Читать книгу Engineering Physics of High-Temperature Materials - Nirmal K. Sinha - Страница 62

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.

As mentioned in Section 2.2.2, in oxide glasses, the oxygen atoms form bridges and network formers, such as silicon, boron, phosphorus, or germanium, form strong bonds with them in a randomly arranged network structure. Network modifiers, such as sodium and calcium, generally sit in ionic form within interstitial holes. Such ions and intermediates are added to optimize the properties of glass for specific functions and/or aesthetics. A striking example is the addition of small amounts of specific metal ions to add color to glass: manganese for purple, iron for green, copper for blue, etc. The glass constituents play a role not only in the final product, but also during the production process. Fluxes made from sodium or potassium carbonate can lower the melting point and glass transition temperature of the formers while stabilizers, such as calcium oxide, provide water/humidity resistance to intermediates, such as sodium silicate.

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).

Modern glasses can have quite a complex chemical composition and the final structure is made even more complex by the potential bonding arrangements. Figure 2.8 shows the composition of a typical container glass examined by X‐ray fluorescence by Hsich (1980).

Figure 2.8 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).