Igor Veremchuk

ZT enhancement in solution-grown Sb(2-x)BixTe3 nanoplatelets.

We report a solution-processed, ligand-supported synthesis of 15-20 nm thick Sb(2-x)BixTe3 nanoplatelets. After complete ligand removal by a facile NH3-based etching procedure, the platelets are spark plasma sintered to a p-type nanostructured bulk material with preserved crystal grain sizes. Due to this nanostructure, the total thermal conductivity is reduced by 60% in combination with a reduction in electric conductivity of as low as 20% as compared to the bulk material demonstrating the feasibility of the phonon-glass electron-crystal concept. An enhancement in the dimensionless thermoelectric figure of merit of up to 15% over state-of-the-art bulk materials is achieved, meanwhile, shifting the maximum to significantly higher temperatures.

Thermoelectric Properties of Lead Chalcogenide Core–Shell Nanostructures

We present the full thermoelectric characterization of nanostructured bulk PbTe and PbTe-PbSe samples fabricated from colloidal core-shell nanoparticles followed by spark plasma sintering. An unusually large thermopower is found in both materials, and the possibility of energy filtering as opposed to grain boundary scattering as an explanation is discussed. A decreased Debye temperature and an increased molar specific heat are in accordance with recent predictions for nanostructured materials. On the basis of these results we propose suitable core-shell material combinations for future thermoelectric materials of large electric conductivities in combination with an increased thermopower by energy filtering.

Facile General Route toward Tunable Magneli Nanostructures and Their Use As Thermoelectric Metal Oxide/Carbon Nanocomposites

Engineering nanoscale interfaces is a requisite for harnessing electrical and thermal transports within nanostructured materials, especially those destined for thermoelectric applications requiring an unusual combination of low thermal conductivity and electrical resistivity. Nanocomposites open up possibilities in this area, but are still bound to a very narrow range of materials. Here, we report a new approach combining the sol-gel process toward hybrid materials with spark plasma sintering (SPS) to yield functional nanocomposites based on substoichiometric titanium oxides Ti(n)O(2n-1), so-called Magnéli phases. The potential of this new approach is demonstrated by three results. First, multiple Ti(n)O(2n-1) compounds (n = 3, 4, 5, 6, 8) are obtained for the first time as sole nano-Magnéli crystalline phases with controlled specific surface areas from 55 to 300 m(2)·g(-1), classified as potential thermoelectric n-type metal oxides and paving the way toward advanced systems for energy-harvesting devices and optoelectronics. Second, this work combines the use of sol-gel and SPS processes to yield percolated nanocomposites based on metal oxide nanoparticles embedded in a carbon matrix with low electrical resistivity (2 × 10(-4) Ω·m for a Ti(4)O(7) compound) and reduced thermal conductivity (1 W·m(-1)·K(-1)) with respect to bulk phases. Finally, the discovered materials are reliable with thermoelectric figures of merit (ZT = 0.08) relatively high for n-type Ti-O-based systems and metal oxides. Thereby this study represents a proof of concept for the development of promising, cheaper, and more efficient thermoelectric conversion devices.

Cs8−xSi46: A Type‐I Clathrate with Expanded Silicon Framework

The synthesis of the new binary Cs(8-x)Si(46) (shown here) completes the series of binary alkali metal silicides with a clathrate-I structure M(8-x)Si(46) (M = Na, K, Rb, Cs). In contrast with the lighter homologues, Cs(8-x)Si(46) can be prepared only at elevated pressures. The compound was obtained at 1200 degrees C between 2-10 GPa and the Cs content rises with applied pressure.

Diffusion-Controlled Formation of Ti2O3 during Spark-Plasma Synthesis

The spark-plasma-sintering (SPS) technique has successfully been applied for the single-step direct synthesis of Ti2O3 from a mixture of powders of rutile/anatase with titanium. The components react by diffusion through the grain boundaries, forming several intermediate phases locally. A single-phase material of titanium(III) oxide is obtained in compact bulk form after 180 min of SPS treatment at 1473 K. The electrical and thermal transport properties of such a SPS-prepared material measured in the temperature range between 300 and 800 K reflect the known semiconductor-to-metal transition above 400 K. The observed metallic-like electrical and thermal conductivity above this temperature is in good agreement with previously reported results. A maximum of the thermoelectric figure-of-merit ZT = 0.04 is achieved at 350 K.

Structural evolvement and thermoelectric properties of Cu3−xSnxSe3 compounds with diamond-like crystal structures

Polycrystalline samples of Cu(3-x)Sn(x)Se3 were synthesized in the composition range x = 0.87-1.05. A compositionally induced evolvement from tetragonal via cubic to monoclinic crystal structures is observed, when the composition changes from a Cu-rich to a Sn-rich one. The Cu(3-x)Sn(x)Se3 materials show a metal-to-semiconductor transition with increasing x. Electronic transport properties are governed by the charge-carrier concentration which is well described by a linear dispersion-band model. The lattice component of the thermal conductivity is practically independent of x which is attributed to the opposite influence of the atomic ordering and the inhomogeneous distribution of the Cu-Se or Sn-Se bonds with different polarities in the crystal structure. The highest thermoelectric figure of merit ZT of 0.34 is achieved for x = 1.025 at 700 K.

Synthesis, Structure, and Properties of Two Zintl Phases around the Composition SrLiAs

Two atomic arrangements were found near the equiatomic composition in the strontium-lithium-arsenic system. Orthorhombic o-SrLiAs was synthesized by reaction of elemental components at 950 °C, followed by annealing at 800 °C and subsequent quenching in water. The hexagonal modification h-SrLi(1-x)As was obtained from annealing of o-SrLiAs at 550 °C in dynamic vacuum. The structures of both phases were determined by single-crystal X-ray diffraction: o-SrLiAs, structure type TiNiSi, space group Pnma, Pearson symbol oP12, a = 7.6458(2) Å, b = 4.5158(1) Å, c = 8.0403(3) Å, V = 277.61(2) Å(3), R(F) = 0.028 for 558 reflections; h-SrLi(1-x)As, structure type ZrBeSi, space group P6(3)/mmc, Pearson symbol hP6, a = 4.49277(9) Å, c = 8.0970(3) Å, V = 141.54(1) Å(3), RF = 0.026 for 113 reflections. The analysis of the electron density within the framework of the quantum theory of atoms in molecules revealed a charge transfer according to the Sr(1.3+)Li(0.8+)As(2.1-), in agreement with the electronegativities of the individual elements. The electron localizability indicator distribution indicated the formation of a 3D anionic framework [LiAs] in o-SrLiAs and a rather 2D anionic framework [LiAs] in h-SrLi(1-x)As. Magnetic susceptibility measurements point to a diamagnetic character of both phases, which verifies the calculated electronic density of states.

Homo- and Heterovalent Substitutions in the New Clathrates I Si30P16Te8–xSex and Si30+xP16–xTe8–xBrx: Synthesis, Crystal Structure, and Thermoelectric Properties

The new cationic clathrates I Si(30)P(16)Te(8-x)Se(x) and Si(30+x)P(16-x)Te(8-x)Br(x) were synthesized by the standard ampule technique. The Si(30)P(16)Te(8-x)Se(x) (x = 0-2.3) clathrates crystallize in the cubic space group Pm3̅n with the unit cell parameter a ranging from 9.9382(2) to 9.9696(1) Å. In the case of the Si(30+x)P(16-x)Te(8-x)Br(x) (x = 1-6.4) clathrates, the lattice parameter varies from 9.9720(8) to 10.0405(1) Å; at lower Si/P ratios (x = 1-3) the ordering of bromine atoms induces the splitting of the guest positions and causes the transformation from the space group Pm3n to Pm3. Irrespective of the structure peculiarities, the normal temperature motion of the guest atoms inside the oversized cages of the framework is observed. The title clathrates possess very low thermal expansion coefficients ranging from 6.6 × 10(-6) to 1.0 × 10(-5) K(-1) in the temperature range of 298-1100 K. The characteristic Debye temperature is about 490 K. Measurements of the electrical resistivity and thermopower showed typical behavior of p-type thermally activated semiconductors, whereas the temperature behavior of the thermal conductivity is glasslike and in general consistent with the PGEC concept. The highest value of the thermoelectric figure of merit (ZT = 0.1) was achieved for the Br-bearing clathrate Si(32.1(2))P(13.9(2))Te(6.6(2))Br(1.0(1)) at 750 K.

Thermoelectric properties of Eu- and Na-substituted SnTe

Abstract Thermoelectric properties of spark-plasma-sintering prepared bulk materials Eu x Na y ▪ z Sn 1– x – y – z Te above room temperature were investigated. The implementation of Eu and Na into the SnTe lattice was monitored by refinement of lattice parameters as well as energy dispersive X-ray spectroscopy (EDXS). The binary SnTe achieved the highest ZT value of 0.63 at 786 K, and showed a hole concentration of 4.6×10 19 cm −3 at 300 K. In comparison with pristine SnTe, the samples containing Eu had lower carrier mobility but higher Hall carrier concentration. For Eu- and Na-substituted samples, the increased hole concentration was unfavorable for the improvement of thermoelectric properties.

Thermoelectric properties of spark-plasma sintered nanoparticular FeSb2 prepared via a solution chemistry approach

Nanoparticular FeSb2 was prepared in solution from cyclopentadienyl iron(ii) dicarbonyl dimer [Fe(Cp(CO)2)]2 and antimony nanoparticles. Spark plasma sintering was used as consolidation method to maintain the particle size. The thermoelectric performance of FeSb2 is limited by its high thermal conductivity. In this work, the thermal conductivity was suppressed by nearly 80% compared to the bulk value by introducing grain boundary scattering of phonons on the nanoscale. The thermoelectric properties of the consolidated FeSb2 emphasize the possibility of altering thermal transport of promising thermoelectric compounds by phonon scattering by engineering the interfaces at the nanoscale.