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Syntheses And Properties Of Nanostructured Bi2Te3 Based Thermoelectric Materials

Posted on:2006-02-17Degree:DoctorType:Dissertation
Country:ChinaCandidate:X H JiFull Text:PDF
GTID:1101360152971725Subject:Materials Physics and Chemistry
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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 having 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 decrease of the electric conductivity of the materials.In the present work, novel chemical routes such as solvothermal synthesis, hydrothermal synthesis and low temperature aqueous chemical synthesis have been developed to prepare Bi2Te3 based nanopowders. Various hollow shell morphologies of Bi2Te3 including nanotubes and nanocapsules have been obtained and rear-earths (RE) have been alloyed in Bi2Te3 based compounds. The structures and morphologies of the nanopowders were investigated by XRD, SEM and TEM observations. Chemical reactions, nucleation and crystal growth mechanisms during the synthesis of Bi2Te3 have been experimentally investigated and detailedly discussed. Bulk TE materials and nanocomposites have been prepared by vacuum hot pressing. The TE transport properties of hot pressed samples have been measured.The most important result of the present work is the synthesis of Bi2Te3 nanotubes and nanocapsules, as the first report of Bi2Te3 compounds with hollow shell nanostructures. The nanotubes synthesized at 150℃ have diameters between 25 and 100 nm and lengths up to more than a few micrometres. It was found that the tube walls are spiral and titled with the (0 0 3) planes of the Bi2Te3 rhombohedral lattice are at an angle of about 20° to the tube axis. Irregular hollow nanocapsules synthesized by a low-temperature aqueous chemical method are about 10 50 nm in sizes.The second important result of the present work is the suggestion and preparation of advanced Bi2Te3 based nanocomposites with high TE properties. We proposed for the first time the concept of coessential nanocomposites using commercial Bi2Te3 based alloys as the matrix and Bi2Te3 nanotubes as the additive, so that the thermal conducti vities of TE materials could be reduced without the deterioration of the electric properties. A very low thermal conductivity of about 0.8 Wm -K has been obtained for the hot pressed bulk nanocomposites. The highest dimensionless figure of merit reaches 1.25 at about 420K, which was also one of the highest values ever reported for bulk Bi2Te3 based TE materials.RE contained Bi2Te3 TE nanopowders have been prepared via solvothermal synthesis using chlorides or oxides as the RE precursors. The nanopowders were mainly irregular polyhedral particles with diameters of less than 30 nm. It was shown that the hot pressed RE-Bi2Te3 materials have a single-phase structure with the same lattice type and similar lattice constants of binary Bi2Te3, indicating that the RE has been alloyed in Bi2Te3 compounds. It was found that all the RE-Bi2Te3 samples are n-type, suggesting RE elementsare donors in the based semiconductors. The thermal conductivity measurements showed that alloying RE with Bi2Te3 could decrease the thermal conductivities of the Bi2Te3 based compounds. A maximal dimensionless figure of merit of 0.22 has been obtained for the sample Ce-Bi2Te3 at about 45 OK.Various organic solvents, such as ethylenediamine, N,N-dimethylformamide (DMF), pyridine, acetone, ethanol, and distilled water have been used as the reaction medium for the synthesis of Bi2Te3 nanopowders. The effects of the chemical and physical characteristics of the solvents on the formation and morphology of the nanopowders have been discussed. It was found that distilled water is the most appropriate solvent for the synthesis of pure single-phased Bi2Te3, due to the high dielectric constant, high surface tension and th...
Keywords/Search Tags:thermoelectric materials, Bi2Te3, nanotubes, nanocapsules, nanostructures, nucleation and growth mechanism, coessential nanocomposites, solvothermal synthesis, hydrothermal synthesis, low-temperature aqueously chemical synthesis, hot-pressing
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