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stress generation modulus as a counterpart of the stress relaxation modulus
In 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.
In-situ investigations of sintering and crystallization of lithium aluminosilicate glass-ceramics
The sintering and crystalhzation behaviour of a Li₂O-Al₂O₃-SiO₂ glass powder containing TiO₂ and ZrO₂ was investigated by the in-situ measuring methods of high-temperature dilatometry, high-temperature X-ray diffractometry and differential thermal analysis at heating rates between 0.5 a n d 20 K/min. By comparing the results of the different methods the shrinkage could be explained as an overlap of sintering and crystallization processes. When increasing the heating rate, the sintering and crystallization processes are separated mor e sharply, so that higher final densities of the sintered samples can be achieved.
Redox chemistry of iron-manganese and iron-chromium interactions in soda-lime-silica glass melts
Glass melts are mostly not in a chemical equilibrium with a coexisting gas phase, it does not adjust before maintaining the melt at a constant temperature and a constant gas phase for hours. But the one-phase equilibrium between several polyvalent Clements in a melt above the glass transition temperature always exists, it adjusts spontaneously. Below the transition temperature the redox State is invariably frozen-in. The calculation of redox states must be based on the matter and Charge balance (stoichiometric equations) and equilibrium relations (mass-action law). Such a calculation of Schreiber's experimental results was consistently possible. Accordingly cooled glasses are free from Cr⁶⁺, if the glass contains Fe²⁺. To achieve this one has to melt oxidized chromium-containing glasses with sufficient iron content.
Nucleation at cordierite glass surfaces: Kinetic aspects
The surface nucleation of high-quartz sohd Solution crystals at fractured surfaces of glasses of the stoichiometric cordierite composition (2MgO · 2AI₂O₃ · 5SiO₂) was studied by optical microscopy. Particular attention was focused on the nucleation kinetics. Α constant nucleation density, Ν ≈ 10⁻⁴ μm⁻², was found not to be significantly influenced by the time and the temperature of nucleation treatment. Even a very fast heating of samples employing heating rates u p to 1200 K/min does not lower Ν substantially. However, for small average crystal diameters ( < 2 0 μm) a distribution of crystal size in the same order of magnitude is detectable indicating a simultaneous appearance of b o t h measurable nucleation rates and growth velocities. It can be concluded that the surface nucleation of µ-cordierite occurs during the thermal treatment from a limited number of preferred nucleation sites; these sites are "used Up" rapidly enough to cause a strong Saturation effect of nucleation, but slow enough to cause a crystal size distribution at the same time. The surface nucleation rate, Is , was calculated from the observed distribution of crystal sizes. Is progressively increases with rising temperature similar to the crystal growth velocity indicating a broad temperature ränge of essential nucleation activity. The latter must be regarded as t he main obstacle to measure or to control surface nucleation density by means of two-step nucleation and growth treatments and must therefore be claimed to be mainly responsible for the observed constancy of N.
Borosilicate glass matrix composites reinforced with short metal fibres
The reinforcement of a borosihcate glass with Hastelloy X short fibres is discussed. A fabrication process consisting of wet mixing of the glass powder and the metallic fibres and hot pressing of the mixtures was developed. Composites containing 15 vol.% fibres homogeneously distributed in a near fully dense glass matrix were obtained. Both Young's modulus and fracture strength increased with increasing volume fraction of fibres. The experimental values for the Young's modulus are in agreement with theoretical predictions. No fibre pull-out during fracture was detected and there is a good interfacial bond. By means of a rule-of-mixture calculation for the fracture energy, a five-fold increase in fracture toughness for composites containing 15 vol.% fibres can be predicted.
Konventionell beheizte Glasschmelzöfen für die Hohl- und Flachglasherstellung im deutschsprachigen Raum - Ergebnisse einer HVG-Umfrage - Teil 1. Beheizung, bauliche Merkmale, Schmelzleistung
[no abstract available]
Thermodynamic approach to viscosity in the glass transition
The glass transition is re-investigated by means of the formahsm of linear nonequilibrium thermodynamics. The process is treated for melts cooled down below their liquidus temperatures in terms of three events. These events are, in the order of decreasing temperature: by-passing of crystallization, freezing-in of stress relaxation, and freezing-in o f near-range structural relaxation. Conditions for the viscosity levels are derived at which each of these events is accomplished. The model is tested against data of five one-component, three simple binary and ternary, and three multicomponent systems. Within the scope of this test, it is confirmed that the model correctly describes the viscosity levels typical of the glass transition, as well as their dependence on the cooling rate. An explanation for the 10¹³dPa*s rule at Tg is included.
Oxygen balance in sulfur-containing glass melts
In sulfur-containing glasses the Fe²⁺/Fe³⁺ ratio at a given oxygen partial pressure shows a considerable shift compared to glasses without sulfur. This effect is caused by an electron exchange and governs the redox behavior of industrial glass melts at fming temperature. The electron exchange is formulated by a thermodynamic approach which can quantitatively describe the redox reactions between melting temperature and room temperature in flint glasses. In amber and green glasses this dependence is superimposed by the dissolution of SO₂-containing bubbles, which results in an additional shift of the oxygen partial pressure in the direction of the sulfur solubility minimum during cooling. From this shift the maximum solubility of sulfite in soda-lime-sihca glass melts can be estimated to be about 0.025 wt% SO₃. Resorption curves of sulfite sulfur in the melt describe the refining quahty. They can serve as a tool for improving the secondary refining in sulfur-containing industrial glass melts.