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Polar Y6Pt13X4 (X = Al, Ga) compounds feature Pt-filled Y6 octahedra embedded in the XPt3 framework.

The present study reports the structural, electronic, elastic, and thermoelectric properties of Cu12Sb3.9Te0.1S13-xSex (x = 0, 0.1, 0.5, 0.75 and 1) tetrahedrite. The thermal conductivity was reduced due to charge carrier compensation (by Te), weakening of the Sb-S bond and local chemical disorder (induced by Se), resulting in a low mean sound velocity. Simultaneously, the power-factor was maintained high from an enhancement of the density of states near the Fermi level due to Se. Consequently, a maximum figure of merit ~0.84 at 673 K was achieved for the Cu12Sb3.9Te0.1S12.5Se0.5 sample due to concomitant optimisation of the power-factor and thermal conductivity.

The thermoelectric efficiency of skutterudite materials can be improved by lowering the lattice thermal conductivity via the uniform dispersion of a nanosized second phase in the matrix of filled C...

Several n- and p-type Half-Heusler (HH) thermoelectric materials (Ti0.5Zr0.5NiSn-based and NbFeSb-based) have been processed by high-pressure torsion (HPT) to improve their thermoelectric performance via a drastic reduction towards ultra-low thermal conductivity. This reduction occurs due to grain refinement and a high concentration of deformation-induced defects, i.e. vacancies and dislocations as inferred by this severe plastic deformation and documented via SEM and TEM investigations. In most cases the figure of merit, ZT, and the thermo-electric conversion efficiency were enhanced up to η ∼ 10% for the thermally stable HPT-processed sample. Raman spectroscopy, backed by DFT calculations, proves that HPT induces a stiffening of the lattice and as a consequence, a blue-shift of the lattice vibrations occurs. Furthermore for all investigated specimens Vickers hardness values after HPT were significantly higher, whereas the change in the elastic moduli was less than 5% in comparison to the HP reference sample.

Filling the voids of cage forming compounds with loosely bound electropositive elements and by incorporating nano-sized secondary phases are promising approaches to enhance the thermoelectric figure of merit of these materials. Hence, in this work, by combining these two approaches-inserting In into the voids of skutterudite Co4Sb12 as well as dispersing nanoparticles (GaSb)-we have synthesized various samples via ball-milling and spark plasma sintering. InSb as the secondary phase of the matrix, mixed with GaSb, forms the solid solution Ga1-xInxSb. Nanocrystalline grains together with a few larger grains (10-30 μm) are found to be spread in In0.2Co4Sb12. The former is comprised of either InSb, GaSb or Ga1-xInxSb. Because of their identical space group and similar lattice parameters, InSb, GaSb and Ga1-xInxSb could not be detected separately in EBSD. High resolution transmission electron microscopy (HRTEM) was used to resolve different phases, which showed GaSb grains of size ∼10-30 nm and InSb grains of size ∼30-100 nm. Scattering of charge carriers at the interfaces of InSb, GaSb and Ga1-xInxSb as well as the matrix phases increased both the electrical resistivity and Seebeck coefficient. The multi-scale size distribution of grains, of both the matrix phase and the secondary phases, scattered phonons within a broad wavelength range, resulting in very low lattice thermal conductivities. As a result, an enhanced figure of merit of 1.4 was achieved for the (GaSb)0.1 + In0.2Co4Sb12 nanocomposite at 773 K.

Electrical resistivity (4.2–800 K), Seebeck coefficient (300–800 K), specific heat (2–110 K), Vickers hardness, elastic moduli (RT) were defined for single-phase nonstoichiometric compounds: Ti2.13Ni2Sn0.87, Zr2.025Ni2Sn0.975, and Hf2.055Ni2Sn0.945.

Germanium tellurides and its pseudo binary compounds offer interesting properties that are important in thermoelectric and phase-change applications. Despite being a class of materials under scrutiny since its discovery, unique properties and functionalities kept on emerging in recent years. In this work, we observed another unique property of Ge-Sb-Te (GST) thin film that can be beneficial in its development for thermoelectric application. A rapid heating and quenching process of the GST film resulted in a metastable rock-salt cubic structure, exhibiting a unique electronic-transition-like behavior. Above the transition temperature at 150 °C, we observed a temperature-induced band modulation, corroborated with changes in its effective mass and valence band position that leads to favorable electronic and thermoelectric properties. Charge transfer between Sb and Te occurred, accompanied by a distorted cubic-to-cubic structural change. The interplay of the electronic and lattice structure born out of the co...

As various branches of industry are interested in good thermoelectric generators, there is a serious demand for an easy, fast, cheap and sustainable production route of excellent thermoelectric materials. In this paper we demonstrate how we could achieve a high-ZT (ZT ∼ 1.45 at 823 K) n-type bulk skutterudite, (Mm,Sm)yCo4Sb12, by processing the industrially produced raw powder using a modified high-pressure torsion (HPT) equipment at elevated temperature under inert gas. For this HPT processed sample as well as for the annealed one, structural properties, especially focused on grain size and dislocation density (backed up by electron probe microanalysis, transmission electron microscopy investigations as well as XPD profile analysis), physical properties (from 300 to 823 K and for the electrical resistivity in addition from 4.2 to 300 K) and mechanical properties (elastic moduli, hardness, fracture resistance) were measured and evaluated and compared with the values of a traditionally prepared bulk material, using hot pressing (HP). As a consequence of severe plastic deformation, we found a change from metallic to semiconducting behavior during measurement induced annealing of the HPT processed skutterudite, which could be explained via the change of lattice distortion and its influence on the band gap. A positive ZT net effect occurred because although the electrical resistivity was enhanced the thermal conductivity was decreased. It turned out that the elastic moduli of the HPT samples were not much different from those of the HP skutterudite, however, that the hardness was significantly increased.

Thermoelectric materials play an important role in the field of renewable energy for their ability of thermal energy conversion into electricity (thermoelectric generator) and reversibly electric energy conversion into thermal energy (thermoelectric heat pump or Peltier cooling). Doped CoSb3 based skutterudites represent one of the very promising categories for the development of highly efficient thermoelectric materials for the conversion of waste heat to electricity. The thermoelectric efficiency, however, is closely related to thermal and phase stability as the current materials contain volatile elements (Sb, Sr, Yb, etc.), which can evaporate at operation conditions and thus cause structure changes and damage the thermoelectric properties. For a better understanding of the thermal behaviour of complex multicomponent CoSb3 based skutterudite systems, a study of the thermal stability of primary CoSb3 skutterudite is necessary. In this work, the thermal and phase stability of primary CoSb3 skutterudite prepared by ball milling and hot pressing was investigated using thermal analysis and Knudsen effusion mass spectrometry performed on a Netzsch STA 409 CD/3/403/5/G apparatus, a specially-adapted type of the commercial STA 409 CD - QMS 403/5 Skimmer Coupling Instrument. Results, including data on phase transformations and those from vapour pressure measurements of antimony, supported by measurements of diffusion profiles and microstructure observations are summarized and used for evaluation of the long term thermal stability of the material.

Crystals of the solid solution Tm(Al1-xMox)B4(x = 0.002 - 0.050) were grown from Tm2O3, metal Mo and crystalline boron powders as starting materials under an Ar atmosphere at 1773 K for 5 h. Tm(Al1-xMox)B4crystals were obtained as a single phase in the case of Al1-xMoxflux (x = 0.005 and 0.010). Tm(Al1-xMox)B4crystals at a maximum size of about 2.2 mm were obtained in the shape of a flake, and together with needle-like crystals of MoAlB and an unknown phase using Al0.970Mo0.030flux. The lattice constants of Tm(Al1-xMox)B4(x = 0.005 - 0.010) crystals were determined to be larger compared to those obtained for TmAlB4. This is related to the fact that the ionic radius of Mo is larger than the ionic radius of Al. When the Al position in the TmAlB4structure was substituted with Mo having a large ionic radius, the hardness hardly changed. In contrast to TmAlB4, no antiferromagnetic transition could be observed for Tm(Al0.995Mo0.005)B4, indicating that the disorder introduced by Mo-doping disrupted the magnetic transition.

Crystals of the quaternary compounds α-Lu(Al1-xTx)B4(T = Fe, Cr) (YCrB4-type, orthorhombic, Pbam), obtained from the nominal composition of Lu(Al1-xTx)B3, were grown by using Al flux mixed with T metal at 1773 K for 5 h under an Ar atmosphere. The maximum dimensions and morphology of the crystals obtained were about 0.7 mm × 0.5 mm for flake-type crystals of Lu(Al1-xCrx)B4and about 5.2 mm × 0.2 mm for prism crystals of Lu(Al1-xFex)B4. The lattice constants determination and chemical analyses of Lu(Al1-xTx)B4(T = Fe, Cr) compounds were carried out for Fe 0.5 - 10.0 at% and Cr 0.5 - 1.0 at%. The lattice constants and the unit lattice volume in Lu(Al1-xTx)B4crystals decreased with increase of the concentration of Fe or Cr. The values of micro-Vickers hardness of Lu(Al1-xFex)B4(x = 0.005 - 0.030) and Lu(Al1-xCrx)B4(x = 0.005 - 0.010) samples are in the ranges of 16(2) - 20(3) GPa and 13(2) - 16(3) GPa, respectively. The hardness values showed a little increase as the solid solution of Fe or Cr was realized in Lu(Al1-xTx)B4, possibly because of distortion to the crystal structure. The magnetic susceptibility of as-grown Lu(Al0.995Fe0.005)B4crystals for example, exhibited diamagnetic behavior with a small ferromagnetic component.

With a small gap in the density of states and a substantially semiconducting behavior half Heusler alloys have drawn attention as thermoelectric materials. For this study we have selected Hf-free compounds, Ti0.5Zr0.5NiSn, Ti0.5Zr0.5NiSn (with a densification aid (DA)) and Ti0.5Zr0.5NiSn0.98Sb0.02 as well their parent alloys TiNiSn and ZrNiSn as cheap thermoelectrics. Electrical resistivity, thermal conductivity and specific heat were evaluated below room temperature (4.2–300 K) in order to get insight into the mechanism of transport properties. SEM and TEM investigations as well as DFT (density functional theory) calculations accompany this research. The fine-grained epitaxial microstructure with a large number of dislocations warrants a low thermal conductivity at ultralow values (∼30 mW/cmK at 300 K) at a narrow band gap with a sufficiently high density of states at the Femi level. High order of components mixing strongly affects the stability of the solid solutions by the configuration entropy term, which causes a shrinkage of the miscibility gap. For the electronic density of states (DOS) the split Zr band and impurity Ni band induce a significant reduction of the effective energy gap and thus explain n-type of conductivity of the compounds and solid solutions studied.

The two La2Pd3Ge5 and Nd2Pd3Ge5 compounds, crystallizing in the oI40-U2Co3Ge5 crystal structure, were targeted for analysis of their chemical bonding and physical properties. The compounds of interest were obtained by arc melting and characterized by differential thermal analysis, scanning electron microscopy, and X-ray diffraction both on powder and on a single crystal (for the La analogue), to ensure the high quality of the samples and accurate crystallographic data. Chemical bonding was studied by analyzing the electronic structure and effective QTAIM charges of La2Pd3Ge5. A significant charge transfer mainly occurs from La to Pd so that Ge species assume tiny negative charges. This result, together with the -(I)COHP analysis, suggests that, in addition to the expected homopolar Ge bonds within zigzag chains, heteropolar interactions between Ge and the surrounding La and Pd occur with multicenter character. Covalent La-Pd interactions increase the complexity of chemical bonding, which could not be adequately described by the simplified, formally obeyed, Zintl-Klemm scheme. Electric resistivity, specific heat, magnetization, and magnetic susceptibility as a function of temperature indicate for both compounds a metallic-like behavior. For Nd2Pd3Ge5, two low-temperature phase transitions are detected, leading to an antiferromagnetic ground state.

Filling the voids of cage forming compounds with loosely bound electropositive elements and by incorporating nano-sized secondary phases are promising approaches to enhance the thermoelectric figure of merit of these materials. Hence, in this work, by combining these two approaches-inserting In into the voids of skutterudite Co4Sb12 as well as dispersing nanoparticles (GaSb)-we have synthesized various samples via ball-milling and spark plasma sintering. InSb as the secondary phase of the matrix, mixed with GaSb, forms the solid solution Ga1-xInxSb. Nanocrystalline grains together with a few larger grains (10-30 μm) are found to be spread in In0.2Co4Sb12. The former is comprised of either InSb, GaSb or Ga1-xInxSb. Because of their identical space group and similar lattice parameters, InSb, GaSb and Ga1-xInxSb could not be detected separately in EBSD. High resolution transmission electron microscopy (HRTEM) was used to resolve different phases, which showed GaSb grains of size ∼10-30 nm and InSb grains of size ∼30-100 nm. Scattering of charge carriers at the interfaces of InSb, GaSb and Ga1-xInxSb as well as the matrix phases increased both the electrical resistivity and Seebeck coefficient. The multi-scale size distribution of grains, of both the matrix phase and the secondary phases, scattered phonons within a broad wavelength range, resulting in very low lattice thermal conductivities. As a result, an enhanced figure of merit of 1.4 was achieved for the (GaSb)0.1 + In0.2Co4Sb12 nanocomposite at 773 K.

The thermoelectric efficiency of skutterudite materials can be improved by lowering the lattice thermal conductivity via the uniform dispersion of a nanosized second phase in the matrix of filled Co

More than hundred diffusion couples between p- and n-type skutterudites and various materials were prepared and interaction zones were investigated after heat treatment at 600 °C for 1100 h. The constitution of reaction/diffusion zones was discussed in terms of: (a) atom site preference in the skutterudite lattice, (b) phase equilibria in multicomponent systems and (c) particularities of the crystal structure of intermediate phases. It could be shown that phase composition and thermo-mechanical properties of bonding can be engineered by chemical substitution. The results obtained allowed the determination of several necessary criteria for the development of chemically and mechanically stable diffusion barriers/couples for skutterudite based thermoelectric (TE) modules.

Thermoelectric leg materials with a high figure of merit, ZT, are the essential basis to build thermoelectric generators, directly converting waste heat into electricity. Skutterudites and half-Heusler alloys are promising candidates because they can be used in a wide temperature range, the starting material is available and cheap and in addition they are environmentally friendly. Severe plastic deformation via high-pressure torsion (HPT) is a technique to achieve very fine grains in micro- or even nano size with small and high angle grain boundaries and in parallel introduces a high level of defects like vacancies and dislocations. Therefore, this method was applied not only to enhance ZT of ball-milled and hot-pressed skutterudites and half-Heusler alloys but so far was also successful to directly produce dense nanostructured bulks from skutterudite powders. Although high-pressure torsion compacted samples are chemically homogeneous, they are not homogeneous with respect to the shear strain increasing from the center to the rim. HPT changes the microstructure and density and thereby not only influences the thermoelectric but also the mechanical properties. In this work an overview is given of the influence of HPT on hardness, elastic moduli, indentation fracture toughness, thermal expansion and thermal shock resistance. The corresponding properties of hot-pressed skutterudites and half-Heusler alloys are compared with those after severe plastic deformation, dependent on the processing properties and position of the specimen in respect to shear strain. Data are collected from earlier investigations of the authors and from the literature, but also newly achieved and evaluated data are included.

Skutterudites are an important class of thermoelectric p- and n-type materials and they have already achieved fair efficiencies for the conversion of heat to electricity. Nevertheless researchers try to further enhance the figure of merit, ZT, by various ways. In this work we study microstructure and mechanical properties of two thermoelectric materials: an industrial n-type (Mm,Sm)yCo4Sb12 skutterudite and an industrial p-type DDyFe3CoSb12 skutterudite, both mixed with 1 wt.% of Ta0.8Zr0.2B. Thin lamellae were prepared from the compacted materials using a focused ion beam. Analytical transmission electron microscopy was used on lamellae to study details of microstructure. A fine dispersion of precipitates was found both at nanograin boundaries and in their interiors. Quasistatic and dynamic nanoindentation tests were carried out on planar polished sections in the range of applied loads from 0.01 to 10 mN. The results were complemented with quantitative modulus mapping of local mechanical properties with 10-nm resolution.

The crystal structure of the novel Al-rich intermetallic phase Ce4Pt9Al13 was determined from single-crystal X-ray diffraction data and confirmed by powder X-ray diffraction. The compound crystallizes with its own structure type, which is a “site exchange” variant of the Ho4Ir13Ge9-type (Pearson symbol oP52, space group Pmmn, No. 59, Z = 2, lattice parameters: a = 4.1826(1) A, b = 11.4424(3) A, c = 19.8475(5) A, V = 949.88(7) A3). The magnetic properties of Ce4Pt9Al13 were determined in the temperature range 1.72–400 K. Down to the lowest temperatures studied, the compound shows a Curie-Weiss paramagnetic behavior with strong crystalline electric field contribution observed below about 200 K. The electrical conductivity in Ce4Pt9Al13 has metallic character, and at low temperatures it is governed by an interplay of strong crystal field and Kondo interactions.

For mass fabrication of thermoelectric generators a fast, cheap and simple method is required to produce high ZT leg material. It is well known that severe plastic deformation (SPD) enhances the density of defects and dislocations, and also refines the grains to nano-size. These are the essential key parameters for low thermal phonon conductivity. Consequently and as a first example, SPD via high-pressure torsion (HPT) at elevated temperatures in protective gas atmosphere was used to directly consolidate and plastically deform commercial p-type skutterudite powder, DDyFe3CoSb12, (DD stands for didymium) into a dense thermoelectric solid. The HPT-sample exhibited a high figure of merit, ZT > 1.3 at 773 K, much higher than that of the hot-pressed reference sample (HP) from the same powder. The ZT achieved is comparable with ZTs of corresponding high energy ball milled and hot pressed samples (HBM-HP) as well as of ball milled, hot pressed plus HPT processed skutterudites (BM-HP-HPT). The thermoelectric efficiency is even 4% higher. Synchrotron measurements were performed at temperatures from 300 to 825 K in order to evaluate the changes in grain size and dislocation density of the CP-HPT samples before, during and after annealing. SEM and TEM images served to give a better insight into the temperature dependent diversification of the new material. Due to grain refinement and enhanced dislocation density, the CP-HPT material exhibits higher hardness values than the reference sample. The elastic moduli are, within the error bar, the same. With large high-pressure torsion facilities at hand, a new, fast and therefore cheap mass production of thermoelectric leg material directly from powders can be envisaged. This new production technique furthermore warrants high figures of merit but excludes time-consuming ball milling and hot-pressing steps.

X-ray single-crystal structure analysis was performed for the novel compound V1.84Fe1.16B2 ≡ (V1-xFex)2FeB2 at x = 0.08 (P4/mbm; a = 0.555931(9) nm, c = 0.306781(5) nm; U3Si2-type). Consequently, structural identity is obvious between (V0.92Fe0.08)2FeB2 and the precipitates V∼2Fe∼1B2 earlier identified in the UGISTAB215XH permanent magnet. Magnetic and 57Fe Mossbauer studies of (V0.92Fe0.08)2FeB2 reveal a magnetically ordered ground state with Tc∼110 K. Mossbauer spectra point towards a ferrimagnetic spin arrangement. Enthalpy of formations (DFT calculations) for (Fe,V), VB, V3B2, and the hypothetical solution V3-xFexB2 (x Calculation of the electron localization function elf yielded a very high value (ϒ ∼0.75) between boron atoms documenting strong covalent bonding. The Young's modulus E (from nano-indentation) for V1.84Fe1.16B2 is 442 GPa. The higher anisotropy in the ternary boride V2FeB2 is concluded from the significantly higher difference between C11 and C33 in V2FeB2 (192.1 GPa) with respect to V3B2 (117.0 GPa).

Mechanical (elastic) properties have been derived for polycristalline TiMn2 from micro-indentation and thermal expansion measurements. With a room temperature (RT) Vicker's hardness of ∼11 GPa, TiMn2 is a rather hard intermetallic material. The elastic modulus is E = 217 ± 4 GPa at RT. Physical properties (from 1.8 K to 300 K) including specific heat, electrical resistivity and magnetic susceptibility reveal temperature independent paramagnetism and metallic conductivity consistent with a Bloch-Gruneisen behavior. Based on experimental data (X-ray powder diffraction, metallography, SEM and EMPA on 30 alloys prepared by various methods employing arc melting, powder reaction sintering in closed crucibles as well as nitriding binary master alloys in 105 Pa nitrogen gas) phase relations in the ternary system Ti-Mn-N have been established and calculated for the isothermal section at 900 °C. The thermodynamic modelling relies on existing thermodynamic assessments of the Mn-N and Ti-N binary systems and on our recent modelling of the Ti-Mn system, which is characterized by a significantly higher thermodynamic stability of the C14-type Laves phase Ti1-xMn2+x than hitherto assumed in the literature. It is this higher stability, which is consistent with the experimental phase triangulation in Ti-Mn-N: as the most important consequence of the higher stability of Ti1-xMn2+x, compatibility exists between the only two congruently melting compounds Ti1-xMn2+x and TiN1-x. The high melting refractory solution TiN1-x dominates the liquidus surface pushing all monovariant melting lines close to the Ti-Mn binary.

We synthesized a new ternary stannide CePd 3 Sn 2 and determined its crystal structure from single crystal x-ray diffraction. This compound crystallizes in the orthorhombic structure with space group Pnma (No.62). One-dimensional chains of the shortest Ce-Ce contacts at 0.47663(1) nm run along the b axis. Both specific heat C and susceptibility χ of polycrystalline samples show a pronounced peak at T N = 0.6 K indicating an antiferromagnetic order. An obvious tail in C ( T ) appears in the temperature range from T N up to 3 K where the magnetic entropy S m reaches Rln2. Meanwhile, the electrical resistivity ρ exhibits an upward curvature without showing any sign of Kondo scattering. The unusual behaviors of C ( T ) and ρ ( T ) above T N are attributed to antiferromagnetic short-range correlations in the Ce-Ce chain.

Although filled skutterudites have a high figure of merit, ZT, it is still an aim to further improve their thermoelectric behavior. One successful route is to homogeneously disperse nanoparticles into the skutterudite matrix and thereby to reduce the lattice thermal conductivity without disturbing the electrical conductivity and the Seebeck coefficient. In this work Ta0.8Zr0.2B was dispersed into n-type (Mm,Sm)yCo4Sb12 powder by ball-milling or high energy ball-milling prior to hot-pressing. Structural and physical properties, covering also the low temperature range, were studied and compared with the recently investigated impact of nano Ta0.8Zr0.2B on p-type skutterudites. ZT was enhanced by 16%. As for thermoelectric applications good mechanical properties are important, we investigated the influence of Ta0.8Zr0.2B nanoparticles, added to (Mm,Sm)yCo4Sb12, on the hardness, elastic moduli and fracture toughness and found an enhancement of all three mechanical properties. It is well known that p-type skutterudites generally have a higher thermal expansion coefficient than their n-type counterpart. We show that dispersed borides in both n-type (Mm,Sm)yCo4Sb12 and p-type DD0.7Fe3CoSb12 can level out this discrepancy.

Investigations of phase relations in the ternary system Ti-Fe-Sb show that the single-phase region of the Heusler phase is significantly shifted from stoichiometric TiFeSb (reported previously in the literature) to the Fe-rich composition TiFe1.33Sb. This compound also exhibits Fe/Ti substitution according to Ti1+xFe1.33-xSb (-0.17 ≤ x ≤ 0.25 at 800 °C). Its stability, crystal symmetry and site preference were established by using X-ray powder techniques and were backed by DFT calculations. The ab initio modeling revealed TiFe1.375Sb to be the most stable composition and established the mechanisms behind Fe/Ti substitution for the region Ti1+xFe1.33-xSb, and of the Fe/Co substitution within the isopleth TiFe1.33Sb-TiCoSb. The calculated residual resistivity of Ti1+xFe1.33-xSb, as well as of the isopleths TiFe1.33Sb-TiCoSb, TiFe0.665Co0.5Sb-TiCoSb0.75Sn0.25 and TiFe0.33Co0.75Sb-TiCoSb0.75Sn0.25, are in a good correlation with the experimental data. From magnetic measurements and 57Fe Mossbauer spectrometry, a paramagnetic behavior down to 4.2 K was observed for TiFe1.33Sb, with a paramagnetic Curie-Weiss temperature of -8 K and an effective moment of 1.11μB per Fe. Thermoelectric (TE) properties were obtained for the four isopleths Ti1+xFe1.33-xSb, TiFe1.33Sb-TiCoSb, TiFe0.665Co0.5Sb-TiCoSb0.75Sn0.25 and TiFe0.29Co0.78Sb-TiCoSb0.75Sn0.25 by measurements of electrical resistivity (ρ), Seebeck coefficient (S) and thermal conductivity (λ) at temperatures from 300 K to 823 K allowing the calculation of the dimensionless figure of merit (ZT). Although p-type Ti1+xFe1.33-xSb indicates a semi-conducting behavior for the Fe rich composition (x = -0.133), the conductivity changes to a metallic type with increasing Ti content. The highest ZT = 0.3 at 800 K was found for the composition TiFe1.33Sb. The TE performance also increases with Fe/Co substitution and reaches ZT = 0.42 for TiCo0.5Fe0.665Sb. No further increase of the TE performance was observed for the Sb/Sn substituted compounds within the sections TiFe0.665Co0.5Sb-TiCoSb0.75Sn0.25 and TiFe0.33Co0.75Sb-TiCoSb0.75Sn0.25. However, ZT-values could be enhanced by about 12% via the optimization of the preparation route (ball-mill conditions and heat treatments).

Investigation of the system Th-Ni-B prompted a novel ternary compound ThNi12B6. X-ray structure analysis of single crystals obtained by the mechanical fragmentation of an as-cast sample revealed a fully ordered CeNi12B6-type structure (space group Cmc21, no. 36; a = 0.95638(1) nm, b = 0.73852(1) nm, c = 1.10195(1) nm; RF2 = 0.0305). Density functional theory (DFT) calculations have been performed comprising heat of formation, electronic band structure and density of states, Fermi surface via Wannier functions, phonon band structure and density of states, phonon and electronic contributions to specific heat and elastic constants Cij. Comparing the parameters evaluated from DFT with the experimental data, an overall satisfactory agreement has been achieved. Measurements of electrical resistivity, magnetic susceptibility and specific heat manifest a Pauli paramagnetic, metallic behaviour for ThNi12B6 without any anomalies, in close match with the isotypic homologue LaNi12B6. Static and dynamic hardness data show rather high values; Young's modulus is in the range of 240 GPa. The Debye temperature, θD = 490 K, gained via elastic constants, is slightly higher than the values extracted from specific heat or electrical resistivity data. A rather low coefficient of thermal expansion, α = 5.5 × 10-6 K-1, was derived from the temperature dependent length change.

Out of several methods, one of the most explored strategies to decrease the lattice thermal conductivity of Co4Sb12-based materials are either filling suitable electropositive elements into the voids or the formation of nanocomposites. These two approaches were combined in this work by filling Yb into the void of Co4Sb12 and preparing nanocomposites of Yb0.2Co4Sb12 and InSb according to the formula (InSb)x + Yb0.2Co4Sb12 (where x = 0.1, 0.2, 0.3, 0.4), via ball-milling and spark plasma sintering. Yb2O3 and CoSb2 as impurity phases were found at the grain boundaries. EBSD and TEM micrographs showed nanocrystalline InSb phase (20–200 nm) dispersed in the matrix grains. The charge transfer from Yb filler with an oxidation state of +3 to Co4Sb12 yielded a low electrical resistivity (ρ) of the matrix. An increase in ρ and Seebeck coefficient (S) in the composites with x = 0.1 and 0.3 occurred due to the higher amount of oxide impurities in these two samples and the scattering of charge carriers at the interfaces induced by the secondary phases. The other two composites with x = 0.2 and 0.4 exhibited ρ(T) and S(T) similar to the Yb0.2Co4Sb12 matrix. The dispersion of the InSb and Yb2O3 phases at the grain boundaries combined with the anharmonicity introduced by the fillers (Yb) in the voids enhanced the scattering of phonons within a broad wavelength range and reduced the lattice thermal conductivity significantly. Hence, a highest zT of ~1.2 at 773 K with a thermoelectric efficiency of 8.89% and 8.28% (423–773 K) were obtained for (InSb)0.1 + Yb0.2Co4Sb12 and (InSb)0.2 + Yb0.2Co4Sb12 nanocomposites, respectively.

Thermoelectric materials are actively considered for waste heat recovery applications. To improve the thermoelectric (TE) performance, nanostructuring to reduce their thermal conductivity has been inferred as an effective approach. Here, we present our investigations of the influence of ball milling and hot pressing process parameters on the chemical and thermoelectric properties of the type-I clathrate Ba 8 Cu 4.5 Si 6 Ge 35.5 . Powders with very small grain sizes could be obtained by ball milling. However, in contrast to other materials, the ultra-fine powders deteriorate the sintering behavior in the hot pressing process. We suggest that this is mainly due to an oxygen-induced local decomposition of the clathrate phase, which might start from the surface of nanoparticles. Partial decomposition also occurs in sintered samples of low bulk density when they are heated under low vacuum conditions. Overall this leads to the loss of Si/Ge and thus to an enhanced Cu content in the remaining clathrate(s), which deteriorates the TE performance in spite of the reduced thermal conductivity. Thus, keeping the composition and structure of the clathrate stable during the nanostructuring process is essential for reaching high TE performance.

Two boron rich phases were successfully synthesized by borothermal reduction of Yb oxide under vacuum. For the new boron-poorer phase, the single phase was established at around [B]/[Yb] = 43.3 at 1500 °C ( Pbam space group; YB 50 -type; a = 16.5811(5) A, b = 17.5950(5) A, c = 9.4647(3) A; powder X-ray diffraction; Rietveld refinement). The crystal structure of the boron-richer phase ([B]/[Yb] = 56.7) has been elucidated by single crystal X-ray diffraction ( F m 3 ¯ c space group; YB 66 -type; a = 23.3587(6) A). Powder X-ray diffraction data of the alloy YbB ~70 annealed at 1825 °C yielded, along with the YB 66 -type compound (a = 23.3691(2) A), β - rh B as a secondary phase ( R 3 ¯ m space group, a = 10.9298(3) A, c = 23.875(1) A), for which the solubility of Yb was found to be below 1 at%. The Yb atoms in β - rh B occupy the D and E voids. Both YbB 43.3 (YB 50 -type) and YbB 56.7 (YB 66 -type) feature complicated boron atom frameworks which exhibit shorter B-B separations both within and between boron clusters as compared to those observed for prototype structures.

Half-Heusler alloys, a non-centrosymmetric structure variant of the Heusler type, create a category of advanced thermoelectric materials. The thermal and phase stability of half-Heusler alloys TiFe1.33Sb and TixNb1-xFeSb (x = 0, 0.15), prepared by various techniques (hot pressing, ball milling or high-energy ball milling and hot pressing, as well as in one case with additional annealing), have been studied by means of differential thermal analysis and the Knudsen effusion method. The results from the measurement of phase transformations and evaporation of antimony, as the volatile element, supported by microstructure measurements and by diffusion profiles are presented and discussed in view of the long-term operation stability of the thermoelectric materials investigated. The alloys TiFe1.33Sb and TixNb1-xFeSb have all evidenced only a slight evaporation of antimony and have proven their long-term stability at temperatures well above the operation temperature of ~873 K.

With a high figure of merit, ZT, described in detail in the introduction, multi-filled CoSb3 skutterudites attract attention for their use as thermoelectric materials. In this work, the thermal and phase stability of n-type skutterudites, Sr0.07Ba0.07Yb0.07Co4Sb12, prepared by ball milling, hot pressing and high-pressure torsion or by combinations thereof, with ZT values of about 1.4, have been studied via differential thermal analysis and Knudsen effusion method. The results from evaporation of antimony, strontium, barium and ytterbium as the volatile elements and those from phase transformation measurements are presented. The information, supported by microstructure investigation and measurement of diffusion profiles, is summarized and the long-term operation stability of the studied bulk and nano-structured thermoelectrics is evaluated. Almost no strontium, barium and ytterbium evaporation and only a slight evaporation of antimony demonstrate a long-term operation stability of the bulk and nano-structured thermoelectrics investigated.

The constitution of the two phase diagrams Nb-Mn and Ta-Mn has been determined from light optical and transmission and scanning electron microscopy (LOM, TEM and SEM) with energy dispersive (EDX) as well as wavelength dispersive (WDX) X-ray spectroscopy, X-ray powder (XPD) and single crystal diffraction (XSCD), differential thermal analysis (DTA) and/or differential scanning calorimetry (DSC). The Laves phases NbMn2 and TaMn2 are the only binary compounds in these systems. High-temperature differential thermal analyses revealed congruent melting for NbMn2 with T,(NbMn2) = 1515 +/- 15 degrees C, whereas TaMn2 melts incongruently with T-m(TaMn2)= 1797 +/- 40 degrees C close to a depleted peritectic reaction. Both Laves phases engage in eutectic reactions l (Mn) + Nb(Ta)Mn-2 (T-eut = 1220 +/- 10 degrees C at 4.9 at% Nb and T-eut = 1234 +/- 10 degrees C at 0.7 at% Ta, respectively). NbMn2 also forms a eutectic with (Nb): l (Nb) + NbMn2 at T-eut = 1493 +/- 15 degrees C and 53.2 at% Nb. Mn shows remarkably large maximum solid solubilities of 19.4 at% Mn in (Nb) as well as of 21.3 at% Mn in (Ta). Detailed atom site distribution has been established for the Laves phases by means of temperature dependent X-ray single crystal data (both C14 - MgZn2-type). Combined data from XPD, EDX/WDX and SEM microstructure indicate that for both Laves phases extended homogeneity regions exist: Nb1+xMn2+x (62.5-73.0 at% Mn at 950 degrees C: -0.19

Two borides, ScRu2B3 and Sc2RuB6, were obtained by argon-arc melting of the elements followed by annealing at 800 °C. ScRu2B3 exhibits a new structure type with the space group Cmcm (a = 3.0195(2) A, b = 15.4056(8) A, c = 5.4492(3) A; single crystal X-ray data; RF2 = 0.0105). Sc2RuB6 adopts the Y2ReB6-type structure (space group Pbam; a = 8.8545(2) A, b = 11.1620(3) A, c = 3.4760(1) A; single crystal X-ray data; RF2 = 0.0185). ScRu2B3 displays an unusual intergrowth of CeCo3B2- and AlB2-related slabs; a striking feature is a boat configuration of puckered boron hexagons within infinite graphite like boron layers (63 nets). Sc2RuB6 presents two-dimensional planar nets of condensed boron pentagons, hexagons, and heptagons sandwiched between metal layers. In Sc/Y substituted Y2ReB6-type, Y atoms are distributed exclusively inside the boron heptagons. Exploration of the Sc–Ru–B system at 800 °C including binary boundaries employing EPMA and powder X-ray diffraction technique furthermore rules out the existenc...

A re-investigation of phase equilibria, crystal structure and homogeneity region of the Half-Heusler (HH) phase in the ternary system Nb-Fe-Sb at 600 °C has solved controversies in the literature confirming the version of Melnyk et al. ( [1] J. Phase Equilibria 20 (2) (1999) 113–118). For the first time transport properties of Half-Heusler (HH) compounds NbFeSb and Nb0.85M0.15FeSb (M = Ti, Zr, Hf) were studied in the full temperature range from 4.2 to 823 K. The semiconducting material NbFeSb has an electronic structure close to a metal-to-insulator transition, which leads to changes of the conductivity type with the composition as well as with increasing temperature. Ti, Zr and Hf doped NbFeSb alloys show metallic behavior and were confirmed to be high ZT p-type thermoelectric materials. Surprisingly, the lattice thermal conductivity for the Zr-doped composition was found to be higher than those of the Ti- and Hf doped materials; this effect can be explained in terms of mass and strain field fluctuations. For the first time we report experimental information on thermal expansion coefficients, specific heat and elastic moduli for these p-type compounds. The mechanical properties show a good compatibility with those previously reported for n-type HH alloys.

Nanostructured thermoelectric materials prepared by mechanical alloying and sintering have already shown enhanced values of the dimensionless thermoelectric figure-of-merit ( ZT ) for different families of materials. Oxide materials, however, are still far from reaching competitive values of thermoelectric efficiency. In this work we report results regarding a nanostructuring approach on thermoelectric transport properties of La 0.875 Sr 0.125 CoO 3 obtained by solid-state reaction combined with ball milling and sintering. This simple method allows us to obtain a set of samples with very different nanostructures, with grain sizes ranging from 19 nm to 0.7 μ m. Both, thermal and electrical properties, were found to be dependent on the grain size. The grain size reduction results in an enhancement of the electrical resistivity whereas the thermal conductivity is strongly reduced. Interestingly, the thermoelectric power of the sample with the smallest grain size increases significantly compared to the microcrystalline sample over the whole temperature range. As a result the thermoelectric figure of merit of the sample with the smallest grain size is enhanced by 30% compared to the microcrystalline sample above room temperature. This finding confirms the validity of the nanostructuring approach on thermoelectric oxides in order to improve their thermoelectric efficiency.

A novel ternary compound, UMo3B7, has been synthesized by arc melting and annealing at 900 °C. Its crystal structure was determined from X-ray single crystal diffraction data (YMo3B7-type structure, space group Pnma; a = 1.10310(8) nm, b = 0.30995(2) nm, c = 1.2792(1) nm, RF2 = 0.0205). The structure is composed of boron filled trigonal prisms as well as unfilled tetrahedra and tetragonal pyramids formed by metal atoms. With respect to boron atoms aggregation, it exhibits a well-developed two-dimensional boron network revealing infinite bands of edge linked boron hexagons. The relationship with the members of the structural series within the V-B system: VnBn+1 = (n-1)VB (CrB-type) + VB2 (AlB2-type) (n = 1, 2, 3, 5) is discussed. Specific heat, magnetic susceptibility and electrical resistivity measurements characterize UMo3B7 as a spin fluctuating system.

Half Heusler alloys are among the most promising materials for thermoelectric generators as they can be used in a wide temperature range and their starting materials are abundant and cheap, the latter as long as no hafnium is involved. For Sb-doped Ti 0.5 Zr 0.25 Hf 0.25 NiSn Sakurada and Shutoh in 2008 have published ZT max = 1.5 at 690 K, a value that hitherto was never reproduced independently. In this paper we successfully prepared Ti 0.5 Zr 0.25 Hf 0.25 NiSn with ZT max = 1.5, however, at higher temperature (825 K). As the main goal is to produce hafnium – free half Heusler alloys, we investigated the influence of niobium or vanadium dopants on Ti x Zr 1−x NiSn 0.98 Sb 0.02 , reaching ZTs > 1.2 and thermal-electric conversion efficiencies up to 13.1%. For Hf-free n-type TiNiSn-based half Heusler alloys these values are unsurpassed. In order to further improve our thermoelectric materials our study is completed by electrical resistivity and thermal conductivity data in the low temperature range but also by mechanical properties (elastic moduli, hardness) at room temperature. The electrical properties have been discussed in comparison with DFT calculations.

The current review summarizes and discusses the latest research activities (after 2014) on p- and n-type skutterudite thermoelectrics.

A new member of the series of Th7 Fe3 -type derivative structures, h-(Pd0.86 Cu0.14 )7 B3 (≡Pd6.02 Cu0.98 B3 , unique structure type Pd6 CuB3 , space group P63 cm, a=12.9426(9) A, c=4.8697(4) A, single-crystal X-ray diffraction (XRD) data) was obtained from as cast alloys and alloys annealed at 600-650 °C. Further substitution of Cu by Pd led to formation of a Mn7 C3 -type structure, o-(Pd0.93 Cu0.07 )7 B3 (≡Pd6.51 Cu0.49 B3 , space group Pnma, a=4.8971(2) A, b=7.5353(3) A, c=12.9743(6) A, single-crystal XRD). Isotypic LT h-(Pt0.70 Cu0.30 )7 B3 (≡Pt4.90 Cu2.10 B3 ) was observed in the Pt-Cu-B system as a low-temperature (LT) phase (T≤600 °C) (powder XRD), whereas the Th7 Fe3 -type (high-temperature (HT) h-(Pt0.73 Cu0.27 )7 B3 ≡Pt5.11 Cu1.89 B3 , space group P63 mc, a=7.4671(1) A, c=4.9039(1) A, powder XRD) proved to be stable at high temperature. The three structures are built of columns of face connected metal octahedra and columns of metal tetrahedra alternatingly fused by common faces and vertices. Boron atoms are found in trigonal prisms formed by metal atoms. The volumes of the three new Th7 Fe3 -type derivative borides relate as 1:2:3. Superconductivity was discovered for Pt4.9 Cu2.1 B3 (Pd6 CuB3 -type) and Pt5.1 Cu1.9 B3 (Th7 Fe3 -type) below 0.67 and 0.66 K, respectively. Despite the close value of the transition temperature the values of the upper critical field at 0 K differ as 0.37 T and 0.27 T for the two compounds.

Skutterudites are known as excellent thermoelectric p- and n-type materials and have already achieved good efficiencies for the conversion of heat to electricity, but nevertheless researchers try to further enhance the figure of merit, ZT. In the present work the aim was to mix p- and n-type skutterudite powders with additives in order to produce an evenly dispersed distribution of sub micron grains, preferable small nano-particles, which enhance the scattering of the heat carrying phonons of different wavelengths and reduce thermal conductivity without changing electrical resistivity and Seebeck coefficient. Various quantities of three groups of materials (a) nonmetallic oxides (Al 2 O 3 ), (b) metallic oxides (Cu 2 O and La 1.85 Sr 0.15 CuO 4 ) and (c) metallic borides (Fe 2.25 Co 0.75 B and Ta 0.8 Zr 0.2 B) were added to industrially produced p-type (DD y Fe 3 CoSb 12 ) and n-type ((Mm,Sm) y Co 4 Sb 12 ) skutterudite powders. First the influence of pre-sieving and various ball milling conditions before hot pressing were studied, using Al 2 O 3 as additive. As a consequence of these studies pre-sieved powders and high-energy ball milling were used for all following experiments. The goal, to enhance ZT was not reached with Al 2 O 3 and Cu 2 O. La 1.85 Sr 0.15 CuO 4 was successful for the n-type, Fe 2.25 Co 0.75 B for the p-type skutterudites, although ZT-enhancement was small, but with Fe 2.25 Co 0.75 B the maximum ZT could be shifted to lower temperatures, a valuable information for device production. Much better results in respect to ZT values were gained with adding 0.5, 1.0 and 1.5 wt.% Ta 0.8 Zr 0.2 B to p-type DD y Fe 3 CoSb 12 . In this series it was possible to enhance ZT (from ZT ∼ 1.2 to ZT ∼ 1.3) as well as to significantly increase the thermal-electrical conversion efficiency η. In addition, we found that all boride additives enhanced the hardness, elastic moduli and fracture resistance.

The crystal structure of Yb 2 PdGe 6 was studied by powder X-ray diffraction technique. The compound in both as-cast and annealed forms crystallizes with the orthorhombic structure of the Ce 2 GaGe 6 -type (space group Cmce ) that can be modelled as a sequence of AlB 2 -type slabs, quadrangular antiprismatic slabs, and distorted αPo-type slabs propagating along the crystallographic c direction. The low-temperature physical properties of Yb 2 PdGe 6 were determined by means of magnetic susceptibility and electrical resistivity measurements. The compound was characterized as an intermediate valent material because of a highly unstable 4 f shell of Yb ions.

The temperature and phase stability of p-type skutterudites, DD