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- 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.
- ItemElectric melting of glass: Influence of cathodic currents on the formation of protective layers on molybdenum electrodes(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1998) Rüssel, Christian; Kämpfer, AndreaThe influence of cathodic currents on the corrosion of molybdenum electrodes during electrical melting of glass was studied with the aid of laboratory experiments. It is shown that cathodic currents lead to the formation of molybdenum silicide layers on the electrode. Best results were obtained using a DC current density of 3.75 mA/cm² at a heating current density of 1 A/cm². Higher DC current densities resulted in molybdenum silicide layers, which dissolve in the melt as silicide particles. Simultaneous to the molybdenum silicide layers, at the counter electrodes MoO₂ layers are formed, which also dissolve as particles in the melt.
- 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.
- 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.
- ItemInfluence of process parameters on the formation of protective MoSiₓ layers on molybdenum electrodes during electric melting of glass(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1998) Rüssel, Christian; Kämpfer, AndreaWith the aid of laboratory experiments, the cathodic passivation of molybdenum electrodes during electrical melting of glass was studied. The formation of molybdenum silicide layers does not only depend on the cathodic passivation currents, but also on time and the AC heating current density If the melt has freshly been prepared from raw materials, the molybdenum silicide layer is thinner than under otherwise same conditions. While at an AC heating current density of 1 A/cm², a cathodic current density of 3.75 mA/cm² led to Optimum layer formation, at a heating current density of 2 A/cm², higher cathodic current densities are required.
- ItemRedox behaviour of polyvalent ions in phosphate glass melts and phosphate glasses(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1998) Matthai, Annegret; Doris, Ehrt; Rüssel, ChristianGlass melts and glasses with the basie composition NaPO₃ · 2Sr(PO₃)₂ doped with V₂O₅ and Fe₂O₃ were studied with the aid of square-wave voltammetry and spectroscopic methods. From the square-wave voltammetry in the glass melt, a linear dependence of the peak potentials on the temperature was obtained. The Standard enthalpy ΔH⁰, the Standard entropy ΔS⁰ and lg(C_ox/C_red) of the transitions Fe³⁺/Fe²⁺ and V⁴⁺/V³⁺ were calculated. The diffusion coefficients calculated from the peak currents measured follow Arrhenius' law. By means of electron spin resonance, the concentration of V⁴⁺ was determined. The corresponding extinction coefficients for the V⁴⁺ bands were calculated from the absorption spectra. Concerning the vanadium-containing glass, the results of square-wave voltammetry and spectroscopy were compared quantitatively. For the iron-doped glass, the comparison was only qualitative.
- 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.
- ItemRheological properties of calcium metaphosphate melts during extrusion(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1999) Yue, Yuanzheng; Carl, Gunter; Rüssel, ChristianAn extrusion method to determine viscosities is described. Here, different stages of the response on the load applied are observed and the viscous heating efFect caused by the mechanical deformation work is taken into account. The method enables the determination of viscosities in the ränge of 10⁶ to 10^(8.5) dPa s. The values obtained are in good agreement with interpolated ones determined by beam bending and cylinder rotation. During extruding of calcium metaphosphate melts, the viscositiy decreases with increasing shear rate. This shear-thinning efFect (or non-Newtonian behaviour) is attributed to the orientation of flow units of the dynamic melt network. It is found that both Newtonian and non-Newtonian flow behaviour of calcium metaphosphate melts was largely influenced by adding a small amount of SiO₂. However, the introduction of a small amount of SiO₂ into calcium metaphosphate melts is beneficial to study the flow behaviour, since by that the crystallization of melts is avoided, which hinders the continuation of extruding processes.
- ItemInfluence of nucleating agents on the crystallization of Mg-Ca-Si-Al-O-N oxynitride glasses(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1998) Deckwerth, Martin; Rüssel, ChristianOxynitride glasses in the system M g - C a - S i - A l - O - N were prepared with the aid of a polymeric preceramic aluminum nitride precursor as nitrogen source and transformed into glass-ceramics at temperatures in the ränge of 950 to 1350°C To support the devitrification process, TiO₂ , Cr₂O₃, MgF₂ and ZrO₂ were added to the batches as potentially nucleating agents. Under the reducing meldng conditions applied. TiO₂ led to undesired reactions in the glass melt and was rejected as nucleadng agent. Cr₂O₃ could only be dissolved in the glasses to a maximum content of 1 wt% and did not lead to obvious effects concerning nucleation and crystal growth. The addition of fluorine results in the formation of a fine-grained microstructure but by analogy to Cr₂O₃, did not lead to phase Separation. Adding ZrO₂, provoked phase Separation in the oxynitride glasses, which was even promoted by the presence of nitrogen. These glass-ceramics possessed an extremely fine-grained microstructure containing nonstabilized tetragonal ZrO₂, which results in an additional nitrogen content independent improvement of the mechanical properties. By comparison to Ti02, Cr203 and MgF2, zirconia has a twofold effect: it is not only an efficient nucleating agent in the case of oxynitride glasses but also an efficient toughening agent for the resulting glass-ceramics.
- 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.