Network structure of sodium and potassium borosilicate glass systems

Molar volumes and optical absorption spectra of Ni²⁺ ions were measured for sodium and potassium borosilicate glasses of the compositions xR₂O·B₂O₃·rSiO₂ (0.01 < x < 2.0; r = 1 and 2), where parameters x and r represent respectively the molar ratios R₂O/B₂O₃andSiO₂/B₂O₃. The presence of structural groups was discussed from the results. It was confirmed that addition of an alkali oxide changed BO₃ units to RBO₄ units in the range x < 0.55 for r=1 and x < 0.6 for r=2, while it created ROSiO₃ units in the range $\text{0.55 < x < 0.8}\text{for}\text{r=1}\text{and}\text{0.6}\text{x}\text{?}\text{< 0.9}\text{for}\text{r=2}$. In the range x > 0.8 for r=1 and x > 9 for r=2, ROSiO₃ and ROBO₂ units were created by the alkali addition. Here ROBO₂ units were also formed as a result of self-decomposition of RBO₄ units. The volume of void associated with a BO₄ unit was calculated and compared with the size of sodium and potassium ions. It was shown that sodium ions were small enough to be accommodated in the void whereas potassium ions were too large. This could explain the composition dependence of the molar volume of the sodium and potassium borosilicate glasses in the composition range x < 0.55 for r = 1 and x < 0.6 for r =2.     

Fabrication of a high lithium ion conducting lithium borosilicate glass

A lithium ion conducting glass, Li₂O-B₂O₃-SiO₂, is fabricated by the conventional melt and quenching technique from a mixture of Li₂CO₃, B₂O₃ and SiO₂ powders. It appears that B₂O₃ decreases the crystallization tendency of the Li₂O-SiO₂ binary glass, resulting in an expanded glass forming region in the Li₂O-B₂O₃-SiO₂ ternary glass. The maximum conductivity is 2 × 10^− 6 S cm^− 1 at 25 °C for the 50Li₂O-38B₂O₃-12SiO₂ glass sample. The observed high conductivity is due to the mixed former effect. The conductivity strongly depends on the Li₂O content, but not on K (SiO₂/B₂O₃) in the Li₂O-B₂O₃-SiO₂ ternary glass.     

Effect of temperature on the extraction of Pd by liquid tin from molten borosilicate glass containing simulated radwaste

Separation of Pd from borosilicate glass-melt containing simulated high level radioactive waste (Pd, Mo, Cs, Sr, Zr, Nd, Ni, Fe) using Sn as the solvent metal was studied on lab scale. The separation of Pd was studied using varying amounts of Sn, and the percentage of extraction of Pd and other elements was investigated by varying the process temperature and the starting material for Pd. The process temperature was observed to have a profound influence on the separation of Pd from the glass melt to the metallic tin phase. The % extraction of Pd was found to be less for the batches heat-treated at temperatures below 900 °C. The decrease in the % extraction was due to the entrainment of Pd in the glass phase for the samples heat-treated at low temperatures. The entrainment of Pd leads to the formation of Pd-rose like structures in the glass phase as observed by SEM and EDAX. Similarly, Mo, Fe and Na also formed separate phases inside glass matrix when the samples were heated below 900 °C. However, entrainment was not seen when the samples were heat-treated above 1050 °C in Ar. At 1200 °C, a recovery of about 99% of Pd was obtained. The formation of a PdSn₄ type intermetallic phase was noticed in tin metallic phase. The extraction behavior of Pd in borosilicate glass melt was also studied by using Pd(NO₃)₂ instead of Pd powder as staring material, and the results showed that the process temperature is the key determinant of the extraction behavior of Pd.     

Thermodynamical properties of glass forming systems: A Nuclear Magnetic Resonance analysis

We present a Nuclear Magnetic Resonance (NMR) study of the thermal evolution of the magnetic properties of three different glass formers: glycerol, o-terphenyl and salol. In particular, we analyze how the response of these liquids to the applied magnetic field changes with temperature. We focus on the total magnetization and on the chemical shift of each protonated group. By means of these quantities we account that the dynamics of the glass forming materials, on decreasing the temperature, is dominated by the onset of well defined local inhomogeneities due to precise microscopic cooperative processes. Just these “dynamical heterogeneities” and their energetic topology determine the dynamic crossover from fragile (super Arrhenius) to strong (pure Arrhenius) glass forming behavior. The specific heat changes evaluable from the measured NMR chemical shift associate this phenomenon, and all the related ones, to local configurational changes.     

Oxidation state and local environment of selenium in alkali borosilicate glasses for radioactive waste immobilisation

The oxidation state and local environment of selenium in alkali borosilicate glasses for high level radioactive waste (HLW) immobilisation were studied by Se K-edge XANES and EXAFS spectroscopy. An inactive surrogate of the UK's “MW” HLW-loaded glass and two waste-free glasses were investigated. Results confirm that the predominant Se oxidation state in all air-melted glasses is Se⁴⁺. Low levels (< 10% each of Se⁰ and Se⁶⁺) were also detected in the simulated HLW-loaded glass. The presence of Se⁶⁺ is consistent with moderately oxidising melting conditions arising from decomposition of nitrates in the waste. Results also suggest small but measurable Fe-Se redox interactions. Imposed atmospheres of air, N₂, or H₂ during melting resulted in increasingly reduced average Se valence as expected. Linear combination XANES fitting to quantify the relative abundances of these species was restricted by the limited availability of appropriate XANES standards for reduced Se species. EXAFS of all air-melted glasses provided robust fits indicating Se-O bond lengths of 1.71 ± 0.1 Å and CN = 3 ± 0.3, consistent with Se⁴⁺ being present in SeO₃²⁻ selenite groups. Therefore, ⁷⁹Se in UK alkali borosilicate HLW glasses is expected to occur predominantly as Se⁴⁺ in SeO₃²⁻ selenite groups.     

Apparent activation energy of structural relaxation for Se70Te30 glass

Differential scanning calorimetry and dc conductivity measurements were used to study structural relaxation of Se₇₀Te₃₀ glass. A single set of Tool-Narayanaswamy-Moynihan (TNM) parameters was obtained from the curve-fitting procedure. The value of apparent activation energy Δh∗ was further confirmed by two non-fitting techniques. Results of the Δh∗ evaluation from the T_g dependence on cooling rate are discussed in terms of how the T_f determination might be influenced by the material’s structure type and by the interference of the crystallization process.     

Development of thermally stable tellurite glasses designed for fabrication of microstructured optical fibers

We optimize the composition of tellurite glass for the manufacture of photonic crystal fibers with a large spectrum of transparency. The glasses, synthesized in four and five component (TeO₂-WO₃-Na₂O-Nb₂O₅ and TeO₂-WO₃-PbO-Na₂O-Nb₂O₅) oxide systems with variable contents of WO₃ (5-38 mol%) and PbO (0-18 mol%), are designed and manufactured, and the transmission properties of the obtained glasses for the spectral range of 200 nm-7 μm have been determined. Thermal expansion coefficients and glass characteristic temperatures are determined by the dilatometer and Leitz heat microscope methods. Differential Scanning Calorimetry (DSC) measurements as well as crystallization tests by isothermal heat treatment are used to measure the thermal stability of the glasses and their ability to crystallize. Diffractive X-ray (XRD) measurements are used to determine the crystalline phases of the glass samples and the glasses with the highest resistance to recrystallization during thermal treatment were selected and used for the manufacture of photonic crystal fibers.     

Preparation of new glass systems by the polymeric precursor method for dental applications

Glass ionomer cements (GICs) are largely employed in Dentistry for several applications, such as luting cements for the attachment of crowns, bridges, and orthodontic brackets as well as restorative materials. The development of new glass systems is very important in Dentistry to improve of the mechanical properties and chemical stability. The aim of this study is the preparation of two glass systems containing niobium in their compositions for use as GICs. Glass systems based on the composition SiO₂-Al₂O₃-Nb₂O₅-CaO were prepared by chemical route at 700 °C. The XRD and DTA results confirmed that the prepared materials are glasses. The structures of the obtained glasses were compared to commercial material using FTIR, ²⁷Al and ²⁹Si MAS-NMR. The analysis of FTIR and MAS-NMR spectra indicated that the systems developed and commercial material are formed by SiO₄ and AlO₄ linked tetrahedra. These structures are essential to get the set time control and to have cements. These results encourage further applications of the experimental glasses in the formation of GICs.     

Effects of Nb2O5 Replacement by Er2O3 on elastic and structural properties of 75TeO2-(10 − x)Nb2O5-15ZnO-(x)Er2O3 glass

Tellurite 75TeO₂-(10 − x)Nb₂O₅-15ZnO-(x)Er₂O₃; (x = 0.0-2.5 mol%) glass system with concurrent reduction of Nb₂O₅ and Er₂O₃ addition have been prepared by melt-quenching method. Elastic properties together with structural properties of the glasses were investigated by measuring both longitudinal and shear velocities using the pulse-echo-overlap technique at 5 MHz and Fourier Transform Infrared (FTIR) spectroscopy, respectively. Shear velocity, shear modulus, Young's modulus and Debye temperature were observed to initially decrease at x = 0.5 mol% but remained constant between x = 1.0 mol% to x = 2.0 mol%, before increasing back with Er₂O₃ addition at x = 2.5 mol%. The initial drop in shear velocity and related elastic moduli observed at x = 0.5 mol% were suggested to be due to weakening of glass network rigidity as a result of increase in non-bridging oxygen (NBO) ions as a consequence of Nb₂O₅ reduction. The near constant values of shear velocity, elastic moduli, Debye temperature, hardness and Poisson's ratio between x = 0.5 mol% to x = 2.0 mol% were suggested to be due to competition between bridging oxygen (BO) and NBO ions in the glass network as Er₂O₃ gradually compensated for Nb₂O₅. Further addition of Er₂O₃ (x > 2.0 mol%) seems to further reduce NBO leading to improved rigidity of the glass network causing a large increase of ultrasonic velocity (v_L and v_S) and related elastic moduli at x = 2.5 mol%. FTIR analysis on NbO₆ octahedral, TeO₄ trigonal bipyramid (tbp) and TeO₃ trigonal pyramid (tp) absorption peaks confirmed the initial formation of NBO ions at x = 0.5 mol% followed by NBO/BO competition at x = 0.5-2.0 mol%. Appearance of ZnO₄ tetrahedra and increase in intensity of TeO₄ tbp absorption peaks at x = 2.0 mol% and x = 2.5 mol% indicate increase in formation of BO.