Effect of intermixing on self-propagating exothermic reactions in Al/Ni nanolaminate foils

Abstract

Exothermic reactions can self-propagate rapidly in multilayered foils, and the properties of these reactions depend strongly on the heat of reaction, the average atomic diffusion distance, and the degree of intermixing at the layer interfaces prior to ignition. By performing low-temperature anneals on sputter-deposited Al/Ni nanolaminate foils, the thickness of the intermixed region between layers was increased and its effects on the heats and velocities of reactions were measured. The intermixed region consisted of the metastable Al9Ni2 phase while the final phase of the foil was Al3Ni2. Analytical and empirical models were used to predict reaction velocities as a function of bilayer thickness and intermixing thickness, and the predictions are in good agreement with the experimental results. Increasing the average thickness of the intermixed region from 2.4 to 18.3 nm reduced the reaction velocity for all of the foils but was most significant for the foils with bilayer thicknesses less than 25 nm. The results indicate that the reaction velocity can be separated into two distinct regimes. The first regime occurs for thicker bilayers in which the average atomic diffusion distance is large. In this regime, reaction temperatures are high and reducing the bilayer thickness increases the reaction velocity. The second regime occurs for thinner bilayers where reaction velocity is dominated by the reduction in available energy due to intermixing. In this regime, reducing bilayer thickness results in a decrease in reaction velocity.