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Author: Geotti-Bianchini, Franco × Has files: Yes × Type: article × Version: publishedVersion × Publisher: Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft ×

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Now showing 1 - 7 of 7
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    Parameters affecting the workability of container glass Part 2. Influence of dissolved gases and of other parameters
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1992) Geotti-Bianchini, Franco
    The hypotheses associated with variations of dissolved gases, structure, macroinhomogeneity (or cordiness), surface composition gradients and radiant heat transfer properties are presented in some detail in the second part of the review. While each of the physicochemical parameters considered was shown to be potentially influential by laboratory experiments, there is still no general agreement on the factors which are most relevant in practice. The most popular candidates are microstructure formation and variations of the content of dissolved gases; however, so far no conclusive evidence has been obtained. The influence of such parameters on stress-induced viscosity variations is also not well clarified. In order to decide what should be improved in the melting practice to achieve a better control of the forming behaviour, it is essential to solve the alternative with suitable experiments on carefully selected production samples.
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    Thermal conductivity of float glass at room temperature
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1993) Geotti-Bianchini, Franco; Lohrengel, Joachim
    When the heat transfer coefficient of multiple glazings is to be determined, it is important to know the exact thermal conductivity value of the glasses used for building purposes. The values given for flat glass lie between 0.8 and 1.15 W/(m K ) . The thermal conductivity of 11 float glass samples of 4 different colours, manufactured in 5 European countries, has been determined in the temperature range between 10 and 90 °C with an uncertainty of less than 1.5 %. At 10 °C, the mean value of all glasses investigated amounts to 1.022 W/ (m K); the individual values vary between 1.033 and 1.017 W/(m K) . On the basis of the composition, the basicity and the density, the measurement results could be well interpreted by known theoretical considerations. The slight variation of the thermal conductivity values obtained by measurement is in agreement with the values calculated from the composition and the density and can be explained by the fact that all float glasses are of nearly the same composition. By interpolation of the measurement data, coefficients could be determined to calculate the thermal conductivity from the oxide composition. The investigation has shown that - at 10 °C - a value of 1.02 W/(m K) can be applied for the thermal conductivity of flat glasses used for building purposes, independent of their colour or manufacturer.
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    New interpretation of the IR reflectance spectra of SiO2-rich films on soda-lime glass
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1991) Geotti-Bianchini, Franco; De Riu, Luca; Gagliardi, Giovanni; Guglielmi, Massimo; Pantano, Carlo G.
    A new interpretation of the reflectance spectra of weathered or silica-coated soda-lime glass between 1250 and 900 cm^-1 is suggested. Traditionally, the presence and intensity of each spectral band observed were attributed to the concentration of a given chemical species in the film. Contributions due to the substrate and to the film thickness were excluded. On the basis of a more rigorous model, with the help of mathematical and physical simulations, it is shown that the penetration depth is much higher than previously assumed. Accordingly, all the spectral features observed for coatings or weathered films are a function not only of the nature of the film, but also of film thickness and type of substrate.
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    Measured angular distribution of the emissivity and calculated radiation heat transfer of architectural coated flat glass Part 1. Theory
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1989) Geotti-Bianchini, Franco; Lohrengel, Joachim
    International standards for the calculation of the central U value of coated multiple glazings are based on a model of radiative heat transfer allowing the behaviour at all angles to be predicted from emissivity measured at normal incidence. In order to provide a background for such standards, a theoretical and experimental study of the angular variation of total directional emissivity at room temperature of the relevant materials (including uncoated float glass and typical architectural coatings) has been undertaken in cooperation with TC 10 of the International Commission on Glass and with the Light and Energy Group of the Groupement Europeen des Producteurs de Verre Fiat. After illustrating the equations which correlate the optical constants with angular reflectivity and emissivity, the possibility of mathematically correlating the normal emissivity with the hemispherical and effective emissivity for different materials is discussed.
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    Parameters affecting the workability of container glass Part 1. Description of the problem - Melting history, rheology and influence of microstructure
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1992) Geotti-Bianchini, Franco
    Poor workability designates sporadic cases in which the forming behaviour of container glass varies unpredictably, impairing quality, productivity and sometimes strength. The literature offers several plausible suggestions to explain such a poor reproducibility of the rheological behaviour, generally associated with physicochemical parameters of the glass which are not routinely analyzed and which are supposed to vary with "melting history" (temperatures, raw materials, melting atmospheres) and to influence viscosity by altering the structure of "flow units", while the analytical composition remains unchanged. Alternative hypotheses are based on various not properly controlled process parameters, such as gob temperature distribution or plunger and mould condition including lubrication. Moreover, recent advances in rheology show that during the most critical stages of the forming process viscosity may change drastically with the deformation rate, providing a further plausible explanation for the observed irregular forming behaviour. One of the most promising physicochemical parameters is the possible presence of microstructure (phase separation, clustering, microcrystals). While in model glasses the correlation between microstructure and rheology is well-established, both at equilibrium and at high shear rates, there is still a lack of conclusive evidence about the presence of microstructure in industrial multicomponent glass.
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    Spectrophotometric determination of the normal emissivity of coated flat glass Report of the International Commission on Glass Technical Committee 10 "Optical Properties of Glass"
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1988) Nicoletti, Fabiano; Geotti-Bianchini, Franco; Polato, Pietro
    The results of the activity of TC 10 of the International Commission on Glass aimed at assessing the reproducibility and accuracy of the determination of normal emissivity from IR reflectance measurements with commercial spectrophotometers are presented. Eight European and American flat glass producers and two glass institutes participate in the activity. Two intercomparisons of near normal spectral reflectance are performed on 4 samples, representative of the current production. The circulated samples are subsequently calibrated in a metrological laboratory. The results show that even with a free integration procedure and reference mirror a satisfactory reproducibility can be achieved (the maximum discrepancy in computed U values between the participants being within 0.1 W/(m2 K)). The agreement significantly improves by using a common reference mirror, showing the importance of the availability of reference mirrors calibrated by some metrological source. After discussing the effect of the integration procedure on the resulting emissivity, a new 30 selected ordinates method is developed and transmitted to ISO (to be incorporated into the standard for the computation of the U values of double glazings).
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    Measured angular distribution of the emissivity and calculated radiation heat transfer of architectural coated flat glass Part 2. Results and discussion
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1989) Geotti-Bianchini, Franco; Lohrengel, Joachim
    The angular emissivity curves of 10 representative architectural coatings and of uncoated float glass are measured at 283 Κ with the help of an angular emissometer up to an angle of 70°. The experimental values can be fitted with the help of Fresnel's equations for a single, optically thick material, and extrapolated up to 90°. The obtained curves are integrated and the results compared with the values computed according to several drafts of standards in course of approval. The principle of computing the hemispherical and effective emissivity from normal emissivity with the help of a suitable multiphcative coefficient is found to be in substantial agreement with the experimental evidence.