First-principles Study About The Electronic Structures And Thermoelectric Properties Of ZrNiPb-based Half-Heusler Compounds

Thermoelectric materials can directly realize the reversible conversion between electricity and thermal energy based on Seebeck effect and Peltier effect,and have high potential for the collection of waste heat or electric cooling.With the advantages such as simple structure,abrasion free,no moving parts,quite,small,safe,maintenance free and etc.,they have extensive application prospects.The low efficiency of thermoelectric device is mainly the results from low performance of thermoelectric materials.The development of TE materials with high performance is of significant urgency for broader range of applications.Half-Heusler thermoelectric materials undergo a rapid development in the past two decades,which are suitable for middle and high temperature region.Half-Heusler alloys are MgAgAs-typed structures with a general formula of ABX,where A is a transition metal,a noble metal or a rear-earth element,B is a transition metal or a noble metal and X is a main group element.Half-Heusler alloys have high symmetry and heavy atoms.These characteristics lead to an easy doping with many potential dopants in the structure and exhibit excellent electrical transport properties.The improvement of thermoelectric performance of Half-Heusler alloys is limited by its high lattice thermal conductivity.It's significant important to optimize the performance of Half-Heusler for broader range of applications.In the present thesis,first-principles calculations were performed to investigate the influence of iso-electron doping on the band structure,lattice thermal conductivity and thermoelectric properties of ZrNiPb.The thesis including the following results:1.The electronic structure and thermoelectric properties of half-Heusler ABPb?A=Hf,Zr;B=Ni,Pd?compounds were investigated by the first-principle calculation.Our results indicate that all of the four compounds are narrow-gap semiconductors.The CBM mainly derived from A-d and B-d orbital,while the VBM mainly contribute by A-d orbital.The optimal p-and n-type doping concentrations with different temperature have been estimated.Comparing the calculated data with the experimental data of ZrNiPb,the relation between? and concentration,the thermopower S,power factor S s and the maximum power factors as a function of temperature were determined.The power factor is in good agreement with experimental data.The power factor as high as 4.22 ?W cm^-2K^-1 below 600 K in both of the p-and n-type.2.Based on first-principles calculations we systematically studied the electronic structure and thermoelectric properties of Zr1-xHfxNiPb?x=0,0.25,0.5,0.75,1?compounds with TB-mBJ and TB-mBJ+spin-orbit coupling methods.Our results indicate that these compounds are thermodynamic stabled narrow-gap semiconductors.The low frequency optical branches intersects with the acoustic modes below 75 cm^-1 for these artificial compounds,and then enhances the phonon scattering and decreases the thermal conductivity.The Hf substitution significantly decreases the lattice thermal conductivity from 13.1 Wm^-1K^-1?x = 0?to 0.23 Wm^-1K^-1?x = 0.25?at 300 K.We confirmed that the Hf doping hardly influence the power factors S²? in both of p-and n-type,but greatly decreases the thermal conductivity ?,i.e.improves the figure of merit ZT.The Zr_0.75Hf_0.25NiPb,Zr_0.5Hf_0.5NiPb and Zr_0.25Hf_0.75NiPb,with ultra low thermal conductivity?0.23,2.2 and 0.48 W/mK at 300 K?and higher power factor,may become new candidates of high performance thermoelectric materials in both of p-and n-type.