The “Thermal Sensors” team contributes substantially to powder technical preparation techno¬logy as well as thermoelectric characterization within the scope of the EU FP5 compound project “NanoThermEl”(STREP) pursuing the development of highly efficient nano-structured thermo¬electrics in the material family of skutterudites, clathrates and zinc antimonide. Skutterudites are a wide class of substances based on the compound cobalt antimonide (CoSb3). In the project, skutterudites were prepared by a novel chemical technique of co-precipitation of components from an aqueous solution at the Royal Institute of Technology, Stockholm. The subsequent thermal processing produces nano-sized powders. The consolidated monoliths of these powders are extremely rich in grain boundaries. Under certain conditions this sort of grain boundaries has a more intense scattering effect on phonons than on charge carriers. Thus, the ratio of electrical to thermal conductivity can be increased, and likewise the quality of the thermoelectric material. For the control of carrier density in the course of material optimisation as well as for the reduction of heat conductivity, substituted and alloyed skutterudites are prepared by means of replacing Co (e.g. by Ni) and Sb (e.g. by Te). Furthermore, combined substitution have lead to solid solutions whose structural and functional properties have been examined. During the project the figure of merit of this kind of materials has been raised above the best-known values of comparable substances. By introducing an additional mechanism of phonon scattering at “rattling atoms” which means interstitial foreign atoms that occupy huge cavities of crystal structures (“cages”) and are bond very loosely to the hosting structure, a further essential improvement in material’s properties may be expected.
Optimized single crystalline Peltier materials
In cooperation with Russian partners (Baikov Institute, Moscow; St. Petersburg Polytechnical University; Ioffe Institute, St. Petersburg) bismuth telluride based solid solutions are being developed that exhibit extremely high homogeneity of the functional properties and highest thermoelectric figure of merit. To stabilise the thermoelectric properties in a wide temperature range at high performance, advantage is taken of a pinning effect of the Fermi level occurring at a resonant impurity level in the conduction band observed at high Sn doping.