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Author: Carl, Gunter × Author: Rüssel, Christian × Start date: 2000 × Has files: Yes × Type: Text ×

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    Preparation of glass-ceramics with needle-like apatite crystals of different aspect ratios and their behavior during extrusion
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2000) Moisescu, Cornelia; Carl, Gunter; Rüssel, Christian
    Glass-ceramics containing needle-like apatite crystals were produced from glasses in the system SiO2-Al2O3-CaO-P2O5-K2O-F- . Crystallization was achieved by rapid heating to 1200°C and keeping the sample for 0.5 to 15 h. With increasing time of heat treatment, length, L, width, W, as well as the aspect ratio, L/W, of the apatite crystals increased. This also led to an increase in the Newtonian viscosity. At higher shear rates, non-Newtonian flow behavior was observed. This effect was more pronounced with increasing aspect ratio. In all cases, the result of the extrusion process was highly oriented glass-ceramics, if it was carried out in the non-Newtonian range.
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    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.