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- ItemInfluence of homologous substitutions of chemical components on the rheological properties and on isochomal workability of Silicate glass melts(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1996) Yue, Yuanzheng; Brückner, RolfGlass melts of various homologous Silicate glass series were investigated by means of the cylinder compression method with respect to their flow behaviour (Newtonian and non-Newtonian), stress generadon modulus (stiffness, brittleness) and critical deformadon rate (high-temperature tensile strength, isochomal workability). The experimental results show that decreasing CaO content and increasing basicity lead to the decrease of the brittleness of Silicate glass melts and to the increase of the critical deformadon rate, i.e. to the improvement of the isochomal workability of these melts. The mixed alkali effect of sodium potassium calcium Silicate glass melts at molar fracdon [K₂O]/[Na₂O+K₂O] = 0.5 has a favourable influence on the isochomal workability The correladon between flow behaviour and workability of glass melts is also discussed in the present paper. It could be shown that the ratio β = E2.8/E3.6 (E is the extinction of the hydroxyl band at 2.8 and 3.6 ηm, respectively) indicates the structural origin of the dependence of the mentioned rheological properties on the chemical compositions, at least within the homologous series.
- ItemAnisotropic optical properties and flow behaviour of mechanically deformed single-phase glass melts(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1994) Brückner, Rolf; Habeck, AndreasStarting from the stress-optical coefficient around and above Tg, the following effects are regarded in order to give a comprehensive picture of the structural response of various glass melts under flow conditions at various temperatures: anisotropy o f glass fibres, the flow birefringence and Maxwell constant of the melt directly and/or indirectly via frozen-in anisotropics by cooling the melt thermally stress-free under mechanical load. Some of the applied various rheological methods allow to study the Newtonian and non-Newtonian viscosity additionally to the birefringence. Four very different glass melts are investigated under these aspects: a potassium-calciumsilicate glass, a float glass, an alkali metasilicate glass and an alkali metaphosphate glass. The results show that there are significant differences in the specific birefringence and in the deviation from Newtonian flow behaviour depending on the special flow units of the glass types. The structure of these anisotropic glasses and melts extends from frozen-in deformations of the network to orientations of chain-like flow units depending on chemical composition, temperature, mechanical stress and deformation rate. Particularly, a close connection between the onset of the non-Newtonian flow behaviour and the alteration o f the specific birefringence was found at certain critical stresses.
- ItemDetermination of the rheological properties of high-viscous glass melts by the cylinder compression method(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1994) Brückner, Rolf; Yue, Yuanzheng; Habeck, AndreasA detailed description of the cylinder compression method and its application on the viscoelastic behaviour of glass melts is given with the help of closed solutions and equations in contrast to algorithmic methods, like e.g. the finite element method. Special attention is paid to the system deformation and system deformation rate of the testing equipment and to the dissipation of mechanical energy within the specimen and the heat flow from the specimen. The limits of this treatment are discussed with respect to the applicability of the theoretical basis (Gent and Nadai equation) and with respect to the experimental determination of the rheological properties of glass melts. On this basis the pure non-Newtonian viscosity, ηnN, can be determined. If very small differences have to be ascertained in the rheological behaviour of glass melts due to small differences in composition or in melting history (e.g. redox condition or bad/good workability), the same conditions concerning the mechanical and thermal equipment and even the same geometry of the glass specimens have to be strictly maintained. Comparison between different glass melts should be made rather on the basis of equal (Newtonian) equilibrium viscosity, η₀, (isochomal conditions) and not on equal temperature. Owing to large thermal effects, the slope of the viscosity-temperature curve at η₀ plays a certain role, particularly for the forming process.
- ItemAnisotropie glasses and glass melts - a survey(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1996) Brückner, RolfΑ general introduction describes intentional and unintentional anisotropic phenomena resulting from chemical and thermal inhomogeneities (internal stresses) as well as anisotropies induced by external elastic stresses and strains in homogeneous glasses for special opdcal purposes. Further the paper will point out the possibilities and various phenomena of producing defmed anisotropies by means of flow stresses and strain rates in glass mehs which may be frozen-in by cooling under load. It will be distinguished between Single - and double-phase glass melts and glasses which show very different kinds and effects of anisotropies. In particular, optical anisotropies (birefringence, specific birefringence) and structural-mechanical anisotropies (densities, shrinkages and expansions during annealing) of single-phase glasses and glass melts will be investigated under defined flow condidons (in-situ measurements directly in glass melts by the classical flow birefringence) and geometries (e.g. cylindric bulk glass samples, glass fibres, glass rods). In double-phase glass Systems the following anisotropic properties will be treated: the form birefringence and the distribution birefringence of decomposed glasses, the orientation of melts with suspended anisometric crystals and birefringence effects of chemically leached microporous glasses. Finally, a new phenomenon will be reported where the frozen-in birefringence of a glass shows an other sign after deformation above Tg than the stress optical constant of the same glass at room temperature before this treatment above Tg.
- ItemElastic and inelastic properties of soda lime Silicate glass melts(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1999) Börnhöft, Hansjörg; Brückner, RolfThe influence of CaO on the elastic and inelastic behaviour of soda lime Silicate glass melts of the series (26 - x) Na₂O · x CaO * 74 SiO (composition in mol% with x = 0 to 16) is investigated. Longitudinal and transversal ultrasonic waves in the MHz frequency ränge are transmitted through the glass melt. Their velocities and damping behaviour are measured as functions of temperature. The elastic moduli, internal friction and relaxation times are calculated and their dependence on temperature and viscosity is shown. These properties describe the transition from an elastic to a viscoelastic and fmally to a pure viscous behaviour of the invesdgated glass melts by a large dispersion ränge. Enhanced values of the moduli and activation energies are determined with an increase of CaO content. Thus, the structure of the melt becomes strenger and stiffer i.e. the mean bonding strength and the linkage of the dynamic network of the Silicate melt is improved. The relaxation behaviour can be described by a Maxwell based model with a distribution function of relaxation times.
- Itemstructure-sensitive investigations on glass fibers from the system SiO₂-Al₂O₃-GaO(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1998) Murach, Jürgen; Makat, Andreas; Brückner, RolfGlass fibers of the system SiO₂-Al₂O₃-CaO were prepared with respect to defined drawing conditions and investigated with structure-sensitive methods (birefringence, radial and axial alterations by thermal treatment at Tg). These glass fibers, which lie in a compositional sense roughly between the disilicate and silica glasses concerning the ratio network modifiers to network formers, exhibit similar optical anisotropies. On the other hand, they show remarkably lower axial and radial alterations which are exclusively contracdons on annealing for all applied drawing conditions in contrast to the silica or the alkali disilicate glass fibers. Obviously, the filling of free volume hollows with and the reinforcement of them by the Ca²⁺ ions and the smaller polarizability of the latter as compared to alkali ions are responsible for the relatively low structural anisotropy. An increase of the optical anisotropy with increasing CaO concentration is observed at comparable viscosities. This is a consequence of the increasing incorporation of the network modifier oxide which produces a weaker network strength and a larger polarizability.
- Itemstress generation modulus as a counterpart of the stress relaxation modulus(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1995) Brückner, Rolf; Hessenkemper, Heiko; Habeck, Andreas; Yue, YuanzhengIn order to measure the time dependence of the stress relaxation modulus, E, a stress-strain deformation has to precede which induces a stress within the viscoelastic sample from which the sample relaxes. The generation of stress is characterized by a strain rate-dependent and relaxation rate-dependent portion which exhibits a maximum value, E_max, which is called "stress generation modulus". E_max was called the "maximum stress relaxation modulus" in earlier papers. Meanwhile, however, it turned out that a better verbal distinction should be made in future by the new term "stress generation modulus" because E_max is about one order of magnitude larger than E.
- ItemOn the different descriptions of the non-Newtonian viscosity (shear-thinning effect) of glass melts with respect to heat dissipation(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1996) Yue, Yuanzheng; Brückner, RolfThe range of non-Newtonian viscosity is usually connected with high strain rates or/and stresses. Therefore, the effect of heat dissipation (or viscous heating) cannot be neglected. In order to regard this effect various correlations were given in former papers, which will be analyzed and discussed in the present paper. It turns out that through the exact thermal elimination of viscous headng one can easily obtain the quantity of the pure, realistic and exclusively structurally determined non-Newtonian viscosity, ηnN, which may be normalized by the Newtonian viscosity, η0, to yield the form, ηnΝ/η0- At small amounts of strain and stress other expressions, such as η/η0,Tmax and η/η0,Tnet, may describe the non-Newtonian flow behaviour approximately where the thermal elimination is not made for the as-measured non-Newtonian viscosity, η, but for the Newtonian values, ηο. (η0,Tmax Stands for the adiabatically corrected Newtonian viscosity, due to the maximum temperature of viscous heating by the total deformation work; η0,Tnet Stands for t h e nonadiabadcally corrected Newtonian viscosity due to the gross temperature of viscous heating, regarding also the heat flow out of t he glass sample.) In contrast to these more or less pure non-Newtonian normalized viscosities the so-called normalized non-Newtonian gross viscisity, η/η0, is that viscosity which involves the viscosity decrease due to the pure non-Newtonian flow, determined by structural orientation, as well as the viscosity decrease due to the viscous heating. The various relationschips are analyzed and discussed with the help of recently developed rheological equations.
- ItemInfluence of non-homologous substitutions of chemical components on rheological properties and on isochomal workability of Silicate glass melts(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1996) Yue, Yuanzheng; Brückner, RolfThe glass melts of a non-homologous series were investigated with respect to flow behaviour and stress generadon behaviour as well as to workability by means of the cylinder compression method. The results show that the competition between the gross viscosity effect and the bonding strength of the dynamic glass melt structure extremely deterrnines the high-temperature tensile strength, σ_ts, the critical deformation rate hc, and by that the isochomal workability. The larger hc values are not always connected to a larger non-Newtonian flow effect or to a larger gross viscosity effect as it is usually the case for homologous series. However, the rule for homologous series that smaller Emax moduli, i.e. smaller stiffness and brittleness of a glass melt, are coupled to a better isochomal workability is also vahd for non-homologous series. The special connections between σ_ts, hc and deformation, Δh/h0, are analyzed.
- ItemIon refraction and stress optical coefficients for di- and metasilicate glasses(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2002) Noll, Claudius; Habeck, Andreas; Brückner, RolfGlasses of the di- and metasilicate stoichiometric composition were melted, with lithium, sodium and potassium being replaced for each other. Additionally, SiO2 and B2O3 glass samples, the DGG standard glass, a float glass and a metaphosphate glass were examined. Molar mass, molar volume and oxygen partial volume were measured and found to increase in the sequence Li < Na < K. While potassium expands the glass network, the lithium ions show a counteracting behavior. The metaphosphate and the pure B2O3 glass are those with the most compact structure (related to one structure unit) and therefore with the lowest values of the studied properties. It was shown quantitatively that the polarizabilities of the nonbridging oxygen atoms are much higher than those of the bridging oxygen atoms. The influence of the cations was usually small. The origin of the small polarizability values of the B2O3 glass may be interpreted in terms of the lack of the nonbridging oxygen atoms and in terms of the denser structure of the oxygen polyhedra as compared with those in the SiO2 glass. The values of the stress-optical coefficients increase from lithium- to sodium- to potassium-dominated glasses, presumably resulting from increasing ion refraction of the nonbridging oxygen atoms. All preloaded samples of the di- and possibly also of the metasilicate composition showed an increase of the stress-optical coefficients with increasing preload at temperatures higher than the glass transformation temperature Tg. The reason is that a certain flow condition of the glass melts is frozen-in.