Vitreous silica diffusion coefficients

The diffusion of metal ions in vitreous silica has not been studied as extensively as that of the gaseous species. The alkali metals have received the most attention because their behavior is important in electrical applications. The diffusion coefficients for various metal ions are listed in Table 5. The general trend is for the diffusion coefficient to increase with larger ionic sizes and higher valences. 

Figure 2.9 Arrhenius plot showing temperature variation of nohle gas diffusion coefficients. Samples 1-3 are glass melts 4, 5, and 14 are vitreous silica 6 is commercial glass 7 and 14 are B203 8-10 are mixtures of alkali oxides with B203, Si02, and A1203 11 and 12 are obsidians 13 and 15 are Si02. Reproduced from Hiyagon (1981).

Although vitreous germania has a structure very similar to that of vitreous silica, its thermal expansion coefficient is approximately an order of magnitude larger. Gas diffusion studies indicate that the free volume of the vitreous germania structure is much smaller than that of the vitreous silica structure. Apparently this difference in free volume restricts the bond bending process which causes the low thermal expansion of vitreous silica. 

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