Molecular Dynamic Simulation Of Material Nucleation Process Regulated By Chemical Reaction And Diffusion

Nucleation process widely exists in the process of phase transformation of materials.It is also a key scientific issue in many disciplines and fields,such as haze control,drug synthesis,material preparation and so on.However,it is difficult to observe materials nucleation directly by experiments.The lack of experimental evidence of nucleation process leads to many assumptions and hypothesis in the developed nucleation theories.So,this dissertation employs the high-temporal-resolution molecular dynamics(MD)simulation to study the nucleation process of materials,to to explore the role and mechanism of chemical reaction and diffusion on the nucleation process.It is expected to develop a dynamic nucleation model regulated by chemicals reaction and diffusion,which would benefit the design and rational synthesis of materials structures,as well as to solve the haze pollution.Firstly,the molecular dynamic simulation model used in this study was established,which includes the introduction of the basic processes of MD simulation including initialization,numerical integration,post-processing,force field calculation and other multi-step processes.Furthermore,the numerical integral of motion equation and the solution of the long-range force were elaborated,and the advantages and disadvantages of different methods were compared.In the choice of force fields,the pair potentials and multi-body potentials were introduced and the selecting strategy of force field was given.Embedded-atom method(EAM)potential was used to simulate silver nucleation under implicit solvent,and Buckingham force field was used to simulate ion interaction in the nucleation process.In addition,the OPLS(Optimized Potentials for Liquid Simulation)force field was used to describe the glycerol used to regulate the ions diffusion in the nucleation process.The simulation was crashed when the original OPLS force field was used to simulate glycerol molecules,which triggered us to modify the force field.The modified force field solved the crashing problem and ran properly in simulating glycerol molecules.Furthermore,the simulated thermodynamic properties and kinetic properties of glycerol fit well with the experiment data.Silver nucleation was simulated in a glycerol solution under the modified force field.The simulated size distribution of silver clusters was in good agreement with the experiment,which verified the stability and effectiveness of the modification force field again.In the following,the role of chemicals diffusion in the process of nucleation was explored.The nucleation process of calcium carbonate solution in different glycerol systems was simulated by the method of all-atom MD simulation.It was found that the nucleation rate of calcium carbonate was exponentially dependent on the diffusion coefficient.The nucleation process of calcium carbonate in different glycerol solution was studied by UV absorption spectroscopy,which verified the simulation results.Then,by applying pulse energy to the calcium carbonate solution to accelerate the ion diffusion rate,the nucleation process in the all-atom model(water-glycerol-CaCO3 system)with different pulse energy was studied by MD simulation at the same supersaturation.The exponential relationship between the nucleation rate and diffusion coefficient was confirmed.By introducing ultrasonic field into calcium carbonate solution,the nucleation rate of calcium carbonate was measured at different ultrasonic pulse energy,which agreed with the results of MD simulation.It was also found that the critical size decreased with the increase of diffusion coefficients.In the classic nucleation theory,the nucleation rate is linearly related to the diffusion coefficient of chemicals.While in our study,both the experiment and simulation results showed that the nucleation rate was exponentially related to the diffusion coefficient.This difference may attribute to the two-step nucleation mechanism of calcium carbonate solution.The increase of nucleation rate caused by the aggregation of nuclei is neglected by the classic nucleation theory.Furthermore,the role of chemicals reaction and diffusion on the nucleation process was studied.In atomic scale and particle scale,the nucleation processes were explored by regulating the concentration and diffusion coefficient of the monomers.At the atomic scale MD simulation,EAM and Coulomb force fields were used to study the nucleation process.Controlling the diffusion rate of silver atom by changing the viscosity of the solvent,we found that the nucleation rate of silver atom is exponentially related to the diffusion coefficient of silver atom,which confirmed the finding above.Then,to regulate the reaction rate,different concentrations of silver ions were used in a simulation.It was found that the nucleation rate is dependent on the concentration of silver atoms,which agreed with the prediction of classic nucleation theory.To the discover the mechanism of chemical diffusion ad reaction in regulating nucleation,the collision number of monomers were calculated.It was found that the nucleation rate under different reaction and diffusion conditions was linearly related to the number of collisions of monomers.At the particle scale MD simulation(60 nm silver particle as the basic nucleation monomers),the similar conclusion with the atom scale were obtained by regulating the reaction and diffusion conditions,that is,the number of monomer collision was linearly related to the nucleation rate(R2=0.98).In addition,the effects of surface charge on the collision and nucleation process and were discovered.No matter the atom or particle scales,the linear correlation between the nucleation rate and the collision probability was obtained.When the surface charge increased,the collision probability decreases and the nucleation rate decreased.The reason may be that the electrostatic repulsion force makes the nucleation monomer spring away,thus reducing the collision probability.Finally,to verify the simulation results,silver crystallization was investigated in an in-situ scanning electron microscope at different reaction and diffusion conditions,which verified the simulation results.Based on the above finding,a dynamic nucleation model regulated by chemical diffusion and reaction was proposed,which may benefit the understanding and control on the nucleation process.