Natural corrosion of the uranium-colored historical glasses

The historical uranium-colored glasses have been collected from the dumps of former glassworks. Buried in the soil the glasses have been exposed to natural weathering for approximately 150 years. Uranium content ranges from 0.089 to 0.556 wt% depending on the particular glass type. Two types of natural corrosion crusts were distinguished: (1) The leached glass that is hydrated and depleted of alkalies: Affected layer is up to 0.56 mm thick. The corrosion involves leaching of alkali ions from the glass. In comparison with the primary glass the corroded part shows higher uranium content. Uranium concentration in the leached layer increases because (UO₂)²⁺ sorption on the residual glass. (2) Laminated outer layer of residual glass and newly formed aluminosilicate along with calcite: The surface layer system of newly formed phases and leached relics of glass is at most 30 μm thick. With respect to the glass composition the research suggests that the Ca²⁺ and Al³⁺ ions forming new phases have been provided by the natural leaching solution. The uranium concentration in the corrosion crust is significantly lower. Alpha spectroscopy demonstrated the isotopes ²³⁸U and ²³⁴U in the products of the glass corrosion are at the radioactive disequilibrium.     

Investigation of gel porosity clogging during glass leaching

A 5-oxide glass (62.5SiO₂, 16.6B₂O₃, 13.1Na₂O, 6.0CaO, 1.8ZrO₂) was leached at 90 °C at a high glass-surface-area-to-solution-volume ratio (SA/V = 80 cm^?1). Its dissolution rate diminished over time until it became unmeasurable. The alteration layer was characterized by ²⁹Si isotopic tracing in the leaching solution. ToF-SIMS elemental profiles showed that glass dissolution ceased due to clogging of the gel porosity at the gel/solution interface. One of the hypotheses proposed to account for the rate drop observed during borosilicate glass alteration is based on morphological changes in the alteration gel over time. Monte Carlo modeling of glass alteration, especially with simple glasses, indicates a clogging of the porosity on the external portion of the gel (near the solution/gel interface) after densification of the layer by silicon precipitation, but this phenomenon had never previously been directly observed experimentally. The initial results obtained by isotopic tracing provide new data that appears to confirm this hypothesis.     

The effect of TiO2 on phase separation and crystallization of glass-ceramics in CaO–MgO–Al2O3–SiO2–Na2O system

The experiments were carried out on studying the effect of phase separation on nucleation and crystallization in the glass based on the system of CaO–MgO–Al₂O₃–SiO₂–Na₂O. In the experiments, TiO₂ was chosen as nucleating agent. Three batches of 5, 8 and 10 wt% TiO₂ substitution were investigated by the techniques of DSC, XRD, FTIR and FESEM equipped with EDS. XRD and FTIR analysis indicated that the super cooled glasses were all amorphous, the heat treatment leading to nucleation would cause a disruption of silica network which followed phase separation. The phase separation followed the generation of crystal seeds Mg(Ti, Al)₂O₆. FESEM observation and EDS analysis revealed that the more TiO₂ content of glass, the more droplet separated phase and crystal seeds after nucleation heat treatment. The main crystal phase is clinopyroxene, Ca(Ti, Mg, Al)(Al, Si)O₆, of crystallized glass.     

Melting enthalpy ΔHm for describing glass forming ability of bulk metallic glasses

A new melting enthalpy ΔH_m criterion for the prediction of glass forming ability (GFA) of alloys is proposed and five Zr–Al–Ni–Cu bulk metallic glasses (BMG) with critical dimension Z_max up to ? 7.5 mm are also developed by us in the light of the optimum ΔH_m of Zr–Al–Ni–Cu alloy system. And then, we researched the relationships between ΔH_m and two GFA parameters (critical cooling rate R_c and Z_max) of five bulk metallic glass (BMG) systems, such as Mg–Ni–Nd, Pd–Cu–Si, La–Al–Ni–Cu, Zr–Al–Ni–Cu and Zr–Ti–Ni–Cu–Be, respectively. The results show that the relationships between ΔH_m and R_c are all concave upward parabolas, and the optimum ΔH_ms for Mg–Ni–Nd, Pd–Cu–Si, Zr–Al–Ni–Cu, Zr–Ti–Ni–Cu–Be and La–Al–Ni–Cu are 10.3960 kJ mol^?1, 21.2202 kJ mol^?1, 19.7146 kJ mol^?1, 18.1455 kJ mol^?1 and 13.1558 kJ mol^?1, respectively. On the contrary, the relationships between ΔH_m and Z_max are all concave downward parabolas, and the optimum ΔH_ms for Mg–Ni–Nd, Pd–Cu–Si, Zr–Al–Ni–Cu, Zr–Ti–Ni–Cu–Be and La–Al–Ni–Cu are 10.5530 kJ mol^?1, 21.0830 kJ mol^?1, 19.6603 kJ mol^?1, 19.7231 kJ mol^?1 and 13.1173 kJ mol^?1, respectively. Furthermore, other BMGs’ R_cs or Z_maxs predicted by above-mentioned relationships satisfactorily agree with the tested results, which indicates that these relationships are reliable. However, the predicted results are reliable only if the main components are similar with the fitted BMGs or the additive is sparkle enough that the alloy’s character does not change. On the whole, the ΔH_m can act as a criterion for quickly predicting the alloy’s GFA and be helpful for the development of new BMGs.     

Heterophase inclusions and dissolved impurities in Ge25Sb10S65 glass

The impurity content and microhomogeneity of Ge₂₅Sb₁₀S₆₅ glass samples, prepared by direct synthesis from elements, were investigated. It was shown that the increase in temperature of synthesis of the glass-forming melt resulted in the increase of the content of impurities of H, Na, Al, Si, K, Ca and transition metals in the prepared glasses. The glasses from the melt, subjected to chemical-distillation purification, were characterized by the low content of gas-forming impurities and the increased content of Al, Si and Cl. The glasses contained heterophase impurity inclusions mainly consisting of SiO₂, and their concentration and size depended on the conditions of glass preparation. The impurity content in the purest glasses was as follows: oxygen – <0.5 ppm wt, carbon – <5 ppm wt, hydrogen – 0.1 ppm wt, Si – <1 ppm wt, transition metals – <0.25 ppm wt, heterophase impurity inclusions with sizes larger than 80 nm – <10² cm³. It was shown that heterophase impurity inclusions behaved as the centers of glass crystallization.     

Thermal analysis of geopolymer pastes synthesised from five fly ashes of variable composition

This paper presents a study on the thermal properties of a range of geopolymers in order to assess their suitability for high temperature applications such as thermal barriers, refractories and fire resistant structural members. Geopolymers were synthesised from five different fly ashes using sodium silicate and sodium aluminate solutions to achieve a set range of Si:Al compositional ratios. The thermo-physical, mechanical and microstructural properties of the geopolymers are presented and the effect of the source fly ash characteristics on the hardened product is discussed, as well as implications for high temperature applications. The amount and composition of the amorphous component (glass) of each of the fly ashes was determined by combining XRD and XRF results. It was found that the Si:Al ratio in the glass of the fly ashes strongly influenced the thermal performance of the geopolymers. Geopolymers synthesised from fly ashes with a high Si:Al (≥ 5) in the glass exhibited compressive strength gains and greater dimensional stability upon exposure to 1000 °C, whereas geopolymers synthesised from fly ashes with low Si:Al (< 2) in the glass exhibited strength losses and reduced dimensional stability upon high temperature exposure.     

Neutron diffraction study of sodium borosilicate waste glasses containing uranium

The effect of uranium oxide on the structure of sodium borosilicate host glasses has been studied by neutron diffraction. The samples were prepared by quenching the melted mixtures of composition 70 wt% [(65 − x)SiO₂ · xB₂O₃ · 25Na₂O · 5BaO · 5ZrO₂] + 30 wt% UO₃ with x = 5, 10 and 15 mol%. It was found, that the U-loaded glasses posses good glass and hydrolytic stability. An enhanced probability for inter-mediate atomic correlations at around 4.8 Å has been established. The RMC simulation of the neutron diffraction data is consistent with a model where the uranium ions are incorporated into interstitial voids in the essentially unmodified network structure of the starting host glass. The U–O atomic pair correlation functions show a sharp peak at around 1.7 Å, and several farther distinct peaks are at 2.8, 3.6 and 4.1 Å. The uranium ions are coordinated by six oxygen atoms in the 1.6–3.4 Å interval.     

Monte Carlo simulations of the dissolution of borosilicate glasses in near-equilibrium conditions

Monte Carlo simulations were performed to investigate the mechanisms of glass dissolution as equilibrium conditions are approached in both static and flow-through conditions. The glasses studied are borosilicate glasses in the compositional range (80 ? x)% SiO₂ (10 + x / 2)% B₂O₃ (10 + x / 2)% Na₂O, where 5 < x < 30%. In static conditions, dissolution/condensation reactions lead to the formation, for all compositions studied, of a blocking layer composed of polymerized Si sites with principally 4 connections to nearest Si sites. This layer forms atop the altered glass layer and shows similar composition and density for all glass compositions considered. In flow-through conditions, three main dissolution regimes are observed: at high flow rates, the dissolving glass exhibits a thin alteration layer and congruent dissolution; at low flow rates, a blocking layer is formed as in static conditions but the simulations show that water can occasionally break through the blocking layer causing the corrosion process to resume; and, at intermediate flow rates, the glasses dissolve incongruently with an increasingly deepening altered layer. The simulation results suggest that, in geological disposal environments, small perturbations or slow flows could be enough to prevent the formation of a permanent blocking layer. Finally, a comparison between predictions of the linear rate law and the Monte Carlo simulation results indicates that, in flow-through conditions, the linear rate law is applicable at high flow rates and deviations from the linear rate law occur under low flow rates (e.g., at near-saturated conditions with respect to amorphous silica). This effect is associated with the complex dynamics of Si dissolution/condensation processes at the glass–water interface.     

Ex situ XRD,TEM, IR,Raman and NMR spectroscopy of crystallization of lithium disilicate glass at high pressure

The structure of Li₂O · 2SiO₂ (LS₂) glass was investigated as a function of pressure and temperature up to 6 GPa and 750 °C, respectively, using XRD, TEM, IR, Raman and NMR spectroscopy. Glass densified at 6 GPa has an average Si–O–Si bond angle ∼7° lower than that found in glass processed at 4.5 GPa. At 4.5 GPa, lithium disilicate crystallizes from the glass, while at 6 GPa new high pressure form of lithium metasilicate crystallizes. This new phase, while having lithium metasilicate crystal symmetry, contains at least four different Si sites. NMR results for 6 GPa indicate the presence of Q⁴ species with (Q⁴)Si–O–Si(Q⁴) bond angles of ∼157°. This is the first reported occurrence of Q⁴ species with such large bond angles in alumina free alkali silicate glass. No five- or six-coordinated Si are found.     

Germanosilicate and alkali germanosilicate glass structure: New insights from high-resolution oxygen-17 NMR

We present here triple-quantum, magic-angle spinning (3QMAS) NMR spectra for ¹⁷O in a SiO₂–GeO₂ binary glass, and for two sodium germanosilicate glasses, all with Si/Ge ratios of 1. In the binary germanosilicate, three NMR peaks are partially resolved, and correspond to the three types of bridging oxygens, Si–O–Si, Si–O–Ge, and Ge–O–Ge. Peak areas indicate that the relative abundances of these species are close to those expected for random mixing of the Si and Ge in the network. In a sodium germanosilicate glass with a relatively low Na content (Na₂O ≈ 8 mol%), the spectra demonstrate the formation of significant fractions of both nonbridging oxygens bonded to Si, and of oxygens bonded to Ge in five- or six-coordination. At higher Na content (Na₂O ≈ 31%), most or all Ge is four-coordinated and network modification is dominated by the formation of NBO on Si and on Ge. Models of physical properties of alkali germanosilicates, in which modifier oxides are distributed between the Si and Ge components of the network in proportion to the Si/Ge ratio, are thus supported, as is extensive mixing of Si and Ge.     

Fluorine laser-induced silicon hydride Si–H groups in silica

Formation and destruction of silicon hydride (Si–H) groups in silica by F₂ laser irradiation and their vacuum ultraviolet (VUV) optical absorption was examined by infrared (IR) and VUV spectroscopy. Photoinduced creation of Si–H groups in H₂-impregnated oxygen deficient silica is accompanied by a growth of infrared absorption band at 2250 cm⁻¹ and by a strong increase of VUV transmission at 7.9 eV. Photolysis of Si–H groups by 7.9 eV photons in this glass was not detected when the irradiation was performed at temperature 80 K. However, a slight destruction of Si–H groups under 7.9 eV irradiation was observed at the room temperature. This finding gives a tentative estimate of VUV absorption cross section of Si–H groups at 7.9 eV as 4 × 10⁻²¹ cm², showing that Si–H groups do not strongly contribute to the absorption at the VUV fundamental absorption edge of silica glass.     

Nanostructural and optical features of amorphous AlxSi0.45−xO0.55 (0 ⩽ x ⩽ 0.05) thin films prepared by RF-magnetron sputter techniques

The nanostructural, chemical, and optical features of Al_xSi_0.45−xO_0.55 (0 ⩽ x ⩽ 0.05) thin films were investigated in terms of Al concentration and post-deposition annealing conditions; the films were prepared by co-sputtering a Si main target and Al-chips, and the annealing was carried out at temperatures of 400–1100 °C. The a-Si_0.45O_0.55 films prepared without Al-chips and annealed at 800 °C contain ∼3.5 nm-sized Si nanocrystallites. The photoluminescence (PL) intensity as well as the volume fraction of Si nanocrystallites increased with increasing the concentration of Al to a certain level. In particular, the intensity of the PL spectra of the Al_0.025Si_0.425O_0.550 films which were annealed at 800 °C increased significantly at wavelengths of ∼580 nm. It is highly likely that the observed increase in the PL intensity is caused by the raise in the total volume of the ∼3.5 nm-sized nanocrystallites in the films. The addition of Al as well as the post-deposition annealing allow adjustment and control of the nanostructural and light-emission features of the a-SiO_x films.     

High-quality silicon/insulator heteroepitaxial structures formed by molecular beam epitaxy using Al2O3 and Si

Heteroepitaxial growth of γ-Al₂O₃ films on a Si substrate and the growth of Si films on the γ-Al₂O₃/Si structures by molecular beam epitaxy have been investigated. It has been found from AFM and RHEED observations that, γ-Al₂O₃ films with an atomically smooth surface with an RMS values of ∼3 Å and high crystalline quality can be grown on Si (1 1 1) substrates at substrate temperatures of 650–750°C. Al₂O₃ films grown at higher temperatures above 800°C, did not show good surface morphology due to etching of a Si surface by N₂O gas in the initial growth stage. It has also been found that it is possible to grow high-quality Si layers by the predeposition of Al layer followed by thermal treatment prior to the Si molecular beam epitaxy. Cross-sectional TEM observations have shown that the epitaxial Si had significantly improved crystalline quality and surface morphology when the Al predeposition layer thickness was 10 Å and the thermal treatment temperature was 900°C. The resulting improved crystalline quality of Si films grown on Al₂O₃ is believed to be due to the Al₂O₃ surface modification.     

Microstructure and field emission properties of the Si–TaSi2 eutectic in situ composites by electron beam floating zone melting technique

The directionally solidified Si–TaSi₂ eutectic in situ composites, which have highly aligned and uniformly distributed TaSi₂ fibers embedded in the Si continuous matrix, are obtained by electron beam floating zone melting (EBFZM) technique at the solidification rate range 0.3–9.0 mm/min. The preferential orientation of the Si–TaSi₂ eutectic is also studied by selected area electron diffraction (SAED), which is [0 1¯ 1¯]Si∥[0 0 0 1]TaSi₂ and (0 1¯ 1)Si∥(0 1¯ 1 1)TaSi₂. Moreover, field emission properties of the Si–TaSi₂ eutectic in situ composites are investigated by transparent anode imaging technology. Approximately straight F–N curves show that this material has excellent field emission properties.     

Preparation and the third-order optical nonlinearities of the sodium borosilicate glass doped with Cu7.2S4 quantum dots

The sodium borosilicate glass doped with Cu_7.2S₄ quantum dots was prepared by using both sol–gel and atmosphere control methods. The formation mechanism and the microstructure of the glass were examined using differential thermal analysis and thermal gravimeter (TG-DTA), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), energy dispersive X-ray spectra (EDX), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED). The results revealed that Cu_7.2S₄ quantum dots in orthorhombic crystal system had formed in the glass, and the size ranged from 9 nm to 21 nm. In addition, Z-scan technique was used to measure the third-order optical nonlinearities of the glass. The results indicated that the third-order optical nonlinear refractive index γ, the absorption coefficient β, and the susceptibility χ⁽³⁾ of the glass were 1.11 × 10^? 15 m²/W, 8.91 × 10^? 9 m/W, and 6.91 × 10^? 10 esu, respectively.     

Aqueous alteration of five-oxide silicate glasses: Experimental approach and Monte Carlo modeling

The alteration behavior of glass comprising five oxides (61 ? x)SiO₂–17B₂O₃–18Na₂O–4CaO–xZrO₂ was studied during static leach tests in a buffer solution at 90 °C and with a glass-surface-area-to-solution-volume (SA/V) ratio of 15 cm^?1. The morphological evolution of altered glasses investigated by small-angle X-ray scattering (SAXS) exhibits a strong dependence with the zirconium content in the glass. The experiments were compared with modeling results using Monte Carlo simulation. The model has been improved to simulate the alteration kinetics and alteration layer morphology, considering zirconium atoms at coordination number 6. The simulations exhibit very good agreement with experimental results, showing relations between the alteration rate and the restructuring altered layer. The model is used to interpret experimental observations by proposing a porosity closure mechanism in the altered layer to account for the diminishing alteration rate. For high zirconium concentrations, the simulation highlights the existence of percolation pathways responsible for a complete alteration of the glass. Zirconium has a hardening effect that limits the dissolution of neighboring atoms; this effect is favorable in terms of the glass alteration kinetics, but by inhibiting silicon recondensation it prevents complete closure of the porosity and the glass is completely altered.     

Bulk glass formability for Cu–Hf–Zr–Ag and Cu–Zr–Ag–Si alloys

The effects of the gradual substitution of Zr by Hf on glass formability and thermal stability in the Cu₄₅Zr_45?xHf_xAg₁₀ alloys and the effects of small additions of Si on glass formability in the Cu₄₅Zr₄₅Ag₁₀ alloy are reported and discussed. The samples were prepared as ribbons of thickness in the range 25–200 μm by melt spinning and as conical bulk shapes, with a length of 50 mm and cone base diameters in the range 2–10 mm, by suction die casting. The alloy Cu₄₅Zr₄₅Ag₁₀ had a critical cylindrical rod diameter for glass formation, d_c, of 3.5 mm but substitution of 1.5 and 3.5 at.% Zr by Hf resulted in substantial increases to 5.5 and 4.5 mm, respectively. However, for x in the range 5–40 at.%, d_c was reduced to <1 mm_. The small addition of Si proved to be beneficial to the glass forming ability (GFA), increasing d_c up to 5.5 mm for 0.5 at.% Si. Thus the chemical and atomic size similarities of Hf and Zr do not guarantee that bulk glass formation will be maintained on substituting large proportions of Zr by Hf though small substitutions of Hf and of minor additions of Si were beneficial to the GFA. These effects are discussed in terms of the possible influence of the alloying additions on the liquid structure and on the number density of heterogeneous nucleants.     

Vapor hydration of SON68 glass from 90 °C to 200 °C: A kinetic study and corrosion products investigation

Corrosion of nuclear waste glass in unsaturated conditions is expected to occur upon the closure of the repository galleries during disposal cell saturation in the proposed French disposal site. The objectives of the present work were to determine the alteration kinetics of the SON68 reference in such conditions. Vapor hydration tests were conducted using thin, polished SON68 glass coupons contained in stainless steel autoclaves. Temperatures ranged between 90 °C and 200 °C and the relative humidity (RH) was maintained at 91 ± 1%. Additional experiments at 175 °C and 80, 85, 90 and 95% RH were also conducted to assess the role of RH on the glass corrosion rate. The nature and extent of corrosion have been determined by characterizing the reacted glass surface with scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS). Elemental profiling of the glass hydrated at 90 °C was studied by TOF-SIMS. The chemical composition of the external layer depends on experimental conditions. The hydration rate at 90 °C (TOF-SIMS analysis) is 10 × higher than the generally accepted final rate of SON68 in water at 90 °C (~ 10^? 4 g m^? 2 d^? 1). This may indicate that the glass hydration process cannot be simulated by experiments in aqueous solution with a high S/V ratio. Subsequent leaching (corrosion in an aqueous solution) of samples weathered in water vapor showed dissolution rate values higher than those of pristine glass. This result indicates that mobile elements are trapped within the alteration products during the hydration step and it gives insight into mobility variations of the considered elements.     

Medium-range order in phospho-silicate bioactive glasses: Insights from MAS-NMR spectra,chemical durability experiments and molecular dynamics simulations

The medium-range order of phospho-silicate bioactive glasses (with compositions (2 ? p)SiO₂ · 1Na₂O · 1.1CaO · pP₂O₅, in which p = 0.10, 0.20, 0.26) has been studied by means of a combined-experimental (MAS-NMR, chemical durability measurements) and computational (classical molecular dynamics (MD)) approach. The structural model obtained by MD is showed to be helpful in the interpretation of the NMR spectra. A small amount of Si–O–P link units has been detected in glasses with low P₂O₅-content, but at high P₂O₅ concentration the percentage of Si–O–P bridges becomes important. However, Qⁿ distributions show that the HP5 (p = 0.20) glass structure is less polymerized with respect to the H (p = 0.10) and HP6.5 (p = 0.26) glasses. These results provide useful explanation of the behavior of these glasses in water and highlight the influence of the medium-range order on a very important property of potential bioactive glasses such as the chemical durability.     

Solid-state multinuclear magnetic resonance investigation of Pyrex®

Pyrex^® is an ubiquitous material that has attained widespread use in laboratory glassware and household utensils due to its properties, cost effectiveness, and ease of fabrication. Despite its importance, the structure of borosilicate glass such as Pyrex^® is not fully understood. Here, we report high resolution ²⁹Si, ²⁷Al, and ¹¹B magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopic measurements at 9.4 and 19.5 T, combined with ²⁷Al and ¹¹B multiple-quantum MAS (MQ-MAS), and ¹¹B Rotor Assisted Population Transfer (RAPT), to identify structural features in Pyrex^®. A distribution of Q⁴ silicon and aluminum sites is observed, and there is no evidence of five or six coordinated aluminum. The ²⁷Al quadrupolar coupling constants for the tetrahedral aluminum varied from 3.5 to 4.5 MHz, while the isotropic chemical shifts varied from 50 to 65 ppm. Boron-11 measurements resolve four distinct distributions in boron environments: two identified as tetrahedral, BO₄, and two as trigonal, BO₃. Tetrahedral sites are distinguished by different second coordination spheres, e.g., BO₄(0 B, 4 Si) and BO₄(1 B, 3 Si), and the trigonal sites based on the symmetry of the boron environments.