One of the significant challenges in the use of nanoparticles is the control of primary particle size and extent of agglomeration when grown from the gas phase. In this paper we consider the role of surface passivation of the rate of nanoparticle coalescence. We have studied the coalescence of bare and H-coated silicon nanoparticles of sizes between 2-6 nm using molecular dynamics simulation at 1000 and 1500 K. We found that coalescence of coated particles consists of two steps, where reaction between particles and relocations of surface atoms near the reacting region, occur in the first step, which comprise an induction period. The second step consists of the nominal coalescence event, which depends on the surface tension and solid-state diffusion in the particle. The hydrogen passivation layer was found to remain on the surface of coalescing pair of the particles during the entire coalescence event. We also develop a mathematical model to describe the dynamics of coalescence of coated particles. The model is able to describe both the initial induction period and the coalescence period, and the role of the extent of surface coverage on the coalescence rate. In general, the entire coalescence time of coated particles is about 3-5 times that of bare particles, and the exothermicity from coalescence is about half that for the unpassivated particles.
Using the classical molecular dynamics method we simulate the mechanochemical behavior of small ͑i.e., core diameterϽ 10 nm͒ oxide coated aluminum nanoparticles. Aluminum nanoparticles with core diameters of approximately 5 and 8 nm are simulated with 1 and 2 nm thick oxide coatings or shells. In addition to thickness the shells are parametrized by varying degrees of crystallinity, density, and atomic ratios in order to study their effect on the ignition of nanoparticle oxidation. The oxide shells are parametrized to consider oxide coatings with the defects that commonly occur during the formation of an oxide layer and for comparison with a defect free crystalline oxide shell. Computed results include the diffusion coefficients of aluminum cations for each shell configuration and over a range of temperatures. The observed results are discussed and compared with the ignition mechanisms reported in the literature. From this effort we have found that the oxidation ignition mechanism for nanometer sized oxide coated aluminum particles is the result of an enhanced transport due to a built-in electric field induced by the oxide shell. This is in contrast to the currently assumed pressure driven diffusion process. This induced electric field accounts for approximately 90% of the mass flux of aluminum ions through the oxide shell. The computed electric fields show good agreement with published theoretical and experimental results.
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