Crystal nucleation in a glass during relaxation well below T_g

Abstract

Until quite recently, in almost all papers on crystal nucleation in glass-forming substances it was assumed that nucleation proceeds in a completely relaxed supercooled liquid and hence at constant values of the critical parameters determining the nucleation rate for any given set of temperature, pressure, and composition. Here, we analyze the validity of this hypothesis for a model system by studying nucleation in a lithium silicate glass treated for very long times (up to 250 days) in deeply supercooled states, reaching 60 K below the laboratory glass transition temperature, Tg. At all temperatures in the considered range, T<Tg, we observed an enormous difference between the experimental number of nucleated crystals, N( t), and its theoretically expected value computed by assuming the metastable state of the relaxing glass has been reached. Analyzing the origin of this discrepancy, we confirmed that the key parameters determining the nucleation rates change with time as a result of the glass relaxation process. Finally, we demonstrate that, for temperatures below 683 K, this particular glass almost fully crystallizes prior to reaching the ultimate steady-state nucleation regime (e.g., at 663 K, it would take 176 years for the glass to reach 99% crystallization, while 2,600 years would be needed for complete relaxation). This comprehensive study proves that structural relaxation strongly affects crystal nucleation in deeply supercooled states at temperatures well below Tg, hence this phenomenon has to be accounted for in any crystal nucleation model.

Effects of Structural Relaxation of Glass-Forming Melts on the Overall Crystallization Kinetics in Cooling and Heating

“…These theoretical concepts have been successfully applied to the interpretation of experimental data on nucleation as shown, e.g., in [23,[33][34][35] and in another contribution to the present special issue [36]. They also allow a new approach to the understanding of hysteresis effects in crystallization in cooling and heating as partly discussed already in [37].…”

“…Note that, accounting for the interplay of relaxation and crystal nucleation and growth, even at isothermal conditions, the general relations, Equations ( 10), (11), and (24), have to be employed, as a rule, for the description of overall crystallization since the nucleation and growth rates may vary with time due to relaxation. The simple relations, Equations ( 14) and (25) as described above, are not applicable any more (see also [34][35][36]).…”

“…In the latter mentioned paper, we drew attention to the dependence of the steadystate nucleation rate on the interplay of relaxation and crystal nucleation in cooling and heating. Here, this analysis is extended to the description of the effect of deviations of the glass-forming liquid from metastable equilibrium and its relaxation on the kinetics of overall crystallization in cooling and heating, going considerably beyond the analysis of the effects of the interplay of relaxation and crystallization on the overall crystallization under isothermal conditions performed in [34,35].…”

Special Topic Preface: Nucleation—Current understanding approaching 150 years after Gibbs

“…Nucleation from an amorphous glass is the subject of a number of experimental studies. Abyzov et al 23 examine the often-employed assumption that crystal nucleation in glass-forming substances occurs in a completely relaxed supercooled liquid. By studying nucleation in a lithium silicate glass treated for long periods in deeply supercooled states, they show that structural relaxation strongly affects crystal nucleation in deeply supercooled states at temperatures well below the glass transition.…”