| Thermoelectric (TE) materials, also called as material with electric potential in different temperature, are one kind of function materials which can directly convert thermal energy into electrical energy through the transportation of electric carriers in solid. The thermoelectric materials are regard as potential medium for energy source. The devices made by thermoelectric materials have advantages such as small size, noiseless, contaminant free, no moving part as well as high reliability. Therefore, thermoelectric materials will have been applied in important applications in fields such as power generation using waste heat and solid-state refrigeration. The quality of a thermoelectric material is judged by its dimensionless Figure of merit (ZT) which is determined by the electrical conductivity, Seebeck coefficient, and thermal conductivity of the material. A promising thermoelectric material with high ZT value should have a large Seebeck coefficient, a high electrical conductivity and meanwhile a low thermal conductivity. In recent years, employing elements doping and nanostructure engineering to enhance the ZT value of thermoelectric materials have become focus. Due to the advantages of low cost, less device-dependence and large-scale synthesis, hydrothermal method has been widely used to prepare nanopowders of thermoelectric materials.In this paper, we synthesized Bi2Te3nanopowders with different morphologies by hydrothermal method using different surfactants, Te sources, reaction times and temperatures. And a possible nucleation and growth mechanism of Bi2Te3with different morphology is suggested. The obtained nanopowders were hot-pressed into pellets with high density. The effects of different morphologies and doping with different rare earth elements on the thermoelectric properties of the bulks were investigated. So far, n-type Bi2Se0.3Te2.7materials have not shown significant improvement. So we synthesis n-type rare earth elements doped R0.2Bi1.8Se0.3Te3(R=Ce, Y and Sm) nanopowders attempting to improve the thermoelectric properties of such bulk samples prepared from these nanopowders. Through preparing Bi2Te3nanopowders with different morphologies by hydrothermal method, we found that the growth of Bi2Te3crystals is a very complex procedure. It can be affected by surfactants, Te sources, Bi sources as well as synthesis methods and procedures. In general, EDTA surfactant can accelerate the growth rate along the basal plane of Bi2Te3, while SDBS surfactant can block the growth along the basal plane of Bi2Te3. The effect of PVP surfactant is some what between the EDTA and SDBS. For the Te sources, Na2TeO3can have a fast soluble speed in the solution than Te powders. This is beneficial for the nucleation of a larger amount of small Bi2Te3crystals. In order to control the size and structure of the Bi2Te3nanopowders suitable surfactants, material sources, as well as synthesis methods and procedures should be carefully selected.Bi2Te3nanowires were synthesized by hydrothermal method at different reaction times and temperatures. It is found that the addition of SDBS is beneficial to form nanowires, and temperature and duration have remarkable effects on morphologies of Bi2Te3nanopowders. At150℃for24h pure Bi2Te3nanowires can be obtained and the width of them is about30nm. The EDTA also has remarkable effects on morphologies of Bi2Te3nanopowders. When the content of EDTA is lower, the products are nanoparticals. With the increase of the EDTA content, the flower-like nanosheets can be formed. We found an optimum value of the EDTA content with which pure Bi2Te3flower-like nanosheets can be formed.The morphologies of the starting nanopowders will have remarkable effects on the bulks prepared from them. It is shown that a suitable microstructure of the bulk should scatter phonons effectively but not scatter carriers much. The flower-like nanopowders synthesized by hydrothermal method using EDTA as surfactant and Te powder as Te source seem to meet this requirement. The microstructure of the hot-pressed bulk prepared from the flower-like nanopowders is a mixture of larger grains and small grains which scatters the phonons effectively and results in a high ZT value with lower thermal conductivity, high electrical conductivity and a relatively larger Seebeck coefficient. As contrast, the bulks prepared from nanoparticals and nanowires have relative lower ZT value. The main reason for this is that there are a lot of crystal boundaries in the bulks prepared from nanoparticals and nanowires which lower their electrical conductivity.In order to improve the ZT values of flower-like nanopowders further, we attempt to synthesize rare earth elements doped n-type R0.2Bi1.8Te3(R=Ce, Y and Sm) flower-like nanopowders. It is shown that Ce, Y and Sm doping has significant effects on the morphologies of the nanopowders and is not helpful to the formation of the flower-like nanopowders. The possible reason may be that the replacing Bi by rare earth elements can change the bonds strength and affect the growth rate along a-axis, b-axis and c-axis. The Ce, Y and Sm doping will not be helpful to enhance the electrical resistivity and the Seebeck coefficient, but they will be helpful to reduce the thermal conductivity. Among these three elements the Ce seems to be more effective to reduce the thermal conductivity. Although flower-like nanopowders was not obtained, the ZT values of R0.2Bi1.8Te3bulk sample can reach1.29at398K, which is higher than zone-melting ingots and shows the advantages of combining the elements doping and nanotechnology.Compared with the p-type Bi0.5Sb1.5Te3materials which already show significant progress in recent years, the n-type Bi2Seo.3Te2.7materials have not shown significant improvement. Based on the results of the previous experiments, we synthesized n-type rare earth elements doped R0.2Bi1.8Se0.3Te3(R=Ce, Y and Sm) nanopowders attempting to improve their thermoelectric properties. CexBi2-xSe0.3Te2.7(x=0-0.3) nanopowders were synthesized by the hydrothermal method. The doping of Ce, Y and Sm will not cause impure phases, but with the increase of the doping content the morphologies of the nanopowders will change remarkably. We found that the optimum doping content should be around x=0.2. Especially, the Y doping not only helps to reduce the thermal conductivity but also helps to increase the electrical conductivity. The main reason is that Y doping can improve the carrier concentration and also make the bulk have an ideal microstructure which scatters phonons effectively but not scatters carriers much. The Y0.2Bi1.8Se0.3Te2.7bulk sample has a high ZT value and can reach1.21at413K. |
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