Crystallization kinetics of non-stoichiometric glass-forming compositions

Abstract

Crystallization kinetics of non-stoichiometric glass-forming compositions Vladimir M. Fokin, 2012. The complex thermodynamics, kinetics and mechanisms of crystallization of glass-forming melts remain a burning problem of glass science. Being the alternative to glass formation, crystallization underlies the development and production of traditional and advanced glass-ceramics. For several decades most studies of crystallization relied upon the use of stoichiometric (model) glasses. Thus, a detailed study of the kinetics of non-stoichiometric crystallization is urged from both practical and theoretical point of views. The present project proposes the investigation of non-stoichiometric crystallization. We plan to employ glasses of the pseudo binary joint lithium meta-silicate (LS) - calcium meta-silicate (CS), which has a simple eutectic (see Fig.1). Different general problems remain unsolved and need a detailed investigation. For instance: 1. In any nucleation theory the strongest parameter determining the thermodynamic barrier for nucleation, and the nucleation rate, is the specific surface energy, s, of the critical nucleus/melt interface. Unfortunately there are no direct methods to measure the surface energy of the critical nuclei. As far as we know, the surface energy of critical nuclei was estimated independently of nucleation rate measurements only in very few cases. Commonly s values are estimated from experimental nucleation rates as a fit parameter. Taking into account the shortage and importance of data on s, a systematic study of the variation of surface energy with the change of some parameters is quite desirable and will be made. 2. Heterogeneous surface nucleation follows the general laws of CNT - that is the kinetics of heterogeneous nucleation is determined by the nucleation time-lag and the steady-state nucleation rate. As we have shown earlier, in the glass of eutectic composition of the LS-CS joint, calcium meta-silicate (CS) crystals precipitate at relatively low temperatures only at an advanced stage of crystallization. That is, at a fixed temperature, there is some induction time tind when the LS primary phase no longer forms due to kinetic reasons and the CS crystals still do not appear. A similar effect also takes place at non-isothermal heating regime and results in two separated exothermic peaks in a DSC curve corresponding to the crystallization of LS and CS together with LS. We suppose that this induction time characterizes the time-lag for nucleation of CS. Thus the estimation of tind as a function of temperature is of interest for the understanding of CS nucleation and overall crystallization. 3. We plan to carry out a detailed investigation of crystallization of glasses belonging to both sides of the eutectic point of the phase diagram. Moreover, the assumed non-monotonic dependence of the LS nucleation rate on glass composition at crossing the eutectic composition is of special interest. 4. Crystal size distribution and its time evolution contain information on the nucleation and growth processes even after the nucleation process has stopped. Glasses with compositions on the LHS of the phase diagram enriched by lithium reveal extremely high nucleation rates. Therefore there are wide temperature intervals of overlapping nucleation and growth rate. Thus we plan to obtain for these temperatures the LS crystal size distribution to track the evolution of crystallization process from nucleation to the growth stage. 5. Finally, the influence of the heating and cooling rates on the sequence of phase formation should give some important clues about the overall mechanism of non-stoichiometric crystallization. (AU)