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Spinel crystallization is known to be detrimental to the operation of Joule heated ceramic melters during the vitrification of iron‐rich high‐level nuclear wastes (HLW) into borosilicate glasses. The literature on this subject focuses on tackling the problem by developing empirical constraints to design compositions, which limit the fraction of spinels formed in the melter or by developing empirical models to predict the settling behavior of spinels in the melter as a function of the glass composition. While these empirical models can predict the behavior of most of the compositions, they are not failsafe as there are always some compositions, whose behavior is beyond the predictive ability of these models. This can lead to undesirable situations during the vitrification of the nuclear waste, and therefore an in‐depth investigation of the chemo‐structural descriptors controlling the crystallization behavior in these glasses is warranted. Accordingly, the present study aims to understand the impact of non‐framework cation mixing (i.e., Li+/Na+ and Ca2+/Na+) on the structure (through Raman spectroscopy and Mössbauer spectroscopy) and crystallization behavior (through XRD, SEM‐EDS, and vibrating sample magnetometry) of iron‐rich model HLW glasses in the system: (mol.%) x MyO–(25−x) Na2O–9.12 B2O3–6.4 Al2O3–51.25 SiO2–7.22 Fe2O3–0.38 MnO–0.08 Cr2O3–0.55NiO (MyO = Li2O or CaO). [ABSTRACT FROM AUTHOR]

In this review, we provide a perspective on the science and technology of vitrification of waste. First, we provide a background on the general classes of wastes for which vitrification is currently used for immobilization or is proposed, including nuclear and industrial hazardous wastes. Next, we summarize the issues surrounding solubility of waste ions and resulting uncontrolled crystallization or phase separation. Some newer waste form designs propose a controlled crystallization, resulting in a glass-ceramic. A summary of glass systems and glass-ceramic systems is given, with the focus on immobilizing waste components at high waste loading. Throughout, design and processing considerations are given, and the difference between uncontrolled undesirable and controlled desirable crystallization is offered. [ABSTRACT FROM AUTHOR]

Glass-ceramic (GC) materials have the formability advantages of glasses and the opportunity for controlled crystallization of functional ceramic phases. Here, we obtain GCs containing the hard magnetic phase Sr-hexaferrite (SrFe12O19) from a borate glass. Ten compositions in the B2O3-Fe2O3-SrO system were explored, varying B2O3 and Sr/Fe ratio. Compositions forming glass on quenching, according to X-ray diffraction (XRD), were subsequently heat treated to promote crystallization. Three selected compositions were investigated with vibrating sample magnetometry (VSM), thermal analysis, and electron probe microanalysis (EPMA). Phases identified by XRD after air heat treatment included α-Fe2O3, Fe3O4, SrB2O4, and SrFe12O19. Glasses were also crystallized in situ in a VSM in an argon environment, measuring magnetic properties during heating to 650°C after in situ heat treatment at 800°C. In samples with SrFe12O19, wasp-waisted loops were observed. First order reversal curve (FORC) measurements confirmed low (magnetite) and high (Sr-hexaferrite) coercivity phases. Room temperature VSM measurements of argon in situ treated samples were compared with two ex situ air heat treated protocols. The microstructures of the three investigated air heat treated GC materials were completely different, and compositional position on the phase diagram appeared to influence crystallization progress. These results suggest that careful control of composition as well as heat treatment protocol including atmosphere is necessary for crystallization of desired magnetic phases. Also, VSM was confirmed to be sensitive to magnetic phases at low concentrations not visible to XRD. [ABSTRACT FROM AUTHOR]

Ion irradiation effects are studied on the Fe-based core-shell nanocluster (NC) films with core as Fe and shell as Fe3O4/Fe3N. These NC films were deposited on Si substrates to thickness of ~0.5 μm using a NC deposition system. The films were irradiated at room temperature with 5.5MeV Si2+ ions to ion fluences of 1015 and 1016 ions/cm2. It is found that the irradiation induces grain growth, Fe valence reduction in the shell, and crystallization or growth of Fe3N. The film retained its Fe-core and its ferromagnetic properties after irradiation. The nature and mechanism of oxide shell reduction and composition dependence after irradiation were studied by synthesizing additional NC films of Fe3O4 and FeO+Fe3N and irradiating them under the same conditions. The presence of nanocrystalline Fe is found to be a major factor for the oxide shell reduction. The surface morphologies of these films show dramatic changes in the microstructures due to cluster growth and agglomeration as a result of ion irradiation. [ABSTRACT FROM AUTHOR]

Abstract: Lithium and sodium aluminosilicates are important glass‐forming systems for commercial glass‐ceramics, as well as being important model systems for ion transport in battery studies. In addition, uncontrolled crystallization of LiAlSiO4 (eucryptite) in high‐Li2O compositions, analogous to the more well‐known problem of NaAlSiO4 (nepheline) crystallization, can cause concerns for long‐term chemical durability in nuclear waste glasses. To study the relationships between glass structure and crystallization, nine glasses were synthesized in the LixNa1‐xAlSiO4 series, from x = 0 to x = 1. Raman spectra, nuclear magnetic resonance (NMR) spectroscopy (Li‐7, Na‐23, Al‐27, Si‐29), and X‐ray diffraction were used to study the quenched and heat‐treated glasses. It was found that different LiAlSiO4 and NaAlSiO4 crystal phases crystallize from the glass depending on the Li/Na ratio. Raman and NMR spectra of quenched glasses suggest similar structures regardless of alkali substitution. Li‐7 and Na‐23 NMR spectra of the glass‐ceramics near the endmember compositions show evidence of several differentiable sites distinct from known LixNa1‐xAlSiO4 crystalline phases, suggesting that these measurements can reveal subtle chemical environment differences in mixed‐alkali systems, similar to what has been observed for zeolites. [ABSTRACT FROM AUTHOR]

In this work, the vision and need for a fully ceramized long-wave infrared (LWIR)-transmitting glass ceramic have been articulated. Three sulfide systems were explored including two with La 2 S 3 in hopes of imparting strong bonds from this refractory sulfide, and two containing GeS 2 in hopes of widening the glass-forming region. Attempts were made to produce glasses in the Ga 2 S 3 –La 2 S 3 –(ZnS,CaS) system, the GeS 2 –La 2 S 3 system, and the GeS 2 –Ga 2 S 3 –CdS system. Water quenching produced glasses of Ga 2 S 3 –La 2 S 3 –CaS and GeS 2 –Ga 2 S 3 –CdS. Microstructural and thermal analyses were used to explore nucleation and growth in these systems and infrared transmission and mechanical hardness showed potential for LWIR window use. The GeS 2 –Ga 2 S 3 –CdS system showed good LWIR transmission and pre-crystallized hardness superior to chemical vapor deposited ZnS. The Ga 2 S 3 –La 2 S 3 glasses did not appear to be viable candidates at this time due to a small temperature window between crystallization and glass transition temperatures and problems with oxygen contamination in the La 2 S 3 source. Suggestions are made for two alternative methods for producing fully ceramized LWIR-transmitting glass ceramics. [ABSTRACT FROM AUTHOR]

This study presents the characterization of salt phases that formed on simulated low-activity waste glass melts during a rhenium solubility study. This study with rhenium salts is also applicable to real applications involving radioactive technetium salts. In this synthesis method, oxide glass powder is mixed with the volatile species, vacuum-sealed in a fused quartz ampoule, and then heated in a furnace. This technique restricts the volatile species to the headspace above the melt but still within the sealed ampoule, thus maximizing the concentration of these species that are in contact with the glass. Above the previously determined solubility of Re7+ in this glass, a molten salt phase segregated to the top of the melt and crystallized into a solid layer. This salt was analyzed with X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, as well as wavelength dispersive spectroscopy and was found to be composed of alkali perrhenates ( NaReO4, KReO4) and alkali sulfates. Similar crystalline inclusions were found in the bulk of some glasses as well. [ABSTRACT FROM AUTHOR]

Abstract: Sodium- and aluminum-rich high-level nuclear waste glasses are prone to nepheline (NaAlSiO4) crystallization. Since nepheline removes three moles of glass-forming oxides (Al2O3 and SiO2) per mole of Na2O, the formation of this phase can result in severe deterioration of the chemical durability in a given glass. The present study aims to investigate the relationships between the molecular-level structure and the crystallization behavior of sodium alumino-borosilicate-based simulated high-level nuclear waste glasses with infrared spectroscopy (FTIR) and X-ray diffraction, respectively. The molecular structure of most of the investigated glasses comprise a mixture of Q 2 and Q 3 (Si) units while aluminum and boron are predominantly present in tetrahedral and trigonal coordination, respectively. The increasing boron content has been shown to suppress the nepheline formation in the glasses. The structural influence of various glass components on nepheline crystallization is discussed. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

The purpose of this review is to compare and contrast silicate natural glasses with so-called un-natural glasses, bringing together literature and common issues in geology and technical glass science and engineering. The review is organized into three proposed 'frontiers' of glass science of interdisciplinary interest: 1) extreme conditions for glass formation, 2) crystallization in complex glasses, and 3) long-term glass alteration. In each frontier, examples of natural (geologic) as well as un-natural (accidental anthropogenic) glasses are presented, and their characteristics are compared and contrasted. Since crystallization is an important consideration, not only fully amorphous glasses but also partially crystallized glasses are considered. Natural glasses discussed include primarily impactites, obsidian, and basaltic glass, though glasses formed by other means are also considered. Un-natural glasses discussed include those created in atomic blasts, vitrified Iron Age hillforts, and slags from metallurgical processes. Conditions for extreme glass formation in different types of glasses formed by shock metamorphism are compared to modern time-resolved shock and high-pressure experiments for observing polyamorphism and densification changes. Silicate magmas producing obsidian and basaltic glasses are presented and the differences in crystallization pathways are discussed as they relate to changes in flow properties and eruptions. Vitrified hillfort materials and combustion natural glasses are then compared with respect to their felsic-mafic liquid phase separation and crystal assemblage. Finally, an appeal is made for the use of natural and un-natural glasses for understanding long-term glass corrosion as a function of composition. Glass corrosion models are considered as applied to both types of glasses, particularly relating to the neo-formed crystals, biologically-induced glass corrosion, and nuclear waste management. It is hoped that this unorthodox presentation will foster dialogue between earth scientists and technical glass scientists and further research into these and other frontiers of natural glass science. • Natural and un-natural glasses are juxtaposed for interdisciplinary consideration • Glass frontiers: extreme conditions for formation, crystallization, and alteration • Natural glasses discussed: impactites, magmatic glasses, combustion glasses • Un-natural glasses discussed: trinitite, vitrified hillforts, slags [ABSTRACT FROM AUTHOR]

Crystallization of rhenium salts in a simulated low-activity waste borosilicate glass: Erratum Corresponding author: Brian Riley; email id: brian.riley@pnnl.gov There is a typographical error in the introduction of this paper. Crystallization of rhenium salts in a simulated low-activity waste borosilicate glass. [Extracted from the article]

Abstract The influences of boron on silicate glass dissolution behavior and crystallization have long been important in the nuclear waste glass and technical glass communities. Boron is a challenging component to model due to this ability to assume three- and four-fold coordinations with oxygen, as well as its ability to form complex structural units with silicate tetrahedra. Furthermore, the low atomic mass limits the available characterization techniques. In the present work, recent literature findings on the compositional influence on boron speciation are implemented to interpret new data on alkali boroaluminosilicate glasses studied with 11B nuclear magnetic resonance (NMR), synchrotron X-ray scattering (s-XRS), and chemical durability measurements. The purpose of this study is to understand the influence of boron on the crystallization behavior of sodium and lithium aluminosilicate systems, NaAl 1-x B x SiO 4 and LiAl 1-x B x SiO 4 , as applied to problems in nuclear waste vitrification. Comparisons are drawn between the alkali coordination number and crystallization behavior as a function of boron addition, as obtained through modeling of synchrotron X-ray total scattering using Empirical Potential Structural Refinement. Highlights • EPSR modeling of glasses with systematic boron addition and comparison to NMR. • N4 increases with increasing boron. • Crystallization propensity reduced with increasing alkali coordination. • Increase of glass corrosion at higher boron fractions due to greater interruption of glass network. [ABSTRACT FROM AUTHOR]

• X-ray nano-computed tomography (nano-CT) is powerful technique to explore glasses. • Complex features were imaged in nuclear waste, natural, and chalcogenide glasses. • Nano-CT enabled 3D study of crystal, phase separation, pore, & fracture morphology. • Features from 100 to 20 µm were visualized with 3D videos and 2D images. • 3D measurements included dimensions, orientation, shape, and crystal number density. X-ray nano-computed tomography (nano-CT) is a powerful technique to characterize and visualize 3 dimensional (3D) phenomena in complex glasses at the nanoscale. This technique can offer a unique opportunity to explore the intricate morphology of multicomponent glass and glass-ceramic samples, due to its low X-ray energy and high spatial resolution (down to 50 nm). In the current demonstration paper, nano-CT provided insight into crystallization and phase separation, as well as a variety of other phenomena, including corrosion, fracture, and porosity. At scales ranging from 100 to 20 μm, the microstructures of phase separation in borosilicate, complex silicate, and chalcogenide glass compositions were examined. In addition, crystallites present in simulant nuclear waste glasses, volcanic glasses, and other related samples were explored. Nano-CT analysis can add to the understanding of (1) the formation processes, (2) distributions, interactions, and compositions of phases, and (3) the mechanisms, structures, and growth of crystallization and phase separation. Nano-CT offers wide potential in the field of glass science, especially when (1) common characterization methods are insufficient, (2) simple sample preparation is required, (3) 3D rendering is needed, or (4) compelling images of small, complex features are desired. [ABSTRACT FROM AUTHOR]

In alkali aluminosilicate glasses, additions of 4+ cations like Zr and Ti are often added to promote crystallization. In this study, Zr, Ti, or Sn are progressively substituted for Si in nepheline (NaAlSiO 4) glass to determine their impact on the crystallization behavior. For glasses homogeneous on quenching, up to NaAlZr 0.075 Si 0.925 O 4 , NaAlSn 0.100 Si 0.900 O 4 , and NaAlTi 0.300 Si 0.700 O 4 , respectively, crystallization temperatures were investigated by thermal analysis. A subset of compositions was subjected to additional thermal analysis, varying heating rate and particle size, to investigate the subtleties of crystallization behavior. Subsequent heat treatment of glass powders was performed to maximize crystallization, and glass-ceramic microstructure was assessed by optical microscopy and electron microprobe, while crystalline phases were determined by X-ray diffraction. In all cases the major crystalline phase was orthorhombic carnegieite, which accommodated Ti in its structure but not Zr or Sn, and excess 4+ cations formed MO 2 , i.e., brookite, baddeleyite, and cassiterite, respectively. [ABSTRACT FROM AUTHOR]

Glasses with compositions x NaCl-(50- x /2)K 2 SO 4 -(50- x /2)ZnSO 4 , x = 0 to 30 (in mol%), were characterized via vibrational spectroscopy and X-ray pair distribution function (xPDF) analysis. The xPDF's were modeled using Empirical Potential Structure Refinement (EPSR). From the modeled xPDF, the deconvoluted xPDFs were obtained along with cation-anion coordination numbers. The sulfate environment was elucidated through analysis of the vibrational spectroscopy paired with xPDFs. As determined via these techniques, the SO 4 2− oxyanions remain as distinct tetrahedra and dominate the overall structure of these ionic glasses, even with up to 30 mol% addition of NaCl. Addition of NaCl lowers the glass transition temperature (T g) of these materials and increases the temperature between T g and the first crystallization temperature upon heating, as determined by differential thermal analysis (DTA). The composition-dependent xPDFs indicate that chloride ions are distributed equally and increasingly around Zn, K, and Na cations as x increases. • Ionic glasses in the system NaCl-ZnSO 4 -K 2 SO 4 were synthesized and characterized. • Sulfate units are non-bridging tetrahedra according to PDF and Raman • Increasing NaCl addition decreases the glass transition temperature. • Bond distances in the glass are similar to those in crystalline sulfate salts. [ABSTRACT FROM AUTHOR]

The effects of La 2 O 3 on microstructure and properties of the CaO-MgO-SiO 2 -Al 2 O 3 glass ceramics, made from Bayan Obo West Mine tailing and fly ash, were investigated. Results showed that La 2 O 3 additions up to 2 wt% promoted augite crystallization. Above this concentration, La 2 O 3 suppressed augite crystallization by rather forming lanthanum oxyapatite that acted as a barrier for the growth of augite. Also Cr ions, the key element for the augite nuclei, were enriched in lanthanum oxyapatite. The La3+ cations were found to partially substitute for Ca2+ at the M2 structural sites in clinopyroxene (augite). Nano crystalline clusters, containing La3+, Na+, Ca2+, Mg2+ and Fe3+ in augite-like proportions, were found uniformly dispersed in the residual glassy phase. The glass ceramic with 2 wt% La 2 O 3 shows optimum mechanical properties and chemical durability. The bending strength and Vickers hardness are 238 MPa and 7.57 GPa, acid and alkali-resistance are 97.5% and 98.9%, respectively. • La3+ in CaO-MgO-SiO 2 -Al 2 O 3 glass-ceramics affects chemo-mechanical properties. • La3+ in augite maximizes strength. • Excess La 2 O 3 forms La oxyapatite (Cr doped) and makes large grains. • Nano-augite clusters form in residual glass. [ABSTRACT FROM AUTHOR]