Theoretical Study On The Thermoelectric Properties Of Several Ternary Zintl Phase Compounds

The increasingly serious energy shortage and environmental pollution problems have seriously affected human survival,and the development and utilization of green and renewable energy has more and more urgent.The discovery of the Seebeck effect,Peltier effect and Thomson effect in the 19^th century provide key theoretical support for human research on thermoelectric conversion technology.Thermoelectric materials are a type of functional materials which can directly convert thermal and electrical energy into each other.However,the current low thermoelectric conversion efficiency limits the extensive application of thermoelectric devices.The conversion efficiency of thermoelectric materials is measured by the dimensionless figure of merit ZT.The higher the ZT value,the higher the thermoelectric conversion efficiency.High thermoelectric conversion efficiency requires good electrical transport and poor thermal transport.However,the conductivity,Seebeck coefficient and electronic thermal conductivity that affect the figure of merit are coupled through the carrier concentration and the energy band effective mass,giving rise to difficulties in improving the thermoelectric conversion efficiency by regulating the carrier concentration through doping,defects,alloying and other means.Moreover,complex control methods at atomic,nano,and mesoscopic scales reduce lattice thermal conductivity while weakening electrical properties.Therefore,the design concept about thermoelectric materials with the characteristic of"Phonon Glass-Electron Crystal（PGEC）"emerges,which means that ideal thermoelectric materials ought to have phonon transport properties like glass and electron transport properties like crystals.The proposal of this concept plays an important guiding role in designing and optimizing high performance thermoelectric materials.Zintl phase compounds are ideal thermoelectric materials which satisfy the"Phonon Glass-Electronic Crystal"characteristics.Its excellent electrical transport performance and low lattice thermal conductivity have attracted attention.Although many experimental and theoretical studies prove that Zintl phase compounds have intrinsic low lattice thermal conductivity and good electrical transport properties,further research is needed to design higher ZT values and explore the micro mechanisms which result in the good thermoelectric properties of Zintl phase compounds.This thesis adopts the first principles method combined with semi-classical Boltzmann transport theory to discuss the origin of low lattice thermal conductivity about Ba₃Al Sb₃ and Sr₃Al Sb₃.A more universal design rule for layered thermoelectric materials is constructed by utilizing the lattice structure,charge balance,and characteristics of single-layer two-dimensional materials about Zintl phase compounds.The research work in this thesis is mainly divided into the following two parts:Employing density functional theory combined with electron and phonon Boltzmann transport theories,we report that Sr₃Al Sb₃ and Ba₃Al Sb₃ within the Zintl 3-1-3 compositional family exhibit lattice thermal conductivities of 0.78 and 0.55 W/m K at room temperature,respectively.These low thermal conductivities are rooted in low-energy optical phonon modes with strong anharmonicity and the emergence of flat optical phonon modes.Moreover,heavier cationic atoms are found to soften low-lying optical phonon modes,which enhance phonon scattering and further favor a lower thermal conductivity.These combined characteristics lead to high and balanced figure of merit values around 2.3 for Zintl compound Ba₃Al Sb₃,about 1.82 and2.34 for Sr₃Al Sb₃ at both optimal p-type and n-type doping and 800 K,respectively.Our work highlights the important role of flat optical phonon modes on designing promising thermoelectric materials with intrinsic low thermal conductivity.In recent years,layered thermoelectric materials favour lower lattice thermal conductivities,low-dimensional electrical properties,and highly anisotropic electron transport behaviours owing to the two-dimensional crystal structure with strong intralayer and weak interlayer bonds,thus offering great opportunity to disentangle the interconnected thermoelectric parameters.According to the characteristic of layered thermoelectric materials,we propose a design concept for layered thermoelectric materials:Searching for a stable charged monolayer,the metal ionic sublayer can be viewed as cation atoms occupying specific lattice points,and act as electron supplier to fulfill the charge balancing rule when surrounding the charged monolayer（s）and thus forming interlayer ionic bond.Following this new line of thinking,ionic layered thermoelectric materials can be considered as cation stabilizing charged monolayer network.This offers two unique freedoms to design the crystal structures,which potentially leads to novel layered thermoelectric materials,and better thermoelectric performance.The first is the structural type and repeating number of encapsulated monolayers,the second is the element type and occupying lattice positions of the surrounding cations.Based on the design concept combined with the electron and phonon Boltzmann transport theory,we successfully predict a series of 1-2-2 Zintl phase families after extensive screening and use the new phase Ba Zn₂As₂as an example to study the lattice structure and thermoelectric properties,which exhibits the lattice thermal conductivity of 2.61 W/m K at room temperature,close to the lowest lattice thermal conductivity of the block material.Coupled with good intralayer electrical transport performance,the maximum ZT value reaches 2.12 at room temperature.Our work provides theoretical guidance for designing efficient layered thermoelectric materials.