Theoretical Investigation On Thermoelectric Properties Of Several Low Thermal Conductivity Half-Heusler Materials

The increasing depletion of non-renewable resources such as fossil fuels and the environmental pollution caused by the burning of fuels are the two major problems facing human beings today.It has become an urgent matter to explore new and environmentally friendly energy materials.As a new type of energy material,thermoelectric materials can realize the direct mutual conversion between thermal energy and electrical energy without moving parts or emissions,and have attracted great attention from scientists.However,the wide application of thermoelectric materials is limited by the low conversion efficiency,which is measured by the dimensionless thermoelectric figure of merit（ZT）:the larger the ZT value,the higher the conversion efficiency.In recent years,through the unremitting efforts of researchers,the development of thermoelectric materials has made great progress.However,large-scale commercial applications still face huge challenges,mainly due to the relatively weak mechanical strength of the material,relatively low ZT value,and poor thermal stability at high temperatures.Half-Heusler materials have the advantages of high mechanical strength,good electrical transport properties and thermodynamic stability,which make half-Heusler materials a promising thermoelectric material.However,the intrinsic lattice thermal conductivity of half-Heusler materials,which is widely concerned in experiments,is generally high（10 W/mK）.Although the thermal conductivity is greatly reduced by doping and other means,its thermal conductivity is still much higher.For materials with low intrinsic lattice thermal conductivity（such as SnSe with an intrinsic thermal conductivity of 0.5 W/mK at 300 K）,this limits the improvement of the thermoelectric performance of the half-Heusler system and its wide application.Therefore,this thesis adopts the first-principles calculation method,taking several half-Heusler compounds（CoNbSi,CoNbSnand PCdNa）with intrinsic lattice thermal conductivity as the research objects,and exploring the materials through detailed thermal transport property calculations.The physical mechanism with low lattice thermal conductivity,combined with the analysis of electrical transport properties,found that CoNbSi and PCdNa materials are potential high-performance thermoelectric materials.This provides theoretical guidance for finding and designing thermoelectric materials with intrinsically low thermal conductivity,and promising candidate thermoelectric materials for experimental synthesis.The main work is as follows:1.Two low-cost,environmentally friendly half-Heusler thermoelectric materials:CoNbSi and CoNbSnare systematically investigated using density functional theory.The phonon spectrum of the material is calculated by the supercellular finite displacement method,and it is proved that it has thermodynamic stability.After solving the phonon Boltzmann transport equation,Sheng BTE was used to obtain the lattice thermal conductivity,and it was found that the thermal conductivity of CoNbSi containing light elements was lower than that of heavy CoNbSn.Analysis of the reasons found that CoNbSi compound has weak bond,strong anharmonicity and large anharmonic scattering rate,resulting in its relatively low lattice thermal conductivity.In addition,by studying the electronic structure and electrical transport properties of the material,it is found that the energy difference between the first valence band and the second valence band of CoNbSi is very small,which means that high degeneracy can be obtained through energy band engineering,and then high electrical transport parameters.Finally,at 1000 K,combined with the electrical transport parameters and thermal conductivity,the thermoelectric figure of merit ZT of p-type CoNbSi can reach 2.1,and the optimal carrier concentration is 5.0×10²⁰cm^-3.The results show that CoNbSi is a very promising thermoelectric material due to its low lattice thermal conductivity and excellent electrical transport properties.2.Exploration of the physical mechanism of the ultra-low lattice thermal conductivity of PCdNa.A novel half-Heusler compound with low lattice thermal conductivity:PCdNa was calculated by first-principles calculations.Its lattice thermal conductivity is 2.6 W/mK at room temperature.Molecular dynamics simulations were performed using VASP to demonstrate its thermal stability.Through the calculated material phonon spectrum,it is found that the low-energy acoustic branch is mainly contributed by Cd atoms,and the existence of the acoustic branch and the optical branch avoids the crossover phenomenon,which indicates the rattling of Cd atoms.According to the calculated interatomic force constants,it is found that the bonding between Cd atoms and other atoms is weak,and Cd vibrates like a guest atom in the framework structure formed by P atoms.The rattling vibrational modes of Cd atoms enhance the anharmonicity of the material and reduce the phonon group velocity,resulting in lower lattice thermal conductivity,which is the physical mechanism for the ultra-low lattice thermal conductivity of PCdNa.In addition,its high valence band degeneracy and wrinkled Fermi surface are both beneficial to obtain good electrical transport properties.Based on the low lattice thermal conductivity and good electrical transport properties,the highest ZT value of p-type PCdNa at 900 K can reach 3.3.This suggests that the rattling vibration of Cd atoms is responsible for the ultralow lattice thermal conductivity of PCdNa and that PCdNa is a promising p-type high-temperature half-Heusler thermoelectric material.