Ring Size Distribution Research Articles

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.

Unraveling the relationship between the structural features and solubility properties in Sr-containing bioactive glasses

Understanding the effect of glass network structure on the solubility properties of bioactive glasses is fundamental for designing new compositions with tailored biological functions and applications. This paper bridged the composition-structure-property relationship utilizing experimental measurements and molecular dynamics simulations. The Sr atom reorganized the network structures through a partial transformation of tetrahedral to trigonal boron units and the change of the ratio of bridged oxygen/non-bridged oxygen, which modified the glass structures at longer lengths scales, such as through changing the ring size distribution. Our results demonstrated that the evolution of solubility properties can be linked to the atomic structure change in short- and medium-range ordering. The structure-property relationship can be used to design and tune bioactive glass composition with specific properties.

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.

Evolution of the hydrogen-bonded network in methanol-water mixtures upon cooling

The hydrogen-bonded structure of methanol – water mixtures is investigated over the entire alcohol concentration range (from xMethanol = 0.1 to 1.0) at several temperatures, from 300 K down to the freezing point of the given mixture. Classical molecular dynamics simulations have been carried out, using the all-atom OPLS-AA force field for methanol and the TIP4P/2005 model for water molecules. Simulation trajectories (‘particle configurations’) obtained have been analyzed, in order to characterize the hydrogen-bonded network in the mixtures. The temperature and concentration dependence of the average hydrogen bond (H-bond) numbers between different types of molecules, the donor/acceptor roles of water and methanol molecules, and hydrogen bond number distributions have been revealed. The topology of the total system, as well as that of the water and methanol subsystems, has been investigated by calculating the cluster size distributions, the number of primitive rings, and ring size and ring type distributions. It has been found that upon cooling, the average number of H-bonded water molecules increases at every concentration and temperature investigated. As far as the connectivity of the hydrogen-bonded network is concerned, the percolation threshold has been shown to be above xM = 0.9 already at room temperature.

Study of beyond nearest-neighbor environment and intermediate-range order in a sodium aluminosilicate geopolymer using Reverse Monte Carlo modeling

A large scale three-dimensional structural model of a geopolymer with an approximate composition of NaAlSi2O6.5.5H2O obtained by Reverse Monte Carlo (RMC) modeling based on experimental high-energy x-ray diffraction is presented for the first time in order to obtain information regarding beyond nearest-neighbor environment and intermediate-range order (IRO). RMC model exhibits a three-dimensional network consisting of randomly cross-linked AlO4 and SiO4 tetrahedral units with uniformly distributed Na atoms and H2O molecules. The bond angle distributions, i.e. T-O-T, O-T-O and Na-O-Na angles, are somewhat distorted with lower values compared to analogous crystal systems. The origin of the first peak in the structure factor indicating IRO is investigated using the partial structure factors; Na-Na, Si-Na, Al-Si, O-H, and H-H atom pairs are found to be the main contributors. Ring size distribution analysis demonstrates that the structure mainly involves 6-, 7- and 8-membered rings. The coherence length of these IRO characteristics is ∼10.3 Å.

Ring compaction as a mechanism of densification in amorphous silica

The structure of permanently densified silica glass is investigated by neutron and x-ray diffraction, and by the production of atomistic models that reproduce the diffraction results. The focus is on the nature of the ordering on an intermediate length scale that originates from the formation of $n$-rings, where $n$ is the number of Si or O atoms within a ring. The densification process has a large effect on this ordering, as identified by pronounced changes to the first sharp diffraction peak (FSDP) in the measured structure factors. A clear and systematic dependence of the distribution of ring sizes on the position and shape of the FSDP could not be found, i.e., the ring statistics are not directly encoded into the form of this peak. Instead, a significant contribution to the glass densification originates from compaction of the $n$-rings, a metric that is quantified by the radius of gyration and lifetime of the rings. We thereby uncover and quantify a key densification mechanism in amorphous materials.

The network structure of the corneal endothelium.

A generic network model is applied to study the structure of the mammalian corneal endothelium. The model has been shown to reproduce the network properties of a wide range of systems, from low-dimensional inorganic glasses to colloidal nanoparticles deposited on a surface. Available extensive experimental microscopy results are analyzed and combined to highlight the behavior of two key metrics, the fraction of hexagonal rings (p6) and the coefficient of variation of the area. Their behavior is analyzed as a function of patient age, the onset of diabetes, and contact lens wearing status. Wearing contact lenses for ∼10 years is shown to change the endothelium structure by the equivalent of ∼30 years contact lens-free. Model network configurations are obtained using a Monte Carlo bond-switching algorithm, with the resulting topologies controlled by two potential model parameters (the bond and angular force constants) and the Monte Carlo temperature. The effect of systematically varying these parameters is investigated. In addition, the effect of constraining the ring size distribution is investigated. The networks generated with relatively weak bond force constants are shown to correlate best with the experimental information. The importance of extracting the full ring size distribution (rather than simply the fraction of hexagons) is discussed.

Effect of pressure quenching on the structures and properties of borosilicate glasses: Insights from molecular dynamics simulations

Combining thermal and pressure effect represents a novel approach to modify glass properties. However, the microscopic structural origin of these property modifications is complex and far from fully understood, especially in multicomponent glasses with mixed glass formers. In this paper, we have utilized classical molecular dynamics simulations with a set of composition dependent potentials to investigate pressure-quenching effect on sodium borosilicate glasses. Processes including hot compression, cold compression and subsequent annealing on the structures and properties are investigated and compared. It was found that applying pressure up to 10 GPa at the glass transition temperature led to permanent densifications and a dramatic increase of elastic moduli by 90%, while thermal annealing reversed the increase and applying pressure at ambient temperture did not increase the modulus. The main structural change of the hot compressed sample is the increase of four-fold coordinated boron while silicon remains four-fold coordinated. The sodium environment shows an increase of coordination number and a decrease of Na O and Na Na bond distances. Medium range structure is also changed with an increase of 8-membered rings. These results provide atomistic insights of the pressure quench effect on borosilicate glasses. • Compared cold compression, compression near T g and annealing effect on glass structure and properties • Up to 90% increase of elastic moduli due to hot compression while cold compression led to little change of elastic properties • Hot compression increases [4] B and Na coordination while a decrease of Na O bond distance • Medium range structure change observed in ring size distribution • Combing thermal and pressure effect a novel way to modify glass properties

Composition Dependence of the Atomic Structures and Properties of Sodium Aluminosilicate Glasses: Molecular Dynamics Simulations with Reactive and Nonreactive Potentials.

Understanding the composition-structure-property relations of glass materials is essential for their technological applications. In this study, the structures and properties of a series of sodium aluminosilicate glasses with varying Al2O3/Na2O ratios ((35 - x)Na2O-xAl2O3-65SiO2, x = 0, 5, 10, 15, 17.5, 20) covering peralkaline to peraluminous compositions, have been studied by using molecular dynamics simulations with two types of interatomic potentials: a fixed partial charge pairwise potential (Teter) and a reactive diffusive charge reactive potential (DCRP). The short and medium structural features such as bond lengths, coordination numbers, Qn distributions, and ring size distributions were obtained and compared with experimental data. It was found that silicon remained fourfold-coordinated throughout the compositional range, while a noticeable amount of fivefold-coordinated aluminum together with oxygen triclusters (TBO) are present in compositions with higher Al2O3 contents (RAl/Na > 1). In addition, the simulation results from both potentials show a certain level of violation of the Al avoidance rule by exhibiting a non-negligible amount of [AlOx]-[AlOx] polyhedral connections. Neutron and X-ray diffraction structure factors of the simulated glasses were calculated and compared with available experimental data. The mechanical properties, including Bulk, Shear, and Young's modulus, were calculated and found to increase with increasing RAl/Na, in good agreement with the experiments. Correlations of the properties with glass structures as a function of glass compositions and the advantages as well as potential issues of the two sets of potentials in modeling sodium aluminosilicate glasses are discussed in the context of features of glass structures and the prospect of future simulations of glass-water reactions.

Relationship between diffraction peak, network topology, and amorphous-forming ability in silicon and silica

The network topology in disordered materials is an important structural descriptor for understanding the nature of disorder that is usually hidden in pairwise correlations. Here, we compare the covalent network topology of liquid and solidified silicon (Si) with that of silica (SiO2) on the basis of the analyses of the ring size and cavity distributions and tetrahedral order. We discover that the ring size distributions in amorphous (a)-Si are narrower and the cavity volume ratio is smaller than those in a-SiO2, which is a signature of poor amorphous-forming ability in a-Si. Moreover, a significant difference is found between the liquid topology of Si and that of SiO2. These topological features, which are reflected in diffraction patterns, explain why silica is an amorphous former, whereas it is impossible to prepare bulk a-Si. We conclude that the tetrahedral corner-sharing network of AX2, in which A is a fourfold cation and X is a twofold anion, as indicated by the first sharp diffraction peak, is an important motif for the amorphous-forming ability that can rule out a-Si as an amorphous former. This concept is consistent with the fact that an elemental material cannot form a bulk amorphous phase using melt quenching technique.

Revealing the structural role of MgO in aluminosilicate glasses

The addition of MgO to aluminosilicate glasses usually leads to significant changes for many macroscopic properties. The underlying microscopic structure origin of this effect, however, remains elusive. We herein conduct systematic studies to reveal the structural role of MgO in sodium aluminosilicate glasses over a wide composition range from peralkaline to peraluminous using molecular dynamics simulations. Our results provide pieces of evidence to show that MgO plays a dual role of glass network modifier/charge compensator and network former, with the balance of these two roles strongly depending on the composition of other oxides in the glass. Specifically, Mg2+ tends to play a network modifier/charge compensator role when the glass simultaneously contains large amount Al atoms that need to charge balanced and a low concentration of other modifier oxides. In contrast, Mg2+ exhibits a more pronounced network-forming role in Al-poor glasses. Additionally, by considering the structural role of MgO in the glass network, we further demonstrate that the non-linear evolution of the isokom annealing temperature upon increasing Al2O3 content in the peraluminous regime can be rationalized in terms of the combined effects of short-range connectivity and medium-range ring size distribution. The findings provide insights to further exploit MgO in making glasses with advanced properties.

Continuous network structure of two-dimensional silica across a supporting metal step edge: An atomic scale study

The network structure of a silica bilayer film at a monolayer-bilayer transition and across a supporting metal step edge was studied at the atomic scale by scanning tunneling microscopy. The ring size distribution, ring-ring distances, and height profiles are analyzed across the step edge region. Density functional theory proposes two models to explain the observed network structure: a pinning of the lower layer to the substrate and a carpetlike mode. The results indicate a continuous coverage of the silica bilayer film across the step edge.

Experimental method to quantify the ring size distribution in silicate glasses and simulation validation thereof.

Silicate glasses have no long-range order and exhibit a short-range order that is often fairly similar to that of their crystalline counterparts. Hence, the out-of-equilibrium nature of glasses is largely encoded in their medium-range order. However, the ring size distribution-the key feature of silicate glasses' medium-range structure-remains invisible to conventional experiments and, hence, is largely unknown. Here, by combining neutron diffraction experiments and force-enhanced atomic refinement simulations for two archetypical silicate glasses, we show that rings of different sizes exhibit a distinct contribution to the first sharp diffraction peak in the structure factor. On the basis of these results, we demonstrate that the ring size distribution of silicate glasses can be determined solely from neutron diffraction patterns, by analyzing the shape of the first sharp diffraction peak. This method makes it possible to uncover the nature of silicate glasses' medium-range order.

Neural Network Quantum Molecular Dynamics, Intermediate Range Order in GeSe2, and Neutron Scattering Experiments.

A remarkable property of certain covalent glasses and their melts is intermediate range order, manifested as the first sharp diffraction peak (FSDP) in neutron-scattering experiments, as was exhaustively investigated by Price, Saboungi, and collaborators. Atomistic simulations thus far have relied on either quantum molecular dynamics (QMD), with systems too small to resolve FSDP, or classical molecular dynamics, without quantum-mechanical accuracy. We investigate prototypical FSDP in GeSe2 glass and melt using neural-network quantum molecular dynamics (NNQMD) based on machine learning, which allows large simulation sizes with validated quantum mechanical accuracy to make quantitative comparisons with neutron data. The system-size dependence of the FSDP height is determined by comparing QMD and NNQMD simulations with experimental data. Partial pair distribution functions, bond-angle distributions, partial and neutron structure factors, and ring-size distributions are presented. Calculated FSDP heights agree quantitatively with neutron scattering data for GeSe2 glass at 10 K and melt at 1100 K.

Hydrogenation of C24 Carbon Clusters: Structural Diversity and Energetic Properties.

This work aims at exploring the potential energy surfaces of C24Hn=0,6,12,18,24 using the genetic algorithm in combination with the density functional based tight binding potential. The structural diversity was analyzed using order parameters, in particular the sum of the numbers of 5- and 6-carbon rings R5/6. The most abundant and lowest energy population was designated as the flake population (isomers of variable shapes, large R5/6 values), characterized by an increasing number of spherical isomers when nH/nC increases. Simultaneously, the fraction of the pretzel population (spherical isomers, smaller R5/6 values) increases. The fraction of the cage population (largest R5/6 values) remains extremely minor while the branched population (smallest R5/6 values) remains the highest energy population for all nH/nC ratios. For all C24Hn=0,6,12,18,24 clusters, the evolution of the carbon ring size distribution with energy clearly shows the correlation between the stability and the number of 6-carbon rings. The average values of the ionization potentials of all populations were found to decrease when nH/nC increases, ranging from 7.9 down to 6.4 eV. This trend was correlated to geometric and electronic factors, in particular to carbon hybridization. These results are of astrophysical interest, especially regarding the role of carbon species in the gas ionization.

Predicted structures of calcium aluminosilicate glass as a model for stone wool fiber: effects of composition and interatomic potential

Characterization of compositionally-complex aluminosilicate glass particles and fibers such as stone wool, and their interfaces with water and ions, is significant to a range of areas regarding dissolution phenomena. Knowledge of atomic level structures of these interfaces is critical to elucidating their dissolution traits. Molecular simulations can provide these details, complementing experimental efforts. However, prediction of the structure of stone wool fiber has been hampered by a lack of suitable inter-atomic potentials. Here, two candidate potentials are evaluated for their ability to recover experimental structural data of calcium aluminosilicate (CaO-Al2O3-SiO2) glass of compositions relevant to stone wool fibers. Both potentials produce structures that are broadly consistent with experimental data, including defect concentrations, aluminium avoidance, and ring size distributions, and either could provide a suitable basis for modelling dissolution of these materials.

Persistent homology in two-dimensional atomic networks.

The topology of two-dimensional network materials is investigated by persistent homology analysis. The constraint of two dimensions allows for a direct comparison of key persistent homology metrics (persistence diagrams, cycles, and Betti numbers) with more traditional metrics such as the ring-size distributions. Two different types of networks are employed in which the topology is manipulated systematically. In the first, comparatively rigid networks are generated for a triangle-raft model, which are representative of materials such as silica bilayers. In the second, more flexible networks are generated using a bond-switching algorithm, which are representative of materials such as graphene. Bands are identified in the persistence diagrams by reference to the length scales associated with distorted polygons. The triangle-raft models with the largest ordering allow specific bands Bn (n = 1, 2, 3, …) to be allocated to configurations of atoms separated by n bonds. The persistence diagrams for the more disordered network models also display bands albeit less pronounced. The persistent homology method thereby provides information on n-body correlations that is not accessible from structure factors or radial distribution functions. An analysis of the persistent cycles gives the primitive ring statistics, provided the level of disorder is not too large. The method also gives information on the regularity of rings that is unavailable from a ring-statistics analysis. The utility of the persistent homology method is demonstrated by its application to experimentally-obtained configurations of silica bilayers and graphene.

Ab initio molecular dynamics simulation of the structural and electronic properties of aluminoborosilicate glass

AbstractIn this study, the structural and electronic properties of aluminoborosilicate glass, which has a wide range of applications in fields such as microelectronics and displays, were examined using ab initio molecular dynamic simulations. Computing models containing 220 atoms correctly described the local structure of the glass. The reliability of the computing models was verified by the consistency between the experimental results, obtained using high‐energy X‐ray diffraction and solid‐state nuclear magnetic resonance, and the simulation results pertaining to structural factors, pair distribution functions, Qn distribution, and elastic properties. The presence of B and Al increased the flexibility and asymmetry of the system, as shown by the bond angle and ring size distributions. Based on the electronic properties, we observed that the introduction of Al and B atoms into the network could also cause covalent interactions with the O atoms, similar to that with Si atoms. However, the Na and Mg atoms still interacted with all kinds of atoms in the network via charge transfer and exhibited highly non‐localized effects on the charge of the network formers. These results extend our understanding of the structure of aluminoborosilicate glass and have guiding significance for improving and designing new types of this glass.

Impact of pressure on structure and properties of hot‐compressed Na2O–Al2O3–SiO2 glass by molecular dynamics simulations

AbstractWe conduct systematic studies to investigate the impact of pressure on structure and properties of Na2O–Al2O3–SiO2 glass system using molecular dynamics simulations. Glass compositions range from peralkaline, to charge‐balancing, to peraluminous. Hot‐compression operation is applied to as‐prepared glasses with pressure up to 4 GPa. Several key learnings are obtained from the study: (a) Density and elastic moduli increase with increasing Al/Na ratio (RAl/Na) and pressure, which are mainly driven by average internetwork bond angle reduction and void size reduction upon compression. (b) Population of three bonded oxygen (TBO) show some presence in the peralkaline composition space, while gain significantly in the peraluminous regime. Pressure essentially encourages conversion of bridging oxygen to nonbridging and three bonded oxygens. Local nearest neighbor environment of TBO is predominately Al atoms rather than Si atoms regardless of RAl/Na and applied pressure. (c) Analysis of first sharp diffraction peak (FSDP) from neutron scattering suggests that medium‐range structure is compacted and small size rings are noticeably distorted over the hot‐compression process. (d) Ring size distribution analysis shows that pressure has a more pronounced effect on altering medium‐range structure for peralkaline glasses as compared to peraluminous glasses.

Two-dimensional Wide-Angle X-ray Scattering on a Cm-doped borosilicate glass in a beryllium container.

The two-dimensional wide-angle X-ray diffraction technique was applied to a Cm-doped borosilicate glass in a beryllium container. The experiment involved a high-energy X-ray beam and an image plate. It is shown that it is possible to extract the structure factor of the radioactive glass successfully from diffraction patterns and compare it with that of the pristine one. Striking differences appear under the first diffraction peak, revealing new sub-structures for the radioactive glass. It is suggested that they could be related to structural changes in the medium-range order, in particular the size distribution of rings or chains under the influence of mixed interactions between the glass network, α-particles and recoil nuclei.