Potassium silicate glasses

Figure 37. Nd decay times in potassium silicate glasses [from Ref. (131). ...

The most important property of sodium and potassium silicate glasses and hydrated amorphous powders is their solubility in water. The dissolution of vitreous alkali is a two-stage process. In an ion-exchange process between the alkali-metal ions in the glass and the hydrogen ions in the aqueous phase, the aqueous phase becomes alkaline, due to the excess of hydroxyl ions produced while a protective layer of silanol groups is formed in the surface of the glass. In the second phase, a nucleophilic depolymerization similar to the base-catalyzed depolymerization of silicate micelles in water takes place. 

Figure 1 The atomic scale structure of the potassium silicate glass containing 20 mol % K20 (Reproduced with permission from reference 17. Copyright 1991 American Institute of Physics.)...

Figure 6.09. Ionic conductivity data for sodium potassium silicate glasses fitted to Hendrickson-Bray function (After Hendrickson and Bray, 1972).

Potassium Silicate Glass (62.2Si02-37.2K2O) 820-902 0.1 0.007-0.055 Gas exchange Emiched O2 gas... 

Critical temperatures often follow a simple trend for systems of related compositions. The critical temperature of alkali silicate melts, for example, decreases in the order of increasing radius of the alkali ion present, such that lithium and sodium silicate melts clearly exhibit metastable immiscibility, while the existence of immiscibility in potassium silicate glasses has not been conclusively established. There is no evidence for the existence of phase separation in the rubidium or cesium silicate systems. 

Glasses containing two or more alkali oxides display the mixed-alkali effect, as shown in Figure 8.3 for sodium-potassium silicate glasses. 

Figure 8.3 The mixed-alkali effect on the electrical conductivity at 300 °C of sodium-potassium silicate glasses containing 20 mol% total alkali oxide. (Data supplied by J. J. Noonan)...

Sodium silicates are produced as glasses having SiOjtNajO molar ratios of 1.6-3.9. These are sold as lump or pulverized form, partly hydrated powders, and concentrated solutions. Potassium silicate glasses have SiOjrKjO molar ratios of 2.83-3.92 and are sold in pulverized, flake or solution form. 

Since most soluble silicates are made by dissolving the corresponding sodium or potassium silicate glasses, it seems appropriate to review some of the investigations of glasses that have been made since Vail s comprehensive. survey in 1952 (I). However, it should be kept in mind that the structures occurring in these glasses bear little or no relation to the nature of silica in the derived aqueous solutions beyond the effect of the SiOj Na,0 ratio. 

In sodium (or potassium) silicate glasses of SiOj NajO molar ratios of 2 1 to 4 1 such vitron units, if actually present, might disperse without complete depolymerization, giving the colloidal species known to exist in solution. However, since a 3.3 1 ratio solution appears to have the same properties whether made from glass or by dissolving amorphous fine silica in alkali solution, it seems unlikely that the same vitrons are formed spontaneously in solution. Possibly a detailed examination of 3.3 or 3.8 ratio solutions of sodium silicate made by the two methods might still reveal a persistent difference. 

Figure 13. Comparison of the experimental and computer-simulated spectra for potassium silicate glass (K20-4Si02). The upper trace represents the experimental spectrum after subtracting the broad underlying resonance. The lower trace is the computed best-fit spectrum (for the central fine-structure transition) with g = 2.0, A g is = -87 G, Do/gpu = 220 G, = 70 G, AZ)/gpB = 80 G, AF/gpe = 30 G, v = 8.9 GHz, A5pp = 7 G (Lorentzian line-shape). Adapted with permission from Khava and Purans (1980).

---