Rongpei Shi

Self-organization of core-shell and core-shell-corona structures in small liquid droplets

It was recently discovered that core-shell and core-shell-corona microstructures in immiscible liquid alloys, which were previously obtained only in outer space, can be fabricated under gravity condition on earth using conventional gas atomization. The origin was attributed solely to Marangoni motion driven by temperature-dependence of interfacial energy. We found in this letter, with the aid of computer simulation, that coupled processes of spinodal decomposition, decomposition-induced fluid flow, collision and collision-induced-collision among second-phase droplets all play critical rules at different stages in the formation of these structures. Their contributions relative to the Marangoni effect are analyzed as function of system size.

Phase-field simulation of thermally induced spinodal decomposition in polymer blends

A phase-field simulation of thermally induced phase separation with various initial average concentrations under different quench depths is systematically carried out in this paper. The resultant morphology of the phase-separated materials is determined by the quench temperature and the average concentration. A detailed understanding of the effects of quench depth during the phase separation process of spinodal decomposition (SD) is presented. The coarsening mechanism with various average concentrations is investigated. In addition, hydrodynamic effects play an important role in phase separation in liquid immiscible alloys. Therefore, a systematic comparison between systems with and without hydrodynamic effects under different conditions is also considered. The model developed and the results obtained could shed light on using SD in immiscible polymer systems to obtain desired nanostructures for advanced applications.

Simulation Study of Heterogeneous Nucleation at Grain Boundaries During the Austenite-Ferrite Phase Transformation: Comparing the Classical Model with the Multi-Phase Field Nudged Elastic Band Method

In this work, molecular dynamics (MD) simulations have been used to study the heterogeneous nucleation occurring at grain boundaries (GBs) during the austenite (FCC) phase to ferrite (BCC) phase transformation in a pure Fe polycrystalline system. The critical nucleus properties (including size, shape, and activation energy) determined by classical nucleation theory are compared with those obtained by using a combination of the multi-phase field method (MPFM) and the nudged elastic band (NEB) method. For nucleation events that exhibit low-energy facets completely embedded within the parent FCC phase, there is a good agreement between the MD and the MPFM result with respect to the critical nucleus size, shape, and nucleation energy barrier. For systems where the emerging nucleus contains facets that cross the GB plane, the MPFM-NEB, when compared to MD, yields a better prediction than the classical approach for the nucleus morphology. New observations from the MPFM-NEB method indicate that the critical nucleus shape may change with volume and therefore depends on the nucleation driving force (undercooling).

Predicting epitaxial orientations and lattice structure in ultrathin magnetic thin films

Metastable phases, such as bcc Co or Ni and hcp Fe or Ni, reportedly possess extraordinary magnetic properties for epitaxial ultra-thin films. To understand phase stability of these epitaxy-oriented phases upon substrate lattices, we calculated novel phase diagrams of Co, Fe, and Ni ultrathin films by considering the chemical free energy, elastic strain energy, and surface energy. Verified by experimental data in the literatures, the stable epitaxy-oriented phases are readily identified from the phase diagrams. The stabilization of these metastable phases is determined by the interplay between orientation-dependent elastic strain energy and surface energy.