Lag time to crystal nucleation of supercooled lithium disilicate melts: A test of the classical nucleation theory

• Lag time τ to crystal nucleation of supercooled lithium disilicate melts is studied.
• Classical nucleation theory underestimates τ by up to seven orders of magnitude.
• A reduced thermodynamic driving force is proposed for nanocrystals.
• Above the melting point of the metastable nanocrystal nucleation is absent.

A noticeable discrepancy between the observed lag times to crystal nucleation of continuous cooling experiments and the predictions of the classical nucleation theory (CNT) is evident. In particular, in these experiments nucleation at free surfaces and in contact with noble metal is delayed by many orders of magnitude with respect to CNT even if one neglects the heterogeneous character of the phase transition in the kinetic analysis. In this paper it is proposed that delayed nucleation is a consequence of a smaller Gibbs free energy of the evolving critical nucleus as compared to the growing macrocrystal. Considering lag times in scales of the reduced melting temperature T/Tm from 0.5 to 0.92 a difference in the free energy of crystallization of 6.7 kJ mol− 1 and a melting point depression of 146 K is approximated. The results are in line with the description of heterogeneous systems as introduced by the generalized Gibbs approach but can be also a hint to metastable polymorphs at the nanoscale.