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

Improved homogeneity of various glasses by gas film levitation

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

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

Nucleation and growth of precipitates in demixing glass melts in the system NA2O-CAO-SIO2.

As an example of a first order phase transition with conserved order parameter the demixing of glass-forming quasibinary systems has been investigated by in situ measurement of light scattering during heat treatment. The relative supercooling is consistent with theoretical expectations. Using a two stage heat treatment, nucleation and growth are decoupled. Nuclei formed at a nucleation temperature Tn can grow to sizes detectable by light scattering at a higher temperature Tgr, where no more nuclei are formed. Reducing the nucleation time, one at last ends up with an incubation time, the temperature dependence of which is described by an Arrhenius ansatz. For low particle densities, the growth mechanism can be deduced from the initial increase of the scattering intensity. The proportionality of the latter to t5 is characteristic for diffusion controlled growth of spheres in a depleted zone. From extrema in the scattering a definite size is assigned to the aggregate. Then, the number of particles can be deduced from the absolute intensity. For the systems investigated, however, growth time is limited by the occurrence of crystallisation, thus allowing only the determination of relative nucleation rates

Comparative Atomic Force and Scanning Electron Microscopic Studies Disclosing Nanocrystallinity in Cordierite Glass-Ceramics : examples of Surface Modification

The atomic force microscopy (AFM), because of its unique features, can be used for a variety of applications and provides excellent research and development opportunities in the area of nanoscience and nanotechnology of glass and glass-ceramics. In this study, microscopic experiments have been carried out using both AFM and SEM on polished and etched as well as fracture cordierite glass-ceramic surfaces to unfold their comparative capabilities. Most importantly, it has been exhibited that the AFM is capable to ascertain single tiny crystallites originated at the beginning of crystallization of glasses before scanning electron microscope (SEM) or X-ray diffraction (XRD) detects them. AFM provides extraordinary two-dimensional (2D), three-dimensional (3D), and quick surface plot (QSP) formats of images with unobscured (since no coating is necessary) views of nanostructures rather than obscured (since conducting coating is essential for insulators such as glass and glass-ceramics) and only 2D microstructural profiles of SEM. Development of interatomic forces (extended up to tens to hundreds of angstorms from the sample surface) between the atoms of the very sharp tip (probe) and those of surface (sample) during measurement results in unprecedented resolution (similar to0.1 nm) of images in AFM. While SEM produces images (resolution similar to10 nm) based on secondary electron emission from the sample surface. By comparing the results with those of SEM experiments, the AFM is established as a simple and powerful technique for the characterization of nanostructures of glass-ceramics particularly of early stages of crystallization

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

Influence of small additions of Li₂O raw materials on glass melting

For 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

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