Entropy engineering and interface optimization in materials for highly effective thermoelectric energy conversion

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Generators from thermoelectric materials can convert waste heat into electricity and contribute to a sustainable energy supply by reducing the consumption of fossil fuels and the emission of CO2. However, the exploitation of thermoelectric generator technology remains limited due to moderate efficiency, unsolved challenges in material-specific joining technologies and concerns on sustainability because of the current involvement of toxic and rare elements. A concerted effort of leading laboratories from AGH-UST (Poland), UDE and DLR (Germany) will address these challenges, aiming at developing highly effective thermoelectric materials from the families of argyrodites and magnesium silicide-based solid solutions, qualifying them for the usage in inexpensive and eco-friendly energy converters. We propose the following strategies: 1. Exploitation of an Entropy Engineering (EE) approach to stabilize the favorable high-symmetry a-phase in argyrodites at lower temperatures, to allow for the synthesis of novel argyrodites with a favorable electronic band structure and to increase the dopant solubility for both argyrodites and magnesium silicide-based solid solutions. 2. Synthesis of multiphase samples with structuring at the nm-scale. Nanostructuring can be achieved by controlled unmixing making use of the miscibility gap (for magnesium silicide based solid solutions) or secondary phases (argyrodites) by adjustment of the configurational entropy. Enhancement of the thermoelectric properties of the materials can be expected due to a combination of increased phonon scattering and an energy filtering of detrimental charge carriers at formed internal interfaces. The effect of advanced strategies like multiple, composition-dependent doping in multiphase composites can be evaluated directly employing Kelvin Probe Force Microscopy for electrical and Scanning Thermal Microscopy for thermal characterization with a spatial resolution of several 10 nm, rarely applied to thermoelectric materials before. 3. These AFM-based scanning techniques will also be employed to determine electrical and thermal contact resistances of Mg2X and argyrodites joined to selected electrode materials as these electrodes are a prerequisite to operate the functional materials in a device. Making use of the superb spatial resolution we’ll trace the microscopic origin of the thermal along with the electrical resistances of external interfaces and identify experimental levers to reduce them. Local measurements, microstructural characterization and transport modelling will be used to understand the integral thermoelectric performance. Experimental efforts in material development will be supported by first-principle based electronic band structure calculations. Finally, a magnesium silicide/argyrodite pn-uni-couple prototype shall be fabricated so that the tremendous application potential of this promising material combination can be evaluated also from an assembly point of view.