First Principles Investigation Of Point Defect-related Properties In Mo₂GeC

MAX phase is a ternary layered carbide or nitride,and this layered structure result in a combination of outstanding properties of both metals and ceramics.This unique properties provide a broad array of application of MAX phases.For example,they can be used in defense material,portable electronic devices,medical application,sensors,aerospace,as well as protective coatings in future nuclear reactors..We found that Mo₂GeCexhibits good mechanical from investigation which make that Mo₂GeCcan be considered as a potential material for protective coatings and cladding materials in the future nuclear industry.MAX phases may unintentionally produce different kinds of defects during growth in high-irradiation or exposure to external environmental.Defects formation and interaction between them can induce various changes in the physical properties of structural materials.Therefore,as the potential material to use in nuclear reactors,the deep understanding of the physical properties for various defect will have a crucial influence.In this paper,We study the formation energy of various point defects in Mo₂GeCand the change of physical properties of Mo₂GeCdue to the existence of point defects by density functional theory,so as to judge whether materials are possible to be applied to nuclear reactions.The second chapter discusses the development process and theoretical framework of density functional theory.Firstly,we introduce the density functional theory,then we introduce the correlation function and the pseudo-potential.At least,we introduce the method of defect formation energy.In Chapter three,the formation energies of intrinsic defects（vacancy,interstitial,antisite,Frenkel pairs）and their effects on mechanical and electronic properties are studied.Among all intrinsic defects,only C vacancy defect has negative formation energy,andGe_FPhas the largest formation energy（5.82eV）.This shows that C vacancy is the most likely to be generated autonomously,andGe_FP plays a dominant role in the stability of Mo₂GeC.The formation of vacancies cause the structure shrink,while the existence of self-interstitial,antisite and Frenkel defect makes the structure expand.In order to better understand the stability of intrinsic defects,the density of state different intrinsic defects were calculated.The generation of vacancies and antisite defects lead to decrease of the density of states at the Fermi level,while the generation of self-interstitial and Frenkel defects lead to increase.In addition,the existence of C vacancies,_iC¹ and Mo-G eantisite pair result in produce a small pseudo gap at the Fermi level.In Chapter 4,the influence of impurity atoms H,He on the structure properties of Mo₂GeCwere discussed.After relaxation,there are two types of stable H interstitial defects,the results show that the formation energy of H interstitial is negative and the formation volume is positive.The He atom has two stable interstitial positions,the formation energy and formation volume of He interstitial defects are both positive,which shows that the existence of He interstitial defects lead to the expansion of crystal.The fifth chapter summarizes the work of this paper and makes preliminary assumptions for further work on this basis.