| As one of the most excellent thermoelectric materials near room temperature, bismuth telluride based compounds are extensively used in the area of aerospace, medical appliance, microelectronic devices, biologic slug and so on. However, the thermoelectric performance of traditional Bi2Te3-based bulk materials has been stagnated for a long time. With the development of nanotechnologies, nanostructured thennoelectric materials with high figure of merits over unit are reported endlessly. Studies have shown that nanostructure can enhance the electronic density of states near Fermi level and nanostructured thermoelectric materials have a large quantity of boundaries that will strongly scatter the phonons and carries. Therefore the Seebeck coefficient is improved, the thermal conductivity reduced, and the thermoelectric properties improved. As a new kind of materials, graphene has high conductivity, high carrier mobility and excellent mechanical properties. So, it has been widely used to prepare various kinds of functional composites.In this thesis, graphene/Bi2Te3based thermoelectric materials were prepared by hydrotheramal synthesis method and spark plasma sintering (SPS). And their thermoelectric properties were studied. As a raw material, graphene was prepared by two sources. One was direct purchased, and the other was prepared in the lab. The X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM) were used to study the phase composition and microstructure of materials. Electrical resistivity, Seebeck coefficient and thennal conductivity measurement were applied to analysis the thennoelectric properties. The influences of Sb doped and graphene composited on the performance of thermoelectric materials were studied. The main research was as follows:The influence of hydrothennal synthesis conditions and graphene on phase, morphology, particle size of Bi2Te3nanopowders was studied. The results showed that raising reaction temperature was beneficial, but too high temperature would lead to the grain growth, which was unfavourable for nanometer size of the grain. Similarly, extending the holding time was also a key factor to promote grain growth. What’s more, the existence of chelating agent can reduce the reaction temperature and the reaction time. The existence of graphene inhibitted Bi2Te3grain growth and with the increase of the concentration of the graphene, the size of Bi2Te3particles in the composite powder decreased.Spark plasma sintering (SPS) technique was employed to fabricate bulk material. The results showed that as the content of graphene increased, the relative density of the sintered bulk samples decreased gradually. This was caused by the introduction of graphene, which influenced the sintering densification process of the Bi2Te3powder. In addition, by observing the microstructure of sintered sample, we found that as graphene increased, the size of sample also tended to get smaller. Compared with the hydrothermal-synthesized powder, grain size didn’t make obvious growth after sintering, which was relevant with the high heating speed and short sintering time of SPS technology.The thermoelectric properties of Bi2Te3/Graphene composites were studied. It was found that the composited powder composed with0.2v%graphene had the highest ZT value of0.21, under the475k, which was more than31%higher than that of the pure Bi2Te3powder.Ternary Bi2Te3based alloys have the lower thermal conductivities due to the stronger scattering to the short wave phonons, and the optimized band structure. In this paper, hydrothermal preparation was adopted to synthesize P-type Bio.5Sb1.5Te3ternary alloy, and its composition was characterized and analysed. Compared with pure Bi2Te3, it was found that after doping with Sb, the increase of Seebeck and the reduction of thermal conductivity made ZT value of Bio.5Sb1.5Te3higher than that of pure Bi2Te3. The influence of graphene on Bio.5Sb1.5Te3powder and the microstructure of the bulk material was similar to the influence on Bi2Te3. However, the thermal conductivity of Bio.5Sb1.5Te3/Graphene composite was much higher than that of pure Bi0.5Sb1.5Te3. Then lattice thermal conductivity and electronic thermal conductivity were analyzed. The result showed that lattice thermal conductivity exerted a greater influence on Bio.5Sb1.5Te3/Graphene composite. |
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