Structural, vibrational, and elastic properties of a calcium aluminosilicate glass from molecular dynamics simulations: the role of the potential

Functional medium entropy alloys for joint replacement: An atomistic perspective of material deformation and a correlation to wear, corrosion, and biocompatibility

The present study adopts a multi-facet approach to design bio inspired concentrated alloys for potential application as articulating surfaces in joint replacements. A series of equiatomic, Nb rich and Ti rich TiMoNbZr based medium entropy alloys (MEAs) were processed via arc melting and their mechanical, in-vitro corrosion, wear, and in vitro and in vivo biocompatibility were investigated. Equiatomic MEA had primarily bcc with minor hcp phases where the single bcc was achieved with the addition of Nb. The single bcc Nb rich alloy resulted in 13 % elongation, much higher than equiatomic or Ti rich alloy. All the MEAs showed comparatively higher yield strength due to the climb of edge dislocations which is the main rate limiting mechanism at 300 K, as evident molecular dynamics (MD) simulation. The locally fluctuating energy landscape promotes kinks on edge dislocation, and at local minima nanoscale segments gets pinned. Upon yielding the entangled kink leaves a trail of vacancies/interstitials and escapes via climb motion to render high yield strength. The higher corrosion and pitting resistance of Nb enriched alloys can be attributed to the stable ZrO2, Nb2O5, TiO2, and MoO3 oxides, high polarization resistance (106-105 Ωcm-2), and low defect densities (1016-1018). In vitro cell-materials interaction using MC3T3-E1 showed bioinert but cytocompatible nature of the MEAs. The wear rate of the MEAs was in the range of 7-9 × 10-5 mm3N-1m-1. The wear debris did not show any tissue necrosis when implanted in rabbit femur rather new bone regeneration can be seen around the particles. STATEMENT OF SIGNIFICANCE: In the present work, a noble Nb enriched MEAs with superior mechanical, in vitro wear, corrosion and cytocompatibility properties was designed for articulating surfaces in joint replacement.

Structural Causes of Brittleness Changes in Aluminosilicate Glasses with Different Cooling Rates

Numerous sources have already demonstrated that varying annealing rates can result in distinct toughness and brittleness in glass. To determine the underlying mechanisms driving this phenomenon, molecular dynamic (MD) simulations were employed to investigate the microstructure of aluminosilicate glasses under different cooling rates, and then uniaxial stretching was performed on them under controlled conditions. Results indicated that compared with short-range structure, cooling rate has a greater influence on the medium-range structure in glass, and it remarkably affects the volume of voids. Both factors play a crucial role in determining the brittleness of the glass. The former adjusts network connectivity to influence force transmission by manipulating the levels of bridging oxygen (BO) and non-bridging oxygen (NBO), and the latter accomplishes the objective of influencing brittleness by modifying the environmental conditions that affect the changes in BO and NBO content. The variation in the void environment results in differences in the strategies of the changes in BO and NBO content during glass stress. These findings stem from the excellent response of BO and NBO to the characteristic points of stress-strain curves during stretching. This paper holds importance in understanding the reasons behind the effect of cooling rates on glass brittleness and in enhancing our understanding of the ductile/brittle transition (DTB) in glass.

Adhesion Performance of Ettringite at the Interface with Silane and GO/Silane: Insights into Molecular Dynamics Simulations

The application of silane in sulfoaluminate cement repair materials can improve its waterproof, permeability, freeze-thaw, and other properties, but it would reduce the mechanical properties of sulfoaluminate cement-based materials, making it unable to better meet the engineering requirements and durability indices. The modification of silane with graphene oxide (GO) can effectively address this issue. However, the failure mechanism of the interface between silane and sulfoaluminate cement-based materials and the modification mechanism of GO remain unclear. In this paper, the interface-bonding mechanical models of isobutyltriethoxysilane (IBTS)/ettringite and GO-IBTS/ettringite are established by molecular dynamics method to study the source of interface-bonding properties of IBTS, GO-IBTS, and ettringite, as well as the failure mechanism of interface bonding, to reveal the mechanism of GO-modifying IBTS to improve the interface-bonding properties of IBTS and ettringite. This study finds that the bonding properties of the IBTS, GO-IBTS, and ettringite interface are derived from the amphiphilic nature of IBTS, which can only produce unilateral bonding with ettringite, thus becoming a weak link in interface dissociation. The double-sided nature of GO functional groups enables GO-IBTS to interact well with bilateral ettringite, thus enhancing the interface-bonding properties.

Efficient and Broadband Emission in Dy3+-Doped Glass-Ceramic Fibers for Tunable Yellow Fiber Laser

Yellow lasers are of great interest in biology, medicine and display technology. However, nonlinear emission of near-infrared lasers at yellow still presents particularly complex optical alignment to date. Here, to the best of our knowledge, we demonstrate the fabrication of a NaLa(WO₄)₂: Dy³⁺ glass-ceramic fiber (GCF) for the first time. More importantly, the emission band of the GCF, which is around 575 nm, has a wide full-width half maximum (FWHM) of 18~22 nm, which is remarkably larger than that of the Dy³⁺-doped YAG crystal (<7 nm). The precursor fiber (PF) was drawn using the molten core drawing (MCD) method. In particular, benefiting from the in situ nanocrystals fabricated in the amorphous fiber core after thermal treatment, the resultant glass-ceramic fiber exhibits a five-times enhancement of luminescence intensity around 575 nm, compared with the precursor fiber, while retaining its broadband emission. Overall, this work is anticipated to offer a high potential GCF with prominent bandwidth for the direct access of a tunable yellow laser.

Modeling Oxygen Tricluster Formation in Calcium Aluminosilicate Supercooled Liquids and Glasses

Calcium aluminosilicate glasses have technological importance for a variety of industrial applications. However, the short-range structure of this glass system remains widely debated regarding the formation of oxygen triclusters. It is argued that triclusters are observed in high percentages within molecular dynamics simulations because of the high melting temperatures and correspondingly high fictive temperatures. This work explores the formation of such structural units by first simulating various compositions at different liquid temperatures to understand thermodynamic factors affecting the formation of such species. Structural results are then implemented into a statistical mechanical model which can predict the formation of triclusters at a given fictive temperature. Results show temperature and composition dependence of these structures, with aluminum charge modification favored in the peraluminous regime. It is concluded that oxygen triclusters are the preferred method of charge compensation even when extrapolating to laboratory fictive temperatures, indicating that triclusters are not a byproduct of simulation timescales.

Passivation of Mid-Gap Electronic States at Calcium Aluminosilicate Glass Surfaces upon Water Exposure: An Ab Initio Study

When a clean glass surface is exposed to humid air, a thin water layer forms on the hydrophilic surface. Using ab initio molecular dynamics, we simulate the changes in the electronic structure of a CaO-Al₂O₃-SiO₂ glass model upon vacuum fracture and subsequent exposure to H₂O. When the glass is fractured, dangling bonds form, which lower the band gap of the surface by ∼1.8 eV compared to the bulk value due to mid-gap surface states. When H₂O adsorbs onto the vacuum-fractured surface, the band gap increases to a value closer to that of the bulk band gap. Using two different hydroxylation methods, we find that the calculated band gap of the glass surface depends on the hydroxylation state. Surfaces with ∼4.5 OH/nm² have smaller band gaps due to unfilled surface states, and surfaces with ∼2.5 OH/nm² have larger band gaps with no apparent unfilled surface states. The resulting changes in the electronic structure, quantified by electron affinity and work function values, are hypothesized to play an important role in the electrostatic charge transfer based on the principles of surface state theory, which posit that the density of electronic surface states determines the amount of electronic charge transfer to or from material surfaces.

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, Q_n 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 (R_Al/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 [AlO_x]-[AlO_x] 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 R_Al/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.

Role of steric repulsions on the precipitation kinetics and the structure of calcium-silicate-hydrate gels

The microstructure and properties of calcium-silicate-hydrate (C-S-H) gels are largely controlled by the physicochemical environment during their precipitation. However, the role of the steric repulsive environment induced by the pore solution chemistry on the kinetics, structure, and properties of C-S-H gels remains unclear. Here, we develop two potential formalisms, namely sinusoidal and polynomial, to simulate the role of steric repulsions in C-S-H. The results show excellent agreement with experimental observations of precipitation kinetics and elastic properties. We demonstrate that the repulsive interactions result in delayed precipitation and percolation, and an open and branched microstructure. Interestingly, the elastic properties (which are equilibrium properties) are also significantly affected by these second-neighbor interactions. Overall, the present study demonstrates that the kinetics, structure, and equilibrium properties of colloidal gels are controlled by the steric repulsions induced by the chemical environment.

Ab initiobased interionic interactions in calcium aluminotitanate oxide melts: structure and diffusion

Calcium aluminotitanate (CaO-Al₂O₃-TiO₂) ternary oxides are of fundamental interest in Materials as well as Earth and environmental science, and a key system for several industrial applications. As their properties at the atomic scale are scarcely known, interionic interactions for the melts are built from a bottom up strategy consisting in fitting first only Al₂O₃, CaO and TiO₂single oxide compounds separately with a unified description of the oxygen charge and O-O interaction term. For this purpose, a mean-square difference minimization of the partial pair-correlation functions with respect to theab initioreference was performed. The potentials for the ternary oxide are finally built straightforwardly by adding purely Coulomb terms for dissimilar cation-cation interactions without further fit. This general and unified approach is transferable and successfully describes the structural and diffusion properties of the three single oxides as well as the ternary melts simultaneously. A possible underlying structural mechanism at the origin of the diffusion evolution with TiO₂content is proposed based on the formation of Ti induced triply bonded oxygen.

Topological origin of phase separation in hydrated gels

HYPOTHESIS

Depending on their composition, hydrated gels can be homogeneous or phase-separated, which, in turn, affects their dynamical and mechanical properties. However, the nature of the structural features, if any, that govern the propensity for a given gel to phase-separate remains largely unknown. Here, we argue that the propensity for hydrated gels to phase-separate is topological in nature.

SIMULATIONS

We employ reactive molecular dynamics simulations to model the early-age precipitation of calcium-alumino-silicate-hydrate (CASH) gels with varying compositions, i.e., (CaO)_1.7(Al₂O₃)_x(SiO₂)_{1 -x}(H₂O)_{3.7 +x}. By adopting topological constraint theory, we investigate the structural origin of phase separation in hydrated gels.

FINDINGS

We report the existence of a homogeneous-to-phase-separated transition, wherein Si-rich (x ≤ 0.10) CASH gels are homogeneous, whereas Al-rich (x > 0.10) CASH gels tend to phase-separate. Furthermore, we demonstrate that this transition is correlated to a topological flexible-to-rigid transition within the atomic network. We reveal that the propensity for topologically-overconstrained gels to phase-separate arises from the existence of some internal stress within their atomic network, which acts as an energy penalty that drives phase separation.

Elucidating the constitutive relationship of calcium-silicate-hydrate gel using high throughput reactive molecular simulations and machine learning

Prediction of material behavior using machine learning (ML) requires consistent, accurate, and, representative large data for training. However, such consistent and reliable experimental datasets are not always available for materials. To address this challenge, we synergistically integrate ML with high-throughput reactive molecular dynamics (MD) simulations to elucidate the constitutive relationship of calcium–silicate–hydrate (C–S–H) gel—the primary binding phase in concrete formed via the hydration of ordinary portland cement. Specifically, a highly consistent dataset on the nine elastic constants of more than 300 compositions of C–S–H gel is developed using high-throughput reactive simulations. From a comparative analysis of various ML algorithms including neural networks (NN) and Gaussian process (GP), we observe that NN provides excellent predictions. To interpret the predicted results from NN, we employ SHapley Additive exPlanations (SHAP), which reveals that the influence of silicate network on all the elastic constants of C–S–H is significantly higher than that of water and CaO content. Additionally, the water content is found to have a more prominent influence on the shear components than the normal components along the direction of the interlayer spaces within C–S–H. This result suggests that the in-plane elastic response is controlled by water molecules whereas the transverse response is mainly governed by the silicate network. Overall, by seamlessly integrating MD simulations with ML, this paper can be used as a starting point toward accelerated optimization of C–S–H nanostructures to design efficient cementitious binders with targeted properties.

Dynamics of confined water and its interplay with alkali cations in sodium aluminosilicate hydrate gel: insights from reactive force field molecular dynamics

This paper presents the dynamics of confined water and its interplay with alkali cations in disordered sodium aluminosilicate hydrate (N-A-S-H) gel using reactive force field molecular dynamics. N-A-S-H gel is the primary binding phase in geopolymers formed via alkaline activation of fly ash. Despite attractive mechanical properties, geopolymers suffer from durability issues, particularly the alkali leaching problem which has motivated this study. Here, the dynamics of confined water and the mobility of alkali cations in N-A-S-H is evaluated by obtaining the evolution of mean squared displacements and Van Hove correlation function. To evaluate the influence of the composition of N-A-S-H on the water dynamics and diffusion of alkali cations, atomistic structures of N-A-S-H with Si/Al ratio ranging from 1 to 3 are constructed. It is observed that the diffusion of confined water and sodium is significantly influenced by the Si/Al ratio. The confined water molecules in N-A-S-H exhibit a multistage dynamic behavior where they can be classified as mobile and immobile water molecules. While the mobility of water molecules gets progressively restricted with an increase in Si/Al ratio, the diffusion coefficient of sodium also decreases as the Si/Al ratio increases. The diffusion coefficient of water molecules in the N-A-S-H structure exhibits a lower value than those of the calcium-silicate-hydrate (C-S-H) structure. This is mainly due to the random disordered structure of N-A-S-H as compared to the layered C-S-H structure. To further evaluate the influence of water content in N-A-S-H, atomistic structures of N-A-S-H with water contents ranging from 5-20% are constructed. Qⁿ distribution of the structures indicates significant depolymerization of N-A-S-H structure with increasing water content. Increased conversion of Si-O-Na network to Si-O-H and Na-OH components with an increase in water content helps explain the alkali-leaching issue in fly ash-based geopolymers observed macroscopically. Overall, the results in this study can be used as a starting point towards multiscale simulation-based design and development of durable geopolymers.

Structural and dynamic properties of aluminosilicate melts: a molecular dynamics study

In the present work, the structural and dynamic properties of aluminosilicates (Al₂O₃) _x -(SiO₂)_1-x (AS) as a function of the Al₂O₃ concentration x are studied by means of molecular dynamics simulations. Firstly, the parametrization of the Born-Mayer-Huggins type potential developed recently for the more general CaO-Al₂O₃-SiO₂ ternary system is assessed. Comparison of local structural properties, such as the x-ray structure factor, partial pair-correlation functions, distributions of coordination numbers and bond angles, as well as the dynamics through the viscosity and self-diffusion coefficients to experimental data and other molecular dynamics simulations found in the literature, shows that this potential is transferable to AS melts for all compositions and is more reliable than other empirical potentials used so far. The evolution of viscosity with temperature in stable liquid and undercooled regions is studied in the whole composition range and results show a progressive increase of the fragility with increasing Al₂O₃ content correlated to that of local structural entities like the triply bonded oxygen (TBO), AlO₅ and AlO₆.

Thermodynamics and structural properties of CaO: A molecular dynamics simulation study

A detailed theoretical study of CaO in the solid and liquid states by means of combined classical and ab initio molecular dynamics simulations is presented. Evolution of the specific heat capacity at constant pressure as a function of temperature is studied, and the melting temperature and enthalpy of fusion are determined. It is shown that an empirical Born-Mayer-Huggins potential gives a good representation of pure CaO in the liquid and solid states as compared to available experimental data and density functional theory calculations. Consistency of the predicted results obtained for CaO with the data available in commercial thermodynamic databases and experimental values in the literature is discussed. The present methodology and theoretical results provide a new accurate basis for calculations of thermodynamic properties in a temperature range that is hardly accessible by experiments.

Predicting the Young's Modulus of Silicate Glasses using High-Throughput Molecular Dynamics Simulations and Machine Learning

The application of machine learning to predict materials’ properties usually requires a large number of consistent data for training. However, experimental datasets of high quality are not always available or self-consistent. Here, as an alternative route, we combine machine learning with high-throughput molecular dynamics simulations to predict the Young’s modulus of silicate glasses. We demonstrate that this combined approach offers good and reliable predictions over the entire compositional domain. By comparing the performances of select machine learning algorithms, we discuss the nature of the balance between accuracy, simplicity, and interpretability in machine learning.

Modifier clustering and avoidance principle in borosilicate glasses: A molecular dynamics study

Oxide glasses are typically described as having a random, disordered skeleton of network-forming polyhedra that are depolymerized by network-modifying cations. However, the existence of local heterogeneity or clustering within the network-forming and network-modifying species remains unclear. Here, based on molecular dynamics simulations, we investigate the atomic structure of a series of borosilicate glasses. We show that the network-modifying cations exhibit some level of clustering that depends on composition-in agreement with Greaves' modified random network model. In addition, we demonstrate the existence of some mutual avoidance among network-forming atoms, which echoes the Loewenstein avoidance principle typically observed in aluminosilicate phases. Importantly, we demonstrate that the degree of heterogeneity in the spatial distribution of the network modifiers is controlled by the level of ordering in the interconnectivity of the network formers. Specifically, the mutual avoidance of network formers is found to decrease the propensity for modifier clustering.

Elucidating the formation of Al–NBO bonds, Al–O–Al linkages and clusters in alkaline-earth aluminosilicate glasses based on molecular dynamics simulations

Exploring the reasons for the initiation of Al–O–Al bond formation in alkali-earth alumino silicate glasses is a key topic in the glass-science community.

Structure of Strontium Aluminosilicate Glasses from Molecular Dynamics Simulation, Neutron Diffraction, and Nuclear Magnetic Resonance Studies

The structure of strontium glasses with the composition (SiO₂)_{1-2 x}(Al₂O₃) _x(SrO) _x ( R = [SrO]/[Al₂O₃] = 1) and (SiO₂)_{1-4 x}(Al₂O₃) _x(SrO)_{3 x} ( R = 3) has been explored experimentally over both short- and intermediate-length scales using neutron diffraction, ²⁷Al and ²⁹Si nuclear magnetic resonance, and classical molecular dynamics simulations in model systems containing around 10 000 atoms. We aim at understanding the structural role of aluminum and strontium as a function of the chemical composition of these glasses. The short- and medium-range structure such as aluminum coordination, bond angle distribution, Q^{( n)} distribution, and oxygen speciation have been systematically studied. Two potential forms of the repulsive short-range interactions have been investigated, namely, the Buckingham and Morse forms. The comparison of these forms allows us to derive general trends independent of the particular choice of the potential form. In both cases, it is found that aluminum ions are mainly fourfold coordinated and mix with the silicon network favoring the Al/Si mixing in terms of Al-O-Si linkages. For the R = 1 glass series, despite the full charge compensation ([SrO] = [Al₂O₃]), a small fraction of fivefold aluminum is observed both experimentally and in MD simulations, whereas the concentration of sixfold aluminum is negligible. MD shows that the fivefold aluminum units AlO₅ preferentially adopt a small ring configuration and link to tricoordinated oxygen atoms whose population increases with the aluminum content and are mainly found in OAl₃ and OAl₂Si configurations. The modeled Sr speciation mainly involves SrO₇ and SrO₈ polyhedra, giving a range of average Sr²⁺ coordination numbers between 7 and 8 slightly dependent on the short-range repulsive potential form. A detailed statistical analysis of T-O-T' (T, T' = Al,Si), accounting for the population of the various oxygen speciations, reveals that both potentials predict a nearly identical Al/Si mixing.

New insights into the atomic structure of amorphous TiO2 using tight-binding molecular dynamics

Amorphous TiO₂ (a-TiO₂) could offer an attractive alternative to conventional crystalline TiO₂ phases for photocatalytic applications. However, the atomic structure of a-TiO₂ remains poorly understood with respect to that of its crystalline counterparts. Here, we conduct some classical molecular dynamics simulations of a-TiO₂ based on a selection of empirical potentials. We show that, on account of its ability to dynamically assign the charge of each atom based on its local environment, the second-moment tight-binding charge equilibration potential yields an unprecedented agreement with available experimental data. Based on these simulations, we investigate the degree of order and disorder in a-TiO₂. Overall, the results suggest that a-TiO₂ features a large flexibility in its local topology, which may explain the high sensitivity of its structure to the synthesis method being used.

Thermodynamic and structural properties of binary calcium silicate glasses: insights from molecular dynamics

The thermodynamic and structural properties of calcium-modified binary silicate glasses have been computed using molecular dynamics simulations.

Topological Control on Silicates' Dissolution Kinetics

Like many others, silicate solids dissolve when placed in contact with water. In a given aqueous environment, the dissolution rate depends on the composition and the structure of the solid and can span several orders of magnitude. Although the kinetics of dissolution depends on the complexities of both the dissolving solid and the solvent, a clear understanding of which structural descriptors of the solid control its dissolution rate is lacking. By pioneering dissolution experiments and atomistic simulations, we correlate the dissolution rates-ranging over 4 orders of magnitude-of a selection of silicate glasses and crystals to the number of chemical topological constraints acting between the atoms of the dissolving solid. The number of such constraints serves as an indicator of the effective activation energy, which arises from steric effects, and prevents the network from reorganizing locally to accommodate intermediate units forming over the course of the dissolution.

Nature of radiation-induced defects in quartz

Although quartz (α-form) is a mineral used in numerous applications wherein radiation exposure is an issue, the nature of the atomistic defects formed during radiation-induced damage has not been fully clarified. Especially, the extent of oxygen vacancy formation is still debated, which is an issue of primary importance as optical techniques based on charged oxygen vacancies have been utilized to assess the level of radiation damage in quartz. In this paper, molecular dynamics simulations are applied to study the effects of ballistic impacts on the atomic network of quartz. We show that the defects that are formed mainly consist of over-coordinated Si and O, as well as Si-O connectivity defects, e.g., small Si-O rings and edge-sharing Si tetrahedra. Oxygen vacancies, on the contrary, are found in relatively low abundance, suggesting that characterizations based on E' centers do not adequately capture radiation-induced structural damage in quartz. Finally, we evaluate the dependence on the incident energy, of the amount of each type of the point defects formed, and quantify unambiguously the threshold displacement energies for both O and Si atoms. These results provide a comprehensive basis to assess the nature and extent of radiation damage in quartz.