Stoichiometry-Induced Control of the Reaction Front in Ni/Al Reactive Multilayers

Reactive multilayers are a class of metastable materials that exhibit remarkable thermal, chemical, and mechanical properties, making them highly versatile across diverse applications. One of the most intriguing aspects of reactive multilayers is their inherent ability to undergo self-sustained, exothermic reactions with high temperatures. This phenomenon arises from the stacking of alternating thin films of constituents with a large negative enthalpy of mixing. Leveraging this behavior, reactive multilayers find applications as intrinsic heat sources in many fields, including thermal batteries, biological hazard neutralization, joining processes, micro-propulsion, thin-film healing, and the microelectronic industry. Despite numerous investigations on the control of their reaction behavior, further studies are required to broaden the application and enhance the performance of reactive multilayers.<br/>The objective of this study is to control and optimize the thermal management of Ni/Al reactive multilayers through the investigation of the impact of the stoichiometry on their reaction behavior. To this end, a wide range of Ni/Al ratios were fabricated and studied contributing to a substantial expansion of the current reported Ni/Al systems. Furthermore, the role of the bilayer thickness (B) was analyzed and batches of systems with 50, 70 and 90 nm-B were fabricated. This research focused on the evaluation of reaction kinetics and reached temperatures, characterizing alongside the mechanical properties, structure and composition of the systems through nanomechanical testing, differential scanning calorimetry (DSC), scanning transmission electron microscopy (STEM), and X-Ray Diffraction (XRD). MD simulations were conducted to investigate the kinetics and mechanical properties, revealing a consistent trend comparable to the experimental findings. When the concentration of Al exceeds the equimolar ratio with Ni, there is a noticeable decrease in the propagation rate. On the contrary, an increase in Ni concentration results in a reduction in the stoichiometry-dependent nature of propagation. The results obtained from this research will offer valuable insights into the design of Ni/Al reactive multilayers with customized thermal characteristics.