Skutterudites are among the most exciting thermoelectric (TE) materials that could be used for various intermediate temperature applications. This study summarized our recent work on n-type partially filled skutterudites. By combining theoretical and experimental approaches, we revealed the underlying mechanism of void filling in the intrinsic lattice voids in CoSb3. With that, the electronegativity selection rule is established for the current stable filled skutterudites and further used for the discovery of a few novel filled CoSb3 compounds. The correlation between the thermal/electrical transport properties and impurity fillers in n-type partially filled skutterudites was also carefully investigated. Our results provide fundamental understanding to how those filler impurities affect electronic structures and lattice dynamics. Based on these basic understanding on transport mechanisms and sophisticated strategy in materials synthesis, TE figure of merit for n-type materials were continually increased from 1.1 to 1.4 and then to 1.7 for single-, double-, and triple-filled skutterudites.

We report fabrication of compacted Bi0.5Sb1.5Te3 nanoplatelets using hydrothermal methods followed by cold pressing and sintering in an evacuated ampoule at various temperature of 300–380 °C. The compacted Bi0.5Sb1.5Te3 sintered at 340 °C has the highest power factor of 11.6 μW/cm·K2 and its thermal conductivity is 0.37 W/m·K at 295 K, which is very low as compared to the typical value of 1 W/m·K. The resulting dimensionless figure of merit ZT is 0.93 at 295 K.

Most state-of-the-art thermoelectric (TE) materials contain heavy elements Bi, Pb, Sb, or Te and exhibit maximum figure of merit, ZT∼1–2. On the other hand, oxides were believed to make poor TEs because of the low carrier mobility and high lattice thermal conductivity. That is why the discoveries of good p-type TE properties in layered cobaltites NaxCoO2, Ca4Co3O9, and Bi2Sr2Co2O9, and promising n-type TE properties in CaMnO3- and SrTiO3-based perovskites and doped ZnO, broke new ground in thermoelectrics study. The past two decades have witnessed more than an order of magnitude enhancement in ZT of oxides. In this article, we briefly review the challenges, progress, and outlook of oxide TE materials in their different forms (bulk, epitaxial film, superlattice, and nanocomposites), with a greater focus on the nanostructuring approach and the late development of the oxide-based TE module.

(Bi,Sb)2Te3 + 4 mol%PbTe was quenched in water and on a rotating copper wheel (melt spinning). It was found that PbTe was immiscible in (Bi,Sb)2Te3 when the material is quenched in water and that the thermoelectric figure of merit increases by annealing. Natural nanostructures (nns) were found in melt-spun (Bi,Sb)2Te3, whereas they were hard to detect in (Bi,Sb)2Te3 alloyed with PbTe. There is a correlation between the orientation of the strain field and the nns. Within the grains of melt-spun (Bi,Sb)2Te3 alloyed with PbTe, the chemical composition was homogeneous. An enrichment of Pb was found at the grain boundaries. Quenched (Bi,Sb)2Te3 alloyed with 0.3 wt%PbTe have been spark plasma sintered (SPS). After optimization, the Seebeck coefficients of the melt-spun SPS (MS-SPS) materials were larger than for materials quenched in water and sintered (QW-SPS) materials. In addition, the mobility increases with the carrier concentration in MS-SPS materials, whereas it decreases in QW-SPS materials.

This research investigates the combination of electrochemical deposition and postdeposition vapor annealing as a method for the fabrication of Bi2Te3 layers. The galvanostatic deposition of Bi2Te3 thin films is characterized as a function of electrolyte composition and deposition-current density. Material with near-stoichiometric composition can be synthesized from electrolytes containing 20 mM Te and 30 mM Bi ions and a deposition-current density of 3.75 mA/cm2. All deposited samples show n-type behavior with Seebeck coefficients around −55 μV/K. An equilibrium annealing process in Te atmosphere is used to readjust the composition of the material after the deposition, consistently leading to tellurium-rich Bi2Te3 with a Te content of 60.4 ± 0.4 at%. At a temperature of 250 °C, an annealing duration of 60 h is sufficient for the material properties to reach a steady state, with a Seebeck coefficient of −130 μV/K.

A combined melt-spinning and spark-plasma-sintering (SPS) procedure has proven to be effective in preparing high-performance (Bi,Sb)2Te3 thermoelectric (TE) nanocomposites via creating and optimizing their resulting multiscale microstructures. (Bi,Sb)2Te3 possesses a highly anisotropic crystal structure; therefore, it is important to investigate any potential correlation between the SPS conditions, the as-formed microstructures, and the resulting TE properties. In this work, we investigate the correlation between the SPS pressure, the microstructure texture, and the anisotropy of the total thermal conductivity in these melt-spun spark-plasma-sintered (Bi,Sb)2Te3 compounds. The thermal conductivity has been measured in directions that are both perpendicular and parallel to the pressing (or force) direction by rearranging the sample geometry as described in the text. The results show that the anisotropy of thermal conductivity is ∼0, 2–3, 6–7, and 13–15% for the samples sintered at pressures of 20, 30, 45, and 60 MPa, respectively. These results are consistent with an increasing degree of orientation observed by x-ray diffraction and electron microscopy.

This article reports on the role of annealing on the development of microstructure and its concomitant effects on the thermoelectric properties of polycrystalline AgPbmSbTe2+m (m = 18, lead–antimony–silver–tellurium, LAST-18) compounds. The annealing temperature was varied by applying a gradient annealing method, where a 40-mm-long sample rod was heat treated in an axial temperature gradient spanning between 200 and 600 °C for 7 days. Transmission electron microscopy investigations revealed Ag2Te nanoparticles at a size of 20–250 nm in the matrix. A remarkable reduction in the thermal conductivity to as low as 0.8 W/mK was also recorded. The low thermal conductivity coupled with a large Seebeck coefficient of ∼320 μV/K led to high ZT of about 1.05 at 425 °C for the sample annealed at 505 °C. These results also demonstrate that samples annealed above 450 °C for long term are more thermally stable than those treated at lower temperatures.

To answer the fundamental questions of p-type skutterudites, systematical study on the influence of chemical composition on the electrical transport properties of RyFexCo4−xSb12 (R = Ce and Yb) has been carried out. By adjusting the filling fraction of fillers, the optimized electrical properties are obtained at the specific Fe content. It is found that the hole concentration increases with Fe content. Fe doping can also enhance the effective mass of holes significantly, which is beneficial for improving electrical performance. Because of the limit of electron supply, for trivalent Ce filling system CeyFexCo4−xSb12, the maximum figure of merit (ZT) value is achieved when Fe content is around x = 3, and for divalent Yb filling system YbyFexCo4−xSb12, the maximum ZT value is obtained even at lower Fe content. At high temperature above 700 K, the bipolar diffusion leads to great increase of total thermal conductivity and therefore deteriorates the thermoelectric properties.

Indium has attracted much attention as a beneficial addition to cobalt–antimony-based skutterudites as a result of good thermoelectric performance. In this study, as-cast InxCo4Sb12 with x = 0.05, 0.2 were examined using x-ray diffraction analysis and scanning electron microscopy. For x = 0.2 we found, besides the skutterudite main phase, nanometer-sized regions of secondary phases distributed along the grain boundaries, which exhibit substructures. As-cast material with x = 0.05 does not show visible precipitates. We further observed that changing one of the heat treatment parameters of In0.2Co4Sb12 has a major effect on the microstructure and shape of the precipitates, but minor influence on the skutterudite matrix composition. Energy dispersive x-ray spectroscopy analysis by transmission electron microscopy) reveals that indium is to a large extent distributed into the skutterudite structure. Measurements of short-term sintered material confirm that the addition of indium and particularly the modification of the synthesis parameter entails to an enhanced ZT.

The temperature-dependent thermoelectric (TE) and structural properties of n-type filled skutterudites were measured from 300–625 K. In0.2Co4Sb12, and In0.2Ce0.05Yb0.1Co4Sb12 exhibited figure of merit (ZT) values as high as 1.2 at 625 K and In0.2Ce0.15Co4Sb12 showed ZT values of ∼1.4 at 625 K. The room temperature Young’s modulus, Poisson’s ratio, and coefficient of thermal expansion (at 298–673 K) of In0.2Ce0.15Co4Sb12, In0.2Co4Sb12, and In0.2Ce0.05Yb0.1Co4Sb12 compositions were found to be lower than that for the unfilled Co4Sb12 skutterudite material. It was discovered that thermal cycling of n-type In0.15Ce0.1Co4Sb12 and In0.2Ce0.17Co4Sb12 materials from 323–673 K (200 cycles) actually increased their power factors by 13.6–36% at 510–525 K without appreciably changing the Young’s modulus or the Poisson’s ratio. The transport and structural properties characterized in this work are critical to transitioning these materials into operating TE devices and systems.

In- and Yb-doped CoSb3 thin films were prepared by pulsed laser deposition. Process optimization studies revealed that a very narrow process window exists for the growth of single-phase skutterudite films. The electrical conductivity and Seebeck coefficient measured in the temperature range 300–700 K revealed an irreversible change on the first heating cycle in argon ambient, which is attributed to the enhanced surface roughness of the films or trace secondary phases. A power factor of 0.68 W m−1 K−1 was obtained at ∼700 K, which is nearly six times lower than that of bulk samples. This difference is attributed to grain boundary scattering that causes a drop in film conductivity.

We studied nanoprecipitates (NPs) and defects in p-type filled skutterudite CeFe4Sb12 prepared by a nonequilibrium melt spinning plus spark plasma sintering method using transmission electron microscopy. NPs with mostly spherical shapes and different sizes (from several nanometers to several tens of nanometers) have been observed. Among these, two types of NPs were most commonly observed, Sb-rich superlattices and CeSb2. The Sb-rich superlattices with a periodicity of about 3.6 nm were induced by the ordering of excessive Sb atoms along the c-direction. These NPs typically share coherent interfaces with the surrounding matrix and induce anisotropic strain fields in the matrix. NPs with compositions close to CeSb2, on the other hand, have been shown to be much larger in size (∼30 nm) and have orthorhombic structures. Various defects were typically observed on the interfaces between these NPs and the matrix. The strain fields induced by these NPs are less distinct, possibly because part of the strain has been released by defect formation.

We report a novel approach to realize the formation of well-distributed nanodispersions in n-type filled skutterudite through the manipulation of metastable void fillers by a designed sophisticated process of materials synthesis. Metastable Ga filling in CoSb3 is proved to happen at high temperature. The subsequent controlled annealing procedure drives Ga out of the crystal voids and finally leads to the homogeneous dispersion of GaSb nanodots with an average size of 11 nm in CoSb3 matrix. The grain size of nanodispersions can be manipulated by the controlled cooling procedure. The well-distributed nanodispersions are observed to enhance Seebeck coefficients and reduce lattice thermal conductivity at low temperature. Therefore, the thermoelectric performance of nanocomposite is improved in the whole temperature range. The highest figure of merit (ZT) is obtained to be 1.45 at 850 K, and an average ZT of 0.99 in 300−850 K is achieved for Yb0.26Co4Sb12/0.2GaSb nanocomposite.

We have synthesized single- and polycrystal Ba8AlxSi46−x clathrates to compare their thermoelectric properties. Single-crystal sample was prepared by Czochralski method in an argon atmosphere. Polycrystal sample was prepared by arc melting and annealed at 850 °C for 100 h in an argon atmosphere. The Seebeck coefficients of single- and polycrystal Ba8Al12Si34 at 500 °C were 44.5 and 53.0 μV/K, respectively. The Seebeck coefficients of both samples were almost the same because the Seebeck coefficients depend on carrier concentration, which is related to aluminum content. The electrical resistivity of the single-crystal sample with 0.49 mΩcm was lower than that of the polycrystal sample with 0.95 mΩcm because of the reduction of electron scattering. Therefore, the power factor of the single-crystal sample with 4.0 × 10−4 V2/K2Ωm was higher than that of the polycrystal sample with 3.0 × 10−4 V2/K2Ωm at 500 °C. It is suggested that single crystallization is efficient for improvement of the thermoelectric property in the Ba8AlxSi46−x clathrate.

Ba8Ga16−xGe30+x is the clathrate with the highest thermoelectric figure of merit known to date. However, no p-type material could be obtained by conventional synthesis from the melt. Here we show that the time and cost-effective melt spinning technique can produce Ba8Ga16−xGe30+x in a metastable state, where x can be varied continuously from negative to positive values, resulting in both p- and n-type materials. The quenched phases were characterized by x-ray powder diffraction and transmission electron microscopy. It was surprising that they were perfectly crystalline, with large grain sizes of the order of a micrometer. Temperature dependent measurements of the electrical resistivity, Hall effect, thermopower, and thermal conductivity are presented and discussed in terms of a two-band model.

The influence of annealing time and annealing temperature under controlled partial pressure of selenium on the in-plane electrical transport properties of specimens of [(PbSe)0.99]1[WSe2]1 turbostratic nanolaminates was studied. The annealing treatments were found to be very effective in reducing carrier concentrations and improving carrier mobility in the annealed films, which is attributed to the reduction of compositional and structural defects. As a result, room temperature Hall mobilities greater than 60 cm2 V−1·s−1 are observed in spite of the small in-plane domain sizes (on the order of 10 nm) that are related to the turbostratic disorder. The technique appears promising for decreasing the concentration of kinetically trapped defects in these and related self-assembled nanostructures, a key challenge to evaluating the expected potential for controlling electrical and thermal transport properties via designed nanostructure in these and related materials.

SiGe alloys belong to the class of classic high temperature thermoelectric materials. By the means of nanostructuring, the performance of this well-known material can be further enhanced. Additional grain boundaries and point defects added to the alloy structure result in a strong decrease in thermal conductivity because of reduced lattice contribution to the overall thermal conductivity. Hence, the figure of merit can be increased. To obtain a nanostructured bulk material, a nanosized raw material is essential. In this work, a new approach toward nanostructured SiGe alloys is presented where alloyed nanoparticles are synthesized from a homogeneous mixture of the respective precursors in a microwave plasma reactor. As-prepared nanoparticles are compacted to a dense bulk material by a field assisted sintering technique. A figure of merit of zT = 0.5 ± 0.09 at 450 °C and a peak zT of 0.8 ± 0.15 at 1000 °C could be achieved for a nanostructured, 0.8% phosphorus-doped Si80Ge20 alloy without any further optimization.

Si80Ge20B0.6 thermoelectric alloys with minute erbium (Er) addition were prepared by the spark plasma sintering technique. The samples with different amounts of Er additions were analyzed by x-ray diffraction, x-ray fluorescence, and x-ray photoelectron spectroscopy. The thermoelectric properties were measured from 400 to 900 K. Effects of the amount of Er addition on the thermoelectric properties of Si80Ge20B0.6 alloys were investigated. New findings indicate that the Er-added alloys have larger carrier concentrations than the pristine sample. The larger carrier concentration appears to make a significant contribution to the electrical conductivity. Seebeck coefficient decreases with the increase of carrier concentration, whereas the power factor increases with increasing electrical conductivity. It was generally believed that the scattering of phonons by carriers may result in the thermal conductivity reduction. The samples with Er addition exhibit better figure of merits than the pristine sample. The optimal ratio of Er addition is actually in the range of 0.085–0.125 at.%.

A systematic investigation of the intermetallic phase Ru1-yIn3 (0 ≤ y ≤ 0.025) and its substitution derivatives RuIn3-xSnx and RuIn3-xZnx (x = 0.01, 0.025, 0.05, and 0.1) is performed. The samples were prepared from a liquid–solid reaction of components with subsequent spark plasma sintering treatment. Ru1-yIn3 exhibits n- and p-type behavior crossing over from low to high temperatures. Substitution of indium by tin or zinc up to 2.5 at.% leads to an increase of the charge carrier concentration, with negative (Sn) or positive (Zn) Seebeck values, respectively. The electrical resistivity was P changed from semiconductor- to metal-like properties by substitution, whereas the thermal conductivity was reduced down to 50% of that of RuIn3. Higher values of the thermoelectric figure of merit were achieved by chemical substitution (RuIn3-xSnx, RuIn3-xZnx), opening up a possibility for tuning the thermoelectric properties in this class of materials.