1995, Brückner, Rolf, Hessenkemper, Heiko, Habeck, Andreas, Yue, Yuanzheng

In order to measure the time dependence of the stress relaxation modulus, E, a stress-strain deformation has to precede which induces a stress within the viscoelastic sample from which the sample relaxes. The generation of stress is characterized by a strain rate-dependent and relaxation rate-dependent portion which exhibits a maximum value, E_max, which is called "stress generation modulus". E_max was called the "maximum stress relaxation modulus" in earlier papers. Meanwhile, however, it turned out that a better verbal distinction should be made in future by the new term "stress generation modulus" because E_max is about one order of magnitude larger than E.

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.

1988, Hessenkemper, Heiko, Brückner, Rolf

Starting from the basic experiments first of all the statements of the cylinder compression method will be regarded from which information will be obtained about flow behaviour, relaxation modulus and hot-fracture strength as important criteria for the workability of high-viscosity glass melts. On the basis of calibration measurements it is shown that viscosities can be determined from 1 · 10^14 to 2 · 10^5 Pa s due to the given limits of the apparatus. For the standard glass I of the Deutsche Glastechnische Gesellschaft and for a commercial float glass the purely rheological decrease of viscosity (non-Newtonian or structural viscosity) of earlier investigations could be confirmed, the investigated viscosity range could be essentially enlarged and a clear separation could be made from thermally dependent viscosity decrease by dissipation energy. The effect of non-Newtonian flow behaviour increases clearly with temperature at equal axial stresses and shows a non-monotonous course in the range 10^9 to 10^7 Pa s. An interpretation of these observations is given in connection with a load-dependent alteration of the structure of the glass melt and the non-monotonous course is thought to be connected with crystal nucleation processes.

1990, Hessenkemper, Heiko, Brückner, Rolf

The influence of various redox ratios on the isothermal workability of glass melts is investigated by means of ultrasonic waves and mainly by the cylinder-compression method. The experiments were made with laboratory glass melts of float glass composition containing 0.22 wt% iron oxide for which the redox ratio Fe2+/Fetotal was varied between 14 and 84 % (melting history or redox prehistory). Properties are measured being responsible for the brittleness or stiffness and for the isothermal workability of a glass melt (non-Newtonian flow behaviour, relaxation modulus and high-temperature tensile strength). The main results are that a reduced glass melt shows a better isothermal (and isochomal) workability than an oxidised one. This means a better high-temperature tensile strength and a lower relaxation modulus and therefore a lower brittleness and rigidity of the reduced melt as well as lower compression and elastic moduli. Besides that, interesting alterations within the melts could be detected by means of spectroscopic investigations such as a shift of the iron absorption bands depending on the thermal, pressure and redox prehistory. The influence of different OH- contents and the problem of the origin of the amber colour were considered intensively.

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.

1990, Hessenkemper, Heiko, Brückner, Rolf

Criteria are given for the isothermal or isochomal workability of glass melts and discussed by means of a schematic example which is based on numerous measurements. Α thesis for a simple connection of this kind of workability is presented and examined by means of so far existing measurements. The isothermal or isochomal workability, which becomes more important with increasing deformation rate, is in contrast to the non-isothermal or athermal workability. The differences are based on certain connections between the stiffness, the relaxation modulus of a glass mek, its high-temperature fracture strength and the value of the quantity Β in the Vogel-Fulcher-Tammann (VFT) equation if the constants are evaluated from a measurement of the whole viscosity-temperature range, from 10^1.5 to 10^12 Pa s.

1990, Hessenkemper, Heiko, Brückner, Rolf

Glass melts are usually treated in literature as thermorheologically simple fluids. The temperature-time equivalence of the relaxation behaviour is described by the WLF equation. This also for silicate melts well-known behaviour will be verified in the present paper with the help of the cylinder-compression method up to 240 Κ above Tg for very different melts. In addition it is shown that the relaxation behaviour is load-dependent above a certain load limit indicating a deviation from the conception of the thermorheological simplicity. The load-dependent relaxation behaviour is investigated for glass melts with very different network structures (chains, cross-linked chains and three-dimensionally connected structures). Α clear correlation with structure is demonstrated in that way that the relaxation ability and its load dependence increase with decreasing degree of structural interconnection from a three-dimensionally to a one-dimensionally linked potential network structure.

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.

1990, Habeck, Andreas, Hessenkemper, Heiko, Brückner, Rolf

The influence of liquid-solid (Zerodur glass-ceramic melts) and liquid-liquid (PbO-B2O3 melts) two phase systems on the mechanical properties is investigated. The load-dependent mechanical properties which characterise the isothermal workability are: Newtonian and non-Newtonian flow behaviour, relaxation behaviour and therefore the stiffness as well as the high-temperature tensile strength of the melts. From the rheological point of view two systems (1 and 2) have to be regarded: 1) the liquid-solid and that part of the liquid-liquid system with the high-viscous inclusions within a low-viscous matrix on the one hand and 2) that part of the liquid-liquid system with the low-viscous inclusions within a high-viscous matrix on the other hand. These two systems show among various of the mentioned other special properties and relations a general decrease of high-temperature strength and isothermal workability with increasing portion of the distributed minor phase. System 1) shows with respect to constant equilibrium viscosities a constant non-Newtonian viscosity decrease, an increase of the relaxation modulus (stiffness) and a decrease of high-temperature tensile strength with increasing portion of solid and/or high-viscous inclusions. System 2) under the same aspect exhibits a strong increasing non-Newtonian viscosity decrease, a decrease of the relaxation modulus and a decrease of high-temperature tensile strength with increasing portion of the low-viscous inclusions.

1989, Hessenkemper, Heiko, Brückner, Rolf

The intention of this paper is to investigate the frequently in practice used conception "brittleness of a glass melt" and to find a method for quantitative measurements of this kind of property. The simple compression of cylindrical glass samples has been applied as a measuring technique, by which information can be obtained on the high-temperature (ht)-fracture strength, the stiffness and the relaxation behaviour of glass melts. In this way the brittleness of a glass melt can be defined in a quantitative manner. Α clear correlation has been found between the ht-fracture tensile strength and the relaxation modulus for some industrial commercial glasses and for various laboratory glasses with successive substitution of CaO by MgO in such a way that a high relaxation modulus is connected to a low ht-fracture tensile strength and vice versa at comparable (Newtonian) viscosities and deformation rates. Differences up to more than 300 % have been measured for silicate glass melts of various compositions. Thus, a powerful method has been developed which allows to understand and to measure the isothermal workability of a glass melt and to investigate the effect of various parameters on it. In contrast to the pure elastic and viscoelastic range the brittle fracture behaviour within the viscous range is a volume property.