A joint research effort between the University of Mainz (Institute of Physics, Prof. Gerhard Jakob), the Max Planck Institue for Chemical Physics of Solids (Prof. Clauda Felser, Dresden), and the University of Stuttgart (Institute for Materials Science, Prof. Anke Weidenkaff, Dr. Benjamin Balke) is dedicated to the improvement of thermoelectric materials. Thermoelectric generators can convert otherwise wasted heat to useful electrical energy. Waste heat as energy source is abundant in car engines and in power plants. To convert even a small part of this huge energy 'reservoir' to electric energy will reduce greenhouse gas emissions considerably.
Our cooperation is focussed on half-Heusler compounds as model systems for nanostructured thermoelectric materials. In a combined effort of theory and experiment we implement half-Heusler nanostructures by complementary “top down” and “botttom up” approaches. The nanostructured materials are prepared as artificial superlattice structures using a “bottom up” strategy and as spontaneously phase separated bulk materials in a “top down” approach.
The main aim of this investigation is the enhancement of the thermoelectric figure of merit via reduction of the thermal conductivity. As demonstrated in a left figure below, an employment of artificial TiNiSn/HfNiSn superlattices leads to the reduction of the thermal conductivity (black circles) compared to bare materials (blue bars). Interestingly, at the period of 3.2 nm a crossover between the coherent and incoherent phonon transport is observed. It is manifested by a minimum of the thermal conductivity. As expected, reduced thermal conductivity of superlattices improves the thermoelectric figure of merit ZT (right figure below). The key results were published in [1-4].
- Komar et al., APL Mater. 4, 104902 (2016).
- Komar et al., Phys. Status Solidi A 213, 732-738 (2016).
- Hołuj et al., Phys. Rev. B 92, 125436 (2015).
- Jaeger et al., Semicond. Sci. Technol. 29, 124003 (2014).