Abstract:
Despite melting being studied for well over one hundred years, there still
remains no complete microscopic theory of how melting occurs, despite many
models being proposed over the decades. There remain even fewer when
considering the melting of metal nanowires, and their stability during solidliquid
phase coexistence. Using classical molecular dynamics simulations
(MD) and continuum modelling, these questions are investigated.
The model developed is based on a Landau-Ginzburg type of free energy
functional, where bulk and surface melting temperatures for cylindrically
symmetric nanowires are derived. Predictions are made based on the
equations which suggest that the undercooling of bulk and surface melting
temperatures agree with the accepted trend of being proportional to the
reciprocal wire radius. Additionally, the theory predicts the existence of a
critical radius below which surface melting will not occur.
The analytic model is then tested against molecular dynamics simulations
on nickel and aluminium which are known to be wetting and partially wetting
respectively. Fits to bulk melting temperatures of nickel are good but require
a sharp interface, whereas in aluminium the model performs poorly, but
nevertheless agree with the sentiment of the developed model.
Nanowire stability during solid-liquid phase coexistence is studied in copper
using MD simulations. It is found that the preferred mode destabilising
the solid is approximately the wire length, with the presence of shorter wavelengths
due to the anisotropy of the solid-liquid interface. A dynamical
theory derived from capillary wave fluctuations is used to support these
arguments.