A lattice chain model for the thermal restructuring of nanoparticle chain aggregates

Chain aggregates composed of nanoparticles of carbon black, silica, titania and other metal oxides are produced commercially using aerosol reactors. Properties of nanoparticle chain aggregates (NCA), such as restructuring when heated in gaseous suspension, may play an important role during synthesis. It is hypothesized that in some cases a separate step in NCA creation is the restructuring of the chains initially formed by rigid body addition processes, for example cluster-cluster aggregation. A model of a freely rotating bead chain (zero activation energy for particle rotation) is adapted here from the polymer literature to describe nanoparticle chain restructuring resulting from interaction with the surrounding gas. The link between the physical properties of the surrounding gas and the dynamics of the nanoparticle chain is provided through the diffusion coefficient of the chain center of mass. We have studied the restructuring dynamics for primary particles of various diameters, different chain lengths and at different temperatures, and the model predicts a fast transition from a non-equilibrium initial chain configuration to a relaxed state. For a stretched chain of 64 particles, each 35 nm in diameter, in air at 1800 K and atmospheric pressure, the characteristic time for relaxation was 0.34 ms. This time is consistent with a 12 ms upper limit in restructuring time for soot aggregates reported in the literature. It is possible that in practice the approach to equilibrium may be delayed by constraints on particle rotation.