Study On Mg₂(Si,Sn) And Bi₂Sr₂Co₂O₉ Thermoelectric Materials

Thermoelectric materials have promising protentials in thermoelectric power generation and cooling applications, as they can convert thermal energy into electrical energy directly or vice versa. Future commercial applications at large scales require thermoelectrial materials be efficient, environmental-friendly and cost-effective. Mg₂(Si, Sn) based materials and Bi₂Sr₂Co₂O₉ oxides are those candidates. Thus, in this thesis the study on those two material systems about the relationship among the compositions, micro-structure, thermoelectric properties and fabrication routes are covered. The highlights of the results are as follows.The ZT of Mg₂(Si, Sn) based materials can be optimized by tuning electrical and thermal properties via adjusting composition and charge carrier concentration. The electrical properties and ZT of Mg₂Si_0.45Sn_0.55 are impoved by La and Sb doping. The peak ZT value is 0.6. In addition, a study about the effects of La+Sb co-doping on Mg₂Si_0.6Sn_0.4 showed that co-doping can further increase the electrical properties as well as ZT, which is 0.7 in this case. All the results showcased a propect for future commercial applications of Mg₂(Si, Sn) in power generation at intermediate temperature range.Mg₂(Si, Sn) based materials require a cost-effective fabrication method to produce stoichiometric thermoelectric materials. It has been a headache for researchers due to the high saturated vapor pressure and volatility of Mg as well as the big difference among the melting points of Mg(650℃), Si(1414℃) and Sn(232℃). In this thesis, a flux method using B₂O₃ and a hybrid method were reported. Both lead to the stoichiometric Mg₂(Si, Sn) thermoelectric materials with reasonable economic cost. Especially the flux method using B₂O₃ featured its simple procedures, low fabrication temperature and samples with good thermoelectric properties. The highest figure of merit amounts to 0.8.Bi₂Sr₂Co₂O₉ layer oxide has relatively low thermal conductivity for its unique natural supperlattice structures. Thus, this thesis covers the efforts to improve the electrical properties of Bi₂Sr₂Co₂O₉ by La and Sn doping. Bi₂Sr₂Co₂O₉ samples exhibit improved the electrical conductivity and Seebeck coefficient for the same sample. Meanwhile figure of merit reaches 0.1, revealing a future application of power generation in high temperature range.