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Thermoelectrics enabling a direct conversion of waste heat into useful electricity is widely investigated for renewable energy applications. n-type half-Heusler (HH) MNiSn (M = Ti,Zr,Hf) thermoelectric (TE) elements are known as attractive semiconducting candidates for such purposes. Yet, both electronic and phonon scattering optimization are still required for fulfilling their full potential. In the current research, Ti0.3Zr0.35Hf0.35Ni1.005Sn separating into a main Ti0.3Zr0.35Hf0.35NiSn HH matrix and a minority full-Heusler (FH) Ti0.3Zr0.35Hf0.35Ni2Sn phase is reported. Adverse electronic effects of the metallic FH phase are nearly avoided by its small relative amount and dimension, while maintaining nearly optimal electronic TE performance along with large phonon scattering, minimizing the lattice thermal conductivity. Consequently, a very high maximal TE figure of merit, ZT, of ∼1.04 is obtained, which is among the highest ever reported for n-type MNiSn HH compounds. [ABSTRACT FROM AUTHOR]

In an attempt to reduce the reliance on fossil fuels, associated with severe environmental effects, the current research is focused on the identification of the thermoelectric potential of n-type Pb1-xTixTe alloys, with x values of up to 3%. A solubility limit of 0.5 at. % Ti in PbTe was identified, while beyond this composition, a precipitation of a TiTe2 phase was occurred. An impressive maximal dimensionless thermoelectric figure of merit ZT of ~1.2 was obtained upon 0.1% Ti doping at 500 °C, indicating a ~9% efficiency enhancement compared to an undoped PbTe. It is shown that generating a functionally graded material based on undoped PbTe as a low temperature segment and a 0.1% Ti doped PbTe as a high temperature segment has a potential to enhance the efficiency by ~14% compared to the undoped sample. [ABSTRACT FROM AUTHOR]

The search for alternative energy sources is nowadays at the forefront of applied research. In this context, thermoelectricity for direct energy conversion from thermal to electrical energy plays an important role, in particular, for the exploitation of waste heat [G. J. Snyder and E. S. Toberer, Nat. Mater. 7, 105 (2008); M. S. Dresselhaus et al., Adv. Mater. (Weinheim, Ger.) 19, 1043 (2007)]. Materials for such applications should exhibit thermoelectric potential as well as mechanical stability. PbTe based alloys have been considered for many years as state of the art thermoelectric materials for mid-temperature power generation (500–900 K), with efficiency values that are still being improved by both alloying [P. F. P. Poudeu et al., Chem., Int. Ed. 45, 1 (2006); J. R. Sootsman et al., Chem. Mater. 18, 4993 (2006); P. F. P. Poudeu et al., Chem. Soc. 126, 14347 (2006); J. Androulakis et al., Adv. Mater. (Weinheim, Ger.) 18, 1170 (2006); K. F. Hsu et al., Science 303, 818 (2004)] and doping [Y. Gelbstein et al., Physica B (Amsterdam) 363, 196 (2005); Y. Gelbstein et al., Physica B (Amsterdam) 396, 16 (2007)] optimizations. However, the mechanical properties of PbTe based materials are highly dependent on the conductivity type (n or p) and carrier concentrations [Y. Gelbstein et al., Scr. Mater. 58, 251 (2008)]. This paper puts forward the mechanical durability of thermoelectric materials and, in particular, of PbTe as a dominant factor that is nondetachable from the transport properties, which should be considered in the search for high quality thermoelectric materials. Here we discuss the microhardness enhancement of p-type PbTe alloys with hole concentrations higher than 5×1018 cm-3. This anomaly is obtained while all the other investigated n-type (up to 1020 cm-3) and p-type (up to 1018 cm-3) compositions maintained a constant microhardness value of ∼30 HV. The origin of this microhardness enhancement is not yet understood on a fundamental level, however two possible mechanisms are discussed. One deals with the elastic interaction between dislocations and impurities with higher covalent radius than the sublattice vacancy. The other is correlated with the existence of a second valence band of heavy holes in PbTe, which begins to fill up at the same concentration where a hardness enhancement was observed. These mechanisms correlating between mechanical end electronic properties of PbTe based alloys can serve as guidelines for the search for potential candidates, obtaining both thermoelectric potential and mechanical stability for thermoelectric applications. [ABSTRACT FROM AUTHOR]

In an attempt to enhance the thermal to electrical conversion efficiency, novel complicated thermoelectric alloys are constantly reported. Most of these reports, correlate the low lattice thermal conductivity values, attributing to the enhanced efficiencies, to nanofeatures apparent in their systems. Yet, since most of the highly efficient thermoelectric materials ever reported are based on complicated alloys, a major reduction of the lattice thermal conductivity can be solely attributed to alloying/disordering effects. The current manuscript, explores by combined experimental and theoretical, using density functional theory and analytical modeling, approaches the lattice thermal conductivity values originated solely by alloying/disordering effects in the highly thermoelectrically efficient p-type Ge xPb1- xTe alloys. By comparing these calculated results to various reported experimental values following different synthesis routes, it is shown that solution-treated samples fit well to the calculated values while for phase-separated samples, a significant lattice thermal conductivity reduction of ∼50% might be expected. [ABSTRACT FROM AUTHOR]

Since the 1960s, the TAGS system, namely (GeTe)(AgSbTe), with two specific compositions x = 0.8 and 0.85, known as TAGS-80 and TAGS-85, respectively, was identified as containing highly efficient p-type thermoelectric materials. Recently, another highly efficient p-type GeTe-rich composition, namely 3% BiTe-doped GePbTe, achieving thermoelectric properties comparable to TAGS-based solid solutions, was also reported. Since all of these compositions were obtained by different manufacturing approaches, a comparison between the transport and mechanical properties of these alloys, prepared by the same manufacturing techniques, is required to identify the advantages and disadvantages of these compositions for practical thermoelectric applications. In the current research, the thermoelectric and mechanical properties of three highly efficient GeTe-rich alloys, TAGS-80, TAGS-85, and 3% BiTe-doped GePbTe, following hot pressing, were investigated and compared. Maximal ZT values of ∼1.75, ∼1.4, and ∼1.6 at 500°C were found for these compositions, respectively. Improvement of the mechanical properties was observed by increasing the GeTe content. The influence of the GeTe relative amount on the transport and mechanical properties was interpreted by means of the phase-transition temperatures from the low-temperature rhombohedral to the high-temperature cubic phases. [ABSTRACT FROM AUTHOR]

Half-Heusler (HH) and especially TiNiSn-based alloys have shown high potential as thermoelectric (TE) materials for power generation applications. The reported transport properties show, however, a significant spread of results, due mainly to the difficulty in fabricating single-phase HH samples in these multicomponent and multiphased systems. In particular, little attention has been paid to the influence of the various minority phases on the TE performance of these compounds. A clear understanding of these issues is mandatory for the design of improved and stable TE HH-based composites. This study examines the structural and compositional influence of the residual metallic (Sn) and intermetallic phases (mainly Ti6Sn5 and the Heusler compound TiNi2Sn) on the TE properties of the TiNiSn HH compounds processed by spark plasma sintering. [ABSTRACT FROM AUTHOR]

GeTe-based alloys have been considered as p -type thermoelectric materials for decades. Yet the fact that they possess intrinsically high concentration of cation germanium vacancies have triggered the thermoelectric community in finding alternatives and strategies to optimize their efficiency. While most of the published studies mainly discuss the stoichiometric alloy, here we demonstrate the introduction of excess germanium/tellurium to the system. The nonstoichiometric alloy is doped with bismuth, acting as a donor simultaneously with an increased number of vacancies, enabling higher solubility of other impurities (iodine/indium). This strategy and the mechanisms involved, has been investigated and correlated to the microstructural characteristics. ZT of ~1.6 was obtained but more importantly, it opens the opportunity for a potential route for further thermoelectric enhancement. • GeTe -based alloys shows ZT enhancement throughout all temperature range, reaching a 60% improvement compared to pure GeTe. • Achieved by electronic optimization using doping nonstoichiometric Ge x Te 1-x with both BiTe and cationic/anionic impurities. • General effective media evaluation enables the assessment of other routes for further transport properties enhancement. [ABSTRACT FROM AUTHOR]

The MNiSn (M = Ti; Zr; Hf); half-Heusler semiconducting alloys have a high potential for use as n-type thermoelectric materials at elevated temperatures (~1000 K). The alloys' durability is crucial for their commercial handling and use, and therefore it is required to characterize their surface oxidation behavior and stability at the working temperature. X-ray photoelectron spectroscopy was utilized to study the surface composition and oxidation of the ZrNiSn alloy at room and elevated temperatures. It was found that during heating in a vacuum, Sn segregates to the surface in order to reduce the surface energy. Exposing the alloy to oxygen resulted mainly in the oxidation of the zirconium to ZrO2, as well as some minor oxidation of Sn. At room temperature, the oxidation to ZrO2 was accompanied by the formation of a thin ZrO layer at the metal-oxide interface. In contrast to TiNiSn, where most of the oxide was formed on the surface due to oxygen-enhanced segregation of Ti, and in the case of ZrNiSn, the formed oxide layer was thinner. Part of the oxide is formed due to Zr segregation to the surface, and in part due to oxygen dissolved into the alloy. [ABSTRACT FROM AUTHOR]

• half-Heusler (HH) High entropy alloy (HEA) MNiSn (M=Ti,Zr,Hf) with addition of Al and Sc in the M sub-lattice was studied. • The thermoelectric figure of merit showed an improvement of ~40% in comparison with (Ti 0.3 Zr 0.35 Hf 0.35)NiSn. • Ab-initio DFT calculations are applied to calculate the binary interaction parameters in the M sub-lattice. • A new HEA parameter for multi-component alloying in only one sub-lattice of a compound is proposed. [Display omitted] Half-Heusler (HH) High entropy alloy (HEA) MNiSn (M=Ti,Zr,Hf) with addition of Al and Sc in the M sub-lattice was studied. The micro-structure of (Ti 0.33 Zr 0.33 Hf 0.33 Al 0.005 Sc 0.005)NiSn compound was examined and its transport properties were measured. The measurements showed the improvement of ~40% in the thermoelectric figure of merit in comparison with a similar composition of (Ti 0.3 Zr 0.35 Hf 0.35)NiSn without Al and Sc. The thermodynamic stability of the alloy was examined using the HEA methodology, including a new proposed HEA parameter for multi-component alloying in only one sub-lattice of the compound. Ab-initio density functional theory (DFT) calculations were carried out to obtain the binary interaction parameters between the components in the sub-lattice of M atoms. The thermodynamic stability of the alloy as a function of the atomic fraction of the elements occupying the same sub-lattice in the compound is analyzed using the new HEA parameter. [ABSTRACT FROM AUTHOR]