The Electronic Structure And Thermoelectric Properties Of Several Materials From Theoretical Studies

Energy is the basis for people of survival and development. energy supply relates to the nationalsecurity and the national economic development. At present, such as petroleum, natural gas and coal arestill our major source of energy. The use of fossil fuels in the project play an important role in theenvironment. For example, the global climate change and endangering the ecological balance. Therefore,exploration for new energy and research of new energy materials are a focus for concern. Thermoelectricmaterial can convert energy between thermal energy and electrical energy, it is a new kind of energymaterial. As thermoelectric generators are solid-state devices with no moving parts, they are silent,reliable and scalable, making them ideal for small, distributed power generation. Therefore,thermoelectric materials is an extremely wide range of applications.The thermoelectric performance of a given material is characterized by the materials dimensionlessfigure of merit ZT=S2σT/(ke+kl), where S is the Seebeck coefficient, σ is the electrical conductivity, T isthe absolute temperature, and keand klare the electric and lattice thermal conductivity, respectively.Therefore, a good thermoelectric material should have a large Seebeck coefficient, a high electricalconductivity, and a low thermal conductivity. Metals always have very high electrical conductivity, buthigh thermal conductivity. Glasses have low thermal conductivity, but very low electrical conductivity.Therefore, ideal thermoelectric material should be a phonon glass and an electron crystal. Transitionmetal (TM) silicides are potential materials for various high temperature applications due to their highmelting points, rich resources, low price, and chemical stability at elevated temperatures. Transitionmetal nitrides have been receiving great attention for their fundamental physics and technological applications. Extensive experimental work and theoretical calculations have been performed on theircrystal structures, elastic properties, and phase transitions.In this work, We have employed first-principles calculations and Boltzmann transport theory topredict CrSi2, β-FeSi2, IrN2, Ba3Al3P5, Ba3Ga3P5structures and explore their thermoelectric properties. Itis found that the transport properties of p-type CrSi2are better than that of n-type. The highthermoelectric performance of p-type CrSi2are mainly due to the high anisotropy of its electricalconductivity with p-type doping. For CrSi2, the effective mass of holes along the z direction is smallerthan that along the x direction, and consequently the hole mobility along the z direction is higher thanthat along the x direction. A high thermoelectric performance of CrSi2can be achieved hole doping withconcentration range of1-6×1021cm-3. The transport properties of n-type β-FeSi2are better than p-type one.Using first principles and Boltzmann transport calculations, we analyze the thermoelectric properties ofIrN2. For elucidating its thermoelectric performance, we determine its Seebeck coefficient, conductivity,carrier concentration, power factor, and a\maximum" thermoelectric gures of merit ZeT. For pure IrN2,Seebeck coefficient as large in magnitude as260μV/K was found at720K, and the largest value ZeT is0.83at800K. Such high Seebeck coefficient and ZeT suggest that excellent thermoelectric propertiesmay be found in IrN2. By studying the carrier concentration dependence of transport properties, we foundthat p-type doping is better than that n-type doping, although both they have good thermoelectricproperties. Moreover, the thermoelectric properties along the x direction is better than that along the yand z directions. By studying the electronic structure, we conclude that the good thermoelectricproperties of IrN2may come from its larger dispersion and a valley degeneracy in valence band.Ba3Al3P5and Ba3Ga3P5also have good thermoelectric properties.