1999, Müller-Fildebrandt, Constanze, Priller, Stefan, Frischat, Günther Heinz

Three types of glass fibers, differing mainly in their alumina, alkaline earth and alkali oxide contents, were investigated by secondary neutral mass spectrometry. The investigation was carried out on fibers as-prepared and after exposure to humidity and to attack in distilled water. Concentration-depth profiles of the fibers obtained by secondary neutral mass spectrometry showed that the asprepared fibers were already pre-experimentally weathered in a ≤50nm deep surface zone. The pattern of chemical change and leaching was found for each fiber to be similar after exposure to humidity and short-term attack by water, respectively, however with enhanced leaching observed in water, where the leached elements are removed from the fiber surfaces. Different leach mechanisms could be established for the different fiber types and changes of leach mechanisms with time could be observed. Altogether, the depth profiling method proved to be a powerful tool to elucidate the comphcated leach mechanisms also in the case of glass fibers.

1998, Holtmann, Klaus Heinrich, Frischat, Günther Heinz, Ruppert, Karl

Stray light on the windshields of automotive vehicles, caused by the impact of small particles, imposes severe safety hazards during night driving. Due to reduced contrast by stray light, nonilluminated objects are perceived much later than through pristine windshields. The objective of this paper therefore is to show the dependence of stray light on impact velocity, impact angle, particle shape and the mechanical properties of the impacting particle and the target material. Single and multiple impacts of SiC (≈1 mg/particle) and quartz sand (≈ 0.7 mg/particle) on ordinary and strengthened float glass under angles of 30°, 45° and 90° and velocities between 50 and 200 km/h were investigated. Profilometer and various methods of stray light measurement, such as total integrated scattering, were used. Based on the correlation between surface roughness and stray light, it can be proved that the amount of stray light is proportional to the ratio of impact volume to the area of the undamaged surface. Caiculating the contact pressure during impact on the target for Sharp and rounded particles in dependence on velocity and impact angle and applying the fracture mechanical criteria for the formation of lateral cracks allows the modeling of the impact volume for the various combinations of particle and target contacts and thus an estimation of the stray light to be expected. Comparison of the model calculation with experimental data exhibits good agreement. Both sets of data show an increase in the amount of damage with increasing velocity and steeper impact angle. The damage caused by SiC is always more severe than the damage by quartz sand. Increasing the fracture toughness of the target material reduces the extent of the damage by about 10%. Α more pronounced reducdon of stray light and mechanical deterioration of up to 60% in comparison to normal float glass can be achieved by a newly developed composite material, consisting of a 0.2 mm thick glass, a 0.36 mm thick layer of PVB, and a float glass Substrate of 3 mm thickness.

1999, Wünsche, Christine, Rädlein, Edda, Frischat, Günther Heinz

Fracture surfaces of Herasil (silica) and Duran (borosilicate) glass rods were examined with an atomic force microscope. Generally the roughness in the fracture surface increases with growing distance from the origin of fracture. The morphology displays a variety of features. In the smooth fracture mirror near the origin of fracture the surface consists of small hillocks ≈25 nm in diameter and ≤ 1 nm high. With growing distance these hillocks increase in size and height. In the mist zone bigger outcrops occur with steep flanks. Steps can also be discovered in the fracture surface. Parts of the surface are covered by ripples, which are proposed to result from a local melting of the glass near the crack tip during fracture.

1999, Heidrich, Roland Paul, Frischat, Günther Heinz

The Christiansen-Shelyubskii method used in this work was optimized experimentally. Different tests were performed with two grain fractions of standardized crushed sheet glass. Maximum transmissions, half-widths and their Standard deviations were optimized, and it was further shown that sampling, an appropriate cleaning of the granulated glass and the quality of the optical cells used are of great importance, too. Α variety of industrial glasses was investigated and the Christiansen-Shelyubskii homogeneity numbers were compared with the homogeneity data provided by the glass manufacturers using industrial homogeneity control methods for Container, tube and flat glasses, respectively. Although the physical principles of the homogeneity methods compared were dissimilar, mostly qualitatively similar trends were found. Only in the case of the Mach-Zehnder interferometry method, which is based on a similar principle as the Christiansen-Shelyubskii method, a quantitative correlation analysis was possible. Reasonable agreement between the homogeneity values from both methods was achieved. It was further shown that the Christiansen-Shelyubskii method can monitor the homogeneity during glass melting with external cullet, both in freshly prepared and in composted states.

1998, Schmitz, Regine, Frischat, Günther Heinz

The electron gas secondary neutral mass spectrometry, operated in the high-frequency mode, is especially suitable for the analysis of electrically insulating materials. This is demonstrated for the sol-gel coating Systems Li₂O · 2 SiO₂ and BaO · 2 SiO₂ on silica glass Substrates as examples. The Li₂O · 2 SiO₂ coating is crystalline and displays fluctuations in composition and/or phases. This heterogeneity is confirmed on atomic force microscope Images. Depending on heat treatment the BaO · 2 SiO₂ coatings are glassy and crystalline, respectively. They do not show any fluctuations; however, the depth profiles display some AI₂O₃ having diffused from the silica Substrates into the films during sol-gel consolidadon. Atomic force microscope Images reveal grooves in the Substrate surfaces in which AI₂O₃ remnants of the grinding process have been captured. The depth of the AI₂O₃ profile from the SNMS measurement corresponds to the depth of Channels found using the atomic force microscope. It is assumed that AI₂O₃ originates from remnants of the grinding material used to produce the silica Substrates.

1999, Hauk, Rainer, Frischat, Günther Heinz, Ruppert, Karl

Glass surfaces including those of automotive glazing show insufficient Scratch resistance. In order to improve this, a sol-gel process has been developed by means of which scratch-resistant and transparent corundum coatings (α-Αl₂O₃) may be deposited on glass surfaces. Coatings which were produced by using commercial boehmite(AlOOH) dispersions could also be completely transformed into α-Αl₂O₃, but they were not dense enough at about 500 nm thickness. Coatings produced by an alkoxide-gel process from aluminium butoxide were dense at a thickness of maximal 100 nm and adhered very well to the glass surface which had been slightly roughened by HF etching. Α Scratch test with a quartz grain showed that the damage of the glass surfaces coated in this way was less than 2 % compared to that of uncoated surfaces. Transmission of these coated glasses was about 90 % in the visible wavelength ränge.

1998, Larson, Sandra Christiane, Frischat, Günther Heinz

Oxynitride 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.

1998, Janke, Achim, Frischat, Günther Heinz

Heavy-metal fluoride glasses of the system ZrF₄-BaF₂-LaF₃-AlF₃-NaF and soda-lime-silica glasses both from industrial and laboratory crucible production were processed by the gas film levitation technique. In this process the glass melt is suspended contactlessly on a thin gas film, avoiding any reaction with the crucible material. This prevents chemical contamination, surface defects and heterogeneous nucleation, which is especially advantageous for the preparation of specialty glasses. The overall homogeneity of the glasses was strongly improved, too, which was characterized quantitatively by the Christiansen-Shelyubskii method.