Multiscale modeling and simulation of material phase change problems: ice melting and copper crystallization

The primary objective of this work is to propose a state-of-the-art physics based multiscale modeling framework for simulating material phase change problems. Both ice melting and copper crystallization problems are selected to demonstrate this multiscale modeling and simulation. The computational methods employed in this thesis include: classical molecular dynamics, finite element method, phase-field method, and multiscale (nano/micro coupling) methods.

Classical molecular dynamics (MD) is a well-known method to study material behaviors at atomic level. Due to the limit of MD, it is not realistic to provide a complete molecular model for simulations at large length and time scales. Continuum methods, including finite element methods, should be employed in this case.

In this thesis, MD is employed to study phase change problems at the nanoscale. In order to study material phase change problems at the microscale, a thermal wave method one-way coupling with the MD and a phase-field method one-way coupling with MD are proposed. The thermal wave method is more accurate than classical thermal diffusion for the study of heat transfer problems especially in crystal based structures. The second model is based on the well-known phase-field method. It is modified to respond to the thermal propagation in the crystal matrix by the thermal wave method, as well as modified to respond to temperature gradients and heat fluxes by employing the Dual-Phase-Lag method. Both methods are coupled with MD to obtain realistic results.

It should be noted that MD simulations can be conducted to obtain material/thermal properties for microscopic and/or macroscopic simulations for the purpose of hierarchical/sequential multiscale modeling. These material parameters include thermal conductivity, specific heat, latent heat, and relaxation time. Other type of interfacial parameters that occur during the phase change process, such as nucleus shape, interfacial energy, interfacial thickness, etc., are also obtained by MD simulation since these have so far been too difficult to measure experimentally.

I consider two common phase change phenomena, ice melting and copper crystallization, in this thesis. For the case of ice melting, MD is first employed to study its phase change process and obtain thermal properties of ice and water. Several potential models are used. I conduct simulations of both bulk ice and ice/water contacting cases. It is found that various potential models result in similar melting phenomena, especially melting speed. Size effects are also studied and it is found that the melting time is longer for larger bulk ice segments but that the average melting speed is size dependent. There is no size effect for the melting speed at ice/water interface at the nanoscale if the same temperature gradient is applied. The melting speed of ice should depend on the temperature gradient. To study ice melting at the microscale, the thermal wave model is employed with parameters obtained from MD simulations. It is found that ice melting speed is scale, for both length scale and time scale, dependent.

For the case of copper crystallization, an EAM potential is first employed to conduct MD simulations for studying the copper crystallization process at the nanoscale. I obtain thermal properties and interfacial parameters, including thermal diffusion coefficient, latent heat, relaxation time, interfacial thickness, interfacial energy and the anisotropy coefficients, and nucleus shape etc. A central symmetry parameter is used to identify an atom in solid state or liquid state. And then an initial nucleus shape is obtained and used as the input for microscale simulation, in which the phase-field method is used to study copper crystallization at the microscale.