53 research outputs found
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Improved homogeneity of various glasses by gas film levitation
Heavy-metal fluoride glasses of the system ZrFâ-BaFâ-LaFâ-AlFâ-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
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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
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Influence of different Al2O3-containing batch materials on melting, fining and properties of soda-lime-silica glass
Proceeding from the composition (in wt% ) : 72.5 SiO2, 1.5 AI2O3, 10.5 CaO, 2.5 MgO, 12.5 Na2O, and 0.5 K2O the batch raw materials calcined and hydrated alumina, nepheline syenite, Calumite, and Ecomelt, respectively, were employed in order to introduce aluminum oxide to the glass. Batch calculations assured constant chemical composition, sand/sulfate ratios, sulfate/carbon ratios, redox numbers, and total sulfur contents. The glasses and melts obtained were investigated with respect to different properties, e. g. viscosity, homogeneity, optical transmission, and residual sulfur content. The results display similarities, however, partly also significant differences with respect to the AI2O3 source used. Thus especially the glasses containing the blast furnace slags revealed distinct characteristics compared to glasses with nepheline syenite and hydrated alumina. With increasing melting time the differences are reduced
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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
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Natrium-Tracerdiffusion in wasserarmem synthetischem Kieselglas
An dem wasserarmen synthetischen Kieselglas Suprasil W (Na-Gehalt 0,55 ppm) wurde zwischen 240 und 965 °C die 22Na-Diffusion gemessen. Das Arrhenius-Diagramm des erhaltenen Diffusionskoeffizienten lĂ€Ăt sich nur durch eine gekrĂŒmmte Funktion darstellen. Der Mechanismus der Na-Diffusion kann, wie bei dem ebenfalls wasserarmen (aus Quarz hergestellten, 8,1 ppm Na enthaltenden) Infrasil, in einem Austausch 22Na+ aus dem Tracer gegen 23Na+ aus dem Glas gesehen werden, wĂ€hrend bei dem stark wasserhaltigen synthetischen Suprasil I (0,14 ppm Na) möglicherweise auch eine Wechselwirkung mit H+ -Ionen auftritt.
Bei Suprasil W und Suprasil I, nicht aber bei Infrasil, tritt bei allen Temperaturen neben dem durch einen eindeutigen Diffusionskoeffizienten beschreibbaren Volumeneffekt ein Steilabfall der AktivitĂ€t in oberflĂ€chennahen Schichten < 10 bis 20 ”m auf. Dieser OberflĂ€cheneffekt wird nach einem Vergleich der Absolutkonzentrationen zwischen 22Na-Tracer und Na in den GlĂ€sern als Folge einer Verletzung der Anfangsbedingung einer âmomentanen Quelle" diskutiert
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Sperrwirkung ionenausgetauschter GlasoberflÀchen
Durch eine einstĂŒndige Behandlung eines Na-haltigen Silicatglases in einer K-Salzschmelze bei 425 °C ( <Tg) wird infolge von K-Na-Ionenaustausch eine dĂŒnne OberflĂ€chenschicht des Glases in Struktur und Eigenschaften verĂ€ndert. Diese Schicht wirkt sich auf ein 22Na-Diffusionsexperiment im Sinne einer partiellen Sperrung aus. WĂ€hrend die Matrix der Austauschschicht etwa die Eigenschaften des unverĂ€nderten Ausgangsglases beibehĂ€lt, wandert zwischen 440 und 180 °C ungefĂ€hr die HĂ€lfte der Ionen mit einem um den Faktor 20 bis 100 kleineren Diffusionskoeffizienten. Aufbau der Austauschschicht und Auswertung der Diffusionsprofile werden diskutiert. Dabei zeigt sich, daĂ auch hier Ă€hnlich wie bei anderen Transportprozessen (elektrische LeitfĂ€higkeit und innere Reibung) an homogenen und oberflĂ€chenbehandelten GlĂ€sern in erster Linie nur der Ăquivalentbruch der K-Ionen fĂŒr die auftretenden Effekte verantwortlich ist
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Ionentransport in Siliciumdioxid
Dieser Bericht gibt einen Ăberblick ĂŒber die bisher an SiO2 ausgefĂŒhrten Transportuntersuchungen. Neben der Bestimmung der Selbstdiffusion der Grundbausteine Silicium und Sauerstoff spielt auch die chemische Diffusion eine groĂe Rolle, wobei zu unterscheiden ist, ob die zusĂ€tzlich eingefĂŒhrten Ionen lediglich als Mineralisatoren und/oder Stabilisatoren wirken oder ob es sich um chemische Reaktionen mit Bildung neuer Produkte handelt. Da auch die reinsten Formen von SiO2 in die Struktur eingebaute Verunreinigungen im ppm-Bereich aufweisen, stellen deren Beweglichkeiten ebenfalls typische Kennwerte dar. Von besonderem Interesse sind dabei die Spuren OH-, Na+, Ca2+ und Al3+.
Bisherige Untersuchungen dieser Art wurden fast ausschlieĂlich nur an kristallinem Quarz und an nichtkristallinem Kieselglas ausgefĂŒhrt. Dabei interessierte neben der Ermittlung der kinetischen GröĂen insbesondere auch der im Atomaren ablaufende Mechanismus. Eine endgĂŒltige KlĂ€rung konnte noch nicht in allen FĂ€llen erzielt werden
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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 CaSOâ 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 AIâSâ 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
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Influence of small additions of LiâO raw materials on glass melting
For a container glass composition up to 1 mol% of the alkali oxide was substituted by LiâO in steps of â0.2 %. The lithia was added as LiâCOâ and as spodumene, respectively. As shown earlier LiâO reduces the viscosity of the melts considerably, irrespective of the LiâO source. However, melt formation, carbonate decomposidon, silicate and silicate melt formation processes during raw materials reactions are considerably accelerated by LiâCOâ addition only, whereas LiâO 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 LiâO 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 LiâO content. The Liâș ions are less polarizable and, due to their high Dietzel field strength, the glass structure is dghtened compared with that of LiâO-free glass. In conclusion, the substitution of small amounts of LiâO instead of NaâO and/or KâO 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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Kationenselbstdiffusion in SilicatglÀsern
An einem Glas aus dem ternĂ€ren System Na2O-CaO-SiO2 wird unterhalb Tg die 22Na- und 45Ca-Diffusion mit Hilfe einer RestaktivitĂ€tsmethode bestimmt. Die Ergebnisse lassen sich fĂŒr die Na-Selbstdiffusion zwischen 221 und 513 °C durch D0 = 8,31 â 10^-3 cm2/s und Q = 22,8 kcal/mol und fĂŒr die Ca-Selbstdiffusion zwischen 474 und 545 °C durch D0 = 39,7 cm2/s und Q = 53,7 kcal/mol beschreiben. Verschiedene bisher an SilicatglĂ€sern gewonnene Ergebnisse der Kationenselbstdiffusion werden vergleichend zusammengestellt und bezĂŒglich des Einflusses der Glaszusammensetzung, der GröĂe der Frequenz- und Korrelationsfaktoren und der im Glas möglichen Defektstruktur diskutiert
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