Phase separation in sodium silicates observed by solid-state MAS-NMR

Phase separation in glasses has been extensively studied due to its occurence in silica-rich glasses; however, some aspects as the first steps of the nucleation of domains with different compositions remain to be clarified as they are difficult to characterize with most analytical techniques. Thereafter, it will be demonstrated that these first steps of phase separation can be studied using solid-state magic angle spinning nuclear magnetic resonnance (MAS-NMR). First, the measurement of the spin-lattice relaxation (T-1) in such glasses without added paramagnetic impurities helps for the identification of phase separation and of small Q(4) clusters. Second, two-dimensional Si-29-Si-29 correlation experiments characterize the Si-O-Si network after various heat treatments and are used to obtain the compositions of the two demixing phases. The first phase is made of nearly pure silica (SiO2) and the second with a composition that tends towards the Na2Si2O5 composition. A link can be made with the crystallizing phases when longer heat-treatments are applied. (C) 2014 Elsevier BM. All rights reserved.