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- ItemSpectroscopic investigations of barium aluminophosphate glasses containing pairs of transition elements(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1987) Zirkelbach, Karl; Brückner, RolfIn barium-aluminophosphate glasses containing two transition elements (two redox pairs) vanadium and iron ions are oxidized by manganese (combinations vanadium-manganese and iron-manganese), iron ions by vanadium (combination iron-vanadium). For low and intermediate concentrations of transition metal ions the redox equliibrium shift is complete during cooling from the melting temperatures, but for high concentrations it remains mostly incomplete (depending on the specific combination). The influence on Tg is given as an example for medium concentrations of Fe-V (8 mol).
- 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.
- ItemSome aspects of the workability of glass melts(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1990) Hessenkemper, Heiko; Brückner, RolfCriteria are given for the isothermal or isochomal workability of glass melts and discussed by means of a schematic example which is based on numerous measurements. Α thesis for a simple connection of this kind of workability is presented and examined by means of so far existing measurements. The isothermal or isochomal workability, which becomes more important with increasing deformation rate, is in contrast to the non-isothermal or athermal workability. The differences are based on certain connections between the stiffness, the relaxation modulus of a glass mek, its high-temperature fracture strength and the value of the quantity Β in the Vogel-Fulcher-Tammann (VFT) equation if the constants are evaluated from a measurement of the whole viscosity-temperature range, from 10^1.5 to 10^12 Pa s.
- ItemZum Redoxgleichgewicht Chrom — Mangan in Silicat- und Boratgläsern(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1984) Lee, Jeong-Hoon; Brückner, RolfIn Natriumsilicatgläsern nimmt die Dissoziation in der Reihenfolge Fe → Mn → Cr zu, während in Alkaliboratgläsern gilt: Fe → Cr → Mn. Dies liegt daran, daß die Polarisierbarkeit der Sauerstoffionen nicht nur von den Netzwerkwandler-, sondern auch von den Netzwerkbildnerionen und ihren Konzentrationen stark beeinflußt wird. Bei Analogieschlüssen von einem System auf andere, wie z. B. auch von wäßrigen Lösungen auf Oxidgläser, ist daher Vorsicht geboten.
- 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.
- ItemDer strukturmodifizierende Einfluß des Hydroxylgehaltes in Kieselgläsern(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1970) Brückner, RolfDie Anomalie des negativen Volumen-Temperaturverhaltens von hydroxylarmen, „natürlichen" Kieselgläsern (Typ I/II) im Temperaturbereich 1000 bis 1500 °C mit dem Hochtemperaturminimum bei 1550 °C tritt bei hydroxylreichen, „synthetischen" Kieselgläsern (Typ III) nicht auf. Die Volumen-Temperaturkurve zeigt einen Verlauf, der zwischen dem des Typs I/II und den binären Alkali- und Mehrkomponenten-Silicatgläsern liegt. Das Tieftemperaturminimum, je nach thermischer Vorgeschichte bei - 60 bis - 120 °C, bleibt dagegen erhalten und wird durch den Hydroxylgehalt zu tieferen Temperaturen verschoben. Die Ergebnisse werden mit dem Konzept der Vorordnungsbereiche, der strukturellen Spannungen und der Entkoppelungswirkung der Hydroxylgruppen in Verbindung mit der Vorstellung von Smyth über die bevorzugten transversalen Schwingungen im SiO2-Verband gedeutet.
- ItemLoad-dependent relaxation behaviour of various glass melts with different structural configurations(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1990) Hessenkemper, Heiko; Brückner, RolfGlass melts are usually treated in literature as thermorheologically simple fluids. The temperature-time equivalence of the relaxation behaviour is described by the WLF equation. This also for silicate melts well-known behaviour will be verified in the present paper with the help of the cylinder-compression method up to 240 Κ above Tg for very different melts. In addition it is shown that the relaxation behaviour is load-dependent above a certain load limit indicating a deviation from the conception of the thermorheological simplicity. The load-dependent relaxation behaviour is investigated for glass melts with very different network structures (chains, cross-linked chains and three-dimensionally connected structures). Α clear correlation with structure is demonstrated in that way that the relaxation ability and its load dependence increase with decreasing degree of structural interconnection from a three-dimensionally to a one-dimensionally linked potential network structure.
- 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.
- ItemCorrosion of refractory material under the action of forced convection flow by means of the rotating cylinder face area at 1500 °C(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1987) Dunkl, Michael; Brückner, RolfThe corrosion rates of various refractory materials in a container glass melt (sodium-calcium silicate basis) were determined quantitatively under the action of forced convection flow by means of the rotating cylinder face area at 1500 °C as compared with those at 1400 °C. The applied method not only gives a good comparison between the various types of bricks, but it also elucidates the influence of microstructure and of occasionally occuring crystalline or textural alterations during corrosion, which possibly can be detected only in experiments of extended duration. Present and earlier results on corrosion rates under the action of forced convection flow will be compared with those obtained from the same refractories under the action of free density and interfacial convection flows at 1400 and 1500 °C, which yield a very different dependence on temperature for the three convection flow types. The interfacial convection-driven corrosion shows the largest temperature dependence. Very critical are swelling effects, which sometimes can be observed only in separate long-time investigations.
- ItemSiC- and carbon-fibre-reinforced glass under alternating bending stress loadings(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1992) Klug, Thomas; Brückner, RolfAlternating bending stress experiments are described which were performed with various fibre/DURAN-glass composites reinforced by Nicalon NL 202 SiC fibres and by two different types of carbon fibres, a high-tensile strength (ht) and a high-modulus (hm) fibre. Also the influence of unidirectionally homogeneous and inhomogeneous fibre distribution as well as the bidirectional 0°/90° ply distribution are studied. In contrast to previously investigated strain-controlled experiments the present stress-controlled experiments show after the pretreatment of 1000 alternating tensile-compressive stress cycles fatigue (beginning damage) already at amplitudes below the limit of pure elasticity (below bendover stress) of a simple bending-load experiment. Despite of this difference the "training effect" of the previous investigation, the increase of the bendover stress with increasing stress amphtude after 1000 alternating-load cycles, is found to be similar and can be established also for the present experiments. The carbon-fibre-reinforced composites show a better tolerance of damage due to their smaller fibre diameters than the SiC-fibre-reinforced composites.