Energy accumulation in nanoparticle collision and coalescence processes

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. Two such cases are investigated in this paper: (1) low pressure growth of Si nanoparticles and (2) high volume loading growth of TiO2 nanoparticles. 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/or high volume loadings.