Preparation And Performances Of Bi₂Te₃ Based Thermoelectric Material Obtained By Hot Pressing

Thermoelectric material is a kind of function materials which can convert electricity into heat and have been widely used for cooling and temperature control in virous areas. Bismuth telluride based materials have a rhombohedral structure. However, the weak Van der Waals bonding between Te-Te layers causes difficulties in fabrication process and potential problems in the reliability for the applications. The materials also have excellent thermoelectric and mechanical properties, which for the current thermoelectric materials research and development of related devices are of great significance. It is of great significance to investigate two aspects of research and composition optimization, the following research work has been carried out in this paper.1. The ultrarapid quenching process of P-and N-type bismuth telluride based alloys has been researched. The thickness of the ultrarapid quenching foils are 5-20μm. The morphology of contact and free surface of obtained foils are significantly different:in the free surface, the arborescent crystal interlaces with each other and have some lacuna; the contact face is smooth. The fluctuation of composition between contact and free surface can be found by energy-dispersive X-ray spectroscopy(EDS) analysis.2. The ultrarapid quenching foils were grinded and sifted and powder was obtained. The bulk materials with good thermoelectric were prepared by hot presing. In this paper, the initial powder size of the milling process and the temperature, pressure and time of hot pressing on the P-type bismuth telluride based thermoelectric materials were researched. According to the overall thermal performance, they were determined that 44-74μm particle size for the best initial,440℃,50MPa and 40min for the best hot pressing process.Metallographic photos and cross-section shape of the bulk materials show that the particle orientations of the bulk materials obtained by ultrarapid quenching with hot pressing are higher than the ordinary sintering's. Compared with the materials of the ordinary sintering, the grain boundary introduced has significant effect on scattering phonons for the sintered materials, which can greatly decrease the thermal conductivity. It is believed that such a high mechanical strength should provide good ability to sustain the cutting load and improve the reliabilty for the applications especially in miniature thermoelectric modules.3. The thermoelectric properties of excess Te for P-type Bi0.5Sb1.5Te3 and excess Se for N-type Bi2Te2.7Se0.3 materials had been studied in this paper. The matrix of P-type and N-type materials were the solid solutions of Bi2Te3, Sb2Te3 and Bi2Te3, Bi2Se3, and the excess Te and Se were used as dopant.For P-type thermoelectric materials, with the excess Te complement, the first role of excess Te is to compensate the volatile elements due to the lack of Te. With the increase of excess Te content, electrical conductivity increases and then showed a trend of decrease, the Seebeck coefficient showed a trend of decreases and then increases, which corresponds to 2wt% excess Te content have a maximum power factor of about 3.14×10-3 W/mK2. For the N-type Bi2Te3 Department of materials, with the increase of the excess Se content, electrical conductivity at first decreases and then increases the trend, the Seebeck coefficient at first increases and then decreases, corresponding to the Se content in excess of 0.8wt% with a maximum power factor about 3.23×10-3W/mK2.