Study On Thermoelectric Properties Of New Antimonides Based On First-principles

Thermoelectric materials are a kind of functional material which can realize the mutual transformation of heat and electricity,and the conversion does not need complex engineering system.Thermal energy can come from the heat released by various latest energy sources,such as sunlight,geothermal,nuclear energy,and so on,so thermoelectric materials have potential for application.The properties of thermoelectric materials can be reflected by the size of ZT.The ZT value of some traditional thermoelectric materials has been hovering around 1.0,and its maximum conversion efficiency is difficult to exceed 20%,which restricts the large-scale application of thermoelectrics.Nevertheless,more and more technologies have emerged in recent years,which have greatly improved the situation,such as nano-materials,low-dimensional materials,directional crystal growth,heterojunction composite materials and other new structures and processing methods,making thermoelectric materials can reach more than 2.0 in experiments.Bi-based materials with high thermoelectric properties have been extensively studied,such as Bi₂Te₃,Bi₂Se₃,Bi Cu Se O,etc.However,Sb-based compounds have been less studied.Sb is more than twice as abundant on the earth as Bi,and its price is relatively lower,and its mechanical stability is stronger than Bi.Therefore,it is very important to explore the possibility of antimonides in thermoelectric applications.In this paper,the thermoelectric properties of four kinds of antimonides,including Zintl phase compounds Mg₃Sb₂ and KSnSb,half-Heusler material Lu Pt Sb and alkaline metal antimonide Li₂NaSb,have been systematically studied by first-principles method combined with the accurate electron-phonon coupling theory.By the analysis of power factor and ZT value,the reason why these Sb-based compounds have excellent thermoelectric properties is found.The main results are as follows:1.In the field of thermoelectricity,many Zintl phases have not only good electrical transport properties but also low lattice thermal conductivity because of its special ionic structure.Mg₃Sb₂ is a widely studied Zintl phase,but its ZT value is difficult to reach up to 2.0.By comparing the thermoelectric properties of Mg₃Sb₂bulk and two-dimensional thin films with the first-principles calculation method,it is found that the"low-dimension"technology can greatly improve the thermoelectric properties,and the ZT value of nano-Mg₃Sb₂ thin films can reach 2.5.2.Another new layered Zintl material KSnSb has been predicted to have outstanding thermoelectric properties by high throughput screening.In order to verify this hypothesis,the thermoelectric properties of the compound were calculated by the electron-phonon coupling theory and the first-principles method.The multi-energy valley effect at the bottom of the conduction band of this material is obvious,which makes the Seebeck coefficient and power factor very high.At the same time,the material contains a variety of inter-atomic interaction forces,which makes the phonon scattering enhanced and the lattice thermal conductivity low.As a result,its ZT value can reach 2.2.3.Some topological materials have good thermoelectric properties,such as BiSe,Bi₂Te₃ and Bi₂Se₃,so there seems to be some relationship between the topological state and thermoelectric properties.In order to study this coupling relationship,the half-Heusler semi-metal Lu Pt Sb is taken as the research object to explore the possibility of its thermoelectric application.Because Lu Pt Sb is a semi-metal,its conductivity is very high and Seebeck coefficient is very small,its ZT value is only about 0.3.After hydrostatic pressure was applied,the topological phase transition occurs and the band gap is opened into a semiconductor.When hydrostatic pressure of18.3 GPa is applied to Lu Pt Sb,its ZT value can be optimized to 1.5.4.Li₂NaSb compound is a semiconductor with proper band gap.Because of the contribution of Sb around Fermi energy,the effective mass of density of states is large.The direct reflection of this is that the Seebeck coefficient increases.Meanwhile,the proper band gap makes its conductivity large.In sum,its thermoelectric power factor is high and its ZT value is 1.1.Therefore,the thermoelectric performance of Li₂NaSb is better but not excellent.Its high lattice thermal conductivity makes its ZT value low,but the average free path of phonon can be reduced by"nano-structure"technology.The theoretical prediction shows that the ZT value of nano-Li₂NaSb can be optimized to 1.7.