Numerical Simulation Of Nanoparticle Agglomerate Breakage, Restructuring, And Coagulation

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.