The Controlling Morphology And Doping Of Nanostructures Bi₂Te₃Based Thermoelectric Materials

Thermoelectric (TE) materials that can convert thermal energy into electrical energy directly or reversely are a kind of function materials. It is a kind of static energy conversion materials and can make thermal energy and electrical energy convert each other without any moving parts. Thermoelectric materials are the key of core components to manufacture thermoelectric converters.The devices made by thermoelectric materials have a great deal of advantages such as the small size, noiseless, non-pollution, no moving parts and maintenance free. Thermoelectric materials will have been applied in many important applications in fields such as power generation by different temperature, temperature control, thermolelectric refrigeration and temperature measurement. Thermoelectric properties of thermoelectric materials are judged by their dimensionless Figure of merit (ZT), which is determined by the electrical conductivity, the Seebeck coefficient, and the thermal conductivity of the material. In recent years, it becomes the focus to search for high performance thermoelectric matericals.Bismuth telluride based compouds and its solid solution are known as the best thermoelectric materials currently available near room temperature. They have been applied in fields such as optoelectronic devices, the national defense, the medical equipment, aeronautics and astronautics and the environmental protection. People have researched Bi2Te3materials for a long time, but its ZT is still around1, so people try to improve and optimize its thermoelectric properties through a variety of methods, In recent years, nanotechnology and energy-band engineering are the effective means to enhance the ZT value of thermoelectric materials and have become the focus.This thesis combined nanotechnology with energy-band engineering focused on investigating the effect of element doping and morphologies controlling of Bi2Te3nanopowders on their thermoelectric properties. The main research contents and results are as follows:(1) The synthesis of Bi2Te3nanopowders were prepared by the hydrothermal method. It was reported that good thermoelectric properties can be obtained by hot-pressing flower-like morphology powders. However, the growth of Bi2Te3crystals is a very complex procedure, and a lot of factors can influence the formation of the flower-like nanopowders. The focuses this thesis is to study the effects of processing paraments on the morphologies of the Bi2Te3nanopowders. It is shown that the amount of EDTA surfactant, the reaction time as well as the reaction temperature will have remarkable effects on the purity of Bi2Te3powders and the powders morphology. Bi2Te3nanopowder is a mixture of irregular small nanoparticles and some nanosheets when the amount of EDTA is lower.With the excess of the EDTA, there is a lot of impurities in the powders. Therefore, a suitable amount of the EDTA is very crucial to the formation of the flower-like morphology nanopowders. When the reaction time is shorter and the temperature is lower, the powder has impurities. With suitable reaction time and reaction temperature, crystal particles can gain more energy and grew up into larger nanosheets which is beneficial to form flower-like morphology nanopowders.(2) Y and Ce doping were investigated. It is shown that Y and Ce doping has significant effects on the morphologies of the nanopowders, it is harmful to the formation of the flower-like nanopowders. The main reason may be that Y and Ce doping can change the bonds strength and affect the growth rate along a-axis, b-axis and c-axis. However, when increasing the amount of EDTA appropriately as compared with the undoped samples, flower-like nanopowders can be still formed even with Y or Ce doping. The reason for this is that EDTA can accelerate the growth rate along a-axis and b-axis and is helpful to form larger nanosheets, which is benifacal to the formation of flower-like morphology nanopowders.(3) The Y and Ce doped flower-like nanopowders were hot-pressed into bulk pellets and to study the effects of element doping on the thermoelectric properties. The microstructure of hot-pressed bulk pellets prepared from the flower-like nanopowders is the mixture of larger sheets and small grains. The larger sheets are ideal to decrease the electrical resistivity, which can improve the transport properties of carrier.In addition, this nanostructure can intensity scattering of the phonons and is beneficial to low the thermal conductivity. It is shown that the Y0.25Bi1.7tTe3bulk sample prepared from such flower-like nanopowders has a lower electrical resistivity than than the undoped sample, but Seebeck coefficient is not decreased. As results, the Y0.25Bi1.75Te3bulk sample have a high ZT value which can reach1.23at410K, which is a high performance material among the n-type Bi2Te3semiconductor thermoelectric. The Ce0.1Bi1.9Te3bulk sample can reach1.22at386K which is higher than the commercial Bi2Te3bulk.