Thermodynamic assessment of the pseudoternary Na2O–Al2O3–SiO2 system

Vitrified high‐level radioactive waste that contains high concentrations of Na₂O and Al₂O₃, such as the waste stored at the Hanford site, can cause nepheline to precipitate in the glass upon cooling in the canisters. Nepheline formation removes oxides such as Al₂O₃ and SiO₂ from the host glass, which can reduce its chemical durability. Uncertainty in the extent of precipitated nepheline necessitates operating at an enhanced waste loading margin, which increases operational costs by extending the vitrification mission as well as increasing waste storage requirements. A thermodynamic evaluation of the Na₂O–Al₂O₃–SiO₂ system that forms nepheline was conducted by utilizing the compound energy formalism and ionic liquid model to represent the solid solution and liquid phases, respectively. These were optimized with experimental data and used to extrapolate phase boundaries into regions of temperature and composition where measurements are unavailable. The intent is to import the determined Gibbs energies into a phase field model to more accurately predict nepheline phase formation and morphology evolution in waste glasses to allow for the design of formulations with maximum loading.     

Understanding the structural origin of crystalline phase transformations in nepheline (NaAlSiO4)‐based glass‐ceramics

Nepheline (Na₆K₂Al₈Si₈O₃₂) is a rock‐forming tectosilicate mineral which is by far the most abundant of the feldspathoids. The crystallization in nepheline‐based glass‐ceramics proceeds through several polymorphic transformations — mainly orthorhombic, hexagonal, cubic — depending on their thermochemistry. However, the fundamental science governing these transformations is poorly understood. In this article, an attempt has been made to elucidate the structural drivers controlling these polymorphic transformations in nepheline‐based glass‐ceramics. Accordingly, two different sets of glasses (meta‐aluminous and per‐alkaline) have been designed in the system Na₂O–CaO–Al₂O₃–SiO₂ in the crystallization field of nepheline and synthesized by the melt‐quench technique. The detailed structural analysis of glasses has been performed by ²⁹Si, ²⁷Al, and ²³Na magic‐angle spinning — nuclear magnetic resonance (MAS NMR), and multiple‐quantum MAS NMR spectroscopy, while the crystalline phase transformations in these glasses have been studied under isothermal and non‐isothermal conditions using differential scanning calorimetry (DSC), X‐ray diffraction (XRD), and MQMAS NMR. Results indicate that the sequence of polymorphic phase transformations in these glass‐ceramics is dictated by the compositional chemistry of the parent glasses and the local environments of different species in the glass structure; for example, the sodium environment in glasses became highly ordered with decreasing Na₂O/CaO ratio, thus favoring the formation of hexagonal nepheline, while the cubic polymorph was the stable phase in SiO₂–poor glass‐ceramics with (Na₂O+CaO)/Al₂O₃ > 1. The structural origins of these crystalline phase transformations have been discussed in the paper.     

Nepheline Crystallization in Nuclear Waste Glasses: Progress Toward Acceptance of High-Alumina Formulations

Historical data have been critically compiled and analyzed for investigating the quantity of nepheline (NaAlSiO₄) precipitated as a function of composition in simulated nuclear waste glasses. To understand compositional effects two primary methods were used: (1) investigating the Al₂O₃–SiO₂–Na₂O ternary while filtering for different B₂O₃ levels and (2) creating a quadrant system consisting of compositions reduced to two representations: (i) the nepheline discriminator (ND) which depends only on the SiO₂ content by weight normalized to the total weight of the Al₂O₃–SiO₂–Na₂O submixture and (ii) the optical basicity (OB) which contains contributions from all constituents in the glass. Nepheline precipitation is expected to be suppressed at high SiO₂ levels (ND > 0.62) or at low basicities (OB < 0.55–0.57). Changes in sodium aluminosilicate glass OB values due to additions of CaO and B₂O₃ correlate with the observed effects on nepheline formation. It is proposed that additional composition space is available for formulating high waste loading, high-Al₂O₃ nuclear waste glasses when consideration is given to location on the Al₂O₃–SiO₂–Na₂O submixture as well as OB.     

Influence of Al2O3/SiO2 ratio in multicomponent LNAS glasses and glass-ceramics on the crystallization behavior,microstructure and mechanical performance

Transparent glass-ceramics containing eucryptite and nepheline crystalline phases were prepared from alkali (Li, Na) aluminosilicate glasses with various mole substitutions of Al₂O₃ for SiO₂. The relationships between glass network structure and crystallization behavior of Li₂O–Na₂O–Al₂O₃–SiO₂ (LNAS) glasses were investigated. It was found that the crystallization of the eucryptite and nepheline in LNAS glasses significantly depended on the concentration of Al₂O₃. LNAS glasses with the addition of Al₂O₃ from 16 to 18 mol% exhibited increasing Q⁴ (mAl) structural units confirmed by NMR and Raman spectroscopy, which promoted the formation of eucryptite and nepheline crystalline phases. With the Al₂O₃ content increasing to 19–20 mol%, the formation of highly disordered (Li, Na)₃PO₄ phase which can serve as nucleation sites was inhibited and the crystallization mechanism of glass became surface crystallization. Glass-ceramics containing 18 mol% Al₂O₃ showed high transparency ～84% at 550 nm. Moreover, the microhardness, elastic modulus and fracture toughness are 8.56 GPa, 95.7 GPa and 0.78 MPa m^1/2 respectively. The transparent glass-ceramics with good mechanical properties show high potential in the applications of protective cover of displays.     

ANALYSIS OF RECENT MEASUREMENTS OF THE VISCOSITY OF GLASSES.—II1

Viscosity of two series of four component glasses .—The results recently reported by S. English² are here analyzed by the method used in a previous paper.³ (1) Series 6SiO₂, 1.2Na₂O, 0.8(CaO+MgO). The temperature at which the difference between the viscosity of a glass and the viscosity of 6SiO₂,2Na₂O at the same temperature makes a sharp bend is called the aggregation temperature Ta . It seems to correspond to the devitrification temperature. For this series Ta reaches a sharp minimum for the composition 6SiO₂, 1.15Na₂O, 0.45MgO, 0.39CaO. The viscosity for any temperature also reaches a minimum at or near this composition. (2) Series 6SiO₂, 1.1Na₂O, 0.9(CaO+Al₂O₃). The curves are similar to those for the first series, TA reaching a sharp minimum for the composition 6SiO₂, 1.11Na₂O, 0.81Ca0, 0.14Al₂O₃.     

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.     

Thermal expansion and characteristic points of –Na2O–SiO2 glass with added oxides

A differential method with an optical lever system was used to determine the thermal expansion of the glasses based on Na₂O–SiO₂ and containing the following oxides: Al₂O₃, CaO, TiO₂, ZnO, Al₂O₃–TiO₂, CaO–TiO₂, or ZnO–TiO₂. The determination was carried out on both as-drawn and annealed rods. The influence of the thermal history as well as of the added oxide(s) on the thermal expansion of glasses is dealt with in this paper. The experimental results for the thermal expansion of annealed rods show that the addition of CaO–TiO₂ in the glass has the lowest value while the addition of CaO in the glass has the highest value for the coefficient of linear expansion. At the same time, the characteristic points of the annealed glasses, determined from the expansion-temperature curves, show that the addition of ZnO in the glass has the lowest values while the addition of TiO₂ in the glass has the highest values.     

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.     

Non-isothermal crystallisation behaviour of fluorophlogopite/nepheline glass ceramics

Abstract

Fluorophlogopite/nepheline glass ceramics were formed from the system of SiO₂–Al₂O₃–MgO–K₂O–Na₂O–F, and the thermodynamic, crystallisation behaviour and microstructure were investigated using differential thermal analysis, X-ray diffraction, and scanning electron microscopy. It was found that fluorophlogopite crystals and nepheline crystals crystallised simultaneously, and bulk nucleation was the main crystallisation mechanism. Crystal growth was prone to follow the two-dimensional direction and controlled by diffusion. Activation energy for glass transition was 797·43 kJ mol^?1, and crystallisation activation energy was 433·16 kJ mol^?1.     

Effect of Al2O3 and K2O content on structure,properties and devitrification of glasses in the Li2O–SiO2 system

The effect of Al₂O₃ and K₂O content on structure, sintering and devitrification behaviour of glasses in the Li₂O–SiO₂ system along with the properties of the resultant glass–ceramics (GCs) was investigated. Glasses containing Al₂O₃ and K₂O and featuring SiO₂/Li₂O molar ratios (3.13–4.88) far beyond that of lithium disilicate (Li₂Si₂O₅) stoichiometry were produced by conventional melt-quenching technique along with a bicomponent glass with a composition 23Li₂O–77SiO₂ (mol.%) (L₂₃S₇₇). The GCs were produced through two different methods: (a) nucleation and crystallization of monolithic bulk glass, (b) sintering and crystallization of glass powder compacts.Scanning electron microscopy (SEM) examination of as cast non-annealed monolithic glasses revealed precipitation of nanosize droplet phase in glassy matrices suggesting the occurrence of phase separation in all investigated compositions. The extent of segregation, as judged from the mean droplet diameter and the packing density of droplet phase, decreased with increasing Al₂O₃ and K₂O content in the glasses. The crystallization of glasses richer in Al₂O₃ and K₂O was dominated by surface nucleation leading to crystallization of lithium metasilicate (Li₂SiO₃) within the temperature range of 550–900 °C. On the other hand, the glass with lowest amount of Al₂O₃ and K₂O and glass L₂₃S₇₇ were prone to volume nucleation and crystallization, resulting in formation of Li₂Si₂O₅ within the temperature interval of 650–800 °C.Sintering and crystallization behaviour of glass powders was followed by hot stage microscopy (HSM) and differential thermal analysis (DTA), respectively. GCs from composition L₂₃S₇₇ demonstrated high fragility along with low flexural strength and density. The addition of Al₂O₃ and K₂O to Li₂O–SiO₂ system resulted in improved densification and mechanical strength.     

Seeded Stage III glass dissolution behavior of a statistically designed glass matrix

The dissolution rate of some glasses accelerates after prolonged time spent at a slow, residual dissolution rate. This phenomenon is referred to as Stage III behavior. The acceleration in glass dissolution rate linked to Stage III behavior is significant and may be the most impactful behavior to long-term performance of glass in a repository. This work is aimed at understanding the effect of glass composition on Stage III behavior to add a level of technical defensibility to glass disposal. To this end, a set of 24 glass compositions were statistically designed, where eight glass components (SiO₂, B₂O₃, Al₂O₃, CaO, Na₂O, SnO₂, ZrO₂, and Others) have been independently varied in order to study the individual effects of each glass component. These glasses have been subjected to static dissolution tests at 90°C in deionized water and then seeded with zeolite Na-P2 28 days into the testing to induce Stage III behavior. The response of the glasses to the zeolite seeds fell into four primary types: (1) no response to seeds; (2) an immediate linear sustained acceleration in the rate; (3) an immediate linear acceleration in the rate followed by a decrease; and (4) a progressive acceleration in the rate that is concurrent with the addition of the seeds. The main glass components observed to influence these behaviors were CaO, Al₂O₃, B₂O₃, and ZrO₂, where (1) CaO influenced which glasses showed a Stage III response to seeds (high CaO: types 2, 3, and 4) or did not respond to seeds (low CaO: type 1), (2) Al₂O₃ and B₂O₃ influenced which glasses showed a sustainable Stage III response (high Al₂O₃: types 2 and 4) versus transitory response (low Al₂O₃ and high B₂O₃: type 3), and (3) ZrO₂ concentration influenced whether glasses showed a linear (high ZrO₂: type 2) versus progressive (low ZrO₂: type 4) response to seeds.     

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.     

Role of MgO in lowering glass transition temperature and increasing hardness of lithium silicate glass and glass-ceramics

The effect of variation of MgO (1.5, 4.5 and 7.5 mol%) content on glass structure, crystallization behavior, microstructure and mechanical properties in a Li₂O–K₂O–Na₂O–CaO–MgO–ZrO₂–Al₂O₃–P₂O₅–SiO₂ glass system has been reported here. Increased amount of MgO enhanced the participation of Al₂O₃ as a glass network former along with [SiO₄] tetrahedra, reducing the amount of non-bridging oxygen (NBO) and increasing bridging oxygen (BO) amount in glass. The increased BO in glass resulted in a polymerized glass structure which suppressed the crystallization and subsequently increased the crystallization temperature, bulk density, nano hardness, elastic modulus in the glasses as well as the corresponding glass-ceramics. MgO addition caused phase separation in higher MgO (7.5 mol%) containing glass system which resulted in larger crystals. The nano hardness (～10 GPa) and elastic modulus (～127 GPa) values were found to be on a much higher side in 7.5 mol% MgO containing glass-ceramics as compared to lower MgO containing glass-ceramics.     

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.     

The Fluxing Effect of Mineral Material – Cullet Compositions on Enamel Glass

Enamel glasses are synthesized based on the Na₂O – B₂O₃ – Co₂O₃ – SiO₂ system, including perlite, nepheline concentrate, and cullet. The physicochemical and technological properties of the resulting glasses and one-layer coatings based on these glasses are considered. A comparative analysis of the fluxing effect of the nepheline - cullet and perlite-cullet compositions on glasses is carried out. The reference composition areas are shown to be suitable for the development of one-coat powdered glass enamels.     

PRELIMINARY STUDY ON PORTIONS OF SYSTEMS Na2O-CaO-AI2O3-Fe2O3 AND Na2O-CaO-Fe2O3-SiO2*

Portions of the quaternary system Na₂O-CaO-Al₂O₃-Fe₂O₃ have been studied by the exploration of (1) the plane CaO-4CaO.Al₂O₃°Fe₂O₃-(Na₂O + 3Al₂O₃) and (2) planes above the base system CaO.5CaO.3Al₂O3–2CaO.Fe₂O₃ which contain successively increasing amounts of Na₂O up to 6%. A portion of the quaternary system Na₂O-CaO-Fe₂O₃-SiO₂ has been studied by the exploration of a plane containing 5% of Na₂O above the base system CaO-2CaO.SiO₂-CaO.Fe₂O₃. In the pseudosystem CaO-4CaO.Al₂O₃.Fe₂O₃-(Na₂O + 3A1₂O₃) the compound Na₂O.-8CaO.3A1₂O₈ was found to exist as a primary phase, and the area in which the plane cuts the Na₂O.8CaO.3A1₂O₃ primary-phase volume was established. Three points on uni-variant curves were located. The iron phase (4CaO.A1₂O₃.Fe₂O₃ solid solution) was observed to exist in a solid-solution series. In the system Na₂O-CaO-5CaO.3Al₂O_3--2CaO.Fe₂O₃ it was found that the compound Na₂O.8CaO.3Al₂O3 appears at an Na₂O concentration of 4.2%. As soda, however, is taken into solid solution by other phases, it was not feasible at this time to determine the invariant point for Na₂O.8CaO.3A1₂O₃, 3CaO.Al₂O₃, 5CaO.3A1₂O₃, and 4CaO.Al₂O₃.-Fe₂O_a solid solution. In the system Na₂O-CaO-2CaO.SiO₂-CaO.Fe₂O₃ no ternary compounds were observed up to the 5% limit of Na₂O employed. A soda-containing phase occurred in solid solution with α-2CaO.SiO₂, which may precipitate on cooling, forming inclusions in the ß-2CaO.SiO₂, or enter into reaction with the glassy phase.     

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.     

Synthesis and characterization of newly developed phosphate-based glasses through experimental gamma-ray and neutron spectroscopy methods: Transmission and dose rates

In this study, four new phosphate-based glasses with the compositions of CaO–Na₂O–K₂O–P₂O₅ (PN system), CaO–Na₂O–K₂O–Al₂O₃–P₂O₅ (PA system) and CaO–Na₂O–K₂O–Al₂O₃–SiO₂–P₂O₅ (PS system) were synthesized and characterized through experimental gamma-ray and neutron spectroscopy methods. Glass densities were then measured experimentally and evaluated theoretically. Next, a high purity Germanium (HPGe) detector was used for determining the fundamental gamma-ray transmission parameters in 35.4–383 keV gamma-ray energies emitted from ¹³³Ba source (Radioactivity: 3Ci). Additionally, the experimental setup was used to determine the equivalent dose (EAD) to get a better knowledge of fast neutron attenuation. Our findings indicate that experimental gamma-ray transmission measurements are consistent with standard theoretical data (EpiXS). Consequently, PA10 was shown to have higher gamma-ray and neutron attenuation capabilities when compared to the other glass samples studied. Our outcomes showed that increasing the molar contribution of Al₂O₃ to the phosphate-based glasses increased not only their transparency but also their gamma-ray and neutron attenuation capacities. It can be concluded that substituting Al₂O₃ for P₂O₅ is a functional and monotonic tool for improving the optical, gamma-ray, and neutron attenuation of phosphate-based glasses, which are being evaluated as prospective shielding materials for medical and industrial radiation facilities.     

Understanding the structural origin of intermediate glasses

Intermediate glasses show nearly constant elastic moduli with temperature and/or pressure. These glasses would prove useful in designing a-thermal optical fibers for enhanced telecommunication, fiber sensing applications, and in designing glass products for applications where a broad range of thermal and mechanical stimulation is expected. In this study, intermediate glasses belonging to the Na₂O–SiO₂, Na₂O–Al₂O₃–SiO₂, and Na₂O–TiO₂–SiO₂ glass systems were identified from in situ high-temperature Brillouin light scattering (BLS) experiments. Glasses important for engineering applications like the international simple glass (ISG) and the less brittle glass (LBG) were also found to exhibit intermediate behaviors. In situ Raman spectroscopy was used to investigate their structural evolution from room temperature to temperatures beyond T_g. Raman spectra along with molecular dynamics simulations revealed common structural signatures that intermediate glasses with different compositions possess. Our study showed that the intermediate elastic behaviors come from a delicate balance between the stiffening effect associated with conformation changes in the medium-range flexible rings and the softening effect due to the weakening of short-range chemical bonds with temperature.