Phase Stability And Property Prediction Of M_n+1AlNn Phase Ceramics Based On First Principle

The ternary layered ceramic may also be referred to a M_n+1AX_n phase ceramic,where X is C or N,A is an A-group element,M is early transition metal.The ternary layered ceramics exhibit an unique combination of both metal-and ceramic-properties,such as good electrical and thermal conductivities,high melting point,high Young's moduli,high fracture toughness,excellent thermal shock,oxidation and corrosion resistance,and so on.These excellent properties render it potential candidates for aviation,ship and high-temperature structural materials.The stable existence of compounds can be used in life and production.First-principle method based on DFT?density functional theory?is used to calculate the stability of M_n+1AlN_n phase compounds,the basic properties of M_n+1AlN_n phase compounds are studied.The thermodynamic stability of M_n+1AlN_n phase compounds was studied by linear programming method.In order to prove the reliability of the thermodynamic stability calculation,the thermodynamic stability of M_n+1SiN_n and M_n+1SiC_n compounds were also calculated.The theoretical results were in good agreement with the experimental results.The lattice dynamics behavior of the compounds was analyzed.The results show that the formation enthalpy of Ti₂AlN?Ti₄AlN₃?V_n+1AlN_n?Zr_n+1AlN_n?Hf_n+1AlN_n Zr₃AlN₂?Zr₄AlN₃ and Ti₃SiC₂ compounds is less than zero.So these compounds are thermodynamically stable.Ti₂AlN?Ti₄AlN₃?V₂AlN?Zr₂AlN?Hf₂AlN and Ta₂AlN have no imaginary frequency in phonon scattering graphs,these compounds is dynamically stable.The N atom appears almost in the high frequency phonon region.The phonon vibration frequency of M and Al atoms are below 18 THZ.In this paper,the equilibrium properties including lattice constant,theoretical density,volume and cohesive energy of M_n+1AlN_n?n=1?3?phase compounds were systematically studied using First-principle calculations.And the effects of VEC?valence electronic concentration?and d electronic shell for M atoms were investigated.The results show that the lattice constants a and c of M_n+1AlN_n phase compounds decrease with the increase of valence electron concentration of M atom,the volume decreases with the increase of valence electron concentration of M atom and the number of d electron layers,and the cohesion energy and theoretical density increase with the increase of valence electron concentration and n value of M atom.The electronic structure,chemical bonding,compressibility and elastic properties of the thermodynamic and intrinsic stability compounds were analyzed.The chemical bond stiffness was quantitatively studied by simple model.The M-Al bond has the lowest chemical bond stiffness in each system.Ti₄AlN₃?V₃AlN₂?V₄AlN₃?Zr₄AlN₃ and Hf₄AlN₃ have lower M-A bond stiffness than 1/2 of corresponding M-N bond stiffness,so they have high damage tolerance and fracture toughness.The vacancy defect energies of V₂AlN?Zr₂AlN and Hf₂AlN compounds are also calculated.The results show that the vacancy defect formation energies of N atoms are the largest.