CD Laboratory for Thermoelectricity

Temperature differences in solids can be converted into electrical voltage. This is linked to the hope of being able to utilise waste heat - for example from car engines - in a meaningful way. This laboratory is researching appropriate materials and methods.

The movement of electrons in metals depends, among other things, on temperature differences. If two different metals are joined together and kept at different temperatures, electrical voltage is generated and current can flow through a consumer. In this way, waste heat could be converted into electrical energy - for example, the on-board electronics of cars could be powered by electricity generated from the waste heat of the car engine. However, the economic realisation of this idea has so far failed due to a lack of suitable materials with sufficient thermoelectric performance.

The research work in this laboratory therefore focusses on so-called filled skutterudites. These are alloys that are produced in various process steps and have a cage-like structure of cobalt, iron and antimony. Additional atoms can be placed in these nano-cages, thereby customising the physical properties of the material. Apart from the improved thermoelectric efficiency, these skutterudites are also considerably cheaper than the systems currently used based on bismuth and tellurium.

The main research objective is to improve the thermoelectric properties of skutterudites and to optimise their manufacturing processes. The most important strategy for this is to create structures and inhomogeneities in the nanometre range in the material. Such nanostructures are extremely effective scattering centres for medium- to long-wave phonons and therefore lead to a significant decrease in thermal conductivity - and consequently to an improvement in thermoelectric efficiency. Another research topic is the systematic search for new thermoelectric materials composed of non-toxic elements that are abundant in the earth's crust.

Sophisticated computer programmes make it possible, for example, to calculate the electronic structure of such materials and also enable good predictions to be made about the actual physical properties of these substances. In this way, promising materials are identified, which are then manufactured at great expense. To do this, the necessary elements are first mixed in exactly the right ratio and then merged. This takes place in various systems ("furnaces") that work on the basis of high-frequency magnetic fields, for example. X-ray methods and electron microscopy are used to determine whether the substance produced in this way is actually the desired one. The parameters that are decisive for the thermoelectric performance of the material are then determined experimentally; these include the electrical resistance, the Seebeck effect and the thermal conductivity. The final step is to improve these properties, with work on the nanostructuring of these materials using a high-pressure torsion device, for example, proving to be the most promising approach to date.