Using small‐angle x‐ray (SAXS), neutron (SANS), x‐ray diffraction and light scattering, we study the structure of colloidal silica and carbon on length scales from 4 Å < q−1 < 107 Å where q is the magnitude of the scattering vector. These materials consist of primary particles of the order of 100 Å, aggregated into micron‐sized aggregates that in turn are agglomerated into 100 µ 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 level of morphology, primary particles arise by a complex nucleation process in which primordial nuclei briefly aggregate into rough particles that subsequently smooth out to become the seeds for the primaries. The primaries aggregate to strongly bonded clusters by a complex process involving kinetic growth, mechanical disintegration and restructuring. Finally, the small‐angle scattering (SAS) data lead us to postulate that the aggregates cluster into porous, rough‐surfaced, non‐mass‐fractal agglomerates that can be broken 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 attribute the scaling to mechanical processing that deforms the fractal aggregates down to the maximum size able to withstand the compaction stress. Finally, we rationalize the observed structure based on empirical optimization by filler suppliers and some recent theoretical ideas due to Witten, Rubenstein and Colby.
A numerical model has been developed to predict the formation of carbon black particles by benzene pyrolysis. The approach, patterned after the work of Jensen, assumes that particle formation is controlled by four steps: gas phase reactions producing radical species, nucleation, growth and coagulation, and oxidation. In the present model, 15 reactions based on the work of Fujii and Asaba are used to describe the gas phase kinetics in which the phenyl radical is treated as the important intermediate in the formation of carbon black. A discrete distribution of 10 particle radii is also used to approximate the simultaneous growth and coagulation of spherical carbon particles. The results of the model show that a self-preserving log normal distribution of carbon particles develops after an initial nucleation and growth period. The model results also compare favorably with reactor data. In particular, the numerical model predicts a mass median carbon particle diameter to within 10% of measured values.
In the present study, the operating characteristics of supersonic particle probes are investigated. In particular, characteristics such as internal wall deposition and pressure recovery are examined. Three basic probe designs were tested in a cold flow experiment designed to simulate the hot, hostile environment of rocket and jet engine plumes. The probe designs consisted of two internal shock probes (Dehne and Colket probes) and one external shock probe (McGregor probe). In the internal shock probes the compression from supersonic to subsonic flow occurred either in a constant area throat (Dehne) or at a sudden expansion (Colket). In the external shock or McGregor probe, the shock was positioned slightly outside the entrance of the probe.From deposition studies performed on the probes, three factors were found to enhance deposition. These factors were: 1) shock-boundary layer interaction, 2) particle-boundary layer interaction, and 3) stagnation zones at sudden expansions. The probe with the lowest deposition was a McGregor probe with a 2.0" divergence angle. Using test particles with diameters of 1.0, 1.5, 2.0, and 2.5 pm, the average losses in the McCregor probe were 14% while in the Colket and Dehne probes the losses were 18% and 22070, respectively. Pressure recovery was also found to be the greatest in the McGregor probe with 48% of the initial stagnation pressure regained. The Colket probe had only a 7% pressure recovery.
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