| Undoped Bi-Sb alloy is one of the best semiconductor refrigerating materials in the range of10～250K due to its excellent electrical properties, large Seebeck coefficient and carrier mobility, which would have been extensively applied in aeronautics, computer chips, infrared detector, superconductivity and civil field. At present, in order to enhance the thermoelectric properties of polycrystalline materials, many preparation methods have been explored, such as melting-quenching followed by long time diffusion annealing, mechanical alloying combined with subsequent densifying process (such as hot pressing and SPS), electrochemical deposition, solvothermal method etc. However, the complex and prolonged preparation procedures coupled with the very poor mechanical property and poor thermoelectric property largely limit its commercial application.In this thesis, we concentrate on the n-type Bi-Sb alloy and explore two novel and efficient preparation methods:melt spinning (MS) combined with spark plasma sintering (SPS) method and low temperature ultrasonic wet chemical method. The effects of processing parameters and reaction conditions on the phase composition, microstructure, the homogeneity of the composition and low temperature thermoelectric properties of Bi1-xSbx have been systematically investigated. The main results are listed as follows:(1) The bulk Bi1-xSbx alloys were synthesized by MS-SPS method within3hours. The effects of different Sb content and cooling rate in MS process on the phase composition, microstructure, band structure, the homogeneity of the composition and low temperature thermoelectric properties (10～300K) of bulk Bi1-xSbx have been discussed and the results are shown below:The low temperature thermoelectric performance test results showed that with the increase of Sb content, the electrical properties and thermal conductivities decreased, the Seebeck coefficient of all the MS-SPS samples varied complicated, reaching peaks at the temperature range of50～100K. The MS-SPS sample Bi91Sb9obtains the highest power factor～5.0×10-3W-m-K-2at150K because of its highest electrical conductivity. In all the MS-SPS samples, Bi85Sb15exhibits a highest Z value which is mainly benefited from its relatively higher Seebeck coefficient and lower thermal conductivity. With the increasing cooling rate, the grains become finer and the distributions of the elements are more homogenous, which have a great effect on the thermoelectric transport properties. As a result, the sample Bi85Sb15with the highest cooling rate shows a peak Z of1.9×lO-3K-1at120K. This value is much lower than the Z of reported single crystals or long time annealed ingots, but comparable to most of the reported values of polycrystalline samples. Additionally, the rapid and energy-saving MS-SPS technique is very favorable for commercial application of Bi-Sb alloys.(2) The Bi1-xSbx alloy nanopowders were synthesized by ultrasonic chemical method. The effects of different Sb content, reaction time, reaction temperature, reductant type and content, solvent, pH value, complexing agent and surfactant on the phase composition, microstructure and the homogeneity of the composition have been investigated and the results are as follows:The uniform spherical Bi1-xSbx (x=0～0.22) nanopowders with diameter of15nm were prepared by low temperature ultrasonic chemistry method at60℃for1h under the conditions of distilled water as solvent, EDTANa2as complexing agent, AOT or PVP as surfactant. Furthermore, in order to improve the dispersity of the samples, the reaction was done under the stirring condition and the uniform spherical Bi1-xSbx (x=0～0.22) nanopowders with diameter20nm were prepared by ultrasonic assisted chemistry method at80℃for5h under the conditions of distilled water as solvent, citric acid as complexing agent, AOT or PVP as surfactant. |
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