Preparation, Crystal Structure And Properties Of Bi₂Te₃ Based Materials

At room temperature, the best thermoelectric material now known is Bi2Te3 based semiconductor, which has the figure of merit ZT=1 for the bulk material. Previous studies showed that the optimization of the phase composition and microstructure can improve the thermoelectric transport properties of Bi2Te3 based materials. The micro-sized and nano-sized alloy powders are synthesized by vacuum melting and hydrothermal method in the thesis. Using these powders, bulk samples are prepared by spark plasma sintering system or hot extrusion sintering, whose microstructures and thermoelectric properties are systematically investigated.Bi2Te3 based alloy samples are prepared by the methods of vacuum melting, high energy ball milling and spark plasma sintering. The effects of Sb and Se on the properties are studied. Results indicate that solid solution of Bi2(Te1-xSex)3 exhibits n-type semi-conduction, and the highest thermoelectric properties with the ZT value of 0.62 is acquired when x is from 0.10 to 0.15 at room temperature. It is also noted that the semi-conduction of solid solution (BixSb1-x)2 is transformed from p to n and the ZT value at room temperature reaches 0.91 with x of 0.2.Considering different conditions, Nano-structured Bi2Te3 powders are prepared by hydrothermal synthesis. Analytical results.by XRD and SEM. indicate that reaction temperature and time play an important role in the composition and microstruture of the powders. With the increase of temperature and time, higher extent reaction is explored, but not facilitating to form nano-sized powders. The composition is single phase, with the particle size from 100 to 500nm when temperature and reaction time is 120℃and 20h, respectively. Ternary compounds Bi2(Te1-xSex)3 and (BixSb1-x)2Te3 are synthesized by hydrothermal method. The mineralogical analysis shows that the products are composed of two phases with the same structure and different lattice constants, exhibiting lamellar structure of hexahedral. Furthermore, the length and thickness of particle is about hundreds of nm and some dozens of nm, respectively.Thermoelectric performance of bulk Bi2Te3 alloys can be improved by introducing nano-sized grains to the matrix. Mixtures with different ratios of nano-sized powder to micro-sized one by weight are used to produce Bi2(Te0.90Se0.10)3 and (Bi0.2Sb0.8)2Te3 bulks. Great influence is brought by introducing nano-sized grains on the thermoelectric properties. The microstructures of samples are lamellar and in the large grain layer interspersed with a number of fine grains, exhibiting higher density and thermoelectric transport properties because the scatter of the phonons and carriers is increaed. For the Bi2(Te0.90Se0.10)3 samples, the optimal dosage of nano-powder is 12%, with the ZT value of 0.74 at room temperature. The ZT value of (Bi0.2Sb0.8)2Te3 can reach 1.22 when the dosage of the nano-powder is 9%.As the arrangement of grains with certain orientation, thermoelectric properties of bulk Bi2Te3 based alloys are anisotropic. Generally single crystals have better thermoelectric properties than the polycrystalline, but the poor mechanical properties limit its application. N-type Bi2Te3 based bulk materials with strong grain orientation are prepared by powder hot extraction when its orientation factor F up to 0.49. Compared with the single grains, the mechanical properties of hot extruded samples are greatly improved, with a bending strength of 58MPa. Through the improvement of extrusion mold and optimization of extrusion craft, the difference of thermoelectric properties caused by structure gradient are significantly reduced. The effects of extrusion temperatures on the thermoelectric properties of samples are also explored. Results indicate that Se element in solid solution will be leached with lower or higher temperature. The ZT value up to 0.75 when the extrusion temperature at 400℃.In this thesis, the anisotropy of composite powder SPS samples and extrusion samples are systematically studied. The orientation factor F is used to quantitatively evaluate the anisotropic characteristics of samples. The results show that the degree of grain orientation reduces with the increase of nano-sized content and the decrease of extrusion temperature. The electrical conductivity of extrusion samples perpendicular to the pressing direction are 2.4 times of the one parallel to the pressing direction. And there is no obvious difference of Seeback factor at different directions.