The Research And Rediction Of High Thermoelectric Performance Of Ba₂ZnPn₂?Pn=As?Sb?Bi? And FeZr/Hf_xNb_1-xSb_1-ySn_y

Because the serious increase of energy crisis and environmental deterioration,it is necessary for exploring and developing new energy.Thermoelectric materials can perform direct conversion between thermal and electrical energy,and have a good application prospect.In general,the thermoelectric materials with figure of merit?ZT?above 1 have a large-scale application.In fact,many traditional thermoelectric materials have a low thermoelectric conversion efficiency.Hence,it is important to seek new thermoelectric compounds and optimizing current thermoelectric properties are an important research field.Zintl phase compounds and Half-Heusler alloys are two new types of medium-and high-temperature good thermoelectric materials.In my thesis,the electronic structures and thermoelectric properties of the FeNbSb alloy?half-Heusler alloy?and Ba₂ZnPn₂?Pn=As,Sb,Bi??Zintl phase?were studied by using the density functional theory and semiclassical Boltzman theory.Main research results are concluded as follow:We calculated the relaxation times of Ba₂ZnPn₂?Pn=As,Sb,Bi?based on the deformation potential?DP?theory,and successfully predicted high ZT values of>2 along z-direction for p-type Ba₂ZnAs₂ and Ba₂Zn Sb₂.The Seebeck coefficient?S?of Ba₂ZnAs₂ monotonously increases with increasing temperature,which is favorable for achieving high thermoelectric performance in high temperature range.We also found that the four approximately degenerated bands?N_v=4?near the valence band edge mainly originating from the interaction between Zn atoms and Pn atoms,and the different strengths of Zn-Pn bonding lead to the different energy range spanned of the four bands.The weak Zn-As bonding decreases the dispersion of the four bands and leads to a sharply increased total density of states near the valence band edge,which will largely increase the S of Ba₂ZnAs₂.The strong Zn-Bi bonding increases the dispersion of the four bands near the VB edge,which reduces the effective mass of valence bands near the VB edge,and will be in favor of the carrier mobility.The coexistence of two heavy bands and two light bands near the VB edge contributes to their simultaneous high Seebeck coefficient and high electrical conductivity at the optimum carrier concentration.Moreover,the calculation results show that the ZTs of the p-type doping Ba₂ZnAs₂and Ba₂ZnSb₂ along the z-direction are all greater than 2.The previous experimental work showed that Hf-or Zr-doping has remarkably improved the thermoelectric performance of FeNbSb.Here,the first-principles method was used to explore the possible reason for such phenomenon.The substitution of X?Zr/Hf?atoms at Nb sites increases effective hole-pockets,total density of states near the Fermi level?E_F?,and hole mobility,largely enhancing electrical conductivity.It mainly originates from the shifting the E_F to lower energy and the nearest Fe atoms around X atoms supplying more d-states to hybrid with X d-states at the vicinity of the E_F.Moreover,we find that the X atoms indirectly affect the charge distribution around Nb atoms via their nearest Fe atoms,resulting in the reduced energy difference in the valence band edge,enhancing Seebeck coefficient.In addition,the further Bader charge analysis shows that the possible reason of more holes by Hf-doping than Zr in the experiment is most likely derived from Hf atoms losing less electrons and the stronger hybridization between Hf atoms and their nearest Fe atoms.Furthermore,we predict that Hf/Sn co-doping may be an effective strategy further optimizing the thermoelectric performance of half-Heusler?HH?compounds.