Phonon Transport In Zintl Phase Thermoelectric Matetials With Lowdimensional Structure

Due to the continuous aggravation of energy shortage and environmental pollution in the world,the issue of finding green and renewable energy has attracted widespread attention.Thermoelectric material is a new kind of green renewable energy conversion material,which can realize the direct conversion between heat energy and electric energy through the transport of carriers and phonons in solids.It has the advantages of no noise,no pollution,high reliability and long service life,and has application prospect in the fields of automobile exhaust waste heat,industrial waste heat,solar power generation and refrigeration.However,the current low thermoelectric conversion efficiency limits the use of thermoelectric technology only in aerospace and military fields.Thermoelectric figure of merit（ZT）is usually used to measure the conversion efficiency of thermoelectric materials:the higher the ZT value,the higher the thermoelectric conversion efficiency of materials.There are generally two methods to improve the ZT value of thermoelectric materials,regulating existing thermoelectric materials and exploring new materials.With the maturity and development of quantum mechanics,theoretical calculation methods have been continuously optimized,and the calculation speed has been rapidly improved,and the research on various properties of thermoelectric materials based on first principles has gradually matured.Since it is not limited by experimental conditions,the method of verifying and guiding experiments through theoretical calculation has attracted much attention.For thermoelectric materials,due to the mutual coupling between the Seebeck coefficient,electrical conductivity and electronic thermal conductivity in the thermoelectric parameters,it is difficult to decouple the thermoelectric properties to achieve regulation.For the important lattice thermal conductivity of thermoelectric parameters,it can be independently regulated,so finding thermoelectric materials with intrinsic low thermal conductivity is one of the effective ways to improve ZT.Zintl phase materials have the characteristics of phononic glass-electronic crystals,which have good electrical transport properties and low lattice thermal conductivity,and are potential high-performance thermoelectric materials.Zintl phase materials are rich in structure,variety and composition.According to the structural characteristics,they can be divided into layers,chains,cages,disorder and so on.Although experimentalists have done extensive research on Zintl phase materials,the physical origins of their low thermal conductivity remain unknown.In response to this,this thesis mainly studies Zintl phase thermoelectric materials with low-dimensional structures（chained and layered-like）based on first-principles,and reveals the physical mechanism of its intrinsic low lattice thermal conductivity.The results are as follows:1.First-principles study of phonon transport in chain Zintl phases Ca₃AlSb₃ and Ca₅Al₂Sb₆Systems with the same elements but different stoichiometric ratios in thermoelectric materials usually exhibit different behaviors in thermal conductivity.For Ca₃AlSb₃ and Ca₅Al₂Sb₆ which belongs to Zintl phase systems with chain-like structures,both compounds have low lattice thermal conductivities and are found to be very close experimentally.We investigate the physical causes of its low lattice thermal conductivity from both lattice dynamics and phonon transport.The theoretical results show that the average lattice thermal conductivity of Ca₃AlSb₃(0.80 Wm^-1K^-1)and the average lattice thermal conductivity of Ca₅Al₂Sb₆(0.84Wm^-1K^-1)at 300 K are very similar,and the average lattice thermal conductivities of both are relatively low.The reason for the close thermal conductivity of the two systems is that the phonon group velocity,phonon lifetime and Grüneisen parameter are similar.The low average lattice thermal conductivity is mainly attributed to cutoff frequencies of all three acoustic branches below 3 THz and strong optical phonon-acoustic coupling.At the same time,the lattice thermal conductivity of Ca₃AlSb₃ and Ca₅Al₂Sb₆ in the x direction（along the chain direction）is more than twice that in the z direction（perpendicular to the chain direction）,which is caused by the strong Al-Sb covalent bond along the chain direction.We use heavy elements Tl and Bi replacing Aland Sb along the chain direction in order to further reduce the average lattice thermal conductivity of system.We also found that the potential energy surface can significantly reduce along the chain direction,indicating that the atomic vibration frequency in the x direction decreases,which can further reduce the lattice thermal conductivity.This work elucidates the physical mechanism of the phonon transport behavior of Ca₃AlSb₃ and Ca₅Al₂Sb₆,and also provides theoretical guidance for the discovery and design of materials with low lattice thermal conductivity through the substitution of heavy atoms in AlSb₄ tetrahedra along the chain direction.2.First-principles study of phonon transport in layered Zintl phases Ba₂ZnAs₂ and Ba₂ZnSb₂Based on first-principles phonon calculations,we systematically study the phonon transport of Ba₂ZnX₂（X=As,Sb）with layered structure.The calculated results show that the phonon spectrum of Ba₂ZnBi₂ has an imaginary frequency,which indicates that it is thermodynamically unstable,which is consistent with the experimentally observed instability of Ba₂ZnBi₂.For Ba₂ZnAs₂ and Ba₂ZnSb₂ systems,the phonon spectrum has no imaginary frequency,indicating that the two materials are thermodynamically stable and can be synthesized under certain experimental conditions.By solving the phonon Boltzmann transport equation,it is found that Ba₂ZnSb₂ has lower phonon group velocity(<4.5 kms^-1),larger Grüneisen parameter,shorter phonon relaxation time（<5.5 ps）,lower optical modes（～1.5 THz）,and stronger optical-phonon-acoustic phonon coupling than Ba₂ZnAs₂.These inherent phononic properties can greatly hinder the heat transport ability of Ba₂ZnSb₂,resulting in the ultralow thermal conductivity of 0.11 Wm^-1K^-1 for Ba₂ZnSb₂ at 300 K.In addition,we gain a deeper understanding of the origin of low lattice thermal conductivity from small bulk elastic and shear moduli from elastic properties.This study provides a physical explanation for the low thermal conductivity of Ba₂ZnX₂（X=As,Sb）,and provides important theoretical guidance for the search for efficient thermoelectric materials based on layered Zintl phase compounds.