Characterizing solid-state ignition of runaway chemical reactions in Ni-Al nanoscale multilayers under uniform heating

Nanoscale layers of nickel and aluminum can mix rapidly to produce runaway reactions. While self-propagating high temperature synthesis reactions have been observed for decades, the solid-state ignition of these reactions has been challenging to study. Particularly elusive is characterization of the low-temperature chemical mixing that occurs just prior to the ignition of the runaway reaction. Characterization can be challenging due to inhomogeneous microstructures, uncontrollable heat losses, and the nonuniform distribution of heat throughout the material prior to ignition. To reduce the impact of these variables, we heat multilayered Ni/Al foils in a highly uniform manner and report ignition temperatures as low as 245 °C for heating rates ranging from 2000 °C/s to 50 000 °C/s. Igniting in this way reveals that there are four stages before the reaction is complete: heating to an ignition temperature, low temperature solid-state mixing, a separate high temperature solid-state mixing, and liquid-state mixing. Multiple bilayer spacings, heating rates, and heating times are compared to show that the ignition temperature is a function of the bilayer spacing. A symmetric numerical diffusion model is used to show that there is very little chemical mixing in the first 10 ms of heating but significant mixing after 50 ms. These predictions suggest that ignition temperatures should increase for the slowest heating rates but this trend could not be identified clearly. The modeling was also used to examine the kinetic parameters governing the early stages of solid-state diffusion and suggest that grain boundary diffusion is dominant.