NON-ISOTHERMAL CRYSTALLISATION AND NANOSTRUCTURING IN POTASSIUM NIOBIUM SILICATE GLASSES

Potassium niobium silicate (KNS) glasses the compn. of which is characterized by the K2O/Nb2O5 molar ratio 0.85-1.2 and SiO2 50-54 mol% were examd. to clarify the effects of chem. compn. on formation of a transparent nanostructured state. DTA, X-ray diffraction and SEM were used to study the nonisothermal crystn. of the KNS glasses as well as their morphol. features. All glasses devitrify in 3 steps forming unidentified phases in the 1st 2 steps while, at higher temp. (1000-1100ﾰC), the crystn. of K3Nb3O6Si2O7 takes place. For prolonged heat treatment time (>5 h) at high temp. (1050-1100ﾰC) the transformation of this phase into the KNbSi2O7 ferroelec. one occurs in some extent. Nanostructuring occurs at the first stage of the devitrification process. It results from two partially overlapping processes: amorphous phase sepn. and subsequent crystn. It was shown that only for the glass with K2O/Nb2O5 molar ratio equal to 0.85 and SiO2 50 mol% is it possible to sep. the above processes by isothermal heat treatments at 680ﾰC obtaining fully transparent nanostructured samples. These samples contain nanocrystals 10 times smaller than the amorphous inhomogeneities of the phase sepd. matrix in which they are dispersed.