Sonochemical Synthesis Of Nanostructured Bi₂Te₃ Based Thermoelectric Compounds

Bi₂Te₃ and its alloys, which have the highest dimensionless figure of merit close to unity, are known as the best thermoelectric (TE) materials currently available near room temperature. The figure of merit of TE materials could be significantly improved if the materials were nanostructured, such as nanowires and nanotubes, since the thermal conductivity could be decreased more significantly than the electrical conductivity of the materials.In the present work, various Bi₂Te₃ based nanopowders have been sonochemically synthesized from the precursors of BiCl₃ and Te powder or telluride via various chemical routes. The structures and morphologies of the nanopowders were investigated by XRD and TEM. Chemical reactions, nucleation and crystal growth mechanisms 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 properties of the hot-pressed samples were measured. Some important results of the present work are listed as fellows.1. Bi₂Te₃ based nanopowders about 20-30 nm had been prepared by sonochemical synthesis using BiCl₃ and Te powder as the precursors, distilled water as the solvent, NaBH₄ as the reductant, NaOH as the pH-value controller. The effect of the reductant, time and additive during sonochemical synthesis on the particle sizes and compositions of Bi₂Te₃ nanopowders had been investigated. It was found that the single phase Bi₂Te₃ nanopowders could be sonochemically synthesised at 250W for 16h. The addition of EDTA had a weak effect on the particle sizes, but significantly prolonged the formation of single phase Bi₂Te₃ nanopowders.2. Two possible nucleation and growth mechanisms of Bi₂Te₃ nanopowders were proposed during the process of sonochemical synthesis. One is that the Bi₂Te₃ molecules are formed by the chemical reaction between Bi and Te atoms (or ions) unite and grow into steady Bi₂Te₃ nucleus. The other is the insertion of Bi atoms into the layer of Te crystal to form the Bi₂Te₃ nucleus. The growth rate of Bi₂Te₃ crystal along the c axis was slowerr than that along the base plane because of the anisotropic lattice structure, and it is easier to form polyhedral particles.3. The tellurium source effect on the formation of Bi₂Te₃ nanotube had been investigated during sonochemical synthesis, with the power of 150 W and reaction time of 36h. The analyses showed that all the used tellurium source can synthesize single phase Bi₂Te₃ nanopowders, but had a visible effect on the morphology of the powders. Bi₂Te₃ nanotubes of 20 nm in diameter, 200nm in length and 5 nm in wall thickness could be obtained with Na₂TeO₃ as the reactants, while the powders using Te as the reactant source consisted of 20³0 nm granules.4. The single phase Bi₂Te₃ nanopowders could be obtained with EDTA and SDBS added respectively, using BiCl₃ and Na₂TeO₃ as the precursors, distilled water as the solvent, NaBH₄ as the reductant, NaOH as the pH-value controller. The powders consisted of nanoparticals ofabout 10-30 nm. The nanotubes with the diameter of 20-90 nm, length of 100-300 nm and wall thickness of 5-10 nm could be synthesized with EDTA added.5. The differences between sonochemical methods and low-temperature aqueous chemical methods for synthesizing Bi2Te3 nanopowders were analyzed and compared. The similitudes are that these two methods have simple equipments, easy to operate and have mild reactive enviroment. The differences are that the synthesized powders consist of nanotubes by sonochemical methods and pearl chain granular by low temperature aqueous chemical methods, when the proportion of the raw materials is the same.6. The bulk samples were obtained from the sonochemically synthesized powders by hot-pressing. Thermoelectric property measurement shows they were n-type conductive. The Seebeck coefficients decreased with temperature with the maximal value of 122 jaV / K at room temperature. The electrical conductivies goes up to the maximum value of 1700 / Q. m at 220°C. The thermal conductivity near room temperature for the nanocomposites samples with the addition of 5% and 10% sonosynthesized nanopowders had the minimum values of 0.66 and 0.55 W/mK respectively.