Thermoelectrics in Energy Efficiency, Materials and Resources

Methods for Characterization of Transport Phenomena in Energy Materials: Methods and concepts for material development

Investigators: Tommy Hofmann, Katharina Fritsch, Klaus Habicht
Thermoelectric materials have a large potential for Peltier cooling or electric energy generation by converting heat into electricity and may significantly impact on the global energy scenario. Hence, research on novel, highly efficient thermoelectric materials targets a sustainable and economical usage of energy resources and a reduction of emissions by using waste heat. Along with high thermopower and electrical conductivity, low thermal conductivity is key for high performance of thermoelectric materials. The emphasis of the research on thermoelectric material properties within the working group “Methods for Characterization of Transport Phenomena in Energy Materials” is on basic research aspects. A fundamental understanding of the relation of structure to functionality and, in particular, of the dynamic interplay of charge and quasi-particle transport is a necessary prerequisite for optimised design strategies for novel thermoelectric materials. Basic research on the interaction processes on the level of elementary excitations in the solid state is key to designing transport properties while neutron scattering techniques provide invaluable tools for investigating the underlying dynamics.

The most efficient route for improving the thermoelectric figure of merit is the suppression of thermal transport by phonons. The working group “Methods for Characterization of Transport Phenomena in Energy Materials” has specialised expertise in neutron resonance spectroscopy allowing the highest energy resolution for the investigation of elementary excitations in solid-state matter and thus providing access to qualitatively new information about the lifetime of quasi-particles. In the past, the group has made essential contributions to the development of the neutron resonance spin echo (NRSE) method for investigating elementary dispersive excitations, which profit greatly from the NRSE method [1-3]. Electron-phonon interaction was studied with the NRSE technique. Detailed investigations of the phonon lifetime allowed a comparison of ab initio calculations with experimental data and gave insights into the mechanisms determining the energy gap in a superconductor [4,5]. The methodological progress made now provides access to the fundamentals of transport phenomena.

The efficiency of lowering thermal transport by phonon scattering by nanostructuring and doping with guest filler atoms, and the relation of these phenomena to the functional properties of thermoelectric materials will be studied in Si-based and skutterudite materials. A first branch of research activities at HZB focuses on mesoporous structuring (Fig. 1) of Si-based materials by chemical etching techniques building on existing expertise in our group [6,7]. This allows to investigate the relationship between phonon scattering and mesoscale lattice disorder following the phononic thermal conductivity from the single crystalline Si into the region of porous Si. A second branch of research activities will concentrate on skutterudites where the filling of lattice cages by guest atoms and the coupling between phonon modes leads to a decreased phononic thermal conductivity. Filled skutterudites with general formula AT4X12 (A = alkaline, alkaline earth, lanthanide or actinide atoms, T = Fe, Ru, Os, and X = P, As, Sb) have recently attracted much attention [8]. Skutterudites are promising materials that can have figures of merit values as large as ZT~1.8 and can operate at temperatures up to 700° C. Essential prerequisites for a large ZT of n-type skutterudites is an optimal filling level of the electropositive elements in the oversized cages, providing a reasonably high electrical conductivity in combination with the reduced phonon thermal conductivity due to scattering of heat carrying phonons on the loosely bound guest filler atoms.

Fig.1: REM image of a porous Si surface. Pores have been etched electrochemically into a (100) Si wafer (from Ref. [6]).

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[3] F. Groitl, K. Kiefer, and K. Habicht, Physica B  406, 2342 (2011).
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[6] A.V. Kityk, T. Hofmann, K. Knorr, Phys. Rev. Lett.  100, 036105 (2008).
[7] T. Hofmann, P. Kumar,M. Enderle and D. Wallacher, Phys. Rev. Lett. 110 065505 (2013).
[8] B.C. Sales, D. Mandrus, R.K. Williams, Science 272, 1325 (1996).