Diffusion of the S isotope in soda-lime-silica and sodium trisilicate glass melts

The diffusivity of sulphur in nominal 10 Na₂O-16 CaO-74 SiO₂ (NCS) and 26 Na₂O-74 SiO₂ (NS3) melts was investigated in the temperature range 1273-1473 K using the ³⁵S radioactive isotope in a sandwich setup. Samples were sealed in platinum capsules and run with vertical alignment at 100 MPa confining pressure in an internally heated gas pressure vessel. Using the lowest diffusion coefficient D (m² s^− 1) for each temperature the Arrhenian relations logD = −(4.6 ± 0.3) − (216 ± 7) kJ mol^− 1/RT for NCS and logD = −(6.3 ± 0.6) − (167 ± 17) kJ mol^− 1/RT for NS3 were determined. Viscosity of the melts was used to test the applicability of the Stokes-Einstein (SE) and the Eyring (EY) equations to sulphur diffusion. The SE equation yields unrealistically low radii of the diffusing particles, a consequent of the non-molecular structure of the silicate melts. On the other hand, the Eyring relation yields reasonable jump distances of 550 pm (NCS) and 750 pm (NS3) by fitting the diffusion data to the EY equation. These large values imply that sulphate ions (ionic diameter = 290 pm) migrate as large entities through the silicate network.     

Redox reactions during temperature change in soda-lime–silicate melts doped with copper and iron or copper and manganese

Glasses with the base composition 16Na₂O · 10CaO · 74SiO₂ doped with copper and iron or copper and manganese were studied by high temperature UV–vis–NIR spectroscopy. The spectra exhibited distinct absorption bands attributed to the respective transition metal ions present (Cu²⁺, Fe²⁺, Fe³⁺, Mn³⁺). In glasses doped with only one polyvalent element, the absorption decreases linearly with increasing temperature, the absorption bands are shifted to smaller wave numbers and get broader. In glasses doped with two types of transition metals, the situation is the same up to a temperature of around 550 °C. At larger temperature, the Cu²⁺-absorption in glasses also co-doped with iron increases again, while in glasses doped with both copper and manganese the absorption is approximately the same as in glasses solely doped with copper. It is shown that this is due to redox reactions between polyvalent species. These reactions are frozen in at temperatures <550 °C.     

Surface damage of soda-lime-silica glasses: indentation scratch behavior

Contact mechanics problems are of fundamental interest both to understand the process of surface damage and matter removal in brittle materials, and to develop a method to evaluate their scratch resistance. In order to get insight into these problems in the case of soda-lime-silica glasses, a classical indentation apparatus was used, and an original scratch experimental setup was designed, allowing for a monotonic loading (or unloading) of the indenter combined with a controlled sliding of the specimen beneath the indenter. The influences of the normal load, the moisture level and the glass composition have been studied, and clear relationships were established between the glass compositions and the indentation-scratching behavior. The indentation and scratching characteristics such as the critical-crack-initiation loads and the transition loads between the different scratch regimes were correlated and interpreted in the light of the brittleness index and structural considerations.     

Cation diffusion in soda-lime-silicate glass melts

Cation diffusion was experimentally investigated in soda-lime-silicate glass melts (composition in mol%: 74SiO₂–16Na₂O–10CaO) at temperatures from 1000 to 1200 °C using the diffusion couple technique. One half of each diffusion couple was doped with 11 trace elements (500 ppm by weight of Rb, Cs, Sr, Zn, Cd, Nd, Eu, In, Sn, Ge and 1000 ppm by weight of Fe). Experiments were performed in an internally heated gas pressure vessel at a confining pressure of 100 MPa to avoid convective fluxes in the diffusion samples. The distribution of major elements was analyzed by electron microprobe. IR spectroscopy was used to quantify concentrations of dissolved water in the run products. Trace element diffusion profiles were measured simultaneously employing synchrotron X-ray fluorescence microanalysis. In all analyzed glasses the highest diffusion coefficients were observed for Rb whereas Nd was always the slowest element, e.g. at 1000 °C the diffusivity decreases from (1.51 ± 0.35) × 10⁻¹¹ m²/s for Rb to (1.29 ± 0.34) × 10⁻¹³ m²/s for Nd. The diffusivity of Nd is close to the chemical diffusivity of network former calculated from viscosity data using the Eyring relationship. Surprisingly, the rare earth elements Nd (3+) and Eu (mixed 2+, 3+) diffuse more slowly than the tetravalent Ge. Activation energies for diffusion increase from (132.1 ± 1.5) kJ/mol for Rb to (205 ± 16) kJ/mol for Eu. Based on the diffusion data for Eu, Sr and Nd we estimated that Eu²⁺/Eu_total ratios in soda-lime-silicate glass melts are below 0.04 both at reducing and oxidizing conditions.     

Enhancement in nanohardness of soda-lime-silica glass

For a thin commercial soda-lime-silica glass cover slip a significant increase in nanohardness (e.g. up to 74%) occurred with variations in the loading rate (10-20,000 μN·s^− 1) along with presence of serrations in load-depth plots and deformation band formation inside the nanoindentation cavity. These results are explained in terms of shear stress acting at positions of structural weakness close to the tip of the nanoindenter and the time scale of interaction between the nanoindenter and the local microstructure of the glass.     

Redox behavior of platinum-group metals in nuclear glass

This study highlights the role of two platinum-group metals, ruthenium and palladium, in the redox equilibria of nuclear waste containment glass. Electrochemical measurements in simplified R7/T7 glass melts were used to develop a thermodynamic model of ruthenium redox equilibrium. The oxygen fugacity at equilibrium, corresponding to the coexistence of ruthenium oxide and ruthenium metal dispersed in the molten glass, was measured at different ratios at temperatures ranging from 1000 °C to 1200 °C. Experiments were carried out on glass with and without Pd, revealing the combined role of palladium and tellurium on redox equilibria in the glass. The formation of palladium-tellurium alloys in nuclear glass was observed to result in oxidation of the elements dissolved in the melt.     

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.     

Effect of phase separation on weathering of soda-lime-silica glasses

Accelerated weathering tests on phase-separated and non-separated soda-lime-silica glasses have been made under static conditions of 75% relative humidity at 70°C. Detailed studies of the change of weatherability show that the rate of weathering is reduced if phase separation results in a connected structure of a silica-rich phase, whereas droplets of a silica-rich phase are more liable to weathering. The weathering products were examined by an X-ray wavelength dispersive spectrometer.     

Voltammetric approach to redox behavior of various elements in cathode ray tube glass melts

The redox behavior of various elements (Fe, Sb, Ce, Ti, Zn) was investigated in alkali–alkaline earth-silica CRT (Cathode ray tube: TV panel) model glass melts using the square-wave voltammetry (SWV). The current–potential curve, so called voltammogram, was produced at temperature range of 1000–1400 °C under the scanned potential between 0 and –800 mV at 100 Hz. The Fe, Ti and Zn doped melts exhibited one peak due to reduction reaction. In the case of the Sb doped melts, two traces doubted as peak were found in the voltammogram at low temperature while only one peak existed at high temperature. The peak potential was shifted to the negative direction with decrease of temperature, however, its temperature dependence showed linearity. On the other hand, no peak was found in voltammogram of the melts doped with Ce. Based on the temperature dependence of the peak potential, standard enthalpy (ΔH⁰) and standard entropy (ΔS⁰) for the reduction of Fe³⁺ to Fe²⁺, Sb³⁺ to Sb⁰ and Zn²⁺ to Zn⁰ in CRT model glass melts were calculated.     

Strengthening of soda-lime-silica glass by surface treatment with sol–gel silica

In this study, the failure resistance of soda-lime-silica glass was increased by surface treatment with sol–gel silica. Samples annealed and ion-exchanged in KNO₃ for 24 h at 450 °C were considered. Sol–gel silica coating was carried out by dipping the glass samples into a sol suspension prepared by hydrolysis of Si(OEt)₄ in ethanol/water solution. The deposited layer was consolidated in air for 24 h and subjected to mild thermal treatment at 300 °C for 1 h. The surface treatment increased the fracture resistance of annealed glass of about 35 MPa; conversely, ion-exchanged specimens showed an average increase of about 90 MPa. The strengthening effect induced by the surface treatment was attributed to the reduction of the effective crack length generated by the silica coating. The different strength increase between annealed and ion-exchanged samples is discussed in terms of fracture toughness which, for ion-exchanged glass, is not constant, due to the presence of the surface residual stresses and thus the reduction of the crack length due to the silica coating determines a higher strength increase than for annealed glass.     

Rheological and thermodynamic behavior of calcium aluminosilicate melts within the anorthite–wollastonite–gehlenite compatibility triangle

For melts within the compatibility triangle anorthite–wollastonite–gehlenite (An–Wo–Geh) (CaAl₂Si₂O₈–CaSiO₃–Ca₂Al₂SiO₇), the relationships between the chemical composition, the melt structure, and the rheological and thermodynamic properties are studied for each of the boundary lines, i.e. the An–Wo, Wo–Geh and Geh–An join. The influences of the degree of polymerization of the structural network and the amount of Al–O–Al linkages on the fragility and the configurational entropy of the melts are studied along the boundary lines. For the An–Wo join, the degree of polymerization dominates the responses of both fragility and configurational entropy to compositional variations. For the Wo–Geh and Geh–An joins, the amount of the Al–O–Al linkages dominates those responses. It is found that the depolymerization of the structural network and the formation of Al–O–Al linkages increase the fragility of the melt and the degree of structural disordering.     

Tracer diffusion of 22Na and 45Ca,ionic conduction and viscosity of two standard soda-lime glasses and their undercooled melts

The tracer diffusivities of ⁴⁵Ca in two different high purity standard soda-lime silica glasses have been measured by the radiotracer method below and above their calorimetric glass transition temperatures. Calorimetric glass transition temperatures (T_g) of 845 K and 867 K have been obtained for standard glasses I and II, respectively, using differential scanning calorimetry (DSC) at a heating rate of 20 K/min. In this paper, we focus on the results of ⁴⁵Ca diffusion and conductivity of the two standard soda-lime glasses and compare them with ²²Na diffusivities also obtained in our laboratory [E.M. Tanguep Njiokep, H. Mehrer, Solid State Ionics 177 (2006) 2839, E.M. Tanguep Njiokep, H. Mehrer, Defect Diffus Forum 237–240 (2005) 282]. The ⁴⁵Ca diffusion coefficients obtained are found to follow the Arrhenius law, both below (Tanguep Njiokep and Mehrer, 2006, 2005) and above T_g. In the Arrhenius diagram a change of slope of the ⁴⁵Ca diffusivities appears at 835 K for standard glass I and at 790 K for standard glass II. At the same time, the ionic conductivities display a change in slope at 790 K and 778 K for standard glasses I and II, respectively. These temperatures are somewhat smaller than the calorimetric glass transition temperatures obtained at a heating rate of 20 K/min. Rather, they appear to be close to values of T_g obtained by extrapolation to a vanishing heating rate (Tanguep Njiokep and Mehrer, 2006). The viscosity diffusion of standard glass I is considerably smaller than the conductivity diffusion coefficient and both tracer diffusivities. In both glasses the ionic conductivity is essentially due to the motion of Na ions. The contribution of Ca ions to the conductivity is negligible.     

The optical properties of a soda-lime-silica glass in the region from 0.006 to 22 eV

From the measured absorption and reflection spectra, we have determined the optical properties of a well-characterized (with respect to impurities and homogeneity) high-purity 21.3 wt% Na₂O?5.2 wt% CaO?73.5 wt% SiO₂ glass over the energy range 0.006–22 eV. The origins of the absorption spectra are discussed.     

Copper redox behavior, structure and properties of copper lead borate glasses

Copper oxidation states, structure and properties of xCuO · (50-x)PbO · 50B₂O₃ glasses were investigated. Both infrared (IR) and ¹¹B magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopies were employed to determine the tetrahedral BO₄ fraction in the glasses as a function of CuO content. IR study indicates that the replacement of Pb²⁺ by Cu²⁺ ions increases the BO₃ units by converting BO₄- containing groups into ring type metaborate groups. The oxidation states of copper ions in the glasses have been studied using both X-ray photoelectron spectroscopy (XPS) and the wet chemical method. For high CuO containing (?30 mol%) glasses, high Cu⁺ ion concentrations (Cu⁺/Cu_tot.>0.3) result in a relatively slow disproportionation of B⁴-containing groups because of the small coordination number of Cu⁺ compared to Cu²⁺ ions. Effects of both glass structure and redox states of copper ions on glass properties including density, Vickers’ hardness, coefficient of thermal expansion, and chemical durability have been discussed.     

Composition and polyamorphism in supercooled yttria-alumina melts

By extending recent work on liquid-liquid transitions in supercooled yttria-alumina AYx liquids we draw attention to the compositional dependence of the structure factor of the high density liquid, arguing that this is sufficiently sensitive to discriminate between liquids at the level of a few %. Comparing structure factor differences between liquids of different compositions and in the same liquid AY20 between high and low temperatures straddling the transition at 1788 K between a high density liquid (HDL) and a low density liquid (LDL) enables compositional phase separation to be ruled out. It points instead to kinetic changes in polyhedral configurational order being the drivers for this polyamorphic transformation. Rotor behaviour observed in levitated liquid drops used in the high temperature experiments enables the reversibility of the LLT transition (LLT) and the associated changes in entropy and density to be identified. Evidence for critical-like behaviour in the structural relaxation time and in the fluctuation correlation length is presented. By re-examining recent work which failed to find the structural and thermal signatures for the LLT in liquid AY20 at 1788 K we present evidence for the LLT occurring instead in liquid AY15 at 1940 K, suggesting that the liquid-liquid transition temperature in AYx liquids decreases with increasing yttria content.     

Convection in melts and impurity distribution in semiconductor crystals

Impurity distributions in semiconductor melts and crystals grown from these melts are experimentally and numerically studied on an example of Ga-doped Ge crystals. It is shown that inhomogeneous dopant distribution is observed in the form of striations and is caused by the convective flows in the melt and their nonstationary rearrangement in the vicinity of the crystallization front. The character of heat and mass transfer under the microgravity conditions is predicted. The necessity of precision experiments under terrestrial and, especially, space conditions is emphasized.