First-principles Study Of New MAX Phase Materials

MAX phase is found to combine many of the best attributes of metals and ceramics,so it attracts wide attention and interest.On the one hand,like metals,it has low density,low thermal expansion coefficient,high modulus,high strength and better high temperature oxidation resistance.On the other hand,like metal material,it has good thermal conductivity,conductivity,easy processing,plasticity and thermal shock characteristics.Because the MAX phase has this unique performance,it has hope to apply in harsh conditions,especially in high temperature environments.Therefore,it is important to study of MAX phase,especially through theoretical calculation.The new MAX phase can not only study the basic properties of atomic size,but also save research cost.First,the properties of the new MAX phase Mo2GeC material under high pressure conditions are studied by first principle calculation,namely the structure stability,mechanical properties and electrical properties of Mo2GeC under different pressure conditions.The results show that Mo2GeC is much more compressible along the a direction than along the c direction.The amount of compression along a or b axis is almost twice times as that along c axis.This strong resistance to the compression along the c direction is confirmed by the calculation of elastic constants,in which C33 increases rapidly with increasing pressure.The difference between the directional bulk modulus is much greater,which clearly illustrate this mechanical anisotropy.Moreover,Mo2GeC is dynamically stable from 0 to 50 GPa,because all phonon frequencies are positive.Finally our electronic structure calculation explains the existence of the covalent Mo-C bonds and with increasing pressure,the covalent Mo-Ge bonds and the metallic Mo-Mo bonds.Then,the new MAX phase V2AIN is studied versus existing V2AIC,based on the density functional perturbation theory.The phonon frequency is studied,and the thermodynamic properties of V2AlC and V2AIN are studied.The results found that both V2AIC and V2AIN are predicted to be metallic and the replacement of N for C results in the decrease in the DOS at Fermi level.In addition,since there is a higher value at the Fermi level of the DOS for V2AlC compared to V2AlN,the former can exhibit larger conductivity.With the substitution of C by N,the band structure energy decreases,so V2AIN is more stable than V2AIC.Bader charge analysis shows that the replacement of N for C leads to the increase of V and X charge and the decrease of A1 charge.Investigation of dynamical stability via phonon dispersion calculations shows that the V2AIX(X= C or N)are dynamically stable.Moreover,it is found that V and Al atoms have the important impact for the phonon modes in the low frequency region,but the lightest carbon or nitrogen mainly dominate in the high frequency region.Our results show that the thermodynamic properties of the V2AIN as a function of temperature have some differences compared to the V2AIC.Heat capacity,thermal expansion coefficient and bulk modulus of the V2AIN are higher than the V2AIC,mainly due to the stronger V-X(X= C or N)bonds and the modes downshift for the former.Finally,the paper has a brief summary,and prospects the theoretical research and potential application of the MAX phase.