternary derivative of binary CoSb₃, displays anomalous electrical and thermal transport properties because of considerable modifications in the band structure induced by Fe and significant mixed valence state (namely Fe²⁺ and Fe³⁺) scattering of phonons. The substitution of Te for Sb generates more holes without notably affecting the band structure, while markedly improving the electrical conductivity and retaining a high Seebeck coefficient due to the enhanced density of states, thereby leading to dramatically increased power factors. Furthermore, the heat carrying phonons are strongly scattered with increasing x value because of the formation of solid solutions between two end members: □FeSb₂Te and □FeSb₃ (where □ can be viewed as a vacancy). Consequently, high thermoelectric figures of merit were achieved in the FeSb_2+xTe_1?x compounds, with the largest ZT value reaching ～0.65 for the sample with x = 0.2. This is the highest value among all p-type unfilled skutterudites and is comparable with some filled compositions. Prospects for further improving the performance of p-type FeSb₂Te-based skutterudites are discussed.     

Electronic transport properties of Te-doped CoSb<Subscript>3</Subscript> skutterudites prepared by hot pressing

Te-doped CoSb₃ (CoSb_3−yTe_y) skutterudites were prepared by hot pressing and their electronic transport properties examined. A single δ-phase was successfully obtained. The Seebeck and Hall coefficients confirmed that all the Te-doped CoSb₃ showed n-type conduction. The Te atoms successfully acted as electron donors by substitution of the Sb atoms. The carrier concentration increased an order of 10²⁰ cm⁻³ by Te doping, whereas the carrier mobility decreased as the doping content increased. The Seebeck coefficient and electrical resistivity decreased with an increase in the Te content. The doping considerably reduced the thermal conductivity due to electron-phonon scattering. The lattice contribution was dominant over the electronic contribution.     

Synthesis and thermoelectric properties of CoSb3/WO3 thermoelectric composites

In this study, nano-sized WO₃ powder was dispersed into CoSb₃ powder by ball milling and CoSb₃/WO₃ thermoelectric composites were fabricated using hot-pressing sintering. The results showed that the WO₃ phase distributed uniformly in the form of clusters and the average size of cluster was lower than 4 μm. As the content of WO₃ increased, the electrical conductivity and Seebeck coefficient of CoSb₃/WO₃ composites decreased. The thermal conductivity of composites decreased obviously which resulted from the phonon scattering by the WO₃ inclusions locating on the grain boundaries of CoSb₃ matrix. The highest thermoelectric figure of merit ZT = 0.40 was achieved at 650 K for CoSb₃/2%WO₃ composite.     

Experimental determination of phase equilibrium in the Fe–Co–Sb ternary system

Phase stability and phase transformation were studied in the Fe–Co–Sb ternary system for the three sections: CoSb–Fe_0.56Sb_0.44, 30 at.% of Sb and 75 at.% of Sb. In the first section, we find a continuous solid solution without any secondary phase. The unit cell volume increases as a function of X_Fe/(X_Fe + X_Co). At 30 at.% of Sb, the B8 and the bcc-A₂ phases are obtained across the whole Fe–Co section. On the CoSb₃ side (75 at.% of Sb), Fe atoms cannot completely substitute for Co atoms in the skutterudite structure. Below 5 at.% of substitution of Fe for Co in CoSb₃, only the D0₂ phase is present while for high concentration of Fe, marcasite and Sb phases coexist.     

Enhanced thermoelectric performance of dual-element-filled skutterudites BaxCeyCo4Sb12

Single-phase polycrystalline dual-element-filled skutterudites Ba_xCe_yCo₄Sb₁₂ (0 < x < 0.4, 0 < y < 0.1) are synthesized by the melting–quenching–annealing and spark plasma sintering methods. The electrical conductivity, Seebeck coefficient, thermal conductivity and low-temperature Hall data of these compounds are reported. Our results suggest that there is essentially no difference in electrical transport properties between the dual-element-filled Ba_xCe_yCo₄Sb₁₂ and single-element-filled Ba_yCo₄Sb₁₂ systems. The Ba–Ce co-filling is more effective in lattice thermal conductivity reduction than Ba single filling in the temperature range of 300–850 K. Very low lattice thermal conductivity values less than 2.0 W m^?1 K^?1 are obtained at room temperature. Consequently, enhanced thermoelectric figure of merits (ZT) for these dual-element-filled CoSb₃ skutterudites are achieved at elevated temperatures, in particular ZT = 1.26 at 850 K for Ba_0.18Ce_0.05Co₄Sb_12.02.     

Thermoelectric properties of MxMo6Te8 （M=Ag, Cu）

Ag and Cu filled Chevrel phase MxMo6Te8 (x=1.0, 2.0) samples were synthesized by direct solid state reaction and spark plasma sintering. The electrical and thermal properties were investigated in the temperature range of 300-800 K. The results show that both the electrical and thermal properties are affected by filler atoms. Although the electrical conductivity of MxMo6Te8 is slightly higher than that of state-of-the-art thermoelectric material, such as filled skutterudites, the absolute value of Seebeck coefficient is relatively low. Due to the phonon scattering by the filler atoms, the decrease of the thermal conductivity and the lattice thermal conductivity is obvious. As a result, the dimensionless figure of merit(ZT) is improved over the whole temperature region. The highest ZT value is 0.034 at 800 K for the AgMo6Te8 sample.     

Effects of Sn-doping on the electrical and thermal transport properties of p-type Cerium filled skutterudites

p-type Sn-doped CoSb₃-based skutterudite compounds have been prepared using melting-quenching-annealing method and spark plasma sintering technique. Sn atoms in our samples are completely soluted on Sb-site with a fixed charge state and non-magnetic feature, providing a better choice to ascertain the effect of element doping at the [Co₄Sb₁₂] framework on the electrical and thermal transport properties in p-type skutterudites. Doping Sn at the framework introduces additional ionized impurity scattering to affect the electron transport greatly. Similar electrical transport properties between Ce_0.2Co₄Sb_11.2Sn_0.8 and Co₄Sb₁₁Sn_0.6Te_0.4 suggest that Ce fillers contribute little to the valence band edge. Filling Ce into the voids and doping Sn at the framework introduce additional phonon resonant and point defect scattering mechanisms, thereby reducing lattice thermal conductivity remarkably. Moreover, our data suggest that combining these two effects is more effective to suppress lattice thermal conductivity through scattering broad range of phonons with different frequencies.     

Transport properties on Sn-filled and Te-doped CoSb<Subscript>3</Subscript> skutterudites

Sn-filled and Te-doped CoSb₃ skutterudites (Sn_xCo₈Sb_23.25Te_0.75) were synthesized by the encapsulated induction melting process. Single δ-phase was successfully obtained by subsequent heat treatment at 823 K for 6 days. Structural characterizations were carried out through X-ray diffraction studies. Transport properties such as the Seebeck coefficient, electrical resistivity, thermal conductivity, carrier concentration and mobility were measured and analyzed. The unfilled Co₈Sb_23.25Te_0.75 sample showed n-type conductivity from 300 K to 700K. However, the Sn-filled Sn_xCo₈Sb_23.25Te_0.75 showed n-type conductivity for z=0.25 and 0.5, and p-type conductivity for z=1.0 and 1.5 from 300 K to 700 K. Thermal conductivity was reduced by the impurity-phonon scattering. The dimensionless figure of merit (ZT) was remarkably improved over that of untreated CoSb₃. However, the ZT value decreased when filling with z≥1.0 because the conductivity type was changed from n-type to p-type, thereby allowing bipolar conduction. The details are discussed in terms of the two-band model and the bipolar thermoelectric effect.     

Synthesis and thermoelectric properties of double-filled skutterudites CeyYb0.5−yFe1.5Co2.5Sb12

Ce–Yb double-filled skutterudites Ce_0.5?yYb_yFe_1.5Co_2.5Sb₁₂ (y = 0.1, 0.2, 0.4 and 0.5) were synthesized by a melting method with subsequent annealing. The thermal conductivity, electrical conductivity and Seebeck coefficient were measured from room temperature up to 773 K. The thermal conductivities of all the double-filled skutterudites were found to be lower than the Yb single-filled skutterudites. An enhancement in the dimensionless thermoelectric figure of merit ZT was also observed in all the double-filled skutterudites as compared to the Yb single-filled skutterudite. Ce_0.3Yb_0.2Fe_1.5Co_2.5Sb₁₂ has the highest dimensionless figure of merit ZT of 0.32 at 723 K, which is 55% higher than the Yb single-filled skutterudite at the same temperature.     

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 CoSb_2.75Ge_0.25?xTe_x (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 30–80 nm grains. After spark plasma processing the average grain size of the bulk samples is about 200 nm. High-resolution transmission electron microscopy images show an in situ nanostructure consisting of circular, 15 nm diameter dots of Te- and Ge-enriched skutterudite phase embedded in the skutterudite matrix. Transport properties were measured from 2 to 800 K 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 750 K.     

New p- and n-type skutterudites with ZT > 1 and nearly identical thermal expansion and mechanical properties

Using thermoelectricity to directly convert (waste) heat energy into useful electricity faces a number of challenges. Not only are optimised thermal and electrical transport properties required resulting in a high figure of merit ZT and a high thermal–electric conversion efficiency η over a wide temperature range, thermoelectric (TE) materials must have sufficient mechanical integrity to survive continuous heating–cooling cycles. Thermal expansion of the material as well as the mechanical properties play an important role, i.e. their values should be as similar as possible for p- and n-type alloys to avoid stresses when used in a TE device. In this paper multiple filled p- and n-type skutterudites (Ba,Sr,DD,Yb)_y(Fe_1–xNi_x)₄Sb₁₂ with a ZT > 1 and η ≈ 13% are presented, for the first time showing, in contrast to hitherto investigated skutterudites, nearly identical thermal expansion coefficients and elastic moduli. The ZT values of these skutterudites could be further enhanced by more than 20% after severe plastic deformation via high-pressure torsion.     

Thermoelectric properties of In<Subscript>z</Subscript>Co<Subscript>4</Subscript>Sb<Subscript>12</Subscript> skutterudites

In this study, indium-filled CoSb₃ skutterudite is synthesized via encapsulated induction melting and subsequent annealing at 823 K for six days, and the crystal structure, lattice constant, filler position, phase homogeneity and stability were investigated. All of the In-filled CoSb₃ samples were n-type conducting samples. The temperature dependence of the electrical resistivity showed In_zCo₄Sb₁₂ is a highly degenerate semiconducting material. The thermal conductivity was reduced considerably by In filling. The highest thermoelectric figure of merit value was achieved when the In filling fraction is 0.25. It was found that the ZT of the In-filled CoSb₃ (In_zCo₄Sb₁₂) was higher than that of the In-substituted CoSb₃ (Co_3.75In_0.25Sb₁₂ and Co₄Sb_11.75In_0.25). This is mainly due to the lower thermal conductivity and higher Seebeck coefficient.     

Electronic transport properties of Ni-doped CoSb<Subscript>3</Subscript> prepared by encapsulated induction melting

Ni-doped CoSb₃ skutterudites were prepared by encapsulated induction melting and their thermoelectric and electronic transport properties were investigated. The negative signs of Seebeck and Hal coefficients for all Ni-doped specimens revealed that Ni atoms successfully acted as n-type dopants by substituting Co atoms. The carrier concentration increased as the Ni doping content increased, and the Ni dopants could generate excess electrons. However, the carrier mobility decreased as the doping content increased, which indicates that the electron mean free path was reduced by the impurity scattering. The Seebeck coefficient and the electrical resistivity decreased as the carrier concentration increased, as the increase in carrier concentration by doping overcame the decrease in the carrier mobility by impurity scattering. The Seebeck coefficient showed a negative value at all temperatures examined and increased as the temperature increased. The temperature dependence of electrical resistivity suggested that Co_1−xNi_xSb₃ is a highly degenerate semiconducting material. Thermal conductivity was considerably reduced by Ni doping, and the lattice contribution was dominant in the Ni-doped CoSb₃.     

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

The synthesis and thermoelectric properties of In_0.1−xCo₄Sb_12−x skutterudite-based nanocomposites with xInSb nanoinclusions are reported. The nanoinclusions reduce the thermal conductivity of the composites considerably compared with nanoinclusion-free In_0.1Co₄Sb₁₂. 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.     

Thermal conductivity of electron-doped CaMnO3 perovskites: Local lattice distortions and optical phonon thermal excitation

The thermal transport properties of a series of electron-doped CaMnO₃ perovskites have been investigated. Throughout the temperature range 5–300 K, phonon thermal conductivity is dominant, and both electron and spin wave contributions are negligible. The short phonon mean free paths in this system result in the relatively low thermal conductivities. The strong phonon scatterings stem from the A-site mismatch and bond-length fluctuations induced by local distortions of MnO₆ octahedra. The thermal conductivity in the magnetically ordered state is enhanced as a result of the decrease in spin–phonon scattering. The results also indicate that above the magnetic ordering temperature, observable thermal excitation of optical phonons occurs. The contribution of optical phonons to thermal conductivity becomes non-negligible and is proposed to play an important role in the glass-like thermal transport behavior (i.e. positive temperature dependence of the thermal conductivity) in the paramagnetic state. These features can be understood in terms of an expression of thermal conductivity that includes both acoustic and optical phonon terms.     

Thermal conductivity analysis of BaUO3 and BaZrO3 by semiempirical molecular dynamics simulation

We performed the semiempirical molecular dynamics (MD) simulation for perovskite type BaUO₃ and BaZrO₃, and analyzed their thermal conductivities. The Morse-type potential function added to the Busing-Ida type potential was employed for the ionic interactions. The interatomic potential parameters were determined based on the experimental values of the lattice parameters of the perovskites. From the MD simulation, it is suggested that BaUO₃ has a larger phonon scattering probability than BaZrO₃, indicating that the thermal conductivity of BaUO₃ is lower than that of BaZrO₃. This result agrees well with the experimental data measured by the present authors.