【作者】 王铮；

【导师】 陈晓平； 刘道银；

【作者基本信息】 东南大学 ， 热能工程， 2018， 硕士

【摘要】 纳米颗粒由于独特的光学、电学和催化特性,被广泛地应用于能源、材料、化工、食品等行业中。在自然状态下,纳米颗粒容易形成聚团结构。在生产或运输纳米颗粒的过程中,常常伴随纳米颗粒聚团的破碎、重组、凝并过程,聚团结构的改变,会对颗粒产品性质、效果产生影响。深入研究聚团尺寸与形貌的变化过程,对调控纳米颗粒聚团结构和尺寸具有重要指导意义。本文采用耦合粘性颗粒碰撞模型的离散颗粒模型（DEM）,实现了颗粒聚团在剪切流场下的破碎与重组模拟方法;基于DEM模拟方法,耦合颗粒布朗运动力,以及颗粒碰撞、粘附、反弹、合并等多个过程,实现了纳米颗粒运动和凝并过程模拟方法。最后,基于表面能最小化和体积守恒方程,建立了纳米颗粒烧结动态模型。主要工作总结如下:考察了剪切强度、颗粒表面能、聚团初始结构对剪切流场中颗粒聚团的破碎与重组的影响,发现聚团的尺寸与形貌是剪切强度、颗粒表面能、聚团初始结构共同作用的结果。通过观察聚团回转半径（R_g）和分形维数（D_f）的变化,发现聚团在流场中的变化主要有以下三个阶段:（1）初始聚团受到流场作用拉伸破碎。（2）聚团碎片发生旋转和碰撞,破碎、团聚和密实化。（3）聚团的尺寸和结构达到动态稳定状态。随着剪切强度的增加,聚团尺寸减小且更加均匀,结构变得更加密实。当剪切强度较高时,增加表面能,聚团更大且更密实,反之当剪切强度较低时,增加表面能,聚团更加密实但尺寸变化不大。聚团初始结构的影响会随剪切强度的增加而逐渐消失。达到稳态时聚团的D_f的范围为1.71到2.65。在表面能和剪切强度均较小条件下,稳定状态时聚团D_f达到最小D_f=1.71。提高剪切速率,或提高表面能,或提高初始分形维数D_f0,,稳定状态时聚团的分形维数均增加。在表面能和剪切强度均较大的条件下,稳定状态时聚团的D_f达到最大,约为D_f=2.65。考察了在不同温度、初始粒径、数密度条件下,纳米颗粒团聚、凝并的演变过程和达到平衡态时聚团尺寸、形貌、与结构参数的分布。颗粒聚并是颗粒碰撞时间和颗粒合并时间两个因素的竞争结果。提高温度,或减小初始粒径,纳米颗粒的特征烧结时间减小,碰撞后更容易发生合并,导致平衡态时纳米颗粒聚团直径较小,聚团分形维数较大,但是聚团内单个纳米颗粒半径增加。提高初始数密度,颗粒自由程减小,碰撞频率增加,颗粒间团聚增强,可以一定程度促进颗粒合并,但是进一步提高数密度,颗粒合并趋势的不变。因此,提高初始数密度,聚团直径增加,聚团含颗粒数增加,聚团的分形维数减小,聚团内单个纳米颗粒半径先略微增加然后保持不变。当温度从873K提高到1273K,平衡时聚团的D_f从1.78增加到3.00。初始颗粒直径从2nm增加至10nm,聚团D_f从3.00减小到1.88。初始数密度从8?10^{2 0}/m³增加到8?10²²/m³,聚团D_f从3.00减小到1.92。针对静止条件下,考察了纳米颗粒聚团烧结的动态过程,分别模拟了一对等直径的纳米颗粒,一对不等直径的纳米颗粒,和几百个纳米颗粒构成的分形聚团的烧结过程,并与理论计算结果进行对比。在烧结初期,聚团的自由表面积减小速率非常快,之后迅速减小,最终达到稳定,烧结完成。等直径的纳米颗粒对烧结过程和纳米颗粒聚团烧结过程的自由表面积的减少均符合Koch and Friedlander模型在将来的研究中,可以将该烧结动态模拟与纳米颗粒运动和凝并模型耦合,研究在运动过程中颗粒烧结的详细过程。更多还原

【Abstract】 Nanoparticles have been widely used in the energy,material,chemical,food and other industries due to their unique optical,electrical and catalytic properties.In nature,Nanoparticles usually exist in the form of agglomerates that are not easily broken.In product and process engineering,the structure of agglomerates undergoes a series of changes,e.g.,agglomeration,breakage,restructuring and sintering,which determines their characteristics and influences device performance.Therefore,it is meaningful to study the changing process of agglomerates size and morphology,which provide an approach to controlling the size and structure of agglomerates.In this paper,DEM with a cohesive contact model is used to simulate the agglomerate breakage and restructuring of agglomerate under shear flow.DEM with a coagulation model is used to simulate the coagulation process of the nanoparticles in Brownian motion.Brownian motion is calculated by the Langevin equation.Finally,a dynamic sintering model is established based on the minimization of free energy.The key points of this thesis are summarized as follows:The effects of shear gradients,surface energies,and initial structures on the dynamic evolution of agglomerates under shear flow are simulated.The results show that the size and structure of agglomerates at a steady state are the results of competition among the shear gradient of the flow field,the surface energy and the initial structure of the agglomerates.Based on the variation of the radius of gyration（R_g）and fractal dimension（D_f）of agglomerates with time,the following three stages can be distinguished:（a）a stage dominated by stretch and breakage,（b）a stage dominated by agglomeration and densification,and（c）a stage characterized by the steady size and structure of agglomerates.With increasing shear gradient,the agglomerate fragments at the steady stage become smaller,more compact and more uniform.When the shear gradient is high,increasing the surface energy can result in larger fragments with a more compact structure.However,when the shear gradient is low,increasing the surface energy does not lead to larger fragments.The effect of initial structure on the stable size and structure of fragments disappears gradually with the increase in shear gradient.TheD_f ranges from 1.71 to 2.65.AD_f of 1.71 is found for the case with the smallest surface energy and shear gradient.When increasing the shear rate,surface energy,or D_f0,of the initial agglomerate,D_f increases.AD_f of 2.65 is found for the case with the largest surface energy and shear gradient.The effects of temperatures,initial particle size,and particle number density on the agglomeration and coagulation of agglomerates are simulated.The coagulation between particles is the competition result between particle collision time and characteristic sintering time.With increasing temperature or decreasing the initial particle size,the characteristic sintering time of the nanoparticles decreases,which means particles are more likely to coagulation when stick together.The agglomerates at the steady state become smaller,and the fractal dimension become larger,but the particles size increase.When temperature increases from 873K to 1273K,theD_f increases from 1.78 to 3.00.When initial particle diameter increases from 2nm to 10nm,theD_f decreases from 3.00 to 1.88.With increasing initial number density,the particles free path decreases and collision frequency increases,which promote agglomeration and coagulation but the effect is limited.Therefore,increasing initial number density lead to larger agglomerates,more particle number per agglomerate and lower D_f.Particles size increase slightly and remain unchanged.When initial number density increases from8?10^{2 0}/m³ to 8?10²²/m³,theD_f decreases from 3.00 to 1.92.In order to study the dynamic sintering process of static nanoparticle,we simulated the sintering of two nanoparticles of equal and unequal diameters,agglomerate consist of hundreds of nanoparticles,and the results were compared with theoretical results.The surface area of agglomerate is reduced very fast in the beginning,then gradually slows down,and reaches stability finally.The decrease of surface area of two equal size nanoparticles and agglomerate is in agreement with the theoretical results of Koch and Friedlander model.In future,this dynamic sintering model can be coupled with the DEM model to study the sintering process of particle.更多还原