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- 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.
- 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.
- ItemSol-gel derived coatings with completely reversible photochromism(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2000) Fachet, Regina; Müller, Matthias; Bürger, Herbert; Kriltz, AntjeBesides the photochromic glasses based on silver halides especially the cuprous halide containing materials are of practical importance. Due to their high absorption coefficient the cuprous halides are particularly suited for the preparation of thin photochromic films. Using the sol-gel technique silicon alkoxide based coating solutions with continuously distributed photoactive components were prepared. From these sols single or multilayer coatings were produced by dipping. The precipitation of the cuprous halide microcrystals as carriers of the photochromic properties of the coatings proceeded during a subsequent heat treatment (temperature range of 100 to 800 °C). Thus it was possible for the first time to produee photochromic coatings with a pronounced darkening and with complete reversibility of the photochromic reactions at room temperature.
- 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.