Alkali distribution in borosilicate glasses

The alkali distribution in borosilicate glass has been discussed on the basis of the different quantities obtained by various experimental methods. The NBO fraction was chosen as a common quantity for evaluating the alkali distribution models. The NBO fractions in sodium borosilicate glasses were estimated from 11 B and 29 Si NMR as well as XPS. However, the best model was not found among the existing models. The amounts of sodium oxide consumed for the NBO formation in borate and silicate networks and for the formation of BO4 units were separately estimated from the 11 B and 29 Si NMR data. A new trend was found in the alkali distribution suggesting the possibility for a new universal distribution model. Borosilicate glass consists of borate and silicate networks formed by BO3 and SiO4 units, and the addition of network modifiers such as alkali and alkaline earth oxides to the glass leads to complicated structural changes. When alkali ions are associated with a silicate network, nonbridging oxygens (NBOs) are formed in the network. When associated with a borate network, fourfold coordinated borons (B4) and NBOs are produced in the borate network. Hence, the amount of the structural species present in the glass is dependent on the amount of the alkali ions associated with silicate and borate networks. Several alkali distribution models have been proposed for alkali borosilicate