Synthesis Of Bi₂Te₃ And Bi₂Te₃/ MWNT Through Chemical Methods And Study On Their Thermoelectric Properties

Bismuth telluride (Bi2Te3) nanostructured bulk thermoelectric (TE) materials, which own high figure of merit and have large-scale commercial application due to simple synthesis process, are the excellent candidates of the future TE materials. Chemical methods are used to prepare Bi2Te3 nanoparticles, and they have following advantages:The diverse microstructures make it more easy to find TE materials with high property. Secondly, the Seebeck coefficient, electrical conductivity and thermal conductivity could be quantized when the particle size is approximate to the crystal lattice. Thirdly, the thermal conductivity will reduce because abundant grain boundary could scatter the long-wave phones. But the products usually have different morphology and wide size distribution. This will make the chemical and physical properties of grain boundary unstable, and will badly influence the TE properties.In this thesis, Bi2Te3 nanoparticles was synthesized by both hydrothermal and microwave-polyol (MP) methods, and Bi2Te3/MWNT (multiwalled carbon nanotubes) nanocomposites was yielded through two different progresses. The major factor, which influenced the uniform morphology of Bi2Te3 in hydrothermal method, was studied. The chemical reaction mechanism of Bi2Te3 synthesized by MP process was revealed. The important results are shown as following:Through a systemic experiments, it is found that pH value is the key factor which controls the uniform morphology of Bi2Te3. Bi2Te3 nanoplates with width of～10 nm are yield in the absence of NaOH, which then form "flower-like" heterostructures self-assembly. Bi2Te3 nanocrystals, nanorods and nanoplates appear when NaOH is added. When the adding sequence of Te, BiCl3, ethylene diamine tetraacetic acid (EDTA) NaBH4 and NaOH was changed, little difference in morphology and substance. The reaction mechanism of Bi2Te3 by hydrothermal method concludes:Firstly, Te powder is reduced into Te nanoparticles by NaBH4. Secondly, bismuthide is reduced into Bi. Finally, Bi and Te react to form Bi2Te3.Synthesis of Bi2Te3 is conducted under different temperature and reaction time, with different concentration of NaOH. It is found that abundant NaOH could accelerate the reaction speed of Bi2Te3. The standard electrode potential of equation 3Te+6OH-(?)2Te2-+TeO32-+3H2O is negative, which means that this chemical reaction could not take place from left to right, except the pH is very high, according to Nernst equation. The Te nanoparticles obtained improve the reaction rate of Bi2Te3.Bi2Te3/MWNT nanocomposites are firstly prepared through MP process aid with addition. Bi(NO3)3·5H2O and Na2TeO3 solution, of which the ratio is 2:3, are added into the mixture of MWNTs and ethylene glycol when 150℃. The adding rate is～1 mL/min. After reacting for 40 min under 170℃, the Bi2Te3/MWNT could be yielded. The major morphology is that～5 nm Bi2Te3 nanoplates and part of their aggregates attach onto surfaces of MWNTs. Bi2Te3 nanoparticles enter into the inner of relatively slender MWNTs.Two steps method is adopted to synthesize MWNT supported Bi2Te3 nanospheres by MP process. Firstly, Bi(NO3)3·5H2O and Na2TeO3 solution, of which the ratio is 2:3, are added into the mixture of MWNTs, ethylene glycol and water when 155℃, and react for 30 min. The intermediate product is oxide containing Bi and Te elements. Secondly, use NaBH4 to reduce this oxide. After 30 min, the Bi2Te3 nanospheres are obtained.