Preparation And Property Optimization Of Multi-dimensional Bi₂Te₃-based Thermoelectric Materials

As one of the most frequently used thermoelectric materials near room temperature, Bi2Te3-based compounds have been broadly applied in many fields, such as minicooler for micro-and opto-electronics, medical device, electronic consumables, 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, thermoelectric materials are facing new opportunities and rejuvenation. As pointed out by previous researchers, thermoelectric performance of nano-materials could be greatly improved owing to the strong quantum confinement effect. Based on a detailed review to the research status and developing trend of thermoelectric materials, the research topic of this thesis is focused on Bi2Te3-based multi-dimensional thermoelectric materials, the preparation techniques of multi-dimensional bismuth telluride based thermoelectric materials were explored and their thermoelectric performance was optimizedElectrochemical atomic epitaxy(ECALE), microwave assisted wet chemical synthsis (MAWCS), mechanical alloying plus hot pressing (MA-HP) and mechanical alloying plus plasma activated sintering (MA-PAS) were subjected to fabricate the nano-films, nano-particles and Bi2Te3 bulk materials respectively. FE-SEM, EDS, XRD and HRTEM were performed to characterize the morphology, composition and structure of Bi2Te3-based multi-dimensional materials respectively. Energy band gap and transport properties, such as Seebeck coefficient, electrical resistivity, charge carrier concentration, mobility and thermal conductivity, were measured to study the effect of processing conditions on thermoelectric performance of Bi2Te3-based compounds. Based on these results, thermoelectric performance of the materials was optimized.Electrochemical aspects of bismuth and selenium on different substrates and each other are carefully investigated. The optimal deposition conditions are determined for the ECALE process of Bi2Se3 on polycrystalline Pt substrate and single crystal Au substrate. A 400 ECALE-cycle and 200 ECALE-cycle Bi2Se3 thin film deposits were grown on polycrystalline Pt substrate and single crystal Au substrate respectively. The 400-cycle nanofilm on polycrystalline Pt substrate shows an orthorhombic structure and a smooth morphology. While a rhombohedral Bi2Se3 film, was obtained on the single-crystal Au substrate. The different structure of Bi2Se3 deposits should be ascribed to the crystal structure and surface situation of the substrates. Due to the quantum confinement effect, the band gap of the 200-ECALE-cycle Bi2Se3 film on Au substrate is blueshifted.Sb2Te3 and Sb2Se3 binary compounds were synthesized successively by the means of microwave assisted wet chemical method (MAWCS). Effect of synthesizing conditions on the composition of the reaction products was studied, and the reaction mechanisms of microwave assistant synthesis are analyzed. Hexagonal Sb2Te3 nano-plates with rhombohedral structure were synthesized by MAWCS; while Sb2Se3 compound with orthorhombic structure, which has a nanorod morphology, was obtained by MAWCS. The unique morphology of the compound is determined by the growing speed difference between different crystal planes.Bio.4Sb1.6Te3 and Bi2Te2.5Se0.5 ternary compounds were synthesized via MAWCS for the first time in this work. The strength of KOH and KBH4 plays a critical role in the synthesis of Bi0.4Sb1.6Te3 and Bi2Te2.5Se0.5. The morphology of Bi0.4Sb1.6Te3 powders was tuned from irregular nano-particles to nano-plates structure with addition of a certain amount of sodium dodecyl sulfonic (SDS) to the solution system; while Bi2Te2.5Seo.5 hexagonal nano-plates could be obtained without addition of any surfactant.The Bi0.4Sb1.6Te3 nano-plates synthesized by MAWCS are mixed with the as-MAed Bi0.4Sb1.6Te3 powders and then consolidated with hot press. Effect of the content of nano-plates on thermoelectric properties of the bulk (Bi,Sb)2Te3 alloys was investigated. When the doping content is 7.5wt.%, the total thermal conductivity and the phonon thermal conductivity of the sample reduced 7.2% and 9.8% respectively than those of the sample without addition of nano-plates. The maximum figure of merit was achieved as ZT=1.31 at room temperature with addition of 7.5wt.% nano-plates.P-type Bio.4Sb1.6Te3 bulk thermoelectric materials were fabricated by MA-PAS. Effects of processing conditions of PAS were investigated. Thermoelectric performance parallel to the pressure direction outweighs that of perpendicular to the pressure direction of the samples. The dependence of thermoelectric performance on sintering temperature and current were researched, and the maximal Seebeck coefficient of 244.8μV·K-1was obtained at 323K in the sample sintered at 653K, and it also has the maximum power factor of 5.7×10-3Wm-1K-2. The maximum ZT of 1.42(T=323K) was achieved in the sample sintered at 653K.TEM observation shows that there are twin crystals and layered stripes within the grains of the samples prepared by MA-PAS, and further HRTEM observation reveals confirmed crystal lattice distortion and crystal defects exist around those stripes. The results of TEM and HRTEM observation demonstrate the layered stripes are caused by diffractioin contrast due to dramatic crystal lattice distortion and lattice defects. Crystal lattice distortion, crystal defects within the grains, together with nano non-crystal areas greatly depress crystal lattice thermal conduction, which results in decrease of thermal conductivity and improvement of thermoelectric performance.