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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−1.99 −0.71 0–9b 15.8 41.5 PbO −4.85 −3.17 −2.24 0–12b 23.4 17.6 Li2O −35.54 −30.04 −26.45 0–3a 16.0 80.4 Na2O −12.65 −9.19 −7.06 10–17a 22.4 37.3 K2O −5.93 −4.17 −3.53 0–9a 37.6 23.4 Error ±4.7 K ±3.4 K ±3.2 K — n.s. n.s. Graphs depict the temperature change brought about by a replacement of 1 percent of SiO2 by another oxide in the base glass composition 74SiO2, 10CaO, 16Na2O. (left) for oxide amounts by wt. (right) for molar amounts.

      3.2 Liquidus Temperatures

      Most industrial glass‐melting processes run in a continuous way 365 days per year over periods of 2–15 years. This operation time depends on the type of glass and on the corrosion wear of both the refractory lining of the melting tank and channels guiding the melt to the working stations. A maximum temperature of about 1500 °C is needed to achieve homogenous fusion; at the exit of the furnace, the melt still is at about 1350 °C. This temperature makes it necessary to cool steadily the melt down to T(3.0) while keeping a safety margin ∆T above the liquidus temperature Tliq of the particular composition to prevent precipitation of crystals. This yields a constraint of a minimum temperature of T(3.0) + ΔT required to ensure a crystal‐free glass. This constraint is especially critical for continuous glass fiber production (Chapter 1.5), but it applies to any other process as well before the forming step.

      Traditionally, liquidus temperatures have been determined experimentally to be represented graphically for simple‐enough systems in the form of phase diagrams (Chapter 5.2, [2]). For complex compositions of industrial interest, they are generally determined with a gradient furnace whereby a series of 5–10 samples are heated for typically 24 hours in a temperature gradient spanning the expected range of Tliq. After quenching, the samples are examined by optical microscopy. The liquidus temperature is then bracketed by the treatment temperatures of the last homogeneous glass and that of the first sample in which crystallites are observed. In a more accurate approach, samples from the gradient furnace containing tiny amounts of crystals are reheated in a heating‐stage microscope at a rate well below 1 K/min, and the temperature at which the last crystal dissolves is adopted as Tliq.