Stabilizing effects of oxidation on propagating formation reactions occurring in nanometer-scale metal multilayers

Abstract

Reactive rare-earth/transition metal multilayers exhibit a variety of complex reaction behaviors depending on surrounding gaseous environment and material design. Small period (< 100 nm bilayer), 5 μm-thick Sc/Ag multilayers undergo self-sustained formation reactions when ignited in air or in vacuum. High-speed videography reveals unstable reaction waves in these samples, characterized by the repeated, transverse passage of narrow, spin bands. Intermediate Sc/Ag designs – with multilayer period between 100 and 200 nm – only react in air. These multilayers exhibit propagating reactions with alternating unstable and stable characteristics. Narrow, spin bands advance the reaction front stepwise. Soon after the passage of a transverse band, a trailing oxidation wave encroaches on the intermetallic reaction front temporarily pushing the stalled wave forward in a uniform manner. Viewed in full, these events repeat giving rise to a new oscillatory behavior. Sc/Ag multilayers having a large period (> 200 nm bilayer) also react exclusively in air but exhibit a different propagating mode. The oxidation of Sc combined with the exothermic reaction of metal species results in continually-stable waves characterized by a smooth wavefront morphology and uniform velocity. The flame temperatures associated with propagating waves are estimated using measured heats of reaction and enthalpy-temperature relationships in order to provide insight into the possible phase transformations that occur during these different exothermic reactions.