The lattice dynamics in as‐cast and nanocrystalline thermoelectric Bi2Te3 based p‐type and n‐type material were investigated using inelastic neutron scattering. Generalized densities of phonon states show substantial agreement between the lattice dynamics in as‐cast samples and previous studies. The lattice dynamics in the nanocrystalline materials differ significantly from its as‐cast counterparts in the acoustic phonon regime. In nanocrystalline p‐type and n‐type compounds, the average acoustic phonon group velocity was found to be reduced to 80(5)% and 95(2)% of the value in as‐cast material. It is argued that point‐defect and strain contrast scattering may play an important role for the understanding of lattice thermal conductivity in (nanocrystalline) Bi2Te3 based thermoelectrics beside the observed decrease of sound velocity. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Bi2Te3 and CoSb3 based nanomaterials were synthesized and their thermoelectric, structural, and vibrational properties analyzed to assess and reduce ZT‐limiting mechanisms. The same preparation and/or characterization methods were applied in the different materials systems. Single‐crystalline, ternary p‐type Bi15Sb29Te56, and n‐type Bi38Te55Se7 nanowires with power factors comparable to nanostructured bulk materials were prepared by potential‐pulsed electrochemical deposition in a nanostructured Al2O3 matrix. p‐type Sb2Te3, n‐type Bi2Te3, and n‐type CoSb3 thin films were grown at room temperature using molecular beam epitaxy and were subsequently annealed at elevated temperatures. This yielded polycrystalline, single phase thin films with optimized charge carrier densities. In CoSb3 thin films the speed of sound could be reduced by filling the cage structure with Yb and alloying with Fe yielded p‐type material. Bi2(Te0.91Se0.09)3/SiC and (Bi0.26Sb0.74)2Te3/SiC nanocomposites with low thermal conductivities and ZT values larger than 1 were prepared by spark plasma sintering. Nanostructure, texture, chemical composition, as well as electronic and phononic excitations were investigated by X‐ray diffraction, nuclear resonance scattering, inelastic neutron scattering, Mössbauer spectroscopy, and transmission electron microscopy. For Bi2Te3 materials, ab‐initio calculations together with equilibrium and non‐equilibrium molecular dynamics simulations for point defects yielded their formation energies and their effect on lattice thermal conductivity, respectively. Current advances in thermoelectric Bi2Te3 and CoSb3 based nanomaterials are summarized. Advanced synthesis and characterization methods and theoretical modeling were combined to assess and reduce ZT‐limiting mechanisms in these materials.

Nanoparticles (NPs) of Ba8Ga16Ge30 clathrate‐I were synthetized via sol–gel‐calcination/chemical‐reduction route. A solution of the metallic cations was used to prepare an acryl‐amid‐based gel. The gel is dried and calcined to obtain nanocrystalline powders of precursor oxides. The oxides are reduced by reacting with CaH2 to produce the clathrate, which is embedded in a CaO matrix. CaO is removed by a washing step to obtain the clathrate NPs. The shape and size of the precursor oxide NPs can be modified by addition of complexing agents, surfactants or by varying the pH or the metal and surfactant concentration in the gel. Powder X‐ray diffraction and SAED patterns confirm the clathrate‐I‐type crystal structure of the products. SEM/TEM investigations show that the size and morphology of the oxides are retained in the clathrate NPs after the reduction. The clathrate NPs exhibit morphology of thin plates ∼300 × ∼300 nm2 and thickness of ∼50 nm, or sphere‐like morphology with ∼200 nm diameter, depending on the sol–gel synthesis conditions. The clathrate NPs were compacted via spark plasma sintering (SPS) to pellets with 53–93 % of crystallographic density. The total thermal conductivity (κ) of the pellet with 93 % density shows a reduction of 25 % in comparison to the reported κ in bulk clathrate. Preliminary characterization of the Seebeck (S) and electrical resistivity (R) of the low density sample (53 %) indicates n‐type conduction and semiconductor behavior of the Ba8Ga16Ge30 clathrate‐I. The transport properties of Ba8Ga16Ge30 clathrate‐I with 3‐, 4‐ or 5‐layer slabs and [100] surface termination as well as of the bulk material were calculated by using the semi‐classical Boltzmann transport theory within the constant scattering approximation. Our results show an increase in S for the geometries with reduced dimensions in agreement with the experimental observations.

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