Atom Cluster Model Research Articles

Concurrent quantum/continuum coupling analysis of nanostructures

A general multiscale computational framework that concurrently couples the quantum-mechanical model with the continuum model is developed. This approach is then used to study the electronic properties of single-walled carbon nanotubes (CNTs) influenced by geometry and deformation. The electronic properties of the CNT, such as the band structure and band gap, are intimately connected to its electrical and thermal properties, mechanical stiffness, failure strength, chemical reactivities and many others. The CNT geometry is featured by a cylindrical-shaped structure, which leads to a mixture of the electronic structures that were originally present in graphite. These important effects are incorporated in a coarse-grained tight-binding model based on an extension to the Bloch theorem and the virtual atom cluster model developed previously by the authors. With this approach, we study the correlation among the electronic structures/properties, CNT geometry and applied deformation for a wide variety of tubes. The major technical ingredients and findings are (1) Compared with the computational cost of O ( n e 3 ) of the full-scale tight-binding calculation with n e being the electronic base functions used, we have developed an efficient concurrent approach as it scales with O( N G ) with N G being the number of quadrature points; (2) We have established a new coarse-grained cluster model that provides a direct way of coupling deformation mapping with the quantum-mechanical model. A unique feature of this coarse-grained model is that it does not use any stress and strain concepts as in a standard continuum approach; (3) We report strong influences of CNT geometry on its electronic properties. For zigzag tubes with chiral index of ( n, 0) and radius r, the band gap E g ∝ r −0.695 when the remainder of n divided by 3 (mod( n, 3)) is 1 and E g ∝ r −1.109 when mod( n, 3) = 2; (4) We report significant changes in the band gap and electrical conductance as functions of the applied tensile strain and twist. Transitions from metal to semiconductor and vice versa are observed; (5) There is a weak coupling between the band gap, electrical conductance and applied bending angle. Based on extensive studies, we conclude that the electron–mechanical coupling relations obtained in this work are more robust than the previous analytical studies in that it takes into account the important effects of curvature and relaxation. The simulation results highlight the importance of the concurrent coupling among the electronic properties, CNT geometry and mechanical deformation.

Electronic theoretical study on the corrosion and passivation mechanism of Ti metal

The atomic cluster models of the crack in αTi formed by dislocation accumulation were set up. The electronic structure of Ti was calculated by using recursion method, and the influence of O, Cl, and Pd on the electronic structure of Ti was studied. The calculated results show that, the total density of states near Fermi level is lowerd due to the existence of O, which leads to the decrease of the chemical activity of Ti. The binding energy of Ti is reduced by oxygen. The affinity between O and Ti is large, so O is easy to react with Ti to form the oxide film. The stability of Cl in Ti and the affinity with Ti are not as good as O. Cl is difficult to substitute the O atom on the surface of Ti, so the oxide film of Ti is very stable, the phenomenon of over passivation can not occur. The environment-sensitive embedding energy of Pd in the crack is smaller than in αTi grain, so Pd is easy to diffuse to the crack and makes the environment sensitive embedding energy of H rise obviously, which leads to the weakening of H diffusion to the crack and the improvement of the stress-corrosion resistance of Ti.

The influence of impurities on segregation of the (110) surface of O/RhxPt1-x alloy system

The atomic cluster models of (110) surface of RhxPt1-x disordered binary alloy were constructed under the condition that there is interaction between adsorption and segregation. The coverage of O is 0.5. Also，the models that may have an impact on the alloy debased with Cu，Ni and W were set up by computer respectively. It followed that impurities replaced every Rh which should be the nearest neighbor of O. The environment sensitive inlaid energy (EESE) and the electronic structure of the alloy surface doped with Cu，Ni and W was calculated by recursion method. The results of EESE show that, the influence of Ni，Cu and W on Rh-Pt alloy are all to reverse the segregation of the alloy surface. Furthermore，Ni is the most effective imupurity，and then Cu and W. Doped with impurities Cu，Ni and W，the covalent interaction between Rh and O is reduced，which reverses the surface segregation，so Pt segregates on the surface.

Theoretical study of ignition-proof mechanism of RE element in Mg alloys

The atomic cluster models of α-Mg, liquid Mg and the interface between liquid/solid have been founded by computer program. The environment-sensitive embedding energy of RE elements in α-Mg, liquid Mg, liquid/solid interface has been calculated by recursion method. The atomic affinity energy between Mg, La, Y with O has been defined and calculated. The calculated results show that the solid solubility of La and Y is very small in α-Mg, because their higher environment-sensitive embedding energy leads to instability in α-Mg crystal. The RE elements diffuse to the liquid Mg, which has lower environment-sensitive embedding energy than the solid, and congregate on the surface of liquid Mg as the alloy solidifies. Because the atomic affinity energy of RE-O is lower than Mg-O(The atomic affinity energy between Mg, La, Y with O are Mg-O：-14.9338eV，La-O：-19.0608 eV，Y-O：-19.5050 eV, respectively), the RE elements congregating on the surface of liquid Mg will priorily combined with O, forming compact RE oxides, which prevent Mg alloys from burning.

Lacunars: Nonfractal molecules with fractional mass dimension

Mass dimension D has been studied analytically and by computer simulation within the framework of the atomic cluster model for large numbers of lacunary molecules (lacunars) from the class of polycyclic hydrocarbons and polyphenyls. These molecules have a central framework and radial peripheral fragments. Due to the presence of lacunas (voids) between the peripheral fragments and because of growth of lacunas with increasing length of these fragments, these lacunary molecules, which are not self-similar fractal objects, have fractional dimension 1 < D c < 2 in the region of cores. It is established that D c(q) depends on the number q and length of peripheral fragments, on the core shape and the local symmetry and local anisotropy of the core, and on the character of space occupation by the molecule. For both classes of molecules, analytical dependences D c(q) have been obtained that explain the results of computer simulation. Similarity and differences between lacunars and fractals are discussed.

Simulation investigations in the binding energy and me-chanical properties of HMX-based polymer-bonded explosives

The molecular simulations of the well-known high explosive β-HMX (cyclotetramethylene tetranitramine) and its fluorine containing polymer-bonded explosives (PBXs) were carried out with the combination method of quantum mechanics, molecular mechanics and molecular dynamics. The atomic cluster model, containing the β-HMX molecule and the polymer molecule whose chain dimension was about the same as β-HMX’s, was fully optimized by AM1 and PM3 semi-empirical molecular orbital and molecular mechanical methods using COMPASS and PCFF force field. Then the calculated binding energy is found to be linearly correlated to each other. Molecular dynamics simulations using COM-PASS force field were performed for β-HMX crystal and the PBXs involving β-HMX and a series of fluorine containing polymers. Their elastic coefficients, moduli and Poisson’s ratios were calculated. It is found that the mechanical properties of β-HMX can be effectively improved by blending with fluorine containing polymers in small amounts.

First-principles study of rare-earth effects on grain growth and microstructure inβ−Si3N4ceramics

Rare earth (RE) and group III oxide additions are frequently used to optimize densification during the processing of ceramics. Silicon nitride ceramics frequently serve as model cases, and in these systems the effects of rare earths are important. Additions often determine the morphology of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Si}}_{3}{\mathrm{N}}_{4}$ crystallites that grow in the multiphase ceramic, thereby affecting the microstructure and mechanical toughness of the ceramic. The influence of different rare earths has recently been experimentally characterized in terms of their effects on grain growth aspect ratios. In the study reported here, a new energy parameter is introduced that provides a first-principles based understanding of these effects. Grain growth aspect ratios measured for various RE additions in silicon nitride correlate well with corresponding differential binding energies (DBE) calculated within the partial wave self-consistent field atomic cluster model. The DBE provides a second-difference measure of relative site stabilities of RE vs $\mathrm{Si}$ atoms in regions of variable $\mathrm{O}∕\mathrm{N}$ content. The physical mechanism that underlies anisotropic grain growth is found to originate from the site competition between REs and $\mathrm{Si}$ for bonding at $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Si}}_{3}{\mathrm{N}}_{4}$ interfaces and within the $\mathrm{O}$-rich glass. The different segregation strengths exhibited by rare earth elements in oxynitride glasses are simply a reflection of their different local chemistries in $\mathrm{O}$, $\mathrm{N}$ environments. Elements that segregate to the prism planes of the embedded $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Si}}_{3}{\mathrm{N}}_{4}$ grains impede the attachment of $\mathrm{Si}$-based silicon nitride growth units, and the extent of this limitation leads to the observed grain growth anisotropy.

A multiscale projection method for the analysis of carbon nanotubes

The main objective of this paper is to develop a multiscale method for the analysis of carbon nanotube systems. The multiscale coupled governing equations are first derived based on a multiscale partition, which consists of a coarse scale component represented by meshfree approximation, and a fine scale component to be resolved from molecular dynamics. It is noted that the coarse scale representation is present in the entire domain of the problem, and coexists with the fine scale representation in the enriched region. Using the projection properties of the partition, the decoupled non-linear set of equations are obtained and solved iteratively using Newton's method. In this multiscale analysis, a virtual atom cluster (VAC) model is also proposed in the coarse scale treatment. This model does not involve a stress update scheme employing the Cauchy–Born hypothesis, which has been widely used in the crystal elasticity theory. Finally, to approximate a curved surface at the nanoscale, meshfree approximations are introduced to interpolate a single layer of atoms. Unlike traditional shell or continuum elements, the geometric constraint is only imposed in two dimensions. The high order continuity property of the meshfree shape functions guarantees an accurate description of the geometry and thus the energy of the atomic bond. The accuracy and efficiency of the method are illustrated in the post-buckling analysis of carbon nanotube structures. To our knowledge, this is the first multiscale post-buckling analysis presented for such a nanoscale system.

A Virtual Atom Cluster Approach to the Mechanics of Nanostructures

A virtual atom cluster (VAC) model that represents the effect of interatomic bonding is developed as the constitutive model for crystal systems. In contrast with the crystal elasticity model, the proposed VAC model is distinguished by the following features: i) It does not build any constitutive relations that involve any stress concept, and ii) it does not use the homogeneous deformation assumption, or equivalently, the Born hypothesis. As a consequence of these attributes, the energy density of the system is embedded in the VAC model and directly related to the deformation mapping. The deformation mapping is constructed through the use of meshfree or finite element shape functions. The high-order continuity property of the meshfree shape functions guarantees the accuracy in describing the geometry and thus the energy of the atomic bond. The resulting formulation computationally more efficient than the continuum-based approach. Finally, the robustness of the method is illustrated through example problems involving various nanostructures.

Surface passivant effects on electronic states of the band edge in Si-nanocrystals

We studied the effect of the surface passivants fluorine (F), chlorine (Cl), oxygen (O) and oxygen-related OH on the energy band edge states of clusters with the same Si 29 and Si 47 core by means of the atomic cluster model and density functional theory (DFT). The results confirm that the electronic states of the band edge in clusters are sensitive to these passivants, and the passivant O that may form double bonded structure affects the band edge states most strongly. The results may be helpful for understanding and controlling the electrical and optical properties of nanocrystalline silicon.

Hydrogen chemisorption on a boron-terminated diamond (100) surface: an ab initio study

Ab initio molecular orbital theory and an atomic cluster model are used to study the structures and energetics of a boron-terminated diamond (100) surface, and hydrogen chemisorption on this surface. A 2×1 B–B dimer reconstruction is shown for the boron-terminated C(100) surface and the strength of B–B dimer bond is found to be stronger than that of a π bond in the CC dimer of a clean C(100)2×1 surface. Chemisorption of hydrogens on the boron-terminated C(100)2×1 surface breaks the surface B–B dimer bonds without activation energy which is in contrast to that on the clean C(100)2×1 surface. The strength of a B–H bond on the boron-terminated C(100) surface is found to be weaker than that of a C–H bond on the clean C(100) surface and so is the dehydrogenation energy.

AB INITIO STUDY OF ABSORPTION AND EMISSION PROPERTIES OF BGO AND BSO

Bismuth ortho-germanate (BGO) and bismuth ortho-silicate (BSO) crystals are described within an atomic cluster model. Electronic ground and first excited state for both crystal structures are calculated through unrestricted Hartree–Fock and configuration interaction methods, where the associated cluster geometries are optimized. The theoretical transition energies corresponding to the absorption and emission processes are very revealing. The displacement of one of the oxygen ions away from the bismuth in the excited state, together with the distributed spin density clearly shows that the excitation process cannot be understood as a deformed bismuth ion excitation. Instead, a molecular-like excitation is proposed.

Theoretical study of CH bonds of a-C:H

Several factors influencing the CH bonds of a-C:H were investigated with atomic cluster models using ab initio molecular orbital theory. The lengths of CH bonds were optimized and the Mulliken bond orders of CH were performed. The results show that the doped atoms and boundary dangling bonds can weaken and lengthen CH bonds largely and the presence of lattice vacancies only influences the CH bonds to a small extent. The results are discussed on the basis of cloud distributions and the changes of dipole moments of the clusters. Our results agree reasonably well with some of the experimental ones obtained by other groups [1] (Liu, Y. C., Demichelis, F. and Tagliaferro, A., Solid Slate Communications, 1996, 100, 579).

An Atom Cluster Model ofF-Type Icosahedral Quasicrystals, Derived from the Structure of 1/1 Cubic Approximant Phase (α-(AlPdMnSi))

We propose an atom cluster model of F -type icosahedral quasicrystals based on the structure of an α-(AlPdMnSi) phase, which is a p / q =1/1 cubic approximant of icosahedral quasicrystals. In the model, large dodecahedral and small icosahedral atom clusters, which are located at the (1/2, 1/2, 1/2) and (0, 0, 0) positions in the α-(AlPdMnSi) structure, respectively, are placed at twelvefold sites classified into two groups in the three-dimensional Penrose lattice. A diffraction pattern of this model reproduces well the characteristic features of an electron diffraction pattern of the F -type Al–Pd–Mn icosahedral quasicrystal.

Ab initio cluster model study of polymer–metal interactions

The interactions of hydrocarbon clusters representing polyethylene with Al, Cu and Zn are studied using an ab initio atomic cluster model. Relativistic effective core potentials and full point-group symmetry were employed. A single polyethylene chain is taken to be represented by two hydrocarbon clusters, one cluster is comprised of 3 carbon and 8 hydrogen atoms, the other hydrocarbon cluster is comprised of 5 carbon and 12 hydrogen atoms. The nature of the interactions are investigated by computing potential energy surfaces, electron populations and orbital energies of systems of interest. It is found that the interaction of hydrocarbon cluster with Al is stronger than that with Cu and Zn. This study suggests the feasibility of carrying-out relativistic effective core potential calculation to numerically characterize the comparative adhesion properties of polymers with different types of metal surfaces.

Higher Order Conditional Probabilities and Thermodynamics of Binary Molten Alloys

The recently introduced four atom cluster model is used to obtain higher order conditional probabilities that describe the atomic correlations in some molten binary alloys. Although the excess free energy of mixing for all the systems studied are almost symmetrical about the equiatomic composition, most other thermodynamic quantities are not and thus, the study enables us to explain the subtle differences in their physical characteristics required to describe the mechanism of the observed strong heterocoordination in Au–Zn or homocoordination in Cu–Ni within the same framework. More importantly, we obtain all calculated quantities for the whole concentration range thus complimenting experimental evidence.

Isoelectronic Trap Like Luminescence Centers Of Ingan

AbstractThis paper describes the characteristics of the luminescence centers observed in the various photoluminescence (PL) and photoluminescence excitation (PLE) spectra of hexagonal InxGa1−xN microcrystals, synthesized by the nitridation of sulfide in the In concentration range of 2%&lt;x&lt;6%. The grown crystals showed macroscopic and microscopic inhomogeneity. A series of component bands expressed by Gaussian functions with discrete peak energies selected from the frequently observed peaks in the PL spectra and full width at half maximum obtained from the narrowest PL bands were fitted well to observed broad emission bands. As some of the peak energies of the component bands coincide with those of the bands that are attributed to the decay of excitons at the localized states caused by the In content fluctuation in the literature, we concluded the luminescence center for the component bands also originates from the fluctuation of the In contents. Although the In contents of the samples determined by the X ray diffraction method were compatible the gap energies from the PLE measurements were different. The PLE spectra showed that the Stokes shifts were large and the gap energies were irrelevant to the emission band peaks. The excitation bands for the emission bands in the range of 2.8 to 2.48 eV were located at 3.36, 3.26, and 3.21 eV. These results indicate that the states of the luminescence center are localized and the energy levels are discrete. To obtain the microscopic structure of the localized states in InxGa1−xN, we propose the isoelectronic atom cluster model comparable with Te clusters in ZnSeTe.

Hardening behavior of amorphous alloy Al 90RE 5Ni 5 dispersion by nanoscale α-Al during crystallization

The crystallization process of amorphous alloy Al 90RE 5Ni 5 can be written as: Am Al 90RE 5Ni 5 — nano α-Al + Am AlRENi — α-Al + Al 3Ni + Al 11RE 3. The structure of the Al-based amorphous alloy was explained by an atomic cluster model. The growth of nanoscale α-Al in an amorphous matrix was reported. The hardening process can undergo two stages. One is controlled by a shear deformation mechanism of the amorphous matrix. Another is decided by the dislocations in the nanoscale α-Al grain. The microhardness of the amorphous alloy dispersion by nanoscale α-Al depends strongly on the average grain sizes D and the spacing δ between the grains in an amorphous matrix.

Ordering Phenomena in Na-Ga and Na-Sn Molten Alloys

Abstract The quasilattice approximation (QLA) has been applied to study the alloying behaviour of Na-Ga and Na-Sn molten alloys by assuming the formation of complexes of the form Na5Ga8 and Na4Sn3. This has been used to obtain the concentration dependence of the free energy of mixing, the long wavelength limit of the concentration-concentration fluctuations and the ordering parameter α1. Our calculations yield for both alloys, qualitative agreement with experiment about the concentrations at which phase separation or compound formation occurs in the alloy. In the case of Na-Ga, however, because of the relatively large asymmetry in the concentration dependence of its free energy of mixing and possibly the nature of the complex chosen, the calculated results when compared with experiment are not as good as that for Na-Sn. For Na-Ga, we have also calculated the higher order conditional probabilities using the four atom cluster model (FACM) this has made it possible to calculate another value for α1. A compar...

Nucleation and growth of platinum and nickel films on amorphous carbon substrates

The nucleation and growth of Pt and Ni films deposited on amorphous carbon (a-C) substrates by ion-beam sputtering (IBS) have been followed from an early stage through to complete substrate coverage. The influence of IBS parameters and substrate temperature Ts on deposition rate and metal particle (island) size distributions have been measured. Metal island growth is shown to be predominantly two dimensional in character. Soft-x-ray reflectivity measurements on Ni/a-C multilayers detect the single atom layer increments in Ni island thickness tNi′ during growth from the initial (critical) value tNi′ = 0.87 nm which is consistent with a theoretical 22 atom cluster model. The influence of the metal island growth stages on the soft-x-ray reflectivity of multilayer mirrors is considered.