Narrow Results

The advancement in new materials processing and fabrication techniques has made it possible to better control the atomistic level of structures in a way, which was not feasible only a decade ago. If one can couple this atomic control with a good understanding of the relationship between structure and properties, this will in the future lead to a significant contribution to the synthesizing of tailor-made materials. In this paper we have focused on, the structurally related nanolayered ternary compounds M_N+1AX_N, (MAX) where N = 1, 2 or 3, M is an early transition metal, A is an A-group (mostly IIIA and IVA) element, and X is either C and/or N, which has attracted increasing interest owing to their unique properties. The general relations between the electronic structure and materials properties of MAX phases have been elaborated based on ab initio calculations.

We have investigated theoretical Vickers hardness, thermodynamic and optical properties of four zirconium metal-based MAX phases Zr₂AC (A = Al, Si, P and S) for the first time in addition to revisiting the structural, elastic and electronic properties. First-principles calculations are employed based on density functional theory (DFT) by means of the plane-wave pseudopotential method. The theoretical Vickers hardness has been estimated via the calculation of Mulliken bond populations and electronic density of states. The thermodynamic properties such as the temperature and pressure dependent bulk modulus, Debye temperature, specific heats and volume thermal expansion coefficient of all the compounds are derived from the quasi-harmonic Debye model. Further, the optical properties, e.g., dielectric functions, indices of refraction, absorption, energy loss function, reflectivity and optical conductivity of the nanolaminates have been calculated. The results are compared with available experiments and their various implications are discussed in detail. We have also shed light on the effect of different properties of Zr₂AC as the A-group atom moves from Al to S across the periodic table.

MAX phases captured attention ever since they were synthesized. They are suited for different applications, especially in high-temperature and extreme environments. So knowledge of mechanical properties is essential for our applications of MAX phases in different situations. Cr₂AC (A = Al, Ge) compounds belong to M₂AX phases. But, few of their physical properties have been studied. This work demonstrated the compression behavior of Cr₂AC (A = Al, Ge) by first-principles method. The elastic constants were calculated in the range of 0–50 GPa, it is shown that they satisfy the requirement of mechanical stability. The values of bulk moduli, shear moduli, Young’s moduli, Possion’s ratio and Lame’s constant were calculated. We also studied brittleness/ductility of Cr₂AC(A = Al, Ge). For the more, we studied the dependence of a/a₀, c/c₀ and bulk moduli on the pressure below 50 GPa. In the end, we calculated the Mulliken bond overlap and theoretical hardness. We also discussed the bonding and antibonding.

Using ab initio calculations, we have studied the structural and elastic properties of M₂InC, with M=Sc, Ti, V, Zr, Nb, Hf and Ta. Geometrical optimization of the unit cell is in agreement with the available experimental data. We have observed a quadratic dependence of the lattice parameters versus the applied pressure. The elastic constants are calculated using the static finite strain technique. We derived the bulk and shear moduli, Young's moduli and Poisson's ratio for ideal polycrystalline M₂InC aggregates. We estimated the Debye temperature of M₂InC from the average sound velocity. This is the first quantitative theoretical prediction of the elastic properties of Sc₂InC, Ti₂InC, V₂InC, Zr₂ InC, Nb₂InC, Hf₂InC and Ta₂InC compounds, and it still awaits experimental confirmation.

The structural, elastic and electronic properties of Ti₂InC and Ti₂InN compounds have been calculated using the full-potential linear muffin-tin orbital (FP-LMTO) method. The exchange and correlation potential is treated by the local density approximation (LDA). The calculated ground state properties, including, lattice constants, internal parameters, bulk modulus and the pressure derivative of the bulk modulus are in reasonable agreement with the available data. The effect of pressure, up to 40 GPa, on the lattice constants and the internal parameters is also investigated. Using the total energy-strain technique, we have determined the elastic constants C_ij, which have not been measured yet. The band structure and the density of states (DOS) show that both materials have a metallic character and Ti₂InN is more conducting than Ti₂InC. The analysis of the site and momentum projected densities shows that the bonding is achieved through a hybridization of Ti-atom d states with C (N)-atom p states. Otherwise, it has been shown that Ti–C and Ti–N bonds are stronger than Ti–In bonds.

Mo₂Ga₂C, which is a new member of MAX phases family discovered recently, has been regarded as the first compound of M₂A₂X family considering that no members of M₂A₂X phases have been found. Our work conducted first-principles calculations of new nanolaminate Mo₂Ga₂C followed by comparison with Mo-containing M₂AX phases (Mo₂GaC). Our results have shown that electric conductivity and ductile properties are greatly increased by adding Ga-layer, while theoretical hardness is retained. The Mo₂Ga₂C is found to possess more metallicity.

The geometric structure, energy barrier and electronic properties of H-incorporated ${\mathrm{\text{Ti}}}_3{\mathrm{\text{SiC}}}_2/\mathrm{\text{Zr}}$ heterojunctions were investigated by first-principles calculations. Hydrogen atom settles in ${\mathrm{\text{Ti}}}_3{\mathrm{\text{SiC}}}_2/\mathrm{\text{Zr}}$ as interstitial impurity due to its small radius. Through calculating and analyzing the total energies of H-incorporated ${\mathrm{\text{Ti}}}_3{\mathrm{\text{SiC}}}_2/\mathrm{\text{Zr}}$ heterojunction, a much higher potential barrier (1.75 eV) was found when H atom diffuses from the interface into the ${\mathrm{\text{Ti}}}_3{\mathrm{\text{SiC}}}_2$ material than that (0.25 eV) into the Zr metal. The encountered potential barriers of H atom diffusing from vacuum into the ${\mathrm{\text{Ti}}}_3{\mathrm{\text{SiC}}}_2$ and Zr metal are also calculated, and they are both positive. These findings indicate that ${\mathrm{\text{Ti}}}_3{\mathrm{\text{SiC}}}_2$ is a suitable coating material to prevent the hydrogen embrittlement and corrosion in Zr metal. The electronic properties and valence bond properties of H-incorporated ${\mathrm{\text{Ti}}}_3{\mathrm{\text{SiC}}}_2/\mathrm{\text{Zr}}$ were analyzed based on the band structure, electronic density of states and Mulliken distribution. The calculated results show that all the H-incorporated ${\mathrm{\text{Ti}}}_3{\mathrm{\text{SiC}}}_2/\mathrm{\text{Zr}}$ heterojunctions exhibit metallic, covalent and ionic properties. These investigations may provide new insight into the underlying mechanisms of hydrogen diffusion in the ${\mathrm{\text{Ti}}}_3{\mathrm{\text{SiC}}}_2/\mathrm{\text{Zr}}$ heterojunction.