Preparation And Property Optimization Of Mg₂Sn-based Thermoelectric Materials

Thermoelectrical (TE) materials are functional materials for thermal-to-electrical energy conversion. An ideal TE material should not only have a high ZT but also be eco-friendly and low-cost. Mg2Sn-based TE materials are new promising class TE materials in 400-800K temperature range as their constituent elenments are nontoxic, abundantly available and their special physical properties.In this thesis, for the characteristics of the Mg2(Si, Sn)-based and Mg2(Ge, Sn)-based thermoelectric materials, the system studys on them about the relationships among the preparation of stoichiometry, phase structure, the n-type and p-type doping optimization are coverd. The main results are summarized as follows:The experimental data of Mg2Si0.4Sn0.6 thermoelectric materials prepared by the economical and simple B2O3-flux method indicate a good amount of Mg excess is beneficial for single-phase materials, and the single-phase Mg2Si0.4Sn0.6 with Mg excess of 4% obtains the minimum thermal conductivity and the minimum lattice thermal conductivity.The experimental data of Mg2Si1-xSnx prepared by B2O3-flux method show that a miscibility gap exists at 0.2<Sn<0.45 in room temperature range, compared to other reported data in the literature, which is related to the synthesized method of materials and the amount of Mg excess. Mg2Si1-xSnx-based thermoelectric materials in the boundary of miscibility gap achieve better performance, and Mg2Si0.55Sn0.45 obtains the minimum lattice thermal conductivity of 1.4W·m-1·K-1.Mg2Ge1-xSnx-based thermoelectric materials have been prepared by the repatable Ta-tube method. This indicates that there is not miscibility gap in Mg2Ge1-xSnx-system. And Mg2Ge0.4Sn0.6 is suitable to p-type doped, because its Seebeck coefficient of n-type in the high temperature region is apt to transform into p-type; Mg2Ge0.5Sn0.5 obtains the minimum lattice thermal conductivity of 2.0W·m-1·K-1.The samples of Mg2Si0.55-xSn0.45Sbx and Mg2Si0.6Sn0.4-xInx prepared by B2O3-flux method are n-type semiconductor. The thermal conductivity of Mg2Si0.55-xSn0.45Sbx increases, but its electrical performace is improved dramatically, and its ZT value is significantly increased, achieving ZTmax=0.6 in the high temperature region. Then, the repeated tested and annealed samples show that there are a certain amount of Mg evaporation in the process of testing and annealing, resulting in the reduction of their electrical conductivities, but the annealed samples enhance ZT value slightly up caused by their higher Seebeck coefficient. Unlike Sb-doping sample, the thermal conductivity of In-doping sample decreases, and its electrical conductivity decreases with increasing In.The samples of Ag-dopoing Mg2-xSi0.5Sn0.5Agx and Mg2-xGe0.4Sn0.6Agx. prepared by Ta-tube method are successfully transformed into p-type semiconductor, and their electrical performances have been significantly improved. As Ag doping induces defects scattering, the thermal conductivity of sample is also down. The thermoelectric figure of merit ZT has been significantly improved, and Mg1.9sGe0.4Sn0.6Ag0.02 achieves ZTmax=0.4 at 700K.