Revealing thermally-activated nucleation pathways of diffusionless solid-to-solid transition

Solid-to-solid transitions usually occur via athermal nucleation pathways on pre-existing defects due to immense strain energy. However, the extent to which athermal nucleation persists under low strain energy comparable to the interface energy, and whether thermally-activated nucleation is still possible are mostly unknown. To address these questions, the microscopic observation of the transformation dynamics is a prerequisite. Using a charged colloidal system that allows the triggering of an fcc-to-bcc transition while enabling in-situ single-particle-level observation, we experimentally find both athermal and thermally-activated pathways controlled by the softness of the parent crystal. In particular, we reveal three new transition pathways: ingrain homogeneous nucleation driven by spontaneous dislocation generation, heterogeneous nucleation assisted by premelting grain boundaries, and wall-assisted growth. Our findings reveal the physical principles behind the system-dependent pathway selection and shed light on the control of solid-to-solid transitions through the parent phase’s softness and defect landscape.

Kinetics of Crystallisation of Polymers - A Review

A review covering nucleation modes and models of polymer crystallization kinetics. The classical models assume the rate of crystallization to be related to temperature only. For materials exhibiting low molecular mobility, e.g., polymers, time effects appear justified. Ziabicki's model (51-53) allows the rate to be related to time. In thermal nucleation, this relation stems from the delay of the steady-state condition to become established under specific external conditions. The athermal mechanism of nucleation produces another time effect. It involves no potential barriers to be overcome by a cluster to become a nucleus and proceeds only on account of the change in the criterion for the nucleus stability (critical size) as external conditions are modified. Experiments showed the (iso and non-isothermal) crystallization rate to be directly related to time. The underlying phenomenon involves the athermal nucleation occurring on crystal residues left in the melt and the relaxation effect upon subsequent thermal nucleation. The applicability of Ziabicki's model is demonstrated.

Transient and Athermal Nucleation of Solids in Rapidly Quenched Liquid Si

ABSTRACTWe have theoretically investigated the nature and kinetics of solid nucleation in supercooled liquid Si within the framework of the classical nucleation theory, and corresponding to the cases in which the liquids are quenched at extremely high quenching rates (from 109 to 1011 K/s). In doing so, we identify and draw a general conclusion that in addition to the well-treated phenomenon of transient nucleation, one must also consider the mechanism of athermal nucleation in order to properly elucidate the situations that are encountered at such high quenching rates. Moreover, contrary to the common notion that the transient effect is relevant at low temperatures where sluggish kinetics prevail, it is noted that the effect can also become prominent at near-equilibrium conditions due to the increase in the time needed by the embryos to reach the exceedingly large critical size.