Preparation And Sintering Properties Of Nano-sized Bi₂Te₃ Materials

Thermoelectric (TE) materials are a kind of semiconductor functional materials, which can be used to convert heat energy directly into electricity or reversely. Bi2Te3-based compounds exhibit the best thermoelectric performance around room temperature and have been widely used for cooling and temperature control in various areas. The figure of merit of TE materials can be significantly improved if the materials are created in nanostructured form, since the thermal conductivity can be decreased more significantly than the decrease of the electric conductivity of the materials.In this work, a novel mechanical cryogenic grinding method was employed to prepare the nano-sized Bi2Te3 powder. Compared with the traditional mechanical ball mill, the ultra-low temperature characteristic and the advantages of simple processes made cryogenic grinding easily carry out to preparation of ultrafine uniform nanopowder. In order to investigate microstructure evolution mechanism of Bi2Te3 nanoparticles during cryogenic grinding process, particle morphology and microstructure observation were performed by X-ray Diffraction (XRD) and Transmission Electron Microscopy (TEM). Then nano-sized Bi2Te3 powders were sintered by Spark Plasma Sintering (SPS) apparatus. Morphology and microstructure observation were characterized by a Field Emission Scanning Electron Microscopy (FE-SEM), and the thermal properties of bulk materials were measured. The main results of the present work are listed as follows:1. The nano-sized Bi2Te3 particles of 10～15nm were prepared using N-type Bi2Te3 zone-melted ingots as starting materials after 15h cryogenic grinding, and the cryogenic grinding mechanism of Bi2Te3 particles were discussed. Then according to the Griffith breaking theory (GBT), the breaking model of Bi2Te3 particles during the cryogenic grinding process was set up2. The sintering behaviors of the as-prepared nano-sized powders under different experimental procedures are very different. The results indicated that the holding temperature of 360℃was more conducive to the sintering of nano-sized Bi2Te3 particles than that of 390℃. The further FE-SEM micro-morphology showed that:low temperature sintering can not obtain dense bulk materials; the grain morphology changes at high sintering temperature were detrimental to the thermoelectric performance; the matching micro/nano structure of as-sintered samples could be textured at a sintering temperature between 330℃and 360℃, which introduced an acoustically mismatched second phase to reduce the lattice thermal conductivity and enhance the thermoelectric performance.3. The thermoelectric performance of the Bi2Te3 bulk nanocrystalline material at different holding temperature was measured. It is found that the lattice thermal conductivity of samples sintered at 330℃and 360℃decrease dramatically due to numerous boundaries or interfaces scattering, respectively, reaching to 0.245 W/mK and 0.264 W/mK, which is closed to the theoretical minimum lattice thermal conductivity.