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- ItemGraded pore size by ion exchange of phase-separated 6.5 Na2O · 33.5 B2O3 · 60 SiO2 glass(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2000) Flügel, Alexander; Rüssel, ChristianΑ phase-separated glass with the composition (in mol%) of 6.5 Na2O · 33.5 B2O3 · 60 SiO2 was ion-exchanged in molten KNO3 at temperatures in the range of 520 to 660 °C. At temperatures of 640 and 660 °C, the interdiffusion coefficient measured did not depend on the K2O concentration in the glass, while at lower temperatures, an increase in the diffusion coefficient with increasing K2O concentration was observed. The ion exchange and the subsequent heat treatment led to a change in the microstructure. In the bulk of the glass, where no K2O was detected, a fairly coarse microstructure was observed. At the surface, a much finer microstructure occurred. Within the diffusion layer, from the surface to the bulk, a continuous coarsening was observed. When the K2O was subsequently re-exchanged against Na2O in a second step, layers with a gradient in the pore size distribution were obtained, which were mechanically stable after leaching the boron-rich phase.
- 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.
- ItemElectrochemical study on the redox behavior of selenium-containing soda-lime-silica melts(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2001) Rüssel, ChristianDecolorization of white container glasses nowadays is usually achieved by the addition of selenium compounds to the batch. Most of the quantity added, however, is evaporated during melting which is strongly influenced by the redox state of the melt. Therefore, the redox behavior in selenium-doped soda-lime-silica melts was studied by square-wave-voltammetry. The current-potential curves recorded showed two peaks, attributed to the reduction of SeO3 to SeO2 and SeO2 to Se^0. Under reducing conditions, selenium predominantly occurs as Se^0, which gives rise to the pink color. When melted under oxidizing conditions or, however, when the melt is kept at high temperature in air, the cooled glasses are nearly colorless due to the re-oxidation of Se^0 to SeO2. Keeping the melt at high temperatures for up to 50 h did not result in a notable evaporation of selenium compounds. Highest selenium contents were achieved by adding ZnSe to the batch.
- 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.
- 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.
- ItemHigh-temperature spectroscopic study of redox reactions in iron-and arsenic-doped melts(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2003) Schirmer, Henning; Müller, Matthias; Rüssel, ChristianGlasses with the basic composition 16Na2O - 10CaO - 74SiO2 doped with iron or with both iron and arsenic were studied by means of high-temperature UV-VIS-NIR spectroscopy. Increasing temperatures led to a shift of the UV absorption edge caused by Fe3+ -charge transfer bands to larger wavelengths. All other bands, especially the Fe2+ absorption band at around 1100 nm, decreased in intensity at higher temperatures. For glasses, solely doped with iron, the temperature dependency of the extinction coefficient was quantitatively determined. Glasses doped with both arsenic and iron showed a different behaviour: the intensity of the bands decreased up to a temperature of 600 to 650 °C and then increased again. This can be explained by the temperature-dependent redox reaction 2Fe3+ + As3+ ⇌ 2Fe2+ + As5+. Increasing temperatures lead to a shift of the reaction to the right. This reaction is in equilibrium at temperatures > 650 °C and gets frozen in at smaller temperatures, depending on the respective iron and arsenic concentrations. The latter is explained by a numerical simulation assuming the redox reactions to be controlled by diffusion.
- ItemThermodynamics of the Sn2+/Sn4+ equilibrium in alkali-alkaline earthsilicate melts(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2003) Benne, Darja; Rüssel, ChristianMelts with the basic compositions I6R2O - 10CaO - 74SiO2 (R = lithium, sodium, potassium), 20Na2O ∙ xCaO ∙ (80-x)SiO2 (x = 0, 10, 20), 16Na2O - 10MgO - 74SiO2 and 15Na2O - 85SiO2 all doped with 0.25 mol% SnO2 were studied using square-wave voltammetry at temperatures in the range from 1000 to 1600 °C. Over a wide temperature range, the measured peak potentials decreased linearly with temperature. The peak potentials are most negative for the 15Na2O - 85SiO2 melt and least negative for the 16Na2O - 10MgO - 74SiO2 melt. The effect of the glass composition on the Sn2+/Sn4+ redox equilibrium is much less pronounced than in the case of the Fe2+/Fe3+ equilibrium.
- 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.
- ItemTitania, antimony-doped tin oxide and tin-doped indium oxide sol-gel spray coatings(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2001) Löser, Carsten; Rüssel, ChristianTiO2, antimony-doped dn oxide and tin-doped indium oxide coatings were prepared from sols using a spray coating procedure. Coatings prepared from ethanolic sols exhibited high surface roughness. Adding butanediole to the sols, however, resulted in smooth layers. Supposedly, butanediole prevents the evaporation of high quantities of ethanol from the surface of the droplets initially formed during spraying. The electrical conductivity of the layers formed is in the same range as those of dip coatings.
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