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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.
- 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.
- ItemAlumina and zirconia as inhibitors of crystallization during sintering of borosilicate glasses(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2002) Pascual, M. Jesus; Pascual, Luis; Durán, Alicia; Wange, Peter; Rüssel, ChristianThe glass composition (in mol%) 16 Na2O ∙ 24 B2O3 ∙ 60 SiO2 has suitable viscosity and dilatometric properties to be used as sealing material for the gas manifolds of molten carbonate fuel cells (MCFC). Nevertheless, quartz crystallization takes place during sintering of the borosilicate glass powder at temperatures between 600 and 700 °C. The quartz crystallization leads to the formation of pores and a variation in the thermal expansion coefficient; both giving rise to defects and tensions in the seal. In this work, the possibility of inhibition of quartz crystallization by substituting AI2O3 or ZrO2 for 2 or 5 mol% of SiO2 is described. The thermal properties and the viscosity-temperature curve of the new compositions, especially those containing alumina do not differ much from the original glass.
- ItemChemical solubility of phosphate glasses in the system Na2O-CaO-MgO-P2O5-Al2O3-TiO2 in aqueous solutions of different pH values(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2004) Vogel, Jürgen; Wange, Peter; Knoche, Silke; Rüssel, ChristianPhosphate glasses of different composition ranges show very different solubility behaviour in aqueous solutions of pH 4 and 5.8. With inereasing P2O5 concentrations between 38 and 52 mol% , in the system Na2O-CaO-MgO-P2O5-Al2O3-TiO2 , the dissolution rate generally increases, however, the most notable increase is observed at P2O5 concentrations larger than 48 mol%. Additives of TiO2 and AI2O3 increase the stability to corrosion. Glasses with compositions in the metaphosphate range show larger dissolution rates in slightly acidic medium than glasses near the invert glass range. Glasses in the invert glass range show a stronger increase in the dissolution rates with decreasing pH value and also deceleration of the dissolution with time.
- ItemGlasses for the preparation of gradient index lenses in the Na2O-Al2O3-B2O3-SiO2 system - hydrolytic durability, thermal and optical properties(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2004) Hornschuh, Sandra; Rüssel, Christian; Messerschmidt, Bernhard; Possner, Torsten; Possner, UlfGlasses in the system Na2O-Al2O3-B2O3-SiO2 were melted from the raw materials and studied with respect to their chemical durability, their crystal growth velocities, their refractive index and dispersion, their glass transition temperature and their thermal expansion coeffieient. With inereasing Na2O concentration, the crystal growth velocity increases and the chemical durability decreases. Equimolar subtitution of B2O3 for SiO2 results in a decrease in both the chemical durability and in crystal growth velocities. Equimolar subtitution of AI2O3 for SiO2 leads to increasing chemical durability and decreasing crystal growth velocities. Optimum glass compositions to produce gradient index lenses should possess B2O3 concentrations of around 12.5 to 15 mor% and molar AI2O3 concentrations some percent larger than the Na2O concentration. In these glasses, nonbridging oxygen does not occur.
- ItemThermodynamics of the Sn2+/Sn4+ equilibrium in alkali-alkaline earthsilicate melts(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2003) Benne, Darja; Rüssel, ChristianMelts with the basic compositions I6R2O - 10CaO - 74SiO2 (R = lithium, sodium, potassium), 20Na2O ∙ xCaO ∙ (80-x)SiO2 (x = 0, 10, 20), 16Na2O - 10MgO - 74SiO2 and 15Na2O - 85SiO2 all doped with 0.25 mol% SnO2 were studied using square-wave voltammetry at temperatures in the range from 1000 to 1600 °C. Over a wide temperature range, the measured peak potentials decreased linearly with temperature. The peak potentials are most negative for the 15Na2O - 85SiO2 melt and least negative for the 16Na2O - 10MgO - 74SiO2 melt. The effect of the glass composition on the Sn2+/Sn4+ redox equilibrium is much less pronounced than in the case of the Fe2+/Fe3+ equilibrium.
- ItemStructurization of a partially crystalline melt by a stationary ultrasonic wave(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2001) Gerlach, Sandra; Rüssel, Christian; Avramov, IsaakAt a partially crystalline melt composed by a lead borate glass and crystalline SnO2, a stationary ultrasonic wave was attached at a temperature of 600 °C. This led to an accumulation of the SnO2 particles at the pressure nodes. After cooling the melt, a layered structure was obtained. The effect of the particle size upon the particle distribution is shown. The movement of the particles in the stationary ultrasonic wave is described by a differential equation which is numerically solved. Results of the numeric simulation are compared with experimental results and discussed.
- 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.