Atomic Distribution Function Research Articles

Using the Ab Initio Molecular Dynamics Method for Simulating the Peculiarities in the Temperature Dependence of Liquid Bismuth Properties

The specific features pertinent to the temperature dependence of the electronic and atomic properties of liquid bismuth that have been observed in experiments are investigated according to the ab initio molecular dynamics method using the SIESTA open software package. The density of electronic states, the radial distribution function of atoms, and the self-diffusion coefficient are calculated for the temperature range from the melting point equal to 545 K to 1500 K. The calculated data are in good agreement with the experimental data. It is found that the position of the first peak in the radial distribution function of atoms and the self-diffusion coefficient are characterized by a nonmonotonic dependence under the conditions of superheating by approximately 150 K above the melting temperature. In the authors’ opinion, this dependence feature is attributed to a change in the liquid short-range order structure.

Atomistic and Experimental Investigation of the Effect of Depth of Cut on Diamond Cutting of Cerium.

The ultra-precision diamond cutting process exhibits strong size effects due to the ultra-small depth of cut that is comparable with the cutting edge radius. In the present work, we elucidate the underlying machining mechanisms of single crystal cerium under diamond cutting by means of molecular dynamics simulations, with an emphasis on the evaluation of the effect of depth of cut on the cutting process by using different depths of cut. Diamond cutting experiments of cerium with different depths of cut are also conducted. In particular for the smallest depth of cut of 0.2 nm, shallow cutting simulations varying the sharpness of the cutting edge demonstrate that an atomically sharp cutting edge leads to a smaller machining force and better machined surface quality than a blunt one. Simulation results indicate that dislocation slip is the dominant deformation mechanism of cerium under diamond cutting with each depth of cut. Furthermore, the analysis of the defect zone based on atomic radial distribution functions demonstrates that there are trivial phase transformations from γ-Ce to δ-Ce occurred in both the machined surface and the formed chip. It is found that there is a transition of material removal mode from plowing to cutting with the increase of the depth of cut, which is also consistent with the diamond cutting experiments of cerium with different depths of cut.

STRUCTURE AND PHASE COMPOSITION OF W-Si MULTILAYER X-RAY MIRRORS

X-ray diffractometry in a hard region (l~0.154 nm) was used to study the phase structure, composition and construction of W/Si multilayer X-ray mirrors (MXMs) with thicknesses of tW<10 nm for tungsten layers obtained by direct-current magnetron sputtering. Two series of samples were fabricated with different tungsten deposition rates, which differ approximately by a factor of 4: ~0.60 nm/s and ~0.15 nm/s. It is shown that tungsten layers have a polycrystalline (BCC) structure at thicknesses tW>2.7 nm, and at tW<1.9 nm they are amorphous. Using the sin2Y-method, it was found that in thin crystalline layers of tungsten (tW<10 nm), more than 3 at.% Si can be contained. Tensile stresses in the layers of crystalline tungsten do not exceed 1.1 GPa. The construction of the radial distribution functions of atoms made it possible to establish that amorphous layers of tungsten have an arrangement of atoms close to b-W. In all samples, formation of silicide interlayers is observed at the interfaces, as a result of which the actual thickness of the tungsten layers is less than the nominal one. Amorphous silicide layers, necessarily formed at the stage of MXM manufacturing, contain tungsten disilicide. Depending on the deposition rate, disilicide can have an arrangement of atoms close to either the tetragonal phase, t-WSi2 (~0.6 nm/s), or to the hexagonal phase, h-WSi2 (~0.15 nm/s). An improved model for the construction of amorphous W/Si MXMs is presented. Mechanisms for the formation of silicide layers are proposed, according to which the bottom silicide interlayers (W-on-Si) are formed mainly by ballistic mixing of tungsten and silicon atoms, and the top ones due to diffusion inermixing. The interdiffusion coefficients were estimated, which made it possible to establish that the deposited surface of the layers can be heated at least 250° above the substrate temperature. The ways of reducing the interface interaction are suggested.

Investigation of the structure of CaO-Al2O3-SiO2 melts as a basis for the development of new agglomerated welding fluxes and industrial refractories

A significant number of welding fluxes and industrial ceramics are produced in the Ukraine. The production of these materials is sufficient for both domestic needs and significant export. It is assumed that the ternary CaO-Al2O3-SiO2 system may become the basis for the development of agglomerated welding fluxes and technical ceramics. Three samples of ternary CaO-Al2O3-SiO2 system were studied by means of high-temperature X-ray diffraction above the melting point: 23.3CaO- 14.7Al2O3-62.0SiO2 wt. % (sample 1, eutectic), 9.8CaO-19.8Al2O3-70.4SiO2 wt. % (sample 2, eutectic), 15.6 CaO- 36.5Al2O3-47.9SiO2 wt. % (sample 3). Experimental scattering intensity curves, structure factors and the radial distribution function of atoms were obtained. The structural parameters of short-range order were calculated using melt structure models obtained by Reverse Monte Carlo method. The existence of microparticle associates of the mullite or sillimanite type immersed in the slag matrix is assumed. It is assumed that the calcium ions coordinated aroundthese associates are surrounded by oxygen melt anions. Such a structure can be considered as a colloidal solution. The microparticle of mullite is the core of the aggregate that is surrounded by calcium and oxygen ions from the molten medium.

Conservative algorithms for non-Maxwellian plasma kinetics

We present a numerical model and a set of conservative algorithms for Non-Maxwellian plasma kinetics with inelastic collisions. These algorithms self-consistently solve for the time evolution of an isotropic electron energy distribution function interacting with an atomic state distribution function of an arbitrary number of levels through collisional excitation, deexcitation, as well as ionization and recombination. Electron-electron collisions, responsible for thermalization of the electron distribution, are also included in the model. The proposed algorithms guarantee mass/charge and energy conservation in a single step, and is applied to the case of non-uniform gridding of the energy axis in the phase space of the electron distribution function. Numerical test cases are shown to demonstrate the accuracy of the method and its conservation properties.

Fast and scalable prediction of local energy at grain boundaries: machine-learning based modeling of first-principles calculations

We propose a new scheme based on machine learning for the efficient screening in grain-boundary (GB) engineering. A set of results obtained from first-principles calculations based on density functional theory (DFT) for a small number of GB systems is used as a training data set. In our scheme, by partitioning the total energy into atomic energies using a local-energy analysis scheme, we can increase the training data set significantly. We use atomic radial distribution functions and additional structural features as atom descriptors to predict atomic energies and GB energies simultaneously using the least absolute shrinkage and selection operator, which is a recent standard regression technique in statistical machine learning. In the test study with fcc-Al [110] symmetric tilt GBs, we could achieve enough predictive accuracy to understand energy changes at and near GBs at a glance, even if we collected training data from only 10 GB systems. The present scheme can emulate time-consuming DFT calculations for large GB systems with negligible computational costs, and thus enable the fast screening of possible alternative GB systems.

Suprathermal oxygen atoms in the Martian upper atmosphere: Contribution of the proton and hydrogen atom precipitation

This is a study of the kinetics and transport of hot oxygen atoms in the transition region (from the thermosphere to the exosphere) of the Martian upper atmosphere. It is assumed that the source of the hot oxygen atoms is the transfer of momentum and energy in elastic collisions between thermal atmospheric oxygen atoms and the high-energy protons and hydrogen atoms precipitating onto the Martian upper atmosphere from the solar-wind plasma. The distribution functions of suprathermal oxygen atoms by the kinetic energy are calculated. It is shown that the exosphere is populated by a large number of suprathermal oxygen atoms with kinetic energies up to the escape energy 2 eV; i.e., a hot oxygen corona is formed around Mars. The transfer of energy from the precipitating solar-wind plasma protons and hydrogen atoms to the thermal oxygen atoms leads to the formation of an additional nonthermal escape flux of atomic oxygen from the Martian atmosphere. The precipitation-induced escape flux of hot oxygen atoms may become dominant under the conditions of extreme solar events, such as solar flares and coronal mass ejections, as shown by recent observations onboard NASA’s MAVEN spacecraft (Jakosky et al., 2015).

Analysis of Nanoprecipitates in a Na-Doped PbTe-SrTe Thermoelectric Material with a High Figure of Merit.

The dimensionless figure of merit, ZT, of bulk thermoelectric materials depends mainly on the transport properties of charge carriers and heat-carrying phonons. PbTe-4 mol % SrTe doped with 2 mol % Na (Pb0.94Na0.02Sr0.04Te) is a nanostructured material system that exhibits a ZT higher than 2. The precipitate size distribution of SrTe precipitates is believed to play a key role. This raises the question of whether its performance is limited by precipitate coarsening (Ostwald ripening) at elevated temperatures. Herein, we utilize an atom-probe tomography (APT) to study the number density and mean radii of precipitates in concert with partial radial distribution functions (RDFs) of individual atoms. We find that the SrTe precipitates actually contain oxygen: SrTe1-xOx. We correlate this information with the overall ZT performance, specifically focusing on the electrical and lattice thermal conductivities after isothermal heat treatments at 300 and 400 °C for 7 days, followed by furnace cooling. Comparison of the samples annealed at 400 and 300 °C demonstrates significant coarsening of SrTe1-xOx precipitates as well as strong segregation of oxygen impurities in the SrTe1-xOx precipitates. Additionally, on the basis of the partial RDFs, the Na dopant atoms cluster with other Na atoms as well as with Pb, Te, and Sr atoms; clustering depends strongly on the annealing temperature and concomitantly affects the overall ZT values. We found that the coarsening slightly increases the lattice thermal conductivity and also increases the electrical conductivity, thereby having little or even a beneficial effect on the ZT values. Importantly, these findings demonstrate that APT enables quantitative analyses in three dimensions of the PbTe-4 mol % SrTe samples in addition to correlation of their properties with the thermoelectric performance.

Molecular Gibbs Surface Excess and CO2-Hydrate Density Determine the Strong Temperature- and Pressure-Dependent Supercritical CO2-Brine Interfacial Tension.

In CO2 geological storage, the interfacial tension (IFT) between supercritical CO2 and brine is critical for the storage capacitance design to prevent CO2 leakage. IFT relies not only on the interfacial molecule properties but also on the environmental conditions at different storage sites. In this paper, supercritical CO2-NaCl solution systems are modeled at 343-373 K and 6-35 MPa under the salinity of 1.89 mol/L using molecular dynamics simulations. After computing and comparing the molecular density profile across the interface, the atomic radial distribution function, the molecular orientation distribution, the molecular Gibbs surface excess (derived from the molecular density profile), and the CO2-hydrate number density under the above environmental conditions, we confirm that only the molecular Gibbs surface excess of CO2 molecules and the CO2-hydrate number density correlate strongly with the temperature- and pressure-dependent IFTs. We also compute the populations of two distinct CO2-hydrate structures (T-type and H-type) and attribute the observed dependence of IFTs to the dominance of the more stable, surfactant-like T-type CO2-hydrates at the interface. On the basis of these new molecular mechanisms behind IFT variations, this study could guide the rational design of suitable injecting environmental pressure and temperature conditions. We believe that the above two molecular-level metrics (Gibbs surface excess and hydrate number density) are of great fundamental importance for understanding the supercritical CO2-water interface and engineering applications in geological CO2 storage.

Dynamical investigation of formation of Ni[sbnd]Pt nanoclusters in gas phase

The inert gas condensation process has been investigated for the formation of PtxNiy nanoclusters using molecular dynamics (MD) simulations and the obtained results have been averaged for the different formed clusters after 10ns of the simulation time. We have examined the different thermodynamic and structural properties in this work. The results indicate that the stability of the nanoalloys decreases as the concentration of Pt increases. Also, the sphericity of the nanoclusters tends to decrease as the concentration of Pt in the nanocluster increases. Using the results of radial distribution function (RDF) and number of the surface atoms, we found that the Pt atoms tend to lie at the outer layers of the clusters and the Ni atoms tend to lie in the core of the clusters. The nanoclusters also show solid-solid deformations at 20 and 50% of Pt in the clusters.

Molecular dynamic simulation of core–shell structure: study of the interaction between modified surface of nano-SiO2 and PAMAA in vacuum and aqueous solution

The interaction between acrylamide acrylicacid copolymer (PAMAA) and the modified surface of nano-SiO2 is investigated using the molecular dynamic (MD) simulation. The binding energies (Ebinding) of interface, the concentration profiles of PAMAA and functional groups (carboxyl and acylamino) of corresponding model, the mean square displacements (MSD) and diffusion coefficients (D) of PAMAA in four systems with different modifiers are all calculated at 325 K in vacuum. Vinyl trimethoxy silane (VTEOS) shows best modification effect in the systems mentioned above. Furthermore, the effects of temperature on the interaction between VTEOS modified surface of nano-SiO2 and PAMAA are studied at 300, 325, 350, 375 and 400 K in aqueous solution. Interesting results show that, water molecular layer reduces with the increase of temperature, and then improves the interaction between PAMAA and VTEOS modified surface of nano-SiO2. The corresponding Ebinding of interface, the radial distribution functions (RDF) of carbon atoms on the surface and oxygen atoms of water molecules, the concentration profiles of PAMAA on the surface of nano-SiO2, the MSD and D of PAMAA are all studied seriously to find the reason of this counterintuitive phenomenon.

All-atom molecular dynamics analysis of kinetic and structural properties of ionic micellar solutions

All-atom molecular dynamics simulation results regarding aqueous sodium dodecyl sulfate (SDS) solutions have been presented. Both salt-free solutions with different SDS concentrations and those containing calcium chloride additives have been studied. The simulation has shown that surface-active SDS ions form stable premicellar aggregates. The obtained molecular dynamics trajectories have been used to describe both the kinetic and structural properties of solutions containing SDS molecular aggregates and the properties of individual aggregates. Aggregation kinetics has been investigated, and the characteristic sizes of the aggregates have been calculated by different methods. It has been found that the size of a premicellar aggregate with aggregation number n = 16 in a salt-free solution virtually does not depend on surfactant concentration. Radial distribution functions (RDFs) of hydrogen and oxygen atoms of water molecules relative to the center of mass of an aggregate have no local maxima near the aggregate surface; i.e., the surface is incompletely wetted with water. Corresponding RDFs of carbon atoms have one, two or three maxima depending on the surfactant concentration and the serial number of a carbon atom in the hydrocarbon radical of the surface- active ion. The study of the potentials of mean force for the interaction of sodium and calcium ions with an aggregate having aggregation number n = 32 shows that only calcium ions can be strongly bound to such an aggregate.

Molecular dynamics and MesoDyn simulations for the miscibility of polyvinyl alcohol/polyvinyl pyrrolidone blends

ABSTRACTThe miscibility of polyvinyl alcohol (PVA)/polyvinyl pyrrolidone (PVP) blends is investigated by atomistic molecular dynamics (MD). Specific volumes of five PVA/PVP blends (with mass ratio (ω, ω = mPVA/mPVP) at 25%, 50%, 100%, 200% and 400%) as well as pure PVA and PVP are reviewed at the temperature of 298 K. The Flory–Huggins parameter determined from the cohesive energy densities and the radial distribution functions of the inter-molecular atoms suggest that PVA/PVP blends can be miscible at all sample. This is further supported by the morphologies of PVA/PVP blends. Then the phase separation kinetics of PVA/PVP blends was examined using density profiles calculated from the MesoDyn approach to examine mesoscopic morphology of the blends, and the result strengthening the view that the blends can be miscible with any sample mass fraction by the enthalpy contributions of H-bonds.

Atomic-scale simulation study of structural changes of Fe-Cu binary system containing Cu clusters embedded in the Fe matrix during heating

Nano-size Cu precipitates are the main products of irradiation embrittlement of nuclear reactor pressure vessel steels. Molecular dynamics simulation within the framework of embedded atom method is performed to study atomic packing change in Fe-Cu binary system, where the small Cu clusters are embedded in the crystal body centered cubic (BCC) Fe lattices. As the temperature increases, atomic packing change occurs in the Fe-Cu binary system. The mean square displacement of Cu atom, pair distribution function of the Cu atoms, and the atomic density profile along the radial direction are calculated. The atom packing structures in pure Cu region, Fe-Cu interface region, and pure Fe matrix are analyzed. The simulation results show that the packing structures in the Cu cluster and the Fe matrix are greatly affected by the sizes of these clusters and the volume of the Fe matrix containing these clusters. The structural changes present apparent differences, for the Fe matrixes contain these confined Cu clusters with different atom numbers during heating. As the Fe matrix can only provide small space to accommodate the Cu atoms, packing patterns in many Cu atoms are disordered for the Febulk-Cu135 system. In this binary system, strain region in the Fe matrix is adjacent to the Cu cluster. In the meantime, there are a lot of vacancy defects and strain regions in the matrix. For the Febulk-Cu141 system, although the Cu cluster contains more atoms, the Fe matrix can accommodate Cu atoms in a larger space, and the majority of these Cu atoms are located at the BCC crystal lattices. With increasing the temperature, the changes can be observed that the number of the strain regions decrease, whereas the sizes of some strain regions increase.

Crystal structural and diffusion property in titanium carbides: A molecular dynamics study

Titanium carbides were studied via molecular dynamics simulation to characterize TiC[Formula: see text] structures with respect to the carbon diffusion properties in this study. The effect of carbon concentration on atomic structures of titanium carbides was investigated through discussing the structure variation and the radial distribution functions of carbon atoms in titanium carbides. The carbon diffusion in titanium carbides was also analyzed, focusing on the dependence on carbon concentration and carbide structure. Carbon diffusivity with different carbon concentrations was determined by molecular dynamics (MD) calculations and compared with the available experimental data. The simulation results showed an atomic exchange mechanism for carbon diffusion in titanium carbide.

Molecular dynamics study on the liquid phosphorus tribromide by applying force-fields derived from quantum chemical approach

The purpose of this paper is to study the molecular dynamics simulation of phosphorus tribromide (PBr3) as the condensed inorganic liquid. To this aim, four regional intermolecular potential functions of phosphorus tribromide in the gas phase were considered. The potential energy data of 9 phosphorous tribromide configuration dimers were calculated at the B3LYP/6-31+G (d) level of theory. The calculated potential data were employed to construct a 4-site all-atom force field model. Then, the force field parameters were used to perform the molecular dynamics simulations and compared the simulation results with experimental data. Quantitative agreements for the atomic number density, internal energy, enthalpy, heat capacity and radial distribution function over a wide range of experimental conditions were obtained as a consequence of the parameters force field validation without using experimental data a priori.

Molecular modeling of OH− transport in poly(arylene ether sulfone ketone)s containing quaternized ammonio-substituted fluorenyl groups as anion exchange membranes

We present classical molecular dynamics (MD) and first principles molecular dynamics (FPMD) studies to investigate the transport mechanisms of hydroxide ions through poly(arylene ether sulfone ketone)s containing quaternized ammonio-substituted fluorenyl groups (QPE) as anion exchange polymers used in applications such as alkaline fuel cells. The effect of the number of repeating units in QPE on the ion conductivity of hydroxide ions was investigated by classical MD. Calculated ion conductivities are almost the same regardless of the number of repeating units in QPE and are consistent with the experimental values. The microscopic structures of water distribution in hydrated QPE were also investigated. Analysis of radial distribution functions (RDFs) of atoms calculated by classical MD implies the possibility of two different interacting conformations of hydroxide ions and ammonium groups in hydrated QPE. Distinct peaks in the RDF of O(hydroxide ion)–N(QPE) means that diffusion of hydroxide ions in hydrated QPE is mainly governed by surface diffusion, i.e., hopping between the ammonium groups of QPE, when the vehicle transport occurs. Transport of hydroxide ions based on the Grotthuss mechanism was also investigated by FPMD. The results show that hydroxide ion transport occurs with the formation of H3O2− through the hydrogen network of the water molecules. The formed H3O2− cleaves into H2O and OH−, resulting in proton exchange between a water molecule and a hydroxide ion, which is similar to the proton transfer mechanism observed in proton exchange membranes.

Mean-field theory of atomic self-organization in optical cavities

Photons mediate long-range optomechanical forces between atoms in high finesse resonators, which can induce the formation of ordered spatial patterns. When a transverse laser drives the atoms, the system undergoes a second order phase transition, that separates a uniform spatial density from a Bragg grating maximizing scattering into the cavity and is controlled by the laser intensity. Starting from a Fokker-Planck equation describing the semiclassical dynamics of the $N$-atom distribution function, we systematically develop a mean-field model and analyse its predictions for the equilibrium and out-of-equilibrium dynamics. The validity of the mean-field model is tested by comparison with the numerical simulations of the $N$-body Fokker-Planck equation and by means of a BBGKY hierarchy. The mean-field theory predictions well reproduce several results of the $N$-body Fokker-Planck equation for sufficiently short times, and are in good agreement with existing theoretical approaches based on field-theoretical models. Mean-field, on the other hand, predicts thermalization time scales which are at least one order of magnitude shorter than the ones predicted by the $N$-body dynamics. We attribute this discrepancy to the fact that the mean-field ansatz discards the effects of the long-range incoherent forces due to cavity losses.

Local Structural Distortion Induced Uniaxial Negative Thermal Expansion in Nanosized Semimetal Bismuth.

The corrugated layer structure bismuth has been successfully tailored into negative thermal expansion along c axis by size effect. Pair distribution function and extended X-ray absorption fine structure are combined to reveal the local structural distortion for nanosized bismuth. The comprehensive method to identify the local structure of nanomaterials can benefit the regulating and controlling of thermal expansion in nanodivices.

ANALYSIS OF COVALENT BONDS MULTIPLICITY DEPENDENCE ON CRYSTAL STRUCTURES OF NANO-SCALE PHASES OF ALLOTROPIC CARBON, RELEVANCE TO THE CLEAN ENERGY AND SOME OTHER APPLICATIONS

The approach based on the atomic radial distribution functions (RDF) theoretical building for volume, surface, and linear atomic structures was developed, accounting the escape depth of the analyzed signal, for interpretation of electron energy loses fine structure spectroscopy (EELFS) data. Basing on the made experiments and the theoretical investigations, the covalent bonds multiplicity values were determined for two carbine modifications formed on the different metal substrates with the different methods.