DSpace :: Browsing by Author "Rüssel, Christian"

Browsing by Author "Rüssel, Christian"

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Alumina 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, Christian
The 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.
Chemical 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, Christian
Phosphate 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.
Corrosion of molybdenum electrodes in an aluminosilicate glass melt
(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1991) Holzwarth, Stefan; Rüssel, Christian; Tomandl, Gerhard
The corrosion of molybdenum electrodes in an aluminosilicate melt was studied with the aid of electrochemical methods at temperatures up to 1600 °C. The glass melt investigated contained polyvalent ions which enabled redox reactions with the electrode material. A layer of molybdenum oxide is formed on the electrode in contact with the glass melt. The layer partially dissolves in the glass melt and eventually evaporates into the furnace atmosphere. Dissolution of the oxide layer is assumed to be the step which determines the rate of the corrosion process.
Crystalization in High Iron Containing Silicate Glasses - Electron Microscopy Investigation
(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2004) Völksch, Günter; Harizanova, Ruzha; Rüssel, Christian; Mitsche, Stefan; Pölt, Peter
[no abstract available]
Crystallization 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, Christian
Glasses 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.
Effect of additives on the structure of SiO2 sol-gel spray coatings
(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2000) Löser, Carsten; Rüssel, Christian
SiO2 coatings were prepared from alkoxide sols by means of spray coating. Coatings prepared from sols solely containing ethanol as solvent showed heterogeneous structures. This is supposedly due to solvent evaporation during spray coating. When reaching the substrate, the droplets possess too high viscosity to enable them to disperse on the substrate. However, when additives, such as 1.3-butanediol, ethylene glycol or glycerol, with high boiling temperatures were used, smooth coatings exhibiting small surface roughness were obtained. It is shown that best results were obtained when 1.3-butanediol was used. With increasing additive concentration, the surface roughness decreased.
Effect of TiO2 , ZrO2 and ZnO addition on the hydrolytic durability, viscosity and crystallization behaviour of fibre glasses
(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2005) Tauch, Diana; Wange, Peter; Rüssel, Christian
Glasses of the System Na2O-K2O-MgO-CaO-BaO-AI2O3-SiO2-B2O3 with additions of TiO2, ZrO2 and ZnO were melted from the raw materials. The glasses were characterized with respect to their hydrolytic durabilities, the working ranges (difference of temperatures attributed to viscosities of 10^2 and 10^4 dPa s), the temperatures attributed to the working point (η = 10^3.5 dPa s), and the crystal growth velocities. In the most cases, the additions led to an increase in viscosity and to a slight decrease in the workability range. The crystal growth velocities decreased with the addition of Ti02 and/or Zr02, while they increased if adding ZnO. The addition of ZnO or of TiO2 and ZrO2 resulted in better chemical durability.
Electric 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, Andrea
The 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.
Electrochemical study on the corrosion of molybdenum electrodes in lead glass melts
(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1988) Rudolph, Thomas; Balazs, G. Bryan; Rüssel, Christian; Tomandl, Gerhard
The corrosion of molybdenum in lead glass melts is a two step process. In the cathodic part of the ac heating cycle lead ions of the glass melt are reduced to metallic lead. During the anodic part molybdenum is oxidized. Cyclic voltammetry revealed Mo3+ ions as the intermediate state in the oxidation process. During corrosion experiments a porous layer of Mo(VI) Compounds and MoO2 were detected. A drastic reduction of the corrosion rate was obtained when a small anodic dc potential was superimposed on the ac heating current. Slow voltammetric scans showed passivation of the molybdenum at potentials higher than -300 mV. The protection layer consists most likely of molten K2Pb(MoO4)2.
Electrochemical study on the redox behavior of selenium-containing soda-lime-silica melts
(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2001) Rüssel, Christian
Decolorization of white container glasses nowadays is usually achieved by the addition of selenium compounds to the batch. Most of the quantity added, however, is evaporated during melting which is strongly influenced by the redox state of the melt. Therefore, the redox behavior in selenium-doped soda-lime-silica melts was studied by square-wave-voltammetry. The current-potential curves recorded showed two peaks, attributed to the reduction of SeO3 to SeO2 and SeO2 to Se^0. Under reducing conditions, selenium predominantly occurs as Se^0, which gives rise to the pink color. When melted under oxidizing conditions or, however, when the melt is kept at high temperature in air, the cooled glasses are nearly colorless due to the re-oxidation of Se^0 to SeO2. Keeping the melt at high temperatures for up to 50 h did not result in a notable evaporation of selenium compounds. Highest selenium contents were achieved by adding ZnSe to the batch.
EPR 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, Christian
Mixed 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.
Glass-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, Erwin
Glass-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.
Glasses 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, Ulf
Glasses 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.
Graded pore size by ion exchange of phase-separated 6.5 Na2O · 33.5 B2O3 · 60 SiO2 glass
(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2000) Flügel, Alexander; Rüssel, Christian
A phase-separated glass with the composition (in mol%) of 6.5 Na2O · 33.5 B2O3 · 60 SiO2 was ion-exchanged in molten KNO3 at temperatures in the range of 520 to 660 °C. At temperatures of 640 and 660 °C, the interdiffusion coefficient measured did not depend on the K2O concentration in the glass, while at lower temperatures, an increase in the diffusion coefficient with increasing K2O concentration was observed. The ion exchange and the subsequent heat treatment led to a change in the microstructure. In the bulk of the glass, where no K2O was detected, a fairly coarse microstructure was observed. At the surface, a much finer microstructure occurred. Within the diffusion layer, from the surface to the bulk, a continuous coarsening was observed. When the K2O was subsequently re-exchanged against Na2O in a second step, layers with a gradient in the pore size distribution were obtained, which were mechanically stable after leaching the boron-rich phase.
High temperature investigation of soda-lime-silicate glasses doped with manganese and chromium
(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2004) Kido, Ladislav; Müller, Matthias; Rüssel, Christian
[no abstract available]
High-temperature spectroscopic study of redox reactions in iron-and arsenic-doped melts
(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2003) Schirmer, Henning; Müller, Matthias; Rüssel, Christian
Glasses 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.
High-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, Christian
Absorption 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.
High-temperature UV-VIS-NIR spectroscopy of chromium-doped glasses
(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2001) Gödeke, Dieter; Müller, Matthias; Rüssel, Christian
Chromium-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.
Impedance of molybdenum electrodes in a soda-lime-silica glass melt
(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1991) Rüssel, Christian
Impedance measurements at molybdenum electrodes in a soda-lime-silica glass melt were carried out in a frequency range of 0.1 to 10^5 Hz. The spectra obtained were quite different depending on the DC potential superimposed. Thus, spectra obtained from anodically passivated electrodes could be clearly distinguished from those of electrodes with a MoSix-surface layer and from unprotected electrodes. Although the physical interpretation of the spectra obtained is very difficult, and up to now unsatisfactory, impedance measurements seem to be suitable for measuring and controlling passivating layers at molybdenum electrodes.
Induced crystallization of glass-forming melts : Part 1. Heterogeneous nucleation. Effect of noble metal microcrystals on the crystallization of calcium metaphosphate glasses
(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2004) Guencheva, Victoria; Stoyanov, Emil; Gutzow, Ivan; Carl, Gunter; Rüssel, Christian
The possibilities of initiating crystallization in glass-forming melts are analyzed, using experimental evidence, obtained with several model glass-forming systems. In the present Part 1 of the investigation the process of heterogeneous nucleation, catalyzed by insoluble crystallization cores is studied, using a new theoretieal thermodynamie model. The nucleation activity coefficient, Φ, of the aubstrates is described by the adhesion energy, β, at the interface crystallization core/overgrowing crystal. Lattice disregistry is accounted for as an additional correlation factor, influencing the thermodynamie work of adhesion. The crystallization of calcium metaphosphate glass-forming melts, initiated by noble metal microerystals (Ag, Au, Pd, Pt, Rh, Ru, Ir, Os), is studied by differential thermal analysis, optical and scanning electron microseopy. Ir microerystals show highest nucleation activity promoting intensive bulk crystallization in the Ca(PO3)2 systems investigated. The activity of other crystallization cores decreases in the following sequence: Ru > Rh > Os > Pt > Pd > Au > Ag. The above outlined thermodynamic theory of nucleation activity is used to correlate experimental data and to calculate activity coefficients, Φ, in dependence on the properties of the substrates employed. Under tangential stress, applied by pressure in extrusion experiments, the metal microcrystals in the glass samples are stratified in concentric rings with particle density increasing towards the sample surface. Thus a new method of formation of pre-oriented glass-ceramic materials is indicated. In Part 2 of the investigation the effect of soluble oxide additives on nucleation and crystallization in glass-forming melts is considered.

Browsing by Author "Rüssel, Christian"

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