The Role of Interfacial Reactions in Aluminum/Nickel Reactive Multilayer Performance

Interfacial reactions play an important but often overlooked role in the performance of reactive material systems. These are systems where the reactants must be in physical contact for the reaction to occur. Consequently, the initial stages of the macroscopic reaction are extremely sensitive to the microscopic mechanisms operating at the interface between reactants. In the realm of intermetallic formation reactions, the aluminum/nickel system is one of the most frequently studied reactive materials systems. The literature abounds with data on this reaction, including experimental data spanning a large range of heating rates, overall compositions, and ignition methods, as well as a growing body of molecular dynamics simulations. Despite this substantial body of evidence, literature review reveals a gap in the experimental parameter space at heating rates between those studied by calorimetry and those observed in self-propagating reactions. In this dissertation I explore two approaches aimed at addressing this gap. Inert-mediated reactive multilayers are reactive multilayers to which inert material is added to suppress the maximum temperature. Taking experimental data, we use the theoretically predicted scaling of flame speed with flame temperature to infer that the rate-limiting process in the Al/Ni self-propagating reaction changes with decreasing temperature. In unmediated reactive multilayers (highest temperatures) the reaction rate appears to be limited by the diffusion of Ni in liquid Al. However, when the reaction temperature is reduced the reaction rate becomes limited first by the interfacial dissolution of Ni atoms and ultimately by the solid-state diffusion of Ni through a layer of intermetallic product. Nanocalorimetry is a small-scale thermal analysis technique capable of very high heating rates. Here we use it to study the Al/Ni interfacial reaction from 103 K/s – 105 K/s. Included in this effort is the development of in situ nanocalorimetry, which combines nanocalorimetry measurements with time-resolved electron microscopy for structural characterization. Results from these experiments show that while the first phase to form does not change between 103 K/s and 105 K/s, it is possible to drive the reaction into a regime where the nucleation of the first phase is controlled entirely by a parameter called the critical concentration gradient. This condition represents the minimum amount of mixing that is required before there is a positive driving force for nucleation, and has been predicted to play a role in phase suppression at higher heating rates. Taken together, these new experimental techniques and results provide valuable insights into the interplay between thermodynamics and kinetics in determining the progression of the Al/Ni interfacial reaction. This insight, in turn, will be valuable in understanding other reactive materials and in predicting reaction performance, particularly ignition