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Sol-gel preparation of scratch-resistant AI₂O₃ coatings on float glass
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.
Preparation and properties of R₂O - CaO - SiO₂ - Si₃N₄ oxynitride glasses and melts
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.
Application of the Christiansen-Shelyubskii method to determine homogeneity and refractive index of industrial glasses
The Christiansen-Shelyubskii method has been applied to determine the homogeneity of both colorless and colored technical glasses. It could be confirmed that this method is sufficiently sensitive to changes by the melting process of flat, container and special glasses. The homogeneity factor, which essentially is the standard deviation of the refractive index, can be obtained with a precision of about ±5 %. The measurement simultaneously delivers the mean refractive index with high accuracy. This property possibly could be used to substitute density measurements to control the constancy of glass composition. The Christiansen-Shelyubskii method can be standardized and highly automated. About 15 samples can be measured per day. Thus it shows all features of a method to be used for industrial quality control.
Optimizing the Christiansen-Shelyubskii method and its comparison with industrial control methods for homogeneity determination of glasses
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.
Drawing of oxynitride glass fibers
Glass fibers were drawn from three MgO-Al₂O₃-Y₂O₃-SiO₂-based oxynitride glass melts. Α single-hole bushing process was used to spin the melts containing nitrogen contents between about 13 and 16at.%. The drawing process is described in detail, and it is shown that besides melt viscosity the high surface tension of the oxynitride glass melts strongly controls the fiberization. Thi s is analyzed in terms of Reynolds and Weber numbers. Glass fibers up to a length of about 30 cm can be drawn for Reynolds numbers between about O.Ol and 0.2 and Weber numbers between about 2.6 and 3.1, however, even there the fiber diameter oscillates to some extent. For smaller Reynolds and Weber numbers it is impossible to draw fibers at all. In this instability regime only droplets leave the nozzle outlet. The oxynitride fibers obtained have excellent mechanical properties and a high chemical resistance to alkaline attack.
Laboratory trials to incorporate sulphur in the vitrification of filter ashes from incineration plants and hydrolytic investigation of the melt products
Incineration of refuse in incineration plants produces toxic dusts in the waste gas filters which have to be disposed of Vitrification is one possibility of immobilization of such dusts which are contaminated by heavy metals, sulphates and Chlorides. Under oxidizing melting conditions, alkaline sulphates a n d Chlorides would volatilize and would have to be precipitated again. Therefore, in this work reducing melts were used. Thus, it was possible to reduce the sulphate to sulphide and to incorporate the latter in the glass matrix to an efficiency of up to 100%. This is experimentally proved by melting batch compositions of a filter ash from a refuse incineration plant with glass forming additives as well as model glasses. The reduction of CaSO₄ and formation of sulphides is induced by means of suitable additions of the Clements carbon, iron, zinc, titanium, manganese, Silicon, and aluminium, separately and in combination. The sulphides CaS, FeS, ZnS, MnS and AI₂S₃ are present in the melt product mainly in giassy form and only for a small part in a crystalline phase. The melts with additions of manganese and Silicon show the best properties with respect tc vitrification, incorporation of the sulphur and hydrolytic resistance of the product. By means of leaching tests using the Soxhlet equipment, products of great hydrolitic resistance are proved falling under the hydrolytic class III at least.
Morphology of silica and borosilicate glass fracture surfaces by atomic force microscopy
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.
Low-Tg phosphate glasses with improved water resistance for the coating of fluoride glass fibers
Starting from a multicomponent lead phosphate glass composition given in the literature, a glass was developed suitable for the protective overclad of heavy-metal fluoride glass fibers. Besides a low glass transition temperature < 270 °C, a high thermal expansion coefficient > 18 * 10⁻⁶ K⁻¹ the phosphate glass material developed had a water resistance at 60°C for 10 h comparable to that of commercial container glass. The final composition developed was (in mol%): 54.7 P₂O₅, 19.8 PbO, 18.0 Na₂O, 1.2 MgO, 1.2 BaO, 1.2 CaO, 0.4 V₂O₅, 1.5 AI₂O₃, 2.0 Bi₂O₃. Besides the influences of these different components the acdon of the melting conditions, especially melting temperature and trace SiO₂ content from the crucible material, have to be taken into account. Fiber drawing experiments including the in-situ apphcation of the phosphate glass overclad were successful.
Homogenization of glass melts by bubbling on a laboratory scale
In a typical melter, the molten glass tends to be inhomogeneous due to the heterogeneity of the raw materials. One means of yielding more homogeneous glass is bubbling air through the glass melt through nozzles at the base of the melter. The induced fluid flow dissolves cords and homogenizes the glass melt. This bubbling process was investigated on a laboratory scale both from an experimental and a theoretical point of view. A standard soda-lime-silica glass was bubbled with argon in a platinum crucible at 1400 C. The samples treated were tested with regard to their optical homogeneity, using an improved version of the Christiansen-Shelyubskii method. The corresponding fluid flow phenomena were simulated by a suitable mathematical model. Due to the axial symmetry of the bubbling equipment and the high viscosity of the glass melt (creeping flow), the problem can be reduced to the solution of a differential equation of the fourth order with the stream function as independent variable. The numerical treatment superposes Gegenbauer functions matching the given boundary values for the velocity and tension, respectively. The homogeneity strongly increased with bubbling time and its local variation showed good correlation with the calculated flow pattern in the crucible.
Analysis of as-prepared and corroded glass fibers by secondary neutral mass spectrometry
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.