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- ItemUV-VIS spectroscopic investigations of amber glass at high temperatures(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1999) Müller, Matthias; Rüssel, Christian; Claußen, OlafThe absorption spectra of iron-containing glasses and an amber glass were recorded in the temperature ränge of 25 to 700 °C. Α shift of the UV absorption edge towards lower wave numbers with increasing temperature was found. The amount of the shift depends on the Fe³⁺ concentration. Here, the UV absorption edge may superimpose or even cover up the absorption bands of colouring ions with increasing temperatures. For an amber glass sample, with increasing temperature, the intensity of the absorption bands of the amber chromophore decreases slightly. However, above 550 °C the decomposition of the chromophore is observed. All changes are reversible when temperature is decreased again. The decomposition of the chromophore and the attributed temperature ränge are in agreement with thermodynamic calculations.
- 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.
- 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.
- 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.
- 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.
- 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.
- ItemVoltammetric investigations of the redox behaviour of Fe, Ni, Co and Sn doped glass melts of AR® and BK7® type(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2000) Matthai, Annegret; Ehrt, Doris; Rüssel, ChristianThe redox behaviour of iron, nickel, cobah, and tin ions was studied in glass melts with compositions near AR® and BK7® type by means of square-wave voltammetry (SWV). From the linear dependence of the peak potentials on the temperature, the standard enthalpies, ΔH°, and the standard entropies, ΔS° of the transitions Fe3+/Fe2+, Co2+/Co0, Ni2+/Ni0, Sn4+/Sn2+ and Sn2+/Sn0 were calculated. The reduced states are formed in the AR glass melt already at less reducing conditions than in the BK7 melt. The dependence of the redox ratios upon temperature is larger in the AR melt. As colouring ion Co2+ is more advantageous than Ni2+ because of its higher stability against reduction. It is possible to reduce the Fe3+/Fe2+ ratio without formation of Co0 in a disturbing concentration.
- 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.
- 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.
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