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- Itemstress generation modulus as a counterpart of the stress relaxation modulus(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1995) Brückner, Rolf; Hessenkemper, Heiko; Habeck, Andreas; Yue, YuanzhengIn 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.
- ItemGlass conditioning by viscosity controi(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1996) Roger, Ulrich; Hessenkemper, Heiko; Roth, PeterDuring the forming process of a glass melt the viscosity is the dominant process parameter. Up to now in the industrial Container glass production only temperature-controlled Systems have been used. With these control elements only thermally induced variations of the viscosity are visible. Variations in the composition, the redox State , the water and the SO₃ content in the melting process cause a shift in the viscosity Up to now these changes have not been detectable and therefore, it has not been possible to compensate them. Α measuring device for t he continuous determinadon of the viscosity in the glass was developed and tested for several months in a Container glass plant. All measured variations of the viscosity can now be corrected by the existing control elements for the thermally controlled glass conditioning. Dependent on the posidon of the viscosimeter in the feeder Channel , a feedforward or feedback Controlling concept is practicable.
- ItemTracing of primary melt formation and reactions involved in glass batch melting(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1997) Suwannathada, Pichanon; Hessenkemper, HeikoThe primary melt formation and the reactions involved therein were traced by means of electrical conductivity measurement. After the temperature of the furnace had reached 1200 °C, 700 g of soda-lime-silica glass batches, either with or without carbon addidon, were loosely charged into an induction furnace. An electrical conductivity sensor connected with a thermocouple was put immediately into the batch; then the conductivity in terms of voltage drop and the temperatures were continuously recorded. Not only the changes in t h e reaction atmosphere were detected, but also at least four events leading to the occurrence of the primary melts could be identified. They were the double salt (Na₂Ca(CO₃)₂) with a soda-rich NaCa mixed crystal eutectic, the sodium distilicate-silica eutectic, the double salt melting temperature, and the soda melting temperature. The temperature at which the conductivity curve passed its original baseline is proposed as the primary melt formation temperature. The first highest sodium ion diffusion rate, or hopping region on the conductivity-temperature curve, is also proposed as the primary glass-forming melt temperature.
- ItemInfluence of water dissolved in the structure of soda-lime-silica glass on melting, forming and properties: state-of-the-art and controversial issues Report of the International Commission on Glass (ICG) Technical Committee 14 "Gases in Glass"(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1999) Geotti-Bianchini, Franco; Brown, John T.; Faber, Anne J.; Hessenkemper, Heiko; Kobayashi, Sho; Smith, Ian H.The paper summarizes the information available about the water content of soda-lime-silica glass, the factors that influence it and the influence exerted by water in glass on redox and sulphur retention, on fming, on the melting and forming behaviour, workability and the physical and chemical properties of the melt. Such issues were discussed by the authors in their presentations at the first Forum on Water in Glass, organized in 1997 by Technical Committee 14 (TC 14 "Gases in Glass") of the International Commission on Glass (ICG). The present paper reports the conclusions of the final discussion, highlighting the issues on which consensus was reached (IR spectroscopy is suitable to monitor the water content; the water content of conventionally fired Containers is 300 to 400 ppm and daily oscillations are moderate; water, redox and sulphur interact mutually; water lowers the viscosity; the influence of water content variations on the radiative thermal conductivity is moderate) and the still controversial issues (influence of water and hydroxides in the batch on the final water content; influence of water in the batch and atmosphere on the redox State of the glass; influence of oscillations of OH level on the workability; influence of OH level on ease of forming, product quahty and strength).
- ItemInfluence of water content and basicity on redox ratio - Consequences on radiation heat absorption and emission of glass melts during fusion and processing(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1993) Brückner, Rolf; Hessenkemper, HeikoIt 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.
- ItemElastic constants of glass melts above the glass transition temperature from ultrasonic and axial compression measurements(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1991) Hessenkemper, Heiko; Brückner, RolfIt 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.
- ItemNucleation and crystallization kinetics on float glass surfaces(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1992) Deubener, Joachim; Brückner, Rolf; Hessenkemper, HeikoThe 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.
- ItemSome aspects of the workability of glass melts(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1990) Hessenkemper, Heiko; Brückner, RolfCriteria 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.
- ItemInfluence of microheterogeneities on the mechanical properties of high-viscous melts(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1990) Habeck, Andreas; Hessenkemper, Heiko; Brückner, RolfThe 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.
- ItemLoad-dependent flow behaviour of silicate glass melts(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1988) Hessenkemper, Heiko; Brückner, RolfStarting 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.