Melting/solidification of nanoparticles: new scale effects, thermally activated surface nucleation and bi-stable states
Kamran Samani (Iowa State University), Valery Levitas (Iowa State University)
Mechanics of Phase Transforming and Multifunctional Materials
Wed 9:00 - 10:30
CIT 219
Melting/solidification of nanoparticles and surface-induced premelting and melting are fundamental problems with significant applied interest. Phase field approach is an ideal continuum tool to study all the above phenomena. Despite the significant progress in PFA to melting of nanoparticles, there is one important drawback: while PFA resolves finite width interfaces and surface molten layer, external surface is considered as the sharp one, while it has comparable width. Here we advanced the phase field approach to melting by introducing the finite width of the external surface layer (particle-gas interface) as the new scale parameter, which leads to revealing various phenomena and previously unknown scale effects. Strong dependence of the melting temperature for nanoparticles of various radii and the width of the molten surface layer on is found. In addition to traditional continuous barrierless surface melting, barrierless jump-like surface melting and thermally activated surface melting via critical nucleus are revealed. Very rich temperature - transformation diagram is found, which includes various barrierless and thermally activated transformations between solid, melt, and surface melt, and complex hysteretic behavior under various temperature and trajectories. Kinetic results differ essentially from results based on equality of energies and barrierless nucleation. It also introduces kinetic transition from melt to surface melt, which cannot occur barrierlessly. Bi-stable states between solid and melt is found for 2 nm particle and between solid and surface melt for up to 5 nm particles, in a - dependent temperature range. Our result for size range of bi-stability between solid and melt are in agreement with MD simulations. Obtained results open unexplored direction of controlling surface melting and melting/solidification by controlling width of the external surface and utilizing predicted phenomena.