Searching for correlations between vibrational spectral features and structural parameters of silicate glass network

Infrared (IR) and Raman spectroscopic features of silicate glasses are often interpreted based on the analogy with those of smaller molecules, molecular clusters, or crystalline counterparts; this study tests the accuracy and validity of these widely cited peak assignment schemes by comparing vibrational spectral features with bond parameters of the glass network created by molecular dynamics (MD) simulations. A series of sodium silicate glasses with compositions of [Na₂O]_x[Al₂O₃]₂[SiO₂]_98−x with x = 7, 12, 17, and 22 were synthesized and analyzed with IR and Raman. A silica glass substrate and a crystalline quartz were also analyzed for comparison. Glass structures with the same compositions were generated with MD simulations using three types of potentials: fixed partial charge pairwise (Teter), partial diffuse charge potential (MGFF), and bond order-based charge transfer potential (ReaxFF). The comparison of simulated and experimental IR spectra showed that, among these three potentials tested, ReaxFF reproduces the concentration dependence of spectral features closest to the experimentally observed trend. Thus, the bond length and angle distributions as well as Si–Qⁿ species and ring size distributions of silica and sodium silicate glasses were obtained from ReaxFF-MD simulations and further compared with the peak assignment or deconvolution schemes—which have been widely used since 1970s and 1980s—(a) correlation between the IR peak position in the Si–O stretch region (1050-1120 cm⁻¹) and the Si–O–Si bond angle; (b) deconvolution of the Raman bands in the Si–O stretch region with the Qⁿ speciation; and (c) assignment of the Raman bands in the 420-600 cm⁻¹ region to the bending modes of (SiO)_n rings with different sizes (typically, n = 3-6). The comparisons showed that none of these widely used methods is congruent with the bond parameters or structures of silicate glass networks produced via ReaxFF-MD simulations. This finding invokes that the adequacy of these spectral interpretation methods must be questioned. Alternative interpretations are proposed, which are to be tested independently in future studies.     

Phase separation and crystallization in glasses of the lithium silicate system xLi2O · (100 ? x)SiO2 (x = 23.4, 26.0, 33.5)

The phase separation in ultimately homogenized glasses of the lithium silicate system xLi₂O · (100 − x)SiO₂ (where x = 23.4, 26.0, and 33.5 mol % Li₂O) has been investigated. The glasses of these compositions have been homogenized using the previously established special temperature-time conditions, which provide the maximum dehydration and the removal of bubbles from the glass melt. The parameters of nucleation and growth of phase_separated inhomogeneities and homogeneous crystal nucleation have been determined. The absolute values of the stationary nucleation rates I _st of lithium disilicate crystals in the 23.4Li₂O · 76.6SiO₂ and 26Li₂O · 74SiO₂ glasses with the compositions lying in the metastable phase separation region have been compared with the corresponding rates I _st for the glass of the stoichiometric lithium disilicate composition. It has been established that the crystal growth rate have a tendency toward a monotonic increase with an increase in the temperature, whereas the dependences of the crystal growth rate on the time of low-temperature heat treatment exhibit an oscillatory behavior with a monotonic decrease in the absolute value of oscillations. The character of crystallization in glasses with the compositions lying in the phase separation region of the Li₂O-SiO₂ system is compared with that in the glass of the stoichiometric lithium disilicate composition. The inference has been made that the phase separation weakly affects the nucleation parameters of lithium disilicate and has a strong effect on the crystal growth.     

Structure-property relations in crack-resistant alkaline-earth aluminoborosilicate glasses studied by solid state NMR

The effect of the average ionic potential ξ = Ze/r of the network modifier cations on crack initiation resistance (CR) and Young's modulus E has been measured for a series of alkaline-earth aluminoborosilicate glasses with the compositions 60SiO₂–10Al₂O₃–10B₂O₃–(20−x)M₍₂₎O–xM’O (0 ≤ x ≤ 20; M, M’ = Mg, Ca, Sr, Ba, Na). Systematic trends indicating an increase of CR with increasing ionic potential, ξ, have been correlated with structural properties deduced from the NMR interaction parameters in ²⁹Si, ²⁷Al, ²³Na, and ¹¹B solid state NMR. ²⁷Al NMR spectra indicate that the aluminum atoms in these glasses are essentially all four-coordinated, however, the average quadrupolar coupling constant <C_Q> extracted from lineshape analysis increases linearly with increasing average ion potential computed from the cation composition. A similar linear correlation is observed for the average ²⁹Si chemical shift, whereas the fraction of four-coordinate boron decreases linearly with increasing ξ. Altogether the results indicate that in pure alkaline-earth boroaluminosilicate glasses the crack resistance/E-modulus trade-off can be tailored by the alkaline-earth oxide inventory. In contrast, the situation looks more complicated in glasses containing both Na₂O and the alkaline-earth oxides MgO, CaO, SrO, and BaO. For 60SiO₂–10Al₂O₃–10B₂O₃–10MgO–10Na₂O glass, the NMR parameters, interpreted in the context of their correlations with ionic potentials, are consistent with a partial network former role of the MgO component, enhancing crack resistance. Altogether the presence of MgO in aluminoborosilicate glasses helps overcome the trade-off issue between high crack resistance and high elasticity modulus present in borosilicate glasses, thereby offering additional opportunities for the design of glasses that are both very rigid and very crack resistant.     

Mixed modifier effects on structural,mechanical, chemical,and mechanochemical properties of sodium calcium aluminosilicate glass

Glass properties are governed by the interplay between network formers and network modifiers; for a given composition of network formers, the ratio of different cationic modifiers compensating the anionic species in the network has a profound effect, which is often nonlinear, called a mixed modifier effect (MME). We have investigated the MME of sodium (Na) and calcium (Ca) in an aluminosilicate (NCAS) glass series following the formula [Na₂O]_30−x [CaO]_x [Al₂O₃]₁₀ [SiO₂]₆₀, where x = 0, 7.5, 15, 22.5, and 30. A nonadditive trend was observed in hardness and indentation toughness, with aqueous corrosion resistance exhibiting a shift from incongruent to congruent corrosion, whereas the network structure determined by molecular dynamics simulations revealed no significant trend with composition. Additionally, the NCAS glass containing both [Na₂O] and [CaO] within an intermediate range exhibited superior resistance to wear at high humidity, a clear MME phenomenon previously only observed in soda–lime silica.     

Diffusion of cations in sodium-potassium and sodium-barium silicate melts

Diffusion coefficients of sodium and potassium ions in molten glasses with the composition (25 ? x)Na₂O · xK₂O · 75SiO₂ have been determined by the sectioning method with measurement of the residual activity ²²Na and ⁴²K isotopes. Diffusion of sodium and ¹³³Ba ions has been studied in melts of the (30 ? x)Na₂O · xBaO · 5Ga₂O₃ · 65SiO₂ system. It has been found that the concentration dependences of diffusion characteristics in melts containing alkali and alkaline-earth cations principally differ from the regularities diffusion in dialkali melts.     

Assessment of interatomic parameters for the reproduction of borosilicate glass structures via DFT-GIPAW calculations

Borates and borosilicates are potential candidates for the design and development of glass formulations with important industrial and technological applications. A major challenge that retards the pace of development of borate/borosilicate based glasses using predictive modeling is the lack of reliable computational models to predict the structure-property relationships in these glasses over a wide compositional space. A major hindrance in this pursuit has been the complexity of boron-oxygen bonding due to which it has been difficult to develop adequate B–O interatomic potentials. In this article, we have evaluated the performance of three B–O interatomic potential models recently developed by Bauchy et al [J. Non-Cryst. Solids, 2018, 498, 294–304], Du et al [J. Am. Ceram. Soc. https://doi.org/10.1111/jace.16082 ] and Edèn et al [Phys. Chem. Chem. Phys., 2018, 20, 8192–8209] aiming to reproduce the short-to-medium range structures of sodium borosilicate glasses in the system 25 Na₂O x B₂O₃ (75 − x) SiO₂ (x = 0-75 mol%). To evaluate the different force fields, we have computed at the density functional theory level the NMR parameters of ¹¹B, ²³Na, and ²⁹Si of the models generated with the three potentials and the simulated MAS NMR spectra compared with the experimental counterparts. It was observed that the rigid ionic models proposed by Bauchy and Du can both reliably reproduce the partitioning between BO₃ and BO₄ species of the investigated glasses, along with the local environment around sodium in the glass structure. However, they do not accurately reproduce the second coordination sphere of silicon ions and the Si–O–T (T = Si, B) and B-O-T distribution angles in the investigated compositional space which strongly affect the NMR parameters and final spectral shape. On the other hand, the core-shell parameterization model proposed by Edén underestimates the fraction of BO₄ species of the glass with composition 25Na₂O 18.4B₂O₃ 56.6SiO₂ but can accurately reproduce the shape of the ¹¹B and ²⁹Si MAS-NMR spectra of the glasses investigations due to the narrower B–O–T and Si-O-T bond angle distributions. Finally, the effect of the number of boron atoms (also distinguishing the BO₃ and BO₄ units) in the second coordination sphere of the network former cations on the NMR parameters have been evaluated.     

Degree of Polymerization of Aluminosilicate Glasses and Melts

This paper presents the results of analyzing the data available in the literature on the structure and properties of silicate glasses and melts that contain Ti⁴⁺, Al³⁺, and Fe³⁺ cations in addition to alkali and alkaline-earth cations. It is established that the aforementioned multivalent cations in glasses and melts have a coordination number of four and play the role of network-formers. Aluminosilicate glasses and melts with the mole fraction ratio Al₂O₃/M ₂(M)O = 1 are of special interest. For these glasses, the structure is considered to be completely polymerized and, contrary to traditional concepts, their properties depend on the concentration ratio Al₂O₃/SiO₂. Taking into account that the structure of aluminosilicate glasses involves unusual structural units (such as triclusters) and a certain number of nonbridging oxygen atoms, a formula is proposed for calculating the degree of polymerization. The proposed formula is used to calculate the degree of polymerization for a number of Na₂O · Al₂O₃ · mSiO₂ glasses and the CaO · Al₂O₃ · 2SiO₂ glass. It is demonstrated that the calculated degrees of polymerization correlate with the experimentally measured viscosities of the relevant melts.     

Kinetics of crystal nucleation of Na<Subscript>2</Subscript>O · 2CaO · 3SiO<Subscript>2</Subscript>-based solid solutions in glasses of the Na<Subscript>2</Subscript>SiO<Subscript>3</Subscript>—CaSiO<Subscript>3</Subscript> pseudobinary join

The kinetics of crystal nucleation is investigated in sodium calcium silicate glasses of two compositions (22.4 and 24.4 mol % Na₂O), which belong to the Na₂SiO₃—CaSiO₃ pseudobinary join and, according to the phase diagram, lie in the region of the formation of solid solutions between the compositions Na₂O · 2CaO · 3SiO₂ and Na₂O · CaO · 2SiO₂. The stationary rate of crystal nucleation of Na₂O · 2CaO · 3SiO₂-based solid solutions is measured as a function of temperature. It is shown that the maximum stationary rate of nucleation increases with an increase in the sodium oxide content in the initial glasses. The experimental data are analyzed in the framework of the classical nucleation theory.Original Russian Text Copyright © 2004 by Fizika i Khimiya Stekla, Soboleva, Yuritsyn, Ugolkov.     

Statistical mechanical model for the formation of octahedral silicon in phosphosilicate glasses

Unlike ambient pressure silicate glasses, some phosphosilicate glasses contain sixfold-coordinated silicon (Si⁶) units even when prepared at ambient pressure. The variation in the fraction of Si⁶ with composition remains a topic of interest, both for technological applications of phosphosilicate glasses and for fundamental understanding of the glass structure. In this work, we use statistical mechanical modeling to predict the composition–structure relationships in Na₂O–P₂O₅–SiO₂ and CaO–P₂O₅–SiO₂ glasses. This is achieved by accounting for the enthalpic and entropic contributions to the interactions between each pairwise modifier ion and structural unit. The initial enthalpy parameters are obtained based on experimental structural data for binary Na₂O–SiO₂, CaO–SiO₂, Na₂O–P₂O₅, and CaO–P₂O₅ glasses, which can then be transferred to predict the structure of mixed former glasses. This approach has previously been used to predict the short-range structure of borosilicate and aluminoborate glass systems. However, here we show that the formation of Si⁶ must be specifically included to make accurate predictions of the composition–structure relationships in phosphosilicate glasses. After incorporating the formation mechanism of Si⁶ in the statistical mechanics model, we find an excellent agreement between model predictions and experimental structure data for Na₂O–P₂O₅–SiO₂ and CaO–P₂O₅–SiO₂ glasses.     

Optimal Strategy Searching for a Component Composition of Synthetic Sodium Silicates Used in the Production of Microspheres

The results of synthesis of a sodium silicate batch for microspheres in an aqueous medium using sol-gel technology are described. It is demonstrated that crystalline phases marked on the Krachek diagram have been formed without high-temperature treatment. A nomogram has been developed, which is recommended for designing glasses to model a phase composition based on a preset silica modulus and a structure cohesion factor taking into account the effect of Na₂O and SiO₂ on glass properties, to determine the range of probable glass formation, and to optimize the technological process.     

Effects of alkali oxides and ion-exchange on the structure of zinc-alumino-silicate glasses and glass-ceramics

Types and contents of alkali metal ions play important role on crystallization and ion-exchange properties in glasses. In this work, effects of Na₂O/K₂O ratio on crystallization and ion-exchange properties of zinc-alumino-silicate glasses were investigated. The crystalline phases precipitated in glasses changes from ZnO to β-Zn₂SiO₄ with the progressive replacement of K₂O by Na₂O in parent glasses. Ion-exchange depth of layer (DOL) decreases gradually with the increase in Na₂O content in parent glasses. Precipitation of ZnO and β-Zn₂SiO₄ nanocrystals facilitated the ion-exchange and enlarged the DOL. Na⁺ and K⁺ ions were doped into ZnO and β-Zn₂SiO₄ nanocrystals during heat-treatment, and the extent of doping was facilitated by ion-exchange. Vickers hardness were improved significantly with the crystallization and ion-exchange. Results reported here are valuable for the controlled preparation and chemical strengthening of ZnO and β-Zn₂SiO₄ glass-ceramics.     

On the Glass Formation in the Na2O–SiO2–H2O System

The glass formation in the Na₂O–SiO₂–H₂O system is considered in the framework of the free volume theory of glasses. The boundaries of the glass transition range in the given system are determined from the dependences of the specific volume and the degree of connectivity on the water content in the system. The differences in the structure of hydrated glasses prepared by different methods are revealed. It is demonstrated that the molecular packing coefficients of the structures formed by drying of sodium silicate solutions are smaller than those of glasses prepared by hydration of anhydrous glasses. The densities are determined and the molar volumes are calculated for hydrated glasses in the Na₂O–SiO₂–H₂O system with a water content of 12–23 wt % (33–51 mol %) at a constant molar ratio SiO₂ : Na₂O = 2.8. The correlation relationship for the partial molar volume of water as a component of hydrated glasses is proposed as a function of the water content in the system. The use of this dependence allows one to calculate the molar volumes (densities) of hydrated glasses with different water contents.     

Ab initio Modeling of the Electronic Structures and Physical Properties of a‐Si1−xGexO2 Glass (x = 0 to 1)

The amorphous silica (a‐SiO₂) and germania (a‐GeO₂) have a wide range of applications in glass industry. Based on a previously constructed near‐perfect continuous random network model with 1296 atoms and periodic boundary conditions, we extend our study to amorphous Si_1?xGe_xO₂ models of homogeneous random substitution of Si by Ge with x ranging from 0 to 1. We have calculated the structural, electronic, mechanical, and optical properties for the series by using the first‐principles density functional theory methods. The x‐dependence of the variations in the properties is analyzed and critically compared with available experimental data. The mass density, volume, total bond order density, bulk mechanical properties, and refractive index are found to vary linearly as a function of x. For x = 0.5, we have also constructed six different kinds of particle immersion models to test the effect of inclusion of spherical particles of one glass of different sizes in the medium of the other glass on their physical properties. It is shown that particle sizes do affect the properties of particle immersion. Our calculations provide deep insight on the properties of mixture and nanocomposites of a‐SiO₂ and a‐GeO₂ glasses.     

Effect of Nitrogen on Properties of Na2O–CaO–SrO–ZnO–SiO2 Glasses

Glasses in the Na₂O–CaO–SrO–ZnO–SiO₂ system have previously been investigated for suitability as a reagent in Al‐free glass polyalkenoate cements (GPCs). These materials have many properties that offer potential in orthopedics. However, their applicability has been limited, to date, because of their poor strength. This study was undertaken with the aim of increasing the mechanical properties of a series of these Zn‐based GPC glasses by doping with nitrogen to give overall compositions of: 10Na₂O–10CaO–20SrO–20ZnO–(40?3x)SiO₂–xSi₃N₄ (x is the no. of moles of Si₃N₄). The density, glass‐transition temperature, hardness, and elastic modulus of each glass were found to increase fairly linearly with nitrogen content. Indentation fracture resistance also increases with nitrogen content according to a power law relationship. These increases are consistent with the incorporation of N into the glass structure in threefold coordination with silicon resulting in extra cross‐linking of the glass network. This was confirmed using ²⁹Si MAS‐NMR which showed that an increasing number of Q² units and some Q³ units with extra bridging anions are formed as nitrogen content increases at the expense of Q¹ units. A small proportion of Zn ions are found to be in tetrahedral coordination in the base oxide glass and the proportion of these increases with the presence of nitrogen.     

EFFECT OF SUBSTITUTING MgO FOR CaO ON PROPERTIES OF TYPICAL SODA-LIME GLASSES*

In base glasses containing (a) Na₂O 16, CaO 10, and SiO₂ 74% and (b) Na₂O14, CaO 12, and SiO₂ 74%, MgO was substituted in steps of 2% for CaO. The effects of the substitution on liquidus temperature, viscosity, fiber softening point, and the resistance of the glasses to dilute acid and to distilled water are described.     

Evaluation of novel composition (SiO2–CaO–Na2O–P2O5, SrO–CuO) degradable amorphous silicate glasses properties and comprehensive characterization of the thermal features

The current work presents and discusses the findings of a comprehensive study on the structural, chemical and thermal properties of SrO and CuO incorporated SiO₂–CaO–Na₂O–P₂O₅ amorphous silicate glass with a novel composition. Here, fundamental features (experimental density, oxygen density, and hardness) of all glasses were determined and chemical as well as phase composition of the glasses was verified with XRF and XRD, respectively. Moreover, the thermal behavior (viscos flow and crystallization kinetics) of amorphous silicate glass was investigated by non-isothermal methods using DTA analysis. The activation energies of glass transition (E_g) were calculated in the range of 546–1115 kJ/mol by Kissinger method, whereas the activation energies of crystallization (E_c) were calculated in the range of 164–270 kJ/mol by three different methods (Kissinger, Ozawa, Yinnon and Uhlmann). Avrami exponent (n) values ranged from 1.17 to 3.28 demonstrated that amorphous silicate glasses have different crystallization mechanism. Working temperature, which is one of the parameters indicating glass stability, increased with the incorporation of Sr and Cu from 187 °C to 245 °C. The initial dissolution measurement has been applied to study the degradability behavior of Sr and Cu incorporated amorphous glasses in vitro. Quantitative evaluation of Si⁴⁺ (0.156–0.373 kV), Ca²⁺ (0.043–0.332 kV), Na⁺ (0.044–0.329 kV), P⁵⁺ (0.057–0.289 kV), Sr²⁺ (0.134–0.385 kV), and Cu²⁺ (0.090–0.203 kV) depending on the ion activation energy (E_a-ion) and ion concentration at different temperature values (24, 37 and 55 °C) was performed in contact with Tris-HCl solution by ICP-OES analysis. The results revealed that investigated glasses were degradable and incorporation of Sr and Cu affected the glass initial dissolution. Overall, investigated glasses are suitable for various application such as hot-working production, glass-ceramic manufacturing, and glass or glass-ceramic scaffolds fabrication, due to wide working temperature ranges and high crystallization tendencies of the developed glasses.     

CERAMIC ABSTRACTS SECTION

Data are presented on the properties of twenty-seven Na₂OSrO. Al₂O₃.SiO₂ glasses covering the following percentage composition ranges: SiO₂ 66 to 78; SrO 4 to 16; Al₂O₃ 0,2, and 4; and Na₂O 14, 15.5, and 17. The effects of the systematic substitution of SrO for SiO₂, A1₂O₃, and Na₂O are shown for liquidus temperature, viscosity, coefficient of expansion, deformation temperature, density, and resistance to dilute acid and distilled water. Comparisons are shown between these SrO glasses and corresponding CaO glasses for those properties previously described.     

The role of MgO on the structural properties of CaO–Na2O(MgO)–P2O5–CaF2–SiO2 derived glass ceramics

The effect of increasing MgO/Na₂O replacements (on mole basis) on the crystallization characteristics of glasses based on the CaO–Na₂O(MgO)–P₂O₅–CaF₂–SiO₂ system were studied by using DTA, XRD, and SEM. The crystallization characteristics of the glasses, the type of crystalline phases formed and the resulting microstructure were investigated. The main crystalline phases formed after controlled heat-treatment of the base glass were diopside, wollastonite solid solution, fluoroapatite and sodium calcium silicate phases. The increase of MgO at the expense of Na₂O led to decrease the amount of sodium calcium silicate phase. The Vicker's microhardness values (5837–3362 MPa) of the resulting glass–ceramics were markedly improved by increasing the MgO-content in the glasses. The obtained data were correlated to the nature and concentration of the crystalline phases formed and the resulting microstructure.     

Elucidating the Effect of Iron Speciation (Fe2+/Fe3+) on Crystallization Kinetics of Sodium Aluminosilicate Glasses

We report on the influence of Fe₂O₃ on the crystallization kinetics of nepheline (Na₂O·Al₂O₃·2SiO₂)‐based sodium aluminosilicate glasses. A series of glasses with varying Al₂O₃/Fe₂O₃ content were synthesized in the system 25Na₂O–(25–x) Al₂O₃–xFe₂O₃–50SiO₂ (x varies between 0 and 5 mol%) through melt‐quench technique. A systematic set of experiments were performed to elucidate the influence of iron speciation (Fe²⁺/Fe³⁺) on the crystallization kinetics of these glasses including: (1) obtaining the details of nonisothermal crystallization kinetics by differential scanning calorimetry, (2) determining the influence of heat treatment on the structure and iron coordination in glasses by X‐ray photoelectron spectroscopy and wet chemistry, and (3) following the crystalline phase evolution in glasses in air and inert environments by X‐ray diffraction and scanning electron microscopy. The crystallization of two polymorphs of NaAlSiO₄—carnegieite (orthorhombic) and nepheline (hexagonal)—was observed in all the glasses, wherein the incorporation of iron promotes the formation of nepheline over carnegieite while shifting the crystallization mechanism from surface to volume. The influence of environment (air versus inert) and iron content on the crystallization kinetics of these glasses is contextualized from the perspective of the devitrification problem usually observed in sodium‐ and alumina‐rich high level nuclear waste glasses.     

New calcium‐free Na2O–Al2O3–P2O5 bioactive glasses with potential applications in bone tissue engineering

Sodium aluminophosphate glasses were evaluated for their bone repair ability. The glasses belonging to the system 45Na₂O–xAl₂O₃‐(55‐x)P₂O₅, with x = (3, 5, 7, 10 mol%) were prepared by a melt‐quenching method. We assessed the effect of Al₂O₃ content on the properties of Na₂O–Al₂O₃–P₂O₅ (NAP) glasses, which were characterized by density measurements, DSC analyses, solubility, bioactivity in simulated body fluid and cytocompatibility with MG‐63 cells. To the best of our knowledge, this is the first investigation of calcium‐free Na₂O–Al₂O₃–P₂O₅ system glasses as bioactive materials for bone tissue engineering.