Preparation And Properties Optimization Of (Bi,Sb)₂(Te,Se)₃ Based Thermoelectric Materials

As one of the most excellent thermoelectric materials near room temperature, bismuth telluride based compounds are extensively used as Peltier cooler for microelectronic devices, photoelectronic devices, biologic slug, medical appliance, military equipment and etc. Currently, they are usually prepared by unidirectional crystal growth methods. Although the resulting single crystal materials present excellent thermoelectric properties along the growth direction, the Te(1)–Te(1) layers bonded with weak Van der Waals force,which aligns orientationally in the single crystal materials, makes them more fragile and difficult for machining. Therefore, the isotropic mechanically strong materials with good thermoelectric properties are more preferable. Powder metallurgical methods, which produce randomly oriented polycrystalline and fine microstructure, and thus good mechanical properties for bismuth telluride based materials, are extensively studied.p-type and n-type Bi₂Te₃ based thermoelectric materials were prepared by mechanical alloying (MA) and different sintering processes, including hot pressing (HP), plasma activated sintering (PAS) and equal channel angular extrusion (ECAE) in the present work. The X-ray diffraction (XRD), differential thermal analysis (DTA), field emission scanning electron microscope (FE-SEM), energy dispersive X-ray spectroscopy (EDS), transmission electron microscope (TEM), infrared spectroscopy (FTIR), positron annihilation technology (PAT) , electrical resistivity, Seebeck coefficient, carrier concentration, mobility and thermal conductivity measurement were applied to analysis the microstructure and thermoelectric properties. Effects of sintering process parameters, doping on the microstructure and thermoelectric properties of Bi₂Te₃ based alloys were investigated.Based on a detailed review on the research and development status for Bi₂Te₃ based thermoelectric materials, the research goal, task and content of this work were then put forward. The effect of MA process parameters on the alloying process of Bi₂Te₃ based alloys was investigated and the MA process parameters were optimized. The effects of sintering time, sintering temperature on the microstructure and thermoelectric properties of p-type Bi_0.5Sb_1.5Te₃ and n-type Bi₂Te_2.85Se_0.15 alloys were researched. A preferentially orientated microstructure with the basal planes (0 0 Z) perpendicular to the pressing direction was formed in the sintering samples. The maximum figures of merit (Z) of the as-PASed p-type Bi_0.5Sb_1.5Te₃ and n-type Bi₂Te_2.85Se_0.15 alloys at room temperature were 2.85×10-3K-1 and 1.8×10-3K-1, and the maximum figures of merit (Z) of the as-HPed p-type Bi_0.5Sb_1.5Te₃ and n-type Bi₂Te_2.85Se_0.15 alloys at room temperature were 2.84×10^-3K^-1 and 1.4×10^-3K^-1, respectively.The effects of nominal Sb₂Te₃ concentration and Te doping on the microstructure and thermoelectric properties of p-type (Bi₂Te₃)1-x(Sb₂Te₃)x alloys were investigated. For the first time, both twin crystals and spinodal decomposition were observed in the HPed and PASed Bi₂Te₃ based alloys, which enhanced the phonon scattering contribution of crystal defects and thus reduced the crystal thermal conductivity. The positron annihilation spectroscopy was firstly applied to analyze the concentration of deffect of the HPed, PASed polycrystal materials and the single crystal Bi_0.4Sb_1.6Te₃. The vacancy concentration of the as-PASed alloy was lower than that of the as-HPed alloy but higher than that of the single crystal. FTIR analysis showed that the band gap decreased from 0.26eV to 0.23eV with increasing the nominal concentration of Sb₂Te₃. The maximum figure of merit (Z) of the as-PASed Bi_0.4Sb_1.6Te₃ alloys reached 5.26×10-3K-1 at 300 K. To our knowledge, this was also the highest figure of merit that has ever been reported for p-type Bi₂Te₃ based bulk materials. Te doping deteriorated the thermoelectric properties of Bi_0.4Sb_1.6Te₃, and the figure of merit (Z) decreased from 5.26×10-3K-1 of the undoping alloys to 4.44×10-3K-1 with 8wt.% Te dopant.The effects of the concentration of Se, Sb and AgI, CuBr2 dopants on the thermoelectric properties of n-type Bi₂Te₃ based alloys were also investigated. A maximum figure of merit (Z) of the n-type alloys Bi₂Te_2.85Se_0.15 with 0.2wt.% AgI dopant was obtained as 1.86×10-3K-1.By the ECAE process, an obvious preferentially orientated microstructure with the basal planes (0 0 l) in parallel to the extrusion direction was formed in the p-type Bi_0.4Sb_1.6Te₃ and n-type Bi₂Te_2.85Se_0.15 alloys. The maximum orientation factor of the p-type and n-type alloys reached 0.36 and 0.28. The formation of preferentially orientated microstructure resulted in the thermoelectric properties along the parallel direction to the extrusion was more optimized than that of the perpendicular direction to the extrusion. The maximum dimensionless figures of merit (ZT) of p-type Bi_0.4Sb_1.6Te₃ and n-type Bi₂Te_2.85Se_0.15 alloys reached 0.979 and 0.66 at 343K, respectively.The nano-sheet and ellipsoid shaped nano-powders Bi₂Te₃ were prepared by microwave assistant wet-chemical technique and arc-plasma deposition technique respectively and then they were added into the as-MAed powders for consolidation. The effects of the concentration of nano-sheet and ellipsoid shaped nano-powders Bi₂Te₃ on the thermoelectric properties of bulk Bi₂Te₃ were investigated. When the doping concentration was 15wt.%, the crystal thermal conductivity reduced 20% than the undoping alloys.