47 results
Search Results
Now showing 1 - 10 of 47
- ItemGravitational lensing in astronomy(Berlin : Springer, 1998) Wambsganss, J.Deflection of light by gravity was predicted by General Relativity and observationally confirmed in 1919. In the following decades, various aspects of the gravitational lens effect were explored theoretically. Among them were: the possibility of multiple or ring-like images of background sources, the use of lensing as a gravitational telescope on very faint and distant objects, and the possibility of determining Hubble's constant with lensing. It is only relatively recently, (after the discovery of the first doubly imaged quasar in 1979), that gravitational lensing has became an observational science. Today lensing is a booming part of astrophysics. In addition to multiply-imaged quasars, a number of other aspects of lensing have been discovered: For example, giant luminous arcs, quasar microlensing, Einstein rings, galactic microlensing events, arclets, and weak gravitational lensing. At present, literally hundreds of individual gravitational lens phenomena are known. Although still in its childhood, lensing has established itself as a very useful astrophysical tool with some remarkable successes. It has contributed significant new results in areas as different as the cosmological distance scale, the large scale matter distribution in the universe, mass and mass distribution of galaxy clusters, the physics of quasars, dark matter in galaxy halos, and galaxy structure. Looking at these successes in the recent past we predict an even more luminous future for gravitational lensing.
- ItemRedox chemistry of iron-manganese and iron-chromium interactions in soda-lime-silica glass melts(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1998) Nölle, GüntherGlass 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.
- ItemOxygen balance in sulfur-containing glass melts(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1998) Müller-Simon, HayoIn 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.
- ItemWater content of sulfate-fined industrial soda-lime glass and its influence on workability(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1998) Geotti-Bianchini, Franco; De Riu, LucaThe aims of the paper are: a) to systematically determine by IR spectroscopy the water content of sulfate-fmed industrial soda-lime glass (mainly Container and float, besides that household and technical glass) produced in Italy; b) to highlight the influence of redox State , melting conditions, batch and cullet addition on the water content; c) to check the consistency with time and between different feeders of the water content in the glass produced by a given furnace; d) to assess possible correlations between water content and workability in Container glass. The water content ranges between 300 and 400 ppm in conventionally fired glass, with electrical melting it decreases to 150 to 200 ppm, with oxycombustion it increases to 500 to 600 ppm. In the case of conventional flame melting the main factor influencing the water content is the redox State , due to a water/sulfate exchange. When a glass with a given color and redox State is produced in a given furnace, the daily oscillations of the OH level are moderate, with negligible effects on viscosity. In all the workability problems analyzed the contribution of water content oscillations was negligible as compared to other factors. According to the results obtained, significant water-related viscosity variations are expected to take place only when the meldng atmosphere or the redox conditions are significantly altered.
- ItemInfluence of nucleating agents on the crystallization of Mg-Ca-Si-Al-O-N oxynitride glasses(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1998) Deckwerth, Martin; Rüssel, ChristianOxynitride glasses in the system M g - C a - S i - A l - O - N were prepared with the aid of a polymeric preceramic aluminum nitride precursor as nitrogen source and transformed into glass-ceramics at temperatures in the ränge of 950 to 1350°C To support the devitrification process, TiO₂ , Cr₂O₃, MgF₂ and ZrO₂ were added to the batches as potentially nucleating agents. Under the reducing meldng conditions applied. TiO₂ led to undesired reactions in the glass melt and was rejected as nucleadng agent. Cr₂O₃ could only be dissolved in the glasses to a maximum content of 1 wt% and did not lead to obvious effects concerning nucleation and crystal growth. The addition of fluorine results in the formation of a fine-grained microstructure but by analogy to Cr₂O₃, did not lead to phase Separation. Adding ZrO₂, provoked phase Separation in the oxynitride glasses, which was even promoted by the presence of nitrogen. These glass-ceramics possessed an extremely fine-grained microstructure containing nonstabilized tetragonal ZrO₂, which results in an additional nitrogen content independent improvement of the mechanical properties. By comparison to Ti02, Cr203 and MgF2, zirconia has a twofold effect: it is not only an efficient nucleating agent in the case of oxynitride glasses but also an efficient toughening agent for the resulting glass-ceramics.
- ItemPhotochromic materials via sol-gel process(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1998) Fachet, Regina; Müller, Matthias; Bürger, Herbert; Kriltz, AntjePowders and thin films of photochromic glasses containing AgBr or AgCl microcrystals were synthesized via sol-gel process. Up to 10 wt%, of Ag⁺, bonded with suitable complexing agents, were introduced into the matrix system consisting of xSiO₂-(1 - x)Al₂O₃ (x = 0.7... 1). After drying and heat treatment the AgX microcrystals (X = Cl, Br) were characterized by X-ray diffraction and high-resolution TEM. The photochromic properties were measured using a diode array UV-VIS spectrometer.
- ItemOxynitride glasses(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1998) Navarro, José María FernándezThe present work is a review of the numerous nitridation experiments which have been carried out in different glass forming Systems. After a brief introduction to the topic of the solubility of nitrogen in glass melts, the nitridation procedures and the different ways of incorporating nitrogen into the glass network are described. Also considered is the increasing cross-linking structure due to the incoφOΓation of nitrogen. This involves a rise of the transition temperature, a decrease of the thermal expansion coefficient, and an improvement of the mechanical properties as well as of the chemical durability of the oxynitride glasses, depending closely on the nitrogen content. Some experiments on preparation of oxynitride glasses via the sol-gel procedure are also described. Due to their properdes the oxynitride glasses have been employed in several specific technical fields.
- ItemREAPOR® - Sintered open-pore glass foam as a high-strength sound absorber(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1998) Gödeke, Holger; Fuchs, Helmut V.The market for sound insulation products and Services, accounting for some 6000 mill. DM per annum in Germany alone, is covered to the extent of more than 90% by developments which are over 40 years old. Experts at the Fraunhofer Insdtute for Building Physics (IBP) (Director: Prof Dr. Dr. h. c. mult. Dr. E. h. mult. K. Gertis) have, together with an increasing number of small to medium-sized licensees, concentrated on the development of alternative, fibre-free absorbers (ALFA). The aim is to thus reduce noise pollution with more efficient sound-proofing technologies even in cases where traditional sound insulation materials are at a disadvantage as regards fitting and durabihty.
- ItemPreparation and properties of R₂O - CaO - SiO₂ - Si₃N₄ oxynitride glasses and melts(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1998) Larson, Sandra Christiane; Frischat, Günther HeinzOxynitride glasses, or rather melts, of the molar composition (74-3x) SiO₂, xSi₃N₄, 16 R₂O, 10 CaO with R₂O = Na₂O, K₂O show a significant increase in the glass transition temperature Tg, viscosity η and density ρ with increasing nitrogen content, whereas the thermal expansion coefficient α decreases. These data confirm the assumptions of the structural model in which trivalent nitrogen Substitutes for bivalent oxygen to produce a more tightly-linked glass network and most, if not all, of the nitrogen in the glass is linked to Silicon. ²⁹Si-MAS-NMR measurements on glasses of the System Na₂O-CaO-SiO₂-Si₃N₄ prove that the number of links between the [SiO₄]⁴¯ tetrahedra increase with increasing nitrogen content. Mixed alkali glasses, or rather melts, of the molar composition (74-3x) SiO₂, xSi₃N₄, 16(R₂O + K₂O), 10 CaO with R2O = Na₂O, Li₂O and different ratios of Na₂O:K₂O or Li₂O:K₂O show a deviation from the linearity in the glass transition temperature Tg, thermal expansion α, viscosity η and density ρ, which is shown in a minimum or maximum in the curves at a molar composition of about 10 R₂O:6 K₂O.
- Itemstructural investigations of SiO₂-containing phosphate glasses(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1998) Vogel, Jürgen; Jana, Carsten; Hartmann, PeterThe incorporation of SiO₂ into phosphate glasses is interesting in view of the development of phosphatic biomaterials. The present investigation is concerned with the incorporation of SiO₂ into phosphate base glasses possessing different phosphate structures. Due to their structural characteristics, they can be classified into three groups: glasses with long phosphate chains or rings formed by Q² phosphate structural groups, glasses with shorter phosphate chains formed by Q² and Q¹ groups and glasses with phosphate invert glass structure, possessing ortho(Q°)- and pyrophosphate groups (two Q¹ groups) exclusively. The structures of these glasses in dependence on the incorporated silica were investigated by transmission electron microscopy and by ³¹P MAS NMR. The principles of incorporation are discussed.