Bottom-up modeling of Al/Ni multilayer combustion: Effect of intermixing and role of vacancy defects on the ignition process

Vapor deposited multilayered aluminum/oxide and bimetallics are promising materials for Micro Electro Mechanical System technologies as energy carriers, for instance, microinitiators or heat microsources in biological or chemical applications. Among these materials, the Al/Ni couple has received much attention both experimentally and theoretically. However, the detailed relation between the chemical composition of the intermixed interfacial regions and its impact on the ignition capabilities remains elusive. In this contribution, we propose a two-fold strategy combining atomistic density functional theory (DFT) calculations and a macroscopic 1D model of chemical kinetics. The DFT calculations allow the description of the elementary chemical processes (involving Al, Ni atoms and vacancies basic ingredients) and to parameterize the macroscopic model, in which the system is described as a stack of infinite layers. This gives the temporal evolution of the system composition and temperature. We demonstrate that the amount of vacancies, originating from the deposition process and the Al and Ni lattice mismatch, plays a critical role on both the ignition time and the temperature. The presence of vacancies enhances the migration of atoms between layers and so dramatically speeds up the atomic mixing at low temperatures far below ignition temperature, also pointing to the relation between experimental deposition procedures and ageing of the nanolaminates.