Effect of irradiation on the atomic structure of borosilicate glasses

Borosilicate glasses incorporating high-level nuclear waste are exposed to high-energy radiations during their storage in the deep geological repositories. However, the effect of radiation on the atomic structure of borosilicate glasses remains poorly understood. Herein, using molecular dynamics simulations, we study the irradiation-induced structural changes of a series of calcium-sodium borosilicate glasses with varying Si/B molar ratios—ranging from pure silicate to pure borate glasses. We observe that irradiation leads to an increase in disorder, both in the short- and medium-range, as evidenced by the enthalpy, coordination number, and ring distribution. In particular, the impact of the change in the atomic structure (due to radiation) on the glass volume is investigated. Interestingly, we observe a composition-dependent transition in the volumetric response of borosilicate glasses under irradiation—wherein borate-rich compositions tend to swell, whereas silica-rich glasses tend to densify. Through a detailed analysis of the structure, we demonstrate two competing mechanisms contributing to the volume change, i.e., a decrease in the coordination number of boron atoms and a reduction in the average silicon inter-polytope angle. We also show that the increase in the disorder in the medium-range order may play a major role in governing the volumetric changes in the irradiated structure in a non-trivial fashion. Altogether, the present study highlights that irradiation has a non-trivial effect on borosilicate glasses, which, in turn, could impact their corrosion kinetics.     

Influence of the composition and temperature of heat treatment of porous glasses on their structure and light transmission in the visible spectral range

The pore structure and light transmission of high-silica porous glasses in the visible spectral range have been investigated as a function of the heat treatment temperature and the composition of the initial two-phase alkali borosilicate glass. The character of light transmission in porous glasses has been analyzed in the framework of the concepts of structural features of their pore space and the processes occurring in the porous glass during heating. It has been demonstrated that an increase in the temperature of heat treatment of porous glasses with different compositions leads to an increase in the pore size and a decrease in their specific surface area (with a nearly constant total porosity), which is associated with the processes of overcondensation of pores due to the rearrangement and the change in the packing density of secondary silica particles. It has been revealed that the introduction of phosphate and fluoride ions into the initial sodium borosilicate glass results in an increase in the light extinction coefficient of porous glasses due to the increase in the sizes of phase-separated inhomogeneity regions in the initial two-phase glasses, the formation of larger pores, and the presence of nanosized microcrystalline phases in porous glasses.     

Interfacial structures of spinel crystals with borosilicate nuclear waste glasses from molecular dynamics simulations

Spinel crystal formation presents a critical issue and glass formulation in nuclear waste glass processing. In this paper, the interfacial structures of the model borosilicate nuclear waste glasses, the international simple glass (ISG), with two types of spinel crystals, namely the MgAl₂O₄ and NiFe₂O₄, were studied using classical molecular dynamics simulations with effective partial charge potentials and recently developed composition-dependent boron-related parameters. The simulation results revealed the structural features of the borosilicate nuclear waste glasses and their interfaces with the two types of spinel crystals. It was found that there exist notable structural changes of glasses close to the interfacial region, affected by the adjacent crystal structures, terms of preferential segregation and ordering of cations, as well as ctaion coordination numbers. Specifically, the fraction of fourfold coordinated boron (B³) in glass near the interface decreases as compared to the bulk glass. In addition, the amount of fourfold coordinated Al decreases while fivefold Al increases in the glass region close to the glass-crystal interface, which suggests indication of initial stage of crystal growth as Al adopts higher (sixfold) coordination like in the crystal as compared to majority of fourfold coordination in the glass. These interfacial structure changes obtained from MD simulations provide evidence of the influence of the precipitated crystals on the surrounding melt and glass and the initial stage of crystal growth.     

Surface structures of sodium borosilicate glasses from molecular dynamics simulations

Surface plays an important role in the physical and chemical properties of oxide glasses and controls the interactions of these glasses with the environment, thus dominating properties such as the chemical durability and bioactivity. The surface atomic structures of a series of sodium borosilicate glasses were studied using classical molecular dynamics simulations with recently developed compositional dependent partial charge potentials. The surface structural features and defect speciation were characterized and compared with the bulk glasses with the same composition. Our simulation results show that the borosilicate glass surfaces have significantly different chemical compositions and structures as compared to the bulk. The glass surfaces are found to be sodium enriched and behave like borosilicate glasses with higher R (Na₂O/B₂O₃) values. As a result of this composition and associated structure changes, the amount of fourfold boron decreases at the surface and the network connectivity on the surface decreases. In addition to composition variation and local structure environment change, defects such as two‐membered rings and three‐coordinated silicon were also observed on the surface. These unusual surface composition and structure features are expected to significantly impact the chemical and physical properties and the interactions with the environments of sodium borosilicate glasses.     

Redox Chemistry in Candidate Glasses for Nuclear Waste Immobilization

An electrochemical series of redox couples, originally developed for Savannah River Laboratory glass frit 131 (SRL-131) as a reference composition, has been extended to two other alkali borosilicate compositions that are candidate glasses for nuclear waste immobilization. Because no dramatic differences were ascertained in the redox chemistry of selected multivalent elements in SRL-131 vs that in Savannah River Laboratory glass frit 165 (SRL-165) and in West Valley glass frit 205 (WV-205), the comprehensive electrochemical series can readily be applied to a range of nuclear waste glass compositions. In order to alleviate potential problems with foaming and precipitation of insolubles during the processing of the nuclear waste in these glass melts, the [Fe²⁺]/[Fe³⁺] ratio of the melt should be between 0.1 and 0.5.     

Anti-acid corrosion mechanism of yttrium oxide doped barium borosilicate glass

Designing glass with excellent acid resistance is a prerequisite for developing high-performance terminal electrode pastes. Herein, we fabricated Y₂O₃ doped BaO–B₂O₃–SiO₂ glass with excellent anti-sulfuric acid corrosion properties. The anti-corrosion mechanism of glass in acid environment was investigated by spectra and microstructure analysis. The passivating gel layer with a porous structure was formed on the glass surface during the corrosion process. The average pore diameter of the porous gel could be reduced by increasing the content of Y₂O₃. The smaller pore size of the porous gel would considerably increase the collision frequency between solvent molecules and the pore wall, which could effectively inhibit the ion migration in the gel layer, reduce the corrosion rate, and improve the acid resistance of the glass. This study contributes to the understanding of the corrosion mechanism of the glass and provides theoretical guidance for rationally designing anti-acid corrosion glass for terminal electrode pastes.     

硼硅酸盐玻璃浸出前后结构变化的研究

硼硅酸盐玻璃作为最具工业化应用前景的高放射性废物的固化基材,其中的放射性核素主要以浸出方式进入生物圈。本工作采用了MCC-1静态浸泡法对在90 ℃去离子水中浸泡28 d后的硼硅酸盐玻璃的浸出行为进行了研究。借助电感耦合等离子体发射光谱(ICP-OES)、傅里叶红外光谱(FTIR)、拉曼光谱(Raman)以及掠入射X射线衍射(GIXRD)等方法对玻璃的浸出性能及微观结构进行研究。此外,还使用了扫描电镜观测了浸出样品的截面。FTIR、Raman及GIXRD结果表明,浸出后,样品表层中的Si-O-Si结构明显增多,而硼结构单元则消失,表层的微观结构与熔融石英的结构较为相似。此外,在扫描电镜中观察到浸出7 d后的样品表面出现腐蚀层,同时在玻璃基底和腐蚀层中间出现裂隙,由此导致了浸出表面积的增加,从而最终引起浸出7 d时元素归一化标准浸出率的异常增大。     

Model for Leaching Behavior of Aluminosilicate Glasses Developed as Matrices for Immobilizing High-Level Wastes

A field trial initiated in 1960 has shown very low release rates of ¹³⁷Cs and ⁹⁰;Sr from aluminosilicate glasses developed as potential matrices for the immobilization of radioactive fission products. Recent laboratory work in closed glass-water systems shows that certain sodium-calcium aluminosilicate glasses effectively cease to dissolve in water after a short time. This behavior is thought to be associated with the formation of a protective mineralized surface layer. Models were developed for the release of radionuclides from a glass covered by a mineralized surface layer, with leaching in both closed and open systems. The leaching equation for release in an open system is fitted to ⁹⁰Sr release data.     

Accelerated Leach Testing of GLASS (ALTGLASS): I. Informatics approach to high level waste glass gel formation and aging

Glass corrosion data from the ALTGLASS^™ database were used to determine if gel compositions, which evolve as glass systems corrode, are correlated with the generation of zeolites and subsequent increase in the glass dissolution rate at long times. The gel compositions were estimated based on the difference between the elemental glass starting compositions and the measured elemental leachate concentrations from the long-term product consistency tests (ASTM C1285) at various stages of dissolution, ie, reaction progress. A well-characterized subset of high level waste glasses from the database was selected: these glasses had been leached for 15-20 years at reaction progresses up to ~80%. The gel composition data, at various reaction progresses, were subjected to a step-wise regression, which demonstrated that hydrogel compositions with Si^*/Al^* ratios of <1.0 did not generate zeolites and maintained low dissolution rates for the duration of the experiments. Glasses that formed hydrogel compositions with Si^*/Al^* ratios ≥1, generated zeolites accompanied by a resumption in the glass dissolution rate. The role of the gel Si/Al ratio, and the interactions with the leachate, provides the fundamental understanding needed to predict if and when the glass dissolution rate will increase due to zeolitization.     

Lightweight glass–ceramic tiles from the sintering of mining tailings

Modern thermally-insulating building façades comprise lightweight structural panels, in turn mostly composed of porcelain stoneware with engineered porosity. Sintered glass–ceramics may represent a valid alternative, mainly considering layered articles, with a dense surface layer on a highly porous body that could be manufactured by double pressing. In this paper we present a low cost route to lightweight tiles, developed starting from mining tailings, such as waste from the mining of boron-rich minerals and basalt rock, and recycled glasses, such as common soda-lime glass and pharmaceutical borosilicate glass. A highly porous body was obtained by direct sintering of mixtures of mining tailings and soda-lime glass; despite the homogeneity of porosity and the formation of new crystal phases (at only 1000 °C), favorable to good mechanical properties, the water absorption remained far above the limits (>2 wt%). The water absorption was minimized by introduction of a dense glaze, associated to the firing of mixtures coated by a thin layer of recycled borosilicate glass powders; both color and shrinkage were optimized by the mixing of borosilicate glass with powders of zircon mineral and vitrified boron waste/basalt/soda-lime mixture.     

Structural changes on the surfaces of borosilicate glasses induced by gamma-ray irradiation

Vitrification is a kind of glass that can solidify high-level radioactive waste (HLW). As the basic material of vitrification, borosilicate glass was studied extensively. To keep HLW away from the biosphere, the tolerance of borosilicate glass to irradiation is important. In this work, various samples of borosilicate glass with different compositions were irradiated with gamma rays at ambient temperature to study their stability. The hardness, moduli, and microscopic changes on surfaces of the borosilicate glasses were measured at specific absorbed doses. Upon the gamma irradiation, the structural changes on surfaces of borosilicate glasses were identified, which were strongly influenced by the composition of borosilicate glasses. The results demonstrate that gamma irradiation, as well as beta irradiation, might strongly influence the properties of vitrification. The irradiation effects on vitrification induced by gamma irradiation should be paid more attention to than before.     

Characterization and Sintering of a Porous Glass-Ceramic in the System Na2O-B2O3-Sc2O3

Clear glasses which included droplet-like microphases were produced when SiO₂ in sodium borosilicate glasses was replaced by Sc₂O₃. Phase separation and/or crystallization occurred after heat treatment. The porous skeleton of leached glasses consisted of hexagonal ScBO₃. The specific surface areas and pore radii are comparable to those of porous SiO₂ glass. The sintering temperature of porous Sc-based material is higher than that of porous SiO₂. Alumina contamination influenced the structure of the porous material.     

Some Features of Leaching of Two-Phase Alkali Borosilicate Glass Containing PbO

The paper reports the results of a comparative investigation into the leaching kinetics of two-phase alkali borosilicate glasses containing lead oxide in 3 M HCl solution at 100°C and the glasses free from lead oxide. It is revealed that the mechanisms of leaching of the studied glasses exhibit a common feature which resides in the fact that the temporary precipitation of the products of dissolution of the glass components within the leached layer occurs as an intermediate stage of the glass leaching. Unlike glasses free from PbO, the precipitation within the porous layer of a leached lead-containing glass is observed visually, and the presence of crystalline boron-containing precipitates in this layer is confirmed by X-ray powder diffraction analysis. Compared to glasses prepared from two-phase alkali borosilicate glasses free from PbO, the porous glasses produced from two-phase glasses containing PbO are characterized by considerably smaller sizes of micropores and their larger specific surface at the same total porosity.     

Bicontinuous porosity in ceramics utilizing polymer spinodal phase separation

This paper demonstrates how the combination of inorganic and organic polymers can be used to form bicontinuous porosity in ceramics with pore sizes larger than 5 μm. Spinodal phase separation of pseudo-binary polymer mixtures allows to form larger bicontinuous pore structures than spinodal phase separation of inorganic glasses. Addition of salts allows even more complex compositions of ceramics and glasses to be formed. Here, bioactive glasses are presented that were produced via sol–gel processing of a pseudo-binary mixture of an inorganic and an organic polymer. Due to the addition of an organic polymer to the gelling sol and the spinodal phase separation at a specific equilibrium temperature, both an inorganic polymer ceramic phase and organic polymer-rich phase are formed. The evaporation of the solvent and the burnout of the organic polymer produce a microstructure of interconnected and nearly uniform porosity, which can be controlled by several processing parameters. The dependency of pore size and connectivity is best predicted by polymer phase separation rather than glass melt separation. Results suggest that polymer spinodal phase separation could be useful for the manufacture of a variety of porous ceramics.     

Predicted Mechanism for Enhanced Durability of Zinc Containing Silicate Glasses

Experimental studies have reported that zinc oxide improves the durability of glasses used for nuclear waste immobilization. Here, molecular dynamics simulations are used to predict the atomic structures of sodium silicate glass with and without zinc. A simulated melt‐quench procedure is used to generate glass structures. Pair distribution functions, ring size distributions, and alkali clustering are then examined. This allows insights into the structural role of zinc oxide within the glass and helps distinguish between its reported functions as a network former and a network modifier. Changes in the sodium ion distribution and clustering behavior within the glass are observed, due to zinc oxide addition. This affects the local and intermediate‐range structure of the glass and provides a possible explanation for enhanced durability.     

Characterisation of porous glasses prepared from Cathode Ray Tube (CRT)

Display tubes such as those used in TV receivers and computer monitors have an evacuated glass envelope, which consists mainly of a screen (front component) and a funnel (back component hidden inside the TV set or monitor). These two components have different compositions: the screen is composed of lead-free glass with strontium and barium oxides, whereas the funnel is composed of glass with lead oxides. In order to comply with future government measures, a method is required for the recycling or re-utilisation of CRT (Cathode Ray Tube) glasses in end-of-life electronic goods. One open-loop recycling method is to create foam glasses from CRTs using a reducing agent. The results for the chemical compositions of these glasses and their physical properties showed that foam glasses can be prepared from glasses from various CRT glassmakers. In this paper, we use several methods to determine the structures of these foam glasses. We use helium pycnometry, Hg porosimetry, specific surface area measurements and scanning electron microscopy as direct methods for determining foam glass structure. These methods provide information about the morphologies and reactivities of these porous materials. Densities, porosities and pore size distributions were measured, which enable us to suggest some potential applications for the fabricated foam glasses.     

A Preliminary Investigation of the ISG Glass Vapor Hydration

During the geological disposal of high-level waste, the nuclear glass is expected to be first hydrated in water vapor prior to liquid alteration. In the present work, we investigated the vapor hydration of the International simple glass (ISG) at 175°C and different relative humidities (60%, 80% and 98%). The glass hydration was investigated by nuclear reaction analysis (NRA) and Fourier transform infra-red spectroscopy. The chemical and mineralogical compositions of the alteration products were studied using scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM-EDS) and μ-Raman spectroscopy, respectively. The NRA results gave water diffusion coefficients of 2.31–7.34 × 10⁻²¹ m²/s_, in good agreement with the literature data on borosilicate glasses altered in aqueous media. The glass hydration increased with relative humidity percentage and the SEM-EDS analysis showed a slight enrichment in Si and loss of Na in the hydrated glass layer compared with the pristine glass. The hydration rate of the ISG glass was little higher than that of the French SON68 glass hydrated using water vapor. The corrosion products were analcime, tobermorite, and calcite, which were typical of the SON68 glass hydrated in similar conditions.     

Thermophysical Properties of Multiphase Borosilicate Glass‐Ceramic Waste Forms

Multiphase borosilicate glass‐ceramics represent one candidate to contain radioactive nuclear waste separated from used nuclear fuel. In this work, the thermophysical properties from room temperature to 1273 K were investigated for four different borosilicate glass‐ceramic compositions containing waste loadings from 42 to 60 wt% to determine the sensitivity of these properties to waste loading, as‐fabricated microstructure, and potential evolutions in microstructure brought about by temperature transients. The thermal expansion, specific heat capacity, thermal diffusivity, and thermal conductivity are presented. The impact of increasing waste loading is shown to have a small but measurable effect on the thermophysical properties between the four compositions, contrasted to a much greater impact observed when transitioning from predominantly crystalline to amorphous systems. Thermal cycling below 1273 K was not found to measurably impact the thermophysical properties of the compositions investigated here.     

Dissolution Rates of Silicate Glasses in Water at pH 7

Dissolution rates of different glasses in buffered solutions of constant pH of 7 were measured by weight change, profilometry, and ion implantation with Rutherford backscattering. Rates with different techniques agreed within experimental error. Natural obsidian glass dissolved most slowly, at a rate comparable with those of quartz and crystalline aluminosilicate minerals. Commercial soda–lime glass containing alumina dissolved slowly, at about the same rate as vitreous silica; soda–lime silicate glasses both commercial and laboratory without alumina dissolved much more rapidly. Pyrex borosilicate glass dissolved at a rate intermediate between those of soda–lime silicate glasses with and without alumina; at room temperature Pyrex borosilicate glass dissolved about 100 times faster than a commercial soda–lime glass containing alumina. We suggest that surface structure is the main factor determining the relative dissolution rates of silicate glasses. Glasses with transformed surface layers caused by hydration dissolve most rapidly; phase separation and openness of the glass structure are also important factors.     

Molecular Dynamics Simulations of the Structure and Properties of Low Silica Yttrium Aluminosilicate Glasses

Yttrium aluminosilicate glasses are of technological importance in photonics, nuclear waste disposal, and as a delivery vehicle for radiation therapy. Their structures are also of great interest in glass science to elucidate the principles of glass formation and structures. We provide classical molecular dynamics simulation results of a series of yttrium aluminosilicate glasses with low silica concentration. Detailed structure analyses including coordination number, pair and bond angle distributions, Si–O and Al–O network structures, and primitive ring statistics are reported. It is found that the average aluminum coordination number decreases from 4.7 to 4.2 with increasing silica content from 5 to 20 mol%, while maintaining alumina at 55 mol%. Four-coordinated aluminum ions increase from 40% to 84% in the series and the fraction of edge-sharing alumino-oxygen polyhedra decreases, indicating an increased network former role of aluminum ions with increasing silica content. Physical properties such as elastic constant, bulk modulus, Young's modulus, and Possion's ratio were calculated. The results show a decrease of bulk, Young's, and shear moduli with increasing silica and decreasing yttria contents that is in good agreement with experimental data.