The occurrence of a unique 3D nanoscale network in Ni–Cr–S, treatedviaspark-plasma sintering, was discovered with a variety ofex situandin situTEM and XRD techniques.

The crystallinity and the lattice dynamics in elemental modulated Sb2Te3 films are investigated microscopically using high energy synchrotron radiation diffraction combined with Sb-121 nuclear inelastic scattering. The correlation length is found to be finite but less than 100 A. The element specific density of phonon states is extracted. A comparison with the element specific density of phonon states in bulk Sb2Te3 confirms that the main features in the density of phonon states arise from the layered structure. The average speed of sound at 40 K, 1.74(2)km s(-1), is almost the same compared to bulk Sb2Te3 at 20 K. 1.78(2)km s(-1). Similarly, the change in the acoustic cut-off energy is within the experimental detection limit. Thus, we suggest that the lattice thermal conductivity in elemental modulated Sb2Te3 films should not be significantly changed from its bulk value. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Here we report for the first time on a complete simulation assisted "material to module" development of a high performance thermoelectric generator (TEG) based on the combination of a phase change material and established thermoelectrics yielding the compositions (1 - x)(GeTe) x(Bi(2)Se(0.2)Te(2.8)). For the generator design our approach for benchmarking thermoelectric materials is demonstrated which is not restricted to the determination of the intrinsically imprecise ZT value but includes the implementation of the material into a TEG. This approach is enabling a much more reliable benchmarking of thermoelectric materials for TEG application. Furthermore we analyzed the microstructure and performance close to in-operandi conditions for two different compositions in order to demonstrate the sensitivity of the material against processing and thermal cycling. For x = 0.038 the microstructure of the as-prepared material remains unchanged, consequently, excellent and stable thermoelectric performance as prerequisites for TEG production was obtained. For x = 0.063 we observed strain phenomena for the pristine state which are released by the formation of planar defects after thermal cycling. Consequently the thermoelectric performance degrades significantly. These findings highlight a complication for deriving the correlation of microstructure and properties of thermoelectric materials in general.

Thin films of Bi2Te3 and Sb2Te3 were synthesized by the nanoalloying approach, which has recently been proven to yield V–VI compounds with good thermoelectric properties and has several advantages over “conventional” growth on hot substrates. Firstly, repeating layers of the elements Bi, Sb and Te with a thickness in the range between 0.2 nm and 2.4 nm were deposited on BaF2 (111) substrates in an MBE system at room temperature with different deposition patterns for different samples, i.e. in bilayer and quintuple stacks, with different starting layer thicknesses and different Te contents. Subsequently, the element layer stacks were annealed in order to induce crystallization and compound formation of Bi2Te3 and Sb2Te3 thin films. The annealed thin films were characterized using X-ray diffractometry (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The transport properties, i.e. electrical conductivities, carrier concentrations, carrier mobilities, Seebeck coefficients and thermal conductivities, were determined at room temperature for several sets of starting layer thicknesses and deposition patterns depending on the Te content. The texture was found to be strongly influenced by the starting thicknesses of the elemental layers in the deposition pattern. Results of temperature dependent measurements of the Seebeck coefficient and electrical conductivity on one sample of nanoalloyed Bi2Te3 and Sb2Te3 together with results from temperature dependent in situ XRD investigations are presented.

Sb2Te3 and Bi2Te3 thin films were grown at room temperature on SiO2 and BaF2 substrates using molecular beam epitaxy. A layer-by-layer growth was achieved such that metallic layers of the elements with 0.2nm thickness were deposited. The layer structure in the as-deposited films was confirmed by X-ray diffraction and was seen more clearly in Sb2Te3 thin films. Subsequent annealing was done at 250°C for 2h and produced the Sb2Te3 and Bi2Te3 crystal structure as confirmed by high-energy X-ray diffraction. This preparation process is referred to as nano-alloying and it was demonstrated to yield single-phase thin films of these compounds. In the thin films a significant texture could be identified with the crystal c axis being almost parallel to the growth direction for Sb2Te3 and tilted by about 30° for Bi2Te3 thin films. In-plane transport properties were measured for the annealed films at room temperature. Both films yielded a charge carrier density of about 2.6×1019cm−3. The Sb2Te3 films were p-type, had a thermopower of +130μVK−1, and surprisingly high mobilities of 402cm2V−1s−1. The Bi2Te3 films were n-type, showed a thermopower of −153μVK−1, and yielded significantly smaller mobilities of 80cm2V−1s−1. The chemical composition and microstructure of the films were investigated by transmission electron microscopy (TEM) on cross sections of the thin films. The grain sizes were about 500nm for the Sb2Te3 and 250nm for the Bi2Te3 films. In the Bi2Te3 thin film, energy-filtered TEM allowed to image a Bi-rich grain boundary phase, several nanometers thick. This secondary phase explains the poor mobilities of the Bi2Te3 thin film. With these results the high potential of the nano-alloying deposition technique for growing films with a more complex layer architecture is demonstrated.

Samples in the antimony rich region of binary system SbxTe1-x were prepared as thin films using a molecular beam epitaxy (MBE) system. Layer thickness of the elements was varied for different samples within the range 10 to 33 angstrom to yield different compositions. Energy dispersive X-ray (EDX) spectroscopy was employed to determine the composition of the samples, of which the content of Te ranged from 40 at.% to 51 at.%. For all samples studied, the electrical conductivity, charge carrier mobility and concentration and Seebeck coefficient were measured at room temperature. A power factor (PF) of 20 mu W cm(-1)K(2) was observed at room temperature for the annealed film with Te content of 41.8 at.%. Crystalline phase formation and transitions were observed by in-situ X-ray diffraction on the as-deposited samples. Rietveld analysis of the XRD data combined with high resolution transmission electron microscopy (HRTEM) images on the annealed samples exhibit an intrinsically disordered polycrystalline structure due to the deviation in compositions from stoichiometric Sb2Te3. In the current studies, the physical properties and their correlation with micro-structure are also discussed.

The thermoelectric transport properties of Bi2Te3/Sb2Te3 superlattices are analyzed by means of first-principles calculations and semiclassical Boltzmann theory. The anisotropy of the thermoelectric transport under electron and hole doping was studied in detail for different superlattice periods at changing temperature and charge carrier concentrations. A clear preference for thermoelectric transport under hole doping, as well as for the in-plane transport direction was found for all superlattice periods. At hole doping the electrical transport anisotropies remain bulklike for all investigated systems, while under electron doping quantum confinement leads to strong suppression of the cross-plane thermoelectric transport at several superlattice periods. In addition, insights on the Lorenz function, the electronic contribution to the thermal conductivity, and the resulting figure of merit are given.

Sb2Te3 and Bi2Te3 thin films were grown at room temperature on SiO2 substrates using MBE and were subsequently annealed at 250 degrees C. The films were stoichiometric, polycrystalline, textured, and yielded strikingly low charge carrier densities of about 2.7 X 10(19) cm(-3). The in-plane transport properties were measured at room temperature, the thermopower was 130 mu VK-1 for Sb2Te3 and - 153 mu VK-1 for Bi2Te3 thin films. The small charge carrier densities are explained by a reduced antisite defect density due to the low temperatures to which the thin films were exposed during annealing. (c) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

V-VI thermoelectric compounds like Bi2Te3-based alloys are well known for room temperature applications like Peltier coolers or thermogenerators. Their anisotropic physical properties and mechanical weakness are a problem for the manufacturing. To overcome the mechanical problem Spark Plasma Sintering (SPS) was used. 2 inch Wafers of polycrystalline bismuth telluride based n-type and p-type thermoelectric materials were successfully fabricated through SPS-technique, starting from commercially available zonemelted ingots. This paper will report on the milling technique and the influence of the SPS process on the microstructure, the thermoelectric and mechanical properties of the polycrystalline materials. A preferentially oriented microstructure was formed keeping the high quality of the the starting material. Through the polycrystalline structure, the bending strength of the sintered material was increased about three times compared with coarse crystalline ingots.

Here we report on the first results of PbTe epitaxial films with high Bi content above doping concentrations grown on BaF2 in order to obtain nanoscale precipitates. The crystal structure is investigated by x-ray diffraction (XRD) measurements and no other phases than well-oriented PbTe could be found. These layers have been investigated by Hall-effect and Seebeck-effect measurements. The dependence of the Seebeck coefficient on the carrier concentration cannot be explained by simple Boltzmann statistics. Fourier-transform infrared (FTIR) transmission spectra also show some irregularities. These phenomena are interpreted as hints to nanoscale inclusions. The thermal conductivity is measured with the time-domain thermal reflectance method, developed by D. Cahill, to complete the thermoelectric characterizations.

We report on the temperature stability of the unusual conduction type of PbTe and PbSe epitaxial thin films alloyed with tin. Also the origin of the n-type conduction is investigated. In addition investigations have been done to determine the temperature dependent thermoelectric properties (electrical conductivity and thermopower) in the temperature range between 300 K and 600 K. The data derived from thin film material prove that beside the well known p-IV-VI-materials also n-type lead-tin-chalcogenides are suitable for thermoelectric application. To use this material in bulk devices it is necessary to able to transfer the properties to bulk samples. Thus the properties of similar bulk IV-VI lead-tin-chalcogenides were analyzed and compared to the thin film material.

Here we report on the growth mechanism, the structural and thermoelectric characterization of phase change materials as Ge 1 Sb 2 Te 4 , Ge 2 Sb 2 Te 4 in amorphous and crystalline states. In comparison the phase change behaviour from amorphous to crystalline state of the well known thermoelectric material (Bi,Sb) 2 Te 3 is investigated.

Over the past decade Fraunhofer-IPM implemented various measurement techniques applicable to thin films and bulk materials. Fraunhofer-IPM is now developing customer oriented measurement systems especially adapted for thermoelectric materials. They are combining accuracy, rapidity, little efforts for sample mounting, and are fully automated as far as the data acquisition is concerned. The following measurement techniques and their implementation at IPM will be presented: an integrated (acquisition and data interpretation) 3 omega-system for the measurement of the thermal conductivity and heat capacity, two push-button measuring-stations for the measurement of the electrical conductivity and the Seebeck coefficient at room temperature, and a new fully automated station for the measurement of the Seebeck coefficient at high temperatures. For the last mentioned measurement method, which will be discussed in detail, pressure contacts are used for contacting the thermocouples to the sample and the sample to the heat sink, making the measurement of both thin films and bulk materials possible, with a great flexibility on their geometrical dimensions. Finally, the validation of all methods is discussed in terms of suitability and availability of test-samples.

We report on the growth and the structural and thermoelectric characterisation of new superlattice (SL) structures (PbSe0.78Te0.22/PbSe0.76Te0.24 ) with periods between 1.5nm and 15nm. The structural (SEM-, EDX- and XRD-analysis) and in particular on thermoelectric properties (Seebeck coefficient and Hall-effect measurements for carrier concentration, conductivity and mobility) of molecular beam epitaxy grown thin films based on PbSe0.78Te0.22/PbSe0.76Te0.24 superlattice structures, grown without additional doping. These SL-structures were grown and analysed as the reduction of the thermal conductivity is of special interest in thermoelectric materials because of the possibility to achieve a high figure of merit. Special care was taken on the in-plane (Volklein method [Volklein, et al., 1987]) and on the cross-plane (3omega-method [Cahill, -]) measurements of thermal conductivity of thin films