NANOSTRUCTURED SKUTTERUDITES FOR THERMOELECTRIC GENERATION – NANOSKUT

The development of alternative technologies, for instance the conversion of waste heat into electrical energy, is of prime necessity to reduce our dependence to fossil fuels and to preserve our climate. In a thermoelectric (TE) generator (TEG), electricity can be generated by the direct action of heat on the constituting materials (n- and p-type semiconductors). These systems are simple, compact, silent and highly reliable (no moving parts). The efficiency of a TEG is related to the dimensionless figure of merit ZT: T is the average temperature of use and Z a parameter intrinsic to each constituting material which depends among others on its Seebeck coefficient (alpha) and thermal resistivity (W): to maximize ZT, alpha and W must be maximized. It could be possible to carry out energy harvesting from thermal engines and industrial processes in the temperature range 400K – 800K using recently developed materials such as the skutterudites with ZT ~ 1.1 – 1.2 at 800K. However, the problem of their corrosion at high temperature must be solved before considering their use in a durable TEG. Adding to these new materials, the concept of nanostructuring has emerged these past years, leading to ZT values exceeding 2. Nanostructuring was initially employed for the tellurides (PbTe, Bi2Te3) and was only very recently applied to the skutterudites where ZT values of 1.4 were obtained. We thus propose to first synthesize bulk nanostructured n- and p-type skutterudites with improved ZT and then to integrate them in an efficient (8 – 12%) and durable (years) TEG operating at 800K. To reach the first objective, n-type skutterudites constituted of nano-sized grains in the 100 – 200 nm range in order to promote phonon scattering at the grain boundaries will be synthesized (IJL). Such a microstructure will be 2 – 5 times thinner than previously obtained microstructures and should lead to a profound increase of the lattice thermal resistivity. To keep the beneficial effect of nanostructuring at high temperature, small amounts of grain growth inhibitors will be added. p-type skutterudites will also be synthesized with nanoprecipitates at their grain boundaries in order to energetically filter the electrons and increase their Seebeck coefficient (ICMPE). All these materials will be densified by spark plasma sintering. The final goal is to make a reliable TEG using these nanostructured skutterudites. It requires the design and making of electrical connections with not only low contact resistances but also mechanical and thermal stablility (IJL). It also requires the protection of the TEG from air corrosion by coatings with thin layers of oxides (IJL). Some of these challenging questions for terrestrial applications have never been deeply addressed. If they were solved, power generation by TE effects could play a major role in renewable energies. The two laboratories involved in this project are well qualified and complementary to fulfill this study. ICMPE and IJL are internationally known for their contributions to the progress of the TE properties of nanostructured skutterudites and are founding members of the GDR CNRS "Thermoélectricité”. IJL bears the experience of making a TE system for harvesting waste heat at high temperature (CNES, programme "DECLIC") and it has also acquired an expertise in the field of oxidation and corrosion of intermetallics which is essential to design a highly reliable demonstrator and simulate service conditions.