| On the basis of the main progress about Mg-Si-based thermoelectric materials, solid-state reaction was selected in the present paper to synthesis Mg2Si-based compounds at low temperatures followed by SPS method for the preparation. The synthesis method and properties of n-type and p-type Mg2Si-based thermoelectric materials as well as the solid solution behavior and control method of the dopants. were studied systematically. The thermoelectric properties of two type materials were mainly tested and analyzed, and some innovative results were then obtained.At first, basic thermodynamic theories were introduced to calculate the Gibbs function changes of Mg-Si-O-C system, which was an effective tool for verifying the variation of composition during the solid-state reaction process. The possibility of the occurrence for solid-state reaction was discussed. The influence of processing parameters on the compositions and properties of the products was explored too. The results showed the reaction between the Mg and Si powders was solid-state reaction and impurities such as MgO, Mg3C2: were usually produced resulting from the reactions with air or graphite crucible since Mg was very active. So the synthesis process must be optimized to avoid the formation of the above impurities and the oxidation of the synthesized Mg2Si. Furthermore, since Mg had great evaporation ability and the total evaporation amount of Mg metal was accessing to the amount of dopants, it must be taken into account when the raw powders were weighted.DTA, XRD, EPMA. X-ray fluorescence analysis and SEM were adopted in the evaluation of structure, composition and morphology. The processing parameters were then decided by the measured results. The results showed that Mg2A (A=Si or Ge) compounds as well as Mg2Si1-xGex solid solution without any carbonization or oxidation were successfully synthesized at 823K for 8hrs under a high purity Ar atmosphere. The products were integrated crystals and their particle sizes were distributed uniformly. After a further reaction at 1073K for 8hrs, Mg2Si1-xGex (x changed from 0 to 1) solid solutions were obtained. The influences of holding time, reaction temperature, particle size of the products and doping sequence on thermoelectric properties of Mg2Si were discussed in detail.Te, Sb and Ag were selected as the dopants in the synthesizing process of Mg2Si, and the influence of dopants and dopants' amount on thermoelectric properties of Mg2Si specimen was studied. The results showed that the Mg2Si-based thermoelectric material doped with Te and Sb was still n-type, but that doped with Ag changed into p-type. With the introduction of the dopants, the effective mass of carriers was changed and the Seebeck coefficient was increased. At the mean time, the dopants reduced the forbidden energy gap, which changed the carrier concentration and thus increased electrical conductivity. By calculating the forbidden energy gap and electrical conductivity of Mg2Si specimen doped with different amount Sb, the mechanism of transference changed abruptly at 625K. The value of the gap decreased with the increasing of dopants, so the energy needed for the electron transference was reduced. As a result, the electric conductivity of Mg2Si was increased with the increasing of the dopant.The figure of merit of the Mg2Si samples doped with 4000ppn Te was 0.69 X 10-3K-1 at 550K, while that of the samples with 5000ppm Sb reached a peak of 1.02 X 10-3K-1 at 650K, which seemed to be the best results ever reported for Mg-Si system till now.In addition, the nanometer-size particles of BN was doped into the Mg-Si-based thermoelectric materials as the second-phase for the first time. The result showed that the introduction of nano BN could decrease the thermal conductivity of the Mg2Si-based thermoelectric material and as a result its power factor were improved.
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