Engineering Physics of High-Temperature Materials. Nirmal K. Sinha. Читать онлайн. Newlib. NEWLIB.NET

Автор: Nirmal K. Sinha
Издательство: John Wiley & Sons Limited
Серия:
Жанр произведения: Техническая литература
Год издания: 0
isbn: 9781119420460
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compounds or they could be solid solution, the nature of which depends on the phase diagrams for the system that constitutes the glass.

      

      Ward‐Harvey (2009) dates the glass industry back to ancient Egypt – more than 3500 years ago. Yet, even today we are making advances in chemical composition and processing that are enabling the growth of technology, such as flexible glasses for displays.

      Silicon ions are the most common network formers, but pure silicate glasses can be hard to work due to quartz's high melting temperature (1996 K). Soda‐lime glasses (originally Na2O (sodium oxide) + CaO (lime) but also typically includes MgO (magnesia) and Al2O3 (alumina)) are thus the norm for multiple uses, such as windows and jars. Borosilicate glasses, which can include 5–13% boron trioxide, are less vulnerable to thermal shock and are used in cookware and labware (e.g. Pyrex). Aluminosilicate glasses (5–10% alumina) also have good thermal resistance, but are harder to shape and so are used for purposes such as fiberglass. Phosphate glasses (phosphorus pentoxide) have been found to be compatible with the organic mineral phase of bone and are finding increasing biomedical uses (Rahaman 2014).

Schematic illustration of composition of a typical soda-lime container glass given in weight percent.

      Source: Modified from Hsich (1980).

      The composition of glass can also be modified post initial hardening. For example, the surface of Corning's well‐known Gorilla glass – used in a variety of smart phones and devices – is toughened by a process called ion exchange (Corning n.d.). The material is immersed in a molten alkaline potassium salt causing smaller sodium ions in the glass to be replaced by larger potassium ions from the bath. The larger ions create a surface layer with high residual compressive stress and increase the surface's resistance to damage. However, the full magic to Gorilla glass comes from controlling the stresses at the surface and throughout the center of the glass through its forming processes (Bushwick 2013).

      Source: Sinha (1971).

      In Section 4.11 of Chapter 4, we briefly describe the tempering of structural and automotive glass and its strength properties. The thermal tempering process involves heating the glass sheet uniformly to temperatures of around 650–700 °C, or measurably higher than the transformation range, T g, and then subjecting it to rapid cooling, usually by jets of air. Since cooling is usually symmetrical about the midplane of the glass plate, this process results in an approximately parabolic stress distribution in the glass plate with compression at the surfaces and tension in the midplane. It is recognized that the midplane tension, called “degree of temper,” is generally represented by the corresponding birefringence (double refraction). The depth‐dependent structural changes that occur in the glass plate during a toughening process induce this birefringence due to changes in the density and refractive indices within the plate, and planes parallel and perpendicular to the surfaces. The process of inducing small, but desirable, changes in the structure of the same type of optical glass by suitable heat treatment can also be applied in the fabrication of optical glass components.

Schematic illustration of temperature dependence of the CLTE for a lapped and polished lath taken from a large commercially annealed sheet of plate glass.