Solvothermal Synthesis Of Nanostructured Rare-earth Substituting Bi₂Te₃ Based Thermoelectric Materials

Thermoelectric (TE) materials are a kind of semiconductor functional materials, which can be used to convert directly heat energy to electricity or reversely. They are of interest for applications in TE cooling devices and power generators. Bi₂Te₃ and its alloys, which have the highest dimensionless figure of merit close to unity, are known as the best TE materials currently available near room temperature. The figure of merit of TE materials could be significantly improved if the materials were nanostructured, since the thermal conductivity could be decreased more significantly than the electric conductivity of the materials.In the present work, various Bi₂Te₃ based nanopowders have been solvothermally and hydrothermally synthesized from the precursors LaCl₃, BiCl₃ and Te powder or telluride via various chemical routes. The structures and morphologies of the nanopowders were investigated by XRD, SEM and TEM. Chemical reactions, nucleation and crystal growth mechanism during the synthesis of Bi₂Te₃ have been experimentally investigated and detailedly discussed. The bulk materials of Bi₂Te₃ based nanopowders have been prepared by vacuum hot pressing. The TE transport properties of hot pressed samples have been measured. Some important results of the present work are listed as fellows.1. Rare-earth element contained thermoelectric La_xBi_2-xTe₃ (x ≤1) nano-sized powders about 30nm have been prepared by solvothermal synthesis using ethanol as the solvent. The temperature and time effect of solvothermal synthesis on the particle sizes and compositions of La_xBi_2-xTe₃ nanopowders has been investigated. It was found that the ternary compounds La_xBi_2-xTe₃ have the same crystal structure and similar lattice parameters as those of binary Bi₂Te₃. The lanthanum contents in La_xBi_2-xTe₃ increase with the synthesis temperature and are independent of the reaction time. This indicates that La atoms would be seated at the substitutional positions of Bi in the Bi₂Te₃ crystal cells.The structures and morphologies of the powerd are related to the layered crystal structure of Bi₂Te₃ based compounds and the temperature effect on the ionic diffusion in the crystal growth. The powders synthesized at low temperatures have an irregular polyhedral morphology, while those synthesized at high temperatures tend to form thin sheets and nanotubes.2. Nanostructured monophase La_xBi_2-xSe_yTe_3-y alloys have been synthesized with a hydrothermal route, NaOH and ethylenediaminetetraacetic disodium salt (EDTA) as the additives. The NaOH and EDTA effects on the particle sizes and compositions of La_xBi_2-xSe_yTe_3-y, nanopowders during the hydrothermal synthesis have been investigated. It was found that an alkaline additive is necessary for the synthesis of a monophase Bi₂Te₃ based alloys. EDTA acts as a soft template for the lateral growth of the nano-sheets and the self-assembly of the petal-like structure by the parallel side-by-side arrangement of the nano-sheets. It was found, however, that the synthesis reaction could be hindered by excessive EDTA.3. La contained Bi₂Te₃ TE ternary and quarternary compounds have been prepared via the hydrothermal synthesis using BiCl₃, LaCl₃, tellurium and selenium powders as the precursors. The crystal structures and TE properties of Bi₂Te₃ alloy compounds have beenstudied. XRD Rietveld refinement results indicated that the ternary and quarternary compounds have the same crystal structure and similar lattice parameters as those of binary Bi2Te3. Se and La atoms would respectively be seated at substitutional positions of Te (3a and 6c positions) and Bi (6c position) in Bi2Te3 crystal cells. It was found that both ternary and quarternary Bi2Te3 compounds are n-type, suggesting La elements are donors in the semiconductors. The quarternary Bi2Te3 compounds exhibit higher electrical conductivities and Seebeck coefficients than the ternary Bi2Te3 compounds, but its high electrical conductivities and relative density result in the high thermal conductivities. The maximum dimensionless figures of merit of 0.58 and 0.51 have been obtained for ternary and quarternary Bi2Te3 compounds - samples at about 450K.4. Nanostructured thermoelectric Bi2Te3 powders with various morphologies were hydrothermally synthesized in the temperature range between 120°C and 250°C, or using different routes and precursors. The effects of the different conditions of hydrothermal synthesis on the polyhedral morphology and composition of Bi2Te3 nanopowders have been investigated. It was found that the sizes of the polyhedral particles increase with the increase in the synthesis temperature due to particle coarsening, but little growth could be observed for the nanorods or nanosheets, suggesting that the latter originate from the fragments fallen from the tellurium precursor. The formation of various morphologies of Bi2Te3 nanopowders was discussed. Various nucleation mechanisms, including molecule growth model, continuous nucleation model and nucleus saturation model, have been proposed for the different synthesis conditions. It was experimentally found that lateral growth, including "surface-nucleation lateral growth", "spirally lateral growth" and "twin-crystal lateral growth", dominates the Bi2Te3 crystal growth during the hydrothermal synthesis due to the anisotropic lattice structure.