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- ItemApplication of the Christiansen-Shelyubskii method to determine homogeneity and refractive index of industrial glasses(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1995) Tenzler, Thomas; Frischat, Günther HeinzThe 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.
- ItemHomogenization of glass melts by bubbling on a laboratory scale(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1994) Högerl, Klaus; Frischat, Günther HeinzIn 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.
- ItemDrawing of oxynitride glass fibers(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1997) Kaplan-Dietrich, Hannelore; Frischat, Günther HeinzGlass 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.
- ItemLaboratory trials to incorporate sulphur in the vitrification of filter ashes from incineration plants and hydrolytic investigation of the melt products(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1997) Schumacher, Leo; Frischat, Günther HeinzIncineration 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.
- ItemLow-Tg phosphate glasses with improved water resistance for the coating of fluoride glass fibers(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1995) Hartmann, Matthias; Bitzer-Schmidt, Sigrid; Frischat, Günther HeinzStarting 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.
- ItemInfluence of small additions of Li₂O raw materials on glass melting(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1995) Tang, Yan; Frischat, Günther HeinzFor a container glass composition up to 1 mol% of the alkali oxide was substituted by Li₂O in steps of ≈0.2 %. The lithia was added as Li₂CO₃ and as spodumene, respectively. As shown earlier Li₂O reduces the viscosity of the melts considerably, irrespective of the Li₂O source. However, melt formation, carbonate decomposidon, silicate and silicate melt formation processes during raw materials reactions are considerably accelerated by Li₂CO₃ addition only, whereas Li₂O in the spodumene silicate does not enhance these reactions. The fining of the melt and the corrosion behavior of refractories are strongly influenced also by the lower viscosity melts. Thus, the corrosion rate may be diminished by temperature reduction at constant viscosity. For technical purposes the glasses are very stable against crystallization, however, thermal analysis shows that the higher the Li₂O content is, the faster the crystal growth proceeds. The surface tension of the glass melts decreases and the water leach resistance of the solid glasses is enhanced with increasing Li₂O content. The Li⁺ ions are less polarizable and, due to their high Dietzel field strength, the glass structure is dghtened compared with that of Li₂O-free glass. In conclusion, the substitution of small amounts of Li₂O instead of Na₂O and/or K₂O shows several advantages. At low temperatures different raw materials reactions are accelerated and at high temperature the decrease in melt viscosity is decisive.