Control of primaryparticle Size and the Onset of Aggregate Formation: The Effect of Energy Release in Nanoparticle Colli
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Control of Primary Particle Size and the Onset of Aggregate Formation: The Effect of Energy Release in Nanoparticle Collision and Coalescence Processes Karl E.J. Lehtinen1 and Michael R. Zachariah* Departments of Mechanical Engineering and Chemistry University of Minnesota, Minneapolis, MN 55345. 1 on leave from VTT Energy, Finland * [email protected]; www.me.umn.edu/~mrz ABSTRACT
During coalescence, the surface area of the particle decreases, resulting in a heat release associated with the resulting lower surface energy. In a growth process particle heating competes with heat transfer by conduction to the cooler carrier gas and radiation. This temperature increase can be extremely important and should be accounted for when modeling collision/coalescence processes. The heat release associated with particle coalescence may reduce the coalescence time by as much as a few orders of magnitude. In addition, under some conditions there is insufficient time for the particles to cool to the gas temperature before another collision event takes place. It is shown that accounting for energy release and heat transfer effects have a dramatic effect on primary particle formation and the onset of aggregate formation. The results of the work indicate that to grow the largest primary particles one should operate at low pressures and high volume loadings. INTRODUCTION The ability to predict and control the primary particle size of nanostructured materials is essential since it is a key variable in many thermal, mechanical and optical properties. Typically in many industrial aerosol processes, a high concentration of very small particles undergoes rapid coagulation. This may lead to the formation of fractal-like agglomerates consisting of a large number of spheroidal primary particles of approximately uniform diameter. The size of the primary particles ultimately is determined by the rates of collision and coalescence. At high temperatures, coalescence occurs almost on contact resulting in large primary particles and hence small surface area. At low temperatures, the collision rate is faster than the rate of coalescence, leading to fractal-like agglomerates consisting of very small primary particles and thus large surface area. Controlling the coalescence rate is possible through a knowledge of the material properties and the time temperature history of the reactor and the collision rate through the volume loading of the material. Zachariah and Carrier (1999) studied the coalescence of Silicon nanoparticles using molecular dynamics (MD) simulation methods. They found that when two particles coalesce, there is a significant increase in particle temperature. This is illustrated in figure 1, in which particle temperature vs. time is shown for a typical coalescence event. Following collision the formation of new chemical bonds between the particles results in heat release and the formation of a neck between the particles. This heat release increases the particle temperature rapidly and thus also speeds up the coalescence. An oval shape is f
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