DFT INSIGHTS INTO Nb-BASED 211 MAX PHASE CARBIDES: Nb2AC (A = Ga, Ge, Tl, Zn)

Abstract

In this thesis, to study the 211 MAX phase carbides, the first-principles calculations were performed: Nb2AC (A = Ga, Ge, Tl, Zn) and the obtained properties of these phases are compared with those of Nb2AC (A = P, In, Cd, Al). With those from the prior studies, the structural characteristics are in appropriate agreement. Vickers hardness, stiffness constants and elastic modulus have been calculated to explore mechanical behavior. Besides, the structural stability of the studied compounds was checked using the phonon dispersion curves. Based on the elastic constants, the selected MAX phases are mechanically stable. Among the studied compounds, the value of Poisson’s ratio and Pugh ratio indicate that Nb2GaC (υ = 0.23 and G/B = 0.67) behaves as brittle solids whereas other studied compounds behave ductile nature. According to the values of Vickers hardness for studied materials can be ordered as follows: Nb2GaC > Nb2ZnC > Nb2TlC > Nb2GeC. On the contrary, Nb2PC (10.02 GPa and 9.31 GPa for GGA PBE and GGA PBEsol, respectively) has higher Vickers hardness than selected MAX phases. The anisotropy of the elastic characteristics was exposed through the 2D and 3D plotting of elastic moduli and calculating anisotropy indices. Utilizing the bond overlap and Mulliken atomic population, the mixture of ionic and covalent bonding has been explained among these carbides. To confirm the metallic behavior, the band structure and density of states (DOS) have been calculated. The discussion of the strength and bonding nature of different states also used Partial DOS. To explore the possible relevance in various fields, the optical characteristics of these selected phases have also been computed and analysed. Nb2AC (A = Ga, Ge, Tl, Zn, P, In, Cd, Al) MAX phases can be considered as prospective absorbing materials in this energy range because of the large absorption coefficients in the high energy range (7-10 eV). In order to bring out the potential relevance in high-temperature technology, the Debye temperature (ΘD), minimum thermal conductivity (Kmin), Grünisen parameter (γ) and melting temperature (Tm) were studied. The findings of the present research suggest that the mentioned carbides are suitable for usage as thermal barrier coating (TBC) and solar radiation-protecting coating materials.