Realization And Regulation Of High ZT Values In Thermoelectric Materials By Lone-Pair Electrons And Electronic Valley Anisotropy Among Others

Exploring and utilizing new clear energy sources may reduce largely the dependence on traditional energies and promote the construction of an environmentally friendly society.New thermoelectric technology can be achieved through the direct conversion between heat and electric energy by utilizing thermoelectric materials.This technology can effectively improve the utilization efficiency of energy and has an important significance in mitigating the current energy crisis.However,the low thermoelec-tric conversion efficiency of the present thermoelectric materials has limited the large-scale application of these thermoelectric technology both in daily life and in industrial production.The discovery of ef-ficient thermoelectric materials and exploration of new mechanisms to enhance their thermoelectric properties are crucial to realize the device applications of thermoelectric materials.To these ends,the present thesis employs the first-principles calculations combined with the semi-classical Boltzmann transport theory to perform a detailed study on the thermoelectric properties of some two-and three-dimensional thermoelectric materials.This thesis also uncovers several new physical mechanisms to enhance the thermoelectric performances and predicts several high-performance thermoelectric mate-rials.Below,we present brief descriptions of this thesis:（Ⅰ）We have uncovered the positive influence of lone-pair electrons on the thermoelectric properties of two-dimensional materials.Our investigations demonstrate that the interaction between lone-pair electrons can effectively enhance the anharmonicity of low-dimensional materials,which in turn re-duces the material candidate’s lattice thermal conductivity.The monolayer Ge₂Y₂（Y=N,P,As,Sb）showsα-phase andβ-phase,however,allα-Ge₂Y₂samples exhibit lower lattice thermal conductivity when having the same elements.Through the comparative study ofα-Ge₂Sb₂andβ-Ge₂Sb₂,we un-cover that the influence of the lone-pair electrons on the lattice thermal conductivity is derived not only from the interaction between the lone-pair electrons around Sb atom and the bonding electrons of the adjacent Ge atom,but also from the interlayer coulomb repulsion of the lone-pair electrons distributed in different layers.Interestingly,the latter results in a strong anharmonicity,which greatly inhibits the lattice thermal conductivity.As a result,α-Ge₂Sb₂exhibits an extremely low lattice thermal con-ductivity of 0.19 W/mK at 300 K,whileβ-Ge₂Sb₂is 5.1 W/mK at the same temperature.Due to the ultra-low lattice thermal conductivity induced by lone-pair electrons,the predicted maximum ZT of n-type and p-typeα-Ge₂Sb₂reaches 1.2 and 1.18,respectively.（Ⅱ）We have discovered a new class of high-performance thermoelectric materials,i.e.,tetrahedral materials-X₄Y（X=B,Al,Ga;Y=C,Si）,which exhibit special electronic band structure and soft phonon dispersion relations.Based on the electronic band structure of Ga₄C,several different band configurations are designed to study the scattering mechanisms caused by the anisotropy of energy valleys and their effects on thermoelectric transport properties.First,Ga₄C not only has a high degen-eracy of the valence band,but also has a large anisotropy of energy valleys.The unique band structure allows Ga₄C to simultaneously ensure a high Seebeck coefficient and high electrical conductivity.At the same time,the flat electron band connecting the two valleys generates strong polar optical phonon scattering,which to some extent reduces the electrical conductivity.However,the significant increase in effective mass and Seebeck coefficient compensates for the decrease in electrical conductivity,re-sulting in a power factor of 6 m W/mK²for Ga₄C at 300K.Second,due to the low phonon group velocity and phonon lifetime,Ga₄C has an extremely low lattice thermal conductivity of 0.87 W/mK at 300 K.Benefitting from the high power factor and low lattice thermal conductivity,the ZT of Ga₄C reaches 3.28 at 600K.In addition,it is found that a small stress can effectively enhance the phonon scattering,reduce the thermal conductivity of the lattice,and preserve the degeneracy of Ga₄C mul-tiple energy valleys.When the strain is adopted as 0.75%,the lattice thermal conductivity at room temperature drops to 0.398 W/mK,and the ZT value under this conditions reaches 4.4.（Ⅲ）The effects of cationic substitution on carrier scattering and thermoelectric properties of AgInSe₂are investigated and can be considered as an effective way to improve its thermoelectric con-version efficiency.It is found that the change of cation can not only regulate the band gap and degener-acy of band,but also effectively regulate the polar optical phonon scattering.AgGaSe₂shows excellent n-type carrier mobility,which can reach 3500 cm²/Vs with low doping concentration.Benefiting from the increase in the valence band degeneracy and the decrease in the strength of polar optical phonon scattering,the power factor of p-type AgGaSe₂is greater than 6 m W/mK²at high temperature,demon-strating its high potential forwards thermoelectric applications.Moreover,the‘avoid-crossing’phonon dispersion feature effectively reduces the group velocity of longitudinal mode acoustic phonons,and AgAlSe₂,AgGaSe₂and AgInSe₂exhibit very low lattice thermal conductivity down to 0.774 W/mK,0.614 W/mK and 0.697 W/mK,respectively.Due to the higher electric transport coefficient and lower lattice thermal conductivity,the ZT values of p-type and n-type AgGaSe₂reach 4.37 and 3.78 at900 K,respectively,indicating that AgGaSe₂is also an excellent thermoelectric material.