Enabling and controlling slow reaction velocities in low-density compacts of multilayer reactive particles

Uniformly-shaped, micron-scale particles with nanoscale layers of Al and Ni and a volume fill of 20% ± 1% were fabricated by depositing Al/Ni multilayer films onto mesh substrates with a square weave. The films break into individual particles at the weave intersections during removal and the resulting particles have lengths equal to three times their widths. Exothermic formation reactions in loose compacts of these particles were shown to self-propagate up to 200 times slower than reactions in continuous multilayer foils with similar chemistries, layer thicknesses and ignition thresholds. The difference is attributed to the time delay associated with heating each subsequent particle to the point of ignition as the reaction propagates. The results presented here demonstrate an ability to separately control the rate of mass diffusion and the rate of thermal diffusion in particle compacts and thereby produce slow and stable self-propagating formation reactions. Such reactions are desirable for both industrial and military applications such as chemical time delays.