Aqueous alteration of five-oxide silicate glasses: Experimental approach and Monte Carlo modeling

The alteration behavior of glass comprising five oxides (61 ? x)SiO₂–17B₂O₃–18Na₂O–4CaO–xZrO₂ was studied during static leach tests in a buffer solution at 90 °C and with a glass-surface-area-to-solution-volume (SA/V) ratio of 15 cm^?1. The morphological evolution of altered glasses investigated by small-angle X-ray scattering (SAXS) exhibits a strong dependence with the zirconium content in the glass. The experiments were compared with modeling results using Monte Carlo simulation. The model has been improved to simulate the alteration kinetics and alteration layer morphology, considering zirconium atoms at coordination number 6. The simulations exhibit very good agreement with experimental results, showing relations between the alteration rate and the restructuring altered layer. The model is used to interpret experimental observations by proposing a porosity closure mechanism in the altered layer to account for the diminishing alteration rate. For high zirconium concentrations, the simulation highlights the existence of percolation pathways responsible for a complete alteration of the glass. Zirconium has a hardening effect that limits the dissolution of neighboring atoms; this effect is favorable in terms of the glass alteration kinetics, but by inhibiting silicon recondensation it prevents complete closure of the porosity and the glass is completely altered.     

Raman spectroscopic investigation of the structure of silicate glasses (II). Soda-alkaline earth-alumina ternary and quaternary glasses

Raman spectra of some ternary and quaternary glasses in the system Na₂OCaOMgOAl₂O₃SiO₂ are presented. The spectra are interpreted in terms of the structural alteration of the glass as the composition is altered from the binary end members to more complicated glasses. Addition of CaO and MgO to soda-silica glasses act only to increase the disorder of the network slightly. Addition of Al₂O₃ greatly modifies the network. In some soda-lime-aluminosiliscate compositions an estimate can be made of the amount of aluminum in four- and six-fold coordination. It is shown that the amounts of four- and sixfold coordinated aluminum depend on the glass composition.     

Monte Carlo simulations of the dissolution of borosilicate glasses in near-equilibrium conditions

Monte Carlo simulations were performed to investigate the mechanisms of glass dissolution as equilibrium conditions are approached in both static and flow-through conditions. The glasses studied are borosilicate glasses in the compositional range (80 ? x)% SiO₂ (10 + x / 2)% B₂O₃ (10 + x / 2)% Na₂O, where 5 < x < 30%. In static conditions, dissolution/condensation reactions lead to the formation, for all compositions studied, of a blocking layer composed of polymerized Si sites with principally 4 connections to nearest Si sites. This layer forms atop the altered glass layer and shows similar composition and density for all glass compositions considered. In flow-through conditions, three main dissolution regimes are observed: at high flow rates, the dissolving glass exhibits a thin alteration layer and congruent dissolution; at low flow rates, a blocking layer is formed as in static conditions but the simulations show that water can occasionally break through the blocking layer causing the corrosion process to resume; and, at intermediate flow rates, the glasses dissolve incongruently with an increasingly deepening altered layer. The simulation results suggest that, in geological disposal environments, small perturbations or slow flows could be enough to prevent the formation of a permanent blocking layer. Finally, a comparison between predictions of the linear rate law and the Monte Carlo simulation results indicates that, in flow-through conditions, the linear rate law is applicable at high flow rates and deviations from the linear rate law occur under low flow rates (e.g., at near-saturated conditions with respect to amorphous silica). This effect is associated with the complex dynamics of Si dissolution/condensation processes at the glass–water interface.     

Proton conduction in glass - an impedance and infrared spectroscopic study on hydrous BaSi2O5 glass

Hydrous barium disilicate glasses (BaSi₂O₅) containing 2.75 and 3.54 wt% dissolved water (corresponding to a molar concentration of hydrogen atoms of 11.1 and 14.2 mol/l, respectively) were synthesized by high temperature fusion in an internally heated gas pressure vessel. Near-infrared spectroscopy gives evidence that both OH groups and H₂O molecules are present in the glasses. The maximum intensity in the range of OH stretching vibrations is at 2800 cm⁻¹ indicating strong hydrogen bonding in the glasses. Electric conductivity measurements were carried out at temperatures up to 523 K without significant alteration of the sample. At higher temperatures, OH groups are converted to molecular H₂O and water diffuses out of the sample resulting in a continuous decrease of the conductivity. An activation energy of 87 kJ/mol was derived for the dc conductivity in the unaltered glasses similar to the activation energy for bulk water diffusion in other silicate glasses. Because the dry barium disilicate glass is an electrical insulator at experimental conditions, we infer that the dc conductivity of the hydrous glasses is due to proton conduction.     

Investigation of gel porosity clogging during glass leaching

A 5-oxide glass (62.5SiO₂, 16.6B₂O₃, 13.1Na₂O, 6.0CaO, 1.8ZrO₂) was leached at 90 °C at a high glass-surface-area-to-solution-volume ratio (SA/V = 80 cm^?1). Its dissolution rate diminished over time until it became unmeasurable. The alteration layer was characterized by ²⁹Si isotopic tracing in the leaching solution. ToF-SIMS elemental profiles showed that glass dissolution ceased due to clogging of the gel porosity at the gel/solution interface. One of the hypotheses proposed to account for the rate drop observed during borosilicate glass alteration is based on morphological changes in the alteration gel over time. Monte Carlo modeling of glass alteration, especially with simple glasses, indicates a clogging of the porosity on the external portion of the gel (near the solution/gel interface) after densification of the layer by silicon precipitation, but this phenomenon had never previously been directly observed experimentally. The initial results obtained by isotopic tracing provide new data that appears to confirm this hypothesis.     

Influence of the electric double layer on glass leaching

When a glass comes into contact with water, an electric double layer forms at the glass-water interface. An analytical model that calculates the electric field and the resulting velocity of univalent alkali ions migrating in the glass matrix is developed. The ion migration velocity is calculated for an 85% SiO₂–15% Na₂O glass and for more complex glasses. The calculations indicate that the process of ion migration leads to normalized leach rates which are small compared to glass network dissolution rates.     

Kinetic fragility of hydrous soda-lime-silica glasses

The effect of hydration on the kinetic fragility of soda-lime-silica glasses was investigated by viscometry in the glass transition range. Water-bearing glasses were prepared from industrial float glass (FG) and a ternary model glass (NCS = 16Na₂O 10CaO 74SiO₂ in mol%) by bubbling steam through the melt at 1480 °C and up to 7 bar. Additionally, a sodium borosilicate glass (NBS = 16Na₂O 10B₂O₃ 74SiO₂ in mol%) was hydrated under equal conditions. As detected by infrared spectroscopy water dissolves in the glasses exclusively as OH-groups. The hydration resulted in a total water content C_W up to ≈ 0.2 wt% for FG, NCS and NBS glasses. Kinetic fragility, expressed by the steepness index m, was determined from the temperature dependence of η at the glass transition. Viscosity data from previous studies on hydrous float glasses (C_W > 1 wt%) were surveyed together with literature data on the (H₂O)–Na₂O–CaO–SiO₂, (H₂O)–Na₂O–SiO₂ and (H₂O)–SiO₂ systems to expand the range of water concentration and bulk composition. We could demonstrate that m decreases for all glasses although water is dissolved as OH and should depolymerize the network. An empirical equation of the general type m = a ? b logC_W where a, b are fitting parameters, enables m to be predicted, for each glass series as function of the water content C_W. The enlarged data base shows that the parameter B of the Arrhenius viscosity-temperature relation decreases much stronger than the isokom temperature at the glass transition.     

A comparison of natural and experimental long-term corrosion of uranium-colored glass

The alteration features of historical U-colored glasses exposed to natural weathering for over 150 years were compared with the experimental alteration of similar glass with ～0.3 wt% of uranium using a long-term (up to 426 days) kinetic laboratory batch leaching test in deionized water. Two types of natural corrosion crusts were identified by a combination of SEM/EDS, HRTEM/SAED, EPMA and XRD: (i) formation of a leached layer (up to ～600 μm thick) depleted in alkalis and enriched in Si with stable concentration of U and Al and (ii) formation of lamellae depleted in alkalis, Si and U and enriched in Al. The presence of newly formed gibbsite (Al(OH)₃) and kaolinite (Al₂Si₂O₅(OH)₄) were confirmed in the second type of corrosion crust by HRTEM. Dissolution of the glass components including uranium was determined during the laboratory leaching test. Several μm thick alkali-depleted alteration zones with stable U content relatively enriched in Si and Al were formed on the glass surface. The PHREEQC-2 modeling also predicted the precipitation of secondary gibbsite and kaolinite in the late stages of the leaching. These phases may form especially when sufficient amounts of Al are available from the environment (e.g., soil). Furthermore, they provide surfaces for sorption and may, in some cases, affect the mobility of U ions released from the glass in dependence on pH and U speciation.     

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.     

Adhesion of ethylene-tetrafluoroethylene copolymer with silicate glasses

The adhesive strength of ethylene-tetrafluoroethylene copolymer (ETFE) was measured on silicate glass substrates. A value of over 300 kg/cm² obtained with a commercial soda-lime-silica glass and this initial adhesiveness was attributed to the marked wettability of molten ETFE and its chemical structure. Since the decrease in the adhesive strength was small after immersion in water for a dealkalized soda-lime-silica glass, Pyrex, and a soda-lime-silica glass coated witn In₂O₃, it was inferred that the sodium ions in the soda-lime-silica glass played a fundamental role in lowering the adhesive strength in the presence of water. Pretreatment with N-(β-aminoethyl)-γ-aminopropyl-trimethoxy silane markedly improved the water durability of the adhesive strength.     

Leaching of some lithium silicate glasses and glass-ceramics by HCl

The leaching of some binary and ternary lithium silicate glasses and their respective glass-ceramics by HCl is reported.The leaching rate of lithium silicate glasses gradually decreases with the decrease of the percentage of Li₂O or by the introduction of small amounts of a third component, e.g. Al₂O₃, MgO, ZnO or B₂O₃. With a further increase in the proportions of B₂O₃ or ZnO the rate of leaching increases. The rate of leaching is also substantially modified by the conversion of glasses into glasses-ceramics.The results obtained are discussed in terms of the effects of the different ions on the rate of the interdifussion of the lithium and hydrogen ions in the glass and the leached layer, the phase separation developed in the glass, the type and concentration of crystalline phases developed in glass-ceramics and the composition of the residual glass phase.     

Electrical and dielectric properties of Sb2O3–V2O5–K2O glasses

Electric measurements, including temperature dependencies of direct electrical conductivity and temperature dependencies of complex electrical modulus, have been implemented using Sb₂O₃–V₂O₅–K₂O glass samples. These glasses absorb ambient humidity but their resistance to water attack depends on composition. The significant decrease of conductivity up to 100 °C can arise from water desorption. Cycling measurements of direct electrical conductivity versus temperature were also implemented. They show that the 30Sb₂O₃–30V₂O₅–40K₂O and 70Sb₂O₃–30K₂O glasses are irreversibly damaged with the formation of the hydrated layer. In addition, it was observed that the evolution of DC conductivity is ruled by Arrhenius relation, while activation energy decreases as Sb₂O₃ concentration increases.     

Variation of optical basicity with probe ion,for Cs2O + B2O3 and Li2O + B2O3 glasses

Optical basicities (Γ) for Cs₂O + B₂O₃ and Li₂O + B₂O₃ glasses have been measured as a function of glass composition, using Tl⁺, Pb²⁺ and Bi³⁺ probe ions. The three probe ions register different values of Γ for glasses of given composition (and also for pure B₂O₃ glass and water). The divergence decreases as the alkali metal ion size decreases.For the Li₂O + B₂O₃ glasses, ideal (calculated) optical basicities agree within experimental precision with experimental values registered by Pb²⁺ (Γ_Pb²⁺) up to about 15 mol% Li₂O. For higher Li₂O contents, and for the Cs₂O + B₂O₃ glasses, ideal optical basicities agree less well with Γ_Pb²⁺, but show similar trends with composition to those shown by Γ_Pb²⁺.     

The role of water vapour on the oxidation of two Ln-Si-Al-O-N glasses (Ln=Y, La)

The oxidation behaviour of LnSiAlON (Ln=Y, La) glasses was studied at different temperatures (990-1150 °C) and under different water vapour pressures (360-2690 Pa). These results were also compared with those obtained under O₂, N₂/H₂O or O₂/H₂O mixtures. When glasses are treated under a N₂/H₂O mixture, optical and SEM observations show porous scales. Transformations of the reaction rate data and a kinetic model show that there is only one limiting process occurring during oxidation. This rate limiting step is the progress of the chemical reaction at the internal interface. Determination of the pressure law dependence and thermodynamics calculations of water vapour molecules dissociation at the investigated temperatures allow us to suggest that the mechanism of oxidation corresponds to decomposition of water molecules on the oxynitride glass surface.     

Optical absorption of cobalt (II) in binary thallium borate glasses

The optical absorption spectra of cobalt (II) in Tl₂OB₂O₃ glasses have been studied and compared with those in binary alkali borate glasses. In thallium borate glasses cobalt (II) may be present in octahedral and/or in tetrahedral symmetry depending upon the composition of the glass. In low thallium borate glasses cobalt (II) is octahedral while the concentration of tetrahedral cobalt (II) increases with increasing Tl₂O content of the glass; the formation of tetrahedral cobalt (II) becomes noticeable when the concentration of Tl₂O reaches above the critical concentration of about 19 mol %. The ligand field parameters: 10Dq and B have been calculated from the absorption spectra of cobalt (II) in different glasses and it has been found that the Racah parameter, B, is more in Tl₂OB₂O₃ glasses than those in Na₂OB₂O₃ or K₂OB₂O₃ glasses of corresponding molar composition. This indicates that the donor capacity of the BO₄ group in thallium borate glasses is lower than that in alkali borate glasses; this is consistent with the NMR results in Tl₂OB₂O₃ glasses containing less than 20 mol % Tl₂O where three BO₄ groups have been found to form with each Tl₂O unit added.     

Silver valence and local environments in borosilicate and calcium aluminoborate waste glasses as determined from X-ray absorption spectroscopy

Silver K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data were collected and analyzed to characterize silver (Ag) environments in borosilicate and Ca-aluminoborate glass formulations developed as potential candidates for the immobilization of certain nuclear wastes. Silver is found in some nuclear waste streams and must be encapsulated in glass during waste vitrification processes. A related concern deals with phase separation within these glasses and whether colloidal silver would be present in the glass melt, which could present processing issues, or in the waste glass product. Characterization of the silver environments provides useful information for optimizing the silver incorporation ability of such glasses. Data were also gathered on four crystalline standards: Ag-foil, Ag₂O, argentojarosite (AgFe₃(SO₄)₂(OH)₆), and AgO. XANES data indicate Ag⁺ as the dominant species in the glasses. XANES and EXAFS data show that the average Ag environment in the Ca-aluminoborate glass is different compared with those in the two borosilicate glasses investigated. EXAFS analyses show that Ag in the borosilicate glasses is coordinated by two oxygens in a similar environment to that in crystalline Ag₂O, except that the associated Ag–O distances are approximately 0.10 Å longer in the glass. Silver in the Ca-aluminoborate glass may be within one highly disordered site, or possibly, several different sites, where the average Ag–O distance, coordination number, and Debye–Waller factor are larger than those determined for the borosilicate glasses. Despite their relatively high silver contents, there is no evidence from XANES or EXAFS of colloidal silver in the glasses investigated.     

Multinuclear NMR spectroscopic studies of structure and dynamics in hydrous NaAlSi3O8 and Ca0.5AlSi3O8 glasses

We studied the local structure and dynamics of dissolved water in hydrous aluminosilicate glasses with the compositions NaAlSi₃O₈·0.3H₂O, NaAlSi₃O₈·1.3H₂O and Ca_0.5AlSi₃O₈·1.3H₂O by using multinuclear NMR spectroscopy. Glasses were produced by melting glass powders with deuterium oxide under high pressure and the resulting ²H/(¹H + ²H) ratio of the hydrous glasses was larger than 80%. NMR spectra reveal clear evidence for the presence of OH groups as well as of H₂O molecules. The motion of these water species in the time range from some μs to some ms was studied by ¹H NMR relaxation rates and the temperature dependence of the ²H NMR spectra. The motion is faster in the Ca-bearing glass than in the Na-bearing glass. The results are compared with those from quasi-elastic neutron scattering.     

Pressure effects on electrical conduction in glasses

Electrical conduction in various inorganic glasses was studied as a function of hydrostatic pressure up to 2000 atm and phenomenologically classified into electronic, ionic and mixed types. In electronically conducting glasses such as AsSe chalcogenide glasses and Fe₂O₃P₂O₅ glass, the conduction is enhanced by application of pressure. On the other hand in ionically conducting glass such as Na₂OB₂O₃ glass, the conduction is suppressed through the concept of an activation volume. The compatibility of electronic and ionic conduction processes in glasses such as Ag-doped AsSe glasses and Bi₂O₃B₂O₃ glass, which have more complex conduction processes, was discussed from these aspects.     

Structural analysis of Na2OGa2O3SiO2 glasses by the X-ray diffraction method

The structure of Na₂OGa₂O₃SiO₂ glasses of four different compositions containing up to 45 mol. % Ga₂O₃ has been determined by the X-ray diffraction method. The radial distribution function D(r) obtained indicates that Ga³⁺ ions are in fourfold coordination and GaO₄ tetrahedra are formed in these glasses. The reduced intensity function S·i(S) and radial distribution function D(r) which were calculated based on a structural model for A-25, 0.25(Ga₂O₃)·0.75(Na₂O·2SiO₂)/3 glass agreed well with the observed ones.