| From the "steam" to the "motor" era, a series of power technology are developed and used. Humans began to realize that the innovation and update of energy technology drove the progress of human societyand played a very important role inpromoting the development of the society. Hence, energy becomes a driving force of social development, and the viewpointis gradually accepted by people. So far, fossil fuels are main energy source, and human get the civilization and progress. However, the excessive consumption of energy causes the environmental pollution and ecological destruction, which is disastrous to human survival and life environment. Therefore, the development of new functional material and energy conversion material, and change the energy structure become a new task faced by human beings. Looking for the "sustainable development" of energy is deeply concerned by governments around the world. A lot of energyis wasted in human life and production, such as industrial waste heat, waste gas, and waste heat ofcar exhaust, etc. If this part of energy could be utilized, it can effectively optimize the industrial structure, save the use of non-renewable energy, and improve the living environment of human beings.Thermoelectric(TE) materials are a new kind of energy conversion functional materials, which make use of transport and interaction of the carriers and the phonons. TE materials can make the mutual coupling and conversion between heat and electrical energy directly. TE materials have advantages of no noise, no pollution, long life, small volume, and security and stability, and thus they receive extensive attention of the scientific community. Therefore, the research and exploitation of TE materials have great significance in optimizing the allocation of resources and improving the ecological environment. The performance of the TE materials is determined by figure of merit(ZT value). The calculation of ZT value depends on the parameters, such as Seebeck coefficient(S), electrical conductivity(σ) and the thermal conductivity(κ), and the formula is: ZT=S2σT/κ, T is kelvin temperature. Among them, S2σ is defined as the power factor(PF), which can describe the merits of electrical properties. Co Sb3 is considered to be the most promising TE materials in medium temperature region.Co Sb3 is very typical skutterudite crystal structure, possessing the symmetry of cubic crystal. There are 32 atoms in each cell, and interstitial voids at the 2a position(12-corrdinated) among the lattice. Co Sb3-based skutterudite has the narrowwidth of forbid bondand high carrier mobility, leading to a good electrical performance. But the high thermal conductivity becomes the high thermoelectric performance bottleneck. In recent years, Slack puts forward a new theory of the "electronic crystal- phonon glass". The theory means that the TE materials possess high electrical conductivityas crystal, and have low thermal conductivity as glass. According to the idea, in Co Sb3-based skutterudite, when a molecule or an atom with weak bound state exists in the void of the cage comprised of other atoms, the molecule or the atom occur an harmonic vibration with a high degree of localization, which we describe it as "rattle". It can effectively reduce the thermal conductivity of TE materials, and then optimize the thermoelectric performance. The main ways to optimize the performance of Co Sb3-based TE material are filling, doping, introducing defects, nanocrystallization and improving the synthesis technology. For filling or doping Co Sb3, to optimize the TE performance, we can change the structure of the materials by introducing impurity atoms, and alter the quality fluctuations and strain field through the difference in radius of quality of the atoms. For now, many methods have beenused to synthesize Co Sb3-based TE materials, such as melting, powder metallurgy, hydrothermal method, spark plasma sintering and so on. In this research, we adopt the high temperature and high pressure(HPHT) synthesis method for the preparation of TE materials. HPHT could synthesize the materialsthat cannot be achieved at atmospheric pressure, and it has the ability to intercept part of the excellent properties gained at high pressure. HPHT method possesses the unique advantages, such as fast reaction, simple operation, good repeatability, good mechanical properties and high purity of sample.In this paper, we use the high temperature and high pressure(HPHT) method to synthesize a series of Co Sb3-based TE materials. The structure and TE properties are studied deeply. Te-Sn double-doped, Te-Se double-doped and combining In filled and Te-Ge double-doped Co Sb3-based TE materials were synthesized by HPHT method. We systematically study the regulation rules of microstructure, TE properties, carrier concentration and so on, and we expect to effectively reduce the thermal conductivity and improve ZT values. In addition, we prepare the composites of carbon nanotubes(CNTs) dispersed Te-doped Co Sb3-based TE materials, to explore the influence of the CNTs on microstructure and TE properties. This thesis mainly discusses the results of crystal structure, microstructure, electronic structure, carrier concentration, carrier mobility, fermi energy and TE properties, etc. The main research results are as follows:1. Under the conditions of pressure in 0.5-3.5GPa,temperature in 900-1000 K,Co4Sb12-x-yTeySnx(x=0.1,0.2,0.3;y=0.5,0.6,0.7,0.8)was successfully prepared by the HPHT method. We systematically studied the phase structure,microstructure and the modulation law of thermoelectric properties. The experimental results show that, with synthesis pressure increasing, the absolute Seebeck coefficient and electrical resistivity increase. At room temperature, Co4Sb11.1Te0.6Sn0.3 synthesized at 0.5GPa obtains the maximum power factor of 11.35μWcm-1K-2. The positions of the main peaks for Co4Sb12-x-yTeySnx synthesized under different pressures are consistent with the standard spectra. The grain size is homogeneous and dense, and the boundary is abundant in our sample. The samples are composed of a lot of nanocrystallines, indicating they are not single crystal structure. What is more, numerous dislocations can be observed. These microstructures could reduce thermal conductivity effectively. With synthesis pressure increasing, the thermal conductivity decreases. At room temperature, Co4Sb11.2Te0.5Sn0.3 synthesized at 3GPa get a minimum thermal conductivity of 2.4Wm-1K-1; Co4Sb11.1Te0.6Sn0.3 synthesized at 2GPa have the greatest Z value of 3.05×10-4K-1.2. Under the conditions of pressure in 1-3.5GPa,temperature in 900-1000 K,Co4Sb11.3Te0.7-xSex(x=0.05,0.075,0.1)was successfully prepared by the HPHT method. We systematically studied the phase structure, microstructure and the modulation law of thermoelectric properties. The experimental results show that,the diffraction peak positions of samples are consisted with Co Sb3(PDF#78-976). No other diffraction peaks were observed. The samples are no cracks and compact, and the grain sizes are even. The grain boundary is clear and rich. The High resolution transmission electron microscopy(HRTEM) test results show that lattice fringes are arranged in different orientations, and a large number of dislocations are observed. This kind of microstructures can significantly scatter phonons, and thus reduce the lattice thermal conductivity effectively. The electrical resistivity and absolute Seebeck coefficient increase as pressure rises. At room temperature, Co4Sb11.3Te0.65Se0.05 synthesized at 1GPa shows the greatest power factor of 14.6μWcm-1K-2. With the pressure increasing, the thermal conductivity declines. In addition, as the concentration of Se increases, the thermal conductivity also decreases. Due to differences in the quality of Sb, Te, Se atom, there is strong quality fluctuation scattering, which could reduce the thermal conductivity to some extent. Co4Sb11.3Te0.6Se0.1 prepared at 3.5GPa get the minimum thermal conductivity of 1.8Wm-1K-1 at room temperature.3. Under the conditions of pressure in 1.5-3GPa,temperature in 900-1000 K,In0.1Co4Sb11Te0.8Ge0.2 was successfully prepared by the HPHT method. We systematically studied the phase structure,microstructure and the modulation law of thermoelectric properties. The experimental results show that,the obtained samples are crystallized in a cubic Co As3-type structure with IM-3 space group. And, all the peaks position is the same. A wealth of grain boundary are found in our sample, and the grains are dense and no crack. The high pressure and composite doping(more nucleation center) effectively inhibits the growth of grain. With the increase of pressure, the grain size decreases. HRTEM images shows the different lattice fringe orientation, suggesting that the sample is not single crystal structure, but is composed of many nanocrystals. At the same time, many dislocatios are discovered in our samples. The absolute Seebeck coefficient and electrical resistivity first increase and then decrease, and the thermal conductivity declines significantly, as the temperature rises. We obtain the maximum power factor of 28.2μWcm-1K-2 @773K, 2.5 GPa. The lowest thermal conductivity of 1.8 Wm-1K-1 and the maximum ZT value of 1.12 are acquired at 773 K, 2GPa.4. Under the conditions of pressure in 2-3GPa,temperature in 900-1000 K,Co4Sb11.5Te0.5 + 0.25vol%CNTs was successfully prepared by the HPHT method. We systematically studied the phase structure,microstructure and the modulation law of thermoelectric properties. The experimental results show that,the major phases and values of Co4Sb11.5Te0.5+ 0.25 vol.% CNTs match very well with the pure Co Sb3 phase(PDF#78-0976). However, several additional small peaks were observed, which can be indexed to impurity phases of Co Sb2 and Co Te2. The impurities in our samples could form a second phase particle leading to an enhancement of phonon scattering. A slight shift of the XRD peaks has been observed, which is owing to the decreased spacing of crystal planes produced by high pressure. The samples possess homogeneous microstructure, and the matrix is densely compacted. The lattice orientation is in a disorderly growth pattern, and many dislocations appeared. CNTs mainly concentrate on the grain boundaries. The resistivity and the absolute value of Seebeck coefficient increase with the temperature increasing. At 773 K, Co4Sb11.5Te0.5 + 0.25vol%CNTs synthetsized at 3GPa obtains the maximum power factor of 31.6μWcm-1K-2. The lattice thermal conductivity and thermal conductivity decline as the temperature and pressure rise. Co4Sb11.5Te0.5 + 0.25vol%CNTs prepared at 3.5 GPa gains the minimum thermal conductivity of 1.76Wm-1K-1 @773K and the largest ZT value of 1.32 @773K.In a word, through a large amount of inquiry experiment to research the TE properties about compositely doped Co Sb3 TE material. The results show the unique advantages of HPHT method via the introduction of pressure combining with doping, filling and compositing, such as improving the crystal structure, microstructure, electronic structure, forming the second phase, optimizing TE performance significantly, etc. High-performance Co Sb3-based TE material could be successfully synthesized by HPHT method during 30 min, which lays a solid foundation for the synthesis of a higher performance of TE materials. |
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