(382c) Aggregate Neck Growth & Coalescence by Multiparticle Sintering

Nanoclusters are made of primary particles that are either firmly - (aggregates) or loosely -attached to each other (agglomerates). Aggregates are attractive for reinforcing as well as in manufacture of optical waveguides and catalyst pellets since they facilitate gas or liquid transport at minimum pressure drop and energy consumption. In contrast, agglomerates decompose easily to their constituents so they are attractive in nanocomposites and paints but they are potentially more toxic than aggregates since they can release nanoparticles (Strobel & Pratsinis, 2007). The distinction between agglomerates and aggregates in practice is largely empirical. In fact it is related commonly to the required effort to breakup agglomerates (Tsantilis & Pratsinis 2004). The degree of agglomeration is still treated as a ?black box?, as such characterization is based only on the performance of the final product powder. Controlling the extent of powder agglomeration would minimize the demand for postgrinding or other costly secondary powder treatments (Heine & Pratsinis, 2007a). It is therefore essential to understand the underlying fundamentals of aggregate/agglomerate formation to design processes which ideally produce in one step the desired product. Kruis et al. (1993) introduced a monodisperse model that is widely used in aerosol reactor design describing the evolution of primary particle and agglomerate sizes by coagulation and sintering. Tsantilis & Pratsinis (2004) distinguished aerosol synthesis of aggregates from agglomerates by the end of particle sintering. Typically the asymptotic fractal-like dimension (D_f = 1.8) of aggregates/agglomerates is used to calculate their motion and collision rate. Akhtar et al. (1994) developed a Monte Carlo (MC) model simulating coagulation and sintering in two dimensions and found that sintering delays the attainment of the asymptotic D_f of the resulting aggregates/agglomerates. Recently Al Zaitone et al. (2009) calculated the dynamic eveolution of D_f by MC simulations of nanostructured aggregates for constant ratios of collision and sintering time using the 2-particle sintering (Koch & Friedlander, 1990) along the aggregate backbone structure. Coagulation of spherical particles into agglomerates and the resulting structure are reasonably well understood (particle-cluster and cluster-cluster agglomeration). The coagulation rate of particles during sintering, however, is not well understood as they experience a continuous change in structure that affects mostly their collision diameter and much less their primary particle diameter. Furthermore the degree of sintering determines the physical properties of the aggregates like their mechanical stability or electrical conductivity. 

Here formation of aggregates with more than the 2-particle contacts of Koch & Friedlander (1990) is investigated for different coordination numbers and D_f by direct numerical simulation (Heine & Pratsinis 2007b). The minimization of the free surface area is the driving force for the sintering, which results in a densification and shrinkage of aggregates. The evolution of the free surface area, particle structure and collision diameter of aggregates undergoing sintering are investigated. It is found that the normalized change in surface area collapses on one line if plotted as a function of dimensionless time. Figure 1 shows the evolution of the normalized free surface area of particles undergoing sintering, where tf is the time until 2 particles fully coalesce. Clearly more compact particles sinter faster.

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