The effect of fluoride on the Fe2+/Fe3+-redox equilibrium in glass melts in the system Na2O/NaF/CaO/Al2O3/SiO2

Liquids with the base compositions (16 − x/2)Na₂O · xNaF · 10CaO · 74SiO₂ (x = 0, 1, 3, and 4) and (10 − x/2) · Na₂O · xNaF · 10CaO · yAl₂O₃ · (80 − y)SiO₂ (x = 0, 1, 3, 5 and y = 5 and 15) doped with 0.25 mol% Fe₂O₃ were studied by means of square-wave voltammetry in the temperature range from 1000 to 1500 °C. With increasing temperature, the redox equilibria were shifted to the reduced state. Also while increasing the alumina concentration, the Fe²⁺/Fe³⁺-redox equilibrium is shifted to the reduced state. In the soda-lime–silica melt the addition of fluoride shifts the equilibrium to the oxidized state, while in the aluminosilicate melts with 15 mol% Al₂O₃, the equilibrium is shifted to the reduced state. In the aluminosilicate melts with 5 mol% Al₂O₃, the equilibrium was not affected by the fluoride concentration. This is explained by the structure of the respective glass compositions.     

Continuous compositional changes of crystal and liquid during crystallization of a sodium calcium silicate glass

This paper deals with a systematic study of crystal nucleation and growth kinetics in a 14.6Na₂O–34.0CaO–51.4SiO₂ mol% glass, which is close to the CaO · SiO₂–Na₂O · SiO₂ pseudo-binary section, just left of the stoichiometric Na₂O · 2CaO · 3SiO₂ (N₁C₂S₃) compound. We show that crystallization begins with nucleation of a Na_4+2xCa_4−x[Si₆O₁₈] (0 < x < 1) solid solution that is enriched in sodium as compared with both parent glass and the N₁C₂S₃ compound; while a fully crystallized sample is composed only by a solid solution that is stable at very high temperatures, but is metastable in the temperatures under investigation. We thus confirm a continuous compositional change of the crystals during the course of crystallization.     

Redox behavior and diffusion of copper in soda-lime-silica melts

Glasses with the base mol% composition xNa₂O · 10CaO · (90 ? x)SiO₂ with x = 10, 16, 20 and 26 were investigated at high temperatures using square-wave voltammetry. The recorded voltammograms exhibit two peaks. That at less negative or (depending on temperature and glass composition) even positive potential is attributed to the reduction of Cu²⁺ to Cu⁺, while that observed at more negative potential is caused by the reduction of Cu⁺ to metallic copper. For both redox steps, the peak potentials decrease linearly with temperature. Those of the composition with 10 mol% Na₂O show the most negative values. The diffusion coefficients can be fitted to Arrhenius equation. If referenced to the same viscosity, the diffusion coefficients decrease with increasing Na₂O-concentration. The effect of composition on the thermodynamics as well as on diffusivities is explained by the incorporation of the copper ions into the melt structure.     

Medium range order in sodium silicate glasses: role of the network modifier

The effect of different concentrations of the network modifier Na₂O on the position of the Boson peak, observed in the low frequency Raman scattering experiments, is analyzed. Glasses containing 25% Na₂O (Na₂O · 4SiO₂) and 33% Na₂O (Na₂O · 2SiO₂) are compared with pure SiO₂ glass. The observed shift of the Boson peak is interpreted in terms of a change of a correlation length, ℓ, corresponding to some medium range order. This length, ℓ, decreases when the amount of sodium oxide increases.     

Spectral studies of Sm3+ and Dy3+ doped lithium cesium mixed alkali borate glasses

The effect of host glass composition on the optical absorption and fluorescence spectra of Sm³⁺ and Dy³⁺ has been studied in mixed alkali borate glasses of the type 67B₂O₃ · xLi₂O · (32 − x)Cs₂O (x = 8, 12, 16, 20 and 24). The Judd–Ofelt intensity parameters (Ω₂, Ω₄ and Ω₆) are calculated. The radiative transition probabilities (A), radiative lifetimes (τ_R), branching ratios (β) and integrated absorption cross-sections (Σ) are computed for certain excited states of Sm³⁺ and Dy³⁺ ions for different x values in the glass matrix. Stimulated emission cross-sections (σ_p) are obtained for certain emission transitions of two ions in these mixed alkali borate glasses. These parameters are compared for different x values in the glass matrix. Variation of these parameters with x in the glass matrix has been studied.     

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.     

Chemical durability and weathering resistance of canasite based glass and glass-ceramics

Classic composition 8.4Na₂O·5K₂O·10.8CaO·64SiO₂·10.5CaF₂·1.3Al₂O₃ (G1/GC1) and high silicon composition 7.6Na₂O·4K₂O·8.4CaO·71SiO₂·8CaF₂·1.0Al₂O₃ (G2/GC2) canasite-based glass and glass-ceramics were prepared, and the chemical durability and weathering of samples were studied with XRD, ICP-AES, SEM and optical microscopy. Interestingly, a kind of color fringe pattern caused by the acid leaching was directly observed on the glass ceramic surface under optical microscopy. The 20 day weight losses of glass and glass ceramic in acid (1 M HCl) and alkali (1 M NaOH/1 M Na₂CO₃) solution were measured. Accelerated weathering was used to demonstrate that increasing silicon content contributes to the weathering performance of glass and glass-ceramics. For different micro-structures and compositions, the weight loss of each glass and glass-ceramic is quite different. In general, through increasing the network interconnectivity of residual glass network and suppressing the crystallization of the less durable canasite phase, the addition of SiO₂ (from 60 mol% to 71 mol%) enhanced the chemical durability of canasite-based glass and glass ceramic relatively under acid, alkali and weathering conditions.     

Optical phonons and Raman scattering in ternary II–VI spheroidal nanocrystals embedded in a glass matrix

Theoretical and experimental studies of the spatial phonon confinement in ternary CdS_xSe_1−x nanocrystals embedded in a glass matrix formed by the composites (40)SiO₂₋(30)Na₂CO₃–(29)B₂O₃–(1)Al₂O₃ (mol%) + [(2)CdO + (2)S + (2)Se] (wt%) were carried out. From the analysis of the surface phonon modes, the theoretical procedure has allowed the determination of the geometrical characteristics of the nanocrystals. The calculated frequencies were compared with the experimental values obtained from the Raman spectra of CdS_xSe_1−x nanocrystals grown under different thermal treatments. A good correlation between the experimental and calculated CdS-like and CdSe-like surface optical modes was observed. The Raman selection rules and their connection with the nature of the surface optical phonons is discussed in order to use Raman spectroscopy as a probe to determine the composition x and the geometrical shape of the semiconductor nanocrystals.     

Structure–property correlations in lead borate and borosilicate glasses doped with aluminum oxide

Glass samples from four systems: xPbO–(100?x)B₂O₃ (x = 30, 40, 50 and 60 mol%), 50PbO–yAl₂O₃–(50?y)B₂O₃ (y = 2, 4, 6, 8 mol%), 50PbO–ySiO₂–(50?y)B₂O₃ (y = 5, 10, 20, 30 mol%) and 50PbO–5SiO₂–yAl₂O₃–(45?y)B₂O₃ (y = 2, 4, 6, 8 mol%) were prepared by a melt-quench technique. Characterization of these systems was carried out using density measurements, UV–visible spectroscopy, differential scanning calorimetry (DSC), and ¹¹B and ²⁷Al magic-angle spinning (MAS) solid-state nuclear magnetic resonance (NMR). Our studies reveal an increase in glass density with increasing lead(II) oxide concentration in pure lead borates and also with addition of silica into 50PbO–50B₂O₃ glass. ¹¹B MAS NMR measurements determine that the fraction of tetrahedral borons (N₄) reaches a maximum for the glass containing 50 mol% of PbO in the PbO–B₂O₃ glass series and that N₄ is sharply reduced upon adding small amounts of Al₂O₃ into lead borate and lead borosilicate systems. ²⁷Al MAS NMR experiments performed on glasses doped with aluminum oxide show that the Al³⁺ are tetra-, penta- and hexa-coordinated with oxygen, even without any excess concentration of Al³⁺ over charge-balancing Pb²⁺ cations. ^[5]Al and ^[6]Al concentrations are found to have unusually high values of up to 30%. The results of UV–visible absorption spectroscopy, DSC and density measurements support the conclusions drawn from the NMR studies, providing a consistent picture of structure–property relations in these glass systems.     

Electrooptical Kerr phenomenon and Raman spectroscopy of one lithium–niobium–silicate glass-forming system

Raman spectra and electrooptical Kerr coefficients of glasses belonging to one lithium–niobate–silicate glass-forming system xNb₂O₅ · (66 ? x)SiO₂ · 19Li₂O · 11K₂O · 2B₂O₃ · 2CdO are studied. It has been found that these glasses demonstrate a record value of electrooptical Kerr coefficient; the glass with x = 35 showed electrooptical Kerr coefficient equal to 266 × 10^?16 m/V². Using Raman spectroscopy combined with the concept of Constant Stoichiometric Groupings, a correlation of electrooptical Kerr coefficients of these glasses with the content of Li₂O · Nb₂O₅ (or 2LiNbO₃) groupings has been demonstrated. The hypothesis that electrooptical Kerr sensitivity of glasses is related to the ordered regions with composition and symmetry corresponding to some of known electrooptical crystals has been verified. These regions, which the authors called ‘Crystal Motifs’, are identified with the groupings found in studying Raman spectra of the glasses.     

Structure and crystallization behavior of mixed potassium-lithium niobiosilicate glasses

Potassium-lithium niobiosilicate (KLiNS) glasses with a composition of (27 ? x)K₂O · xLi₂O · 27Nb₂O₅ · 46SiO₂ (x = 0, 3, 12 and 20) have been synthesized by a melt-quenching method. The glass structure and devitrification behavior have been studied by Raman spectroscopy, DTA, and XRD. By increasing the lithium content, less distorted niobium octahedra increase, indicating a niobium clustering. This change strongly affects the crystallization behavior. In the glasses x = 0 and x = 3, just above T_g, only nanocrystals of an unidentified phase are formed, while for x = 12 and x = 20 potassium lithium niobate (KLN) solid solutions with tetragonal tungsten–bronze structure crystallize by bulk nucleation. In these glasses, LiNbO₃ crystallizes at higher temperature by surface nuclei. Ultimately, it is possible to produce nanostructured glasses based on KLN nanocrystals, by partial replacement of K by Li.     

Phosphate speciation in sodium borosilicate glasses studied by nuclear magnetic resonance

The structure of RNa₂O · B₂O₃ · KSiO₂ · xP₂O₅ (0.5 < R < 2; 0.86 < K < 3) borosilicate glasses has been studied by nuclear magnetic resonance (NMR). ³¹P magic angle spinning (MAS), double quantum-magic angle spinning (DQ-MAS) and ³¹P–¹¹B transfer of populations under double resonance magic angle spinning (TRAPDOR MAS) NMR were used to determine the phosphate speciation in the glasses and their connectivity with the borosilicate network. The structure of the glass network was characterized with ¹¹B, ²⁹Si and ²³Na MAS NMR. Ab initio calculations of the ³¹P chemical shielding were carried out in order to confirm the connectivity between phosphorus and the structural units of the borosilicate glass network. Na₃PO₄ (monophosphate), Na₄P₂O₇ (diphosphate) and P–O–B species (mono- and diphosphate groups with borate units as the next nearest neighbors) are found all along the compositional range studied. The proportion of the P–O–B groups increases as the glass optical basicity decreases, while the proportions of mono- and diphosphate species decrease. The change in the glass transition temperature of the phospho-borosilicate glasses with respect to that of the borosilicate ones is discussed in terms of the structural characterization. The formation of phosphate species gives rise to the increase in T_g, which is attributed to the re-polymerization of the silicate network, while the formation of P–O–B bonds weakens the glass network and produces a decrease in the glass transition temperature.     

Bond character,optical properties and ionic conductivity of Li2O/B2O3/SiO2/Al2O3 glass: Effect of structural substitution of Li2O for LiCl

A glass of composition (20 ? x)Li₂O–xLiCl–65B₂O₃–10SiO₂–5Al₂O₃ where 0 ? x ? 12.5 wt% is prepared using the normal melt-quenching technique. The optical constants and electrical conductivity and their correlation are investigated, furnished and discussed with the substitution of Li₂O for LiCl. The mechanism of the optical absorption and the calculated Urbach energy follow the rule of phonon-assisted transitions. The ionic conduction mechanism is determined by activation energy process. Substitution up to 10 wt% LiCl provides high ionic conductivity (1.9 × 10^?2 Ω^?1 m^?1) due to the high average electronegativity of LiCl which increases the polarizability of lithium ions. The small cation–anion distance approach confirmed the enhancement in ionic conductivity of LiCl containing glass compared to that of Li₂O. Due to the large size of Cl^? ions, there is an expansion of the lattice which in turn broadens the available path windows. For 12.5 wt% LiCl, anomalous density behavior is observed and a reduction in conductivity is occurred, σ = 5.4 × 10^?3 Ω^?1 m^?1. Owing to the model of bond fluctuation, the reduction is attributed to the increase in the alkali halide concentration which creates bottlenecks that hinder the motion of Li⁺ ions. The ionic conductivity character is strongly supported by the behavior of the glass ionicity factor, density, molar volume, refractive index, average boron–boron separation, molar refraction, metallization criterion and non-bridging oxygen concentration of the studied glass.     

Electrical conductivity of silicate glasses with tetravalent cations substituting Si

The effects of substituting Si by M^4 + cations in soda-lime silica glasses were analyzed by impedance spectroscopy in the frequency range of 1 Hz–1 MHz. The glass composition was (mol%) 22Na₂O·8CaO·65SiO₂·5MO₂, M = Si, Ti, Ge, Zr, Sn, and Ce. Although the Na⁺ concentration in the glasses is constant, the Zr-containing glass exhibits the highest dc conductivity and the lowest activation energy, while the Ce-containing glass exhibits the lowest conductivity. The activation energies obtained experimentally agree with those obtained by a theoretical equation proposed by Anderson and Stuart. The differences in electrical conductivity presented by the several M-containing glasses are attributed to the effect that the M^4 + ion has on the mobility of the diffusing Na⁺ ion.     

Properties and structure of (1 − x)Li2O–xNa2O–Al2O3–4SiO2 glass systems

(1 − x)Li₂O–xNa₂O–Al₂O₃–4SiO₂ glasses were studied for the progressive percentage substitution of Na₂O for Li₂O at the constant mole of Al₂O₃ and SiO₂. The crystallization temperature at the exothermic peak increased from 898 to 939 °C when the Na₂O content increases from 0 to 0.6 mol. The coefficient of thermal expansion and density of these as-quenched glasses increase from 6.54 × 10⁻⁶ °C⁻¹ to 10.1 × 10⁻⁶ °C⁻¹ and 2.378 g cm⁻³ to 2.533 g cm⁻³ when the Na₂O content increases from 0 to 0.4 mol, respectively. The electrical resistivity has a maximum value at Na₂O · (Li₂O + Na₂O)⁻¹ = 0.4. The activation energy of crystallization decreases from 444 to 284 kJ mol⁻¹ when the Na₂O content increased from 0 to 0.4 mol. Moreover, the activation energy increases from 284 kJ mol⁻¹ to 446 kJ mol⁻¹ when the Na₂O content increased from 0.4 to 0.6 mol. The FT-IR spectra show that the symmetric stretching mode of the SiO₄ tetrahedra (1035–1054 cm⁻¹) and AlO₄ octahedra (713–763 cm⁻¹) exhibiting that the network structure is built by SiO₄ tetrahedra and AlO₄.     

Soda lime zirconia silicate glasses as prospective hosts for zirconia-containing radioactive wastes

Na₂O–CaO–ZrO₂–SiO₂ glass compositions with ZrO₂ contents of up to 20 mol% were melted. Up to 12.3 mol% ZrO₂ could be dissolved into the glasses. Melting temperatures ⩾1450 °C were required to remove seed and produce a melt that could be cast. Addition of ZrO₂ caused an increase in the glass transition and crystallization temperatures. Glasses crystallized at temperatures ⩾1050 °C with Keldyshite and Parakeldyshite (Na₂O · ZrO₂ · 2SiO₂) as the crystalline phases. Addition of up to 4.6 mol% ZrO₂ caused an increase in the hydrolytic resistance of the glass, with further additions having little effect. The suitability of these glasses as hosts for ZrO₂-containing radioactive wastes is discussed.     

Silver release from hydroxyapatite self-assembling calcium–phosphate glasses

The in vitro behavior of xAg₂O (100 ? x)[50P₂O₅ · 30CaO · 20Na₂O] glasses (0.14 ? x ? 20 mol%) is investigated in simulated body fluid (SBF) mainly with respect to bioactivity and silver ions release. In order to estimate the biodegradability and bioactivity, the samples were soaked in SBF, which has almost equal ions concentration to those of human blood plasma, and kept at 37 °C for fixed periods of time up to 18 days. After the fixed periods of time analyses were performed on the SBF solutions. Calcium and silver ions concentration of SBF after different soaking times of the glass samples were primarily examined. Conductivity data support the assumption that the released silver ions are reduced in SBF and their release is obstructed by growth of the bioactive layer on the glass surface. X-ray diffraction and infrared analysis attest the development on glass surface of a hydroxyapatite type layer.     

Influence of europium ions on structure and crystallization properties of bismuth borate glasses and glass ceramics

Glasses of the xEu₂O₃ · (100?x)[2Bi₂O₃ · B₂O₃] system with 0 ? x ? 25 mol% have been characterized by X-ray diffraction and FTIR spectroscopy measurements. Melting at 1100 °C and the rapid cooling at room temperature permitted us to obtain glass samples. In order to improve the local order and to develop crystalline phases, the glass samples were kept at 625 °C for 24 h. After heat treatment two crystalline phases were put into evidence. One of the crystalline phases was observed for the host glass matrix, the x = 0 mol% sample, and belongs to the cubic system. The second one was observed for the x = 25 mol% sample and was find to be orthorhombic with two unit cell parameters very close to each other. For the samples with 0 < x < 25 mol% there is a mixture of the two mentioned phases. FTIR spectroscopy data suggest that both Bi₂O₃ and B₂O₃ play the glass network former role while the europium ions play the network modifier role in the studied glasses.     

Sodium tracer diffusion in sodium boroaluminosilicate glasses

Sodium tracer diffusion coefficients, D*_Na, have been measured using the radioactive isotope Na-22 in sodium boroaluminosilicate (NBAS) glasses containing either a small amount of As₂O₃ or Fe₂O₃. The chemical compositions of the first type of glasses are given by the formula [(Na₂O)_0.71(Fe₂O₃)_0.05(B₂O₃)_0.24]_0.2[(SiO₂)_x(Al₂O₃)_1 ? x]_0.8 and those of the second type of glasses correspond to the formula [(Na₂O)_0.73(B₂O₃)_0.24(As₂O₃)_0.03]_0.18[(SiO₂)_x(Al₂O₃)_1 ? x]_0.82. Tracer diffusion measurements were performed at different temperatures between 198 and 350 °C. Pre-annealing of the glass samples at their glass transition temperatures in common air was found to lead to changes in the values of sodium tracer diffusion coefficients. For the NBAS glasses containing Fe₂O₃, after pre-annealing for 5 h, the activation enthalpy derived for the sodium tracer diffusion increases almost linearly from 57.5 to 71.3 kJ/mol with a decrease in the alumina content while the pre-exponential factor of the sodium tracer diffusion coefficient increases from 2.1 · 10^? 4 to 5.3 · 10^? 4 cm²/s. For the iron-free NBAS glasses pre-annealed for 5 h, the activation enthalpy varies between 63.9 and 71.4 kJ/mol while the pre-exponential factor varies between 1.5 · 10^? 4 and 1.2 · 10^? 3 cm²/s. In addition, it was observed that the considered glasses take up water when annealed at 300 °C in wet air with P_H2O = 474 mbar.     

Preparation and ionic conductivity of transparent glass−ceramics containing a large quantity of lithium−mica

In order to crystallize a large quantity of the lithium?mica in glass?ceramics, 5.1 mass% MgF₂ was added to the starting materials of the parent glasses having chemical compositions of Li_(1+x)Mg₃AlSi_3(1+x)O_10+6.5xF₂ (x = 0.5 and 1.0). Transparent glass?ceramics, in which a large quantity of lithium?mica with particle size of <50 nm was separated, could be prepared from the MgF₂-added parent glass with x = 0.5. While the parent glass, which had a binodal phase separation structure, did not exhibit electrical conductivity, the transparent glass–ceramic was given conductivity by the formation of an interlocking structure of mica. As the separated mica formed a tighter interlocking structure, the conductivity increased and reached a value of 2.0 × 10^?3 S/cm at 600 °C. The MgF₂-added parent glass with x = 1.0 was not transparent because of coarse spinodal phase separation. The conductivity was 4.3 × 10^?4 S/cm at 600 °C but was significantly decreased by the separation of mica.