Session: 12-06-03: Modeling of Nano- and Micro-Scale Phase Change Processes

Paper Number: 166200

Emission and Deposition of Atomic Clusters at a Metal Surface Subjected to Evaporation and Condensation: Molecular Dynamics Study 

Understanding the fundamental mechanisms of evaporation and condensation phenomena at interfaces between liquid metals and vapors is important for applications in thermal management, phase-change heat transfer, and high-temperature material processing and manufacturing, including laser ablation and laser powder bed fusion. In hydrodynamic and kinetic simulations of vapor flows, it is usually assumed that only evaporation of monomers occurs at the liquid surface, while the formation of clusters occurs via nucleation, collisions, and growth processes in the vapor volume.  

The goal of this work is to study the mechanisms of material evaporation and condensation from a liquid metal surface well below the thermodynamic critical temperature of the material.  For this purpose, large-scale molecular dynamics (MD) simulations of equilibrium evaporation and condensation at the interface between liquid copper and copper vapor, as well as non-equilibrium simulation of impact of copper clusters on a liquid copper surface are performed using LAMMPS. The embedded atom method potential is used to accurately capture metallic bonding and interatomic interactions. Simulations were conducted at temperatures ranging from 2000 K to 5500 K. To analyze the cluster content in the vapor phase and to distinguish between stable clusters and unstable atomic complexes, a special cluster identification algorithm is developed and used for post-processing of time-dependent simulation results.

The simulations indicate that, contrary to traditional assumption on the single-atom removal during the evaporation process, direct cluster emission from the liquid metal surface occurs, when the molar fraction of clusters in the evaporation flux can be larger than 20%.  The cluster content is dominated by dimers, although the molar fractions of larger clusters tend to increase with decreasing temperature. The content of clusters larger than trimers in the evaporation flux is substantially higher than the equilibrium fraction of such clusters in the vapor phase. The velocity distribution functions of evaporated monomers and clusters are nearly Maxwellian at a relaxation temperature that is smaller than the system temperature. The equilibrium evaporation coefficient is calculated and found to be significantly smaller than 1 at temperatures greater than ~3000 K.  The probability of condensation of monomers and dimers is calculated as a function of the impact speed, angle of incidence, and liquid surface temperatures in non-equilibrium MD simulations. The non-equilibrium and equilibrium condensation coefficients are found to be only weakly different from each other. In addition, a series of simulations of impact of 2-nm cluster on liquid surface was conducted to determine the number of atoms spattered from the surface and their energy. It was found that the angle of incidence dominates the amount of spattered material.  The results obtained in the present work can be used to parameterize the models of material evaporation from liquid metals.

This work was supported by the National Science Foundation, USA through RII-Track-1 Future Technologies and Enabling Plasma Processes project (award OIA-2148653).

Presenting Author: Saeed Siahtiri The University of Alabama

Presenting Author Biography: Saeed Siahtiri is a Ph.D. student in the Department of Mechanical Engineering at the University of Alabama, specializing in molecular dynamics and numerical simulations of phase-change phenomena.

Authors:

Saeed Siahtiri The University of Alabama
Alexey Volkov The University of Alabama