Coordination Number Distribution Research Articles

Numerical Analysis of Non-Sphericity of Particles of Powder Material and their Effect on Packing Structure for Concentrated Solar Power Applications

Concentrated solar power technology represents a novel approach to generating solar power, characterized by high solar radiation density and uninterrupted operation. To store thermal energy and extend system operating hours, concentrated solar power systems rely on thermal energy storage. Selecting the appropriate heat storage media is crucial for designing a cost-efficient and straightforward system capable of withstanding high processing conditions. Among many powder materials, bauxite powder stands out as alternatives to conventional molten salt for thermal storage media due to its widespread availability and suitability for operating conditions. Particle shape, particularly its sphericity, plays a vital role in efficiently packing particles which directly affects to heat transfer properties. To investigate this effect, three samples of particles with different shape were formed: spheres, ellipses, and cylinders using Discrete Element Method with equal volume and material properties, and only variation of their sphericity factors. The study focused on the analysis of the porosity of the packed bed of particles with different shapes and its influence on the packing structure properties. The analysis of local packing fraction revealed that elliptical and cylindrical particles samples exhibited reduced heap formation and a more uniform distribution along z direction during vertical packing. The coordination number and radial distribution function analyses for these non-spherical particles showed a greater number of contacts between particles and disordered distribution of particles due to mechanical interlocking of non-spherical particles. The results indicated that the deviation of sphericity has a positive impact on the denser packing of particles generated more contacts and higher local packing fraction.

Gray correlation analysis between mechanical performance and pore characteristics of permeable concrete

Pervious concrete is a type of sustainable construction material used in pavement engineering due to its advantages in water permeability, skid resistance, and sound absorption performance. This study investigated the pore characteristics of five types of pervious concrete, along with the uniaxial compression behaviors and semi-circular bending (SCB) fracture performance. The critical stress intensity factor was obtained using the equivalent energy approach. Gray relational analysis was conducted between the mechanical performance and the pore characteristics. Pore indicators include the effective porosity, 2D porosity along the specimen height, 2D coordination number (CN) along the specimen height, 2D pore radius, 2D specific surface area, 3D CN, 3D pore radius, and 3D virtual porosity. Results indicated that from M1 to M3, when cement mortar content decreased from 520 kg/m3 to 408 kg/m3, the effective porosity increased from 18.95% to 27.43%, and the compressive strength and fracture toughness decreased obviously. For the same type of pervious concrete mixture, with the increase of notch length, the critical stress intensity factor decreased accordingly. The 2D porosity distribution, 2D pore size distribution, and 2D CN distribution presented a symmetrical trend of decreasing, then increasing, and then decreasing with the increase of specimen height. Aggregate with the gradation within the range of 9.5 mm to 19.0 mm generated a higher compressive strength, larger effective porosity and larger pore size compared to the gradation of 4.75 mm∼9.5 mm. The results of CN and pore radius generated from the 3D pore network model were larger than the corresponding 2D values. The gray correlation analysis revealed that 2D pore radius and 3D pore radius played critical effects on the uniaxial compressive strength and fracture resistance, while 2D CN and 3D CN were the least significant factors affecting the mechanical performance.

Selective laser sintering of amorphous nanoparticles: Molecular dynamics simulations

The paper investigates the process of liquid-phase sintering of amorphous iron-based nanoparticles by the method of molecular dynamics simulations. The classical molecular dynamics package LAMMPS was used for modeling. Visual analysis of the atomic configurations of nanoparticles during their rapid cooling revealed the self-purification effect of the particles. Partial pair correlation functions and coordination number distribution functions were used to analyze the atomic structure of nanoparticles after sintering. As a result of the analysis of the main structural parameters, which were obtained using the specified functions, differences in the atomic composition and structure of the volume and surface of nanoparticles were established.

Molecular dynamics simulation of microstructure and thermophysical properties of LiCl–CaCl2 eutectic molten salt

Abstract Chloride molten salt systems are widely used as electrolytes for molten salt electrolysis because of their relatively low eutectic temperatures and good thermal stability, but there is a serious lack of data on the thermophysical properties of chloride molten salts at elevated temperatures, whereas the nature of the electrolyte is very important for the electrolysis process. In this paper, the variation of the microstructure and thermophysical properties of the binary mixed LiCl–CaCl2 molten salt system with temperature and composition is calculated using molecular dynamics (MD) simulations based on the BMH potential. The microscopic conformations observed in LiCl–CaCl2 molten salts are mainly irregular, distorted tetrahedra and octahedra, which dynamically coexist, as analyzed by the radial distribution function, coordination number and angular distribution function. In addition, the effects of temperature and composition on the density, ionic self-diffusion coefficient, shear viscosity, and ionic conductivity of the molten salts were investigated, and the relationships of the thermophysical properties of LiCl–CaCl2 molten salts with temperature and composition were obtained, which provide fundamental thermophysical data for the molten salt electrolytes.

Chemdex: quantification and distributions of valence numbers, oxidation numbers, coordination numbers, electron numbers, and covalent bond classes for the elements.

The paper introduces Chemdex, a freely accessible web-based database of over 70 000 compounds characterised by crystallography from across the periodic table. Its software calculates for an atom of interest within each compound classifications including valence number, oxidation number, coordination number, electron number, several covalent bond classifications, and the attached atom set. Users may explore distributions of these classifications by percentages and heat map displays for individual elements or sets of elements, or in several cases for one classification plotted against a second. These properties often display clear periodicity. Based upon distributions across the periodic table of valence numbers, electron numbers, coordination numbers, and attached atom data suggestions are made regarding the placement of hydrogen in the periodic table, membership of group 3 in the periodic table, locations of the early actinoids in the periodic table, and assignments of certain elements as metalloids.

Unveiling the structure of calcium borate glass by neutron diffraction, MAS-NMR, Raman and first-principles calculations

Calcium borate glass is a challenging system, due to the rich variety of borate units and alterable boron coordination number, and hence they are also of particular interest for academic study. In this study, neutron diffraction, solid-state magic angle spinning nuclear magnetic resonance (MAS-NMR) and Raman spectroscopy were combined with first-principles calculations, experience potential structure refinement (EPSR) simulations to investigate the structure of xCaO·(1-x)11B2O3 glass. Property data such as density and glass transition temperature were also measured to study the impact of structural changes on the properties of glasses. The structure of calcium borate glass was analyzed using various methods including structural unit, atomic pair bond length, coordination number, ring structure, chain structure and cavity distribution. Most notably, the atomic labeling method plays a key role in the structural analysis process. Through this method some new conclusions were obtained. For example, the increase of CaO content mainly leads to the conversion of BO to NBO in the glass structure, and has little effect on the content of BO3 and BØ4-. This result further leads to an increase in Ca-NBO interactions. Interestingly, the bond length distribution of Ca-NBO and B-NBO is shorter than that of Ca-BO and B-BO, so the glass structure becomes denser. The structural reason why the density of calcium borate glass is less than the crystal density was also revealed. This study contributes to unveiling the structure of calcium borate glass and exploring the correlation between the structure and properties of calcium borate glass, which provides a valuable reference for developing more valuable amorphous materials.

Insight of temperature and density-driven transition of sawtooth penta-silicene nanoribbons via molecular dynamics study

The density-driven transitions of sawtooth penta-silicene nanoribbon (PSiNR) at different temperature points are investigated using classical Molecular Dynamics (MD). Under the confined z-boundary, we found that the penta-to-tetra and the liquid-to-tetra silicene transitions present a clear dependence on the temperature. Insights of the thermodynamics and structural properties such as ring size (RS) and coordination number (CN) distributions, radial distribution function, buckling, and interatomic distance have been carefully analyzed. Above the melting temperature, the liquid 2D silicene tends to direct aggregate from scattered low-membered ring domains into the tetra-silicene nanoribbon (TSiNR). Meanwhile, the transition in sawtooth penta-SiNR takes place from the boundary into the inner region. A phase diagram indicating a dependence on density and temperature is presented. On the other hand, without the confinement of the z-boundary, the sawtooth penta-SiNRs express the periodic rippling. High temperature and axial compressive strain also could induce the PSiNR roll into the nanotube. Our simulations provided insights into the structural variations involving both the density and temperature of sawtooth PSiNRs.

Multi-scale pore network modelling to evaluate connectivity in ceramic composites

Complex morphologies, such as open or connected feature networks, are present in a wide variety of materials. Characteristics of these networks can impact key performance attributes of the materials themselves, affecting transport properties such as thermal conductivity. Therefore, it is critical to analyze the microstructure of these materials to gain a better understanding of the fundamental characteristics of the morphology. This study utilized pore network modeling as a method to extract morphological information on the solid network formed by boron nitride ceramic flakes in a polymeric resin matrix and uses the characteristics of the model to analyze the connectivity of the flakes. In this work, Micro-CT and FIB/SEM tomography were used in tandem to provide complimentary analyses of the microstructure and nanostructure, respectively, of the flake network to understand how this may contribute to transport properties of the material. Rather than a pore network model (PNM), the flake network model (FNM) was extracted from the tomographic datasets and the coordination number distribution was determined for the flakes detected in each. Micro-CT analysis showed that the flakes had formed a cage-like network around the exterior of the sample with limited connectivity in the interior, likely due to flake agglomeration at the outer surface of the material. A comparison of the full and interior-only Micro-CT FNMs indicated lower connectivity in the interior. This was confirmed by flow rate models generated from the network analysis for the flake contact points. The FNM extracted from the FIB/SEM tomography dataset exhibited similar connectivity compared to the interior-only FNM, indicating that the connectivity of the material was consistent when measured at the micron scale and at the nanometer scale.

Melting Process of the Two-Dimensional Material BN: Insights from Molecular Dynamics Simulations

The structure of the two-dimensional BN containing 9941 atoms has been studied by classical molecular dynamics simulation with Tersoff potential. The periodic boundary condition is applied to the two x and y directions, while the z direction is free. The analysis results of the function of total energy per atom and heat capacity, mean squared displacement, diffusion coefficient, radial distribution function, distribution of coordination number, angle distribution, and ring statistics show that the melting point of the material is about 4600 K. This value is higher than the experimental value as well as the previous simulation results. The observations also show that the melting process begins at the corners and edges and then spreads across the face of the model. The breakage of the B-N bond leads to the formation of clusters of N2 molecules and B with different sizes.

Evolution of Structure and Dynamic of Simulated CdTe Model According to the Melting Process

CdTe model containing 9955 atoms was built and simulated by the method of molecular dynamics (MD) with the Stillinger–Weber (SW) potential. The model was obtained by heating from 300K to 3000K at a rate of 1013 K/s. The structural properties of the model are investigated through the total energy per atom, the partial radial distribution functions (PRDFs), the coordination number distributions and the bond-angle distributions. Calculation results have shown that the model has a melting point of about 1370K and changes from tetrahedral to octahedral structure when the temperature of CdTe model is above the melting point. These results are consistent with the reported experimental and simulation results. In addition, the mobility of Cd and Te atoms is also researched through the mean squared displacement (MSD) and the diffusion coefficients of Cd and Te atoms.

Development of a 3D Computer Simulation Model Using C++ Methods

Development of a 3D Computer Simulation Model Using C++ Methods

Gamma‐ray irradiation effect on microstructure and physical performances of porous silica

AbstractIn this paper, the gamma irradiation effect on the microstructure and physical performances of porous silica, including mechanical, thermal, and optical performances, are systematically investigated by using molecular dynamics and density‐functional theory‐based methods. The study of bond angle distribution, pair distribution function, coordination number distribution, and average ring size distribution show that, after gamma‐ray irradiation, the microstructure of porous silica is obviously modified. The tight packing of SiO2 tetrahedrons in the porous silica network is broken by gamma‐ray irradiation. Defects of three‐coordinated Si and non‐bridging oxygen are induced by gamma‐ray irradiation. Moreover, we find that the defects concentrations rapidly grow as gamma‐ray dose increases. The mechanical, thermal, and optical performances of porous silica are all seriously degenerated by gamma‐ray irradiation. Our results show that, for mechanical performance, Young's modulus, Bulk modulus, and Shear modulus first decrease and then keep stable as gamma‐ray dose increases, but the change of Poisson's ratio is slight. For thermal performance, the thermal conductivity decreases exponentially as gamma‐ray dose increases. For optical performance, light absorption coefficients increase exponentially and light transmittance drops as gamma‐ray dose increases in the working range (photon energy range around 3.5 eV) of inertial confinement fusion. Present work is expected to be valuable for studying the degradation mechanism of silicate materials under gamma radiation and developing gamma‐ray irradiation protection technology.

Local structure and transport properties of MgCl2 molten salt with impurity water based on first principles molecular dynamics simulations

Electrolysis of molten anhydrous magnesium chloride is an efficient way to prepare magnesium. Nowadays, it is still an attractive issue to reduce the production cost of anhydrous magnesium chloride in preparing magnesium process by electrolysis. This work focused on the local structure and transport properties of MgCl2 molten salt with impurity water system under various temperature conditions based on the first principles molecular dynamics simulations. The radial distribution function, coordination number distribution, angular distribution function, static structure factor, and self-diffusion coefficient in the temperature range from 1000 K to 1400 K, were compared with of pure MgCl2 to elucidate the microscopic interaction mechanism of impurity water. Besides, this study indicated that the first principles molecular dynamics simulation can serve as an effective and promising tool for the theoretical study of more complex molten salts and provide basic guidance for the experiment to fill in the gaps.

An X-ray and Neutron Scattering Study of Aqueous MgCl2 Solution in the Gigapascal Pressure Range

The structure of electrolyte solutions under pressure at a molecular level is a crucial issue in the fundamental science of understanding the nature of ion solvation and association and application fields, such as geological processes on the Earth, pressure-induced protein denaturation, and supercritical water technology. We report the structure of an aqueous 2 m (=mol kg−1) MgCl2 solution at pressures from 0.1 MPa to 4 GPa and temperatures from 300 to 500 K revealed by X-ray- and neutron-scattering measurements. The scattering data are analyzed by empirical potential structure refinement (EPSR) modeling to derive the pair distribution functions, coordination number distributions, angle distributions, and spatial density functions (3D structure) as a function of pressure and temperature. Mg2+ forms rigid solvation shells extended to the third shell; the first solvation shell of six-fold octahedral coordination with about six water molecules at 0 GPa transforms into about five water molecules and one Cl− due to the formation of the contact ion pairs in the GPa pressure range. The Cl− solvation shows a substantial pressure dependence; the coordination number of a water oxygen atom around Cl− increases from 8 at 0.1 MPa/300 K to 10 at 4 GPa/500 K. The solvent water transforms the tetrahedral network structure at 0.1 MPa/300 K to a densely packed structure in the GPa pressure range; the number of water oxygen atoms around a central water molecule gradually increases from 4.6 at 0.1 MPa/298 K to 8.4 at 4 GPa/500 K.

Effect of Pore Characteristics on Sound Absorption Ability of Permeable Pavement Materials

Permeable pavement materials are attracting great attention due to the superior benefits in fast drainage, driving safety, and noise reduction effectiveness. This study investigated the sound absorption behaviors of permeable pavement materials, including permeable cement concrete (PCC) and open-graded friction course asphalt mixture (OGFC). The sound absorption coefficients (SACs) of dense-graded cement concrete and dense-graded asphalt mixture were also measured for comparison. The sound absorption test was conducted by the impedance tube method according to ASTM E1050-19. Computed tomography (CT) scanning tests were conducted to obtain the pore characteristics of PCC and OGFC, including porosity distribution, pore size, and coordination number (CN). The porosity distribution, pore size distribution, and CN distribution presented a symmetrical trend of decreasing, then increasing, and then decreasing with the increase of specimen height. When OGFC and PCC showed similar porosities, the pore number of PCC was significantly larger than that of OGFC. Comparisons of the peak SAC and the average SAC were made between PCC and OGFC. Correlations were made between the SAC and the pore characteristics. Results indicated that with increasing porosity, the peak SAC and the average SAC both increased significantly for PCC and OGFC. The characteristic pore size, pore number, and CN distribution together affected the SAC. At the same porosity, OGFC mixtures presented larger average SAC compared to PCC, which was ascribed to the fact that more pores with larger size and a better pore connectivity existed in OGFC mixtures.

Development of bromine-related potentials for molecular dynamics simulations of the oxyhalide photo-thermo-refractive glass

The oxyhalide photo-thermo-refractive (PTR) glass has found various applications in optical devices. However, due to its complex compositions, researches on the structures and structure-property relationship of this kind of materials is still limited. In this work, we applied molecular dynamics (MD) simulations to investigate the structure of the classic PTR glass (excluding Ce, Ag, Sb, and Sn). A set of bromine-related potential parameters has been developed and tested by comparing the simulated crystal structures with those from experiments or ab initio calculations. The PTR glass has then been simulated by using MD simulations with the newly developed potentials. The structure information of the PTR glass, such as the pair distribution function, bond angle distribution, coordination number, and two-dimensional distributions of elements, have been calculated and compared with available experimental data from literature. In addition, the growth of the fluorine/bromine-rich regions in the PTR glass after heat treatment has also been investigated. It is found that the bromine prefers to stay around the fluorine-rich regions and form the phase boundary between the fluorine-rich phase and the oxygen-rich glass matrix. And the presence of sodium ions in the fluorine/bromine-rich regions increase the aggregation tendency of these regions thus lead to the growth of the region size. The results show that the newly developed bromine/fluorine-related potential parameters can describe the PTR glass structures thus provide a new method to help design and improve new applications.

Effect of composition and temperature on microstructure and thermophysical properties of LiCl-CaCl2 molten salt based on machine learning potentials

LiCl-CaCl2 molten salt is a promising candidate for storing and converting energy and capturing CO2. The machine learning potential molecular dynamics simulations have high efficiency and accuracy and can overcome the disadvantages of classical molecular dynamics simulations and first-principles molecular dynamics simulations. Therefore, this study applied machine learning potential molecular dynamics simulations to investigate the microstructure change and thermophysical properties of LiCl-CaCl2 molten salt in response to temperature and composition. Partial radial distribution function, coordination number distribution, and angular distribution function were used for microstructural analysis. It is found that the local structure of LiCl-CaCl2 molten salt hardly changed with temperature rise. However, it suffered from serious octahedral distortion with the increased CaCl2 content from 20% to 80%. Thermophysical properties of LiCl-CaCl2 molten salt were systematically analyzed, including density, self-diffusion coefficient, shear viscosity, electrical conductivity, thermal expansion coefficient, and specific heat capacity. The relationships between properties and temperature or CaCl2 mole fraction were fitted. In conclusion, this study can provide a novel and efficient approach to investigating molten salt in-depth and enrich the data of fundamental properties of LiCl-CaCl2 molten salt.

Effects of the Aspect Ratio and Cross-Sectional Area of Rectangular Tubes on Packing Characteristics of Mono-Sized Pebble Beds

The packing characteristics of a pebble bed are essential to understand the heat- and mass-transfer processes occurring within a granular system. Therefore, the packing characteristics of rectangular prismatic pebble beds randomly packed with mono-sized pebbles are analyzed. In terms of the average and local packing fraction distribution, coordination number, and radial distribution function, the effects of the rectangular tube aspect ratio and cross-sectional area on the packing properties of pebble beds are explored in depth. The findings indicate that the packing structures of the rectangular pebble bed exhibit noticeable fixed-wall effects. The average packing fraction and coordination number gradually decline as the rectangular tube aspect ratio rises. Close to the fixed wall, a noticeable wall effect can be seen in the distribution of axial and local packing fractions and the pebble center distribution. The wall effect has an increasing effect on the axial and local packing fraction distributions in rectangular tubes with increasing aspect ratios. Additionally, the average packing fraction and the average coordination number also increase as the cross-sectional area increases, indicating a gradual weakening of the wall effect as the cross-sectional area increases. Furthermore, the cross-sectional area and aspect ratio of the rectangular tubes affect the RDF values of the rectangular pebble beds but have no impact on the RDF features. The findings reported in this paper will be helpful for designing and optimizing pebble beds in the breeding blanket of fusion reactors.

Structure from glass to melt: a case study along the MgSiO 3 –CaSiO 3 join using neutron and X-ray diffraction

Glass and melt structures are inherently complex and disordered with significant changes expected to occur with temperature. In the present paper, a comparison of the structure of glasses and liquids along the MgSiO3–CaSiO3 join is carried out using neutron and X-ray diffraction (XRD). Empirical Potential Structure Refinement (EPSR) simulations were used to fit the experimental data. The average coordination number (CN) and site distribution is obtained for Mg and Ca showing distinct sites between the two cations and higher coordinated sites in the liquids. The major glass to melt modifications is observed at the scale of intermediate range order (IRO) by rearrangement of the (Ca,Mg)–Si and (Ca,Mg)–(Ca,Mg) connections. The structural evolution with temperature, especially concerning the cationic environments, illustrates the differences between glass and melt organization. These changes highlight the important contribution of cations to thermodynamical properties, diffusion and glass forming ability.

Discrete element method investigation of shear behaviour of 3D geogrid–sand interface

Interface shear characteristics have an important impact on the stability of geosynthetically reinforced soil structures. The shear characteristics of three-dimensional (3D) geogrid–sand interfaces were investigated using large-scale direct shear tests and the discrete element method (DEM). Geogrids were manufactured by 3D printing. The effect of mesh pattern and transverse-rib thickness on the stress–displacement relationship, strength parameters, coordination number and porosity distribution were evaluated. The results showed that the mesh pattern and transverse-rib thickness have an impact on the interface shear characteristics. The peak and residual interface shear strength of the modified geogrid mesh pattern exceeded that of biaxial geogrids. The average coordination numbers of the modified geogrid mesh pattern were greater than those of biaxial geogrids. The variability of particle compactness, as characterised by the porosity distribution, shows how the modified mesh pattern increases the interface shear strength. The interface shear strength of the geogrid–sand interface was improved by thickening the transverse ribs of the modified geogrid mesh pattern.