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- ItemStudies on viscosity of dried sodium water glasses by extrusion(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2003) Maia, Luciana; Dontal, Patricia Trindade; Rakus, Sigrun; Rüssel, ChristianΑ study on viscosity and flow behavior of dried water glasses using an extrusion method is first reported. The rheological properties of water glass samples with water concentrations in the range from 22.2 to 36.4 wt% and also some glycerin-containing samples were studied. The extrusion method used enables the determination of viscosities in the range from 10^7 to 10^10 dPa s. By comparison to glass melts, relatively low temperatures in the range from 50 to 85 °C are used. During extrusion of dried water glasses, the viscosity decreases significantly with increasing shear rate. Lower water concentrations lead to a more pronounced deviation from Newtonian flow behavior. This is attributed to nonlinear flow behavior rather than to localized viscous heating effects by mechanical deformation work. Furthermore, the effect of composition on the viscosity-temperature dependence is studied. The higher the water concentration, the lower is the viscosity. Glycerin additions intensify this effect. Generally, the viscosity decreases with increasing temperature. Vogel-Fulcher-Tammann parameters are obtained by ftting the data of extrusion experiments and the corresponding Tg values.
- ItemThermal radiation of chromium-doped glass melts(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2001) Gödeke, Dieter; Müller, Matthias; Rüssel, ChristianGlass melts are semitransparent radiators whose emission depends on the type and concentration of colouring ions in the melt. Furthermore the temperature profile in the melt especially in regions near the surface is of major importance. In this study, numerical calculations based on experimentally determined absorption coefficients of chromium-doped melts are presented. The model used is that of an isothermal glass melt layer covered by a further layer with a constant temperature gradient. The effect of temperatures, layer thickness and concentrations on the emission spectra is described. Emission spectroscopy should enable the quantitative determination of colouring ions in the melt as well as of the temperature profile near the melt surface.
- ItemEffect of TiO2 , ZrO2 and ZnO addition on the hydrolytic durability, viscosity and crystallization behaviour of fibre glasses(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2005) Tauch, Diana; Wange, Peter; Rüssel, ChristianGlasses of the System Na2O-K2O-MgO-CaO-BaO-AI2O3-SiO2-B2O3 with additions of TiO2, ZrO2 and ZnO were melted from the raw materials. The glasses were characterized with respect to their hydrolytic durabilities, the working ranges (difference of temperatures attributed to viscosities of 10^2 and 10^4 dPa s), the temperatures attributed to the working point (η = 10^3.5 dPa s), and the crystal growth velocities. In the most cases, the additions led to an increase in viscosity and to a slight decrease in the workability range. The crystal growth velocities decreased with the addition of Ti02 and/or Zr02, while they increased if adding ZnO. The addition of ZnO or of TiO2 and ZrO2 resulted in better chemical durability.
- ItemChemical solubility of phosphate glasses in the system Na2O-CaO-MgO-P2O5-Al2O3-TiO2 in aqueous solutions of different pH values(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2004) Vogel, Jürgen; Wange, Peter; Knoche, Silke; Rüssel, ChristianPhosphate glasses of different composition ranges show very different solubility behaviour in aqueous solutions of pH 4 and 5.8. With inereasing P2O5 concentrations between 38 and 52 mol% , in the system Na2O-CaO-MgO-P2O5-Al2O3-TiO2 , the dissolution rate generally increases, however, the most notable increase is observed at P2O5 concentrations larger than 48 mol%. Additives of TiO2 and AI2O3 increase the stability to corrosion. Glasses with compositions in the metaphosphate range show larger dissolution rates in slightly acidic medium than glasses near the invert glass range. Glasses in the invert glass range show a stronger increase in the dissolution rates with decreasing pH value and also deceleration of the dissolution with time.
- ItemGlasses for the preparation of gradient index lenses in the Na2O-Al2O3-B2O3-SiO2 system - hydrolytic durability, thermal and optical properties(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2004) Hornschuh, Sandra; Rüssel, Christian; Messerschmidt, Bernhard; Possner, Torsten; Possner, UlfGlasses in the system Na2O-Al2O3-B2O3-SiO2 were melted from the raw materials and studied with respect to their chemical durability, their crystal growth velocities, their refractive index and dispersion, their glass transition temperature and their thermal expansion coeffieient. With inereasing Na2O concentration, the crystal growth velocity increases and the chemical durability decreases. Equimolar subtitution of B2O3 for SiO2 results in a decrease in both the chemical durability and in crystal growth velocities. Equimolar subtitution of AI2O3 for SiO2 leads to increasing chemical durability and decreasing crystal growth velocities. Optimum glass compositions to produce gradient index lenses should possess B2O3 concentrations of around 12.5 to 15 mor% and molar AI2O3 concentrations some percent larger than the Na2O concentration. In these glasses, nonbridging oxygen does not occur.
- ItemKinetics of phase separation in a 6.5 Na2O ∙ 33.5 B2O3 ∙ 60 SiO2 glass(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2000) Flügel, Alexander; Rüssel, ChristianIndustrially melted glasses with the composition (in mol%) 6.5 Na2O ∙ 33.5 B2O3 ∙ 60 SiO2 were thermally treated at temperatures in the range of 660 to 750 °C. This resulted in phase separation, i.e. in the formation of a silica- and a sodium borate-rich phase with an interconnected microstructure. Both, the volume content of the borate-rich phase and the mean structure thickness (the correlation length) increased with time as well as with temperature. The volume content approached to a limiting value at constant temperature. The correlation length increased with time according to a power law (~ t^1/n). By contrast to previous studies, n was in the range of 1 to 1.2 within the temperature range and time scale studied. The correlation lengths were much larger (up to 12 µm) and the viscosities much lower than in most previous studies. The kinetic law was explained as controlled by visous flow.
- ItemRedox equilibria of polyvalent elements in binary Na2O · xSiO2 melts(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2000) Gönna, Gordon von der; Rüssel, ChristianGlass melts with the basic compositions (in mol%) of 15 Na2O ∙ 85 SiO2 and Na2O ∙ 2 SiO2 were doped with oxides of various polyvalent elements. At temperatures in the range of 800 to 1550°C, square-wave voltammograms were recorded, which exhibit distinct maxima attributed to respective reduction processes. The following redox steps were observed: Sb5+/Sb3+, Sb3+/Sb0, As5+/As3+, As3+/As0, Cu+/Cu0, V5+/V4+, V4+/V3+, Cr6+/Cr3+, Cr3+/Cr2+, Fe3+/Fe2+ and Ti4+/Ti3+. Peak potentials measured in the Na2O ∙ 2 SiO2 glass depended linearly on temperature, while those of the 15 Na2O ∙ 85 SiO2 melt were affected by the crystallization of cristobalit at temperatures < 1300°C. Redox equilibria in the alkali-rich Na2O ∙ 2 SiO2 melt were generally shifted to the oxidized state by comparison to those measured in the 15 Na2O ∙ 85 SiO2 melt.
- ItemHigh-temperature UV-VIS-NIR spectroscopy of chromium-doped glasses(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2001) Gödeke, Dieter; Müller, Matthias; Rüssel, ChristianChromium-doped glasses with the basic composition (in mol%) 16 Na2O · 10 CaO · 74 SiO2 were melted under different redox conditions. From these glasses, UV-VIS-NIR absorption spectra were recorded at temperatures up to 1200 °C. While the intensity of the peak attributed to Cr6+ decreases, some of the peaks caused by Cr3+ increase in intensity at higher temperature. All peaks are slightly shifted to larger wavelengths and get broader with increasing temperature. Glasses melted under oxidizing conditions were slowly cooled as well as quenched. Using EPR spectroscopy, in the quenched sample, Cr5+ was detected in a larger concentration than in the slowly cooled sample. Otherwise, the Cr6+ concentration was larger in the slowly cooled sample. This is explained by a redox reaction, i.e. a disproportionation of Cr5+ to Cr6+ and Cr3+ during cooling. As shown by high-temperature spectroscopy of the quenched sample, this redox reaction is frozen in below 550 °C.
- ItemPhase separation in a glass melt with the composition 6.5Na2O · 33.5B2O3 · 60SiO2 studied by impedance spectroscopy(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2004) Ravagnani, Christian; Keding, Ralf; Rüssel, ChristianΑ glass with the composition 6.5Na2O · 33.5B2O3 · 60SiO2 was studied using impedance spectroscopy in the temperature range of 380 to 925 °C. In order to simulate the spectra obtained, simple equivalent circuits were used. At temperatures above a temperature, Tχ, below which phase separation occurs, the specific conductivities determined can be fitted to the Arrhenius equation. Below Tχ deviations from Arrhenius behaviour are observed, which are due to the formation of an Na2O and B2O3 rich phase with larger specific conductivity and an SiO2 rich phase with lower specific conductivity. The specific conductivities are smaller than expected from an extrapolation from temperatures > 750 °C. Within the temperature range of 700 to 520 °C, the slope in the Arrhenius plot becomes continuously more negative due to the change in the chemical compositions of the respective phases. At temperatures < 520 °C, the chemical compositions do not further change. Here again a constant slope in the Arrhenius plot is observed until it changes again at Tg.
- Itemstudy of structural modification in extruded and heat-treated lithium disilicate glasses by the method of radiation color centers chronospectroscopy(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1998) Arbuz, Valerii I.; Carl, Gunter; Rüssel, Christian; Durschang, BernhardSamples of extruded and non-extruded lithium disilicate glasses were studied. Extruded glass samples were cut out along and across the extrusion axis of fmished cylindrical rods. For all the samples, spectra of radiation-induced absorption, Δα(E), were measured for various time instants after the cessation of X-irradiation. On the data basis obtained, kinetic dependences. Δα = f(t), were plotted and analyzed. In the diagram "Δα versus lgt", they are represented by straight hnes. Each of them is falling down with its own constant slope a. These a's are rate parameters of the decay of radiation color centers (CCs). They appear to be functions of average distances between recombinating electron and hole CCs. The above α-parameter decreases when passing from the longitudinal cut extruded glass sample to the sample of the non-extruded glass and finally to the transversely cut extruded glass sample. These data mean that, in the course of extrusion, the glass structure becomes less dense in the axial direction of extruded glass rods and more dense in the radial one. Α 4-hour heat treatment at 465 °C (≈5°C above the glass transformation temperature, Tg) eliminated the above anisotropy of radiation properties in extruded glasses and forced their anisometric structures to return to the isometric State characteristic of the non-extruded glass.