Diffusion of alkali

The attack of most glasses in water and acid is diffusion controlled and the thickness of the porous layer formed on the glass surface consequently depends on the square root of the time. There is ample evidence that the diffusion of alkali ions and basic oxides is thermally activated, suggesting that diffusion occurs either through small pores or through a compact body. The reacted zone is porous and can be further modified by attack and dissolution, if alkali is still present, or by further polymerisation. Consolidation of the structure generally requires thermal treatment. 

Nagashima et al. have measured the thermal diffusivity of alkali chlorides bromides, and iodidesi79 by this method. 

Non-lattice sites may play an important role in the incorporation of large foreign ions in crystal structures during coprecipitation Pingitore (Chapter 27) discusses the importance of these sites in the formation of coprecipitates of calcium carbonate containing Srz+ or Ba. White and Yee (Chapter 28) discuss the diffusion of alkali ions into defect structures in the surfaces of glasses and crystalline feldspars. 

Figure 10.14 Elemental diffusivities of alkalis in a rhyolitic melt (experimental data of Gabis et ah, 1979).

DIFFUSION OF ALKALI IONS IN NETWORK OXIDE GLASSES... 

Major disadvantages of chemical strengthening by ion exchange include costs, (in time and materials). In addition, not all compositions may be strengthened to useful depth profiles and the temperature used permits stress release by continued diffusion of alkali or structural relaxation of the network. 

Maki and others (1995) investigated the conversion mechanism of quartz to belite, starting with limestone, clay, and laboratory chemicals, heating to 1400 C, and quenching rapidly. With the diffusion of alkalies and lime, the quartz changed to cristobalite surficially, then to a liquid rich in SiO. With increase in lime-silica ratio from the outside, lath-shaped wollastonite was formed, and subsequently belite. With further increase in lime, the quartz and wollastonite eventually disappeared and the belite clusters developed. 

Fig. 11.2 Star crystals of nitrogen triiodide (Top— prepared by diffusion of iodine solution into aqueous ammonia, Bottom— prepared by diffusion of alkali ICl into aqueous ammonia)...

---