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- ItemGlass-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, ErwinGlass-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.
- ItemRheological properties of calcium metaphosphate melts during extrusion(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1999) Yue, Yuanzheng; Carl, Gunter; Rüssel, ChristianAn extrusion method to determine viscosities is described. Here, different stages of the response on the load applied are observed and the viscous heating efFect caused by the mechanical deformation work is taken into account. The method enables the determination of viscosities in the ränge of 10⁶ to 10^(8.5) dPa s. The values obtained are in good agreement with interpolated ones determined by beam bending and cylinder rotation. During extruding of calcium metaphosphate melts, the viscositiy decreases with increasing shear rate. This shear-thinning efFect (or non-Newtonian behaviour) is attributed to the orientation of flow units of the dynamic melt network. It is found that both Newtonian and non-Newtonian flow behaviour of calcium metaphosphate melts was largely influenced by adding a small amount of SiO₂. However, the introduction of a small amount of SiO₂ into calcium metaphosphate melts is beneficial to study the flow behaviour, since by that the crystallization of melts is avoided, which hinders the continuation of extruding processes.
- ItemPreparation 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, ChristianGlass-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.
- Itemstudy of structural modification in extruded and heat-treated lithium disilicate glasses by the method of radiation color centers chronospectroscopy(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1998) Arbuz, Valerii I.; Carl, Gunter; Rüssel, Christian; Durschang, BernhardSamples of extruded and non-extruded lithium disilicate glasses were studied. Extruded glass samples were cut out along and across the extrusion axis of fmished cylindrical rods. For all the samples, spectra of radiation-induced absorption, Δα(E), were measured for various time instants after the cessation of X-irradiation. On the data basis obtained, kinetic dependences. Δα = f(t), were plotted and analyzed. In the diagram "Δα versus lgt", they are represented by straight hnes. Each of them is falling down with its own constant slope a. These a's are rate parameters of the decay of radiation color centers (CCs). They appear to be functions of average distances between recombinating electron and hole CCs. The above α-parameter decreases when passing from the longitudinal cut extruded glass sample to the sample of the non-extruded glass and finally to the transversely cut extruded glass sample. These data mean that, in the course of extrusion, the glass structure becomes less dense in the axial direction of extruded glass rods and more dense in the radial one. Α 4-hour heat treatment at 465 °C (≈5°C above the glass transformation temperature, Tg) eliminated the above anisotropy of radiation properties in extruded glasses and forced their anisometric structures to return to the isometric State characteristic of the non-extruded glass.
- ItemOriented mica glass-ceramic by extrusion and subsequent heat treatment(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1997) Habelitz, Stefan; Carl, Gunter; Rüssel, Christian; Marchetti, Kurt; Roeder, Erwin; Eifler, Dieter; Hergt, RudolphGlass-ceramics with oriented mica crystals have been produced by extruding a fluorophlogopite glass and subsequent therma treatment. During extrusion predominantly the intermediate phase norbergite (Mg₃F₂SiO₄) and besides small quantities of phlogo pite mica crystals (Na/KMg₃F₂Si₃AlO₁₀) crystallized. Heat treatments at temperatures around the maximum crystallization rat resulted in an oriented crystallization of phlogopite as proved by electron microscopy and XRD analysis. The plate-like crystals ar aligned with their basal planes parallel to the direction of extrusion. The degree of orientation was studied by X-ray pole figur measurements. The (003) planes show strong ring-fibre texture. The degree of orientation decreased if the crystallization was realize at lower temperatures and lower crystallization rates. The alignment mechanism is discussed.
- ItemInduced 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, ChristianThe 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.