Reactive materials (RMs) are a class of composite materials that, when ignited, produce a sudden release of energy in the form of heat and pressure. Their performance, which can vary based on the choice of constituent materials, is typically midway between that of a propellant and an explosive. This makes them ideal for use in applications that rely on a quick, precise burst of energy such as ejector seats and airbags—situations where fractions of a second can make a world of difference. Recent advances in RM technology have largely focused on improving formulations, for example by altering the size, morphology, assembly, and ratios of the reactive particles. However, while effective in tailoring reactivity, many of these practices are limited by processing constraints or by diminishing returns in performance.
Christopher M. Spadaccini at Lawrence Livermore National Lab-oratory, Jennifer A. Lewis at Harvard University, and their colleagues have introduced a method of tuning the reactivity of RMs through three-dimensional (3D) printed structures. Their work, published in a recent issue of Advanced Materials (DOI: 10.1002/adma.201504286), makes use of modern-day 3D printing techniques to create unique 3D RM architectures that offer an added degree of tunability in energy transport.
(a) A three-dimensional printed channel (left) and hurdle (right) architectures composed of silver nanoparticle ink before deposition of the reactive material. (b) Snapshots of the propagating flame being assisted (top) or impeded (bottom) by the architecture. Credit: Advanced Materials.
The researchers used Al/CuO (thermite) as the reactive material in this work. They evaluated two device architectures, “channels” and “hurdles,” which offer differences in the orientation of the product expansion relative to the direction of intended propagation. First, a custom electrode is 3D-printed with a concentrated silver nanoparticle ink to define the architecture. A conformal film of Al/CuO nanoparticles is then deposited directly onto the printed electrodes through an electrophoretic deposition process.
The researchers studied the combustion process by monitoring the linear flame propagation velocity, a commonly used metric for comparing reactivity, using high-speed videography and varying device architectures, film thickness, and spacing between structures. They found that the orientation and spacing of the RMs had a significant effect on the propagation velocity. In the case of two parallel channels, when the spacing between them was small enough, local pressure waves produced by combustion of the RMs overlap in the intermediate region and direct hot gases forward, effectively increasing the propagation velocity.
The phenomena occurring in hurdle geometries are a bit more complex, as the expansion process also includes the formation and transport of hot particles from within the flame region. Researchers showed that if the hurdles are situated too closely, the expansion event is interrupted and energy pushback occurs, causing the velocity to be impeded. However, when the spacing between hurdles is increased, they could achieve a higher flame velocity even though the overall mass was decreasing. The underlying reason for this is a result of the architecture, which facilitates transport of these hot particles from hurdle to hurdle to propagate the flame. Kyle Sullivan, lead author of the article, points out “they’re two very simple architectures, but the scaling behavior is opposite due to the fact that the mode of energy transport being controlled in each case is different.”
This work has identified a number of critical geometric design parameters and validated the use of alternative 3D architectures in tailoring the dynamic behavior of reactive materials. As Sullivan explains, “Until now, most of the focus has been on reformulating to achieve a desired performance; what 3D printing brings to the table is the ability to use architecture to make better use of the formulations you already have.”
