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- ItemDepth profiling analysis of lithium and barium disilicate coatings on silica glass(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1998) Schmitz, Regine; Frischat, Günther HeinzThe electron gas secondary neutral mass spectrometry, operated in the high-frequency mode, is especially suitable for the analysis of electrically insulating materials. This is demonstrated for the sol-gel coating Systems Li₂O · 2 SiO₂ and BaO · 2 SiO₂ on silica glass Substrates as examples. The Li₂O · 2 SiO₂ coating is crystalline and displays fluctuations in composition and/or phases. This heterogeneity is confirmed on atomic force microscope Images. Depending on heat treatment the BaO · 2 SiO₂ coatings are glassy and crystalline, respectively. They do not show any fluctuations; however, the depth profiles display some AI₂O₃ having diffused from the silica Substrates into the films during sol-gel consolidadon. Atomic force microscope Images reveal grooves in the Substrate surfaces in which AI₂O₃ remnants of the grinding process have been captured. The depth of the AI₂O₃ profile from the SNMS measurement corresponds to the depth of Channels found using the atomic force microscope. It is assumed that AI₂O₃ originates from remnants of the grinding material used to produce the silica Substrates.
- ItemImproved homogeneity of various glasses by gas film levitation(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1998) Janke, Achim; Frischat, Günther HeinzHeavy-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.
- 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.
- 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.
- ItemAtomic force microscope study of the topography of float glasses and a model to explain the bloom effect(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2002) Moseler, Doris; Heide, Gerhard; Frischat, Günther HeinzThe topography and nanostructure of several technical borofloat and soda-lime-silicate float glasses were investigated by a high-resolution atomic force microscope (AFM). The irregular ripple pattern to be seen on as-received atmosphere and tin bath side surfaces had an average diameter of ≈ 60 nm, heights < 1 nm and root mean square (rms) roughnesses on (1 × 1) µm2 images of < 0.25 nm. Topographies obtained in the mirror region of fracture surfaces displayed a somewhat coarser nanostructure. It could further be assured that there are no specific differences between the interior and the edge of the float glass sample. Inhomogeneities like precipitates, crystals, phase separation or pores are not caused by the in-diffusion of tin into the float glass. After annealing the float glasses in air, several of them showed the already long known phenomenon of bloom, a greyish haze produced by a wrinkling of the tin bath glass surface. The borofloat glasses did not produce bloom under any condition. The same was true for the Fe2O3-rich green and blue glasses. Depending on sample dimensions and annealing conditions only the Fe2O3-poor clear float glasses developed a pronounced bloom effect. It is known that annealing of the glasses in air causes an oxidation of Sn2+ to Sn4+, which acts as a network former. This causes a change in glass properties near the surface. However, the precisely measured in-depth profiles of all relevant species in the nanometer and the micrometer regions of the float glasses showed that only in the ease of the Fe2O3-poor silicate float glasses a reversed Sn2+ diffusion from the interior to the surface is caused by the air annealing, forming a very high and steep tin (Sn4+) enrichment in a superficial layer with a thickness between 50 and 150 nm. In analogy to thin film technology a simplified model was developed and a free buckling length of ≈ 2.3 µm was estimated for the bloom surface, which is in reasonable agreement with the experimental finding. It was further shown that a sol-gel derived SiO2 coating of the bloom surface could enhance the optical transmission of the glass considerably.
- ItemWeathering of tin oxide coated glass with low IR emissivity(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2002) Rädlein, Edda; Buksak, Anna; Heide, Gerhard; Gläser, Hans Joachim; Frischat, Günther HeinzThe 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.
- ItemPreparation and characterization of Na2O-Al2O3-B2O3 sol-gel glasses with aluminum lactate and formiate as precursors(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2005) Hoyer, Lars P.; Helsch, Gundula; Frischat, Günther Heinz; Zhang, Long; Eckert, HellmutIn this work aluminum lactate and aluminum formiate have been used as precursors to obtain room temperature stable sols and gels and after annealing homogeneous glasses in the System Na2O-Al2O3-B2O3. The local environments and connectivities of boron and aluminum have been investigated by 11B and 27Al solid-state nuclear magnetic resonance (NMR). It was found that the local atomic structures of the sol-gel glasses markedly depend on the precursors and the preparation routes and are also dissimilar to those of melt quenched glasses of the same compositions. Thus, for example, the fractions of BO3/2 and BO4/2 units differ and it is interesting to note that there are no asymmetric ΒO2/2(O-) units present in the sol-gel materials. The 27Al spectra show AI in four-, five- and sixfold coordination, whose relative abundance in a given glass composition is also dependent on the preparadon route. Rotational echo double resonance (REDOR) 11B {27Al} and 27Al {11B} resuhs indicate that the extent of B-O-Al connectivity is diminished in the gel prepared glasses when compared to the melt cooled glasses. Element distributions are reported on the basis of secondary neutral mass spectrometry (SNMS) data, and the nanostructures of surfaces have been characterized by atomic force microscopy (AFM).
- ItemCorrosion of granulated glassy blast furnace slags in aqueous solutions(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2001) Olbrich, Eva; Frischat, Günther HeinzStructure and reactivity of several commercial and laboratory granulated glassy blast furnace slags were investigated. From the compositions NBO/T was calculated, the number of nonbridging oxygens per tetrahedron. Electron spin resonance spectrometry answered questions regarding the coordination numbers of the ions Mn2+ (Mg2+) and Ti3+. Both the hydrolytic reactivity of the slag glasses in water and their corrosion mechanisms in 0.1 mol/1 KOH model pore solution were studied. Scanning electron and atomic force microscopy showed the development of gel-like phases on the slag grains. Although distinct features could be resolved already, only the nanoscale in-depth profiles obtained by secondary neutral mass spectrometry helped to enlighten the complex reaction sequences. Thus, for example, it could be shown that first Mg2+ -rich gel phases developed by the corrosion process, however, after some hours the leach solutions became supersaturated successively with the consequence of forming a secondary calciumsilicate hydrate phase by a redeposition process. This phase showed a tobermorite-like structure. AI2O3- and TiO2-rich blast furnace slags were more stable than the CaO-rich materials and they also displayed reaction mechanisms at least partly dissimilar to the just mentioned.
- ItemMorphology of silica and borosilicate glass fracture surfaces by atomic force microscopy(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1999) Wünsche, Christine; Rädlein, Edda; Frischat, Günther HeinzFracture 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.
- 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.
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