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Phase separation by the spinodal decomposition or nucleation and growth mechanisms is an established method for the generation of thermodynamically stable sub-micron features, capable of both reducing the lattice thermal conductivity, κl, and stabilizing its value, while obtaining high and stable thermoelectric (TE) figure of merit ZT values during practical applications. In the Mg2(Si,Sn,Ge) class of TE materials, a miscibility gap and a thermodynamic tendency of phase separation were reported in the Mg2Si–Mg2Sn quasi-binary section of the ternary phase diagram, capable of enhancing and stabilizing the TE performance, by κl minimization. Yet, no such tendency was ever reported for the Mg2Si–Mg2Ge quasi-binary system, prohibiting the fulfillment of its TE potential. It is currently shown that a similar (yet, less pronounced) tendency of phase separation is also apparent in the Mg2Si–Mg2Ge quasi-binary system, into Mg2Si- and Mg2Ge-rich Mg2Si0.5±δGe0.5±δ phases. This phenomenon is enhanced upon bismuth doping. Upon 1.5, 2, and 2.5% bismuth doping of Mg2Si0.5Ge0.5, following induction melting and hot-pressing, the solubility limit of Bi was found as 1.5–2%, while increasing the bismuth content resulted in significant Mg3Bi2 segregation into grain boundaries. The combined phase separation and segregation effects on κl reduction with the electronic effect of Bi doping resulted in a reasonably high maximal ZT of 0.9, which was observed upon 2.5% Bi doping. [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]

In an attempt to reduce our reliance on fossil fuels, associated with severe environmental effects, the current research is focused on the identification of the thermoelectric potential of p-type (GeTe)1-x(Bi2Te3)x alloys, with x values of up to 20%. Higher solubility limit of Bi2Te3 in GeTe, than previously reported, was identified around ∼9%, extending the doping potential of GeTe by the Bi2Te3 donor dopant, for an effective compensation of the high inherent hole concentration of GeTe toward thermoelectrically optimal values. Around the solubility limit of 9%, an electronic optimization resulted in an impressive maximal thermoelectric figure of merit, ZT, of ∼1.55 at ∼410° C, which is one of the highest ever reported for any p-type GeTe-rich alloys. Beyond the solubility limit, a Fermi Level Pinning effect of stabilizing the Seebeck coefficient was observed in the x=12%-17% range, leading to stabilization of the maximal ZTs over an extended temperature range; an effect that was associated with the potential of the governed highly symmetric Ge8Bi2Te11 and Ge4Bi2Te7 phases to create high valence band degeneracy with several bands and multiple hole pockets on the Fermi surface. At this compositional range, co-doping with additional dopants, creating shallow impurity levels (in contrast to the deep lying level created by Bi2Te3), was suggested for further electronic optimization of the thermoelectric properties. [ABSTRACT FROM AUTHOR]

An experimental demonstration of the combined photovoltaic (PV) and thermoelectric conversion of concentrated sunlight (with concentration factor, X, up to ~300) into electricity is presented. The hybrid system is based on a multi-junction PV cell and a thermoelectric generator (TEG). The latter increases the electric power of the system and dissipates some of the excessive heat. For X≤200, the system's maximal efficiency, ~32%, was mostly due to the contribution from the PV cell. With increasing X and system temperature, the PV cell's efficiency decreased while that of the TEG increased. Accordingly, the direct electrical contribution of the TEG started to dominate in the total system power, reaching ~20% at X ≈ 290. Using a simple steady state finite element modeling, the cooling effect of the TEG on the hybrid system's efficiency was proved to be even more significant than its direct electrical contribution for high solar concentrations. As a result, the total efficiency contribution of the TEG reached ~40% at X ≈ 200. This suggests a new system optimization concept that takes into account the PV cell's temperature dependence and the tradeoff between the direct electrical generation and cooling capabilities of the TEG. It is shown that the hybrid system has a real potential to exceed 50% total efficiency by using more advanced PV cells and TE materials. [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]

A study was conducted to understand the correlation between measured thermoelectric power (TEP) for the influence of the metallurgical states of solid solutions and the microstructure of two-phase Sn/Te alloys. The TEP values were fitted to a coupled model based on the Bergman-Levy and the general effective medium equations and was observed to confirm to the lower bound region and correspond to a parallel addition configuration.

The influence of Al on the thermoelectric properties of the half-Heusler (HH) TiNiSn compound is reported. The research combined ab initio Density Functional Theory (DFT) calculations with experimental microstructure evaluation and measurements of the transport properties up to 750 K. It is shown that Al addition to the Ti sub-lattice results in an increase of the absolute value of the Seebeck coefficient and electrical resistivity in polycrystalline TiNiSn, while preserving the n-type behavior of the ternary compound, in addition to a significant reduction of the thermal conductivity. In (Ti0.99Al0.01)NiSn, upon 1% Al substitution of Ti, an improvement of 17% in the thermoelectric figure of merit (0.42 at 723 K) compared to pure TiNiSn was observed. Theoretical lattice thermal conductivity calculations are applied to shed light on the different scattering mechanisms in this class of materials. It is shown that the major contribution to the lattice thermal conductivity reduction is stimulated by the presence of Sn-rich inclusions, in addition to an influence of mass fluctuation scattering due to substitution of Ti by Al. Although it is shown that in the widely applied polycrystalline TiNiSn, an addition of the acceptor Al dopant could not fully compensate n-type electronic active defects (e.g. grain boundaries) for obtaining p-type materials, the currently reported results pave a route for thermoelectric optimization of MNiSn (M = Ti, Ni, Sn) n-type half-Heusler compounds. [ABSTRACT FROM AUTHOR]

Half-Heusler, HH, alloys are widely used n-type materials in thermoelectric applications. Today, there is a shortage in p-type HH based materials, which may have an inherent compatibility with the HH n-type pair. Al is a good candidate as an acceptor doping element for this purpose, and the results on alloying of TiNiSn-based HH with Al are reported in details. Combination of CALPHAD and ab initio DFT calculations with an experimental validation was carried out. It is demonstrated that low level Al doping leads to p-type conductivity of the material. The solubility of Al was predicted by calculations and experimentally confirmed. The stable phases with compositions above the solubility limit of Al were determined, including an assessment of the maximal Al solubility in the HH (Ti1−cAlc)NiSn phase up to 1400 K. In addition, a Scheil solidification simulation with the known TiNiSn CALPHAD database was used, in order to further understand the as-cast phase structure. It is shown that beyond the solubility limit the stable phases at room temperature are TiNiSn(HH), Sn(BCT) and NiAl(B2). The maximal solubility of Al in the HH phase is estimated as ∼1 at% at 1400 K. These results give the basic route for designing of (Ti1−cAlc)NiSn as a p-type thermoelectric element. [ABSTRACT FROM AUTHOR]