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

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.