Multilevel structure of reinforcing silica and carbon

Using small-angle x-ray (SAXS), neutron (SANS), x-ray diffraction and light scattering, we study the structure of colloidal silica and carbon on lengt h scales from 4 Angstrom < q(-1) < 10(7) Angstrom where q is the magnitude of the scattering vector. These materials consist of primary particles of t he order of 100 Angstrom, aggregated into micron-sized aggregates that in t urn are agglomerated into 100 mu agglomerates. The diffraction data show that the primary particles in precipitated silica are composed of highly defective amorphous silica with little intermediate -range order (order on the scale of several bond distances). On the next le vel of morphology, primary particles arise by a complex nucleation process in which primordial nuclei briefly aggregate into rough particles that subs equently smooth out to become the seeds for the primaries. The primaries ag gregate to strongly bonded clusters by a complex process involving kinetic growth, mechanical disintegration and restructuring. Finally, the small-ang le scattering (SAS) data lead us to postulate that the aggregates cluster i nto porous, rough-surfaced, non-mass-fractal agglomerates that can be broke n down to the more strongly bonded aggregates by application of shear. We find similar structure in pelletized carbon blacks. In this case we show a linear scaling relation between the primary and aggregate sizes. We attr ibute the scaling to mechanical processing that deforms the fractal aggrega tes down to the maximum size able to withstand the compaction stress. Finally, we rationalize the observed structure based on empirical optimizat ion by filler suppliers and some recent theoretical ideas due to Witten, Ru benstein and Colby.