(Invited) Silicon-Based Nanocomposites for Thermoelectric High Temperature Waste Heat Recovery

Abstract

Thermoelectric generators convert heat flows directly into useable electrical energy. In an appealing future scenario of application, waste heat could be partly recovered by thermoelectric generators. It is typically argued that the heat comes without additional costs. Hence, every contribution of a thermoelectric generator results in an increase of the over-all process efficiency. A thermoelectric converter material with sufficient performance at competitive costs is hereby one precondition. A pragmatic way to optimize the thermoelectric performance of a material is the introduction of nanostructural elements into bulk materials. Hereby, the availability of high quality nanopowder fosters the development of novel thermoelectric materials: Materials which have per se an insufficient thermoelectric figure of merit can be optimized in a range that they become interesting substitute materials like nanocrystalline silicon or composites of nanocrystalline silicon and metal silicides. High hot side temperatures require innovative device concepts. Some years ago, a thermoelectric generator concept was proposed, which uses a large area pn junction aligned in parallel to the temperature gradient. This device concept is especially designed for high hot side temperatures and large ΔTs of at least a few hundreds of degree C. All electrical and mechanical contacts on the pn junction device are on the cold side which is beneficial with respect to stability against degradation of the device. Using nanocrystalline silicon as thermoelectric converter material, we have shown that for temperature differences larger than 300°C, the pn junction device performs competitively with an ideal traditional device and outperforms this device if non-ideal contact resistances of a real device are included.