Sintering of aerosol agglomerates

Abstract

Fundamental studies of the sintering of aerosol agglomerates have been undertaken. A model has been developed that closely follows the sintering kinetics of the two-sphere system. The model solves a detailed description of the evolving geometry as the particles sinter, allowing neck growth to be calculated beyond the small neck limit. Contributions due to multiple mechanisms are calculated by integrating the sum of the individual sintering mechanism fluxes. It has been demonstrated that the relative contributions of individual mechanisms can change as the neck grows. Moreover, changes in sintering temperature can drastically alter which of the sintering mechanisms is predominant. Experiments performed on model bisphere show that neck formation occurs quickly, until the difference between the surface free energy and the grain boundary energy of the aggregate becomes small. This metastable configuration depends upon the equilibrium, or dihedral, angle of the particle. Following neck growth, coalescence proceeds slowly. The projected area and hence, the mobility-equivalent diameter of an agglomerate will generally decrease as coalescence occurs. However, experimental observations of larger model agglomerates reveal some unexpected behavior, including apparent increases in the mobility-equivalent size. These effects may be caused by particle orientation in the mobility classifier. It is postulated that the rotation of individual particles to preferred grain boundary orientations leads to an increase in the projected area of the particle during sintering.