Molecular Dynamics Simulation And Time-Dependent Gingzburg-Landau Theory Study On The Solidification Kinetics Of The Cubic Crystals And The Effect Of The Spontaneous Polarization

The solid-liquid interfacial kinetic coefficient ? is one of the fundamental physical properties regulating dendrite morphology and material defects.The study of this property will help the profound developments of the manufacturing industry,the renewable energy industry,and biomedical technology and meet the long-term needs of our country.Research efforts have been devoted to exploring the solidification/crystallization processes and relating issues in various systems,such as laser selective melting,lithium-ion battery stability,crystallization of 2D material,or protein crystals.No doubt,solving any of these issues could bring substantial commercial value and social benefits.However,the breakthrough points of these issues are often hidden in the spatial and time resolution interval of the magnitude order of the nanometer-picosecond.With the help of an advanced microscope and computational/simulation techniques,researchers have monitored the microscopic dynamics of the solid-liquid interfaces in some material systems(such as the nucleation process and rapid solidification so on),yielding qualitative understandings.Nevertheless,to control these dynamic processes accurately,a precise theory of solidification kinetics is needed.Till today,the mechanism of the pure metal solidification has not been fully clarified yet.For those cases where building block particles have more complex properties other than translational degrees of freedom(such as incorporating rotational degrees of freedom,etc.),the relating theory studies at home and abroad are nearly blank.The current thesis uses rapid solidification atomistic simulation based on molecular dynamic methods,combining state-of-the-art interface analysis and calculation techniques.We choose the elemental metals and a model dipolar particle system(which mimics different crystal phase structures,including face-centered cubic(fcc)and body-centered orthorhombic(bco))as our research systems.We have successfully extended the time-dependent Ginzburg-Landau theory to the solid-liquid phase transition system,developed a quantitative kinetic theory model to predict the solid-liquid interfacial kinetic coefficient accurately,and obtained an understanding of the quantitative relationship between the kinetic coefficient(and its anisotropy)and the crystal structure as well as the additional degrees of freedom of particles,the detailed works include:(1)For the first time,we have constructed the time-dependent GinzburgLandau(TDGL)theory framework to solidify fcc crystals.Unlike the only existing TDGL theory of body-centered cubic(bcc)crystal solidification,we have introduced two sets of Ginzburg-Landau order parameters corresponding to 8 nearest neighbors and 6 next-nearest neighbor reciprocal vectors for the fcc crystal structures.In the quantitative analysis of the density relaxation times(reduced by the corresponding structure factors),the necessity of introducing the second set of the Ginzburg-Landau order parameters has been verified.Subsequently,atomistic simulations and TDGL theory frameworks have been employed to investigate interface kinetic coefficients of metal iron(bcc)and nickel(fcc).It is found that the(110)orientations of the two metal systems,the TDGL prediction,agree well with the simulation predictions,while other existing kinetics theories could not provide correct predictions.For the(100)orientations of the iron and the nickel system,we found that the TDGL theory underestimated the simulation data by around13% and 12%,respectively.This discrepancy between the theory and simulation is discussed in detail and is probably arising from the approximations made in the TDGL formalism on density waves(first-order)and the static/dynamic structural properties of the interfacial liquids.Besides,under the time-dependent GinzburgLandau theory framework,we have compared the four metals(iron,nickel,magnesium,and terbium)described with different inter-atomic interaction potential functions.We have found a universal scaling behavior in the shape of the spatial distribution function of Ginzburg-Landau order parameters across the crystal-melt interface,for a particular type of crystal lattice and crystallographic orientation,independent of the potential function.Based on this knowledge,we argue that the TDGL theory of solidification directly predicts the kinetic anisotropy,which originates from the interfacial particle packing geometric information.(2)Using non-equilibrium molecular dynamics simulations,we measure the kinetic coefficients of crystal growth from a melt of model dipolar particles and assess the impact of the inter-particle interaction and CMI structure by systematically varying the particle dipole strength.Three structure types(fcc/melt,ferroelectric bco/melt,and ferroelectric bco/ferroelectric melt)with two CMI orientations are covered in this work.The comprehensive molecular-dynamics simulation results are then used in the development of a quantitative theoretical framework based on ideas from time-dependent Ginzburg-Landau theory,in which non-bcc crystal/melt interface(CMI)structures and particles with orientational degrees of freedom are included.The extended TDGL theory incorporated new order parameters corresponding to amplitudes of density waves for two sets of reciprocal lattice vectors and the lowest order ferroelectric Ginzburg-Landau free energy described by the polarization energy.The validity of the TDGL theory is confirmed by the quantitative agreement between the simulation and theory predictions of the kinetic coefficient.As predicted by the TDGL theory,the non-bcc CMI ? is determined by the square-gradient terms of different order parameter profiles multiply their corresponding dynamic relaxation times.The kinetic anisotropy is not simply governed by the lattice and CMI orientation static structure properties;it is also governed by the dynamic properties,differing from the prediction of the previous theory,which considers only the contribution of the principal reciprocal lattice vectors.Through the atomistic simulations,we demonstrate the collective orientational reorganization of melt particles adjacent to the crystal.Besides,for the ferroelectric-bco/melt CMIs,the TDGL theory identified the dynamic and dielectric properties of the partially ordered liquid as rate-limiting factors in the crystal growth kinetics.The significant impact of orientational ordering(polarization or magnetization)on the crystallization kinetics could be potentially leveraged to achieve solidification kinetics steering through external electric or magnetic fields.Our combined theory/simulation approach provides opportunities for future investigations of more complex crystallization kinetics.Finally,we summarized the deficiencies and proposed the direction for further improvement of the TDGL theory.We discussed the potential possibilities and challenges in the TDGL theory framework's further extensions to the solidification/crystallization processes with more complexities.