The complexity of the structural ordering kinetics, commonly observed in the crystallization of colloidal particles, is caused by the orientational degrees of freedom of the building blocks. To date, extensive research has been conducted to regulate the crystallization processes and explored their potential photonics applications. Unfortunately, accurate prediction of crystallization kinetics associated with different degrees of freedom has remained a major challenge to the research community due to the limitations of the existing frameworks. The vital parameters used to describe the microscopic processes at the melt/crystal interface have been investigated. Among them, the interface kinetic coefficient (μ) is of great importance in determining the evolution morphology of crystal growth. Its magnitude and orientation-dependence anisotropy has been theoretically and computationally investigated. However, because the crystal/melt interfaces are sandwiched between condensed phases, it obtaining direct measurements of μ is very difficult.
Computer-based simulations have been successfully applied to measure μ and its corresponding anisotropy for different close-packed materials. These techniques allow modeling of dendritic growth rates and produce results that agree well with experimental data. In addition to computer techniques, the theoretical development of μ is rapidly progressing. And the majority of the existing kinetic crystallization theory models are based on classical transition-state theory, meaning they depend on fitting parameters for validation. However, modern kinetic crystallization theories are derived from density functional theories of inhomogeneous and freezing liquid systems. Specifically, the recently developed time-dependent Ginzburg-Landau (TDGL) theory has attracted research attention. It can produce a direct prediction of both anisotropy and the magnitude of μ without fitting parameters. Unfortunately, TGGL theory is limited to only body-centered-cubic structure and translation degrees of freedom.
Motivated by the results from previous research, Dr. Xian-Qi Xu and Professor Yang Yang from East China Normal University together with Brian Laird from the University of Kansas, Professor Jeffrey Hoyt from McMaster University, and Professor Mark Asta from UC Berkely, explored the critical role orientational ordering processes play in influencing the crystal growth in polar systems. The authors extended the existing TDGL theory to treat crystal structures BCC, taking into account the translational ordering processes or dipolar crystals during crystallization. The work is published in the journal, Crystal Growth and Design.
Briefly, the international collaboration study focused on crystal/melt interface systems modeled with dipolar particle models capable of mimicking three different crystal/melt interface structures. A new quantitative framework based on TDGL ideas and analytical expressions for interface kinetic coefficient was developed. Through molecular dynamic simulation, useful in resolving crystallization processes, the feasibility of the extended framework was verified. Its capacity to interpret the differences in crystal/melt interface and the dipolar particle dynamics was explored and discussed with respect to the difference in the μ. Additionally, the relationship between the μ and the interparticle interactions was studied by determining the key roles plays by individual parameters.
Results demonstrated a successful prediction of the crystal/melt interface kinetic coefficient using the extended theory. Taking advantage of the effects of orientation ordering on the crystallization kinetics, it was possible to control the solidification kinetics via electric or magnetic fields. The kinetic anisotropy depended on the lattice and crystal/melt interface orientation and well as the dynamic properties. Furthermore, the evolution of the anisotropy and magnitude of the interface kinetic coefficient with the particle diploe strength exhibited a generic bottom-up route for the direct growth of colloidal particles.
In summary, the authors built on the recent development of kinetic theories for crystal growth to extended the TDGL theory to adapt to three different interfaces. The predicted coefficient values agreed well with the simulation results, even without fitting parameters. The TDGL interpreted the origin of the kinetic anisotropy variations and their relationship with the orientational degrees of freedom. In a statement to Advances in Engineering, the authors stated that the study enables advancement of the TDGL theory for broader applications such as studying more complex crystallization kinetics.
Xu, X., Laird, B., Hoyt, J., Asta, M., & Yang, Y. (2020). Kinetics of Crystallization and Orientational Ordering in Dipolar Particle Systems. Crystal Growth & Design, 20(12), 7862-7873.