Performance Optimization And Transport Mechanism Of Mg₂Si Based Thermoelectric Materials

Mg2Si based thermoelectric material is one of promising mid-temperature thermoelectric materials(450~800K) due to its rich aboundace and nontoxity. The ZT value of intrinsic Mg2 Si is lower than 0.1 because of its high lattice thermal conductivity and poor electrical properties. In this work, performance optimization of Mg2 Si has been done by nano-composite and solid solution doping, and its transportation mechanisms were studied based on the SPB(single parabolic band) model as well. The main results are summarized as follows:Thermoelectric properties of silicon nanowire composite Mg2 Si. SiNWs(Silicon nanowire) were prepared by chemical etching method. SiNW-Mg2 Si composite thermoelectric materials were prepared by the method of external mixing. The effect of SiNW and Bi content on the thermoelectric properties of Mg2 Si materials was studied by means of phase structure and thermoelectric properties. The results are summarized as follows: The coupling relationship between the resistivity and Seebeck coefficient of the Mg2 Si material has been relieved by the addition of the intrinsic SiNW, and the thermal conductivity of the sample is reduced, and the thermoelectric performance is improved significantly as well; Bi doping weakens the energy filter effect and the scattering effect of the SiNW in the intrinsic Mg2 Si material, and the change of Seebeck coefficient and thermal conductivity is not obvious.Thermoelectric transport mechanism of Mg2Si0.4Sn0.6-yBiy. In this work, the synthesis of Mg2Si0.4Sn0.6-yBiy(0≤y≤0.03) solid solution thermoelectric materials was successfully achieved by microwave-assisted solid state reaction at low temperature with MgH2 as one reactant instead of Mg.Their transportation mechanisms are studied based on the SPB(single parabolic band) model as well. The results indicate that the volatilization and oxidation of Mg can be suppressed effectively in this process. Fine stoichiometric product has been achieved with nano-lamellar structure around 100 nm. Bi dopant increase carrier concentration and lattice distortion. With the collaboration between the nano lamellar structure and lattice distortion, the phonon has been scattered so effective that the samples own lowest thermal conductivity, κmin of 1.36 Wm-1K-1 based on the fact the phonon scattering is dominant in the heat transfer process. The lower effective doping rate and complex band structure have double-edged effect by reducing the density-of-states effective mass and the carrier relaxation time simultaneously, which leads to the intrinsic excitation in advance with a ZT of 0.66 at 660 K consequently.