1987, Dunkl, Michael, Brückner, Rolf

The corrosion rates of various refractory materials in a container glass melt (sodium-calcium silicate basis) were determined quantitatively under the action of forced convection flow by means of the rotating cylinder face area at 1500 °C as compared with those at 1400 °C. The applied method not only gives a good comparison between the various types of bricks, but it also elucidates the influence of microstructure and of occasionally occuring crystalline or textural alterations during corrosion, which possibly can be detected only in experiments of extended duration. Present and earlier results on corrosion rates under the action of forced convection flow will be compared with those obtained from the same refractories under the action of free density and interfacial convection flows at 1400 and 1500 °C, which yield a very different dependence on temperature for the three convection flow types. The interfacial convection-driven corrosion shows the largest temperature dependence. Very critical are swelling effects, which sometimes can be observed only in separate long-time investigations.

1970, Brückner, Rolf

Die Anomalie des negativen Volumen-Temperaturverhaltens von hydroxylarmen, „natürlichen" Kieselgläsern (Typ I/II) im Temperaturbereich 1000 bis 1500 °C mit dem Hochtemperaturminimum bei 1550 °C tritt bei hydroxylreichen, „synthetischen" Kieselgläsern (Typ III) nicht auf. Die Volumen-Temperaturkurve zeigt einen Verlauf, der zwischen dem des Typs I/II und den binären Alkali- und Mehrkomponenten-Silicatgläsern liegt. Das Tieftemperaturminimum, je nach thermischer Vorgeschichte bei - 60 bis - 120 °C, bleibt dagegen erhalten und wird durch den Hydroxylgehalt zu tieferen Temperaturen verschoben. Die Ergebnisse werden mit dem Konzept der Vorordnungsbereiche, der strukturellen Spannungen und der Entkoppelungswirkung der Hydroxylgruppen in Verbindung mit der Vorstellung von Smyth über die bevorzugten transversalen Schwingungen im SiO2-Verband gedeutet.

1991, Hessenkemper, Heiko, Brückner, Rolf

It will be shown that on the basis of ultrasonic measurements in connection to density, viscosity and axial compression experiments the temperature- and frequency-dependent elastic constants and the maximum of attenuation can be determined. The ultrasonic measurements are extended over the whole temperature range from room temperature up to 1300 °C. Glasses and their melts with different chemical compositions (silicates and phosphates) and properties are investigated. The results are compared and correlated with those mechanical properties of melts which are obtained from axial compression measurements of cylindrical glass specimens, such as relaxation behaviour, brittleness, influence of redox conditions, etc. It is found that the results of the two methods are compatible. The advantage of the ultrasonic method is that it covers a larger viscosity-temperature range than the cylinder-compression method. The disadvantage is that it is restricted to the linear viscoelastic region.

1986, Manns, Peter, Brückner, Rolf

Der elastische Anteil des viskoelastischen Verhaltens unterkühlter Glasschmelzen (Floatglas und Duranglas) wird mit Hilfe einfacher Biegeversuche, die zugleich der Bestimmung der Biegefestigkeit nach der Doppelringmethode dienen, untersucht. Hierzu dient unterhalb Tg der Elastizitätsmodul Ε der Gläser und oberhalb Tg ein gemittelter Elastizitätsmodul (scheinbarer E-Modul E*) an der Sprödbruchgrenze, die in starkem Maße von der kritischen Verformungsgeschwindigkeit vc abhängt, bei der ein Heißsprödbruch auftritt, E* zeigt bei Tg für beide Gläser ein Dispersionsverhalten. Mit zunehmender Deformationsgeschwindigkeit steigt E* an (stärker als Ε unterhalb Tg), und der Bereich der Dispersion wird zu höherer Temperatur verschoben. Dies gilt auch für Proben mit bewußt geschädigter (geschmirgelter) Oberfläche. Oberhalb dieses Bereiches treten charakteristische Unterschiede für die beiden Glasschmelzen auf: bleibt bei Floatglas konstant, bei Duranglas steigt E* mit der Temperatur stark an. Es bestehen qualitative Ähnlichkeiten zur Frequenz- und Temperaturabhängigkeit der Ultraschallgeschwindigkeit.

1973, Brückner, Rolf, Käs, Hans Herbert

Hochfluoridhaltige Glasschmelzen zeigen einen starken Fluorabbrand. Durch diesen Stoffübergang von der Schmelze in die Atmosphäre wird selbst in der kurzen Zeit während des Ausgießens und Abkühlens bis zum Einfrierbereich dieser Gläser eine inhomogene Oberfläche erzeugt, die heftige Grenzflächenkonvektion zur Folge hat, die wiederum rückgekoppelt den Stoff- und Temperaturübergang anfacht und charakteristische Oberflächenströmungen verursacht. Diese Vorgänge führen in Verbindung mit den Unterschieden in der Einfriertemperatur und im Längen-Ausdehnungskoeffizienten zwischen der inhomogenen Oberflächenhaut und den darunterliegenden (homogenen) Schichten zu charakteristischen Faltungen im „lederharten" Zustand, die schließlich als eingefrorene Oberflächenstrukturprofile sichtbar werden. Die Phänomene werden beschrieben, optisch, chemisch, röntgenografisch sowie mineralogisch untersucht und gedeutet.

1992, Klug, Thomas, Brückner, Rolf

Alternating bending stress experiments are described which were performed with various fibre/DURAN-glass composites reinforced by Nicalon NL 202 SiC fibres and by two different types of carbon fibres, a high-tensile strength (ht) and a high-modulus (hm) fibre. Also the influence of unidirectionally homogeneous and inhomogeneous fibre distribution as well as the bidirectional 0°/90° ply distribution are studied. In contrast to previously investigated strain-controlled experiments the present stress-controlled experiments show after the pretreatment of 1000 alternating tensile-compressive stress cycles fatigue (beginning damage) already at amplitudes below the limit of pure elasticity (below bendover stress) of a simple bending-load experiment. Despite of this difference the "training effect" of the previous investigation, the increase of the bendover stress with increasing stress amphtude after 1000 alternating-load cycles, is found to be similar and can be established also for the present experiments. The carbon-fibre-reinforced composites show a better tolerance of damage due to their smaller fibre diameters than the SiC-fibre-reinforced composites.

1993, Klug, Thomas, Fleischer, Veronika, Brückner, Rolf

The thermal expansion coefficients of DURAN glass reinforced by SiC and various carbon fibres are determined in order to investigate the influence of the different types of fibres and the preparation parameters on the thermal expansion behaviour of the composites. The coefficient of thermal expansion decreases with increasing Young's modulus of the fibres and with increasing fibre volume content. Composites with high-modulus carbon fibres and/or with high fibre volume concentration show a transition from contraction at lower temperatures to expansion at higher temperatures. "Zero expansion composites" can be produced for certain temperature intervals by selection of the fibre types and the fibre volume concentration. The experimentally determined expansion coefficients are compared with the theoretically calculated ones by the mixing rule. The agreement is very good for the high-modulus fibres, less good for the high-strength fibres.

1987, Sammet, Manfred, Brückner, Rolf

Structural conclusions are obtained for various homologous series of the above-mentioned glasses by IR spectroscopy. Alterations in vibration frequencies of the fluorine phosphate tetrahedra as a function of chemical composition are interpreted and indicate, that fluorine ions are present in the glass network as POnF4-n units. P-F vibrations show the participation of fluorine in the phosphorus coordination with network forming character. Another remarkable influence of the fluorine ions is the delocalization of the P = O double bonding, which is typical and localized for pure phosphate glasses, and thus, only bridging and non-bridging oxygen atoms are left in the fluorine phosphate glasses. This delocalization is connected with an increase in frequency of all remaining P-O vibrations (stronger P-O bondings), which prevents the phosphorus atoms from carrying a higher charge. Neighbouring aluminum atoms get a chance for a four-fold coordination without the presence of alkali ions by a charge transfer from P5+ to Al3+ via the p-orbital of oxygen. From other frequency shifts is evident the network forming influence of Al3+, Mg3+ and the network modifying character of Na+, Ca2+ and Ba2+.

1993, Brückner, Rolf, Hessenkemper, Heiko

It is shown that water content and basicity of glasses have an influence not only on the intensity of the absorption bands at and near the wavelengths of the characteristic vibration modes of water itself and on those of transition ions but also on other modes and even on remarkable parts of the NIR and the visible spectrum. Additionally it is shown that the water content influences the redox ratio of iron and prefers the ferrous state. An interpretation of this behaviour is given and the consequences for the industrial glass melting and forming process are discussed.

1992, Deubener, Joachim, Brückner, Rolf, Hessenkemper, Heiko

The superficial crystallization kinetics of a commercial float glass is investigated in the temperature range from 665 to 925 °C. The following phases were observed: cristobalite, devitrite, sporadically sodium metasilicate and additionally β-wollastonite if a reducing atmosphere is applied. The devitrification process is divided into a primary crystallization of cristobalite and a secondary appearance of devitrite after an induction time (t > 7 min). The growth rates of devitrite are up to 4 times larger than those of cristobalite, thus, a fractured surface perpendicular to the float glass plate shows a two-stage devitrification image after longer times of heat treatment. The growth rates depend on the furnace atmosphere as well as on the compositional variations of the two float glass surfaces. The activation enthalpies of crystal growth rates are (in kJ/mol): ≈ 110 for cristobalite, ≈ 150 for β-wollastonite and ≈ 220 for devitrite. Α superficial phase separation occurs at the tin bath side if oxidizing furnace atmosphere is applied. This can be related to a redox interaction of the tin ions and has consequences on nucleation, induction period, crystal growth mechanism and "bloom" effect. Heterogeneous surface nucleation arises from a certain number of sites which are specific for the various crystal phases. In the case of cristobalite the time and temperature dependence of the number of crystals are measured and nucleation rates were determined on the atmosphere side under oxidizing conditions.