| Thermoelectric materials can directly convert heat to electrical energy and vice versa. The intermetallic semiconducting compounds, exhibiting an excellent thermoelectric performance, however, are not ideal thermoelectric materials working at high temperaure because of their easy decomposion and oxidization at high temperature, high manufacturing costs, less abundance and inclusion of harmful elements. Because of their high temperature stability, security, non-toxicity and abundant elements, etc, oxide thermoelectric materials have widely attracted the attention of thermoelectric researchers. Layered oxide systems have a natural superlattice structure. The electrical transport layer in these materials is separated from the thermal barriers, which makes it feasible to manipulate electrical and thermal transports seperately. These materials are considered as candidates of phonon-glass electron-crystal thermoelectric materials. Thus the study of layered thermoelectric oxides has become one of the hot research fields in recent years.At present, the study of the layered thermoelectric oxides is focused on thermoelectric properties and low temperature transport properties of polycrystalline samples. There are few systematic studies on the intrinsic physical mechanism of the transport properties of the layered thermoelectric oxides. For example, are the thermal conductivity and electrical conductivity along the same direction in layered thermoelectric oxides simultanesouly manipulated? How much is the anisotropic difference of thermal conductivity and electrical conductivity in layered thermoelectric oxides? How does the isovalent element replacement affect thermal and electrical conductivities? Bearing these questions, two typical layered thermoelectric oxide materials, Bi2AE2Co2Oy (AE=Ca, Sr, Ba) and BiCuSeO material systems, were chosen as the research objects. High quality single crystals were prepared by optical floating zone method and the flux one. The microstructure, electrical transport and thermal transport of these materials were systematically characterized. Above-mentioned questions were studied in depth. The main research work is described as follows:1 Microstructure and thermal transport properties of Bi2AE2Co2Oy (AE=Ca, Sr, Ba) single crystals(1) Microstructure characterizations show that the crystalline quality of Bi2Sr2Co20y is the best, Bi2Ba2Co2Oy has a finite texture along c-axis. Compared to Bi2Sr2Co2Oy and Bi2Ba2Co2Oy, Bi2Ca2Co2Oy has a complex incommensurate modulation structure along b-axis.(2) The anisotropic thermal conductivity at high-temperature shows that thermal conductivities along ab-plane Kab of Bi2AE2Co2Oy (AE=Ca, Sr, Ba) single crystals are quite close, while the thermal conductivity along c-axis Kc of Bi2Sr2Co2Oy is nearly two-fold larger than those of Bi2Ca2Co2Oy and Bi2Ba2Co2Oy. Anisotropic elastic constants measurements demonstrate that when AE changes from Ca to Ba, anisotropic velocity difference is not big, so it can not explain the difference in the thermal conductivity Kc. The results show that iso-valent replacement in this material mainly regulates the thermal conductivity along c-axis, while its effect on thermal conductivity is ab-plane is negligible.(3) The finite texture and incommensurate modulation structure lead to the fluctuation of elastic constants between atoms. The effect of microstructures on the thermal diffusion constant is simulated by a one-dimensional atomic chain model. This model can qualitatively explain the observed difference of thermal conductivity Kc in Bi2AE2Co2Oy (AE=Ca, Sr, Ba) series of crystals. It reveals the intimate correlation between thermal conductivity and microstructure.2 The electrical transport properties of undoped and doped Bi2AE2Co2O8+δ(AE= Ca, Sr, Ba) single crystalsThe low temperature electrical transport characterizations show that when AE changes from Ca to Ba, Bi2Ca2Co2O8+δ, and Bi2Sr2BaxCo2O8+δ (x=0,0.5,1) demonstrate semiconductor-like properties, while Bi2Sr0.5Ba1.5Co2O8+δ and Bi2Ba2Co2O8+δ show a metallic behavior. Analysis of temperature-dependent resistance, magnetoresistance-magnetic field relationship at low temperature, as well as temperature-dependent magnetization shows that the semiconductor-like behavior in Bi2Ca2Co2O8+δ can be attributed to Anderson localization and electron-electron correlation effects, while the semiconductor-like behavior in Bi2Sr2-xBaxCo2O8+δ(x=0, 0.5,1.0) can be attributed to Anderson localization only. It suggests that iso-valent replacement dramatically modifies the ab-plane electrical property in these materials. Therefore iso-valent replacement is an effective method to improve the performance of thermoelectric oxides.3 Thermoelectric Properties of Bi2AE2Co2Oy (AE=Ca, Sr, Ba)The Seebeck coefficient in ab-plane of Bi2AE2Co2Oy single crystals increases as temperature increases. When AE changes from Ba to Ca, Seebeck coefficient increases in this material system. Bi2Ca2Co2Oy has the highest ZT value of 0.24 at 873K.4 Transport properties of BiCuSeO systemBiCuSeO system has an intrinsic low thermal conductivity, how to improve its electrical conductivity is an important issue. We synthesized the BiCuSeO crystals by flux method at different growth temperatures. Electrical transport characterizations show that transport properties of these crystals changed from a semiconducting to a metallic behavior and it is achieved by regulation of growth conditions. The metallic electrical property of BiCuSeO single crystal can be attributed to the deficiency of bismuth and the resultant hole concentration increase. The resistances of stoichiometric and nonstoichiometric BiCuSeO are 0.77 Ω·cm and 0.092 Ω·cm at room temperature.It suggests that optimizing the growth condition can manipulate the electrical transport, and in turn improve the thermoelectric performance.Based on above-mentioned studies, we understand the microstructure, electrical transport and heat transport of the layered Bi2AE2Co2Oy (AE= Ca, Sr, Ba) and BiCuSeO systems in depth. Firstly, in the layered Bi2AE2Co2Oy oxide system, electrical and thermal transport can be independently manipulated by iso-valent replacement, but it can not be simultaneously realized in the same direction. Iso-valent replacement in Bi2AE2Co2Oy can not manipulate thermal conductivity in ab-plane, so the improvement of thermoelectric properties of these layered oxides is difficult to achieve by reducing the thermal conductivity. Then we should choose an oxide material with intrinsically low thermal conductivity as the research system, and then pay more attention to improve its power factor. Secondly, iso-valent replacement dramatically affect the resistance of these systems. Ab-plane electrical property can be tuned from a semiconducting (Bi2Ca2Co2Oy) to a metallic one (Bi2Ba2Co2Oy). Therefore, the isovalent replacement is a viable method to improve the conductivity. These results are significant for improving thermoelectric properties of oxide thermoelectric materials... |
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