Crushing strength of aluminum oxide agglomerates

Aluminum oxide, formed during aluminum-based solid propellent combustion, condenses and solidifies in the exhaust flow to form submicron size particles, which may then adhere together to form agglomerates. Particle sampling, required for motor performance or environmental impact investigations, is usually done using a probe placed in the supersonic exhaust flow field. The bow shock at the sample probe inlet will decelerate the gas flow which introduces a large velocity differential between the gas and the particulate. This differential will result in a sudden increase in the aerodynamic drag on the agglomerates which may cause them to shear apart, thus altering the sampled size distribution.

In this effort, aluminum-oxide agglomerates were formed and then crushed in order to estimate the magnitude of the interparticle forces binding the agglomerate together. The agglomerates were formed by tumbling commercially available aluminum-oxide powder in a jar, and strength was determined by measuring the load required to crush the agglomerate between two flat plates. Analysis indicated that rough, uniformly shaped particles formed the strongest agglomerates. The experimentally derived particle binding forces were in agreement with values predicted by the Van der Waals force equation for closely spaced spheres. Rocket exhaust particulates collected from a test facility exhaust processing system proved to be unusable due to impurities. Further studies are recommended with suitably collected and processed particulates.