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- ItemGlass-ceramic with preferred orientation of Li2Si205 crystals produced by extrusion below crystallization temperature and subsequent heat treatment(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1994) Durschang, Bernward R.; Carl, Gunter; Rüssel, Christian; Marchetti, Kurt; Roeder, ErwinGlass-ceramics with oriented crystals have been produced from a lithium disilicate glass system by extruding the nucleated glass below the crystallization temperature, and subsequent heat treatment to achieve high-crystalline glass-ceramics. The extruded glass was found to possess a small amount of ahgned, elongated lithium disilicate crystallites and anisotropic stresses. The resulting glassceramic shows a high orientation of the needle-shaped Li₂Si₂O₅ crystals with their c-axis parallel to the extrusion direction. The degree of crystallographic orientation was found to be decreased for samples with an additional heat treatment slightly above Tg, placed between the extrusion process and the crystallization. The crystal alignment of specimens extruded below crystallization temperature is significantly higher than the alignment o f extruded glass-ceramic.
- ItemQuantitative in-situ determination of iron in a soda-lime-silica glass melt with the aid of square-wave voltammetry(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1996) Claußen, Olaf; Rüssel, ChristianSquare-wave voltammetric measurements are fairly convenient to determine thermodynamic data and diffusion coefficients o f polyvalent elements in glass melts. In principle, these methods also allow the quantitative in-situ determination of the total concentrations of polyvalent ions. For the in-situ determination of small quantities, however, improvements of the measuring and analyzing procedures were necessary This was achieved by approximating background currents using polynoms and describing Faradaic currents using theoretically derived equations. Simulations using least Square approximations then allow a much more detailed analysis of the current potential curve. Further improvements were possible by preelectrolyzing the melt at negative potentials and then shifting the potential to zero while recording the square-wave voltammogram. Using both procedures, a quantitative determination of iron concentrations as low a s 0.004 mol% Fe₂O₃ is possible.
- ItemVoltammetry in a sulfur and iron-containing soda-lime-silica glass melt(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1997) Claußen, Olaf; Rüssel, ChristianWith the aid of square-wave voltammetry soda-lime-silica melts with sulfate fining were investigated in order to enable a quantitative in-situ determination of sulfur and iron. In this study, glass melts with low iron and high sulfate contents, typical for technical white glasses, were examined. The current-potential curves are predominantly influenced by sulfur and not solely controlled by diffusion. This behavior is supposedly caused by deposition of a sulfur layer on the surface of the working electrode. However, a simultaneous quantitative in-situ determination of iron and sulfur in melts of white glasses is possible.
- ItemOriented mica glass-ceramic by extrusion and subsequent heat treatment(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1997) Habelitz, Stefan; Carl, Gunter; Rüssel, Christian; Marchetti, Kurt; Roeder, Erwin; Eifler, Dieter; Hergt, RudolphGlass-ceramics with oriented mica crystals have been produced by extruding a fluorophlogopite glass and subsequent therma treatment. During extrusion predominantly the intermediate phase norbergite (Mg₃F₂SiO₄) and besides small quantities of phlogo pite mica crystals (Na/KMg₃F₂Si₃AlO₁₀) crystallized. Heat treatments at temperatures around the maximum crystallization rat resulted in an oriented crystallization of phlogopite as proved by electron microscopy and XRD analysis. The plate-like crystals ar aligned with their basal planes parallel to the direction of extrusion. The degree of orientation was studied by X-ray pole figur measurements. The (003) planes show strong ring-fibre texture. The degree of orientation decreased if the crystallization was realize at lower temperatures and lower crystallization rates. The alignment mechanism is discussed.
- ItemEPR and voltammetric studies of iron-containing mixed alkali glasses with the basic composition xNa₂O · (16-x)Κ₂O * 10CaO * 74SiO₂(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1997) Rüssel, ChristianMixed soda-potassium-lime glasses containing iron oxide in a concentration ränge of 0.1 to 2 m o l % were studied by means of EPR spectroscopy and voltammetry. At low iron concentrations a one-step reduction of Fe³ to Fe² was observed. The values of the Standard reaction enthalpy, ΔH⁰, and of the Standard reaction entropy, ΔS⁰, calculated from the temperature dependence of the Standard potentials show maxima if sodium and potassium concentrations are approximately equal. EPR spectra of all glasses investigated show two paramagnetic signals at g = 2.0 and g = 4.3. The ratio of the peak intensities, I₂.₀ /I₄.₃, increases with the Square of the total iron concentration. Here, the intensity of the g = 4.3 peak increases with increasing Na₂O concentration; the behaviour of the mixed alkali glasses, however, cannot be explained as an additive effect.