Effect of coalescence energy release on the temporal shape evolution of nanoparticles - art. no. 205402

The driving force for coalescence of two nanoparticles is the reduction in free energy through a reduction in surface area. The resulting particle als o has a lower total potential energy, which through conservation of energy can lead to a significant increase in particle temperature. In a growth pro cess particle heating competes with heat transfer to the cooler, carrier ga s. In this paper we develop a model that illustrates that this temperature increase can be extremely important and should be accounted for when modeli ng collision/coalescence processes. Our calculations indicate that the heat release associated with particle coalescence may reduce the coalescence ti me by as much as a few orders of magnitude. This is especially true for the final stages of exponential surface area decay toward sphericity, which be comes much faster and qualitatively explains the fact that primary particle s of only a few nanometers in diameter are of spherical shape. We develop i n this analysis a dimensionless ''coalescence heating number,'' which can b e used to assess if the exothermic nature of coalescence should be accounte d for under a given set of conditions. We also show that a simple coalescen ce model, which includes the temperature effect, closely follows our prior molecular dynamics calculations for silicon nanoparticles sintering. This a nalysis also explains a set of experimental results for alumina nanoparticl e production, previously unexplainable by classical methods. Finally, we se e that lower gas pressures result in lower gas-phase heat transfer, which i n turn results in larger primary particles.