SI-29 MAS-NMR STUDY OF THE SHORT-RANGE ORDER IN ALKALI BOROSILICATE GLASSES

Alkali borosilicate glasses have been prepared with the general compos ition RM(2)O.B2O3.KSiO2, where M is an alkali metal (lithium, sodium o r potassium), R represents the molar ratio of alkali oxide to boron ox ide and K represents the molar ratio of silicon dioxide to boron oxide . Si-29 magic angle spinning-nuclear magnetic resonance (MAS-NMR) was performed on these glasses to examine the short-range order about the silicon atoms. In this paper, the resultant chemical shift data of the se glass families were combined and examined in the light of various m odels derived from both B-11 and Si-29 NMR for the sharing of alkali o xide between the berate and silicate glass networks. These models all define an R value, R(0), below which the alkali oxide is entirely asso ciated with the berate network, and above which the alkali oxide is sh ared proportionally between the two glass networks. By combining the d ata from the three glass families, the model with R(0) optimized as a function of K was shown to provide an extremely good fit to the experi mental data, while promising to reconcile both the B-11 and Si-29 NMR data. Of the three alkali borosilicate families studied, only the pota ssium borosilicates showed the effect of CO2 retention from the alkali carbonate starting materials on the Si-29 NMR spectra.