Non-isothermal crystallization and nanostructuring in potassium niobium silicate glasses

Potassium niobium silicate (KNS) glasses the composition of which is characterized by the K₂O/Nb₂O₅ molar ratio ranging from 0.85 to 1.2 and SiO₂ 50-54 mol% were examined in order to clarify the influence of chemical composition on formation of transparent nanostructured state of glasses. Differential thermal analysis, X-ray diffraction and scanning electron microscopy were used to study the non-isothermal crystallization of the KNS glasses as well as their morphological features. It was found that all glasses devitrify in three steps forming unidentified phases at the first two ones while at higher temperature (1000-1100 °C) the crystallization of K₃Nb₃O₆Si₂O₇ takes place. For prolonged heat treatment time (more than 5 h) at high temperature (1050-1100 °C) the transformation of this phase into the KNbSi₂O₇ ferroelectric one occurs in some extent. Nanostructuring occurs at the first stage of the devitrification process. It results from two partially overlapped processes: amorphous phase separation and subsequent crystallization. It was shown that only for the glass with the K₂O/Nb₂O₅ molar ratio equal to 0.85 and SiO₂ 50 mol% it is possible to separate the above processes by isothermal heat treatments at 680 °C obtaining fully transparent nanostructured samples. These samples contain nanocrystals 10 times smaller than the amorphous inhomogeneities of the phase separated matrix in which are dispersed.