Structural dependence of crystallization in phosphorus-containing sodium aluminoborosilicate glasses

The article reports on the structural dependence of crystallization in Na₂O–Al₂O₃–B₂O₃–P₂O₅–SiO₂-based glasses over a broad compositional space. The structure of melt-quenched glasses has been investigated using ¹¹B, ²⁷Al, ²⁹Si, and ³¹P magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, while the crystallization behavior has been followed using X-ray diffraction and scanning electron microscopy combined with energy dispersive spectroscopy. In general, the integration of phosphate into the sodium aluminoborosilicate network is mainly accomplished via the formation of Al–O–P and B–O–P linkages with the possibility of formation of Si–O–P linkages playing only a minor role. In terms of crystallization, at low concentrations (≤5 mol.%), P₂O₅ promotes the crystallization of nepheline (NaAlSiO₄), while at higher concentrations (≥10 mol.%), it tends to suppress (completely or incompletely depending on the glass chemistry) the crystallization in glasses. When correlating the structure of glasses with their crystallization behavior, the MAS NMR results highlight the importance of the substitution/replacement of Si–O–Al linkages by Al–O–P, Si–O–B, and B–O–P linkages in the suppression of nepheline crystallization in glasses. The results have been discussed in the context of (1) the problem of nepheline crystallization in Hanford high-level waste glasses and (2) designing vitreous waste forms for the immobilization of phosphate-rich dehalogenated Echem salt waste.     

Thermal properties of sodium borosilicate glasses as a function of sulfur content

Sulfur trioxide (SO₃) additions, up to 3.0 mass%, were systematically investigated for effects on the physical properties of sodium borosilicate glass melted in air, with a sulfur-free composition of 50SiO₂–10Al₂O₃–12B₂O₃–21Na₂O–7CaO (mass%). Solubility measurements, using electron microscopy chemical analysis, determined the maximum loading to be ~1.2 mass% SO₃. It was found that measured sulfur (here as sulfate) additions up to 1.18 mass% increased the glass transition temperature by 3%, thermal diffusivity by 11%, heat capacity by 10%, and thermal conductivity by 20%, and decreased the mass density by 1%. Structural analysis, performed with Raman spectroscopy, indicated that the borosilicate network polymerized with sulfur additions up to 3.0 mass%, presumably due to Na₂O being required to charge compensate the ionic additions, thus becoming unavailable to form non-bridging oxygen in the silicate network. It is postulated that this increased cross-linking of the borosilicate backbone led to a structure with higher dimensionality and average bond energy. This increased the mean free paths and vibration frequency of the phonons, which resulted in the observed increase in thermal properties.     

Corrosion of Borosilicate Glasses Subjected to Aggressive Test Conditions: Structural Investigations

Sodium borosilicate (NBS) and barium sodium borosilicate (BBS) glasses, used for immobilization of high‐level nuclear waste with compositions (SiO₂)_0.477(B₂O₃)_0.239(Na₂O)_0.170(TiO₂)_0.023(CaO)_0.068(Al₂O₃)_0.023 and (SiO₂)_0.482(B₂O₃)_0.244(Na₂O)_0.220(BaO)_0.054 were subjected leaching experiments under hydrothermal conditions in an autoclave at 200°C for different time durations. Morphological and structural transformations associated with leaching, have been monitored with techniques like XRD, SEM, solid‐state nuclear magnetic resonance. XRD and SEM along with NMR studies have confirmed that, upon leaching, formation of an aluminosilicate phase, Zeolite‐P (Na₆Al₆Si₁₀O₃₂·12H₂O), occurs with NBS glass. BBS glass upon subjecting to the same conditions leads to formation of multiple amorphous phases having Q⁴ (silica rich phase) and Q³ structural units of Silicon along with structurally modified residual glass. Upon leaching BO₃ structural units preferentially get released from BBS glass. Comparison of results with international simple glass confirmed that, for the latter, mass loss rates are one order of magnitude lower.     

The influence of iodide on glass transition temperature of high-pressure nuclear waste glasses

The glass transition temperature (T_g) is a key parameter to investigate for application in nuclear waste immobilization in borosilicate glasses. T_g for several glasses containing iodine (I) has been measured in order to determine the I effect on T_g. Two series of glass composition (ISG and NH) containing up to 2.5 mol% I and synthesized under high pressure (0.5 to 1.5 GPa) have been investigated using differential scanning calorimetry (DSC). The I local environment in glasses has been determined using X-ray photoelectron spectroscopy and revealed that I is dissolved under its iodide form (I⁻). Results show that T_g is decreased with the I addition in the glass in agreement with previous results. We also observed that this T_g decrease is a strong function of glass composition. For NH, 2.5 mol% I induces a decrease of 24°C in T_g, whereas for ISG, 1.2 mol% decreases the T_g by 64°C. We interpret this difference as the result of the I dissolution mechanism and its effect on the polymerization of the boron network. The I dissolution in ISG is accompanied by a depolymerization of the boron network, whereas it is the opposite in NH. Although ISG corresponds to a standardized glass, for the particular case of I immobilization it appears less adequate than NH considering that the decrease in T_g for NH is small in comparison to ISG.     

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.     

Impact of magnesium on the structure of aluminoborosilicate glasses: A solid-state NMR and Raman spectroscopy study

Seven magnesium-containing aluminoborosilicate glasses, with three to five oxides, have been studied through comprehensive multinuclear solid-state NMR (¹¹B, ²⁷Al, ²⁹Si, ²³Na, ¹⁷O, and ²⁵Mg) and Raman spectroscopy. The progressive addition of cations and the substitution of sodium and calcium by magnesium illuminate the impact of magnesium on the glass structure. The proportion of tri-coordinated boron drastically increased with magnesium addition, demonstrating the poor charge-compensating capabilities of magnesium in tetrahedral boron units. Oxygen-17 NMR showed the formation of mixing sites containing both Na and Mg near nonbridging oxygen sites. Furthermore, a high magnesium content appears to result in the formation of two subnetworks (boron and silicon rich) with different polymerization degrees as well as to promote the formation of high-coordination aluminum sites (Al[V] and Al[VI]). Finally, magnesium coordination ranging from 4 to 6, with a mean value shifting from 5 to 6 along the series, suggests that magnesium might endorse an intermediate role in these glasses.     

Effects of magnesium on the structure of aluminoborosilicate glasses: NMR assessment of interatomic potentials models for molecular dynamics

Classical molecular dynamics simulations have been used to investigate the structural role of Mg and its effect when it is incorporated in sodium aluminoborosilicate glasses. The simulations have been performed using three interatomic potentials; one is based on the rigid ionic model parameterized by Wang et al. (2018) and two slightly different parameterization of the core–shell model provided by Stevensson et al. (2018) and Pedone et al. (2020) The accuracies of these models have been assessed by detailed structural analysis and comparing the simulated nuclear magnetic resonance (NMR) spectra for spin active nuclei (²⁹Si, ²⁷Al, ¹¹B, ¹⁷O, ²⁵Mg, and ²³Na) with the experimental counterparts collected in a previous work. Our simulations reveal that the core–shell parameterizations provide better structural models. In fact, they better reproduce the NMR spectra of all the investigated nuclei and give better agreement with known experimental data. Magnesium is found to be five coordinated on average with distances with oxygen in between a network modifier (like Na) and an intermediate network formed (like Al). It prefers to lay closer to three-coordinated B atoms, forming B–NBO bonds, with respect to Si and especially Al. This can explain the formation of AlO₅ and AlO₆ units in the investigated Na-free glass, together with a Si clusterization.     

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.     

Incorporation limit of MoO3 in sodium borosilicate glasses

Optimizing the concentration of molybdenum incorporated in a borosilicate glass matrix is essential in the vitrification of high-level radioactive waste. However, the incorporation limit of MoO₃ in fundamental borosilicate systems has been rarely correlated with the local structure of the molybdenum cations. This study investigates the variations in the incorporation limit of MoO₃ in ternary sodium borosilicate glass upon varying the B₂O₃/(SiO₂ + B₂O₃) ratio (i.e., B)_. The incorporation limit of MoO₃ was less than 3 mol% in the low-B region (B < 0.7), where molybdenum cations mainly existed as [MoO₄]²⁻. However, when B was higher than 0.85, the incorporation limit was higher than 6 mol%, and the Raman spectra indicated the presence of octahedrally coordinated molybdenum cations, essential to stabilize the Mo–O–Mo linkage. The variation in the local structure of molybdenum cations can be explained by the available amount of non-framework cations compensating for the negative charge near [MoO₄]²⁻. These results allow the development of glass compositions with a high incorporation limit of MoO₃ simply by controlling the local structure near the molybdenum cations.     

Structural and thermal influence of niobia in aluminoborosilicate glasses

The addition of small amounts of niobia (Nb₂O₅) in borosilicate glasses was explored. By analysis on thermal and structural changes, we found evidences that niobium integrates the glass structure in octahedral NbO₆ coordination. Adding up to 8.0 mol% of Nb₂O₅, the oxide partially ruptured the glass structure, interfering in the BO₃/BO₄ ratio, but the predominant network configuration was maintained. Thermally, there was an increase in the processing interval and the glasses became more resistant against crystallization, with the presence of niobia. Also, the oxide contributed to a notable decrease in the viscosity of the melts. The improvement of such properties were obtained by the controlled dispersion of the oxide in the glass network structure, avoiding large areas of phase-to-phase separation to preserve the desired ability of these glasses to incorporate a wide range of elements.     

Structural densification of lithium phosphoaluminoborate glasses

Lithium aluminoborate glasses have recently been found to undergo dramatic changes in their short-range structures upon compression at moderate pressure (~1 GPa), most notably manifested in an increase in network forming cation coordination number (CN). This has important consequences for their mechanical behavior, and to further understand the structural densification mechanisms of this glass family, we here study the effect of P₂O₅ incorporation in a lithium aluminoborate glass (with fixed Li/Al/B ratio) on the pressure-induced changes in structure, density, and hardness. We find that P₂O₅ addition results in a more open and soft network, with P-O-Al and P-O-B bonding, a slightly smaller fraction of tetrahedral-to-trigonal boron, and an unchanged aluminum speciation. Upon compression, the cation-oxygen CNs of both boron and aluminum increase systemically, whereas the number of bridging oxygens around phosphorous (Qⁿ) decreases. The glasses with higher P₂O₅ content feature a larger decrease in Qⁿ (P) upon compression, which leads to more non-bridging oxygen that in turn fuel the larger increase in CN of B and Al for higher P₂O₅ content. We find that the CN changes of Al and B can account for a large fraction (around 50% at 2 GPa) of the total volume densification and that the extent of structural changes (so-called atomic self-adaptivity) scales well with the extent of volume densification and pressure-induced increase in hardness.     

X‐ray tomography of feed‐to‐glass transition of simulated borosilicate waste glasses

High‐level waste feed composition affects the overall melting rate by influencing the chemical, thermophysical, and morphological properties of a cold cap layer that floats on the molten glass where most feed‐to‐glass reactions occur. Data from X‐ray computed tomography imaging of melting pellets comprised of a simulated high‐aluminum feed reveal the morphology of bubbles, known as the primary foam, for various feed compositions at temperatures between 600°C and 1040°C. These feeds were formulated to make glasses with viscosities ranging from 0.5 to 9.5 Pa s at 1150°C, which was accomplished by changing the SiO₂/(B₂O₃+Na₂O+Li₂O) ratio in the final glass. Pellet dimensions and profile area, average and maximum bubble areas, bubble diameter, and void fraction were evaluated. The feed viscosity strongly affects the onset of the primary foaming and the foam collapse temperature. Despite the decreasing amount of gas‐evolving components (Li₂CO₃, H₃BO₃, and Na₂CO₃), as the feed viscosity increases, the measured foam expansion rate does not decrease. This suggests that the primary foaming is not only affected by changes in the primary melt viscosity but also by the compositional reaction kinetic effects. The temperature‐dependent foam morphological data will be used to inform cold cap model development for a high‐level radioactive waste glass melter.     

Simulations of the surfaces of soda lime aluminoborosilicate glasses exposed to water

Molecular dynamics simulations of 7 compositionally different sodium calcium alumino‐borosilicate glasses showed formation of ⁴B and ⁵Al more consistent with experimental data without compromising the other structural features that match experimental results observed in recent simulations of these glasses. Analysis of the dry surfaces of these glasses show a lack of ⁴B in the top 5‐6 Å of the surface in comparison to the bulk concentration for all glasses and no ⁵Al. Upon exposure to water, the simulations show that the ³B in the top 5‐6 Å of the glasses are preferentially attacked, decreasing the number of B bonds to O originally from the glass, indicating a change in the glass network. Inclusion of all B–O bonds in the top 5‐6 Å (i.e., including O from water) shows a decrease in ³B but an increase in ⁴B that is consistent with NEXAFS analysis, which the simulations show are hydroxylated. There is an increase in the concentration of ³Al in the dry surface in comparison to the bulk, but exposure to water converts almost all of these ³Al to ⁴Al. Hydroxyl concentrations vary from 2.6/nm² to 4.1/nm², with SiOH and BOH dominating these surface hydroxyls. Upon exposure to water, network linkages to B are preferentially ruptured. This, and the preferential loss of the nonbridging oxygen sites attached to Na, provide atomistic evidence of the initial stages of removal of B and Na from glass surfaces exposed to water.     

Structural impact of nitrogen incorporation on properties of alkali germanophosphate glasses

The structure, atomic packing density, calorimetric glass transition, and hardness of mixed sodium–lithium germanophosphate oxynitride glasses with varying Ge/P and N/P ratios were investigated. The combined influences of nitridation and mixed network former effect (MNFE) on the glass structure were analyzed using Raman spectroscopy, X‐ray photoelectron spectroscopy (XPS), and ³¹P nuclear magnetic resonance (NMR) spectroscopy. Evidence for the existence of germanium in a higher coordination state, i.e., five‐ or sixfold coordination, was obtained by performing XPS analysis of the oxide glasses, with indication of conversion to tetrahedral coordination upon nitridation. Raman spectroscopy measurements implied that the germanate network was modified upon nitridation, including the removal of ring‐like germanate structures and P–O–Ge mixed linkages. The partial anionic N‐for‐O substitution gave rise to the linear dependence of the glass transition temperature (T_g) and hardness (H_V) on nitrogen content (expressed as N/P ratio), especially for lower Ge/P ratio. However, nitridation also caused an unexpected increase in liquid fragility and decrease in density. This suggests that the governing structural parameter for property evolution in such LiNaGePON glasses is not only the increased degree of cross‐linking of the phosphate chains, but rather the short‐ and intermediate‐range structural modifications within the germanate component of the oxynitride glasses.     

Development of boron oxide potentials for computer simulations of multicomponent oxide glasses

Molecular dynamics and related atomistic computer simulations are effective ways in studying the structures and structure–property relations of glass materials. However, simulations of boron oxide (B₂O₃)-containing oxide glasses pose a challenge due to the lack of reliable empirical potentials. This paper reports development of a set of partial charge pairwise composition-dependent potentials for boron-related interactions that enable simulations of multicomponent borosilicate glasses, together with some of the existing parameters. This set of potentials was tested in sodium borate glasses and sodium borosilicate glasses and it is shown capable to describe boron coordination change with glass composition in wide composition ranges. Structure features such as boron N₄ value, density, Q_n species distribution, fraction of non-bridging oxygen around boron and silicon, total correlation function, and bond angle distribution function were calculated and compared with available experimental data. Mechanical properties of the simulated glasses calculated with the new potential also show good agreement with experiments. Therefore, this new set of potential can be used to simulate boron oxide-containing multicomponent glasses including those with wide industrial and technology applications.     

Durability of High-level Waste Glass in Groundwater

This paper reviews the leaching behavior of simulated high-level-waste glass in Japanese groundwater both on an in situ burial site and in our laboratory. During in situ burial leach tests, the glass is immersed in groundwater for up to 1 year and 7 months, retrieved, and analyzed. A leach rate obtained from the in situ burial tests is compared with other leach data under different conditions. Two factors, i.e., flow rate and composition of the groundwater, are extensively studied in the laboratory to supplement the in situ burial data. Effects of γ irradiation on the groundwater chemistry are also examined. Based on the results, mechanisms observed with the leaching of the glass in groundwater are discussed.     

Anisotropic structure of alkali metaphosphate glasses

Orientation anisotropy, which is well known in organic polymers with appropriate network structures, is less common in oxide glasses. We present the intermediate-range order in anisotropic alkali metaphosphate glass which consists of oriented PO₄ tetrahedral chains and intervening alkali cations along the elongation direction. The X-ray total structure factor S(Q) indicates that the inter-chain spacing depends on the size of alkali cations and varies from 5.03 to 6.28 Å. The mixed alkali effect is primarily related to an increase of the separation. The total correlation function T(r) provides the first definite evidence that the anisotropic structure is composed of phosphorus-bridging oxygen bonds (P–O_B) lying along the elongation direction and phosphorus-non-bridging oxygen bonds (P–O_T) oriented perpendicular to the elongation direction. The present result unveils fundamental aspects of the anisotropic structure of an oxide glass and provides essential information for the development of oxide glasses to control structural anisotropy.     

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.