Theoretical Study On The Thermoelectric Properties Of BAgX?X=Ti,Zr,Hf?,HfTe₅ And CuLiX?X=Se,Te?

In today's world,the reserves of fossil fuels are less and less,and in the process of fossil fuel combustion,there is a lot of pollution to the environment.Therefore,it is urgent to find clean and sustainable energy.Moreover,fuels are used inefficiently,and a large part of it is wasted in the form of heat energy,which is undoubtedly a huge waste.Thermoelectric material is a kind of functional material which uses the movement of internal carriers to directly convert the heat energy into electric energy.Thermoelectric materials have the advantages of no pollution,easy to control,and long service life,etc.However,the main reason of limited application in a wide range is that conversion efficiency is not high.The conversion efficiency of thermoelectric materials is mainly determined by the dimensionless quality factor ZT value.Our main task is to find thermoelectric materials with high ZT value.Based on the first principles of density functional theory?DFT?,the semi-classical Boltzmann theory,deformation potential theory,and Slack model are used to study the thermoelectric properties of semiconductor materials in theory,which provides more theoretical guidance for experiments.The specific research work mainly includes the following aspects:1.The lattice structures of Half-Heusler?HH?compounds BAgX?X=Ti,Zr,and Hf?were optimized and the electron properties were calculated.The results show that they are all indirect narrow bandgap semiconductors.Their relaxation time and carrier mobility are obtained by using the deformation potential theory and lattice thermal conductivity is calculated by Slack model.The transport coefficient was calculated by Boltztrap2 software using energy band calculated by first principles.Finally,the ZT values of these materials are obtained.It is found that their relaxation time and carrier mobility increase gradually with the increase of atomic number.When the temperature is 500 K and the carrier concentration is 1.17×10²00 cm^-3,the maximum ZT value of n-type BAgZr can reach 1.2.The ZT value of n-type BAgZr is also close to 1.2 at temperature of 600 K.It indicates that the n-type BAgZr has stable thermoelectric performance in the temperature range of 500K-600K and can be used in the relevant applications of the temperature segment.2.The thermoelectric properties of the compound HfTe₅ were studied.Because of the lattice constant a?b?c of HfTe₅,it has an anisotropic transport property.Due to the layered structure characteristics,we used optb86b-vdw functional with van der Waals interaction to calculate the electronic properties and the band gap obtained was almost consistent with the values in the relevant literature.We studied the thermoelectric transport characteristics in three directions.The Slack model is used to calculate the lattice thermal conductivity,which is in good agreement with the relevant experimental and theoretical values.In the direction of c,when the temperature is 300K and the carrier concentration is 5.80×10¹99 cm^-3,the ZT value of n-type HfTe₅ can reach 2.68.We found that HfTe₅ is a thermoelectric material with excellent thermoelectric properties at room temperature.3.The thermoelectric properties of HH compound CuLiX?X=Se,Te?were studied.The dynamic stability of their structures is proved by phonon spectrum calculation.The calculated results of electronic structure show that they are all direct bandgap semiconductors.Based on the calculation of lattice thermal conductivity and transport coefficients,the relation between ZT value with temperature and carrier concentration is obtained.It was found that when the temperature was 900 K and the carrier concentration was 3.04×10¹99 cm^-3,the ZT value of n-type CuLiTe could reach 2.46.Through our calculations,we found that CuLiTe is a potential high temperature thermoelectric material.