Showing papers by "Morten Mattrup Smedskjær published in 2018"

A new transferable interatomic potential for molecular dynamics simulations of borosilicate glasses

Abstract: Borosilicate glasses are traditionally challenging to model using atomic scale simulations due to the composition and thermal history dependence of the coordination state of B atoms. Here, we report a new empirical interatomic potential that shows a good transferability over a wide range of borosilicate glasses—ranging from pure silicate to pure borate end members—while relying on a simple formulation and a constant set of energy parameters. In particular, we show that our new potential accurately predicts the compositional dependence of the average coordination number of boron atoms, glass density, overall short-range and medium-range order structure, and shear viscosity values for several borosilicate glasses and liquids. This suggests that our new potential could be used to gain new insights into the structure of a variety of advanced borosilicate glasses to help elucidate composition-structure-property relationships—including in complex nuclear waste immobilization glasses.

Predicting the dissolution kinetics of silicate glasses using machine learning

Abstract: Predicting the dissolution rates of silicate glasses in aqueous conditions is a complex task as the underlying mechanism(s) remain poorly understood and the dissolution kinetics can depend on a large number of intrinsic and extrinsic factors. Here, we assess the potential of data-driven models based on machine learning to predict the dissolution rates of various aluminosilicate glasses exposed to a wide range of solution pH values, from acidic to caustic conditions. Four classes of machine learning methods are investigated, namely, linear regression, support vector machine regression, random forest, and artificial neural network. We observe that, although linear methods all fail to describe the dissolution kinetics, the artificial neural network approach offers excellent predictions, even for untrained data, thanks to its inherent ability to handle non-linear data. We further note that the predictive ability of simpler methods, such as linear regression, could be improved using additional physics-based constraints. Such methods, called as physics-informed machine learning can be used to extrapolate the behavior of untrained compositions as well. Overall, we suggest that a more extensive use of machine learning approaches could significantly accelerate the design of novel glasses with tailored properties.

The hydrophilic-to-hydrophobic transition in glassy silica is driven by the atomic topology of its surface

Abstract: The surface reactivity and hydrophilicity of silicate materials are key properties for various industrial applications. However, the structural origin of their affinity for water remains unclear. Here, based on reactive molecular dynamics simulations of a series of artificial glassy silica surfaces annealed at various temperatures and subsequently exposed to water, we show that silica exhibits a hydrophilic-to-hydrophobic transition driven by its silanol surface density. By applying topological constraint theory, we show that the surface reactivity and hydrophilic/hydrophobic character of silica are controlled by the atomic topology of its surface. This suggests that novel silicate materials with tailored reactivity and hydrophilicity could be developed through the topological nanoengineering of their surface.

Hardness of Silicate Glasses: Atomic-Scale Origin of the Mixed Modifier Effect

Abstract: The origin of the various manifestations of the mixed modifier effect in silicate glasses remains poorly understood. Here, based on molecular dynamics simulations, we investigate the origin of a negative deviation from linearity in the hardness of a series of mixed alkaline earth aluminosilicate glasses. The minimum of hardness is shown to arise from a maximum propensity for shear flow deformations in mixed compositions. We demonstrate that this anomalous behavior originates from the existence of local structural instabilities in mixed compositions arising from a mismatch between the modifiers and the rest of the silicate network. Overall, we suggest that the mixed modifier effect manifests itself as a competition between the thermodynamic driving force for structural relaxation and the kinetics thereof.

Structural Compromise between High Hardness and Crack Resistance in Aluminoborate Glasses.

Abstract: Alkali aluminoborate glasses have recently been shown to exhibit a high threshold for indentation cracking compared to other bulk oxide glasses. However, to enable the use of these materials in engineering applications, there is a need to improve their hardness by tuning the chemical composition. In this study, we substitute alkaline earth for alkali network-modifying species at fixed aluminoborate base glass composition and correlate it with changes in the structure, mechanical properties, and densification behavior. We find that the increase in field strength (i.e., the charge-to-size ratio) achieved by substituting alkaline earth oxide from BaO to MgO manifests itself in a monotonic increase in several properties, such as atomic packing density, glass-transition temperature, densification ability, indentation hardness, and crack resistance. Although the use of alkaline earth oxides as modifier enables higher hardness values (increasing from 2.0 GPa for Cs to 5.8 GPa for Mg), their crack resistance is g...

Effect of nanoscale phase separation on the fracture behavior of glasses: Toward tough, yet transparent glasses

Abstract: Despite recent advances, glass still breaks. This remains a major limitation for cover glasses used, for instance, in smartphone screens. Here, the authors explore the possibility of benefiting from nanoscale phase separation to significantly increase the fracture toughness of glass, while retaining its transparency. Based on peridynamic simulations, they investigate the nature of the toughening mechanisms at play and find that nanoscale phase separation can yield up to a 90% increase in fracture energy. This establishes phase separation as a promising route to develop novel tough, yet transparent glasses

Time and humidity dependence of indentation cracking in aluminosilicate glasses

Abstract: The inherent brittleness and poor crack resistance of oxide glasses have always been among their main limitations for many advanced applications. As the formation of cracks leads to amplification of applied tensile stresses and ultimately catastrophic failure, there is an interest in understanding the compositional and structural dependence of crack initiation and growth. The resistance to cracking can conventionally be measured using instrumented indentation that mimics the real-life damage for certain applications. Wada introduced a method to evaluate the crack resistance by counting the number of initiated cracks as a function of the applied load. Experiments have shown that the environmental humidity and the time period between indentation and crack counting both affect the crack resistance value, but unfortunately these parameters are not always reported in literature studies. Here we perform a systematic study of the time and humidity dependence of crack initiation in calcium aluminosilicate glasses. Depending on the experimental conditions (time and humidity), the crack resistance of an aluminosilicate glass can vary by more than a factor of two. Furthermore, the observed radial/median cracks can initiate several hours after indentation. These results therefore indicate the need for a standardized procedure for determination of crack resistance to allow comparison of data from different research groups. We suggest including a sufficiently long waiting period (such as 24 h) between indentation and crack counting, as the majority of the crack initiation will then have occurred.

Combining high hardness and crack resistance in mixed network glasses through high-temperature densification

Abstract: Obtaining a combination of high toughness and strength is crucial for most structural materials, but unfortunately these tend to be mutually exclusive. The search for strong and tough damage-resistant materials has thus typically been based on achieving an acceptable compromise between hardness and crack resistance. Focusing here on brittle oxide glasses, we propose a new strategy for overcoming this conflict by identifying new structural motifs for designing hard and crack-resistant glasses. Specifically, we report that surprisingly there is no decrease in the densification contribution to deformation of a mixed network ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}\ensuremath{-}{\mathrm{B}}_{2}{\mathrm{O}}_{3}\ensuremath{-}{\mathrm{P}}_{2}{\mathrm{O}}_{5}\ensuremath{-}\mathrm{Si}{\mathrm{O}}_{2}$ bulk glass following predensification of the glass at elevated temperature. Hitherto unique to this glass composition, the treatment reduces the residual stress during subsequent sharp contact loading, which in turn leads to a simultaneous increase in hardness and crack resistance. Based on structural characterization, we show that the more densified medium-range order of the hot compressed glass results in formation of certain structural states (e.g., nonring trigonal boron), which could not be reached through any composition or thermal path. This work thus shows that accessing such ``forbidden'' structural states through the identified densification at elevated temperatures offers a way forward to overcome the conflict of strength versus toughness in structural materials.

Predicting Q-Speciation in Binary Phosphate Glasses Using Statistical Mechanics

Abstract: Predicting the compositional evolution of the atomic-scale structure of oxide glasses is important for developing quantitative composition-property models. In binary phosphate glasses, the addition of network modifiers generally leads to depolymerization of the networks as described by the Q-speciation, where Q n denotes PO4 tetrahedra with n number (between 0 and 3) of bridging P-O-P linkages per tetrahedron. Upon the initial creation of nonbridging oxygens and thus partly depolymerized Q species, a variety of network former-modifier interactions exist. Here, on the basis of 31P magic angle spinning nuclear magnetic resonance spectroscopy data from the literature, we present a statistical description of the compositional evolution of Q-speciation in these glasses by accounting for the relative enthalpic and entropic contributions to the bonding preferences. We show that the entire glass structure evolution can be predicted based on experimental structural information for only a few glass compositions in each series. The model also captures the differences in bonding preferences in glasses with different field strengths (charge-to-size ratio) of the modifier cations.

Predictive model for the composition dependence of glassy dynamics

Abstract: The problem of glass relaxation is traditionally known as one of the most challenging problems in condensed matter physics, with important implications for several high-tech applications of glass. In this study, we present a predictive model for the temperature, thermal history, and composition dependence of glassy relaxation dynamics. Our model enables, for the first time, the quantitative prediction of relaxation behavior for new glass compositions. Using the commercial Corning EAGLE XG® alkaline earth aluminosilicate glass as a reference, the model gives accurate predictions of the nonequilibrium viscosity for four other aluminosilicate glasses, covering both alkali-free and alkali-containing compositions, without any free fitting parameters. Using the composition-dependent nonequilibrium viscosity model, only the measured values of the glass transition temperature and fragility are required to predict the nonequilibrium viscosity as a function of both temperature and thermal history. The range of glass transition temperatures of the four verification glasses covers about 200°C, while that of fragility values is about 10. As such, this work gives insights into the structural origin of nonequilibrium viscosity and can enable the future design of glass compositions with tailored relaxation behavior. This article is protected by copyright. All rights reserved.

Pressure-induced structural changes in titanophosphate glasses studied by neutron and X-ray total scattering analyses

Abstract: In this paper, the densification mechanism of two titanophosphate glasses (binary 73TiO2-27P2O5 and ternary 68TiO2-5Al2O3-27P2O5) is investigated. The glasses are densified by isostatic N2-mediated pressure treatment at elevated temperature and the pressure-induced structural changes are studied by neutron and X-ray total scattering analyses. The medium range structure change is revealed from the change in the first sharp diffraction peak (FSDP), where the density increase by pressurization is correlated with the decreased medium range repeating spacing calculated from the position of FSDP. The hot compression treatment also increases the medium range order, as shown by the increased correlation length calculated from the width of FSDP. The short range structure changes are examined by separating P O and Ti O pair distribution functions (PDF) using differential neutron and X-ray PDFs. Pressurization is also found to increase the coordination number of Ti, as previously observed for other network formers in densified glasses.

Nano-phase separation and structural ordering in silica-rich mixed network former glasses.

Abstract: We investigate the structure, phase separation, glass transition, and crystallization in a mixed network former glass series, i.e., B2O3–Al2O3–SiO2–P2O5 glasses with varying SiO2/B2O3 molar ratio. All the studied glasses exhibit two separate glassy phases: droplet phase (G1) with the size of 50–100 nm and matrix phase (G2), corresponding to a lower calorimetric glass transition temperature (Tg1) and a higher one (Tg2), respectively. Both Tg values decrease linearly with the substitution of B2O3 for SiO2, but the magnitude of the decrease is larger for Tg1. Based on nuclear magnetic resonance and Raman spectroscopy results, we infer that the G1 phase is rich in boroxol rings, while the G2 phase mainly involves the B–O–Si network. Both phases contain BPO4- and AlPO4-like units. Ordered domains occur in G2 upon isothermal and dynamic heating, driven by the structural heterogeneity in the as-prepared glasses. The structural ordering lowers the activation energy of crystal growth, thus promoting partial crystallization of G2. These findings are useful for understanding glass formation and phase separation in mixed network former oxide systems, and for tailoring their properties.

Deformation and cracking behavior of La2O3-doped oxide glasses with high Poisson's ratio

Abstract: Oxide glasses pose high theoretical strength originating from their strong ionocovalent bonding, but they experience amplification of tensile stresses around defects under tensile loading and lack efficient stress dissipation mechanisms. Consequently, glasses exhibit low practical strength and fracture toughness, limiting the scope of their applications. Different strengthening and reinforcement approaches have thus been tested, but relatively little success has been achieved with respect to making the glasses intrinsically more ductile through composition optimization. Following earlier literature reports, a possible route to achieve this would be to prepare glasses with high Poisson's ratio above ~0.32. Yet, no oxide glasses with such high Poisson's ratio have been reported and the mechanical properties of oxide glasses with Poisson's ratio ≥ 0.30 are poorly understood. In this paper, we synthesize 25%La2O3–15%Al2O3–60%B2O3, 25%La2O3–15%Al2O3–60%SiO2, and 25%La2O3–15%Al2O3–60%GeO2 glasses (fractions in mol%), all exhibiting high Poisson's ratio values (~0.30). We evaluate the mechanical properties, including elastic moduli, Poisson's ratio, hardness, and resistance to indentation cracking of the as-prepared as well as densified glasses. In addition, the indentation deformation mechanism of the glasses along with the accompanying underlying structural changes is investigated. This study therefore presents insight into the composition-property relations of high Poisson's ratio glasses, which may be used in future design of ductile oxide glasses with potential applications in electronic devices, optical fibers, and load-bearing components of buildings or other constructions.

Competitive effects of modifier charge and size on mechanical and chemical resistance of aluminoborate glasses

Abstract: Lithium aluminoborate glasses exhibit high resistance to cracking under contact loading, but low hardness and poor chemical durability in aqueous media. On the other hand, alkaline earth aluminoborate glasses feature improved chemical resistance and hardness, but lower resistance to cracking. In this work, we investigate the possibility to simultaneously improve the mechanical and chemical resistance of aluminoborate glasses by mixing alkali and alkaline earth modifiers. We study the mixed Li/Ba and Li/Mg aluminoborate glasses, since Li+ and Ba2+ have different charge and size but similar modifier field strength (charge to size ratio), while Mg2+ has the highest field strength among these modifiers due to its small size. The two glass series will thus give insights into the competitive effects of modifier charge and size on glass structure, mechanical properties, and dissolution rates in acidic, neutral, and basic solutions. The substitution of barium for lithium at fixed [Al2O3]/[B2O3] ratio does not affect the network structure and properties, such as hardness and dissolution rate. However, the substitution of magnesium for lithium leads to an increase in hardness and chemical durability for all pH solutions (2, 7, and 14), likely as a result of the increase in the fractions of five- and six-fold coordinated aluminum species and atomic packing density. The glass with 5 mol% Li2O and 20 mol% MgO exhibits the best combination of high hardness and low dissolution rate, while maintaining a good crack-resistance comparable to that of the Li-aluminoborate glass. In both mixed glass series, the lowest dissolution rates are measured in neutral solutions, while those in acidic and especially basic media are higher. The structural origins of the trends in chemical and mechanical properties are discussed based on 11B and 27Al nuclear magnetic resonance (NMR) spectroscopy measurements.

Structural stability of NaPON glass upon heating in air and nitrogen

Abstract: The thermal stability in air and nitrogen of an oxynitride NaPON glass with high nitrogen content (N/P = 0.5) has been investigated with regards to its structural evolution with temperature. The glass transition temperature (Tg) of the powdered glass is found to decrease upon oxidation, especially when the treatment temperature (Ta) is larger than the Tg of the original oxynitride glass. Upon isothermal oxidation, crystalline metaphosphate forms at the interface of the oxide layer as the dominant phase in both the powder and bulk samples, as detected by Raman spectroscopy and X-ray diffraction. A new deconvolution scheme of Raman spectra is proposed, involving a structural model proposed to account for the in situ high temperature changes of the local structural groups. A distinction is made between different oxynitride Qn(P,N) tetrahedral sites, and two separate bands related to tri-coordinated nitrogen speciation (Nt) are distinguished in the oxidized NaPON glass. Nt groups are connected to either one oxygen or one nitrogen, resulting in two separate Raman bands. The position and area of these Nt-related peaks exhibit an opposite trend with temperature in air and N2. Furthermore, the Raman results imply a thermally driven depolymerization of the oxynitride sub-structure, which could involve a nano-scale phase separation of the nitrogen-involved structure network. In terms of technological applications, this work suggests that the oxynitride glasses should be used in the temperature range up to the glass transition temperature, above which the structural stability is lost.

Parametric study of temperature-modulated differential scanning calorimetry for high-temperature oxide glasses with varying fragility

Abstract: Differential scanning calorimetry (DSC) has proven to be a highly versatile technique for understanding the glass transition, relaxation, and crystallization behavior of inorganic glasses. However, the approach is challenging when probing glass samples that exhibit overlapping transitions or low sensitivity. To overcome these problems, temperature-modulated DSC (TM-DSC) can be utilized, in which a sinusoidal heating rate is superimposed on the linear heating rate known from standard linear DSC. Until recently, it has only been possible to perform TM-DSC measurements on commercial instruments at temperatures below 973 K, which is insufficient for many oxide glasses of industrial interest, particularly silicate glasses. However, recent commercially available software now enables TM-DSC measurements to be performed at temperatures far exceeding 973 K. To investigate the suitability of using TM-DSC to study glass transition and relaxation behavior in high-temperature silicate systems, we have performed systematic TM-DSC measurements on three different oxide glass systems with varying glass transition temperature and liquid fragility. We find that relatively large underlying heating rates (2–5 K/min) and modulation amplitudes (4–5 K) are needed in order to obtain data with high signal-to-noise ratios. For these combinations of experimental parameters, we also observe a linear response as found using Lissajous curves. Overall, this study suggests that TM-DSC is a promising technique for investigating the dynamics of high-temperature oxide glass systems with a wide range of liquid fragilities.

Temperature-modulated differential scanning calorimetry analysis of high-temperature silicate glasses

Abstract: Author(s): Bechgaard, Tobias K; Gulbiten, Ozgur; Mauro, John C; Hu, Yushu; Bauchy, Mathieu; Smedskjaer, Morten M | Abstract: Differential scanning calorimetry (DSC) is one of the most versatile probes for silicate glasses, allowing determination of, e.g., transition temperatures (glass, crystallization, melting) and the temperature dependence of heat capacity. However, complications arise for glasses featuring overlapping transitions and low sensitivity, e.g., arising from SiO2-rich compositions with small change in heat capacity during glass transition or the low sensitivity of thermocouples at high temperature. These challenges might be overcome using temperature-modulated DSC (TM-DSC), which enables separation of overlapping signals and improved sensitivity at the expense of increased measurement duration.

Structural Dependence of Chemical Durability in Modified Aluminoborate Glasses

Abstract: Alkali and alkaline earth aluminoborate glasses feature high resistance to cracking under sharp contact loading compared to other oxide glasses. However, due to the high content of hygroscopic B2O3, it is expected that applications of these glasses could be hindered by poor chemical durability in aqueous solutions. Indeed, the compositional and structural dependence of their dissolution kinetics remains unexplored. In this work, we correlate the dissolution rates of aluminoborate glasses in acidic, neutral, and basic solutions with the structural changes induced by varying the aluminum-to-boron ratio. In detail, we investigate a total of seventeen magnesium, lithium, and sodium aluminoborate glasses with fixed modifier content of 25 mol%. We show that the structural changes induced by alumina depend on the network modifier. We also demonstrate a correlation between the chemical durability at various pH values and the structural changes in Mg-, Li- and Na- aluminoborate glasses. The substitution of alumina by boron oxide leads to a general decrease of chemical corrosion in neutral and acidic solutions. The lowest dissolution rate value is observed in Mg-aluminoborate glasses, as a consequence of the intermediate character of magnesium which can increase the network cross-linking. For basic solutions, the chemical durability is almost constant for the different amount of alumina in the three series, likely because B2O3 is susceptible to nucleophilic attack, which is favored in high-OH- solutions.

Temperature-Modulated Differential Scanning Calorimetry Analysis of High-Temperature Silicate Glasses

Abstract: Differential scanning calorimetry (DSC) is one of the most versatile probes for silicate glasses, allowing determination of, e.g., transition temperatures (glass, crystallization, melting) and the temperature dependence of heat capacity. However, complications arise for glasses featuring overlapping transitions and low sensitivity, e.g., arising from SiO2-rich compositions with small change in heat capacity during glass transition or the low sensitivity of thermocouples at high temperature. These challenges might be overcome using temperature-modulated DSC (TM-DSC), which enables separation of overlapping signals and improved sensitivity at the expense of increased measurement duration.