tailieunhanh - Ebook Materials for engineering (3rd edition): Part 2

(BQ) Part 2 book "Materials for engineering" has contents: Glasses and ceramics, organic polymeric materials, composite materials, appendices. Please refer to the detailed content. | 4 Glasses and ceramics Glasses The random three-dimensional network of molecular chains present in a glass is illustrated diagrammatically in Fig. . The chains consist of complex units based on the SiO4 tetrahedral unit. Pure silica can form a glass; it has a high softening temperature, so it is hard to work because its viscosity is high. Commercial glasses consist of silica mixed with metal oxides, which greatly reduce the melting temperature, thus making the glass easier and cheaper to make. There are two important families of commercial glass: soda lime and borosilicate. Soda lime glasses Soda lime glasses have a typical composition (wt%) 70 SiO2, 10 CaO, 15 Na2O and small amounts of other oxides. These glasses are employed for highvolume products such as windows, bottles and jars. The added metal oxides act as network modifiers in the structure of Fig. . Thus, when soda (NaO2) is added to silica glass, each Na+ ion becomes attached to an oxygen ion of a tetrahedron thereby reducing the cross-linking as indicated in Fig. . The effect of soda addition is thus to replace some of the covalent bonds between the tetrahedra with (non-directional) ionic bonds of lower energy. This reduces the viscosity of the melt, so that soda glass is easily worked at 700 °C, whereas pure silica softens at about 1200 °C. By the same token, this alloying of the glass to make it more workable reduces its strength at hightemperature, so that silica glass must be used in applications requiring hightemperature strength – such as the envelopes of quartz halogen lamps. Soda lime glasses are also sensitive to changes in temperature, and, because of their large coefficient of thermal expansion (~ 8 × 10–6 K–1), they may 133 134 Materials for engineering Na– Sketch of the structure of soda lime glass. develop high thermal stresses that can induce cracking. The second important family of glasses were developed to overcome this problem. Borosilicate glasses .