| Thermoelectric(TE) materials are materials which can convert electricity into heat or heat into electricity directly. The TE materials are environment-friendly materials,having superior properties and widely application prospect. Facing three world crises:energy shortage, environmental pollution and ecological damage, the demand for new materials is rapidly increasing, so the study of new materials have attracted more attention. Thermoelectric devices made by thermoelectric materials mainly have two application fields: TE cooling devices and power generator. The efficiency of TE materials can be judged by their dimensionless figure of merit, which is defined by2 ZT ?S T/(??), where S 、 ρ、 ? and T are the Seebeck coefficient, electrical resistivity, thermal conductivity, and absolute temperature respectively. Therefore, we can enhance the ZT value of a TE material by decreasing its thermal conductivity and increasing its power factor(2PF=? ?).So far, among various TE materials, the Bi2Te3 based compounds and its solid solution alloy(include both n and p type) are not only researched earliest, but also the most promising TE materials at room temperature. Bi2Te3 has been obtained wide applications; many refrigeration components use this kind of materials. However, it has not been applied in large-scale because its ZT value is maintained around 1 for a long time. To realize the large-scale application in business, further improving the thermoelectric properties of the Bi2Te3 based TE materials are the research focus at present. In recent years, the main means to improve the TE properties of TE materials is using energy-band engineering and nanotechnology.This thesis combine nanotechnology with element doping and study the effect of Tl and Tl、I doping on thermoelectric properties of Bi2Te3-based alloys. The main study contents and results are as follows:(1) Thallium-doped nanopowders were prepared by the hydrothermal method.The doping effect of the thallium on the micro-morphologies of the synthesized nanopowders was investigated. It was found that the doping of the thallium can significantly change the morphologies of the synthesized nanopowders. When thesynthesized nanopowders with flower-like morphologies were hot-pressed into bulk samples, its thermoelectric properties is also changed greatly. As compared with the un-doped sample, Seebeck coefficient of the doped samples is enhanced but its electrical conductivity is decreased. The power factors of the thallium-doped samples decrease with the increasing of the thallium doping amount, so the ZT value of the doped samples was not improved much. This is attributed to the increase of the electrical resistivity and the disappearing of the flower-like morphologies of the doped nanopowders.(2) The effects of Tl and I dual-doping on the thermoelectric properties as well on the morphologies of the doped nanopowders were investigated. The preparation methods of Tl and I dual-doped flower-like nanopowders are the same as Tl doped. It was found that the amount of EDTA surfactant will have remarkable effects on the powders morphology and the doping is harmful for the formation of the flower-like nanopowders. The main reason is that the replacing Bi by Tl and the replacing Te by I can change the bonds strength and slow the growth rate along a-axis, b-axis, so it’s harmful to forming larger nanosheets. In our previous studies, we find EDTA has a huge effect on morphologies after reaction. Moderate amount can accelerate the formation of nanometer sheet of the crystal and then form flower-like morphologies.Through adjusting the amount of EDTA, we had obtained needed flower-like nanopowder.The obtained Tl and I dual-doped flower-like nanopowders were hot-pressed into bulk pellets and the effects of element doping on the thermoelectric properties were investigated. It was found, as compared with the un-doped samples, the Tl0.2Bi1.8Te2.8I0.2 bulk sample prepared from such flower-like nanopowders has a higher Seebeck coefficient and a lower thermal conductivity, meanwhile the electrical resistivity is not changed greatly. Therefore, the Tl0.2Bi1.8Te2.8I0.2 bulk sample has a high ZT value which can reach 1.3 at 398 K. The reason for this higher ZT value is due to the microstructure of the hot-pressed bulk pellets prepared from the flower-like nanopowders, which is a mixture of larger sheets and small grains. Such microstructure can scatter phonon effectively but not influence electric carrier much and not deteriorate the electric conductivity properties. |
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