Theoretical Studies On The Thermoelectric Properties Of Several Novel Ternary Compounds

In many areas of human life,the rapid growth of energy demand and the corresponding environmental pollution highlight the importance of developing clean and renewable energy.Thermoelectric technology can directly convert between electrical energy and thermal energy through solid materials,thus causing great research interests.Nevertheless,thermoelectric devices have not been widely used at present.This is mainly caused by the very low conversion efficiency of thermoelectric devices.The conversion efficiency of thermoelectric materials is governed by a dimensionless figure of merit ZT.The larger the ZT is,the higher the conversion efficiency of thermoelectric materials will be.Due to the entanglement of several electrical transmission parameters that affect the ZT,the huge increase of ZT has encouraged great challenges.However,the weak coupling between the electrical transport properties and the lattice thermal conductivity makes it possible to find novel the materials with low lattice thermal conductivity and to study its micro mechanism of high thermoelectric properties,as well as accelerate the practical application of novel thermoelectric materials,which has become the main research direction in the field of thermoelectricityTherefore,in this thesis the first-principles calculations based on density functional theory and semi-classical Boltzmann transport theory are used to investigate the electronic transport performance of the novel ternary compounds Ba/Mg/Si(BaMgSi,Ba2Mg3Si4 and BaMg2Si2)and X2YZ(X=Ca,Sr and Ba;Y=Au and Hg;Z=Sn,Pb,As,Sb and Bi),and the dynamic properties of the materials were studied by the density functional perturbation theory.The main research contents in this thesis are as follows1.The electronic structure,phonon transport properties,and thermoelectric properties of BaMgSi,Ba2Mg3Si4 and BaMg2Si2 compounds,which are complexd filling the honeycomb crystal structures with heavy element Ba,are systematically studied.The results show that the special crystal structures of the three compounds can form multi-phonon scattering centers and atomic rattling vibration effects,which reduces the lattice thermal conductivity effectively.With the decrease of Ba content,the transition from semiconductiver BaMgSi,semimetallic Ba2Mg3Si4 to metallic BaMg2Si2 occurred can be observed.Phonon transport properties studies results have shown that the BaMgSi,Ba2Mg3Si4 and BaMg2Si2 materials all have apparent obvious anisotropy in lattice thermal conductivity,and they obtain the minimum average lattice thermal conductivity occurs at 1000 K,which are 0.37W/mK,0.38 W/mK and 1.0 W/mK,respectively.Besides,all the three materials BaMgSi,Ba2Mg3Si4 and BaMg2Si2 all have high Gruneisen parameters,which not only reflects the strong anharmonic interaction,but also shows that the phonon-phonon scattering is very strong.Combining the electrical transport properties and the lattice thermal conductivity of BaMgSi,Ba2Mg3Si4 and BaMg2Si2 shows that BaMgSi has the best thermoelectric performance among the three materials.Further investigation shows that at the optimal carrier concentration,the maximum ZT value of the p-type BaMgSi is 2.96 at 800 K,and the maximum ZT value of the n-type BaMgSi is 1.68 at 1000 K.This thesis successfully finds a most promising thermoelectric material BaMgSi among the novel ternary Ba/Mg/Si compounds,which provided theoretical support for furtherdesign of thermoelectric material,together with the experimental researches.2.Based on first-principles calculations,the electronic structures and transport properties of the novel full-Heusler X2YZ(X=Ca,Sr and Ba;Y=Au and Hg;Z=Sn,Pb,As,Sb and Bi)alloys were investigated.In order to obtain a more accurate band gap,we adopted the GGA-PBE exchange correlation potential,considering the spin-orbit coupling(SOC)and TB-mBJ correction.Firstly,we use different exchange correlation functionals to calculate the band structures of these alloys with and without SOC effect.It is found that SOC not only affects the band gap values but also affects the symmetry and dispersion of the energy bands at valence band top and conduction band bottom,hence it cannot be ignored when calculating the electron transport properties.Secondly,based on the electronic structures calculated through the GGA-PBE+SOC+MBJ,we analyzed the electrical transport properties of these compounds.Finally,we calculated the ZT of X2YZ(X=Ca,Sr and Ba;Y=Au and Hg;Z=Sn,Pb,As,Sb and Bi),and predicted the minimum value of ZT at room temperature more than 2.0.Interestingly,Ba2AuBi and Ba2AuSb have very similar p-type and n-type thermoelectric properties under the same carrier,which is of great significance for commercial thermoelectric device manufacturing.