| Since the beginning of the new century,due to the rapid development of science and technology,the demand for new functional materials in aerospace,information engineering,artificial intelligence and other fields has been increasing.The performance requirements for new materials have also become higher and higher,so the task of developing and developing new functional materials Imminent.However,energy is inevitably used in the development and production of new functional materials.Today,the main source of energy is the combustion of fossil fuels,but the large-scale combustion of fossil fuels has caused various environmental problems.Therefore,finding efficient and environmentally sound energy for sustainable development is also the goal of our generation.Heusler alloy is a highly ordered intermetallic compound.The highly ordered arrangement of atoms brings rich physical properties and application functions to the material.Therefore,in-depth research on Heusler alloy for new functional materials and efficient and sustainable thermoelectric materials The development provides an idea.The structural,elastic,electronic,thermal and magnetic properties of Mn2-XFe1+XSi（X=0 0.25 0.5 0.75 1）Heusler alloys and the electron transport properties,phonon scattering properties,thermoelectric properties of Fe XLi Sb（X=Ti,Zr,Hf）thermoelectric materials were studied in the framework of the density functional theory（DFT）.In summary,the main conclusions of our research can be summarized as the following two aspects:1.The results show that Mn2-XFe1+XSi compounds are mechanically and dynamically stable with Hg2Cu Ti type（x=0,0.25 and 0.5）and Cu2Mn Al type（x=0.75and 1）;the volume of the series compounds decreases and the series compounds can change from brittle to ductile when the Fe content increases;the series compounds have half-metallic properties and the Fermi level gradually moves to the middle of the minority band gap with the increase of x,which makes the half-metal more stable;the magnetic moment of Mn2-XFe1+XSi mainly arises from more Mn-d orbitals than Fe-d orbitals and satisfies the extended Slater-Pauling rule.We also found that whether it is Hg2Cu Ti type,Cu2Mn Al type,or other highly ordered derived Full-Heusler alloys,the source of half-metallic is mainly due to the d orbital heterogeneity between the excessive metal elements at the A and C positions.The source of half-metallic is independent of the type of transition metal.2.At present,researches on thermoelectric materials of Heusler alloys are mainly based on Half-Heusler alloys.We have designed Quaternary-Heusler alloy thermoelectric materials based on the 18-electron rule.A comparative study of Half-Heusler alloy and Quaternary-Heusler alloy found that the figure of merit ZT of Quaternary-Heusler alloy is about double that of Half-Heusler alloy（ie,the thermoelectric conversion efficiency is doubled）and the thermoelectric conversion efficiency of the P-type semiconductor material is higher than that of the N-type semiconductor material.The Boltzmann transport equation was used to calculate the electronic structures and electron transport properties of the two types of Heusler alloys.It was found that the two structures have similar band gap structures near the Fermi level,which indicates that the electron transport properties of the two types of P-type semiconductors are also similar.Through the calculation of lattice thermodynamics,it is found that the lattice thermal conductivity of Quaternary-Heusler is about twice as small as that of Quaternary-Heusler alloy.This indicates that the high figure of merit ZT coefficient of the Quaternary-Heusler alloy is mainly due to the difference in lattice thermal conductivity.Our research on phonon scattering shows that:（1）As the number of atoms in the unit cell of the system increases,there will be more and more low-speed optical phonons in the phonon spectrum.They will carry more heat,but they will have a very low contribution to heat transfer.This is one of the reasons why Quaternary-Heusler has a lower lattice thermal conductivity than Half-Heusler.（2）We calculated the Green Eisen constants of two types of Heusler alloys and found that Quaternary-Heusler has larger Grüneisen constants than Half-Heusler.The material systems with larger Grüneisen constants tend to have strong anharmonic effects,so Quaternary-Heusler has lower intrinsic lattice thermal conductivity than half-heusler.（3）Filling the vacancies in the Half-Heusler becomes a Quaternary-Heusler.This filling can generate low-frequency local vibration,which will reduce the lattice thermal conductivity of the material.All in all,Fe-based Quaternary-Heusler alloys have higher thermoelectric conversion efficiency than Half-Heusler alloys.This research solves the problem of high thermoelectric conversion efficiency,which is difficult for P-type thermoelectric materials,and provides a new idea for designing thermoelectric materials with high conversion efficiency. |
