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Weathering of tin oxide coated glass with low IR emissivity
The weathering of K-glass has been studied by measuring transmission and reflection in the visible (VIS), near infrared (NIR) and infrared (IR) wavelength ranges, by chemical depth profiling with secondary neutral mass spectrometry (SNMS) and by atomic force microscopy (AFM) imaging. K-glass is a commercial tin oxide coated glass with low IR emissivity. The SnO2:F-coated side has been exposed to atmospheric conditions for up to 32 weeks and for up to two years. It could be shown that the optical properties are only affected in the VIS range, but the visible contamination does not influence the high IR reflection. This means that the heat-insulating properties of window glazings with the K-glass coating on the surface do not degrade under atmospheric conditions. Chemical depth profiles did not reveal major changes, except for an increase in the signals of minor elements on the surface, namely sodium, carbon and silicon. AFM showed that after 32 weeks the deep valleys of the rather rough crystalline SnO2:F are partially filled up. The high transmission of new K-glass can be regained since contamination or corrosion products can be removed by washing. The coating itself is chemically stable.
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.
Atomic force microscope study of silicate glass fracture surfaces in air and in water environment
In the present work changes in the topography of less resistant K2O-CaO-SiO2 and more stable Na2O-CaO-Al2O3-SiO2 glasses were studied in-situ by using a high-resolution atomic force microscope (AFM) in contact mode. The images were taken in air and in water, using a special liquid cell in the latter case. Fresh fracture surfaces were prepared in the corresponding media and the samples were transferred immediately into the AFM. The freshly fractured K2O-containing glasses displayed an irregular ripple pattern in air with ripple diameters of (80 ± 20) nm, heights < 1.2 nm and root mean square (rms) roughnesses < 0.8 nm. With ongoing exposure time, these glasses form a gel layer, isolated droplets and larger drops which can even coalesce. Also partially crystalline secondary reaction products can be observed in the drops. The large drops themselves displayed topographies with a ripple pattern similar to that found on the gel layer-coated surfaces. The drops and the gel layers are soft and may be penetrated and moved by the scanning tip. The in-situ investigation of these glasses in water revealed topographies with a ripple pattern similar to freshly prepared surfaces. This pattern did not change with exposure time, obviously because the water in excess rinsed away possible reaction products. The more resistant Al2O3-containing glasses show a dissimilar behaviour. The ripple pattern on the fracture surfaces is smaller and does not change even after days. Due to the absence of capillary forces and lower van der Waals forces the ripple diameters are smaller in water than in air. Glasses with a high degree of network polymerization also show smaller ripples than glasses with more nonbridging oxygens.
Mid-infrared laser absorption spectroscopy for process and emission control in the glass melting industry : Part 2. Difference frequency generation based MIR laser spectrometer for glass melting furnaces
Emerging techniques of mid-infrared absorption spectroscopy offer potentially great sensitivity and selectivity for combustion control and emission monitoring. Beeause of that, a differenee frequency based mid-infrared absorption spectrometer has been considered for application in the glass industry. Based on preliminary tests within laboratory conditions, a spectrometer which operates at wavelengths around 5 μm was applied to online monitoring of the atmosphere of a gas fired glass melting furnace. The CO concentration was measured in order to demonstrate the feasibility of a mid-infrared absorption spectrometer for process control in the glass industry. Α series of measurements was performed in situ as well as crossing the recuperator entry, resulting in general advice on the construction of a prototype device.
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.
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.
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.
Mid-infrared laser absorption spectroscopy for process and emission control in the glass melting industry : Part 1. Potentials
Laser spectroscopic diagnostics of combustion species at elevated temperature has shown considerable progress during the last years and decades. Particularly, techniques of mid-infrared absorption spectroscopy using fundamental rotational/vibrational transitions offer potentially great sensitivity and selectivity for combustion control. However, applications are still limited to mostly laboratoryscale investigations, which is mainly due to drawbacks of light sources and beam guidance materials. On the other band, many efforts are made to facilitate industrial application. In this context, the relevance of mid-infrared absorption spectroscopy for in situ monitoring of minor species in glass melting furnaces is obvious, too. The present paper gives a general review of available techniques and their impact on emission diagnostics in the glass industry, considering conventional as well as emerging light sources, detectors, peripheral devices and spectroscopic techniques.