CAS OpenIR
反应-扩散调控材料成核过程的分子动力学模拟
窦翔宇
Thesis Advisor韩永生
2020-07-01
Degree Grantor中国科学院大学
Degree Name博士
Degree Discipline化学工程
Keyword成核,分子动力学模拟,反应,扩散,碰撞
Abstract

成核广泛地存在于物质转化过程中,是很多学科和领域的核心科学问题,比如雾霾治理、药物合成、纳米材料制备等。但是成核过程从实验上很难直接观测,实验数据的匮乏导致现有成核理论不得不基于诸多假设和猜测。本文采用高时空分辨率的分子动力学(MD)模拟的手段来研究材料成核过程,探索反应和扩散对成核过程的调控作用,给出基于反应和扩散调控的成核动力学模型,为材料结构定向合成提供理论指导,为解决涉及成核的工程问题提供新思路。本文首先建立了适用于本研究的分子动力学模型。包括MD模拟的基本流程,初始化、数值积分、后处理、力场计算等多步过程。详细阐述了运动方程数值积分以及长程力求解,给出不同方法的优劣分析。对分子动力学模型中的力场进行了分类梳理,根据研究体系选取了相应的力场,如嵌入原子势(EAM)描述模拟体系的金属原子间相互作用,Bucking力场适用于描述成核过程的离子间相互作用,OPLS (optimized potentials for liquid simulation)力场用于描述研究体系中的有机溶剂(如甘油)等。由于OPLS力场应用于甘油分子模拟时,出现了势能稳定性问题,因而对该力场参数进行了修正。修正后的力场模拟甘油体系运行正常且能量稳定,解决了非键氢氧原子的重叠问题,在新力场下甘油分子的宏观热力学性质和动力学性质与实验结果吻合更好。将此修正力场应用于银原子在甘油水溶液的成核过程,模拟的银核尺寸分布与实验结果吻合较好,从而进一步验证了修正后力场的有效性。接下来,研究了反应离子的扩散系数对成核过程的调控作用。采用全原子分子动力学模拟办法,在不同甘油体系中,模拟了碳酸钙溶液的成核过程,发现了碳酸钙成核速率与扩散系数成指数正相关。采用紫外吸收光谱方法,研究了碳酸钙在不同甘油体系中的成核过程,验证了模拟结果。而后,通过向碳酸钙溶液中施加脉冲能量加速离子扩散速率,在相同过饱和度的条件下,模拟了不同脉冲能量的碳酸钙在水和甘油溶剂中的成核过程,确定了成核速率与扩散系数的指数关系。在实验中通过在碳酸钙溶液中,引入超声外场,验证了分子动力学模拟结果。比较模拟得到的临界尺寸和扩散系数关系,发现扩散系数越大,临界尺寸越小。在经典成核理论中成核速率与扩散系数呈线性相关,但在本文中实验和模拟结果均表明无论增大或减小扩散系数,成核速率与扩散系数均呈指数关系,这种差异可能来自碳酸钙体系的二次成核,由于晶核聚集导致的成核速率激增是传统结晶理论预测的盲区。进一步,研究了反应和扩散对材料成核过程的调控作用。分别针对原子尺度和颗粒尺度的成核单元,采用调控成核单元浓度和扩散系数的办法,对不同反应和扩散条件下的成核过程进行了MD模拟研究。在原子尺度MD模拟中,采用EAM势和库仑力场来描述原子间的相互作用。采用溶剂粘度调控银原子扩散速率,发现了银成核速率与银原子的扩散系数的指数正相关。通过调控银原子初始浓度来改变银原子的反应生成速率,得到了银原子成核速率与银原子的浓度呈正相关,这一结果符合传统结晶理论预测。进一步统计不同反应和扩散条件下的成核单元碰撞次数,发现了成核速率与成核单元的碰撞次数呈线性关系。以银颗粒为基本成核单元的成核模拟中,在不同反应和扩散条件下,也得到与银原子相似的模拟结果,即银颗粒碰撞次数与成核速率线性相关(R2=0.98)。此外,针对带电体系,发现了其表面电荷对于成核过程的调控规律,不管是原子还是颗粒,当其表面电荷增大时,碰撞概率降低,成核速率也随之降低,这归因于成核单元间的斥力增大。最后采用原位电镜技术研究了不同反应和扩散条件下的银成核过程,验证了反应和扩散对成核过程的调控作用。基于以上发现,提出了碰撞成核动力学模型。本研究提高了对成核过程的理解和调控能力。;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. 

Language中文
Document Type学位论文
Identifierhttp://ir.ipe.ac.cn/handle/122111/49679
Collection中国科学院过程工程研究所
Recommended Citation
GB/T 7714
窦翔宇. 反应-扩散调控材料成核过程的分子动力学模拟[D]. 中国科学院大学,2020.
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