Preparation And Doping Of P-type Thermoelectric Silicides

Thermoelectric materials can convert heat energy to electricity directly or vice versa, which have unique advantage and various applications in thermoelectric power generation and cooling. A good thermoelectric device necessarily requires both n-type and p-type thermoelectric materials with good performance. This thesis is mainly focused on the study of environmentally friendly and cost-effective p-type thermoelectric silicides such as Mg2Si and higher manganese silicide (HMS), having explored the ways to enhance their thermoelectric performance by some certain kinds of elements doping and the optimization in their fabrication methods.Due to the existing fact that Mg-Si based thermoelectric materials are quite tricky to prepare, solid state reaction method employed in the thesis to prepare Mg2Si was carefully controlled and repeatedly tested to find the stable and repeatable reaction conditions. Well-formed pure Mg2Si sample was prepared under the fixed reaction conditions. It is revealed in the measurements of thermoelectric properties that pure Mg2Si sample is n-type semiconductor. The highest ZT value 0.28 has been obtained at 725K.N-type Mg2Si enjoys a relatively good thermoelectric property and has been intensely studied. But due to the fact that the thermoelectric devices with n-type and p-type terminals of the same material system will generate higher efficiency, it is also necessary and meaningful to modify Mg2Si into p-type semiconductor. Solid state reaction method was adopted to prepare Li-doped Mg2Si compound Mg2-xLixSi, with Li13Si4 as a donator of lithium. The thermoelectric properties of Li-doped and undoped Mg2Si samples were measured and compared. All the Li-doped samples were successfully modified into p-type. Compared with the undoped sample, the Li-doped Mg2Si samples have relatively higher thermal conductivity and lower electrical conductivity. The maximum ZT value in Li-doped Mg2Si samples was 0.12 at 725K, obtained in the sample with the theoretical stoichiometric proportion as Mg1.97Li0.03Si.Higher manganese silicide (HMS) is a p-type silicide material with extraordinary thermoelectric performance. Proper elements-doping and improvement in preparation will further enhance its p-type thermoelectric performance. Levitation melting, melt spinning and spark plasma sintering (SPS) were adopted to prepare Ge-substituted higher manganese silicide thermoelectric alloys Mn(Si1-xGex)1.733. XRD patterns showed that there were two incommensurate phases in the obtained HMS samples and the main incommensurate phase was Mn15Si26, and that rapid solidification could reduce the content of MnSi metallic phase. Lattice distortion was produced by the substitution of Ge for Si, rendering a gradual shift of diffraction peaks to low-angle region in XRD patterns. The micro-morphologic differences of the near-roller surface and the free surface in the flake alloys obtained by melt spinning were analyzed by SEM. This kind of micro-structural difference was explained in the thesis with nucleation theory. The shape and distribution of MnSi metallic phase in the samples before and after SPS were also analyzed by SEM. Spark plasma sintering was employed to prepare high density bulk alloy samples from both the levitation melted powder and the melt-spun powder. Thermoelectric performances of these samples were measured and compared. Results revealed that the thermal conductivity decreased in melt-spun samples and the electrical transportation properties optimized because of Ge doping. Optimization in these two aspects would do good to the enhancement of the dimensionless figure of merit ZT. The aggregation of MnSi metallic phase in the rapid solidified samples was observed with SEM analysis, helping to explain the increase of electrical conductivity in these samples.Within the range of Ge substitution ratios in the experiment, the highest ZT value has been obtained when x=0.010. For the levitation melted samples, the maximum ZT value was 0.53 at 850K, and for the melt-spun samples,0.55 at 750K.Flux synthesis method with B2O3 as fluxing agent was employed to prepare Ga-doped Mg2Ge0.4Sn0.6 samples. XRD patterns showed that the single-phased three-component sosoloid was obtained. Thus flux synthesis method proved to be an effective way to prepare Mg-Ge-Sn sosoloid. Most Ga-doped samples exhibited a p-n type conversion phenomenon.