Preparation and Thermoelectric Properties of Bismuth Telluride-based Materials Obtained by Mechanical Alloying

Abstract: Thermoelectric materials interconvert heat and electricity directly. They are used in Power generation and cooling.Bi2Te3-based materials are the best thermoelectric material at room temperature at present. The figure of merit ZT of Bi_2Te_3 is about unit for decades. Traditionally, they are usually prepared by crystal growth methods(e.g.Bridgman method),but their mechanical ability is quite low due to its brittleness. In recent years, some new preparation methods of thermoelectric materials which can obtain materials with excellent performance emerged. But it can not be generalized because of the whole process is complicated and quite high cost.Mechanical alloying (MA) is a customary method in preparation of powders, which can directly alloy the raw materials into target powders in a relatively low cost way. In this thesis, Mechanical alloying was adopted to prepare Bi_2Te_3-based alloy powders. Atmosphere protected sintering is used to consolide the powders into bulk material. Doping was used to optimize and increase thermoelectric performance in different sintering system. In this way, materials with excellent performance were obtained with low cost.In the first part of this thesis, the development of thermoelectric materials and basic theory is summarized first. Also the MA process of Bi_2Te_3-based material was studied by using XRD、EDS and SEM analysis method. The result indicated that above 20 hour MA was proved to be enough to obtain the single phase Bi_2Te_3 alloy powders.Analysis of samples with different MA time were done in this work. It found that the thermoelectric properties will became better and particle size will minish along with the increase of MA time. The particle size were reduced to 1μm when MA time is more than 20 hours. When MA time is 50 hours, the medium diameter (d_(50)) of particles became 115 nm, the largest power factor reached 87.71μw/(K~2·m).Heat treatment is beneficial to improve thermoelectric properties of bulk materials.The effect on thermoelectric properties of sintering system behave in two aspects, sintering temperature and holding time. It shows that the temperature too high or too low do not have the superior performance of materials. In the range of 500-550℃, materials can obtain a excellent performance. Because that, at the low temperature crystals grow incompletely, On the contrary, too high temperature will cause the volatilization of Te and affect material properties. When holding time is 0.5h, material properties are excellent. Prolonging holding time will result in over-grown crystals, This is not good for properties of materials, but helpful for tropism.Thermoelectric properties of Bi_2Te_3 not doped is not high. After doping Se、Sb, thermoelectric properties were impacted significantly. The performance of electrical conductivity and Seebeck coefficient improve obviously. Power factor improve from 51.1μw/(K~2·m) to 786.5μw/(K~2·m) and 508μw/(K~2·m). Through massive experiments, it shows that the N-type,P-type optimal formula are Bi2Te2.75Seo.25 and Bio.49Sb1.51Te3.Ag element doping has also been done in this thesis. Results showed that 0.25 in wt%, the maximum power factor was 352μw/(K~2·m).Finally, thermoelectric modules and small semiconductor thermoelectric generation devices were researched and developed in this thesis…
Key words: Bismuth Telluride-based Thermoelectric materials ; Mechanical all oyin-g(MA); Thermoelectric generation ; Seebeck coefficient

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