Development of boron oxide potentials for computer simulations of multicomponent oxide glasses

Molecular dynamics and related atomistic computer simulations are effective ways in studying the structures and structure–property relations of glass materials. However, simulations of boron oxide (B₂O₃)-containing oxide glasses pose a challenge due to the lack of reliable empirical potentials. This paper reports development of a set of partial charge pairwise composition-dependent potentials for boron-related interactions that enable simulations of multicomponent borosilicate glasses, together with some of the existing parameters. This set of potentials was tested in sodium borate glasses and sodium borosilicate glasses and it is shown capable to describe boron coordination change with glass composition in wide composition ranges. Structure features such as boron N₄ value, density, Q_n species distribution, fraction of non-bridging oxygen around boron and silicon, total correlation function, and bond angle distribution function were calculated and compared with available experimental data. Mechanical properties of the simulated glasses calculated with the new potential also show good agreement with experiments. Therefore, this new set of potential can be used to simulate boron oxide-containing multicomponent glasses including those with wide industrial and technology applications.     

Surface structures of sodium borosilicate glasses from molecular dynamics simulations

Surface plays an important role in the physical and chemical properties of oxide glasses and controls the interactions of these glasses with the environment, thus dominating properties such as the chemical durability and bioactivity. The surface atomic structures of a series of sodium borosilicate glasses were studied using classical molecular dynamics simulations with recently developed compositional dependent partial charge potentials. The surface structural features and defect speciation were characterized and compared with the bulk glasses with the same composition. Our simulation results show that the borosilicate glass surfaces have significantly different chemical compositions and structures as compared to the bulk. The glass surfaces are found to be sodium enriched and behave like borosilicate glasses with higher R (Na₂O/B₂O₃) values. As a result of this composition and associated structure changes, the amount of fourfold boron decreases at the surface and the network connectivity on the surface decreases. In addition to composition variation and local structure environment change, defects such as two‐membered rings and three‐coordinated silicon were also observed on the surface. These unusual surface composition and structure features are expected to significantly impact the chemical and physical properties and the interactions with the environments of sodium borosilicate glasses.     

Structural features of sodium silicate glasses from reactive force field-based molecular dynamics simulations

Atomistic computer simulations can provide insights into silicate glass-environment interactions with the recent development of reactive potentials. However, the accuracy of generated glass structures with these potential was usually not fully examined. In this paper, the capability of the reactive force field (ReaxFF) to describe the short and medium range structure features of sodium silicate glasses in molecular dynamics simulations is investigated by comparing a widely used partial charge pairwise potential and available experimental data. Glass structure information such as pair distribution function (PDF), coordination number, Q_n species, neutron broadened structure factor, and X-ray broadened structure factor of the glass structures from ReaxFF simulations were calculated and compared to evaluate the generated glass structure. Advantages and limitations of the potentials and glass forming procedures, as well as areas of further improvement, were discussed. The results show that the recently refined ReaxFF parameters through the proposed procedure enable the simulations of sodium silicate glass structures with minimal defects, which paves the way to investigate water-glass interaction mechanisms with the reactive enabled potentials.     

Alternative insight into aluminium-phosphate glass network from ab initio molecular dynamics simulations

Phosphate glasses are an important group of materials for wide applications. They have received attention as biomaterials, eco-fertilizers, in optoelectronic devices, waste immobilization, etc.The subject of the studies is binary Al₂O₃–P₂O₅ glass, seen by ab initio molecular dynamics simulations. In the paper, Mayer's Bond Order analysis was employed as a suitable tool to observe the changes in the network due to Al₂O₃ substitutions. As well as, the bonding properties across the atoms in the network, which were characterized by electron localization function. The basic structural properties of the glasses are presented.The simulations showed that oxygen atoms in [PO₄] tetrahedrons create a weak covalent bond between the atoms. The interaction spreads across the whole phosphate network via bridging oxygens. Whereas, there is no such interaction between the atoms in [AlO_x] polyhedrons. In the case of the Al₂O₃ rich glasses, there are evidence non-network oxygen atoms which influence the distribution of phosphate structural units predicted based on the O/P ratio. The possibility of estimation of the oxygens in an experiment is discussed.The atoms building the network are going to take specific values of the total bond orders and the glass network is an interplay between atoms’ affinity to saturate bond orders and the glass network neutrality.     

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

In 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.     

Structure and vibrations of cerium in silica glass from molecular dynamics simulations

Molecular dynamics simulations are performed to investigate the effect of cerium on the structural and vibrational properties of silica glass. At low-concentration levels, the cerium ions tend to generate longer bonds with bridging oxygens than nonbridging ones, the proportion of which is associated with the average bond length varied with cerium coordination. Formed in the presence of cerium, the bond angles exhibit strong dependence on the types of Ce-O bonds that bring about different angular distributions. Despite the discrepancy in the structures between Ce³⁺ and Ce⁴⁺, similar characteristics of vibrations are observed for the two states. In comparison with the glass formers, the vibrations of cerium that contribute primarily to the low-frequency region show a less localized behavior, whereas the acoustic-like and optic-like modes separate at a much smaller frequency.     

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.     

Effects of magnesium on the structure of aluminoborosilicate glasses: NMR assessment of interatomic potentials models for molecular dynamics

Classical molecular dynamics simulations have been used to investigate the structural role of Mg and its effect when it is incorporated in sodium aluminoborosilicate glasses. The simulations have been performed using three interatomic potentials; one is based on the rigid ionic model parameterized by Wang et al. (2018) and two slightly different parameterization of the core–shell model provided by Stevensson et al. (2018) and Pedone et al. (2020) The accuracies of these models have been assessed by detailed structural analysis and comparing the simulated nuclear magnetic resonance (NMR) spectra for spin active nuclei (²⁹Si, ²⁷Al, ¹¹B, ¹⁷O, ²⁵Mg, and ²³Na) with the experimental counterparts collected in a previous work. Our simulations reveal that the core–shell parameterizations provide better structural models. In fact, they better reproduce the NMR spectra of all the investigated nuclei and give better agreement with known experimental data. Magnesium is found to be five coordinated on average with distances with oxygen in between a network modifier (like Na) and an intermediate network formed (like Al). It prefers to lay closer to three-coordinated B atoms, forming B–NBO bonds, with respect to Si and especially Al. This can explain the formation of AlO₅ and AlO₆ units in the investigated Na-free glass, together with a Si clusterization.     

Soret coefficient of a sodium germanate glass melt: Experiment,theory, and molecular dynamics simulation

The Soret effect is a diffusion phenomenon driven by a temperature gradient in a multicomponent system. This effect in condensed systems is not fully understood. Previously, we reported a theoretical model called “adjusted Kempers model” to predict the Soret coefficient in glass melts, and compared the experimental value to the theoretical value for 11Na₂O-89B₂O₃ (mol%) melts. Here, molecular dynamics calculations, as well as theoretical and experimental values, are quantitatively compared in 10Na₂O-90GeO₂ melts. We used a vertical tubular furnace to cause a temperature gradient and heated the sample from top side to reduce the natural convection. We measured the composition of 10Na₂O-90GeO₂ glass samples after 45, 90, and 180 hours of heat treatment under a temperature gradient, and estimated the steady-state Soret coefficient near 1373 K to be 1.09 × 10⁻³ K⁻¹. In addition, we calculated Soret coefficients to be 3.65 × 10⁻³ K⁻¹ and 1.85 × 10⁻³ K⁻¹ in theory and molecular dynamics calculation, respectively. The ratios between the experimental and theoretical Soret coefficients were 1.2 and 3.3 for 11Na₂O-89B₂O₃ melts and 10Na₂O-90GeO₂ melts, respectively. The difference in ratios may be attributed to the mass and size of diffusion species in the glass melts.     

Effect of boron oxide on mechanical and thermal properties of bioactive glass coatings for biomedical applications

Bioactive glass coatings can improve the osteo integration of metallic implants with the host tissue, thereby increasing their lifespan and overall success rate. However, complex composition-structure-property relations in phosphosilicate-based bioactive glasses make experimental determination of these relations and related composition design of bioactive coatings challenging. By applying molecular dynamics (MD)-based atomistic simulations with recently developed effective potentials, this work addresses the challenge by using a material genome approach to obtain the composition and structure effects on various key properties for bioactive coating applications. A series of potential bioactive glass compositions were studied and the composition effects on the mechanical and thermal properties that are critical to these bioactive glasses as a coating to metallic implants were calculated. Particularly, by varying the level of B₂O₃ to SiO₂ substitutions, the effect of composition on various key properties was elucidated. It was found that by using cation in a 1 to 1 ratio (BO_3/2 to SiO₂) instead of the commonly used substitutions (B₂O₃ to SiO₂), the composition effect can be more clearly expressed and, hence, recommended in future composition designs. Together with careful structural analysis, the origin of property changes can be elucidated. The atomistic computer simulation-based approach is, thus, an effective way to guide future bioactive glass designs for bioactive coatings and other applications.     

Thermal history driven molecular structure transitions in alumino‐borosilicate glass

The glass network structure governs various thermos‐physical properties such as viscosity, thermal, and electrical conductivities, and crystallization kinetics. We investigated the effect of temperature on structural changes in a Na₂O‐CaO‐Al₂O₃‐SiO₂‐B₂O₃ glass system using ²⁷Al MAS NMR spectroscopy. Around the glass transition temperature, most of aluminate structures exist as AlO₄, acting as a glass former. When the temperature is above the melt crystallization temperature, the AlO₄ structure is drastically decreased and glass structures are mainly composed of AlO₅ and AlO₆, acting as glass modifiers. Thermodynamic assessment based on Gibbs energy minimization was used to confirm the dependency of aluminate structure's amphoteric characteristic on temperature by calculating the site fraction of aluminate molecular structures at different temperatures. Temperature‐induced aluminate structural variation can also influence silicate and borate structural changes, which have been confirmed by the ²⁹Si and ¹¹B NMR spectra.     

Molecular and thermodynamic basis for EGCG‐Keratin interaction‐part I: Molecular dynamics simulations

Nonspecific binding of small molecules to proteins influences transdermal permeation and intestinal absorption, yet understanding of the molecular and thermodynamic basis is still limited. In this study, we report all‐atom, fully solvated molecular dynamics simulations of the thermodynamic characteristics of epigallocatechin‐3‐gallate (EGCG) binding keratin. Experimental validation is reported in Part II. Herein, 18 µs of simulation sampling was calculated. We show that the binding process is a combination of hydrophobic interaction, hydrogen bonding and aromatic interaction. The umbrella sampling technique was used to calculate the binding free energy of EGCG with keratin segments. By extracting EGCG from the keratin‐EGCG complex using steered molecular dynamics, the rupture force was observed to be linearly related to the binding free energy. Multilayer binding of EGCG clusters to keratin has been shown. The binding free energy of ?6.2 kcal mol^?1 obtained from the simulations was in excellent agreement with the experimental Part II. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4816–4823, 2013     

Modifications of carbon for polymer composites and nanocomposites

The various forms of carbon used in composite preparation include mainly carbon-black, carbon nanotubes and nanofibers, graphite and fullerenes. This review presents a detailed literature survey on the various modifications of the carbon nanostructures for nanocomposite preparation focusing upon the works published in the last decade. The modifications of each form of carbon are considered, with a compilation of structure-property relationships of carbon-based polymer nanocomposites. Modifications in both bulk and surface modifications have been reviewed, with comparison of their mechanical, thermal, electrical and barrier properties. A synopsis of the applications of these advanced materials is presented, pointing out gaps to motivate potential research in this field.