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- ItemProperties of textile glass fibers based on alkali- and boron oxide-free aluminosilicate glasses(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1998) Kim, Young Keun; Brückner, Rolf; Murach, JürgenThe goal of this paper is to examine the possibihty of producing fibers from alkah- and boron oxide-free glasses with properties comparable to those of commercially available glass fibers. Various compositions from the Systems SiO₂ - Al₂O₃ - CaO - RO are investigated with RO = MgO, ZnO, MnO, BaO between 0 and 10 wt%. The fibers are suitable for application as textile fibers and as reinforcement component for composites. The mechanical properties (tensile strength, Young's and shear modulus and Poisson ratio) are comparable to and/or partly better than those of commercially produced glass fibers. The chemical resistivity in 2 mol/1 NaOH is better than that of E-glass fibers. Although the diameter of glass fibers is not the only parameter which determines fiber properties and structure, it is a very important one because it influences both the mechanical and the thermal history of the fibers. For this reason, and in order to approximate commercial fiber drawing conditions, fibers were prepared with roughly the same fiber diameters. Despite this restriction the results provide information for qualitative ratings and for relative comparison of properties and structural interpretations. Of special interest is the opposite dependence of strength and elastic moduli on the fiber diameter. This is explained by means of frozen-in orientation and deformation effects of microcracks and network anisometries due to the mechanical and thermal history of the fibers produced by the fiber drawing process.
- ItemRheological investigations in the decomposition ränge of the system Na₂O-B₂O₃-SiO₂(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1997) Eberstein, Markus; Habeck, Andreas; Brückner, RolfThree glass melts from the Vycor-type widely distributed along the mixing gap of the pseudobinary line SiO₂-(B₂O₃:Na₂O = 84:16) were invesdgated each one with three different thermal histories. The following rheological properties were determined by the cylinder compression method: the Newtonian and non-Newtonian flow behaviour, the stress generadon modulus as a measure of the stiffness, the high-temperature tensile strength and the critical deformation rate at which the first crack appears. The influences of the fundamental glass compositions (SiO₂ content) as well as the thermal pretreatments on these properties are demonstrated and discussed on the basis of isochomal conditions (equal Newtonian viscosities). The results can be interpreted by means of the knowledge about the demixing rules and processes which lead to various rheological two-phase Systems. Most drastical changes of the rheological properties are observed in the middle of the mixing gap where spinodal decomposidon has its optimum and where he differentation between the matrix phase and decomposed phase is lost.
- 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.
- 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.
- 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.
- ItemThermal expansion behaviour of unidirectionally SiC fibre-reinforced MAS and BMAS glass-ceramics(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1995) Reich, Christian; Brückner, RolfThe thermal expansion behaviour of SiC fibre-reinforced Magnesium-Aluminium-Silicate (MAS) and Barium-Magnesium- Aluminium-Silicate (BMAS) glass-ceramics was studied in air as well as in argon furnace atmosphere. The results show that the coefficients of thermal expansion up to a temperature of 800 C are influenced to a minor degree by the furnace atmosphere and to a major degree by the phase composition of the matrices and the fibre concentration. The largest values in air and argon atmosphere are obtained from BMAS(14)/SiC fibre composites with α₁₅₀/₈₀₀ = 3.92 * 10⁻⁶ K⁻¹ and 3.87 * 10⁻⁶ K⁻¹ respectively, at a fibre content of 52 vol.%, while the lowest values are found for BMAS(9)/SiC fibre composites in air with α₁₅₀/₈₀₀ = 3.24 * 10⁻⁶ K⁻¹ and in argon with α₁₅₀/₈₀₀ = 3.11 * 10⁻⁶ K⁻¹ at a fibre content of about 25 vol.%. Increasing fibre concentration leads to increasing thermal expansion. The experimentally determined values were compared with calculated ones by the mixing rule and a good agreement was found, particularly for the BMAS(14) composites.
- 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.
- ItemUltrasonic measurements and complex elastic moduli of silicate glass melts in the viscoelastic and viscous range(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1994) Bornöft, Hansjörg; Brückner, RolfThe temperature- and frequency-dependence of the ultrasonic velocity and attenuation in glass melts is investigated up to a temperature of 1400 C by means of an equipment for measuring the longitudinal and transversal ultrasonic waves. The range of dispersion and the attenuation maxima are determined at frequencies 0.5 to 4 MHz completely for the DGG standard glass I and for the glass melts (composidons in mol%): 7 4 Si0₂ , 16Na₂O, 10CaO and 7 4 Si0₂ , 16 K₂O, 10CaO and partly for the glass melt 7 4 Si0₂ , 16CS₂O, 10CaO and for the B₂O₃ melt. The whole number of elastic constants follows from the ultrasonic velocities and from the density in the usual way The properties of these glass melts are compared along the temperature and viscosity. The transition from the elastic solid to the viscous fluid can be characterized by the elastic modul and interpreted as a viscoelasdc response of an unrelaxed to a relaxed dynamic network which depolymerizes reversibly with increasing and polymerizes with decreasing temperature. The internal frieden, Q⁻¹ characterizes the inelastic pordon during the periodical stress of the ultrasonic waves. The activation energy of Q⁻¹ obtained via frequencies and temperature of the maxima of is nearly twice the amount of the viscosity activation energy in the same temperature region.
- 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.
- 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.