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44997-G10
Crystallization of N-Alkanes in Equilibrium and Under Shear

Jerome Delhommelle, University of South Carolina

During the first year of this project, we developed Hybrid Monte Carlo molecular simulations methods to study the early stages of crystallization (i.e. both the nucleation and the growth steps). We started by applying these methods to study the crystallization of spherical particles modeled by two different types of potential. First, we studied the crystallization from the melt of softly repulsive spheres interacting through an inverse-power-law potential. We worked at fixed supercooling (i.e. at a temperature 25% below the melting temperature) and considered three systems, defined by different values for the inverse power exponent. Modifying the value of this exponent allowed us to study the onset of crystallization in the domain of stability of the body-centered cubic phase and in the domain of stability of the face-centered cubic phase. We showed that, for the three systems, polymorph selection did not take place during crystal nucleation since the structure of the critical nuclei obtained for the three systems is not well defined. However, our results demonstrate that polymorph selection took place during the growth step. Second, we applied these methods to study the crystallization of Aluminum from the supercooled liquid. We simulated the entire crystallization process at P=1atm and at temperatures 20% and 15% below the melting temperature. We demonstrated that, for both supercoolings, nucleation and growth proceed into a random mixing of the hexagonal close packed structure and of the face centered cubic phase, with a predominance of the stable fcc form. The concentration of icosahedral-like atoms in the supercooled liquid was found to remain constant throughout nucleation and growth, showing that Ih-like atoms did not play an active role in the crystallization process. We also found that the crystallization mechanism of Aluminum significantly differed from that observed for simple fluids.

This work was carried out by a graduate student, Caroline Desgranges, partially supported by the PRF grant, and by the PI. Two papers, detailing this work, have been accepted for publication in the Journal of Chemical Physics and the Journal of Physical Chemistry B. As a young faculty, being awarded the PRF grant on the topic of research I want to develop (self-assembly on the nanoscale, with an emphasis here on the issue of the selection of a specific structure) was very encouraging.

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