Microstructure Of Nano Thermoelectric Materials And Its Influence On Thermoelectric Properties

Due to the drying up of fossil energy and serious air contamination, sustainable energy technologies are attracting significant attention. Thermoelectric conversion device fabricated from thermoelectric materials can convert surplus heat, such as the heat from factories and automobile exhaust into electricity. So thermoelectric conversion technology offers us a project for resolving energy crisis and lessening air pollution.At present, how to improve the thermoelectric properties has become the most important topic. The thermoelectric conversion efficiency depends on various factors, such as the electrical resistivity, thermal conductivity, Seebeck coefficient and so on. The traditional method to improve thermoelectric properties is to discover new thermoelectric materials, but thermoelectric properties are improved only in some degree by this method. However, by adjusting and changing the microstructure of the materials, it can lower the thermal conductivity and thus improve thermoelectric properties. However, how the microstructures such as the defects affect the thermoelectric properties is still an unsolved problem and needs extensive study. Positron is a self-searching probe and is particularly sensitive to atomic scaled defects, so it is very appropriate to study the microstructure of materials. It is a very important project to study the defects in thermoelectric materials and their effects on the thermoelectric properties.The nanostructuring approach of thermoelectric materials is effective for improving thermoelectric properties. In the nanostructuring approach, numerous boundaries or interfaces are introduced throughout the thermoelectric materials, and large amounts of defects may exist in those area, such that phonons are highly scattered and the thermal conductivity is lowered. Of course, for the strategy to be successful the electrical resistivity and Seebeck coefficient should not be significantly affected. This has been confirmed by many theories and expriments. In order to confirm the contribution of defects to the thermoelectric properties, in this thesis, we choose nanocrystalline Bi2Te3and In2O3 thermoelectric materials as our sample. The defects of the materials will be studied mainly by positron annihilation spectroscopy. Firstly, microstructure of nanocrystalline Bi2Te3 treated at different temperatures was studied, and the effect of microstructure on the thermoelectric properties was discussed. Secondly, the influence of different doping content on maicrostructure and thermoelectric properties of nanocrystalline Bi2Te3 was studied. Finally, the effect of heat treatment at different tempertures on In2O3 structure and thermoelectric properties was also investigated. The main results are as follows:1. Bi2Te3nanocrystals were synthesized via a hydrothermal method. Part of the powders were treated by spark plasma sintering (SPS) at 350 ? and further annealed between 350 and 500?, and the others were treated by SPS at temperatures between 300 and 500?. X-ray diffraction (XRD) measurements confirm that the samples annealed at different temperatures and the samples treated by SPS at different temperatures all show Bi2Te3 single phase. According to the results of scanning electron microscopy (SEM), the morphologies of the two groups of samples are particle-like, and the sizes of particles increase with increasing thermal treating temperatures. However, according to the results of XRD and high-resolution tranmission electron microscopy (HRTEM), the grain size of samples has little change with increasing treating temperatures. The influence of different treating temperatures on the grain boundary defects has been studied by positron annihilation lifetime spectroscopy (PALS). The results of PALS reveal vacancy defects in all of the samples, which exist most probably in the grain boundary region, and the vacancy concentration decreases after heat treatment. Meanwhile, the total themal conductivity and the lattice thermal conducvivity of Bi2Te3 nanocrystals increase with increasing treating temperatures. The electrical resistivity and Seebeck coefficient have also some changes for the annealed samples. The intimate correlation between vacancy defects and lattice thermal conductivity confirms that the reduction of thermal conductivity in Bi2Te3 nanocrystals is due to phonon scattering by vacancy defects rather than grain size effects.2. Bi2Te3_xSex (x=0-0.6) nanocrystals were synthesized via a hydrothermal method, and then they were treated by spark plasma sintering at 300?. According to the results of XRD, the samples of Bi2Te3-xSex (x=0-0.6) are all single phase, and the grain szie of samples has little change with increasing doping content. Positron annihilation lifetime measurements reveal that the size and the concentration of samples also has little change. Electrical resistivity measurements show that the electrical resistivity decreases with increasing doping content. The samles of x?0.15 display the characteristic of semiconductors, and the samples of x>0.15 display the characteristic of metals. And the lattice thermal condcutivity of samples decreases monotonically with increasing doping content. The correlation between doping content and lattice thermal conductivity confirms that the reduction of thermal conductivity of Bi2Te3-xSex (x=0-0.6) nanocrystals is due to phonon scattering by the mass difference between Te atom and Se atom rather than grain size and vacancy defect effects.3. In addition, we also studied the In2O3nanocrystals in detail. Part of the powders were treated by SPS at 700 ? and further annealed between 700 and 1300 ?, and the others were treated by SPS between 500 and 1000?. The XRD measurements show that the two groups of the samples are single phase, and the diffraction peaks of the samples annealed at% iffernent temperatures become sharper with the increase of annealing temperature, indicating the improvement of crystallinity of materials, and the calculated average grain size increases with annealing temperature. But the average grain size of In2O3 treated by SPS has no obvious change with the increase of sintering temperatures. The microstucture of the samples has been studied by positron annihilation mesurements, and we find that larger vacancy clusters gradually decompose into smaller vacancy clusters with increasing treating temperatures. Electrical resistivity and Seebeck coefficient measurements also show some change during heat treating process. The lattice thermal conductivity increases with the increase of treating temperature, which probably is the result of the vacancy defects. However, the effect of grain size cannot be excluded and it needs further study.