First-principles Study Of The Properties Of M₂GeC And Monolayer M₂C?M=V,Nb,Ta?

MAX is layered ternary machinable ceramics(their general formula is M _n+1AX_n,where M is a transition metal,A is an A-group element,and X is C or N,n=1,2,3),which can be classified according to the value of n,as‘211'for M₂AX?n=1?,‘312'for M₃AX₂?n=2?,and‘413'for M₄AX₃?n=3?.MAX is characterized by the nature of metals and ceramics.For example,it has relatively high stiffness,thermal shock resistance,excellent thermal and electrical conductivity,and machinability.On extreme conditions,it will display the nature that is different from that at atmospheric pressure.It,almost can meet the wide application on extreme conditions,thus people widely studied the application of MAX on extreme conditions?such as high pressure?.At present,the study on phase 211 and 312 in MAX materials is the most extensive,for example,the properties of Ti_n+1AC_n?n=1,2 A=Al,Si?have been revealed under high pressure,but there are few theoretical and experimental studies on M₂AC?M=V,Nb,Ta A=Ge,Si,As?under high pressure.Up to 2011,the experiment shows that MXene material preparation can be made through eroding MAX,MXene material has many better properties than MAX material and the type of graphene,which has great application prospects,and research on MXene is still in its early stage now.The third chapter in this thesis mainly studies the properties of M₂GeC?M=V Nb and Ta?under high pressure through theory.Chapter Four studies the two-dimensional monolayer bare M₂C material properties.Based on the first principle calculation,this paper studies the nature of the crystal structure of M₂GeC was studied under 0 to 100 GPa,which showed that the three kinds of crystal structure at high pressure might be stable.On the basis of M₂GeC lattice parameters in a?c in the direction of compression ratio,it shows that they are more difficult to be compressed in the c direction.This paper studied the electronic properties of M₂GeC and found that under the same pressure,the electrical conductivity of the material is mainly controlled by transition metal atoms.In the same crystal,when the pressure increases,the band structure becomes loose and the band width increases,while the pressure has little influence on the metal properties.The vibrational frequency of phonon spectrum is all positive,indicating that M₂GeC is dynamically stable under high pressure.And from phonon energy band diagram and the phonon density of states?PhDOS?,it can be seen that the quality of the smallest C atom contributes to the high frequency area and the low frequency zone is mainly contributed by atomic mass M and Ge atoms.It also studies the corresponding thermodynamic properties.By the calculation of elastic constant,when the pressure increases,the direction of the elastic constantly increases.In terms of directional modulus,elastic modulus on the direction of a Ba almost is unchanged,and the elastic modulus of the C direction Bc increases,which indicates that under the high pressure M₂GeC increases the mechanical anisotropy.At the same time,this article also systematically studied the two-dimensional monolayer M₂C system by the nature of electronic structure,which revealed they are golden properties,and found the Dirac point in the band diagram.,Afterwards,it analyzed the three kinds of materials of the phonon energy band and the corresponding?PhDOS?.They are no imaginary frequency in the phonon spectrum,which shows that they are dynamic stability and the gap between optical phonon band get wider from V to Ta.Besides,this study presents the Raman and infrared spectrometry in the center of brillouin zone of the vibration frequency,which is perfectly consistent with the theoretical and experimental data,This study also calculates the corresponding thermodynamic properties.Based on the strain-energy and density-Functional Perturbation Theory?DFPT?,the elastic constants and corresponding elastic parameters of the three materials are obtained.The value of young's modulus for Ta₂C is equal to 222 N m^-1,which is much higher than the value of MoS₂(130 N m^-1).Therefore,it can be defined as extremely stiff materials...