Thermoelectric materials, which can generate electricity from waste heat or be used as solid-state Peltier coolers, could play an important role in a global sustainable energy solution. Such a development is contingent on identifying materials with higher thermoelectric efficiency than available at present, which is a challenge owing to the conflicting combination of material traits that are required. Nevertheless, because of modern synthesis and characterization techniques, particularly for nanoscale materials, a new era of complex thermoelectric materials is approaching. We review recent advances in the field, highlighting the strategies used to improve the thermopower and reduce the thermal conductivity.

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Complex thermoelectric materials.

Herein we cover the key concepts in the field of thermoelectric materials research, present the current understanding, and show the latest developments. Current research is aimed at increasing the thermoelectric figure of merit (ZT) by maximizing the power factor and/or minimizing the thermal conductivity. Attempts at maximizing the power factor include the development of new materials, optimization of existing materials by doping, and the exploration of nanoscale materials. The minimization of the thermal conductivity can come through solid-solution alloying, use of materials with intrinsically low thermal conductivity, and nanostructuring. Herein we describe the most promising bulk materials with emphasis on results from the last decade. Single-phase bulk materials are discussed in terms of chemistry, crystal structure, physical properties, and optimization of thermoelectric performance. The new opportunities for enhanced performance bulk nanostructured composite materials are examined and a look into the not so distant future is attempted.

New and Old Concepts in Thermoelectric Materials

Simple rules for the understanding of Heusler compounds

Recently a significant figure-of-merit (ZT) improvement in the most-studied existing thermoelectric materials has been achieved by creating nanograins and nanostructures in the grains using the combination of high-energy ball milling and a direct-current-induced hot-press process. Thermoelectric transport measurements, coupled with microstructure studies and theoretical modeling, show that the ZT improvement is the result of low lattice thermal conductivity due to the increased phonon scattering by grain boundaries and structural defects. In this article, the synthesis process and the relationship between the microstructures and the thermoelectric properties of the nanostructured thermoelectric bulk materials with an enhanced ZT value are reviewed. It is expected that the nanostructured materials described here will be useful for a variety of applications such as waste heat recovery, solar energy conversion, and environmentally friendly refrigeration.

Enhancement of Thermoelectric Figure-of-Merit by a Bulk Nanostructuring Approach

Thermoelectric (Peltier) heat pumps are capable of refrigerating solid or fluid objects, and unlike conventional vapor compressor systems, they can be miniaturized without loss of efficiency. More efficient thermoelectric materials need to be identified, especially for low-temperature applications in electronics and devices. The material CsBi(4)Te(6) has been synthesized and its properties have been studied. When doped appropriately, it exhibits a high thermoelectric figure of merit below room temperature (ZT(max) approximately 0.8 at 225 kelvin). At cryogenic temperatures, the thermoelectric properties of CsBi(4)Te(6) appear to match or exceed those of Bi(2-x)Sb(x)Te(3-y)Se(y) alloys.

https://science.sciencemag.org/content/sci/287/5455/1024.full.pdf

CsBi4Te6: A High-Performance Thermoelectric Material for Low-Temperature Applications

Four efficient n-type dopants have been found for ZrNiSn-based thermoelectric materials. These are Nb or Ta at the zirconium sites, and Sb or Bi at the tin sites. No suitable dopant was found for the nickel sites. In a alloy, a power factor of and a thermal conductivity of were measured at 300 K, resulting in a dimensionless figure of merit ZT = 0.12. These values are increased to and at 700 K.

https://iopscience.iop.org/article/10.1088/0953-8984/11/7/004/pdf

Efficient dopants for ZrNiSn-based thermoelectric materials

$\mathrm{Tl}{}_{9}\mathrm{BiTe}{}_{6}$ exhibits a thermoelectric figure of merit of $\mathrm{ZT}\ensuremath{\sim}1.2$ around 500 K, which significantly exceeds the state-of-the-art materials in this temperature range. The extraordinary thermoelectric performance is mainly due to the extremely low thermal conductivity of $\mathrm{Tl}{}_{9}\mathrm{BiTe}{}_{6}$ [ $0.39\mathrm{W}/(\mathrm{m}\ifmmode \dot{}\else \.{}\fi{}\mathrm{K})$ at 300 K]. In fact, the minimum lifetime of the phonons has to be taken into account to describe the thermal conductivity data.

High Performance Thermoelectric Tl 9 BiTe 6 with an Extremely Low Thermal Conductivity

We report the effect of annealing of sapphire substrate surfaces and explain the development of step patterns with large terraces. Furthermore, the epitaxial growth of Nb(110) on annealed and unannealed sapphire substrates is studied by AFM and x-ray methods. For the explanation of the well known two component line shape of transverse x-ray scans across Nb(110) Bragg peaks, a structural model is suggested.

http://iopscience.iop.org/article/10.1088/0953-8984/11/13/004/pdf

AFM and x-ray studies on the growth and quality of Nb(110) on

NbIrSn and NbIrSb crystallize in a cubic MgAgAs-type structure with lattice parameters of 617.76(2) and 614.97(2) pm, respectively. Both compounds exhibit incongruent melting behaviour. NbIrSn is a p-type semiconductor with an energy gap of 0.28 eV. At room temperature, it has a Seebeck coefficient of and a resistivity of . NbIrSb is metallic with and at room temperature. The formation of the solid solutions has been confirmed.

https://iopscience.iop.org/article/10.1088/0953-8984/10/35/016/pdf

New compounds with MgAgAs-type structure: NbIrSn and NbIrSb

The compounds Tl9BiTe6 (TBT) and Tl9BiSe6 (TBS) crystallize in the tetragonal space group I4/mcm. Tl9BiTe6 has a thermopower of 185 μV/K and an electrical resistivity of 5.5 mΩcm at 300K, resulting in a power factor of S2/ρ = 0.6 mW/mK2. Compared to Bi2Te3 which is the state of the art material at this temperature this is about a factor of 7 lower. At 300 K TBS has a thermopower of 750 μV/K but a high resistivity of 130 Ωcm. To optimize the thermoelectric properties of TBT solid solutions have been formed with TBS. The resistivities and have been measured on Tl9BiTe1-xSex with x = 0.05, 0.08, 0.2 and 0.5. In addition to the electrical properties the lattice constants have been measured by X-ray diffraction. The dependence of the lattice constants on the Te/Se ratio clearly deviates from Vegard’s law. Different affinities of Te and Se towards the two chalcogenide sites in the crystal can explain this behavior.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S1946427400122420

Bernd Wölfing

Papers

Efficient dopants for ZrNiSn-based thermoelectric materials

High Performance Thermoelectric Tl 9 BiTe 6 with an Extremely Low Thermal Conductivity

AFM and x-ray studies on the growth and quality of Nb(110) on

New compounds with MgAgAs-type structure: NbIrSn and NbIrSb

Thermoelectric properties of Tl9BiTe6 / Tl9BiSe6 solid solutions