Environmentally Friendly Chemical Time Delays Based on Interrupted Reaction of Reactive Nanolaminates

Abstract

Chemical time delays, devices that burn over specific times from under a millisecond to several seconds, are widely used in mining as well as civilian and military pyrotechnics and generally are composed of environmentally hazardous materials. Reactive nanolaminates are energetic materials composed of two or more reactants organized in an alternating layered structure, which may be fabricated with environmentally friendly components. Many traits of reactive nanolaminates are desirable for time delay applications, including their long shelf life and their highly repeatable and finely tunable reaction velocities. However, their reaction velocities are generally too high to be broadly applicable in chemical time delays. In this work, we use polymer mesh substrates to produce a constrained network of reactive particles, which reduce the reaction velocity of Al/Ni multilayers approximately 100× from a range of 2.8 to 7 m s–¹ to a range of 7 to 90 mm s–¹. This is accomplished via an interrupted reaction mechanism wherein individual particles on the coated mesh react rapidly but exhibit a delay before igniting neighboring particles due to the required heat transfer at the particles’ interfaces. We present the macroscale reaction velocities as a function of substrate composition and size, as well as reactive coating thickness and bilayer spacing. We employ high-speed videography to probe the ignition delays and finite element analysis simulations to aid in explaining the observed trends. Through selection of substrate and coating properties, time delays spanning a large range of delay times can be packaged into standard form factors.