Browsing by Author "Müller, Ralf"

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    Elastic Stresses, stress relaxation, and crystallisation
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2003) Schmelzer, Jürn W. P.; Müller, Ralf; Möller, Jörg; Gutzow, Ivan S.
    [no abstract available]
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    Nucleation at cordierite glass surfaces: Kinetic aspects
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1996) Müller, Ralf; Reinsch, Stefan; Pannhorst, Wolfgang
    The surface nucleation of high-quartz sohd Solution crystals at fractured surfaces of glasses of the stoichiometric cordierite composition (2MgO · 2AI₂O₃ · 5SiO₂) was studied by optical microscopy. Particular attention was focused on the nucleation kinetics. A constant nucleation density, Ν ≈ 10⁻⁴ μm⁻², was found not to be significantly influenced by the time and the temperature of nucleation treatment. Even a very fast heating of samples employing heating rates u p to 1200 K/min does not lower Ν substantially. However, for small average crystal diameters ( < 2 0 μm) a distribution of crystal size in the same order of magnitude is detectable indicating a simultaneous appearance of b o t h measurable nucleation rates and growth velocities. It can be concluded that the surface nucleation of µ-cordierite occurs during the thermal treatment from a limited number of preferred nucleation sites; these sites are "used Up" rapidly enough to cause a strong Saturation effect of nucleation, but slow enough to cause a crystal size distribution at the same time. The surface nucleation rate, Is , was calculated from the observed distribution of crystal sizes. Is progressively increases with rising temperature similar to the crystal growth velocity indicating a broad temperature ränge of essential nucleation activity. The latter must be regarded as t he main obstacle to measure or to control surface nucleation density by means of two-step nucleation and growth treatments and must therefore be claimed to be mainly responsible for the observed constancy of N.
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    Open-pore sintered glass-ceramics as carrier material for biotechnological use
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1997) Gemeinert, Marion; Müller, Ralf; Wihsmann, Fred Gustav; Schröder, Frank; Kliche, Horst
    Open-pore carriers with defined open porosity up to 45 % for biotechnological use were manufactured by sintering and crystallization of glass powders of cordierite stoichiometry. The stop of the shrinkage caused by a surface crystallization of the glass ("sinter blockade") is utilized to stabilize a desired pore volume without filier or foaming aids. Glass powders of the stoichiometric composition of cordierite (2MgO · 2AI₂O₃ · 5SiO₂) with different grain size distributions were used as starting materials. The efficiency of the sinter blockade and thereby the stabilized porosity of the resulting compact mainly depends on the surface nucleation density at the Single glass particles which can be adjusted by powder processing. Furtheron, the chosen grain size distribution of the glass powders and the sintering process parameters (heating rate, temperature, heating time) are important factors to control the sintering behaviour. Samples of carrier bodies produced by this way are tested and evaluated quantitatively as carriers for biofilms in a biotechnological process for decomposition of pollutants in a synthetic model waste water system.
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    Viscous flow and surface crystallization caused by Vickers indentation
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2004) Chládek, Jiri; Müller, Ralf; Weh, Lothar; Reinsch, Stefan
    Healing and nucleation activity of crack patterns caused by Vickers micro-indentations on the surface of diopside glass (MgO - CaO - 2 SiO2 ) were studied by optical and surface interference microscopy. Minimizing the glass surface during thermal treatments and healing of cracks result in funnel-like depressions symmetrically arranged around the Vickers indentation. Diopside surface crystals predominantly grow from previous crack edges. Thus, double chains of crystals reveal completely healed radial cracks white long range viscous flow at the formation of funnel-like depressions spreads off former double chains to form circular crystal chains.
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    Water concentration and diffusivity in Silicates obtained by vacuum extraction
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2005) Müller, Ralf; Gottschling, Peter; Gaber, Martin
    Water concentration, Cw, and the mean effective diffusion coefficient of water, Dw, were measured by vacuum hot extraction coupled with mass spectrometer evolved gas analysis (VHE). The study includes silica, soda- and potassium-lime-silica, cordierite, and lamp bulb glasses of water concentrations between 0.017 and 0.17 mol H2O/l (122 and 1159 wt. ppm) as well as cordierite single crystals. Cw was obtained by integration of the VHE water degassing rate dQw/dt. For powdered samples (≈ 50 to 150 mg) of silica, soda-lime-silica and cordierite glass with water concentrations between 0.033 and 0.15 mol/1, VHE results well confirm IR spectroscopy within 5 to 15 % accuracy. Dw could be obtained by two methods: dQw/dt of powdered samples was measured during isothermal VHE experiments and dQw/dt fitted versus time with the appropriate diffusion models. Alternatively, powdered samples were pre-annealed in vacuum before measuring their residual water concentradon by VHE as described above. Cw was then analogously fitted versus the vacuum pre-annealing time. The applied methods and samples allowed the measuring of Dw within 10-9 and 10-13 cm2 s-1. Calculated values of Dw for soda-lime-silica glasses agree with literature data and values obtained by IR spectroscopy within a factor of 2 to 4.