Synthesis and characterization of Bi-doped Mg2Si thermoelectric materials

The Mg2Si-based alloys are promising candidates for thermoelectric energy conversion for the middle high range of temperature. They are very attractive as they could replace lead-based compounds due to their low cost and non toxicity. They could also result in thermoelectric generator weight reduction (a key feature for the automotive application field). The high value of thermal conductivity of the silicide-based materials could be reduced by increasing the phonon scattering in the presence of nanosized crystalline grains without heavily interfering with the electrical conductivity of the thermoelectric material. Nanostructured materials were obtained under inert atmosphere through ball milling, thermal treatment and spark plasma sintering processes. In particular, the role of several bismuth doping amounts in Mg2Si were investigated (Mg2Si:Bi=1:x for x=0.01, 0.02 and 0.04 M ratio). The morphology, the composition and the structure of the samples were characterized by FE-SEM, EDS and XRD analyses after each process step. Moreover, the Seebeck coefficient analyses at high temperature and the electrical and thermal conductivity of the samples are presented in this work. The nanostructuring processes were affect by the MgO amount increase which influenced the thermoelectric properties of the samples mainly by reducing the electrical conductivity. With the aim of further increasing the scattering phenomena by interface or boundary effect, carbon nanostructures named Single Wall Carbon Nanohorns were added to the Mg2Si in order to produce a nanocomposite material. The influence of the nanostructured filler on the thermoelectric material properties is also discussed.