55 results
Search Results
Now showing 1 - 10 of 55
- ItemThermodynamic approach to viscosity in the glass transition(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1994) Conradt, ReinhardThe 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.
- ItemBorosilicate glass matrix composites reinforced with short metal fibres(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1994) Boccaccini, Aldo R.; Ondracek, Gerhard; Syhre, ClaudiaThe 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.
- ItemThe mixed alkali effect revisited - A new look at an old problem(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1994) Ingramm, Malcom D.Progress is reported in resolving the long-standing problem of the mixed alkali effect. Previously theories have failed to encompass all aspects of this phenomenon which involves both a loosening up of glass structure and the reduction of ionic diffusivities. The recently proposed "dynamic structural model", however, reconciles the electrical and structural anomalies by recoupling ion hopping processes to localized site relaxations in glass. The appearance of a mismatch energy and site memory effects leads to successful prediction of the diffusivity "crossover". This could be the first step towards establishing a consensus on the mixed alkah effect which is reflected in several recent publications on this subject.
- 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.
- ItemLightweight glass technology: weight reductions and surface coatings(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1994) Meyer, Henning[no abstract available]
- ItemProtection against heat, radiation and glare - New developments in automotive glazings(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1994) Weigt, Paul; Schrey, HelmutTo maintain thermal comfort for driver and passengers in cars body-tinted, coated and insulated glasses were recently developed. Representative glasses of these categories and their thermal properties are presented. For assessment o f the sun-protective properties the spectrophotometric data are taken into account. The use of these glasses with respect to the legal Hght transmittance limits is discussed. The presentation of the heat protection function includes the determination of U values for different driving conditions. The determination of the spectrophotometric and heat engineering data with respect to the International Standard ISO/DIS 9050 is discussed. I t is shown, how far deviations from this standard can lead to misleading evaluations of the same glazings.
- ItemInvestigations into all-electric melting of neutral glass with improved reboil behaviour(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1994) Haft, Rainer; Linz, Hans-JürgenWith regard to chemical stability, great demands are made on neutral glasses because of their pharmaceutical application, which requires a high standard of the quality parameters: bubble numbers, homogeneity, and reboil stability. Nowadays, acceptable results are only reached by fuel-fired special furnaces with very high specific energy consumption and engineering efforts. The economical and ecological advantages of all-electric glass melting could contribute to considerable cost cutting. At present, however, certain quality criteria are not within reach when using this melting technology. The main disadvantages are the comparatively high residual gas contents and bubble numbers. Up to now, efforts have been unsuccessful to melt neutral glass with sufficient glass quality all electrically. The possible advantages, however, are worth treating this problem further. The present conception is based on a separation in melting and fining basin. By using knowledge of all-electric melting of borosilicate glass 3.3, the disadvantages in respect of gas solubility and bubble numbers will be removed in a separate fining cell by use of bubbling. Various laboratory experiments led to the concept of a combination of chemical and physical refining. The parameters for optimizing the refining process have been investigated. As a result of this work, a technical realization is proposed.
- ItemSpectroscopic investigations of glasses with semiconductor microcrystals at higher temperatures(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1994) Kritz, Antje; Müller, MatthiasGlasses containing microcrystals of CuBr or CuCl show two characteristic absorption peaks caused by excitonic transitions. If temperature is increased the peaks become wider and flatter, whereas the complete disappearance o f the excitonic peaks takes place in a narrow temperature range of 30 to 50 K. Absorption intensity in the UV region (at about 30 000 cm⁻¹) increases simultaneously. The reason for these reversible changes is the melting of the microcrystals about 200 K below the melting point of the corresponding bulk crystals. The melting point depends on size and composition of the microcrystals.
- ItemContact between hot glass and wet porous material : Part 1. Theoretical considerations and experimental results(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1994) Trier, WolfgangThe contact behaviour between hot glass and a porous material containing water has been investigated. The mathematical equations which describe the one-dimensional problem have been derived. A test rig has been developed to measure the time-dependent contact temperature and pressure for contact materials such as sintered stainless steel, alumina, zirconia and sintered glass. It could be shown that the contact temperature is dependent on the saturation pressure of the generated steam. This is valid for the tight system. If the system is leaky and steam escapes laterally, the boiling front migrates into the porous medium and the contact temperature rises rapidly above the saturation temperature. The heat flux density originating from the thermal contact has been calculated using the mathematical equations and the experimental data. In the first second of contact, the heat flux density is nearly constant. With a pressure of 3 bar and a porosity of 25% of the medium the heat flux density ( in W/m²) is 140*10³ for stainless steel, 120*10^ for alumina, 115-10³ for zirconia ( ZrO₂ ) and 70*10³ for sintered glass. The leaky system shows higher values of heat flux density of 260* 10³ W/m² for the sintered stainless steel which is dependent upon the mass flow of the escaping steam. After release o f the pressure, the hot water (ϑhw > 100 °C) reboils. Only some of the water boils, the rest being ejected in form of small droplets or foam. In the case of the leaky system, the reboiling and ejection of water droplets are more violent. The tight system develops a steam layer, which prevents direct contact between the glass and the porous material. The resulting glass surface is very smooth. Under certain not yet fully clarified conditions contact with sintered alumina and zirconia produces glass surfaces covered by numerous rounded pits with a diameter of ≈ 5 µ m . These pits are probably generated by the ejected water droplets.
- ItemGlass-ceramics based on phases with NZP-type structure(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1994) Müller, Gerd; Schultz, Sven Gerd; Mildner, Markus[no abstract available]