First Principles Theory Design Of Thermoelectric Transport Properties Of Ti₂-based Heusler Alloys

With the continuous development of industry,the energy crisis and environmental pollution have become difficult problems for human beings today.Thermoelectric materials can directly convert thermal energy into electrical energy which can play an important role in solving the energy crisis in the future.In this paper,we used the first principles calculation based on density functional theory(DFT)to study the structure,electronic structure and thermodynamic properties of several Ti₂-based Heusler alloys.The thermoelectric transport properties were studied in combination with Boltzmann transport theory,and a series of new Ti₂-based Heusler alloys were predicted.Based on density functional theory(DFT),the first principles calculations for electronic band structure and thermodynamic properties were performed by using the Perdew-Burke-Ernzerh of generalized gradient approximation(PBE-GGA)for the exchange and correlation function,and we studied thermoelectric transport properties of Hf?Mo-doped Ti₂Cr Ge high-entropy alloy Ti_0.75Hf Mo_0.25Cr Ge and electronic properties and thermoelectric transport properties of alloys Ti₂Mo Si,Ti₂Mo Pb and Ti₂Nb Sb.The TE transport properties are calculated using the semi-classical Boltzmann transport theory,and the calculated results are as follows:(1)The electron band structure of Ti_0.75Hf Mo_0.25Cr Ge which is Hf and Mo doping in Ti₂Cr Ge is half-metal,and a narrow band gap exists in the spin-down channel.The results of the phonon spectrum indicate that the structure is stable.The thermodynamic entropy of Ti_0.75Hf Mo_0.25Cr Ge is higher than that of the prototype Ti₂Cr Ge.The thermoelectric transport calculation results show that the spin-up state can obtain better thermoelectric performance when electron doping,and the spin-down state can obtain better thermoelectric performance when hole doping.The lattice thermal conductivity and ZT value of Ti_0.75Hf Mo_0.25Cr Ge and Ti₂Cr Ge were calculated by elastic properties.The results show that the lattice thermal conductivity of Ti_0.75Hf Mo_0.25Cr Ge is lower than that of Ti₂Cr Ge,the lattice thermal conductivity of Ti_0.75Hf Mo_0.25Cr Ge at room temperature is2.89Wm^-1K^-1,and the lattice thermal conductivity of Ti₂Cr Ge is 8.46Wm^-1K^-1.The ZT value of Ti_0.75Hf Mo_0.25Cr Ge in the spin-down state has the same trend as that of Ti₂Cr Ge,and the value of Ti_0.75Hf Mo_0.25Cr Ge in the spin-down channel is higher than that of Ti₂Cr Ge,and the maximum ZT values are 0.273 and 0.197,respectively.This result verifies the feasibility of the"entropy"gene regulating thermoelectric properties,and increasing entropy can be used as an effective method to design high performance thermoelectric materials.(2)The electron band structure of the Ti₂Mo X(X=Si,Pb)Heusler alloys indicates that they are all semimetal.Here,the semimetal is that the conduction band minimum is slightly overlapped with the valence band maximum,and there is no band gap.The results of thermoelectric transport show that Ti₂Mo Si can obtain better thermoelectric performance when electron doping,while Ti₂Mo Pb can obtain better thermoelectric performance when hole doping,and the maximum power factor of Ti₂Mo Pb is higher than that of Ti₂Mo Si.The lattice thermal conductivity of Ti₂Mo Si was calculated by elastic properties,and the ZT value was calculated.The results show that the lattice thermal conductivity at room temperature is 1.27Wm^-1K^-1,and ZT reaches a maximum value of 0.3 at 300K.(3)The electron band structure of the Heusler alloy Ti₂Nb Sb indicates that it belong to semimetal.The conduction band minimum is slightly overlapped with the valence band maximum,and there is no band gap.The results of thermoelectric transport show that Ti₂Nb Sb can get good thermoelectric performance by hole doping.When the hole doping concentration is 2.5×10²⁷m^-3,the PF can reach the maximum value(82.5 ?Wcm^-1K^-2)at 300K.