Glass-forming oxides

Although boron phosphate is derived from two of the three most common glass-forming oxides, it exhibits Htde tendency to form a glass itself. Boron phosphate is a primary phase over a considerable portion of the B2O2—Si02—P20 system (159). 

E24.13 BeO, B2O3, and to some extent Ge02 because they involve metalloid and non-metal oxides. Transition metal and rare earth oxides are typically non-glass-forming oxides, many of which have crystalline phases. 

Further the Zachariasen requirement that energy difference between crystalline and amorphous phases of a glass forming oxide should not be very high, has also been proved to be incorrect. In the case of the archetypal Si02 glass, cm model (Figure 2.06) has been found to be... 

Typical and possible glass-forming oxides are listed in Table 1. 

Densities of the common glass forming oxides are less than those of the corresponding crystalline forms of these compounds. Vitreous silica, for example, has a density of 2.20 g cm" This value can be compared with the densities of a-quartz (the room temperature crystalline form of silica) of 2.65 g cm" p-cristobalite (the least dense crystalline form of silica) of 2.27 g cm" and coesite (a very dense crystalline form of silica obtained at high pressures) of 3.0 g cm". If we calculate the free volume, Vf, of the glass using the simple relationship ... 

Figure 21.4b shows the temperature dependence of viscosity for the main glass-forming oxides as a function of temperature. You will notice that from the slope of these lines we can obtain the activation energy for viscous flow,... 

HGURE 21.4 (a) Viscosity as a function of temperature (T) for several silicate glasses units of ii are dPa s. (b) Viscosity as a function of temperature (1/7) for the main glass-forming oxides. Notice the effect that a small amount of water has on the viscosity of silica glass. 

TABLE 4, Energies of Single Bonds in Various Glass-Forming Oxides and Glassy Chalcogenides and Covalent Components of Their Bond Energy [2]... 

Si02 High-quality sand or sandstone Glass-forming oxide, sometimes partially replaced by B2O3... 

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