Exploring the reaction mechanism in self-propagating Al/Ni multilayers by adding inert material

We present a new class of reactive materials termed “inert-mediated reactive multilayers” (IMRMs) that use inert material to decouple the effects of chemistry and maximum temperature in a reactive multilayer. Important considerations in the selection of composition and thickness for reactive and inert material in an IMRM are detailed. We then present the results from a specific set of IMRM samples that we fabricated using 23-nm-bilayer 1:1 Al:Ni reactive sections and 2:3 Cu:Ni inert sections. In these samples we observe a systematic reduction of heats of reaction, maximum reaction temperatures, and reaction propagation velocities as the volume fraction of reactive material is reduced. At the same time, metrics sensitive to the reaction mechanism and products indicate that there is little if any cross-contamination between inert and reactive material in any of the samples. This indicates that the IMRM samples all undergo the same net reaction (Al/Ni → AlNi) but at a range of different flame temperatures (roughly 1950 K to 1300 K). Using existing theoretical models for the relationship between flame temperature and propagation velocity, we analyze the experimental data to obtain the activation energy for the mixing process and find that this value varies significantly as the maximum reaction temperature changes. At high reaction temperatures we observe a very low activation energy (26 kJ/mol) which suggests diffusion of Ni in molten Al is the rate controlling mixing mechanism in agreement with the conclusions of other studies focused on un-mediated Al/Ni reactive multilayers. However, as the reaction temperature decreases the activation energy appears to shift to much larger values implying a change in the reaction mechanism. We postulate that this shift indicates that solid products are able to form earlier in the reaction at these temperatures, impeding atomic diffusion and intermixing.