The use of aluminum in solid propellants improves performance, but theoretical performance is not reached primarily because of two-phase flow losses. These losses could be reduced if particles ignited sooner rather than agglomerating before ignition, or if particles were dispersed when they burned. In order to modify the ignition and combustion of aluminum particles, the use of mechanically activated (MA) composite particles (Al/PTFE 70/30 wt.%) as a replacement for spherical, micron-sized aluminum in a composite solid propellant and their effects on burning rate, pressure dependence, and aluminum ignition, combustion, and agglomeration is considered here. Use of these modified Al particles, instead of micron-sized Al, results in an increase in pressure dependence (from 0.36 to 0.58) and a corresponding 55% increase in burning rate at a pressure of 15 MPa. Atmospheric observation of the burning surface suggests this increased dependence on pressure is a result of dramatically reduced burning particle size and the smaller particles sizes leads to kinetically controlled combustion. Unlike spherical aluminum, Al/PTFE composite particles promptly ignite at the propellant surface and fragment into smaller burning particles that are consumed closer to the surface. To capture these particles from burning propellant, a unique, solvent-free quench technique is used to minimize contamination and sampling error. Analysis of quenched combustion products collected on a substrate at pressure (2.1 to 13.8 MPa) just above the burning surface indicates these smaller burning particles result in a decrease in average agglomerate size from 76 μm to 25 μm (a 96% decrease in agglomerate volume), from the baseline and modified Al propellants, respectively. Analysis of these products from the modified Al propellant using scanning electron microscopy and energy dispersive spectroscopy indicate the presence of a fine (< 1 μm) particle fraction comprised of aluminum oxide and aluminum fluoride as well as a coarse fraction of aluminum oxide agglomerates, in contrast to the baseline propellant that shows significant unburned Al at the same collection height. In propellant applications, these effects are expected to translate to a decrease in two-phase flow loss and reduced slag accumulation, which could result in improved performance.
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