Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов. Читать онлайн. Newlib. NEWLIB.NET

Автор: Группа авторов
Издательство: John Wiley & Sons Limited
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Жанр произведения: Техническая литература
Год издания: 0
isbn: 9781118799499
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[CaAl2Si2O8] qualifies for a successful production. In passing, note that Figure 6b has been calculated by using the thermochemical software and databases FactSage® [23]. The experimental position of the mentioned eutectic is also marked, thus displaying the degree of accuracy that may be expected from the calculation of liquidus for more complex compositions.

Graphs depict the liquidus lines of binary silicate systems all systems comprising a divalent oxide, except BaO, show an extended stable miscibility gap. Graphs depict the ternary phase diagrams in versions of technological relevance; shorthand notation. (Left) the basic system of all commercial hollowware and flat glasses; the triangles mark the positions of the compounds, Na2O·2 SiO2: the circle the position of the base glass 74 SiO2, 10 CaO, 16 Na2O. (Right) the basic system of reinforcement-fiber glasses; industrial compositions flock around the eutectic. Schematic illustration of the miscibility gaps. (a) Extension of stable gaps in ternary borosilicate systems with different oxides as third component; the area shaded in gray refers to BaO; (b) isotherms of the sub-liquidus immiscibility dome in the system Na2O–B2O3–SiO2.

      3.3 Liquid–liquid unmixing

      It is only with glasses known under the trade name Vycor Glass that liquid unmixing is exploited on purpose. Here, after forming by conventional technology to the desired shape, phase separation develops upon annealing at an appropriate temperature to yield two interconnected phases, namely an Na2O‐ and B2O3‐rich glass along with another one that contains more than 96 wt % SiO2. Then the former is leached out by a hot strong mineral acid, leaving behind a nanoporous skeleton of high‐SiO2 glass. This material may then be used directly as filter, for example, or sintered at temperatures below 1300 °C to fabricate dense and almost pure silica glass articles much more readily than with pure SiO2.

      4.1 Property Optimization

      Both search and optimization of glass formulae begin with a given profile of target glass properties. In the following, three properties will be addressed as examples typically targeted in glass development, namely the elastic properties, the thermal expansion coefficient, and the chemical durability. From a scientific point of view, such a task should rest on deep insights on the relationships between chemical composition, glass structure, and glass properties. It is only from such a fundamental approach that ground‐breaking developments of novel glasses with outstanding properties may be expected. But this goal is still a matter of fundamental research as expounded in the following chapters where this most challenging issue is pursued.

      For the time being, however, only few manageable tools and procedures of this kind are available for the technological community. To optimize properties, technologists thus rely largely on empirical approaches whereby, as applied to glass viscosity in Section 3.1, they use incremental oxide factors derived by statistical means from large numbers of experiments. One has, however, to keep in mind that these approaches represent only interpolations of what is already known. Hence, limited areas in compositional space leading to truly outstanding properties should be easily overlooked so that developments similar to the famous low‐expansion metallic alloy Invar are very unlikely to be found this way.

      4.2 Elastic Properties

      Incremental oxide factors for the calculation of the elastic properties from the composition compiled in the right‐hand part of Table 3 are taken from a widely accepted earlier publication [21]; for the sake of clarity, they have been adjusted with respect to the units used, i.e. to cm3/mol for volume, and to GPa for modulus increments. Young's modulus E is then calculated with

Graph depicts the change of Young's modulus E in the base glass composition 74 SiO2 10 CaO 16 Na2O upon the replacement of x wt percent silica by another oxide.