Free energy profiles of Al and La cation distribution in silica and soda silicate glasses

The factors that control the distribution of Al³⁺ and La³⁺ cations in silica and soda silicate glasses is examined by using molecular dynamics (MD) simulations. In particular, the response of the glass network to the presence of metal oxide is probed using a liquid state theory that treats the glass network as a solvent and the metal cation as a solute. MD simulations are used to obtain the mean solvent-solute and solute-solute force. The trajectory used to determine the free energy is analyzed to determine the stable configurations of a cation pair. Details of determining the potential of mean force for an Al cation pair in silica and silicate glass is presented. A comparison of these results with those previously calculated for a La cation pair in the same glass systems is given. The results reveal that there are differences in how the network accommodates the two different size cations. It is found that for the potential used here, based on the Vessal potential, the network wraps itself around the larger La cation forming a solvent shell, whereas, the smaller Al cation is incorporated into the network backbone. In silica and soda silicate glasses, La ion pairs cluster to form La-O-La linkages. In contrast, the glasses favor a separated state of the Al ion pair.     

Molecular dynamics simulation of La2O3-Na2O-SiO2 glasses. II. The clustering of La cations

Clustering of high-field strength rare-earth ions in silicate glasses has been experimentally observed for a wide range of concentrations. Clustering has also been observed by molecular dynamics (MD) computer simulations over a range 1-10 mol% in soda silicate glasses. Although there have been numerous experimental studies, atomic-level details of the mechanisms that lead to clustering remain unclear. Coupling experiment with MD simulations is essential to uncovering the factors that lead to clustering. In this work, MD computer simulations are used to verify that clustering found in previous MD simulations is not an artifact of the simulation method. This work also provides clues as to the mechanism of atomic-level clustering.     

Structure and lithium ion diffusion in lithium silicate glasses and at their interfaces with lithium lanthanum titanate crystals

Solid state lithium ion electrolytes are important to the development of next generation safer and high power density lithium ion batteries. Lithium containing glasses such as lithium silicate glasses have been widely studied due to their high ionic conductivity. Recently, lithium silicate glasses were introduced in polycrystalline lithium lanthanum titanate (LLT) ceramics as intergranular thin films between the crystalline grains to achieve higher lithium ion conductivities in these solid state electrolytes. In this work, we present investigations of the structure and diffusion behavior of lithium silicate glasses and their interfaces with LLT crystals using molecular dynamics simulations. The short and medium range structures of the lithium silicate glasses were characterized and the ceramic/glass interface models were obtained using MD simulations. Lithium ion diffusion behaviors in the glass and across the glass/ceramic interfaces, as well as the effect of atomic structure on diffusion behaviors, were investigated. It was found that there existed a minor segregation of lithium ions at the glass/crystal interface. The interface lithium ion diffusion energy barrier was found to be dominated by the glass phase.     

On the constitution of silicate groupings in binary lead silicate glasses

Direct chemical methods have been used to determine the type and the percentage of silicate groupings present in binary lead silicate glasses within the composition range 4PbO · SiO₂PbO · SiO₂. The results obtained indicate that the constitution and the relative amounts of various silicate groupings change with the changing total amount of SiO₂ in the glass. Glasses containing small amounts of SiO₂ are characterized by the presence of few low-molecular silicate units; increasing the percentage of SiO₂ promotes the formation of higher polymerized silicate groupings. The results are discussed with regard to stoichiometry, and are consistent with Hägg's concept of glassiness.     

The effect of redox state on the local structural environment of iron in silicate glasses: a combined XAFS spectroscopy, molecular dynamics, and bond valence study

A series of 27 silicate glasses of various compositions containing 0.2-2 at.% iron were synthesized at various oxygen fugacity values. The glasses were examined using X-ray absorption fine structure (XANES) spectroscopy at the Fe K-edge in order to determine iron oxidation state and first-neighbor coordination number. Spectral information extracted from the pre-edge region and principal component analysis (PCA) of the XANES region, together with a spectral inversion, were used to derive the end-member spectral components for Fe(II) and Fe(III). Linear trends in the pre-edge features were observed for most compositional series of the glasses examined as a function of Fe(II)/Fe(III) content. These linear trends are believed to be due to the similarity of average coordination numbers for both Fe(II) and Fe(III) end-members in each series. This result is consistent with model simulations of the XANES region and molecular dynamics (MD) simulations for the two end-member compositions which also show that Fe(II) and Fe(III) have similar average coordination numbers. These simulations also suggest the presence of five-coordinated Fe(III) in the melt phase. Based on a bond valence analysis of these MD simulations, a simple model is proposed to help predict the speciation of iron in oxide and silicate glasses and melts.     

Molecular dynamics simulation of La2O3-Na2O-SiO2 glasses. III. The driving forces of clustering

Molecular dynamics simulations have been carried out to determine the mean force and the potential of mean force of two La ions in a soda silicate glass. The reaction coordinate starts from a well separated state where the network accommodates the high-field strength cations and then brings them together to form an La-O-La linkage that can lead to the formation of free O anions. It is found that clustering of La ions is favored, at least for dimer states. Similar calculations for the ion pairs of La-La, Na-Na and La-Na in pure silica reveal that the local molecular environments of La and Na ions are quite distinct.     

Molecular dynamics study of amorphous titanium silicate

The molecular dynamics computer simulation was used to study the local structure of titanium in silica glass. The empirical potential function used in previous simulations of other glasses performed in our lab was extended to study titanium in silica. The results indicate that at low concentrations (<10 mol% TiO₂), titanium is mainly tetrahedral in bulk silica. At higher TiO₂ concentrations, titanium coordination increases. Slower quench rates enhance these trends observed in the changes in titanium coordination. A cluster of overcoordinated species similar to rutile was found in TiO₂: 9SiO₂ and is discussed within the constraints of the MD experiment. Overall, the MD result are consistent with EXAFS data of bond lengths and coordination numbers of TiO₂ in titanium silicate glasses.     

Structure and lattice dynamics of binary lead silicate glasses investigated by infrared spectroscopy

The polar lattices dynamics of seven binary lead silicate glasses have been studied by infrared spectroscopy. The analysis of the reflectivity spectra with a dielectric function model, based on a modified Gaussian profile, allows a quantitative evaluation of the presence of lead cations within different structural sites. From the role of the lead cations versus the degree of polymerization of the silicate network and the comparison with literature results, we may to give a scenario for explaining the observed structural evolution of the glass matrix and more particularly the drastic change occurring around 45% of lead content. Below this threshold, lead cations act only as modifiers of the silicate network. Above, the glass structure is deeply modified; a lead network involving around 10% of the lead content appears in glasses whose composition is just above the threshold and progressively grows at the expense of the silicate network with the increase of lead content. For high lead content, lead cations can act as modifiers of the silicate network or as network formers. Results also show that the analysis of far infrared measurements combined with the knowledge of the UV edge optical response is very promising to characterize the local disorder around cations in glasses.     

Structure of partially reduced xPbO (1 − x)SiO2 glasses: combined EXAFS and MD study

We have studied the structure of partially reduced lead-silicate glasses using combined EXAFS (extended X-ray absorption fine structure) and MD (molecular dynamics) methods. The analysis was performed for glasses of x[(1 − p)Pb pPbO] (1 − x)SiO₂ composition, x = 0.3, 0.5, 0.7, where parameter (1 − p) describes the degree of reduction, i.e. the content of the granular metallic phase, appearing as the result of the reduction process (e.g. annealing in hydrogen atmosphere). In the EXAFS experiment (1 − p) was expressed via the time of reduction realized at 400 °C (1.5 h, 24 h, 70 h), whereas in the MD simulations it was determined precisely by using proper numbers of particles (corresponding to (1 − p) = 0.0, 0.25, 0.5, 0.75 and 1.0). In the paper we describe in detail the local structure around lead atoms and its changes in the function of glass composition and reduction degree. The tendency for agglomeration of Pb⁰ into clusters, the formation of the granular metallic phase, and continuity of silica and lead oxide subnetworks are discussed. A good agreement between EXAFS-extractcd and MD-extracted parameters of the short-range structure encouraged us to preform a medium-range order analysis, based on the MD simulations only. Moreover, combining the EXAFS and MD methods we could correlate the reduction time (technological parameter) with the degree of reduction (1 − p) and the actual state of the granular structure. The latter relation may be useful for controlled production of reduced glasses of pre-requcstcd physical properties.     

Structural study of sol-gel silicate glasses by IR and Raman spectroscopies

A study of the structure and bonding configuration of sol-gel silicate glasses by Raman and infrared spectroscopies is presented. Moreover, a review of the Raman lines and infrared bands assignment, the identification of the non-bridging silicon-oxygen groups and the ring structures are also demonstrated. The evolution of the changes of the bonding configuration in the composition and the stabilization temperature of the bioactive glasses is discussed in terms of the structural and textural characteristics of the glasses. Raman and infrared analyses contribute to the improvement in understanding of the local symmetry for sol-gel silicate glasses. infrared spectroscopy has allowed to identify the vibration bands of the hydroxyl groups associated with various configurations of the terminal silanol bonds on the glass surface and the free molecular water in the glass matrix. Raman analysis has provided an alternative method of quantifying the network connectivity grade and predicting the textural properties of the sol-gel silicate glasses.     

The changes in lead silicate glasses induced by the addition of a reducing agent (TiN or SiC)

Foam glasses can be prepared from lead silicate glasses through reaction with a reducing agent such as TiN or SiC. The environmental impact and specific characteristics of the final product (e.g., density, porosity, and thermal and mechanical properties) depend on the reaction processes that occur during expansion. The mechanisms of foam glass formation are determined in this study from experimental results, which we also use to determine the structural differences between the bulk (lead silicate glass before treatment) and foam glass (after heating the lead silicate glass with a reducing agent). This paper also presents measurements of the variation of the concentration of the divalent cation Pb(II) in the vitreous network (which initially contains 2–5 mol% PbO) during the expansion process. These results were obtained with various methods, including X-ray diffraction, differential scanning calorimetry, photoelectron spectroscopy, and scanning electron microscopy with energy dispersive spectrometry. Our results show that a small proportion (about 20%) of the initial Pb(II) in network modifying sites is reduced to Pb metal, and the remaining proportion is present in the vitreous network as Pb(II) with another particular environment. Increasing the amount of SiC or TiN results in increased reduction of Pb(II). The quantity of the reducing agent can be optimized with respect to the temperature, pressure, and reaction rate.     

Stress corrosion of silicate glass: a review

In the first section of the present paper, some examples, from the field, of the manifestations or consequences of the fatigue of silicate glass are briefly presented. In the second section, the interpretation of fatigue in terms of stress corrosion is reviewed: the role of ambient molecular water is well established. Whatever the details of the mechanism of action of water, it takes place very efficiently in the highly strained material close to the tip of a surface crack. This enables its sub-critical growth to be explained. But it does not explain many other observed effects of the environment on the mechanical behavior of silicate glass, and, in the last section, questions and issues are presented, which would still need to be investigated.     

Structural model of gelation processes of a sodium silicate sol destabilized by calcium ions: combination of SAXS and rheological measurements

This study deals with the physico-chemical processes involved in the formation of basic fractal silica gels derived from a sodium silicate sol destabilized by calcium ions. Using small-angle X-ray scattering and dynamic rheological measurements, structural and viscoelastic properties have been investigated in situ during aggregation and gelation processes. The experimental results lead to a consistent model that describes the structural features and aggregation mechanisms involved in the formation of these gels.     

Stress relaxation of a soda lime silicate glass below the glass transition temperature

Stress relaxation is an important effect in the ion-exchange procedure of glasses, as it controls the stress profile and the strength. Creep and stress relaxation tests have been performed to study the viscoelastic behavior of soda-lime silicate glass at typical ion-exchange temperatures. The experimental data of these tests can be fitted well by the Burger model and a comparison between the viscosity data from both tests was made. The strain and temperature dependences of the stress relaxation process were studied and the glass exhibited a non-linear viscoelastic behavior and an anomalous temperature dependence. In addition, it was found there is a relationship between the glass density and the stress relaxation behavior.     

The surface layer formed on zinc-containing glass during aqueous attack

The glass-water interaction is reviewed via an outline of the processes of bulk dissolution, selective leaching and surface enhancement. Special attention is paid to the nature and effects of surface films. Glasses of three compositions, each containing about one-fifth by weight Zn, are subjected to aqueous attack. Solution analysis confirms the roles of Na, Si and Zn in leaching, dissolution and surface enhancement, respectively. Proton-induced X-ray emission corroborates Zn enhancement in the glass surface and X-ray photoelectron spectroscopy fixes the Zn build-up to the first few nanometres. Powder X-ray diffraction examination identifies the specific structures of hemimorphite and Zn orthosilicate while transmission electron microscopy finds such phases to be distributed in a largely amorphous matrix. Infrared spectra are consistent with the presence of a modified silicate structure and additionally indicate that water is loosely bound in the reacted surface layer. Attack of a non-Zn glass by a Zn-containing solution shows the ion in solution produces a Zn surface layer while attack of a Zn glass by a Zn complexing solution shows a Zn layer forms preferentially to complexation. Hence Zn, in either the solid or solution, induces a thin, protective, solvated and partially crystalline silicate zone on a glass under aqueous attack.     

SAXS study of the micro-inhomogeneity of industrial soda lime silica glass

The microstructure of a variety of industrial multicomponent soda lime glasses for container and glazing and of a lead alkali silicate glass for pressed tableware articles was analyzed using small angle X-ray scattering (SAXS). No inhomogeneity with size >0.5 nm was found in the industrial samples, while ternary soda lime glass samples with a suitable composition when heat treated developed a phase separation which SAXS detected without difficulty. The results obtained disprove earlier reports which attributed problems of rheology and strength affecting container production (‘bad workability’) to microstructure formation.     

Europium environment and clustering in europium doped silica and sodium silicate glasses

The local environment and distribution of rare earth ions are important to the optical properties of rare earth doped oxide glasses. In this paper, we report studies of the structures of europium doped (around 1 mol% Eu₂O₃) silica and sodium silicate glasses using molecular dynamics simulations. By using effective partial charge potentials, systems with over 24,000 atoms were modeled in order to obtain better statistics of rare earth ion distribution. The simulated glass structures were validated by comparing the calculated neutron and X-ray structure factors with experimental data. It was found that europium ions have higher coordination number (5.9 versus 4.8) and more symmetric environments in sodium silicate glasses than in the silica glass. Rare earth ion clustering has been characterized in detail and it was found that the clustering probability of europium ions in sodium silicate glass is consistently less than that predicted from a random distribution, while the probability of clustering in pure silica glass is higher than that of random distribution at the 1 mol% doping level.     

Oriented lithium disilicate glass–ceramics prepared by electrochemically induced nucleation

Glasses with the compositions Li₂Si₂O₅ and 9Li₂Si₂O₅ · BaSiO₃ were crystallized using electrochemically induced nucleation. A platinum wire was inserted into the melt. A dc-current was supplied between the platinum wire (cathode) and the crucible (anode) at various temperatures below the liquidus temperature. This led to nucleation at the cathode and subsequently to radial growth of lithium disilicate crystals. The crystals exhibit dendritic growth. The main crystal growth direction is the crystallographic c-axis of lithium disilicate. This axis is oriented perpendicular to the electrode surface. A crystal growth selection near the cathode results in a highly textured glass–ceramics. The electrochemically induced nucleation is based on the reduction of the silicate glass to elementary silicon and an alloy formation between Si and Pt with a depletion of Si in the melt near the cathode. The crystalline products were characterized and the effects of experimental parameters were studied.     

Mechanism of deleading of silicate glass by 0.5 N HNO3

Mechanism of removal of lead from silicate glass containing 68.5 wt% PbO by 0.5 N HNO₃ was investigated by incorporation of the chemical-analyses/weight-loss data into shrinking-core model (SCM) and minimization of the difference. Scanning electron microscopy (SEM), emission spectrometry with inductively coupled plasma (ICP), X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDS) were used to determine the compositional changes of the lead-silicate glass (LSG) samples. Dual inter-diffusion chemical reaction mechanisms having respective activation energies of 83.49 and 47.80 kJ/mol dominated the deleading process.     

Fluoride-containing bioactive glasses: Fluoride loss during melting and ion release in tris buffer solution

Melt-derived bioactive glasses (SiO₂-P₂O₅-CaO-Na₂O-CaF₂; CaF₂ 0 to 17.76 mol%) lost fluoride during melting, but nominal and analysed CaF₂ contents in the glass correlated linearly. Analysed CaO contents were increased, showing that fluoride was lost as hydrofluoric acid after reaction with atmospheric water during melting. Weight loss on ignition reduced linearly with increasing CaF₂, suggesting that CaF₂ impedes absorption of atmospheric water. pH changes in tris buffer solution showed that pH is controlled by the silicate matrix (via ion exchange processes), and fluoride release contributes less to the overall pH. Glasses formed apatite in tris buffer; phosphate concentration of the glass was the limiting factor, resulting in fluorite formation for increasing fluoride content in the glass and calcite formation for the fluoride-free composition. These results allow for tailoring of novel fluoride-containing bioactive glasses to address specific needs, particularly in dentistry and for remineralising toothpastes.