Structural studies of ThO2 containing barium borosilicate glasses

Barium borosilicate glasses containing as much as 18.6 wt% ThO₂ have been prepared by a conventional melt–quench method and characterized by ²⁹Si and ¹¹B magic angle spinning nuclear magnetic resonance (MAS NMR), Infrared (IR) absorption and differential thermal analysis (DTA) techniques for their structural features. Based on ²⁹Si and ¹¹B MAS NMR and IR investigations, it has been established that the borosilicate network is not affected by such a large ThO₂ incorporation. Whereas Si exists as Q³ and Q² structural units in the ratio of ∼0.7, boron exists in both trigonal (BO₃) and tetrahedral (BO₄) configurations and their relative concentrations are not affected by ThO₂ incorporation in these glasses. The higher extent of ThO₂ incorporation in barium borosilicate glasses compared to borosilicate glasses has been attributed to the increased number of non-bridging oxygen atoms present in the barium borosilicate glasses. Based on DTA measurements it has been shown that there is neither any change in the glass transition temperature nor the occurrence of any crystallization up to 1000 °C, for ThO₂ incorporation up to 18.6 wt% in these glasses. For higher concentrations of ThO₂, there is phase separation during glass formation and fine crystallites of ThO₂ are formed as revealed by XRD. These findings are of significance for the nuclear waste management related to thorium fuel cycle.     

Structure and properties of MoO3-containing zinc borophosphate glasses

ZnO–B₂O₃–P₂O₅ glasses doped with MoO₃ were investigated in the series (100?x)[0.5ZnO–0.1B₂O₃–0.4P₂O₅]–xMoO₃, where bulk glasses were obtained by slow cooling in air within the compositional region of 0 ? x ? 60 mol% MoO₃. The incorporation of MoO₃ into the parent zinc borophosphate glass results in a weakening of bond strength in the structural network, which induces a decrease in chemical durability and glass transition temperature. Raman spectra reflect the incorporation of molybdate groups into the glass network of the studied glasses by the presence of the polarized vibrational band at ≈976 cm^?1 ascribed to the MO_x symmetric stretching vibrations and the depolarized band at ≈878 cm^?1 ascribed to the Mo–O–Mo stretching vibration. The incorporation of molybdate units into the glass network results in the depolymerization of phosphate chains and the formation of P–O–Mo bonds, as reflected in Raman and ³¹P NMR spectra. According to the ¹¹B MAS NMR spectra, tetrahedral B(OP)_4?x(OMo)_x units are formed in the glasses, whereas only a small amount of BO₄ units is converted to BO₃ units in the MoO₃-rich glasses.     

Study of the alkaline environment in mixed alkali compositions by multiple-quantum magic angle nuclear magnetic resonance (MQ–MAS NMR)

45S5 Bioglasses of the composition 46.1 SiO₂–2.6 P₂O₅–26.9 CaO–(24.4 ? x) Na₂O–xMe₂O (Me = Li or K) have been investigated using MAS NMR and MQ–MAS NMR methods. The analysis of the ²⁹Si MAS NMR spectrum revealed two lineshapes whose chemical shift is consistent with two silica Q_n=2,3 species. The ³¹P MAS NMR spectrum reveals the effect of both Na and Ca ions. The chemical shift of the observed ³¹P signal is intermediate between those of Na₃PO₄ (near 10 ppm) and Ca₃(PO₄)₂ (near 3–0 ppm) species. The ²³Na MAS NMR spectra were observed in the alkali oxide composition: 24.4 Na₂O, 12.2 Na₂O–12.2 K₂O and 12.2 Na₂O–12.2 Li₂O. The substitution of Na with Li or K was done to determine the extend of alteration of the glass structure. This goal was best accomplished by ²³Na MQ–MAS NMR. The two-dimensional spectra revealed three sites in the 24.4 mol% Na₂O glass. These sites were not resolved in the 1D MAS NMR spectroscopy. In the mixed glasses, only two sites were obtained.     

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.     

Structural characteristic of Na2OP2O5GeO2 glass systems

Phosphate glasses based on xNa₂O0.5P₂O₅(0.5−x)GeO₂ (0.0 ⩽ x ⩽ 0.5) mol%, were prepared and their structures were characterized by magic angle spinning (MAS) nuclear magnetic resonance (NMR), Raman and IR spectroscopy techniques. It was found that the phosphate network of these glasses is composed of middle (Q²) and branching (Q³) phosphate tetrahedra, whereas germanium part in the network is composed of three- or four-membered GeO₄ tetrahedral rings. It was also found that the germanium tetrahedral are randomly connected to either Q² or Q³ phosphate units in the network. The glass network, especially the Q² units can be modified by the presence of Na ions. This modification is primarily associated with the phosphate. It is found that these glasses behave as if they are formed from a solution of GeO₂ and sodium–phosphate glass with various GeO₄ units and the Q² and Q³ phosphate units randomly distributed in the network.     

Kinetic fragility of hydrous soda-lime-silica glasses

The effect of hydration on the kinetic fragility of soda-lime-silica glasses was investigated by viscometry in the glass transition range. Water-bearing glasses were prepared from industrial float glass (FG) and a ternary model glass (NCS = 16Na₂O 10CaO 74SiO₂ in mol%) by bubbling steam through the melt at 1480 °C and up to 7 bar. Additionally, a sodium borosilicate glass (NBS = 16Na₂O 10B₂O₃ 74SiO₂ in mol%) was hydrated under equal conditions. As detected by infrared spectroscopy water dissolves in the glasses exclusively as OH-groups. The hydration resulted in a total water content C_W up to ≈ 0.2 wt% for FG, NCS and NBS glasses. Kinetic fragility, expressed by the steepness index m, was determined from the temperature dependence of η at the glass transition. Viscosity data from previous studies on hydrous float glasses (C_W > 1 wt%) were surveyed together with literature data on the (H₂O)–Na₂O–CaO–SiO₂, (H₂O)–Na₂O–SiO₂ and (H₂O)–SiO₂ systems to expand the range of water concentration and bulk composition. We could demonstrate that m decreases for all glasses although water is dissolved as OH and should depolymerize the network. An empirical equation of the general type m = a ? b logC_W where a, b are fitting parameters, enables m to be predicted, for each glass series as function of the water content C_W. The enlarged data base shows that the parameter B of the Arrhenius viscosity-temperature relation decreases much stronger than the isokom temperature at the glass transition.     

Structure and properties of potassium niobato-borophosphate glasses

Bulk glasses of the series (1 ? x)[0.5K₂O–0.1B₂O₃–0.4P₂O₅]–xNb₂O₅ with x = 0–45.7 mol% Nb₂O₅ were prepared by slow cooling in air and investigated by Raman, ³¹P, and ¹¹B MAS NMR spectroscopy. The incorporation of Nb₂O₅ into the parent borophosphate glass results in a substantial increase in the glass transition temperature and chemical durability of glasses. Raman spectra showed that Nb atoms form distorted NbO₆ octahedra, which are isolated at low Nb₂O₅ content, whereas at higher Nb₂O₅ content they form clusters. ¹¹B NMR spectra of the glasses revealed the interaction between Nb₂O₅ and BO₄ tetrahedral units, which results in a partial transformation of tetrahedral BO₄ units to trigonal BO₃ units and the formation of mixed B(OP)_4?n(ONb)_n units.     

Effect of minor additions on structure and volatilization loss in simulated nuclear borosilicate glasses

Structural and thermal properties are reported for a range of caesium oxide-containing alkali borosilicate glasses, of the form xCs₂O(100 − x)ZMW (0 < x < 10), where ZMW represents a variety of simulated base-glasses. Glass densities increase and glass transition temperatures decrease with increase in caesium oxide concentration. Mass-loss from the melt is found to depend on composition in the same manner as the fraction of silicon Q³ units, resolved from ²⁹Si MAS NMR, and is related to the presence of danburite medium-range order units, resolved from ¹¹B MAS NMR. Volatilization is shown to occur even in the absence of caesium oxide and the mixed alkali borosilicate composition of the volatile species, evolved from the melt at high temperature, is independent of the starting composition of the glass.     

Catalysis and proton-conduction of novel phosphate glasses

The performance of phosphate glasses as a catalyst for water decomposition and a proton conductor was investigated. Glasses with a composition of 30Na₂O–10BaO–30P₂O₅–(30?x)WO₃–xNb₂O₅ (5 < x < 25) decompose water vapor and generate hydrogen at 500 °C. The best decomposition performance was observed on a specimen with the Nb₂O₅ composition of x = 15. A part of hydrogen produced on the glass surface changes to protons by reducing W⁶⁺ ions and penetrates into the glass. The electron is the dominant charge carrier in the electric conduction of W-rich glasses, whereas proton conduction is predominant in Nb-rich glasses in hydrogen atmosphere. A Raman scattering experiment revealed that Nb contributes to depolymerize the –P–O–P– chains in the phosphate glass producing non-bridging oxygen. A possible model was proposed for the water decomposition and proton conduction processes.     

Quantitation of sulfate solubility in borosilicate glasses using Raman spectroscopy

Raman spectroscopy is used here as an innovative technique to investigate sulfate content in borosilicate glasses. Using Raman spectroscopy after having heated the material, the evolution of sulfate amounts can be followed as a function of temperature, time and chemical composition of the starting matrix. The accuracy of this technique was verified using electron probe micro analysis (EPMA), on two systems of glasses (SiO₂–B₂O₃–Na₂O (SBNa) and SiO₂–B₂O₃–BaO (SBBa)) in order to compare the effect of alkaline or alkaline-earth elements on sulfur speciation and incorporation. To quantitate sulfate content with Raman spectroscopy, the integrated intensity of the sulfate band at 990 cm^?1 was scaled to the sum of the integrated bands between 850 and 1250 cm^?1, bands that are assigned to Qⁿ silica units. Calibration curves were then determined for different samples. The determination of sulfate contents with Raman spectroscopy analysis is possible with an accuracy of approximately 0.1 wt% depending on the composition of the glass. It mainly allows us to follow sulfate removal during the elaboration process and to establish kinetic curves of sulfate release as a function of the viscosity of the borosilicate glass.     

Network structure of multi-component sodium borosilicate glasses by neutron diffraction

Neutron diffraction structure study has been performed on multi-component sodium borosilicate based waste glasses with the composition of (65 − x)SiO_2. · xB₂O₃ · 25Na₂O · 5BaO · 5ZrO₂, x = 5–15 mol%. The maximum momentum transfer of the experimental structure factor was 30 Å⁻¹, which made available to determine the distribution function with high r-space resolution. Reverse Monte Carlo modelling was applied to calculate several partial atomic pair correlation functions, nearest neighbor distances and coordination numbers have been revealed. The characteristic features of Si–O and Si–Si distributions are similar for all glassy samples, suggesting that the Si–O network consisting of tetrahedral SiO₄ units is highly stable even in the multi-component glasses. The B–O correlations proved to be fairly complex, two distinct first neighbor distances are present at 1.40 Å and 1.60 Å, the latter equals the Si–O distance. Coordination number distribution analyzes has revealed 3 and four-coordinated boron atoms. The O–O distribution suggests a network configuration consisting of boron rich and silicon rich regions. Our findings are consistent with a structure model where the boron rich network contains mostly trigonal BO₃ units, and the silicon rich network is formed by a mixed continuous network of ^[4]Si–O–Si^[4] with several different ^[4]B–O–Si^[4] and ^[3]B–O–Si^[4] linkages.     

Structure and properties of Sb2O3-containing zinc borophosphate glasses

ZnO–B₂O₃–P₂O₅ glasses formulated with Sb₂O₃ were investigated in the series 50ZnO–10B₂O₃–40P₂O₅ + xSb₂O₃ (x = 0–70 mol%). With increasing Sb₂O₃ content, the values of glass transition temperature decrease from 492 °C down to 394 °C. The dissolution rate of the glasses reveals a maximum for the glass with x = 15 mol% Sb₂O₃. Raman spectra with increasing Sb₂O₃ content reflect the depolymerisation of phosphate chains. Antimony at low Sb₂O₃ content forms individual SbO₃ pyramids manifested in the Raman spectra by a broad vibrational band at ∼520–690 cm⁻¹. In the glasses with a higher Sb₂O₃ content SbO₃ units link into chains and clusters with Sb–O–Sb bridges manifested in the Raman spectra by a strong broad band at 380–520 cm⁻¹. The ³¹P MAS NMR spectra with increasing Sb₂O₃ content reflect the depolymerisation of phosphate chains at low Sb₂O₃ content and only small changes in the PO₄ coordination at a high Sb₂O₃ content. ¹¹B MAS NMR spectra reveal a steady transformation of B(OP)₄ units into B(OP)_4−x(OSb)_x units, accompanied by the transformation of BO₄ into BO₃ units with increasing Sb₂O₃ content.     

Effects of modifier additions on the thermal properties,chemical durability,oxidation state and structure of iron phosphate glasses

Modified iron phosphate glasses have been prepared with nominal molar compositions [(1?x)·(0.6P₂O₅–0.4Fe₂O₃)]·xR_ySO₄, where x = 0–0.5 in increments of 0.1 and R = Li, Na, K, Mg, Ca, Ba, or Pb and y = 1 or 2. In most cases the vast majority or all of the sulfate volatalizes and quarternary P₂O₅–Fe₂O₃–FeO–R_yO_z glasses or partially crystalline materials are formed. Here we have characterized the structure, thermal properties, chemical durability and redox state of these materials. Raman spectroscopy indicates that increasing modifier oxide additions result in depolymerization of the phosphate network such that the average value of i, the number of bridging oxygens per –(PO₄)– tetrahedron, and expressed as Qⁱ, decreases. Differences have been observed between the structural effects of different modifier types but these are secondary to the amount of modifier added. Alkali additions have little effect on density; slightly increasing T_g and T_d; increasing α and T_liq; and promoting bulk crystallization at temperatures of 600–700 °C. Additions of divalent cations increase density, α, T_g, T_d, T_liq and promote bulk crystallization at temperatures of 700–800 °C. Overall the addition of divalent cations has a less deleterious effect on glass stability than alkali additions. ⁵⁷Fe Mössbauer spectroscopy confirms that iron is present as Fe²⁺ and Fe³⁺ ions which primarily occupy distorted octahedral sites. This is consistent with accepted structural models for iron phosphate glasses. The iron redox ratio, Fe²⁺/ΣFe, has a value of 0.13–0.29 for the glasses studied. The base glass exhibits a very low aqueous leach rate when measured by Product Consistency Test B, a standard durability test for nuclear waste glasses. The addition of high quantities of alkali oxide (30–40 mol% R₂O) to the base glass increases leach rates, but only to levels comparable with those measured for a commercial soda-lime-silica glass and for a surrogate nuclear waste-loaded borosilicate glass. Divalent cation additions decrease aqueous leach rates and large additions (30–50 mol% RO) provide exceptionally low leach rates that are 2–3 orders of magnitude lower than have been measured for the surrogate waste-loaded borosilicate glass. The P₂O₅–Fe₂O₃–FeO–BaO glasses reported here show particular promise as they are ultra-durable, thermally stable, low-melting glasses with a large glass-forming compositional range.     

Photoluminescence and structural studies on Na2O–PbO–SiO2 glasses

PbO_x[(Na₂O)_0.25(SiO₂)_0.75]_1−x glasses (with x = 0.1–0.30) were prepared by a conventional melt-quench method and characterized by UV–visible optical absorption, photoluminescence and ²⁹Si magic angle spinning nuclear magnetic resonance (MAS NMR) studies. It has been confirmed that the increased number of non-bridging oxygen atoms brought about by the increase in Q² structural units of silicon is responsible for the shifting of the wavelength corresponding to the onset of absorption, as PbO concentration increases in the glass. Emission in the visible region (400–450 nm), from these glasses has been attributed to the presence of L-centers, whose the peak maxima and line widths systematically shift to higher values by incorporating Pb²⁺ along with Na⁺ in the network modifying positions. All the Pb²⁺ ions in these glasses occupy only the network modifying positions, as revealed by the identical chemical shift values of Qⁿ structural units of silicon with increase in PbO concentration. In contrast to binary lead silicate glasses, no luminescence has been observed for comparable concentration of PbO, possibly due to the significant quenching of Pb²⁺ ions in the excited state.     

A structural study of sol–gel and melt-quenched phosphate-based glasses

Phosphate-based glasses have recently attracted much interest as a new generation of biomaterials because of their ability to react and dissolve in the physiological environment and eventually to be replaced by regenerated hard or soft tissue. A series of phosphate-based glasses containing 45 mol% P₂O₅ and various amounts of CaO and Na₂O were synthesized by sol–gel and melt-quenching techniques. A comparison between the structure of the sol–gel glass and the structure of the analogous melt-quenched glasses has been undertaken. A broad-based characterization approach combining different techniques has been used to investigate the short-range structure of the glasses and the effect of adding modifier oxides to the network structure (conventional and high energy X-ray diffraction, infra-red spectroscopy, ³¹P solid state magic angle spinning NMR spectroscopy). Sol–gel and melt-quenched glasses appear to have a similar structure, showing similar Qⁿ distributions and atomic correlations.     

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.     

Structure-property correlation in (50 − x)Na2O–50P2O5–xCoCl2 glassy systems

Ternary sodium–cobalt–phosphate glasses of the composition (50 − x)Na₂O–50P₂O₅–xCoCl₂ with x varying between 0 and 15 mol% prepared by melt quenching have been characterized by Fourier transform infrared (FT-IR) spectroscopy and X-ray diffraction (XRD) techniques. Thermal (T_g, T_c) and electrical properties have been investigated. Infrared spectra reveal the formation of metaphosphate glasses (Q² tetrahedral units) with symmetric bridging oxygen (P–O–P) and non-bridging oxygen (P–O⁻). The spectra also indicate the formation of P–O–Co bonds in the metaphosphate glasses that replace P–O⁻–Na⁺ bonds. The results of thermal studies correlate with these FT-IR findings and support the formation of P–O–Co bonds and an increased cross-link density with increasing CoCl₂. This results in enhanced chemical durability and increased T_g and T_c of the glasses. The electrical conductivity parameters upon changing the composition have been correlated with structural changes in the glass matrix.     

Thermal and optical properties of TeO2–ZnO–BaO glasses

We report the results of a systematic study of the thermal and optical properties of a new family of tellurite glasses, TeO₂–ZnO–BaO (TZBa), as a function of the barium oxide mole fraction and compare them with those of TeO₂–ZnO–Na₂O (TZN). The characteristic temperatures of this new glass family (glass transition, T_g, crystallization, T_x, and melting, T_m) increase significantly with BaO content and the glasses are more thermally stable (greater ΔT = T_x ? T_g) than TZN glasses. Relative to these, Raman gain coefficient of the TZBa glasses also increases by approximately 40% as well as the Raman shift from ~ 680 cm^? 1 to ~ 770 cm^? 1. The latter shift is due to the modification of the glass with the creation of non-bridging oxygen ions in the glass network. Raman spectroscopy allows us to monitor the changes in the glass network resulting from the introduction of BaO.     

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.     

Time-resolved fluorescence measurements on Dy3+ and Sm3+ doped glasses

Various fluoride, phosphate and borosilicate glasses with known properties and global structure have been doped with Dy³⁺ (4f⁹) and Sm³⁺ (4f⁵) between 10¹⁸ and 10²¹ cm^?3 and their time resolved fluorescence in the visible range in combination with characteristic physical properties were studied. Different fit procedures were carried out. Although both ions differ in their intrinsic fluorescence lifetime, with 1.5 ms for Dy³⁺ and 6.5 ms for Sm³⁺, their dependence on glass matrix is remarkable similar. Fluoroaluminate glasses with varying phosphate content between 0 and 20 mol% (FPx), a pure phosphate glass (P100), and two borosilicate glasses with low (DURAN^®-like) and high optical basicity (NBS1) were used for investigations. A strongly ionic surrounding by fluorine ligands, as in fluoroaluminate glass samples, provides the longest fluorescence lifetime. It decreases with increasing phosphate content by increasing oxygen surrounding and with increasing RE³⁺ doping. Large differences were detected in the two borosilicate glasses depending on their optical basicity mainly due to differences in the Na₂O/B₂O₃ ratio. Duran-like samples with low Na₂O content have shown phase separation with higher doping concentration. The RE³⁺ ions are accumulated in the borate-rich droplets. Surprisingly only very low concentration-quenching effects were observed. In the opposite of NBS1 samples with high Na₂O content this generated extremely high quenching effect.