Yijin Mao

Evaluation of copper, aluminum, and nickel interatomic potentials on predicting the elastic properties

Choice of appropriate force field is one of the main concerns of any atomistic simulation that needs to be seriously considered in order to yield reliable results. Since, investigations on mechanical behavior of materials at micro/nanoscale has been becoming much more widespread, it is necessary to determine an adequate potential which accurately models the interaction of the atoms for desired applications. In this framework, reliability of multiple embedded atom method based interatomic potentials for predicting the elastic properties was investigated. Assessments were carried out for different copper, aluminum and nickel interatomic potentials at room temperature which is considered as the most applicable case. Examined force fields for the three species were taken from online repositories of National Institute of Standards and Technology (NIST), as well as the Sandia National Laboratories, the LAMMPS database. Using molecular dynamic simulations, the three independent elastic constants, C11, C12 and C44 were found for Cu, Al and Ni cubic single crystals. Voigt-Reuss-Hill approximation was then implemented to convert elastic constants of the single crystals into isotropic polycrystalline elastic moduli including Bulk, Shear and Youngs modulus as well as Poissons ratio. Simulation results from massive molecular dynamic were compared with available experimental data in the literature to justify the robustness of each potential for each species. Eventually, accurate interatomic potentials have been recommended for finding each of the elastic properties of the pure species. Exactitude of the elastic properties was found to be sensitive to the choice of the force fields. Those potentials were fitted for a specific compound may not necessarily work accurately for all the existing pure species.

Effects of Contact Force Model and Size Distribution on Microsized Granular Packing

Granular packing of microsized particles with different size distributions and contactforce models is studied using discrete element method (DEM). Three kinds of size distri-butions, monosized, uniform, and Gaussian, with mean diameter of 50, 60, and 70lm arestudied. Two aspects of microscale particle packing issues are addressed: one is the im-portance of van der Waals force when the particle size approaching to microscale, theother one is the structure variation caused by different contact force models. The resultsindicate that compared with contact force, the van der Waals force contributes very insig-nificantly to the final packing structure. The packing structures obtained using two differ-ent force models are similar to each other. The effects of particle size and its distributionon the packing structure are more significant than the force model.[DOI: 10.1115/1.4025969]Keywords: additive manufacturing, distinct element method, size distribution, granularmatter

Molecular Dynamics Simulation on Rapid Boiling of Thin Water Films on Cone-Shaped Nanostructure Surfaces

Molecular dynamics simulations (MDS) are employed to investigate the effects of the size of a nanocone on rapid boiling of an ultrathin liquid water film that is suddenly heated by a hot aluminum plate. A physically sound thermostat is applied to control the temperature of the aluminum plate and then to heat the water molecules that are placed on the solid surface. The results show that the cone nanostructures drastically enhance heat transfer from the solid aluminum plate to liquid water and the phase change process from liquid water to vapor. They also have significant effects on temperature histories and the density distributions in the system. In all cases studied, the water molecules above the solid surface rapidly boil after contact with an extremely hot aluminum plate and consequently a cluster of liquid water is observed to move upward during the phase change. It is also observed that the separation temperature associated with separation of liquid water film from the solid surface and its final temperature when the system is at equilibrium strongly depend on the height of the nanocone. Furthermore, in all cases, at a specific time after beginning of boiling, a nonvaporized water molecular layer is formed above the surface of the aluminum plate.

Evaluation of Turbulent Models for Natural Convection of Compressible Air in a Tall Cavity

Numerical simulation of turbulent natural convection of compressible air in a tall cavity is carried out. In order to evaluate the accuracy of turbulent models, various turbulent models are applied to solve the natural convection in a tall cavity that has different temperatures imposed on two opposing vertical walls. For the large-eddy simulation (LES) model, Smagorinsky subgrid scale (SGS) and dynamic Smagorinsky SGS are also applied to the same cases in order to investigate the differences in temperature and velocity caused by different turbulent models. It is found that the k–ϵ model has a high accuracy of predicting velocity distribution at various sampled lines by comparing with experimental data at Rayleigh number of 2.03 × 1010 and 3.37 × 1010, while the LES model has good performance in predicting temperature distributions.

Nonequilibrium Molecular Dynamics Simulation of Nanobubble Growth and Annihilation in Liquid Water

The objective of this article is to investigate nanosized bubble growth and annihilation processes in liquid water via nonequilibrium molecular dynamics (MD) simulation and compare the results with that obtained from the Rayleigh-Plesset equation. The TIP3P potential model is chosen to describe the structure of the water molecule. The SHAKE algorithm is used to hold two O-H bonds and H-O-H angle as rigid, and a harmonic bond style and a Charmm angle style are applied. The results show that the hydrodynamic model based on the Rayleigh-Plesset equation is not valid for predicting nanosized bubble growth and annihilation in liquid water.

Prediction of the Temperature-Dependent Thermal Conductivity and Shear Viscosity for Rigid Water Models

The temperature-dependent thermal conductivity and shear viscosity of liquid water between 283K and 363K are evaluated for eight rigid models with the reverse non-equilibrium molecular dynamics (RNEMD). The five-site models (TIP5P and TIP5P-Ew) have apparent advantages in estimating thermal conductivities than other rigid water models that overestimate the value by tens of percent. For shear viscosity, no single model can reproduce all experimental data; instead, five- and four-site models show their own strength in certain temperature range. Meanwhile, all of current rigid models obtain lower values than experimental data when temperature is lower than 298K, while TIP5P and TIP5P-Ew model can relatively accurately predict the values than others at temperature range from 298K to 318K. At higher temperature range, shear viscosity of liquid water can be reproduced with four-site model (TIP4P-2005, TIP4P-Ew) fairly well.Copyright

Melting, Vaporization, and Resolidification in a Thin Gold Film Irradiated by Multiple Femtosecond Laser Pulses

Melting, vaporization, and resolidification in a gold thin film subject to multiple femtosecond laser pulses are numerically studied in the framework of the two-temperature model. The solid-liquid phase change is modeled using a kinetics controlled model that allows the interfacial temperature to deviate from the melting point. The kinetics controlled model also allows superheating in the solid phase during melting and undercooling in the liquid phase during resolidification. Superheating of the liquid phase caused by nonequilibrium evaporation of the liquid phase is modeled by adopting the wave hypothesis, instead of the Clausius–Clapeyron equation. The melting depth, ablation depth, and maximum temperature in both the liquid and solid are investigated and the result is compared with that from the Clausius–Clapeyron equation based vaporization model. The vaporization wave model predicts a much higher vaporization speed, which leads to a deeper ablation depth. The relationship between laser processing parameters, including pulse separation time and pulse number, and the phase change effect are also studied. It is found that a longer separation time and larger pulse number will cause lower maximum temperature within the gold film and lower depths of melting and ablation.

Analysis of Cohesive Microsized Particle Packing Structure Using History-Dependent Contact Models

Granular packing structures of cohesive microsized particles with different sizes and size distributions, including monosized, uniform, and Gaussian distribution, are investigated by using two different history dependent contact models with discrete element method (DEM). The simulation is carried out in the framework of liggghts, which is a DEM simulation package extended based on branch of granular package of widely used open-source code LAMMPS. Contact force caused by translation and rotation, frictional and damping forces due to collision with other particles or container boundaries, cohesive force, van der Waals force, and gravity is considered. The radial distribution functions (RDFs), force distributions, porosities, and coordination numbers under cohesive and noncohesive conditions are reported. The results indicate that particle size and size distributions have great influences on the packing density for particle packing under cohesive effect: particles with Gaussian distribution have the lowest packing density, followed by the particles with uniform distribution; the particles with monosized distribution have the highest packing density. It is also found that cohesive effect to the system does not significantly affect the coordination number that mainly depends on the particle size and size distribution. Although the magnitude of net force distribution is different, the results for porosity, coordination number, and mean value of magnitude of net force do not vary significantly between the two contact models.

Atomistic-Continuum Hybrid Simulation of Heat Transfer Between Argon Flow and Copper Plates

A simulation work aiming to study heat transfer coefficient between argon fluid flow and copper plate is carried out based on atomistic-continuum hybrid method. Navier-Stokes equations for continuum domain are solved through the Pressure Implicit with Splitting of Operators (PISO) algorithm, and the atom evolution in molecular domain is solved through the Verlet algorithm. The solver is validated by solving Couette flow and heat conduction problems. With both momentum and energy coupling method applied, simulations on convection of argon flows between two parallel plates are performed. The top plate is kept as a constant velocity and has higher temperature, while the lower one, which is modeled with FCC copper lattices, is also fixed but has lower temperature. It is found that, heat transfer between argon fluid flow and copper plate in this situation is much higher than that at macroscopic when the flow is fully developed.Copyright

Molecule Dynamics Simulation of Heat Transfer Between Argon Flow and Parallel Copper Plates

Molecular dynamics (MD) simulation aiming to investigate heat transfer between argon fluid flow and two parallel copper plates in the nanoscale is carried out by simultaneously control momentum and temperature of the simulation box. The top copper wall is kept at a constant velocity by adding an external force according to the velocity difference between on-the-fly and desired velocities. At the same time the top wall holds a higher temperature while the bottom wall is considered as physically stationary and has a lower temperature. A sample region is used in order to measure the heat flux flowing across the simulation box, and thus the heat transfer coefficient between the fluid and wall can be estimated through its definition. It is found that the heat transfer coefficient between argon fluid flow and copper plate in this scenario is lower but still in the same order magnitude in comparison with the one predicted based on the hypothesis in other reported work. [DOI: 10.1115/1.4029158]