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.     

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.     

New calcium‐free Na2O–Al2O3–P2O5 bioactive glasses with potential applications in bone tissue engineering

Sodium aluminophosphate glasses were evaluated for their bone repair ability. The glasses belonging to the system 45Na₂O–xAl₂O₃‐(55‐x)P₂O₅, with x = (3, 5, 7, 10 mol%) were prepared by a melt‐quenching method. We assessed the effect of Al₂O₃ content on the properties of Na₂O–Al₂O₃–P₂O₅ (NAP) glasses, which were characterized by density measurements, DSC analyses, solubility, bioactivity in simulated body fluid and cytocompatibility with MG‐63 cells. To the best of our knowledge, this is the first investigation of calcium‐free Na₂O–Al₂O₃–P₂O₅ system glasses as bioactive materials for bone tissue engineering.     

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.     

Effect of K2O on structure-property relationships and phase transformations in Li2O-SiO2 glasses

Glass compositions with formula (71.78 − x)SiO₂-2.63Al₂O₃-(2.63 + x)K₂O-23.7Li₂O (mol.%, x = 0-10) and SiO₂/Li₂O molar ratios far beyond that of stoichiometric lithium disilicate (Li₂Si₂O₅) were prepared by conventional melt-quenching technique to investigate the influence of K₂O content on structural transformations and devitrification behaviour of glasses in the Li₂O-SiO₂ system. The scanning electron microscopy (SEM) examination of as cast non-annealed glasses revealed the presence of nanosized droplets in glassy matrices suggesting occurrence of liquid-liquid phase separation. An overall trend towards depolymerization of the silicate glass network with increasing K₂O content was demonstrated by employing magic angle spinning-nuclear magnetic resonance (MAS-NMR) spectroscopy. The distribution of structural units in the experimental glasses was estimated using ²⁹Si MAS-NMR spectroscopy suggesting the appearance of Q², enhancement of Q³ and diminishing of Q⁴ groups with increasing K₂O contents. X-ray diffraction (XRD) and differential thermal analysis (DTA) were used to assess the influence of K₂O on devitrification process and formation of lithium disilicate (Li₂Si₂O₅) and/or lithium metasilicate (Li₂SiO₃) crystalline phases.     

Submixture model to predict nepheline precipitation in waste glasses

High-alumina high-level waste (HLW) glasses are prone to nepheline precipitation during canister-centerline cooling (CCC). If sufficient nepheline forms, the chemical durability of the glass will be significantly impacted. Overly conservative constraints have been developed and used to avoid the deleterious effects of nepheline formation in U.S. HLW glasses. The constraints used have been shown to significantly limit the loading of waste in glass at Hanford and therefore the cost and schedule of cleanup. A 90-glass study was performed to develop an improved understanding of the impacts of glass composition on the formation of nepheline during CCC. The CCC crystallinity data from these glasses were combined with 657 glasses found in the literature. The trends showed significant effects of Na₂O, Al₂O₃, SiO₂, B₂O₃, CaO, Li₂O, and potentially K₂O on the propensity for nepheline formation. A pseudo-ternary submixture model was proposed to identify the glass composition region prone to nepheline precipitation. This pseudo-ternary with axes of SiO₂ + 1.98B₂O₃, Na₂O + 0.653Li₂O + 0.158CaO, and Al₂O₃ was found to divide glasses that precipitate nepheline during CCC from those that do not. Application of this constraint is anticipated to increase the loading of Hanford high-alumina HLWs in glass by roughly one-third.     

RELATION OF ELECTRICAL CONDUCTIVITY TO CHEMICAL COMPOSITION OF GLASS*

The electrical conductivity of some glasses in the soda-lime-silica system was measured at 400 °C. with an apparatus which is described. The specific resistance at this temperature ranged from 0.06 to 5.0 × 10⁵ ohms per cc. Measurements were made on other glasses containing equivalent amounts of various oxides incorporated in a parent soda-lime-silica glass. It was found that an increase in resistance was produced by additions of MnO, ZnO, B₂O₃, Fe₂O₃, BaO, PbO, TiO₂, and K₂O, the resistances increasing in this order. A decreased resistance was produced by Na₂O, CaO, and Al₂O₃. Previously published data on the power factors of these glasses are reviewed.     

The effect of B2O3, PbO and P2O5 on the sintering and machinability of fluormica glass–ceramics

Sintering, crystallization and machinability behavior of the SiO₂–Al₂O₃–MgO–K₂O–B₂O₃–F glasses were investigated. The optimum fluormica glass–ceramics with desirable sintering behavior and machinability were obtained by addition of PbO and P₂O₅ glass formers. Various parameters, e.g. the morphology of the mica crystal, relative intensity of the mica phase, the particle size distribution of chips obtained by drilling, microhardness, and the strength differences of glass–ceramics before and after drilling (Δσ) were investigated and compared with naked eye experiments.     

Transparent glazes with reduced concentration of B2O3

Na₂O?K₂O?CaO?B₂O₃?Al₂O₃?SiO₂ glasses low in B₂O₃ (14%) and highly resistant to crystallization and liquation under heating are synthesized. They are used for making transparent low-melting heat-resistant glazes with a low boron ion migration rate.     

Crystallization and Nonlinear Optical Properties of Potassium Niobium Silicate Glasses

The crystallization of the xK₂O · xNb₂O₅· (1 – 2x)SiO₂(x= 0.167–0.250) glasses and glasses close in composition to K₂O · Nb₂O₅· 4SiO₂at the ratio K₂O : Nb₂O₅ 1 is investigated. In high-silica glasses, the metastable phase separation followed by the bulk multiphase crystallization are observed at temperatures close to the glass transition point T _g. The nanostructured transparent glasses that exhibit the optical second harmonic generation (SHG) effect are formed at the early stages of phase separation. The surface crystallization of glasses with the precipitation of the KNbSi₂O₇noncentrosymmetric phase occurs at higher temperatures.     

Crystallization process and some properties of novel transparent machinable calcium-mica glass-ceramics

Bulk glass having a calcium-mica composition (Ca_0.5Mg₃AlSi₃O₁₀F₂) is homogeneous glass. The crystallization mechanism of the mica is surface crystallization and transparency is lost completely when crystallization occurs on the surface. In this study, by decreasing SiO₂ and increasing CaO and Al₂O₃ from the chemical composition of Ca_0.5Mg₃AlSi₃O₁₀F₂, and moreover by replacing a small amount of K₂O instead of CaO, the phase separation appears in the glasses. Because of this phase separation, the mica begins to be crystallized not only on the surface but also in the bulk at lower temperatures. Consequently, the novel transparent machinable mica glass-ceramic can be obtained by heating the glasses having the chemical composition of Ca_0.6Mg₃Al_1.2Si_2.8O₁₀F₂ and K_0.01Ca_0.595Mg₃Al_1.2Si_2.8O₁₀F₂. As a larger amount of calcium-mica is separated, the bending strength decreases and the fracture toughness increases. Furthermore, by replacing K⁺ ion instead of Ca²⁺ ion in the interlayer of calcium-mica, the interlayer bonding strength becomes high, resulting in the increase of the bending strength.     

A closer-look on Copper(II) oxide reinforced Calcium-Borate glasses: Fabrication and multiple experimental assessment on optical,structural, physical,and experimental neutron/gamma shielding properties

This study aimed to fabricate six different Copper(II) oxide reinforced Calcium-Borate glasses with different types of substitutions such as Al₂O₃/V2O₅, BaO₂/V₂O₅, and ZnO/V₂O_5. Accordingly, a depth characterization process has been performed for ACV, BCV and ZCV glasses with a nominal composition of 55B₂O₃–35CaO–9Al₂O₃-0.5CuO-0.5V₂O₅, 55B₂O₃–35CaO-8.5Al₂O₃-0.5CuO–1V₂O₅, 55B₂O₃–35CaO–9BaO₂-0.5CuO-0.5V₂O₅, 55B₂O₃–35CaO-8.5BaO₂-0.5CuO–1V2O5, 55B2O3–35CaO–9ZnO-0.5CuO-0.5V2O5, 55B2O3–35CaO-8.5ZnO-0.5CuO–1V₂O₅. Optical, structural, physical, and experimental neutron/gamma shielding properties of synthesized glasses were determined, respectively. Experiments measuring neutron exposure indicated how well glass samples attenuated fast neutrons. The RADACS software was used to record data from a BF3 gas proportional detector from the Canberra NP-100B series and a 241Am/Be neutron source with a 10 mCi activity. The absorption edge belonging to the samples are found between 420 nm and 480 nm. Our findings showed that the optical band gaps for the samples ranged from 1.179 to 2.022 eV. The wavenumber range of 400–1600 cm^-1 was evaluated for the resulting peaks. The region with the highest band formation was approximately 760–1170 cm^-1. While BCV0.5 and BCV1 glasses with 9% and 8.5% BaO₂ insertion have the largest MAC values ranging between 2.255–0.076 and 2.156–0.076 cm²/g, the lowest MAC values varying between 0.361–0.0761 and 0.366–0.076 cm²/g belong to ACV0.5 and ACV1 glasses with 9% and 8.5% Al₂O₃ addition. Our results showed that the BCV glass family has superior material properties among the fabricated glasses. It can be concluded that BaO₂/V₂O₅ glasses may be used in replacement of Copper(II) oxide glasses to provide monotonic behavior on crucial characteristics while maintaining the greatest density increase for large gamma ray shielding properties.     

Thermal Stability,Optical Transmittance,and Refractive Index Dispersion of La2O3–Nb2O5–Al2O3 Glasses

La₂O₃–Nb₂O₅–Al₂O₃ high‐refractive‐index glasses were fabricated by containerless processing, and the glass‐forming region was determined. The thermal stability, density, optical transmittance, and the refractive index dispersion of these glasses were investigated. All the glasses were colorless and transparent in the visible to near infrared (NIR) region and had high refractive index with low wavelength dispersion. Some of these glasses were found to have significantly high glass‐forming ability. These results indicate that the ternary glasses are suitable for optical applications in the visible to NIR region. The effects of the substitution of Al₂O₃ for Nb₂O₅ on optical properties were discussed on the basis of the Drude–Voigt equation. It was suggested that the substitution of Al₂O₃ for Nb₂O₅ increased the molecular density and suppressed a decrease in the refractive index, even when both the average oscillator strength and inherent absorption wavelength decreased in La₂O₃–Nb₂O₅–Al₂O₃ glasses. These results are helpful for designing new optical glasses controlled to have a higher refractive index and lower wavelength dispersion.     

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.     

Tinted Low-Melting Enamels for Aluminum

The possibility of formation of low-melting glasses in the R₂O – RO – Al₂O₃ – B₂O₃ – TiO₂ – P₂O₅ system at a firing temperature of 550°C is investigated. A correlation of glass formation in the considered system with the content of Al₂O₃ and P₂O₅ is established, and the composition of a transition layer with crystalline phases of Na₂O · P₂O₅, Al₂O₃ and Al₂O₃ · P₂O₅ is determined.     

Structural modification associated with Al2O3 addition in oxyfluoride glasses: Thermal and mechanical properties

In this work, the effect of gradual addition of Al₂O₃ substituting SiO₂ on the structural, thermal, and mechanical properties of SiO₂–BaF₂–K₂O–GdF₃–Sb₂O₃‐based oxyfluoride glasses have been studied. The X‐ray diffraction (XRD) patterns and differential scanning calorimetric (DSC) curves indicate that there is a distinct primary crystallization corresponding to BaGdF₅ phase formation in the samples without (0AlG) and with 5 mol% substitution of Al₂O₃ (5AlG) while the sample with 10 mol% of Al₂O₃ (10AlG) does not show such crystallization event. Further, the activation energy (E_a) for fluoride crystal formation is higher for the 5AlG in comparison to the 0AlG glass as determined by Kissinger, Augis‐Bennett and Ozawa models. Fourier transform infrared (FTIR) and Raman spectroscopy analysis confirmed the structural modification with the gradual addition of Al₂O₃ in the glass matrix revealing dominant presence of AlO₄ tetrahedral units in 10AlG sample unlike in 5AlG sample which exhibited the manifestation of AlO₆ units. Such structural variation has further been substantiated from the estimated elastic properties like Young's modulus (E), shear modulus (G), bulk modulus (K), longitudinal modulus (L), and mean ultrasonic velocity (U_m) by showing a decrease for 5AlG sample in comparison with 0AlG sample followed by subsequent increase for 10AlG sample.     

The Effect of Basicity of Glass on Interaction of Variable-Valence Elements

The interaction between two variable-valence elements (As + Fe and As + Cr) simultaneously present in glasses of the four-component system K₂O – B₂O₃ – SiO₂ – Al₂O₃ is investigated. It is demonstrated that as the glass basicity increases, the element to the left of the redox series passes to a higher valence state. Recommendations are issued for correcting concentrations of variable-valence elements for the purpose of optimizing the tint (color shade) in tinted and clear glasses.     

Influence of TiO2, Cr2O3 and ZrO2 on sintering and machinability of fluorophlogopite glass ceramics

Abstract

The influence of Cr₂O₃, TiO₂ and ZrO₂ on the sintering, crystallisation and machinability of SiO₂–Al₂O₃–MgO–K₂O–B₂O₃–F glasses was investigated. Optimum fluoromica glass ceramic compositions with desirable sintering behaviour and machinability were obtained by addition of titanium and chromium oxides to the base glass. Texture and relative intensity of mica phase I_mica/I_Si were determined by XRD analysis and the particle size distribution of chips was studied by drilling. Microhardness and bending strength were also investigated. The relative intensity of mica phase and microhardness were found to be compatible with the experimental results.     

Bismuth and lead oxides codoped boron phosphate glasses for Faraday rotators

New magneto-optical vitreous materials were obtained by melting-quenching technique comprising wet route raw materials preparation. The glass has the following composition in oxide mol. %: 10 Li₂O, 9 Al₂O₃, 5 ZnO, (35; 20; 50) B₂O₃, (35; 50; 20) P₂O₅, 3 Bi₂O₃, 3 PbO, phosphorus and boron oxide being the vitreous network formers. It was also prepared a similar reference glass composition but without Bi₂O₃ and PbO. Optical and structural characterization by ultraviolet-visible (UV–Vis), Fourier Transform Infrared (FTIR) and Raman Spectroscopy of the bulk glasses showed a transmission over 90%, metaphosphate structure of glass together with Q² boron oxide units and P–O?B bonds. The mechanical parameters, hardness (H), Young's modulus (E) and fracture toughness (K_IC) of boron phosphate glasses, evaluated by micro- and nanoindentation techniques, demonstrated mostly higher values in comparison with those for alumino-phosphate glasses due to mixed boro-phosphate network. Thermal behavior was investigated by Differential Scanning Calorimetry (DSC) putting in evidence the vitreous transition temperature which decreases with about 45 °C when Bi and Pb oxides were added and two crystallization effects. The diamagnetic character of a highly transparent Bi and Pb oxide co-doped boron phosphate glass was confirmed by ellipsometry, and the glass presented high magneto-optical properties at the top of the commercial bulk products.     

Revisiting metastable immiscibility in SiO2–Al2O3: Structure and phase separation of supercooled liquids and glasses

Understanding and controlling liquid–liquid phase separation in aluminosilicates is crucial for optimizing glass properties. However, the metastable nature of aluminosilicates’ phase separation has made it difficult to study experimentally, and uncertainty persists regarding the compositional and temperature extents of the miscibility gap. Here, we present new experimental evidence that suggests a consolute temperature between 1440 and 1590°C and endmember compositions of 7 and 62 mol.% Al₂O₃ for the phase-separated glasses. Using containerless melt processing, deeply supercooled liquids over the 0–60 mol.% Al₂O₃ range are probed with in situ small- and wide-angle X-ray scattering, which simultaneously reveals changes in nanoscale density heterogeneity and atomic structure. Correlations between phase separation and atomic coordination environments are compared for liquids and glasses. Pair distribution function analysis shows mean O–(Si + Al) coordination increases with Al₂O₃ content and decreases with temperature.