Overcoming Asymmetric Contact Resistances in Al-Contacted Mg2(Si,Sn) Thermoelectric Legs.

Thermoelectric generators are a reliable and environmentally friendly source of electrical energy. A crucial step for their development is the maximization of their efficiency. The efficiency of a TEG is inversely related to its electrical contact resistance, which it is therefore essential to minimize. In this paper, we investigate the contacting of an Al electrode on Mg2(Si,Sn) thermoelectric material and find that samples can show highly asymmetric electrical contact resistivities on both sides of a leg (e.g., 10 µΩ·cm2 and 200 µΩ·cm2). Differential contacting experiments allow one to identify the oxide layer on the Al foil as well as the dicing of the pellets into legs are identified as the main origins of this behavior. In order to avoid any oxidation of the foil, a thin layer of Zn is sputtered after etching the Al surface; this method proves itself effective in keeping the contact resistivities of both interfaces equally low (<10 µΩ·cm2) after dicing. A slight gradient is observed in the n-type leg's Seebeck coefficient after the contacting with the Zn-coated electrode and the role of Zn in this change is confirmed by comparing the experimental results to hybrid-density functional calculations of Zn point defects.

High efficiency Mg2(Si,Sn)-based thermoelectric materials: scale-up synthesis, functional homogeneity, and thermal stability.

Considering the need for large quantities of high efficiency thermoelectric materials for industrial applications, a scalable synthesis method for high performance magnesium silicide based materials is proposed. The synthesis procedure consists of a melting step followed by high energy ball milling. All the materials synthesized via this method demonstrated not only high functional homogeneity but also high electrical conductivity and Seebeck coefficients of around 1000 Ω-1 cm-1 and -200 µV K-1 at 773 K, respectively. The measured values were similar for all the samples extracted from the ∅50 mm and ∅70 mm compacted pellets and were stable upon thermal cycling. Thermal stability experiments from 168 hours to 720 hours at 723 K revealed no significant change in the material properties. The low thermal conductivity of ∼2.5 W m-1 K-1 at 773 K led to a maximum figure of merit, zT max, of 1.3 at the same temperature and an average value, zT avg, of 0.9 between 300 K and 773 K, which enables high efficiency in future silicide-based thermoelectric generators.

Layered germanium tin antimony tellurides: element distribution, nanostructures and thermoelectric properties.

In the system Ge-Sn-Sb-Te, there is a complete solid solution series between GeSb2Te4 and SnSb2Te4. As Sn2Sb2Te5 does not exist, Sn can only partially replace Ge in Ge2Sb2Te5; samples with 75% or more Sn are not homogeneous. The joint refinement of high-resolution synchrotron data measured at the K-absorption edges of Sn, Sb and Te combined with data measured at off-edge wavelengths unambiguously yields the element distribution in 21R-Ge(0.6)Sn(0.4)Sb2Te4 and 9P-Ge(1.3)Sn(0.7)Sb2Te5. In both cases, Sb predominantly concentrates on the position near the van der Waals gaps between distorted rocksalt-type slabs whereas Ge prefers the position in the middle of the slabs. No significant antisite disorder is present. Comparable trends can be found in related compounds; they are due to the single-side coordination of the Te atoms at the van der Waals gap, which can be compensated more effectively by Sb(3+) due to its higher charge in comparison to Ge(2+). The structure model of 21R-Ge(0.6)Sn(0.4)Sb2Te4 was confirmed by high-resolution electron microscopy and electron diffraction. In contrast, electron diffraction patterns of 9P-Ge(1.3)Sn(0.7)Sb2Te5 reveal a significant extent of stacking disorder as evidenced by diffuse streaks along the stacking direction. The Seebeck coefficient is unaffected by the Sn substitution but the thermal conductivity drops by a factor of 2 which results in a thermoelectric figure of merit ZT = ~0.25 at 450 °C for both Ge(0.6)Sn(0.4)Sb2Te4 and Ge(1.3)Sn(0.7)Sb2Te5, which is higher than ~0.20 for unsubstituted stable layered Ge-Sb-Te compounds.

The solid solution series (GeTe)x(LiSbTe2)2 (1 ≤ x ≤ 11) and the thermoelectric properties of (GeTe)11(LiSbTe2)2.

Exchanging one Ge(2+) with two Li(+) per formula unit in (GeTe)n(Sb2Te3) (n = 1, 2, 3, ...) eliminates cation vacancies, because it leads to an equal number of cations and anions. This substitution results in the solid solution (GeTe)x(LiSbTe2)2 (with x = n - 1, but n not necessarily an integer). For x < 6, these stable compounds crystallize in a rock-salt-type structure with random cation disorder. Neutron data show that a small fraction of Ge occupies tetrahedral voids for x = 2 and 3. For x > 6, (GeTe)x(LiSbTe2)2 forms a GeTe-type structure that shows a phase transition to a cubic high-temperature phase at ca. 280 °C. The thermoelectric properties of (GeTe)11(LiSbTe2)2 have been investigated and show that this compound is a promising thermoelectric material with a ZT value of 1.0 at 450 °C. The high ZT value of the thermodynamically stable compound is caused by a low phononic contribution to the thermal conductivity; probably, Li acts as a "pseudo-vacancy".

Elemental distribution and thermoelectric properties of layered tellurides 39R-M(0.067)Sb(0.667)Te(0.266) (M=Ge, Sn).

The isostructural phases 39R-Ge(0.067)Sb(0.667)Te(0.266) (R3m, a=4.2649(1), c=75.061(2) Å) and 39R-Sn(0.067)Sb(0.667)Te(0.266) (R3m, a=4.2959(1), c=75.392(2) Å) were prepared by quenching stoichiometric melts of the pure elements and subsequent annealing at moderate temperatures. Their structures are comparable to "superlattices" synthesized by layer-by-layer deposition onto a substrate. These structures show no stacking disorder by electron microscopy. The structure of the metastable layered phases are similar to that of 39R-Sb(10)Te(3) (equivalent to Sb(0.769)Te(0.231)), which contains four A7 gray-arsenic-type layers of antimony alternating with Sb(2)Te(3) slabs. Joint refinements on single-crystal diffraction data using synchrotron radiation at several K edges were performed to enhance the scattering contrast. These refinements show that the elemental distributions at some atom positions are disordered whereas otherwise the structures are long-range ordered. The variation of the elemental concentration correlates with the variation in interatomic distance. Z-contrast scanning transmission electron microscopy (HAADF-STEM) on 39R-Ge(0.067)Sb(0.667)Te(0.266) confirms the presence of concentration gradients. The carrier-type of the isostructural metal (A7-type lamellae)-semiconductor heterostructures (Ge/Sn-doped Sb(2)Te(3) slabs) varies from n-type (Ge(0.067)Sb(0.667)Te(0.266)) to p-type (Sn(0.067)Sb(0.667)Te(0.266)). Although the absolute values of the Seebeck coefficient reached about 50-70 µV/K and the electrical conductivity is relatively high, the two isotypic phases exhibit a maximal thermoelectric figure of merit (ZT) of 0.06 at 400 °C as their thermal conductivity (κ≈8-9.5 W/mK at 400 °C) lies interestingly in between that of antimony and pure Sb(2)Te(3).

From metastable to stable modifications-in situ Laue diffraction investigation of diffusion processes during the phase transitions of (GeTe)nSb2Te3 (6 < n < 15) crystals.

Temperature dependent phase transitions of compounds (GeTe)(n)Sb(2)Te(3) (n = 6, 12, 15) have been investigated by in situ microfocus Laue diffraction. Diffusion processes involving cation defect ordering at ~300 °C lead to different nanostructures which are correlated to changes of the thermoelectric characteristics.

Novel solution route synthesis of low thermal conductivity nanocrystalline bismuth telluride.

A novel synthesis approach based on a solution route has been developed for the fabrication of nanocrystalline bismuth telluride. The method consists of dissolving both bismuth and tellurium into the same organic solvent with the assistance of complexing agents and one-step coprecipitation of bismuth telluride. The synthesized nanocrystalline bismuth telluride powders possess rhombohedral crystal structure and are nanosheet/nanorod-like with an average size of between 30 and 40 nm. The thermal conductivity of the hot-pressed compact consolidated from the as-synthesized nanopowders is 0.39-0.45 Wm(-1)K(-1) in the temperature range of 323 to 523 K, which is at most one third of that of bulk bismuth telluride-based materials reported in the literature. Such low thermal conductivity of the investigated bismuth telluride is mainly attributed to substantially high concentration of grain boundaries provided by nanostructuring to scatter phonons intensively.

Synthesis, crystal structure, atomic Hirshfeld surfaces, and physical properties of hexagonal CeMnNi4.

The hexagonal polymorph of CeMnNi(4) has been synthesized using cold crucible, high-frequency induction melting with subsequent Czochralski crystal pulling. Single-crystal X-ray diffraction, multitemperature synchrotron powder X-ray diffraction (90 to 600 K), and neutron powder diffraction data have been measured to establish the detailed crystal structure and in particular the location of the Mn atoms. The neutron diffraction data provide sufficient scattering contrast between Mn and Ni to establish that the 2c site has an occupancy of 13% Mn atoms, while the 3g site has an occupancy of 25% Mn atoms. Thus, the crystal structure is complex with considerable disorder. Rietveld refinement of the multitemperature synchrotron data establishes a near linear thermal expansion coefficient of 13.9(3) × 10(-6) K(-1) and 14.9(3) × 10(-6) K(-1) for the a and c axes, respectively. Atomic Hirshfeld surfaces are introduced as a new tool to investigate the atomic coordination and interactions in intermetallic compounds. The atomic displacement parameters (ADPs) are observed to be much larger for the heavy Ce atom than for the lighter Mn and Ni atoms, and this correlates with the large atomic Hirshfeld volume of Ce relative to Mn and Ni. The fit of a Debye model to the ADPs gives θ(D) = 312(3) K. Magnetic susceptibility data measured between 2 and 350 K indicate ferromagnetic ordering at 122(2) K (Weiss constant) based on a linear fit of the inverse magnetic susceptibility in the paramagnetic region. Transport properties were measured on a polycrystalline sample containing CeO(2) (2.8%) and Ni (7.7%) impurities. The electrical conductivity is observed to be metallic with a distinct kink in the data around 120 K coinciding with the observed Curie temperature. The lattice thermal conductivity (κ(L)) increases from 0.5 W/Km at 2 K to 8 W/Km at 50 K, and the relatively moderate value of κ(L) probably reflects the significant structural disorder.