Vol. 35, Issue 4, pp. 819-827

Abstract

Formation of crystal phases in glass and the kinetics of the crystal growth are commonly interpreted on the basis of classical nucleation theory (CNT), a thermodynamic description of the formation and growth of clusters of atoms (nuclei) of a new phase in heterogeneous system. The high resolution electron microscopy (HREM) has been used to recognize this clusters (nuclei) formation, their nature and structure. The atomic scale mechanism of the formation of the crystal phase structure within the disordered structure of glass near the temperature T_g was studied. HREM and other methods of the early stages of crystallization of the SiO₂–Al₂O₃–MgO glass studies, with TiO₂ as a crystallization activator, revealed that glass crystallization can be realized directly through rearrangement of the parent glass structure elements as disorder-order transformation. Its mechanism is similar to the solid solution decomposition or polymorphic transformation. However in the glass with ZrO₂ admixture, crystallizing in a liquid state, the mechanism of crystal phase formation is different. Zirconia ZrO₂ particles precipitate and cordierite crystals are growing on them. Validity of CNT for describing the near glass transformation temperature T_g crystallization and nano glass ceramics formation appears disputable. Alternative way of explaining this process is proposed.