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.     

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.     

High temperature thermoelectric properties of skutterudite-Bi2Te3 nanocomposites

Nanocomposite engineering has been proved effective in diverse regimes of material research to attain a performance beyond each constituent phase. In this work, Yb-filled CoSb₃ (bulk matrix/host)-Bi_0.4Sb_1.6Te₃ (secondary inclusion) thermoelectric nanocomposites have been synthesized via an ex situ process. Bi_0.4Sb_1.6Te₃ inclusions are mainly distributed at the grain boundaries of Yb_0.2Co₄Sb₁₂ matrix in the composites. In particular, Te diffuses in situ from Bi_0.4Sb_1.6Te₃ through Yb_0.2Co₄Sb₁₂ matrix during the hot pressing process. This, combined with the grain boundary effect, results in favorable changes in the carrier concentration, carrier mobility, electrical resistivity, Seebeck coefficient, and thermal conductivity. Such synergistic changes are notably absent in the stand-alone Te-doped Yb-filled CoSb₃, suggesting the key role of diffusion and grain boundaries. As a result, a maximum ZT value of 0.96 has been attained for Yb_0.2Co₄Sb₁₂-2 wt% Bi_0.4Sb_1.6Te₃ at 650 K. The present work opens a new avenue towards high performance thermoelectric composites via controlled inter-constituent diffusion and grain boundary effect.     

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.     

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.     

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.     

Filler-reduced phonon conductivity of thermoelectric skutterudites: Ab initio calculations and molecular dynamics simulations

The phonon conductivities of CoSb₃ and its Ba-filled structure Ba_x(CoSb₃)₄ are investigated using first-principle calculations and molecular dynamics (MD) simulations, along with the Green–Kubo theory. The effects of fillers on the reduction of the phonon conductivity of filled skutterudites are then explored. It is found that the coupling between filler and host is strong, with minor anharmonicity. The phonon density of states and its dispersion are significantly influenced by filler-induced softening of the host bonds (especially the short Sb–Sb bonds). Lattice dynamics and MD simulations show that, without a change in the host interatomic potentials, the filler–host bonding alone cannot lead to significant alteration of acoustic phonons or lowering of phonon conductivity. The observed smaller phonon conductivity of partially filled skutterudites is explained by treating it as a solid solution of the empty and fully filled structures.     

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.     

Effects of Tl-filling into the voids and Rh substitution for Co on the thermoelectric properties of CoSb3

In order to enhance the thermoelectric (TE) properties of CoSb₃, we tried to reduce the lattice thermal conductivity (κ_lat) by filling Tl into the voids and substitution of Rh for Co. We prepared polycrystalline samples of Tl_x(Co_1−yRh_y)₄Sb₁₂ (x = 0, 0.05, 0.10, 0.15, 0.20 and y = 0.1, 0.2) and examined their TE properties from room temperature to 750 K. All the samples indicated negative values of the Seebeck coefficient (S). Both the electrical resistivity and the absolute values of the S decreased with increasing the Tl-filling ratio. The Tl-filling and Rh substitution reduced the κ_lat, due to the rattling and the alloy scattering effects. The minimum value of the κ_lat was 1.54 W m⁻¹ K⁻¹ at 550 K obtained for Tl_0.20(Co_0.8Rh_0.2)₄Sb₁₂. Tl_0.20(Co_0.8Rh_0.2)₄Sb₁₂ exhibited the best TE performance; the maximum value for the dimensionless figure of merit ZT was 0.58 at around 600 K.     

High thermoelectric performance of Yb0.26Co4Sb12/yGaSb nanocomposites originating from scattering electrons of low energy

n-type filled skutterudite Yb_0.26Co₄Sb₁₂ composites in which p-type GaSb nanostructured inclusions (5–20 nm) were dispersed were fabricated by an in situ method involving the introduction of metastable void-filling impurity Ga and enrichment of Sb in the synthesis procedure. With the homogeneously dispersed GaSb nanoinclusions, which probably result from the scattering of low-energy electrons by the GaSb-related boundary energy barriers, the power factor is enhanced due to the significant enhancement of the Seebeck coefficient. The total thermal conductivity was decreased with the depression of electronic thermal conductivity. As a result, the dimensionless thermoelectric figure of merit ZT value was improved over a wide working temperature range of 300–850 K, and the expected optimal thermal-electric conversion efficiency η_opt 15.0% was obtained for the Yb_0.26Co₄Sb₁₂/0.2GaSb nanocomposite.     

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.     

Phase formation and thermoelectric properties of Zn1+xSb binary system

The phase formation and thermoelectric (TE) properties in the central region of the Zn?Sb phase diagram were analyzed through synthesizing a series of Zn_1+xSb (x=0, 0.05, 0.1, 0.15, 0.25, 0.3) materials by reacting Zn and Sb powders below the solidus line of the Zn?Sb binary phase diagram followed by furnace cooling. In this process, the nonstoichiometric powder blend crystallized in a combination of ZnSb and β-Zn₄Sb₃ phases. Then, the materials were ground and hot pressed to form dense ZnSb/β-Zn₄Sb₃ composites. No traces of Sb and Zn elements or other phases were revealed by X-ray diffraction, high resolution transmission electron microscopy and electron energy loss spectroscopy analyses. The thermoelectric properties of all materials could be rationalized as a combination of the thermoelectric behavior of ZnSb and β-Zn₄Sb₃ phases, which were dominated by the main phase in each sample. Zn_1.3Sb composite exhibited the best thermoelectric performance. It was also found that Ge doping substantially increased the Seebeck coefficient of Zn_1.3Sb and led to significantly higher power factor, up to 1.51 mW·m^?1·K^?2 at 540 K. Overall, an exceptional and stable TE figure of merit (Z_T) of 1.17 at 650 K was obtained for Zn_1.28Ge_0.02Sb.     

Microstructure and electrical properties of Lu2O3-doped ZnO-Bi2O3-based varistor ceramics

Lu2O3-doped ZnO-Bi2O3-based varistor ceramics samples were prepared by a conventional mixed oxide route and sintered at temperatures in the range of 900-1 000°C,and the microstructures of the varistor ceramics samples were characterized by X-ray diffractometry(XRD)and scanning electron microscopy(SEM);at the same time,the electrical properties and V-I characteristics of the varistor ceramics samples were investigated by a DC parameter instrument for varistors.The results show that the ZnO-Bi2O3-based varistor ceramics with 0.3%Lu2O3(molar fraction)sintered at 950°C exhibit comparatively ideal comprehensive electrical properties.The XRD analysis of the samples shows the presence of ZnO,Bi-rich,spinel Zn7Sb2O12 and Lu2O3-based phases.     

Crystal structures and phase transformation of deuterated lithium imide, Li2ND

Half-Heusler compounds of Sn-doped TiCoSb “TiCoSn_xSb_1−x (x = 0.0, 0.01, and 0.05)” were prepared and their thermoelectric properties were measured above room temperature. From the EDX analysis, all the samples have three phases: the TiCoSn_xSb_1−x, Co-rich, and Ti-rich phases. The values of the thermoelectric power increase with Sn doping, and a positive thermoelectric power is obtained in the sample of TiCoSn_0.05Sb_0.95. The thermal conductivity decreases both with increasing temperature and with Sn content. The maximum value of ZT for p-type material is 0.030 at 988 K in the sample of TiCoSn_0.05Sb_0.95.     

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.     

A new generation of p-type didymium skutterudites with high ZT

This work evaluates the influence of single, double and triple filling of didymium, Ca and Ba in Fe₄Sb₁₂ as well as in Fe₃CoSb₁₂ on the thermoelectric performance. Various filling levels, as well as various preparation methods and nanostructuring were used to improve the thermoelectric performance. It is shown that samples prepared via ball milling have a higher ZT (ZT = 1.1) than their hand milled counterparts (ZT ≈ 0.8). Co/Fe-substituted samples have ZT > 1.2 i.e. 25% higher than samples without Co, an average ZT up to 0.93 and an efficiency up to 14% for the temperature gradient of 300–800 K. With this good thermoelectric performance in such a wide temperature range these materials are hitherto the best p-type skutterudites for thermoelectric devices.     

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.     

Effects of Te doping on the transport and thermoelectric properties of Zn4Sb3

The transport and thermoelectric properties of Te-doped Zn₄Sb₃ compounds, Zn₄(Sb_1?xTe_x)₃ (x = 0, 0.005 and 0.02), were investigated. The results showed that thermal conductivity of the Te-doped compounds were reduced remarkably as compared to that of Zn₄Sb₃ presumably due to enhanced impurity (dopant) scattering of phonons. Thermopower S was found to decrease with increase in Te content, which could be ascribed to the excess Zn in the doped compounds acting as p-type dopant that leads to an increase in the carrier concentration. Moreover, it was found that the β to α phase transition of Zn₄Sb₃ could be completely prohibited by Te doping. The figure of merit, ZT, for doped compounds was greater than the un-doped Zn₄Sb₃ for the temperature range investigated. In particular, the ZT of Zn₄(Sb_0.995Te_0.005)₃ reached a value of 1.08 at 680 K, which is 69% greater than that of the un-doped Zn₄Sb₃ obtained in this study.     

Growth of Sb2S3 nanowires synthesized by colloidal process and self-assembly of amorphous spherical Sb2S3 nanoparticles in wires formation

We report the organic synthesis and growth of antimony sulfide (Sb₂S₃) amorphous nanospheres to nanowires via a simple, colloidal synthetic method. Amorphous Sb₂S₃ nanospheres self-assembly in wires formation was dispersed in isopropyl alcohol. With increased heating time, Sb₂S₃ nanospheres grew into Sb₂S₃ nanowires, probably involving both mechanisms of Ostwald-ripening and spherical nanoparticle self-organization through oriented-attachment of individual nanoparticles. Also, the as-synthesized Sb₂S₃ nanowires with different heating times (0, 5 and 10 min.) from the moment of appearance of the Sb₂S₃ precipitate were analyzed. The observed nanowires become longer with increased heating time and are around 100 nm in diameter and 10?C20 ??m in length. UV-Vis absorption spectroscopy reveals that the optical band-gap energy of the Sb₂S₃ nanowires is independent of the heating times and is found to be ??1.5?C1.6 eV. The optical band-gap energy found for amorphous Sb₂S₃ nanospheres was also ??1.5 eV. The structure of Sb₂S₃ samples was refined down to R-factors of 10.82, 11.76 and 12.08%. The refinement showed that Sb₂S₃ powder belongs to the orthorhombic type with space group Pbnm (no. 62) and that Sb₂S₃ nanowires grow along the [010] direction.