Imagine you're driving to work on a rainy day when a distracted, reckless driver crashes into your car out of nowhere. A "boom" deploys an airbag faster than you can blink to save your life.
That airbag inflated quickly thanks to an energetic material called sodium azide, which during a chemical reaction generates nitrogen gas to inflate your airbag. But what is an energetic material?
Energetic materials include propellants, pyrotechnics, fuels and explosives, and they are used in a variety of environments.
The use of energetic materials includes flares, matches, solid rocket boosters, cannon propellants, hot thermite welding used to fuse materials, fireworks and the explosive special effects in your favorite action movie.
Energetic materials come in many shapes and sizes, but they often have a solid shape and release a lot of energy when burned or exploded, depending on their shape and the conditions in which they operate.
I am a mechanical engineering professor who studies energetic materials. Creating energetic materials is not easy, but developments in 3D printing can make customization easier while opening up more potential scientific applications.
The role of geometry
The way energetic materials are made affects the forms they come in and how they release energy over time. For example, solid rocket fuels are made in the same way as a cake where a stand mixer stirs the 'batter', which usually consists of ammonium perchlorate, aluminum and a rubbery binder, before pouring it into a pan. The "cake" will set in the pan while it bakes in the oven.
Typically, rocket propellants have a cylindrical shape, but with a rod in the center. The rod often has a specific cross-sectional shape, such as a circle or a star. When the propellant solidifies, the rod is removed, leaving the core shape.
The shape of the core affects the way the propellant burns, which can affect the thrust of the engine in which it is used. Just by changing the central shape of the propellant, you can make an engine speed up, slow down, or maintain its speed over time.
But this traditional 'cake baking' process limits the shapes you can make. You need to be able to remove the rod after the propellant has solidified, so if the shape of the rod is too complex you can break the propellant, causing it to burn irregularly.
Designing propellant forms that make rockets go faster or fly further is an active area of research, but engineers need new manufacturing methods to create these increasingly complex designs.
3D printing to the rescue
3D printing has revolutionized manufacturing in several ways, and researchers like me are trying to understand how it can improve the performance of energetic materials. 3D printing uses a printer to stack material layer by layer to build an object.
With 3D printing you can create custom shapes, print multiple types of material in one part and save money and materials.
However, it is a major challenge to 3D print energetic materials for several reasons. Some energetic materials are very viscous, meaning it is very difficult to squeeze that mixture out of a tube with a small nozzle. Imagine squeezing clay from a small syringe: the material is too thick to move easily through the small hole.
In addition, energetic materials can be dangerous if not handled properly. They can ignite if there is too much heat during the manufacturing process or during storage, or if they are exposed to a static electric shock.
Recent progress
Despite this, researchers have made much progress over the past decade to overcome some of these challenges. For example, scientists have 3D printed reactive ink on electronics to enable self-destruction if it falls into the wrong hands.
Theoretically, you could also strategically 3D print these inks on old satellites or the aging International Space Station to break these orbiting devices into small enough debris that would burn up in the atmosphere before hitting the ground.
Many researchers are investigating 3D printing of propellants for cannons. Changing the shape of cannon propellants allows bullets to fly further.
Others have tried to use 3D printing to reduce the environmental impact of cannon propellants and detonators, which require harsh solvents to produce. These solvents are unsafe, difficult to remove and can harm the environment and human health.
I showed that it is possible to 3D print solid rocket fuels that have similar properties to traditionally made propellants. With that research, we now have the opportunity to investigate how propellants made from multiple materials burn, which is new territory.
For example, instead of using a rod to create a cross-section in a propellant, you could 3D print a highly reactive material that you could add to the center. Instead of having to remove that center material, you could burn it so quickly that it leaves a core shape. The reactive material would also add energy to the propellant. This would eliminate the need to use and remove a rod to create a center core.
While much of this research is still in its infancy, companies such as X-Bow have 3D printed propellants and conducted successful flight tests with these engines.
Finally, several researchers have studied how 3D printed explosives detonate. When the explosives are pressed into a grid-like grid, they react differently when their pores are filled with air or water. This process produces a safer "switchable" explosive that does not react unless it is in a specific environment.
3D printing of energetic materials is still a new field. Scientists still have a long way to go before we fully understand how 3D printing affects their safety and performance. But every day, scientists like me find new ways to use 3D printed energies for crucial and sometimes life-saving purposes.
This article is republished from The Conversation, an independent nonprofit organization providing facts and trusted analysis to help you understand our complex world. It was written by: Monique McClain, Purdue University Read more: Monique McClain receives funding from the Air Force Office of Scientific Research (AFOSR), the Army Research Office (ARO), and the National Aeronautics and Space Administration (NASA).
