G. Rogl

Concepts for medium-high to high temperature thermoelectric heat-to-electricity conversion: a review of selected materials and basic considerations of module design

Within the last decade, novel materials concepts and nanotechnology have resulted in a great increase of the conversion efficiency of thermoelectric materials. Despite this, a mass market for thermoelectric heat-to-electricity conversion is yet to be opened up. One reason for this is that the transfer of the lab records into fabrication techniques which enable thermoelectric generator modules is very challenging. By closing the gap between record lab values and modules, broad industrial applications may become feasible. In this review, we compare three classes of materials, all designed for medium-high to high temperature applications in the field of waste heat recovery: skutterudites, half-Heusler compounds, and silicon-based materials. Common to all three classes of thermoelectric materials is that they are built from elements which are neither scarce (e.g. tellurium) nor toxic (e.g. lead) and therefore may be the foundation of a sustainable technology. Further, these materials can provide both, n-type and p-type materials with similar performance and thermomechanical properties, such that the fabrication of thermoelectric generator modules has already been successfully demonstrated. The fabrication processes of the presented materials are scalable or have already been scaled up. The availability of thermoelectric materials is only one important aspect for the development of thermoelectric generator modules and heat conversion systems based on this technology. The design and configuration of the thermoelectric generator modules is similarly important. Hence, basic considerations of module configuration and different fundamental layouts of the thermoelectric heat-to-electricity conversion system are discussed within an additional chapter of this review.

Thermal expansion of thermoelectric type-I-clathrates

Thermal expansion was determined for two series of ternary compounds, Ba8MxGe46−x and Ba8MxSi46−x, with M=Cu, Zn, Pd, Ag, Cd, Pt, and Au and for several quaternary compounds for which we investigated the influence of substitution by Zn/Ni in Ba8ZnxGe46−x as well as the dependence of thermal expansion on the Si/Ge ratio in Ba8Cu5SixGe41−x. In the temperature range from 4.2 to 300 K the thermal expansion of all ternary compounds was measured with a capacitance dilatometer, whereas from 300 to 700 K for several selected samples a dynamic mechanical analyzer was employed. The low temperature data compare well with the lattice parameters of single crystals, gained from measurements at three different temperatures (100, 200, and 300 K). For a quantitative description of thermal expansion the semiclassical model of Mukherjee et al. [Phys. Rev. Lett. 76, 1876 (1996)] was used, which also provided reliable accurate values of the Debye and Einstein temperatures. Results in this respect show good agreement with the ...

Compositional dependence of the thermoelectric properties of (SrxBaxYb1 ? 2x)yCo4Sb12 skutterudites

High temperature thermoelectric (TE) properties for triple-filled skutterudites (Sr(x)Ba(x)Yb₁₋₂x)(y)Co₄Sb₁₂ were investigated for alloy compositions in two sections of the system: (a) for x = 0.25 with a filling fraction y ranging from 0.1 to 0.25 and (b) for 0 < x < 0.5 and y = 0.11 + 0.259x. The representation of the figure of merit, ZT, as a function of skutterudite composition, defined the compositional range (0.25 < x < 0.4; 0.18 < y < 0.24) with ZT over 1.4 at 800 K. It was shown that an enhanced TE performance for these triple-filled skutterudites is caused by low electrical resistivities and low lattice thermal conductivities, as well as by a fine tuning of the chemical composition. Low temperature measurements for the samples with the highest ZT values showed that even a small change of the filler ratios changes the contribution of scattering effects, the carrier concentration and the mobility.

Enhanced Thermoelectric Figure of Merit in P-Type DDy(Fe1-XCox)4Sb12

Thermoelectric (TE) properties of skutterudites DDy(Fe1-xCox)4Sb12 for 0.2  x 0.3 were studied in the temperature range from 300 K to 800 K and compared with values for x = 0. Didymium (DD, 4.76 mass % Pr and 95.24 mass % Nd) was used as natural double filler. At Co-concentrations 0.225  x  0.25 maximum TE-performance was obtained with impressive power-factors (4.5 mW/mK2) and ZTs (ZT1.2 at 700 K). Furthermore these skutterudites maintain the high ZT over a broad temperature range providing an excellent p-leg for high-efficiency thermoelectric power generation.

Severe Plastic Deformation, A Tool to Enhance Thermoelectric Performance

High pressure torsion (HPT) is one of the methods introducing severe plastic deformation (SPD). Particularly HPT has been applied to skutterudites and has shown to be a valuable and efficient technique for grain refinement in the nanometer regime via increasing the concentration of point defects, dislocations and high-angle grain boundaries. These microstructural changes significantly enhance phonon scattering. The decrease of crystallite size causes enhanced microhardness, i.e. Hall-Petch strengthening applies. For a thermoelectric material HPT may slightly enhance the Seebeck coefficient and electrical resistivity, but significantly decreases thermal conductivity and concomitantly increases ZT. The article intends to give a comprehensive view on all data available on the influence of the various techniques of severe plastic deformation on thermoelectric materials in general.

High-Pressure Torsion to Improve Thermoelectric Efficiency of Clathrates?

High-pressure torsion (HPT), as a technique to produce severe plastic deformation, has been proven effective to improve the thermoelectric performance of skutterudites. In this report, we present microstructural and thermoelectric properties of the clathrate Ba8Cu3.5Ge41In1.5 processed by HPT. The sample was synthesized from high-purity elements, subsequently annealed, ball milled, and hot pressed, and finally subject to HPT. Compared with the ball-milled and hot-pressed sample, the HPT-processed sample has higher electrical resistivity and Seebeck coefficient, and lower thermal conductivity, electron concentration, and mobility, which is attributed to the reduced grain size and increased density of dislocations, point defects, and cracks. No essential improvement of the dimensionless thermoelectric figure of merit is observed in the investigated temperature range, questioning the universal versatility of this technique for improvement of thermoelectric materials.

Impact of Ball Milling and High-Pressure Torsion on the Microstructure and Thermoelectric Properties of p- and n-Type Sb-Based Skutterudites

For thermoelectrics it is important to produce thermodynamically stable bulk nanostructured materials. Ball milling/hot pressing was shown to reduce the crystallite size by a factor of 100 and to reach about 100 nm with dislocation densities of 1012 – 1013m-2. Thereby thermoelectric properties of single, double and multifilled Sb-based skutterudites were improved significantly leading to figures of merit ZT, which in some cases are twice as high as those of their microstructured counterparts. With HPT treatment the crystallite size can be decreased to even 50 nm with dislocation densities as high as 1015m-2. The small grains as well as the high dislocation density result in a further lowering of thermal conductivity holding a high potential for future enhancement of ZT.

Mechanical properties of non-centrosymmetric CePt3Si and CePt3B

Elastic moduli, hardness (both at room temperature) and thermal expansion (4.2-670 K) have been experimentally determined for polycrystalline CePt3Si and its prototype compound CePt3B as well as for single-crystalline CePt3Si. Resonant ultrasound spectroscopy was used to determine elastic properties (Youngs modulus E and Poissons ratio ν) via the eigenfrequencies of the sample and the knowledge of sample mass and dimensions. Bulk and shear moduli were calculated from E and ν, and the respective Debye temperatures were derived. In addition, ab initio DFT calculations were carried out for both compounds. A comparison of parameters evaluated from DFT with those of experiments revealed, in general, satisfactory agreement. Positive and negative thermal expansion values obtained from CePt3Si single crystal data are fairly well explained in terms of the crystalline electric field model, using CEF parameters derived recently from inelastic neutron scattering. DFT calculations, in addition, demonstrate that the atomic vibrations keep almost unaffected by the antisymmetric spin-orbit coupling present in systems with crystal structures having no inversion symmetry. This is opposite to electronic properties, where the antisymmetric spin-orbit interaction has shown to distinctly influence features like the superconducting condensate of CePt3Si.

Thermoelectric properties of In and I doped PbTe

A systematic study of structural, microstructural, and thermoelectric properties of bulk PbTe doped with indium (In) alone and co-doped with both indium and iodine (I) has been done. X-ray diffraction results showed all the samples to be of single phase. Scanning electron microscopy (SEM) results revealed the particle sizes to be in the range of micrometers, while high resolution transmission electron microscopy was used to investigate distinct microstructural features such as interfaces, grain boundaries, and strain field domains. Hall measurement at 300 K revealed the carrier concentration ∼1019 cm−3 showing the degenerate nature which was further seen in the electrical resistivity of samples, which increased with rising temperature. Seebeck coefficient indicated that all samples were n–type semiconductors with electrons as the majority carriers throughout the temperature range. A maximum power factor ∼25 μW cm−1 K−2 for all In doped samples and Pb0.998In0.003Te1.000I0.003 was observed at 700 K. Doping ...

Physical properties of the ternary borides Ni21Zn2B20 and Ni3ZnB2

β-YbAlB4 is the unique heavy fermion superconductor that exhibits unconventional quantum criticality without tuning in a strongly intermediate valence state. Despite the large coherence temperature, set by the peak of the longitudinal resistivity, our Hall effect measurements reveal that resonant skew scattering from incoherent local moments persists down to at least∼ 40 K, where the Hall coefficient exhibits a distinct minimum signaling another formation of coherence. The observation strongly suggests that the hybridization between f -moments and conduction electrons has a two component character with distinct Kondo or coherence scales TK of ∼ 40 K and 200 K; this is confirmed by the magnetic field dependence of ρxy .