Thermoelectric Materials: Introduction
Thermoelectric (TE) materials are characterized by their ability to directly convert heat into electricity or vice versa electrical power into temperature gradients. These favorable properties classify them as energy conversion materials, which are expected to play a significant role in the global quest for sustainable energy resources and green technologies. Applications are envisioned as thermal converters, which harvest waste heat, otherwise lost to the environment, or solid state refrigerators, which replace the commonly used systems relying on the compression of cryogenic liquids.
Expressed in macroscopic quantities, the electrical conductivity σ, the thermal conductivity κ, temperature T and the Seebeck coefficient S are related to the performance of state-of-the-art TEs defined by the figure-of-merit ZT = σS2/κT . Besides the operating temperatures at the cold and hot ends of a TE generator, this quantity ultimately determines the energy conversion efficiency and the TE performance. As a consequence, substantial efforts are put into strategies to manipulate ZT or, more precisely, electrical and thermal conductivity as well as Seebeck coefficients independently to develop novel superior materials.
Electrical conductivity and Seebeck coefficients must be optimized to increase the power factor σS2 of state-of-the-art materials in their temperature range of operation. This is by far not a trivial task as σ and S are not independent material properties and depend both on the electronic band structure and the intrinsic Fermi levels. Common approaches to directly control the power factor therefore rely on electronic doping but also on sophisticated, complex band structure and band gap engineering .
The crystal lattice contribution to the total thermal conductivity κ = κlattice + κelectronic in TEs is considered most prone to manipulation, whereas the electronic part is mostly unfavorably coupled to the electrical properties as reflected by the Wiedemann-Franz law. Consequently, introducing additional scattering centers for the heat carrying phonons is employed as the main strategy to manipulate the lattice thermal conductivity. For example, in skutterudites atoms are incorporated in the crystal structure to enhance phonon scattering. The signature of these frequently termed “rattler” atoms are discrete Einstein modes appearing in the low-energy region of the phonon density of states. These lattice vibrations interact with acoustic phonons thereby affecting the heat transport . In complex nanostructures , e.g. thin crystalline multilayer films, nanopowders or nanowires, artificially introduced boundaries or interfaces reduce the phonon mean free path and substantially increase diffuse scattering, thereby reducing the thermal conductivity significantly. Phonon band gap engineering is theoretically well understood  and considered as promising route to alter the thermal conductivity of TEs, but experimental implementation is in general more than challenging .
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Thermoelectric Materials: EM-AMCT Research Activities
Our Working Group “Methods for Characterization of Transport Phenomena in Energy Materials” (EM-AMCT) focuses on basic aspects in the research on thermoelectric materials, well prior to device fabrication. It seeks a fundamental understanding of the structure-functionality relation and, in particular, of the dynamic interplay of charge and quasi-particle transport as necessary prerequisite for optimizing design strategies for novel thermoelectric materials. Insights in the interaction processes on the level of elementary excitations in the solid state is considered as key to designing macroscopic transport properties.
The Working Group operates laboratories and X-ray and neutron scattering instruments at HZB’s Lise-Meitner-Campus in Berlin Wannsee and HZB’s Wilhelm-Conrad-Röntgen-Campus in Berlin Adlershof as vital and invaluable parts of its research activities.
The following list gives an overview of the infrastructure operated by the Working Group. For further detailed information we invite the visitors of our webpage to use the links below.
- Laboratory for Thermoelectric Materials Development: Spark-plasma-sintering equipment allows the synthesis of nanostructured bulk TE materials. A Potential-Seebeck-microprobe apparatus and equipment for simultaneously measuring electrical conductivity and Seebeck coefficient as well as in-house developments of 3ω-instrumentation are assets for thermoelectric characterization of novel materials and the rapid evaluation of the merits of alternative synthesis routes.
- Laboratory for Thermoelectric Transport Measurements: A complete instrument suite for thermoelectric transport measurements on novel materials is available. The laboratory includes a Laser Flash Apparatus, Hall measurement equipment, Differential Scanning Calorimetry equipment and a dilatometer.
- FLEXX: The cold-neutron triple axis spectrometer with its innovative multi-energy analyzer option MultiFLEXX and the neutron resonance spin-echo (NRSE) option at BER II provides highest energy resolution and allows for lifetime studies on quasi-particles in solid state samples. Systematic methodological developments of the NRSE spectroscopy applied to single-crystalline materials in recent years [1-5] are now the basis to fundamentally link macroscopic transport in TE to the quasi-particle dispersion and lifetime as part of the Working Group’s scientific mission.
- PEAXIS: The new end-station at BESSY II offers capabilities for resonant inelastic X-ray scattering (RIXS) and angular resolved photoemission spectroscopy (ARPES) measurements. Wavevector-resolved RIXS allows in particular accessing electronic states and their coupling to dispersive excitations in energy materials.
Research activities within EM-AMCT focus currently on three TE material classes:
In all of these materials, the thermal conductivity is lowered by the suppression of phonon propagation by providing additional phonon Umklapp scattering either by introducing additional filler atoms, as for the skutterudites, or by interfaces, as for mesoporous silicon.
Research on thermoelectric materials offers a variety of fascinating scientific questions to be studied. We encourage highly motivated students who are interested in broadening their background in experimental condensed matter physics and in developing their scientific skills to apply for a Bachelor’s thesis project or a Master’s thesis project. Please contact PD Dr. Klaus Habicht (firstname.lastname@example.org) if you are interested.
 K. Habicht, R. Golub, F. Mezei, B. Keimer, T. Keller, Temperature-dependent phonon lifetimes in lead investigated with neutron-resonance spin-echo spectroscopy, Phys. Rev. B 69 (2004) 104301.
 K. Habicht, R. Golub, T. Keller, The resolution function in neutron spin-echo spectroscopy with three-axis spectrometers, J. Appl. Cryst. 36 (2003) 1307-1318.
 F. Groitl, K. Kiefer, K. Habicht, A resolution model for mode multiplets probed with neutron resonance spin-echo spectroscopy, Physica B 406 (2011) 2342-2345.
 P. Aynajian, T. Keller, S.M. Shapiro, L. Boeri, K. Habicht, B. Keimer, Energy gaps and Kohn anomalies in elemental superconductors, Science 319 (2008) 1509-1512.
 T. Keller, P. Aynajian, K. Habicht, L. Boeri, S. K. Bose, B. Keimer, Momentum-resolved electron-phonon interaction in lead determined by neutron resonance spin-echo spectroscopy, Phys. Rev. Lett. 96 (2006) 225501.
 K. Lieutenant, T. Hofmann, C. Schulz, M. V. Yablonskikh, K. Habicht, E. F. Aziz, Design concept of the high-resolution end-station PEAXIS at BESSY II: Wide-Q-range RIXS and XPS measurements on solids, solutions, and interfaces, J. Electron Spectrosc. Relat. Phenom. 210 (2016) 54–65.
 K. Lieutenant, T. Hofmann, C. Zendler, C. Schulz, E. F. Aziz, K. Habicht, Numerical optimization of a RIXS spectrometer using raytracing simulations, J. Phys. Conf. Ser. 738/1 (2016) 012104.