First-Principles Study On Thermoelectric Properties Of Several Layered Materials

Thermoelectric materials could directly achieve the conversion of waste heat and electrical power at both ends of materials,which can work sustainably without any pollutants,effectively alleviate the problems caused by energy crises and environmental pollution.However,thermoelectric materials also have shortcomings such as low energy conversion efficiency and high manufacturing costs.Currently,they are mainly used in aerospace,military,medical and other fields,there is still a long way to go from large-scale applications.Generally,the efficiency of thermoelectric conversion of materials determined by the figure of merit:ZT（28）S ²σTκ_e（10）κ_l,but the mutual coupling between transport parameters makes it very difficult to enhance ZT.After several decades of development,thermoelectric theory has pointed out the main methods to improve the thermoelectric properties:such as modifying the band structure or increase the band degeneracy by band engineering can effectively improve the electron transport performance,selecting appropriate carrier concentration can effectively optimize the relationship between transport parameters,doping heavy atoms can increase phonon scattering rate and reduce phonon lifetime of materials,selecting low dimensional materials can effectively reduce the lattice thermal conductivity for original structures.With the development of nanotechnology,researchers have found that some layered materials have better thermoelectric transport properties due to their unique structures,their thermoelectric properties are significant superior to traditional three-dimensional structural material.Under the circumstances,we utilize the first principles simulation program VASP to predict a series of new 2D layered materials,such as Tl₂O,Cu₂X（X=S,Se）,Au₂X（X=Se,Te）.They all belong to the binary chalcogenides which have low lattice thermal conductivity and preferable transport parameters.Besides,according research the phonon lifetime,group velocity,Grüneisen constant and other parameters that affect the lattice thermal conductivity of above materials,it provides a guideline for revealing the internal transport laws of phonons.The main work of this thesis is as follows:1.Thermoelectric performance of monolayer Tl₂O have been studied utilize first principles simulation calculation.The research shows that monolayer Tl₂O possess ultralow lattice thermal conductivity（0.97W/m K）at room temperature,which is due to its lower phonon group velocity,stronger anharmonic effect and lower Debye temperature.The electrical transport properties indicate that monolayer Tl₂O has a large Seebeck coefficient under p-type doping,which is originate from the high effective mass of hole along the y direction.In addition,we have studied part mechanical properties of monolayer Tl₂O,and the critical strain points along the x and y directions are 13.8%and 16%,respectively.Therefore,the ZT of monolayer Tl₂O is1.21 at room temperature under optimal doping concentration.When the temperature reaches 700K,the maximum ZT can approximately to 2.62.2.This study found that monolayer Cu₂X（X=S,Se）possess ultralow lattice thermal conductivity（3.25W/m K and 1.93W/m K）at room temperature.We have verified the factors affecting their thermal conductivity by the MFP,phonon group velocity,phonon lifetime,Grüneisen constant and Debye temperature.For the electron properties,we replace the carrier effective mass by the density of states effective mass to correct the band degeneracy at the VBM of monolayer Cu₂X（X=S,Se）.The hole effective mass calculated this method is higher than the electron effective mass,resulting in high Seebeck coefficients under p-type doping and high conductivity under n-type doping.Through the above results,the figure of merit of monolayer Cu₂S and Cu₂Se under optimal n-type doping at 700 K can approach 1.85 and 2.82,which are higher than 0.38 and 1.7 under optimal p-type doping.The excellent thermoelectric properties of monolayer Cu₂X（X=S,Se）are comparable to those of many promising thermoelectric materials reported recently and have a good application prospect.3.The theoretical studies have shown that the ZA branch with quadratic greatly affects its lattice thermal conductivity and figure of merit.Based on the first principle calculations,we have achieved complete quadratic for the ZA branch of monolayer Au₂X（X=Se,Te）by applying a rotational invariance for second order force constant.The corrected lattice thermal conductivity of monolayer Au₂Se and Au₂Te significantly decreased,which about 0.04W/m K and 0.07W/m K at room temperature.In addition,we consider the band degeneracy effect at the VBM for the calculation of electrical transport properties,and find that the figure of merit more easily improved under n-type doping.According to the modified results,at the optimal doping concentration and the ZT of monolayer Au₂Se and Au₂Te can reach 6.4 and 4.2 at room temperature,indicating that they are excellent thermoelectric materials.