Preparation, Structure And Properties Of P-type Bi_0.5Sb_1.5Te₃ Nano Thermoelectric Material

Bismuth Telluride and its alloys are one of the most important thermoelectric materials found to date. They are used in state-of-the-art devices for 200～400K, such as thermoelectric refrigeration and thermopiles. Bi₂Te₃ alloys, which are prepared by traditional zone melting method, have got the highest dimensionless figure of merit (ZT) close to 0.8～1. Both theoretical calculation and experimental results show that the ZT value of thermoelectric materials can be significantly increased if the materials are nanostructured. Therefore, for widely commercial application, new fabrication methods should be explored and developed in order to get high performance Bismuth Telluride based thermoelectric materials with nanostructure.Bio.5Sb1.5Te3 bulk materials with fine nanostructure have been prepared by combining melt spinning technique with spark plasma sintering (SPS). In order to investigate how microstructure affects the thermoelectric properties of bulk materials, the microstructure of ribbons and bulk materials are observed by Field Emitted Scanning Electron Microscopy (FESEM) and High-resolution Transmission Electron Microscopy (HRTEM), and the thermoelectric properties of bulk materials are measured.Owning to the variance of the degree of supercooling, the morphology of contact and free surface of ribbons obtain by melt spinning are significantly different: in the contact surface, arborescent crystal interlaces with each other and the loose structure is formed; in the free surface, the interstices between grains are not clear. The fine fluctuation of composition between contact and free surface can be found by energy-dispersive X-ray Spectroscopy (EDS) analysis. HRTEM observations show that the sizes of nanocrystalline are 5～15 nm in the ribbons prepared by melt spinning with the liner speed of 10 m/s, and there are amorphous Te around the nanocrystalline. Compared with zone melting bulk material (ZM-INGOT), the texture phenomenon of SPS samples (SPS-10, SPS-20 and SPS-30) are not obvious, and their grains are random distribution. FESEM observations show the sizes of layered structure in the SPS samples are about 20-50 nm which are far smaller than that of ZM-INGOT. The fine nanostructure with the size of 5～10 nm, defects and distortion of lattice can be observed by HRTEM in the SPS-10 sample.The electrical conductivity of SPS samples is lower than that of ZM-INGOT, and the Seebeck coefficient of SPS samples is larger than that of ZM-INGOT. With the increase of cooling rate during the melt spinning process, the thermal conductivity of SPS samples gradually decrease. Due to the nanostructure and defects induced by the melt spinning intensify the phonon scattering in the lattice, the lattice thermal conductivity of SPS samples decrease remarkably. The greastest ZT value reaches 1.20 at about 300 K. Compared with the ZM-INGOT, it increases by 70% at the same temperature.