The Theoretical Research On Thermoelectric Properties Of ZrNiSn And NbCoSb

Energy is the basis of human existence and development.However,with the development of the global economy,excessive use of fossil energy has resulted in a lack of energy and environmental deterioration.The search for new clean energy is imminent.Thermoelectric material is a new type of energy conversion material,which utilizes the transport and interaction of carriers and phonons in solids to achieve direct mutual conversion between heat energy and electrical energy.Therefore,as a new type of clean energy technology,thermoelectric conversion technology can greatly solve the environmental pollution and energy shortage problems.However,the low conversion efficiency of current thermoelectric materials limits their large-scale applications.Finding and exploring ways and methods to improve the thermoelectric conversion efficiency of materials is of great significance.In this dissertation,the effect of Hf/Sb co-doping and Nb vacancy of ZrNiSn and NbCoSb on their electronic structure and thermoelectric properties was investigated by first-principles method and semi-classical Boltzmann transport.We found that:Existing experimental reports have shown that Hf/Sb co-doping significantly improves the thermoelectric properties of ZrNiSn alloy.We explore the possible reason.First,substituting of Zr by Hf strongly increases the conduction band edge degeneracy and dispersion,to a large extent,which increases the Seebeck coefficient,and increases the electrical conductivity a little.The possible reason is that,after Hf doping,near the vicinity of the Fermi level,the nearest Ni atoms around the Hf and Zr provide more d electrons to hybridize with Ni d electrons.Although Hf doping slightly increases electrical conductivity,the electrical conductivity of ZrNiSn was not so high.Then,when the Sb is further doped at the atomic position of Sn,not only the total density of states?TDOS?near the Fermi level is increased,but also a high carrier mobility is maintained,and thus the electrical conductivity is remarkably improved.In addition,the Bader charge analysis shows the reasons for supplying more electrons by Sb doping.It is derived from that Sb loses more electrons,and Sb-Ni has a stronger hybridization than Sn-Ni.Additionally,the Hf/Sb co-doping not only achieves the degeneracy of the band at the minimum value of the conduction band of 11,but also reduces the energy difference at the edge of the conduction band,increases the band's extreme value,and further increases the approximation degeneracy of the energy band.When the electron concentration increases,the Fermi level moves to higher energy,the conduction band energy valley involved in the transport increases,and the degeneracy is increased.This leads to the fact that the Seebeck coefficient little decrease as the carrier concentration increases significantly.Moreover,we predict that the ZT of Hf_0.5Zr_0.5NiSn_0.98Sb_0.02 at 1000 K can reach 1.37 with the carrier concentration of 7.56×10¹⁸ cm^-3,indicating that Hf/Sb co-doping is an effective approach in optimizing thermoelectric properties of ZrNiSn alloy compounds.Recent work by Zeier et al.shows that the 18-electron rule can be used to understand the nominally19-electron half-Heusler NbCoSb with Nb vacancies.Compared to Nb_0.8CoSb containing Nb vacancies,the Fermi level of NbCoSb enters the conduction band and exhibits typical degenerate semiconductor characteristics.The calculation results show that Nb deficiency of 0.2 can effectively adjust carrier concentration,and make the Fermi level of Nb_0.8CoSb move down,and at the same time,increase the degeneracy and band effective mass of the conduction band bottom X point,resulting an increase in density of states effective mass.Although a large density of states effective mass helps to obtain a high Seebeck coefficient,but at the same time,the high band effective mass can reduce the carrier mobility,thereby reducing electrical conductivity.Meanwhile,it is revealed that compared with the nominal NbCoSb,the appropriate Nb vacancy can not only eliminate impurity phase,but also optimize carrier concentration,thereby significantly improving thermoelectric performance.At 1100 K,the ZT peak of Nb_0.8CoSb reached0.29.This work indicates that Nb vacancy may be an effective method to improve the thermoelectric properties of NbCoSb.HH alloy compounds with 19 valence electrons with intrinsic holes may be new and promising thermoelectric materials.