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

Sodium borosilicate glasses have been prepared with the general compos ition of RNa(2)O.B2O3.KSiO2 where R represents the ratio of sodium oxi de to boron oxide and K is the ratio of silicon dioxide to boron oxide . The R parameter varied from 0 to 8.0 in the families K = 0.5, 1.0, 2 .0, 4.0 and 6.0. Si-29 MAS NMR was performed on these samples in order to determine the short range order (SRO) about the silicon atoms The average chemical shifts of the absorption spectra were tested against various models for the sharing of added sodium oxide between the silic ate and berate glass networks. These models all define an R value, R(o ), below which the sodium oxide is wholly associated with the berate n etwork and above which the sodium oxide is shared equally between the silicate and berate groups. As was observed previously in the lithium and potassium borosilicate glasses, the model with R(o)=O provides a b etter description of the results than other published models yet fails to agree with B-11 NMR studies of these glasses. However a new model is proposed, in which an optimised, non-zero (but small) R(o) varies w ith K and is shown to provide an extremely good fit to the experimenta l data and promises to be reconcilable with the B-11 NMR data. CO2 ret ention from the Na2CO3 starting material had no appreciable affect on the Si-29 NMR results. This behaviour agrees with that of the lithium borosilicate glasses but is in contrast with that of the potassium bor osilicate glasses.