Architecture-Induced Control of the Heat Front Propagation in Ni/Al Reactive Multilayers

Reactive multilayers are metastable heterogeneous nanostructures consisting of the stacking of several layered thin films that have a strong tendency toward mixing. The constituents are characterized by a large negative enthalpy of mixing and a high adiabatic reaction temperature. Upon ignition, the alternating metal thin films react in a highly exothermic reaction front that propagates through the entire system in a self-sustaining manner. Taking advantage of their nanometer size, on-demand ignition, high temperatures reached, and many other features, these materials find application as intrinsic heat sources in many fields, such as thermal batteries, joining, thin-film healing, and in the microelectronics industry. Depending on the application, different propagation velocities and temperatures are required, and for this reason, many investigations on the control of the reaction behavior have been conducted in recent decades [1]. However, to expand their scope and enable these materials to perform in more specific applications, further studies need to be carried out. In our work, we analyze the thermal management of Ni/Al reactive multilayers through architecture design. To this end, molecular dynamics simulations [2] and experimental studies were performed on the effect of the presence of a premixed layer between the two reactants, the presence of interface roughness, and the stoichiometric variation on the heat front propagation velocity and the temperature reached. Molecular dynamics simulations demonstrated a direct correlation between the premixed layer thickness and the heat front propagation velocity. An increase in the premixed layer thickness leads to a decrease in the heat front propagation velocity. The same trend, with a decrease in the combustion temperature, was observed experimentally. A second simulation showed that the introduction of an interface roughness speeds up the heat front propagation. Contrary to modeling predictions, experimentally the reaction behavior appears to be unaffected to variations in roughness. Finally, a stoichiometric variation showed a change in the reaction front behavior in both simulation and experiment. For the experimental study, Ni/Al multilayers were deposited via magnetron sputtering, and the reaction was triggered by a low-power pulse. High-speed IR imaging was performed on the samples as well as structural and compositional characterization.<br/>[1] Schwarz, Fabian, and Ralph Spolenak. "An MD-study on changing the elemental distribution and composition by alloying to control front propagation in Al–Ni multilayers." <i>Journal of Applied Physics</i> 132.6 (2022): 065101.<br/>[2] Schwarz, Fabian, and Ralph Spolenak. "The influence of premixed interlayers on the reaction propagation in Al–Ni multilayers—An MD approach." <i>Journal of Applied Physics</i> 131.7 (2022): 075107.