Thermal to electric energy conversion is clearly identify as a source of alternative “clean” energy and an efficient way to reduce CO2 emissions (by reducing the fuel consumption) in the automotive industry (by scavenging the heat loss on exhaust systems for instance). Evidently, the relative high cost combined with the limited efficiency of these thermoelectric generators is a real drawback to their large scale development. The efficiency of such converter is directly proportional to the thermoelectric figure of merit of materials constituting the device. In turn, this figure of merit labeled ZT defines the thermoelectric quality of a given material. This dimensionless value is equal the α2T/ρκ with T the absolute temperature, α the seebeck coefficient, ρ the electrical resistivity and κ the total thermal conductivity. Usually, the quantities α, ρ, and κ for conventional 3D crystalline materials are interrelated in such way that it is quasi impossible to manipulate these variable independently in order to increase ZT. Whereas α and ρ vary inversely with the carrier concentration, ρ and κ are related by the Weidemann Franz law κel = LT/ρ where L is the Lorenz constant and κel = κ- κlat with κlat the heat transported by the phonon. However, nanostructuring of bulk materials have shown that major reduction in thermal conductivities can be achieved by the multiplication of grain boundaries and therefore by the creation of extensive interfacing between the nanoparticles enhancing the phonon scattering at the grain boundaries, evidently decreasing κlat.
The research of thermoelectric materials nowadays benefits from the possibilities given by non classical methods of synthesis and processing. As our experience tends to prove that all metals can very well be heated via the absorption of microwave, we propose to elaborate intermetallic materials by microwave irradiation. Together with the possibility to reach very high temperature, this very fast heating technique can lead to the synthesis of compound with remarkable micro (and even nano) structural features. Coupled to Spark plasma sintering (SPS) for densification purposes, and both these techniques being very fast, we expect to form and retain special morphological features of nano and/or microstructure. Therefore we also expect that these innovative processing techniques could lead to the possibility of obtaining and tuning certain physical properties. Using a microwave synthetic method for the fabrication of thermoelectric material, not only can reduce the price of the synthesis by reducing its time but also could lead to nanostructured materials with potentially reduced thermal conductivity and therefore improved thermoelectric performance. Special attention will be taken concerning thermal and electrical transport properties in order to probe the thermoelectric properties of selected intermetallics targets.
Image left: fast synthesis of Mg2Si by direct microwave heating of elemental precursors (Si and Mg)
Image right: scanning electron micrograpgh showing the nano-sized Mg2Si powder as synthesised
UCBN - CRISMAT: Microwave and SPS Processing, thermoelectric properties characterisations, Scanning and transmission electronic microscopy.
UW - Waterloo Institute for Nanotechnology : materials synthesis and characterization.
RENAULT TRUCKS: research-to-market approach for the thermoelectric application. Thanks to the output of RENOTER funded project (2008-2011) and by IDS-FunMAt future achievements, we will be able to assess the application performance.