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- ItemEffect of additives on the structure of SiO2 sol-gel spray coatings(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2000) Löser, Carsten; Rüssel, ChristianSiO2 coatings were prepared from alkoxide sols by means of spray coating. Coatings prepared from sols solely containing ethanol as solvent showed heterogeneous structures. This is supposedly due to solvent evaporation during spray coating. When reaching the substrate, the droplets possess too high viscosity to enable them to disperse on the substrate. However, when additives, such as 1.3-butanediol, ethylene glycol or glycerol, with high boiling temperatures were used, smooth coatings exhibiting small surface roughness were obtained. It is shown that best results were obtained when 1.3-butanediol was used. With increasing additive concentration, the surface roughness decreased.
- 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.
- 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.
- 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.
- 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.
- 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.
- ItemHigh-temperature UV-VIS-NIR absorption and emission spectroscopy of soda-lime-silica glasses doped with Nd2O3(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2003) Gödeke, Dieter; Müller, Matthias; Rüssel, ChristianAbsorption spectra were recorded from a glass with the basic composition 16Na2O ∙ 10CaO ∙ 74SiO2 doped with 4 wt% Nd2O3 at temperatures in the range from 25 to 1400°C. The effective width of the observed absorption peaks increased with increasing temperature, while the peak positions remained nearly constant. Some absorption coefficients decreased with temperature while that at a wavelength of 657 nm increased notably. Emission spectra were recorded from glass melts exhibiting a temperature gradient at the surface (cold surface). The spectra showed distinguished minima at those wavelengths where the absorption maxima occurred. Numerical simulation of the emission spectra assuming a constant temperature gradient at the surface is in agreement with the experimental spectra.
- ItemCrystallization and vicosity of phosphate melts in the system Na2O-CaO-MgO-P2O5-Al2O3-TiO2(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2004) Wange, Peter; Vogel, Jürgen; Knoche, Silke; Rüssel, ChristianGlasses in the system Na2O-CaO-MgO-P2O5-Al2O3-TiO2 were studied with respeet to their crystallization and viscosity. Glasses in the metaphosphate range are composed of phosphate chains, and possess a comparably small tendeney to crystallization. Here, AlPO4 in a cristobalite-like modification and Ca2P2O7 are formed. Glasses in the intermediate range between metaphosphate and invert glass structures are composed of smaller phosphate units and show higher erystal growth velocities as well as higher viscosities. Additions of MgO, AI2O3 and TiO2 result in a strengthening of the glass network, and hence, inereasing viscosities and a notably smaller tendeney to crystallization.
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