Surface melting in nanoparticles and nanosystems. 2. Scientific and nanotechnological aspects of the role of surface melting in nanoparticles and nanosystems

Abstract

Taking into account results of our molecular dynamics experiments, we have concluded that of the three commonly considered alternative models of nanoparticle melting (homogeneous melting, liquid shell, nucleation of liquid and growth), the latter is the most adequate. At the same time, a more adequate model corresponds to a combination of continuous melting at the initial stage of the process with its subsequent abrupt completion. In other words, nucleation and growth of a liquid-like surface layer occur until a certain critical radius of the crystalline core of the particle is reached, and then melting is completed very quickly, almost abruptly (in fractions of a nanosecond) at a temperature interpreted as the nanoparticle melting temperature Tm. Then, the role of surface melting in nanoparticle sintering is discussed. According to our results, the sintering of metal nanoparticles at high temperatures cannot be reduced to a single mechanism: a certain role play surface melting, surface and bulk diffusion, deformation in the contact zone, and collective effects associated with the displacements of groups (clusters) of atoms rather than of individual atoms. We also have put forward and substantiated the hypothesis that the previously introduced redetermined Tamman temperature TT=0,5Tm corresponds to the switching of the scenario of sintering of metal nanoparticles from formation of a dumbbell-shaped nanocrystal at low temperatures to the scenario corresponding to coalescence of solid nanoparticles resulting in the formation of a defective nanocrystal of a shape close to spherical.

Keywords

• surface melting
• metal nanoparticles
• melting mechanisms
• coalescence and sintering
• molecular dynamics
• thermodynamics