Low Temperature Aqueous Synthesis And Characterization Of Nanostructured Bi₂Te₃ Based Thermoelectric Materials

Thermoelectric (TE) materials are a kind of semiconductor functional materials, which can be used to convert heat energy directly into 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 can be significantly improved if the materials are nanostructured, since the thermal conductivity can be decreased more significantly than the electric conductivity of the materials.In the present work, a simple low temperature aqueous chemical route has been applied to synthesize nanostructured Bi₂Te₃ thermoelectric materials. 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 discussed in detail. 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. Bi₂Te₃ nanoparticles have been successfully synthesized by a low temperature aqueous chemical route in an open system at 50 60 ℃. Chemical and physical reactions during the synthesis were investigated insitu by pH measurement, color observation of the solution and X-ray diffraction analysis of the powders after series of parallel experiments. It was found that Bi₂Te₃ could be synthesized only in a strong alkaline solution.2. For the first time, Bi₂Te₃ nanocapsules with hollow structure have been successfully synthesized with the addition of EDTA. The nanocapsules are 50 200 nm in diameter and 100 800 nm in length. The tube walls have a thickness of about 6 nm with the normal parallel to the c-axis of Bi₂Te₃ crystal lattice. EDTA can act as a soft-template to facilitate the formation of Bi₂Te₃ nanocapsules in the alkaline solution.3. Bi₂Te₃ nanowires have been successfully synthesized with the addition of SDBS. The nanowires are 50 100 nm in diameter and 0.5 2 μm in length along the <001> direction of Bi₂Te₃ crystal lattice. The growth mechanism is related to the solid-solution-solid transformation of Te and the adsorption of SDBS. The steric effect could keep the Bi₂Te₃ nuclei from growing larger.4. Tiny particles of Bi2Te3 with the sizes of about 10 30 nm in diameter as well as nanotubes with about 50 nm in diameter could also be prepared under the similar process as well as the acidic preparation using Na2TeC>3 as the reactant. Other organic surfactants were used to control the sizes and the shapes of the products.