Thermoelectric transport mechanism of Mg2Si0.4Sn0.6-yBiy prepared by low-temperature microwave reaction

Abstract

According to Debye relaxation, the polarization of electric dipole can be accomplished in 10-10 s under the action of an alternating electromagnetic filed with a frequency of 2.45 GHz, so it is feasible to obtain nano powder by carrying out solid reaction in microwave at low temperature in a short time. In this work, the syntheses of Mg2Si0.4Sn0.6-yBiy (0 ≤ y ≤ 0.03) solid solution thermoelectric materials are successfully achieved by microwave-assisted solid state reaction at low temperature with MgH2 serving as one reactant instead of Mg, and 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 can be achieved with nano-lamellar structure with an interlayer spacing of about 100 nm by carrying out the reaction between MgH2 and Si, Sn in microwave at 400℃ in 15 min. The introduction of Bi dopant can increase carrier concentration and lattice distortion. With the cooperation between the nano lamellar structure and lattice distortion, the phone is scattered so effectively that the sample owns a lowest thermal conductivity, κmin of 1.36 W·m-1·K-1 at 550 K based on the fact that the phonon scattering is dominant in the heat transfer process. The calculated results show that the theoretical κmin is 0.93 W·m-1·K-1, which is lower than 1.36 W·m-1·K-1. Therefore, by further adjusting the process parameters and increasing the effective doping rate of Bi and the density of the lattice defects, it is expected to obtain lower thermal conductivity. The band convergence is also verified by increasing the density-of-state effective mass. The apparent increase in m* is due to a gradual increase in carrier concentration with increasing temperature. Despite the agreement between the data and the model, the irregular behavior between m* and temperature is a very strong indication and the electric transmission performance of the sample is likely to be influenced by the structure of the multi band structure. Owing possibly to the low reaction temperature, there are Bi precipitates at the grain boundary. In addition to the phonon scattering and the alloy scattering, the Bi segregation and the scattering of carrier by nano-lamellar structure make the carrier mobility of the sample slightly lower. The lower effective doping rate and complex band structure make the carrier concentration and density-of-state effective mass low coupled with the low carrier mobility, which leads to low material factor β with a ZT of 0.66 at 600 K consequently.