In the synthesis of nanoparticles the high number concentrations, unavoidable in an even moderate yield process, lead to the formation of agglomerates. This is especially true in the aerosol phase, where stabilization of particles in the unagglomerated state is quite challenging. Upon contact the primary particles of an agglomerate form bonds which can be of different nature, depending on the synthesis process. At sufficiently high temperatures for instance, the viscosity of the particles or the diffusivity allow sintering of the particles in contact, and thus hard aggregates are formed.

Depending on the chemical nature of the particles the formation of chemical bonds between the particle surfaces may also occur. As a baseline of the interparticle energy van der Waals forces are always present, leading to relatively weak agglomerates if no other mechanism contributes to the bond strength. While aggregates are very stable structures that can hardly be broken up, agglomerates can be disintegrated in suitable processes for further use. The distinction between agglomerates and aggregates is significant to estimate the product properties of nanopowders since the strength of interparticle bonds to a large extent determines the physical properties and applicability in subsequent manufacturing steps.

For fumed silica a limit of dispersibility in liquid media is observed, where an increase of the energy input does not lead to further fragmentation. The particles remain at diameters of about 100 nm, while the primary particles are much smaller than this, in the range of 10-20 nm (e.g. Pohl et al. 2005). While electron micrographs show no signs of sintering there are obviously substructures of the agglomerates with high interparticle forces that can not be fragmented. It can be speculated that these substructures are aggregates, formed in regions of high temperature in the synthesis process where minute sintering in the contact regions was possible. These aggregates may then agglomerate in cooler regions to form weak agglomerates.

The aim of this work was the systematic investigation of the interparticle forces in silica agglomerates. To this end silica particles were generated and allowed to agglomerate at room temperature, forming van der Waals contacts. The formation of high energy bonds was then induced by controlled sintering at various temperatures between 1000 and 1500°C at a constant residence time of 30 ms. The bond energies of the primary particles within the agglomerates were determined using the method of impact fragmentation, which was adapted to the nanoscale (Seipenbusch et al., 2002). The method enables fragmentation of agglomerates under variation of the kinetic energy prior to impaction. Analysis of the fragmentation patterns at different initial kinetic energies then yields fragmentation curves that show the distribution of interparticle forces within the agglomerates. In parallel to the fragmentation experiments the evolution of solid state bridges was analysed using electron microscopy.

As a reference Aerosil® 200 was analysed for its fragmentation behaviour. It showed a fragmentability of 30% and a kinetic energy necessary to break 50% of the interparticle contacts of 5.5x1017 J/bond.

The experiments with our synthesized SiO2 showed, that particles agglomerated at room temperature are indeed held together by relatively weak forces and can be deagglomerated almost entirely. At temperatures higher than 1000°C however, the maximum degree of deagglomeration (fragmentability) obtainable in the applied energy range rapidly decreased. When sintering necks eventually became visible at temperatures above 1300°C the fragmentability had already dropped to about 50%. The fragmentation curve of Aerosil® 200 was approximated for temperatures exceeding 1400°C. The interparticle contacts in fumed silica therefore appear to be dominated by solid state necks.