Electronic: Thermoelectric materials

Thermoelectric effects refer to the conversion of a temperature gradient to electric energy or vice versa. At the atomic scale, an applied temperature gradient causes charge carriers in the material to diffuse from the hot side to the cold side. As a result, a potential difference builds up. The basic mechanism is easy to understand — at higher temperature, more phonons are excited and the scattering of charge carriers therefore increases. The different degree of scattering at the hot and the cold ends of the material causes an increased concentration of charge carriers at the cold end, for purely statistical reasons.

Thermoelectric devices promise to play very important roles in future global sustainable energy solutions, since they enable the recovery of energy otherwise just wasted as heat. Furthermore, since thermoelectric devices are fuel-free solid-state devices with no moving parts, they should also be extremely reliable with long lifetimes. Their present day application is however limited due to the low conversion efficiency, which can be expressed using the thermoelectric figure of merit zT. This entity is proportional to the electrical conductivity and the square of the Seebeck coefficient, and inversely proportional to the thermal conductivity. A serious obstacle in improving the thermoelectric figure of merit is the need to optimize conflicting properties — when the Seebeck cofficient increases the electrical conductivity tends to decrease, and so forth. Recently, nanostructuring and fine-tuning of material parameters have demonstrated that significant performance improvements can be achieved, and the area is therefore subject to intense research and development at the moment. A direct effect of nanostructuring is that the thermal conductivity can be decreased through manipulation of the phonon spectrum and phonon lifetimes.

Only recently, HPC resources have become powerful enough to allow advanced calculations, founded in quantum mechanics, of thermoelectric effects. Recent method development has also helped to make this subject come into focus for computations. Specifically we aim at calculating phonon lifetimes using several schemes which require further method development.