Papers by Keyword: Cohesive Energy

Abstract: Recent advances in transmission electron microscopy (TEM) in respect of structural characterization down to atomic scale have enabled confirmation of stabilization of long ignored hexagonal omega (ω) phase in steel. The presence of ω phase is suggested to increase the strength of steel, and one of the factors concerning its stabilization is enrichment caused by the presence of certain solute atoms in the nanometer sized areas. Here, we report a density functional theory study conducted on a (3×3×2) ω –Fe supercell by introducing alloying elements in such a way that at a particular instant, either interstitial or substitutional C co-exist with any one of the elements Mn, Cr, Al, Si, and Co in substitutional position. From total energy calculations, we show that the cohesive energy of ω-Fe supercell increases in general, and the most stable combinations in the decreasing order of stability are C_sub-Cr > C_sub-Co > C_sub-Si. Even though the ferromagnetic state is more stable when compared to non-magnetic and antiferromagnetic configurations, the total magnetism of the supercell decreases as some of the atoms acquire negative magnetic moments. The density of states analysis shows that the d-band width of Fe decreases in presence of alloying elements, and this can lead to increased cohesive energy. Our results elucidate that the presence of minor alloying elements can be a factor in stabilizing the metastable ω-phase in steel.

Abstract: Based on the first principles and quantum mechanics, a new approach is put forward to calculate the cohesive energy of face-centered cubic solid neon, in which both the two-body and the total many-body interaction potentials are reasonably emphasized by a new combination formula. It shows that the new scheme is a simple and accurate tool to understand the high-pressure behaviors of solid neon, and it will be applied to calculate the compression curves of dense Helium, Argon, Krypton and Xenon at very high pressures. It is expected that this method can be applicable to all rare gas, including the gas, solid, and liquid phase regions, even of molecular systems, ionic systems.

Abstract: In this study, equilibrium lattice parameters, heat of formation and cohesive energy of four kinds of typical phases with different structure intermetallic compound in Al-Cu-Mg alloy were investigated by first-principles calculations based on density functional theory via CASTEP software. The calculation results are analyzed and show that ternary strengthening phase Al₂CuMg generated first when Mg content is higher, while binary strengthening phase Al₂Cu or Al₃Cu₂ first generated and more stable when Mg content is low in Al-Cu-Mg alloy which indicates that element Cu and Al alloying capacity significantly higher than that of Mg and Al element.

Abstract: We have investigated the elastic and thermal properties for perovskite SrCo_1-xSc_xO_3-d, by means of Modified Rigid Ion Model (MRIM). We have also computed the second order Elastic constants (SOECs) and their combinations. Besides we have reported the cohesive energy (f), Debye temperature (θ_D) and Gruneisen parameter (γ). The variation of specific heat (C) at temperature 15 K≤x≤1000 K is computed for SrCo_1-xSc_xO_3-d. The computed properties reproduce well with the available data in literature.

Abstract: The first-principles calculation within density functional theory is used to study in detail the electronic structure and ground state properties of alkali-metal oxoargenates A₄[Ag₄O₄] (A= Na, K and Rb). The total energies calculated within the atomic sphere approximation (ASA) were used to determine the ground state properties such as equilibrium lattice parameter, c/a ratio, bulk modulus and cohesive energy. The theoretically calculated equilibrium lattice constants values are in well agreement with the available experimental values. The electronic band structures, total and partial density of states are calculated. The result of electronic band structure shows that the KAgO and RbAgO are direct band gap semiconductors with their gap lying between the Γ-Γ points, whereas NaAgO is found to be an indirect band gap semiconductor with its gap lying between Z-Γ points.

Abstract: The object of this work is to investigate the interface and size effects on the structural phase transition of Nb nanoparticles (NPs) embedded in Cu matrix. By means of X-ray diffraction analysis and high-resolution transmission electron microscopy observation, it is found that higher coherency of the Cu/Nb interface benefits the occurrence of phase transition in Nb NPs with larger sizes. The sufficient conditions for the transition are: (1) the size of Nb NPs should be smaller than 8 nm; (2) the Cu/Nb interfaces should be semi-coherent or coherent. The experimental results are consistent with the predictions of Bond Energy model.

Abstract: The cohesive energy of W and Mo nanoparticles is modeled by considering W and Mo nanoparticles as Wulff construction. The energetic characteristics of Wulff construction is described by accounting for bond number in a system. The model predictions are consistent with the corresponding experimental results, especially when the diameter of nanoparticle is smaller than 1nm, which implies the closed packed structure for small nanoparticles.

Abstract: In this work, we are reporting on the simulation of the beryllium selenide (BeSe) nanowires (NWs) by computational package Q-Espresso / PWSCF according to the ab-initio calculations. Structural and electronic properties, including cohesive energy and Density Of State (DOS) BeSe NWs in two phases on the zinc–blende (ZB) and wurtzite (WZ), using density functional theory based on pseudo-potential approximation and generalized gradient approximation (GGA) up to 20 angstrom in diameter has been calculated. Due to dangling bonds (DBs) in the side surface NWs, cohesive energy is obtained less than the amount of this energy in bulk state of this compound, but with increasing diameter of NWs, the amount of this energy will approach to the bulk state. Comparison of cohesive energy with beryllium selenide NWs in two phases, we find these NWs in WZ phase is more stable and have good compatibility for this result with other results in NWs of similar compounds. The value of energy gap in these NWs on various diameters is obtained less than the amount of the bulk state. It is observed that by increasing the diameter of NWs, the cohesive energy approaches to its value in bulk state.

Abstract: The influence of grain size on the lattice constant in some nanocrystallites was studied by computing the interactive cohesive energy of the nanocrystallites. The relationships of the lattice constant with the grain size in NaCl, CsCl structure ionic crystallites, FCC, BCC structure metal crystallites and FCC, BCC, SCC structure molecular crystallites was studied respectively. The results are in good agreement with the above experimental ones qualitatively.

Abstract: The influence of grain size on the lattice constant in some nanocrystallites was studied by in experiment. It is found that the lattice constant of NaCl, CsCl structure ionic crystallites decreases with the reduction of the grain size. The lattice constant of ultrafine iron and nickel particles increases with the reduction of the grain size. As to the experiment result of g-Fe2O3 nanocrystallites, the lattice constant increases with the decreasing of the grain size. It is opposite to the theoretical result of ionic crystallites, which reason is that the combination of most of atoms in g-Fe2O3 is by the covalent bond other than electrovalent bond.