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Study of surface changes on industrial glasses with AFM, FE-SEM, EDX, SNMS and LM : Part 1. Glass skin and corrosion
By combining different analytieal techniques, including modern high resolution imaging tools such as field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM), new phenomena in the surface, near surface and bulk structure of various industrial glasses (without any cleaning process) have been diseovered. Investigations of soda-lime-silica container glass and lead silicate glass tubings exposed to atmospheric and to dried air are presented. The results of the first part of this paper can be explained with an SiO2 rich skin on the glass, which can protect the glass against the attack of external media. If the skin was mechanically injured, inhomogeneous corrosion products on pm scale grew around the injured site after exposure to humid air for times between days up to one year. Microchannels were formed through the injured site due to restricted ion exchange, followed by a local increase of the pH value and consequently the dissolution of the glass network. Finally some stress in the glass, yielded during manufacture, can be partially released and the cutting behaviour is improved. Faster cooling results in a thinner skin and the ions in the glass are able to migrate to the surface more easily when surrounded by some reactive media. Slower cooling results in thicker skin and wax-like droplets instead of erystallites are formed on the surface at a later stage in humid air. The wax-like droplets can etch the glass skin locally, followed by the growth of inhomogeneous corrosion products similar to the injury induced corrosion. If the fresh glass surface was hot-end treated, the quality of the adjacent coating layer depended strongly on the thickness of the skin.
Time resolved fluorescence measurements on Tb3+ and Mn2+ doped glasses
Tb3+ (4f8) and Mn2+ (3d5) ions, known as active luminescent centres for blue, green and red fluorescence, were doped in various fluoride, phosphate and Silicate glasses with well known structure. Narrow bands of f-f transitions with strong emission of Tb3+ in the blue, green and red and broad bands of d-d transitions of Mn2+ were measured with green emission in high optical basicity glasses with tetrahedrally coordinated Mn2+. Orange to red Mn2+ emission was found in glasses with low optical basicity where Mn2+ is octahedrally coordinated. Lifetimes, τe, in the range of milliseconds were recorded in dependence of glass composition and dopant concentration for both Tb3+ and Mn2+ doped glasses. Fluorescence lifetimes are as well shortened by higher basicity of the glasses as by increasing dopant concentration.
Silicon in container glasses, its solution kinetics and dissolving potential in container glass furnaces
The forming proeess of sihcon stones in differently colored container glasses under the action of different aluminum pieces will be discussed and some special features during the generation reported. The dissolution of formed silicon stones was analyzed in dependence on melting time, temperature and color of the glass. With the knowledge of these parameters the rate constant and the diffusion coefficient of the dissoludon were calculated. The main results are: first, it is not possible to generate large silicon stones anyway, indicated by big aluminum pieces which will be divided when they become liquid. Second, the dissolving of the silicon stones does not go on homogenously about the whole temperature range analyzed. But it can be divided into three fields: an area below, an area around and an area above the melting point of the silicon. The reason for these differences is found in the SiO2-rich layer around the stones which will be formed and dissolved by different rates in colored container glasses. On the basis of these dissolving rates and of the temperature profiles in glass meldng furnaces the dissolving potential in furnaces could be calculated. Three possible routes of silicon stones through special types of furnaces show the respective dissolving potential for silicon. The potential for dissolution of elementary Silicon for the calculated types of Container glass furnaces is nearly the same. The dissolution rates could be improved by raising the temperature of the glass bath in the furnace and by raising the dwell time of the silicon stones in the furnace.
Influence of modifier cations on the radiation-induced effects of metaphosphate glasses
The influence of different modifying cations on the radiation-induced defect generation in metaphosphate glasses was studied by optical absorption and electron paramagnetic resonance spectroscopy. For this task, several model glasses were prepared. These were simple metaphosphates of alkaline earths, aluminum and zinc. The synthesized glass samples were irradiated with X-rays and with UV light from an Xe/Hg lamp. The cation-dependent differences in the structure of the phosphate glasses were found to have a great influence on the concentration of some radiation-induced defects and thus on the optical absorption. The influence of the different cations became especially distinct under UV light Irradiation due to the selective interaction of the UV light with the glass samples. Some additional phenomena were observed for the irradiated zinc and magnesium metaphosphate glasses which were not found by the previously undertaken study of complex phosphate glasses.
Application of electrical heating in forehearth channels
Electrical heating of forehearth channels can be either indirect with radiant heating elements installed above the glass surface, or direct utilizing immersed electrodes. Two types of heating elements are used, made of either silicon carbide or molybdenum disilicide. Electrodes are normally made of molybdenum, but tin oxide is also used, and platinum alloy may be required for some special applications. Direct electrical heating is also used in forehearths to supplement the traditional gas heating, and can provide a significant improvement in the thermal homogeneity of the glass at the end of the channel. Excellent results can also be obtained with full electrical heating. Whatever type of electrical heating is used, the relevant safety standards must be applied.
Glass tank reinforcements
Compared with refractory bricks, molybdenum is far more corrosion resistant to most glass types. Therefore, cladding bricks with molybdenum sheets can protect the critieal areas (e.g. doghouse, throat channel, crosswall) of glass tanks against wear. Maintaining the form of the bricks helps to improve the glass quality and signifieantly extends the service life (campaign) of the glass tank. In order to install molybdenum parts in a cold glass tank and to protect them against oxidation during the up-tempering phase, PLANSEE developed the SIBOR® coating - the only coating which guarantees 100% oxidation protection during the heat-up (initial tempering) of the tank until the shielding is completely immersed in the molten glass.
In situ measurement of batch glass during melting
Α novel optical measuring method is described which was used for the density measurement of model samples of glass batch during the melting process. It used a sample volume of some 1000 mm3, which was suffieiently large to suppress single particle effects. Shape changes of batch samples during heating were taken into account by the optical method. Volumetric changes of soda-limesilica glass batch were monitored between room temperature and 1100 °C. Initially a sintering stage was observed showing seif similar reduction in the sample volume. Thereafter, the sample shape approached the equilibrium surface of drops. This was accompanied by a large increase in volume, which was attributed to the formation of gas bubbles within the batch samples. Α strong influence of heating rate and composition on this volume increase was observed.
Analysis of dissolution of a spherical gas bubble in a glass melt in the presence of a flow and a fast redox reaction
Modeling a process of dissoludon of a slowly moving gas bubble in a glass melt can be reformulated as a problem of dissoludon of a stadonary bubble in a flow. It is shown that in the presence of fast redox reaction (outside the bubble) the effect of the flow on the time of dissolution is negligible. Many authors omit the flow when modeling dissolution of bubbles in glass melts. This analysis specifies the conditions under which such omission can be justified. Due to the presence of fast reaction the problem is singularly perturbed. The boundary function method was used to obtain asymptotic approximation of the solution. Asymptotic results are compared with numerical computations performed for the original statement of the problem.
Preparation and properties of model glasses of historic enamels of the Green Vault Museum, Dresden
Many enamelled art objects of the Green Vauk Museum, Dresden, have been suffering a severe deterioration of the enamel layers during the last 20 to 30 years. The authors obtained three splinters of about 200 mg weight each of green, blue, and opaque turquoise enamel of the Aurangzeb ensemble. Model glasses of these originals were prepared which should match them as closely as possible. The investigations showed that the simple degree of network polymerization related to the SiO2/Na2O ratio is insufficient to understand their chemical properties even if one takes components such as MgO, CaO or AI2O3 also into consideration. To understand further structural influences IR, UV/VIS, and EPR spectroscopy studies were performed on the enamels, and it could be evidenced that components such as MnO, Fe2O3, CuO, CoO, PbO and SnO2 not only act as colorants or opacifiers but also as stabilizers of these materials. Taking this into account the chemical and physical behavior of the respective enamels in the museum can be elucidated.