Coordination Number Distribution Research Articles

A Simulation Study of the Amorphous Germania Structure up to 70 GPa

This work presents the effect of pressure on the local microstructure of amorphous Germania (GeO2). The Molecular Dynamics simulation was performed on systems at 900 K and high pressure up to 70 GPa. The structural properties have been analyzed through the pair correlation function, coordination number distribution, and structure factors. The structural phase transition process starts occurring at 30 GPa. At above 30 GPa, the degree of structural order increases and the intermediate-range order structure depends strongly on pressure. Correlation between the peaks in the plot of structure factor and the topology of basic structural units GeOn(n = 4-6) is also discussed in detail in this paper.

Microstructural differences between naturally-deposited and laboratory beach sands

The orientation of, and contacts between, grains of sand reflect the processes that deposit the sands. Grain orientation and contact geometry also influence mechanical properties. Quantifying and understanding sand microstructure thus provide an opportunity to understand depositional processes better and connect microstructure and macroscopic properties. Using x-ray computed microtomography, we compare the microstructure of naturally-deposited beach sands and laboratory sands created by air pluviation in which samples are formed by raining sand grains into a container. We find that naturally-deposited sands have a narrower distribution of coordination number (i.e., the number of grains in contact) and a broader distribution of grain orientations than pluviated sands. The naturally-deposited sand grains orient inclined to the horizontal, and the pluviated sand grains orient horizontally. We explain the microstructural differences between the two different depositional methods by flowing water at beaches that re-positions and reorients grains initially deposited in unstable grain configurations.

Numerical simulation of particle velocity and coordination number under the slumping regime in a rotating drum

In this paper, a numerical simulation is applied to statistic the particle velocity and coordination number of the surface layer under the slumping regime in a rotating drum. We find that the particle velocity distribution under different coordination numbers in the avalanche always satisfies Maxwell distribution and exponential distribution. This indicated that the distribution of coordination number is the main factor causing uneven avalanche. Furthermore, the mathematical model of particle velocity and coordination number is established, and a novel method for calculating the kinetic energy of particle system is proposed. It is observed that the maximum kinetic energy of particles in different regions during the uneven avalanche is positively correlated with the avalanche duration. These results reveal the particle flow regular in the rotating drum, which provides reference data for improving the control technology of particle material flow and the heat transfer in industry.

Short‐Range Ordered Aluminum Foams

Ordering the bubbles of closed‐cell aluminum foams can contribute to decorative esthetics and create directional mechanical properties that disordered foams do not have. High‐porosity (>80%) aluminum foams prepared by the traditional static gas injection method usually have large and polyhedral cells, which do not form an ordered stacking. Aluminum foams with relatively uniform and small cells (cell size ≈1.2 mm) have been recently obtained by gas injection through a nozzle rotating at high speed. Herein, the stacking of aluminum foams with different cell sizes and a monodisperse aqueous foam are characterized by X‐ray tomography and compared with an ideal face‐centered cubic (FCC) structure. The aluminum foam featuring the smallest cells has a concentrated distribution of cell coordination number with a peak of 12 and the first peaks of the radial distribution function are found to be consistent with those of the monodisperse aqueous foam and an ideal FCC structure. Furthermore, many aligned bubble chains with more than five bubbles are observed on cross‐sectional images. Therefore, aluminum foam can become short‐range ordered whenever the cell size is uniform enough and reduced to around 1.2 mm. Methods for further improving the order of aluminum foam are discussed.

Nano material removal mechanism of 4H-SiC in ion implantation-assisted machining

As a third-generation semiconductor material, four-hexagonal silicon carbide (4H-SiC) finds wide application in industries. The high efficiency and performance machining of 4H-SiC ceramics have attracted researchers’ more attention. Ultra-precision diamond cutting is generally used to machine high-quality nanoscale surfaces. As a typical hard-to-cut brittle material, 4H-SiC causes significant tool wear during machining. It is necessary to study the effective process parameters and machining methods to improve the machining performance. In this study, the molecular dynamic (MD) analysis was used to comparatively investigate the effect of 4H-SiC by implanted different Ga ion implantation doses. The lattice damage, hydrostatic stress, radial distribution function (RDF), and selected area electron diffraction (SAED) were analyzed in detail. The cutting effect of implanted 4H-SiC with different implantation ions was then compared with unimplanted 4H-SiC. The machining properties were characterized by removed material morphological structure, temperature, coordination number, and stress distribution. The results provided an understanding of the mechanism of ion implantation-assisted nano-cutting. Besides, the effect of ion implantation-assisted and stress-assisted combined nano-cutting 4H-SiC was also investigated. The results demonstrated that the combination of ion implantation-assisted and stress-assisted could effectively decrease the cutting force and stresses, thus improving the machining efficiency and machinability of 4H-SiC.

A molecular investigation on lignin thermochemical conversion and carbonaceous organics deposition induced catalyst deactivation

Surface coking is the primary deactivation pattern of metal-based catalyst in biofuel reforming, which hinders the commercial utilisation of biomass. In this study, molecular dynamics simulation with reactive force field is performed to investigate the surface instability induced thermal degradation of Ni nanocatalyst and coke deposition induced catalyst deactivation. Coke deposited on catalyst surfaces is a complex mixture of carbonaceous organics. Coke surrogate models containing polycyclic aromatic hydrocarbons (PAHs) and oxygenated aromatics are proposed to reflect the molecular size and O/C ratio based on the molecular structures identified during lignin pyrolysis and soot inception mechanism. Lindemann index is used to characterize the degree of crystallinity of catalyst. It is found that atoms at unsaturated sites of outer shell show high mobility and tend to modify the coordination number distribution. Mechanisms behind the effects of temperature, PAH size and oxygen content on coke adsorption are revealed from three aspects of molecular collision dynamics, thermal dynamics and kinetics. The modification of crystallinity of catalyst outer shell and the occurrence of seeping after coke adsorption would affect the subsequent catalyst regeneration. This study is expected to provide guidance on the design of anti-coking catalyst, evaluation of catalyst regeneration and reactor optimisation.

Temperature-Dependent Properties of Molten Li2BeF4 Salt Using Ab Initio Molecular Dynamics.

Molten lithium tetrafluoroberyllate (Li2BeF4) salt, also known as FLiBe, with a 2:1 mixture of LiF and BeF2 is being proposed as a coolant and solvent in advanced nuclear reactor designs, such as the molten salt reactor or the fluoride salt cooled high-temperature reactor. We present the results on the structure and properties of FLiBe over a wide range of temperatures, 0–2000 K, from high-throughput ab initio molecular dynamics simulation using a supercell model of 504 atoms. The variations in the local structures of solid and liquid FLiBe with temperature are discussed in terms of a pair distribution function, coordination number, and bond angle distribution. The temperature-dependent electronic structure and optical and mechanical properties of FLiBe are calculated. The optical and mechanical property results are reported for the first time. The results above and below the melting temperature (∼732 K) are compared with the experimental data and with data for crystalline FLiBe. The electronic structure and interatomic bonding results are discussed in correlation with the mechanical strength. A novel concept of total bond order density (TBOD), an important quantum mechanical parameter, is used to characterize the internal cohesion and strength in the simulated models. The results show a variation in the rate of change in properties in solid and liquid phases with anomalous behavior across the melting region. The observed trend is the decrease in mechanical strength, band gap, and TBOD in a nonlinear fashion as a function of temperature. The refractive index shows a surprising minimum at 850 K, among the tested temperatures, which lies above the melting point. These findings provide a new platform to understand the interplay between the temperature-dependent structures and properties of FLiBe salt.

Effect of irradiation on the atomic structure of borosilicate glasses

AbstractBorosilicate glasses incorporating high‐level nuclear waste are exposed to high‐energy radiations during their storage in the deep geological repositories. However, the effect of radiation on the atomic structure of borosilicate glasses remains poorly understood. Herein, using molecular dynamics simulations, we study the irradiation‐induced structural changes of a series of calcium‐sodium borosilicate glasses with varying Si/B molar ratios—ranging from pure silicate to pure borate glasses. We observe that irradiation leads to an increase in disorder, both in the short‐ and medium‐range, as evidenced by the enthalpy, coordination number, and ring distribution. In particular, the impact of the change in the atomic structure (due to radiation) on the glass volume is investigated. Interestingly, we observe a composition‐dependent transition in the volumetric response of borosilicate glasses under irradiation—wherein borate‐rich compositions tend to swell, whereas silica‐rich glasses tend to densify. Through a detailed analysis of the structure, we demonstrate two competing mechanisms contributing to the volume change, i.e., a decrease in the coordination number of boron atoms and a reduction in the average silicon inter‐polytope angle. We also show that the increase in the disorder in the medium‐range order may play a major role in governing the volumetric changes in the irradiated structure in a non‐trivial fashion. Altogether, the present study highlights that irradiation has a non‐trivial effect on borosilicate glasses, which, in turn, could impact their corrosion kinetics.

DEM simulations of vibrated sphere packings in slender prismatic containers

Discrete Element Method (DEM) simulations are carried out to investigate the packing structure of the vibrated mono-sized and polydisperse particle assemblies in slender prismatic containers. Quantities like void fraction distributions, coordination number distribution, Voronoi packing fraction distributions were analyzed. DEM simulations show excellent agreement with the experimental data. The transient response of the packing structure during the vibration of mono-sized spheres reveals that the crystallization starts at the walls. The packing structure's crystallinity enhances during vibration, increasing the packing density of the assembly. Vertical vibration is found to be more effective compared to lateral vibration. The packing structure is found to be independent of the container's orientation during the filling process. The influence of the ratio of the container dimensions to mean particle diameter (X/dm) and polydispersity (λ) on the packing structure is analyzed. The results showed that the increase of X/dm ratio or λ, increases the random arrangement of particles.

Predicting the dissolution rate of borosilicate glasses using QSPR analysis based on molecular dynamics simulations

AbstractQuantitative Structure Property Relationship (QSPR) analysis based on molecular dynamics (MD) simulations is a promising approach for establishing the composition‐property relationships of glasses and other materials with complex structures. A series of 20 borosilicate, aluminosilicate, and boroaluminosilicate glasses have been modeled using MD simulations with recently developed effective potentials. Short‐ and medium‐range structures of these glasses were analyzed and, based on the structural information, QSPR analysis of the initial dissolution rates (r0) of these glasses that were measured at 90°C and pH 9 by using various structural descriptors such as percentage of bridging oxygen species, network connectivity, and average ring size. The structural descriptor, Fnet, which contains both energetic information such as single bond strength and structural information such as cation coordination number and Qn distribution, was also used. It was found that while the overall network connectivity, average ring size and Fnet provide reasonable correlations with r0 of studied glasses, Fnet gives the best correlation among the descriptors. For glasses that show incongruent dissolution, it was found that modification of glass compositions to account for preferential release of modifier cations is necessary to achieve best correlations. The findings were discussed with results of recent studies on evaluating the compositional dependence of glass dissolution behavior using the topological‐constraints‐based models.

Novel high specific heat capacity ternary nitrate/nitrite eutectic salt for solar thermal energy storage

A novel ternary eutectic salt KNO3-NaNO2-KNO2 (KNK) was designed and prepared for thermal energy storage (TES) in a concentrating solar power system (CSP). The thermo-physical properties of KNK such as melting point, decomposition temperature, fusion enthalpy, density, viscosity, thermal conductivity and specific heat capacity were determined respectively. Furthermore the thermal energy storage capacity was evaluated and discussed. Then a comparison of the thermo-physical properties between commercial Hitec and KNK was carried out. KNK showed smaller density, lower viscosity, slightly smaller thermal conductivity, much higher specific heat capacity and bigger thermal energy storage capacity. Raman spectrum displayed that the N-O of NaNO2 frequency peak at about 820 cm−1 in KNK had more significant shift, which could be attributed to a stronger interaction between Na+ and NO2−. Scanning electron microscope (SEM) showed K and Na elements of Hitec were uniformly distributed, while K and Na elements of KNK were in complementary distribution which could be caused by stronger repulsion between Na+ and K+. It was concluded that the stronger interaction and repulsion were correlated to the higher specific heat capacity. Molecular dynamics (MD) simulation was performed to explore the different macroscopic properties of two molten salts from the perspective at molecular level. The characterizations of radial distribution functions (RDF), coordination number and angular distribution functions (ADF) suggested that the interaction between Na+ and NO2− is stronger and that the arrangement of ion clusters was looser in KNK. The results were in good agreement with the analysis of Raman spectrum and SEM.

Effects of coefficient of friction and coefficient of restitution on static packing characteristics of polydisperse spherical pebble bed

The effects of the coefficient of friction and coefficient of restitution on the static packing characteristics of a polydisperse spherical pebble bed are numerically investigated using the discrete element method. Several important static packing characteristics under different coefficients of friction and restitution are presented and discussed. The results show that the coefficients of friction and restitution impose opposite effects on the packing heights and global packing factor. Neither the coefficient of friction nor restitution affected the oscillation width of the wall, whereas their effects are primarily reflected in the oscillation amplitude of the radial local packing factor and the axial local packing factor distribution at the top of the pebble bed. In both the contact force distribution and coordination number distribution, a left-shifted phenomenon appearing as the coefficient of friction occurred, and only the magnitude of the maximum frequency is affected when the coefficient of restitution changed from 0.1 to 0.9. In all simulation cases, the effects of the coefficients of friction and restitution are similar to that of cross-impact.

An interaction potential for zinc selenide: A molecular dynamics study

The structural, thermodynamic, and dynamical properties of zinc selenide, as well as the structural phase transformation induced by pressure, based on a many-body interatomic potential that considers two- and three-body interactions, were studied using molecular dynamics simulations. The potential was able to describe the energetics of the zinc-blende, wurtzite, and rock-salt structures of ZnSe. The effective interatomic potential was parameterized using experimental values of the bulk modulus and cohesive energy at an experimental density. Other properties, not used in the parameterization of the potential, such as the vibrational density of states, were correctly described. Cooling from the liquid, an amorphous phase or a re-crystallized material could be obtained. Pair distribution function, coordination number, volume change, and bond angle distributions are presented and compared with available experimental data. The structural phase transition from zinc-blend to rock-salt induced by hydrostatic pressure was obtained at ∼21 GPa for monocrystals and ∼16 GPa for polycrystals.

Modeling glassy SiC nanoribbon by rapidly cooling from the liquid: An affirmation of appropriate potentials

Modeling glassy SiC nanoribbon by cooling rapidly the SiC liquid from 8000K to 300K is carried out by molecular dynamics (MD) simulations. Two separated MD simulations are performed, one of which uses the Tersoff potential and the other one uses the Vashishta potential. The temperature dependence of various structural and thermodynamic properties of the systems is analyzed and discussed via the radial distribution functions (RDFs), the coordination number distributions, and the ring statistics. We find that the Tersoff potential is clearly more appropriate in order to obtain the glassy SiC when rapidly cooling is employed.

Coordination numbers in natural beach sand

Coordination number controls elastic moduli, seismic velocity, and force transmission in sands and is thus a critical factor controlling the resistance of sands to deformation. Previous studies quantified relationships between coordination number, porosity, grain size, sphericity, and effective stress in pluviated or modeled sands. Here, we determine if these relationships hold in naturally-deposited beach sands. We collect samples while preserving their microstructures and use x-ray computed microtomography images to characterize grain properties. Similar to pluviated and modeled sand studies, we find that average coordination numbers and porosities for freshly deposited natural sands are 8.1 ± 2.8 and 0.37 ± 0.01, respectively. The range and standard deviation in coordination numbers of the natural beach sands are, however, significantly higher than observed in pluviated and modeled sand studies. At the same effective stress and porosities, coordination number is linearly proportional to grain surface area except for the smallest and largest grains. Coordination number depends non-linearly on sphericity. We attribute the higher ranges and standard deviations of coordination numbers in the natural sands to its broader grain size distribution, and we propose that the largest grains limit grain rearrangement, which influences spatial distributions of coordination numbers in natural sands.

To study the structure and thermodynamic properties of silicene materials when melting quickly by molecular dynamics simulation

This paper presents the results of the study of melting silicene by MD simulation with a sample of 6400 atoms. Silicene melting from 300K to 3500K with a rate of 1013K / s. Investigation of energy dependence on temperature shows a jump in the average total energy of molten Silicene at the temperature T = 2500K. The radial distribution function G(r), ring distribution, coordination number distribution, angle distribution, and distance distribution also studied, all show that the phase transition temperature of Silicene is about 2500K. At 3500K, the silicene structure is almost destroyed, but long-term bonds still account for a higher proportion than short-length bonds. The structure is almost homogeneous, symmetrical.

Pore structure characterization and its effect on methane adsorption in shale kerogen

Pore structure characterization and its effect on methane adsorption on shale kerogen are crucial to understanding the fundamental mechanisms of gas storage, transport, and reserves evaluation. In this study, we use 3D scanning confocal microscopy, scanning electron microscopy (SEM), X-ray nano-computed tomography (nano-CT), and low-pressure N2 adsorption analysis to analyze the pore structures of the shale. Additionally, the adsorption behavior of methane on shales with different pore structures is investigated by molecular simulations. The results show that the SEM image of the shale sample obviously displays four different pore shapes, including slit pore, square pore, triangle pore, and circle pore. The average coordination number is 4.21 and the distribution of coordination numbers demonstrates that pores in the shale have high connectivity. Compared with the adsorption capacity of methane on triangle pores, the adsorption capacity on slit pore, square pore, and circle pore are reduced by 9.86%, 8.55%, and 6.12%, respectively. With increasing pressure, these acute wedges fill in a manner different from the right or obtuse angles in the other pores. This study offers a quantitative understanding of the effect of pore structure on methane adsorption in the shale and provides better insight into the evaluation of gas storage in geologic shale reservoirs.

Evolution of Coordination Number of Methane Hydrate Bearing Sediments during Quasi-static Shearing

Quasi-static shearing of methane hydrate bearing sediments (MHBS) was investigated by performing numerical tests using distinct element method (DEM) so as to comprehensively understand mechanical properties of MHBS. Stress-strain response and dilation behaviour of numerical tests were presented and compared with that of physical experiments. And then an examination of the evolution of coordination number (CN) during shearing was also presented, as well as its relation to anisotropy of contact orientations. The research results led us to conclude that (i) DEM can effectively capture the main characteristics of strength and deformation of MHBS; (ii) Distribution of CN shows obvious heterogeneity which is closely associated with shear bands; (iii) The average CN has a strong correlation with total number of contacts, and could be further interpreted from the point of view of anisotropy of contact orientations;

Study on the packing characteristics of a special “J” shape ceramic packed pebble bed based on discrete element modeling

The packing structures of a special “J” – cylinder-tours shape ceramic packed pebble beds, which might be used in the breeding zone of future fusion blanket and the other applications were numerically studied by the Discrete Element Method. The contact force and force chain distribution and the effect of the pebble diameter on the global packing factor have been investigated. Meanwhile, the analyses of radial and axial local packing factor distribution features and coordination number distribution were performed. The results showed that the global packing factor increased with decreasing pebble diameter and the cylindrical part of the packed pebble bed had a slightly higher global packing factor than the torus part. The contact force kept decreasing after a short period of rising and the force chain distribution showed a clear difference between two parts of the packed pebble bed. Both the radial and axial local packing factor exhibited oscillation behavior but a clear difference observed in two parts. The coordination number distribution was also analyzed to investigate the contact status in the packed pebble bed. The presented results of these investigations added to the understandings of the packing structure of irregular shape packed pebble beds, hence, could provide references for fusion facility design and other areas of industry such as chemical engineering research.

Molecular dynamics study on TOTO-based ionic liquids with different cations

Ionic liquids (ILs) have good designability and promising prospect of application. By changing different cations or anions, ILs possessing better and specific properties can be synthesized. In this paper, molecular dynamics (MD) simulation is applied to study ILs with the anion of 2,5,8,11-tetraoxatridecan-13-oate (TOTO), which are named as TOTO-based ILs. Attention is mainly paid to the effect of five different types of cations, including organic choline (Ch), tetraethylammonium (TEA), tetrapropylammonium(TPA), and tetrabutylammonium (TBA) and inorganic sodium (Na) ions, on the structure and properties of TOTO-based ILs. Results about conformation and length distribution of TOTO chain, distance between cations and TOTO anions, distribution of coordination number for cation-anion, and dynamic coordination are analysed, showing that changing cations makes no considerable influence on gauche probability along TOTO chain. All organic cation-TOTO ILs have similar TOTO chain length distribution, and similar radical function distributions between cations and anions. No matter inorganic cations and organic cations, these cations prefer to interact with carboxyl oxygen atoms. For organic cation-TOTO systems, interionic coordination interactions are weakened with the increase of cation size. Due to the hydroxyl group in one Ch alkyl chain, Ch-TOTO system exhibits some particular properties. The introduce of functional group can tune ILs properties and promote the design of task-specific ILs.